EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)

 CERN-PH-EP-2014-115 LHCb-PAPER-2014-020 3 June 2014

Observation of the ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ decay

The LHCb collaboration†††Authors are listed on the following pages.

The first observation of the Cabibbo-suppressed decay
$\Lambda_{b}^{0}\rightarrow J/\psi p\pi^{-}$ is reported using a data sample
of proton-proton collisions at 7 and 8 TeV, corresponding to an integrated
luminosity of 3 $\rm fb^{-1}$. A prominent signal is observed and the
branching fraction relative to the decay mode $\Lambda_{b}^{0}\rightarrow
J/\psi pK^{-}$ is determined to be

$\frac{{\cal B}(\Lambda_{b}^{0}\rightarrow J/\psi p\pi^{-})}{{\cal
B}(\Lambda_{b}^{0}\rightarrow J/\psi pK^{-})}=0.0824\pm
0.0025\>(\text{stat})\pm 0.0042\>(\text{syst}).$

A search for direct CP violation is performed. The difference in the CP
asymmetries between these two decays is found to be

${\cal A}_{CP}(\Lambda_{b}^{0}\rightarrow J/\psi p\pi^{-})-{\cal
A}_{CP}(\Lambda_{b}^{0}\rightarrow J/\psi pK^{-})=(+5.7\pm
2.4\>(\text{stat})\pm 1.2\>(\text{syst}))\%,$

which is compatible with CP symmetry at the $2.2\sigma$ level.

Published in JHEP 07 (2014) 103

© CERN on behalf of the LHCb collaboration, license CC-BY-3.0.

LHCb collaboration

R. Aaij41, B. Adeva37, M. Adinolfi46, A. Affolder52, Z. Ajaltouni5, S. Akar6,
J. Albrecht9, F. Alessio38, M. Alexander51, S. Ali41, G. Alkhazov30, P.
Alvarez Cartelle37, A.A. Alves Jr25,38, S. Amato2, S. Amerio22, Y. Amhis7, L.
An3, L. Anderlini17,g, J. Anderson40, R. Andreassen57, M. Andreotti16,f, J.E.
Andrews58, R.B. Appleby54, O. Aquines Gutierrez10, F. Archilli38, A.
Artamonov35, M. Artuso59, E. Aslanides6, G. Auriemma25,n, M. Baalouch5, A.
Băbeanu38,q, S. Bachmann11, J.J. Back48, A. Badalov36, V. Balagura31, W.
Baldini16, R.J. Barlow54, C. Barschel38, S. Barsuk7, W. Barter47, V.
Batozskaya28, V. Battista39, A. Bay39, L. Beaucourt4, J. Beddow51, F.
Bedeschi23, I. Bediaga1, S. Belogurov31, K. Belous35, I. Belyaev31, E. Ben-
Haim8, G. Bencivenni18, S. Benson38, J. Benton46, A. Berezhnoy32, R. Bernet40,
M.-O. Bettler47, M. van Beuzekom41, A. Bien11, S. Bifani45, T. Bird54, A.
Bizzeti17,i, P.M. Bjørnstad54, T. Blake48, F. Blanc39, J. Blouw10, S. Blusk59,
V. Bocci25, A. Bondar34, N. Bondar30,38, W. Bonivento15,38, S. Borghi54, A.
Borgia59, M. Borsato7, T.J.V. Bowcock52, E. Bowen40, C. Bozzi16, T. Brambach9,
J. van den Brand42, J. Bressieux39, D. Brett54, M. Britsch10, T. Britton59, J.
Brodzicka54, N.H. Brook46, H. Brown52, A. Bursche40, G. Busetto22,s, J.
Buytaert38, S. Cadeddu15, R. Calabrese16,f, M. Calvi20,k, M. Calvo Gomez36,p,
A. Camboni36, P. Campana18,38, D. Campora Perez38, A. Carbone14,d, G.
Carboni24,l, R. Cardinale19,38,j, A. Cardini15, H. Carranza-Mejia50, L.
Carson50, K. Carvalho Akiba2, G. Casse52, L. Cassina20, L. Castillo Garcia38,
M. Cattaneo38, Ch. Cauet9, R. Cenci58, M. Charles8, Ph. Charpentier38, S.
Chen54, S.-F. Cheung55, N. Chiapolini40, M. Chrzaszcz40,26, K. Ciba38, X. Cid
Vidal38, G. Ciezarek53, P.E.L. Clarke50, M. Clemencic38, H.V. Cliff47, J.
Closier38, V. Coco38, J. Cogan6, E. Cogneras5, P. Collins38, A. Comerma-
Montells11, A. Contu15, A. Cook46, M. Coombes46, S. Coquereau8, G. Corti38, M.
Corvo16,f, I. Counts56, B. Couturier38, G.A. Cowan50, D.C. Craik48, M. Cruz
Torres60, S. Cunliffe53, R. Currie50, C. D’Ambrosio38, J. Dalseno46, P.
David8, P.N.Y. David41, A. Davis57, K. De Bruyn41, S. De Capua54, M. De
Cian11, J.M. De Miranda1, L. De Paula2, W. De Silva57, P. De Simone18, D.
Decamp4, M. Deckenhoff9, L. Del Buono8, N. Déléage4, D. Derkach55, O.
Deschamps5, F. Dettori38, A. Di Canto38, H. Dijkstra38, S. Donleavy52, F.
Dordei11, M. Dorigo39, A. Dosil Suárez37, D. Dossett48, A. Dovbnya43, K.
Dreimanis52, G. Dujany54, F. Dupertuis39, P. Durante38, R. Dzhelyadin35, A.
Dziurda26, A. Dzyuba30, S. Easo49,38, U. Egede53, V. Egorychev31, S.
Eidelman34, S. Eisenhardt50, U. Eitschberger9, R. Ekelhof9, L. Eklund51,38, I.
El Rifai5, Ch. Elsasser40, S. Ely59, S. Esen11, H.-M. Evans47, T. Evans55, A.
Falabella16,f, C. Färber11, C. Farinelli41, N. Farley45, S. Farry52, RF Fay52,
D. Ferguson50, V. Fernandez Albor37, F. Ferreira Rodrigues1, M. Ferro-Luzzi38,
S. Filippov33, M. Fiore16,f, M. Fiorini16,f, M. Firlej27, C. Fitzpatrick38, T.
Fiutowski27, M. Fontana10, F. Fontanelli19,j, R. Forty38, O. Francisco2, M.
Frank38, C. Frei38, M. Frosini17,38,g, J. Fu21,38, E. Furfaro24,l, A. Gallas
Torreira37, D. Galli14,d, S. Gallorini22, S. Gambetta19,j, M. Gandelman2, P.
Gandini59, Y. Gao3, J. Garofoli59, J. Garra Tico47, L. Garrido36, C. Gaspar38,
R. Gauld55, L. Gavardi9, G. Gavrilov30, E. Gersabeck11, M. Gersabeck54, T.
Gershon48, Ph. Ghez4, A. Gianelle22, S. Giani’39, V. Gibson47, L. Giubega29,
V.V. Gligorov38, C. Göbel60, D. Golubkov31, A. Golutvin53,31,38, A. Gomes1,a,
H. Gordon38, C. Gotti20, M. Grabalosa Gándara5, R. Graciani Diaz36, L.A.
Granado Cardoso38, E. Graugés36, G. Graziani17, A. Grecu29, E. Greening55, S.
Gregson47, P. Griffith45, L. Grillo11, O. Grünberg62, B. Gui59, E. Gushchin33,
Yu. Guz35,38, T. Gys38, C. Hadjivasiliou59, G. Haefeli39, C. Haen38, S.C.
Haines47, S. Hall53, B. Hamilton58, T. Hampson46, X. Han11, S. Hansmann-
Menzemer11, N. Harnew55, S.T. Harnew46, J. Harrison54, T. Hartmann62, J. He38,
T. Head38, V. Heijne41, K. Hennessy52, P. Henrard5, L. Henry8, J.A. Hernando
Morata37, E. van Herwijnen38, M. Heß62, A. Hicheur1, D. Hill55, M. Hoballah5,
C. Hombach54, W. Hulsbergen41, P. Hunt55, N. Hussain55, D. Hutchcroft52, D.
Hynds51, M. Idzik27, P. Ilten56, R. Jacobsson38, A. Jaeger11, J. Jalocha55, E.
Jans41, P. Jaton39, A. Jawahery58, F. Jing3, M. John55, D. Johnson55, C.R.
Jones47, C. Joram38, B. Jost38, N. Jurik59, M. Kaballo9, S. Kandybei43, W.
Kanso6, M. Karacson38, T.M. Karbach38, S. Karodia51, M. Kelsey59, I.R.
Kenyon45, T. Ketel42, B. Khanji20, C. Khurewathanakul39, S. Klaver54, O.
Kochebina7, M. Kolpin11, I. Komarov39, R.F. Koopman42, P. Koppenburg41,38, M.
Korolev32, A. Kozlinskiy41, L. Kravchuk33, K. Kreplin11, M. Kreps48, G.
Krocker11, P. Krokovny34, F. Kruse9, W. Kucewicz26,o, M. Kucharczyk20,26,38,k,
V. Kudryavtsev34, K. Kurek28, T. Kvaratskheliya31, V.N. La Thi39, D.
Lacarrere38, G. Lafferty54, A. Lai15, D. Lambert50, R.W. Lambert42, E.
Lanciotti38, G. Lanfranchi18, C. Langenbruch38, B. Langhans38, T. Latham48, C.
Lazzeroni45, R. Le Gac6, J. van Leerdam41, J.-P. Lees4, R. Lefèvre5, A.
Leflat32, J. Lefrançois7, S. Leo23, O. Leroy6, T. Lesiak26, B. Leverington11,
Y. Li3, M. Liles52, R. Lindner38, C. Linn38, F. Lionetto40, B. Liu15, G.
Liu38, S. Lohn38, I. Longstaff51, J.H. Lopes2, N. Lopez-March39, P. Lowdon40,
H. Lu3, D. Lucchesi22,s, H. Luo50, A. Lupato22, E. Luppi16,f, O. Lupton55, F.
Machefert7, I.V. Machikhiliyan31, F. Maciuc29, O. Maev30, S. Malde55, G.
Manca15,e, G. Mancinelli6, J. Maratas5, J.F. Marchand4, U. Marconi14, C. Marin
Benito36, P. Marino23,u, R. Märki39, J. Marks11, G. Martellotti25, A.
Martens8, A. Martín Sánchez7, M. Martinelli41, D. Martinez Santos42, F.
Martinez Vidal64, D. Martins Tostes2, A. Massafferri1, R. Matev38, Z. Mathe38,
C. Matteuzzi20, A. Mazurov16,f, M. McCann53, J. McCarthy45, A. McNab54, R.
McNulty12, B. McSkelly52, B. Meadows57, F. Meier9, M. Meissner11, M. Merk41,
D.A. Milanes8, M.-N. Minard4, N. Moggi14, J. Molina Rodriguez60, S. Monteil5,
M. Morandin22, P. Morawski27, A. Mordà6, M.J. Morello23,u, J. Moron27, A.-B.
Morris50, R. Mountain59, F. Muheim50, K. Müller40, R. Muresan29, M. Mussini14,
B. Muster39, P. Naik46, T. Nakada39, R. Nandakumar49, I. Nasteva2, M.
Needham50, N. Neri21, S. Neubert38, N. Neufeld38, M. Neuner11, A.D. Nguyen39,
T.D. Nguyen39, C. Nguyen-Mau39,r, M. Nicol7, V. Niess5, R. Niet9, N.
Nikitin32, T. Nikodem11, A. Novoselov35, D.P. O’Hanlon48, A. Oblakowska-
Mucha27, V. Obraztsov35, S. Oggero41, S. Ogilvy51, O. Okhrimenko44, R.
Oldeman15,e, G. Onderwater65, M. Orlandea29, J.M. Otalora Goicochea2, P.
Owen53, A. Oyanguren64, B.K. Pal59, A. Palano13,c, F. Palombo21,v, M.
Palutan18, J. Panman38, A. Papanestis49,38, M. Pappagallo51, C. Parkes54, C.J.
Parkinson9,45, G. Passaleva17, G.D. Patel52, M. Patel53, C. Patrignani19,j, A.
Pazos Alvarez37, A. Pearce54, A. Pellegrino41, M. Pepe Altarelli38, S.
Perazzini14,d, E. Perez Trigo37, P. Perret5, M. Perrin-Terrin6, L.
Pescatore45, E. Pesen66, K. Petridis53, A. Petrolini19,j, E. Picatoste
Olloqui36, B. Pietrzyk4, T. Pilař48, D. Pinci25, A. Pistone19, S. Playfer50,
M. Plo Casasus37, F. Polci8, A. Poluektov48,34, E. Polycarpo2, A. Popov35, D.
Popov10, B. Popovici29, C. Potterat2, E. Price46, J. Prisciandaro39, A.
Pritchard52, C. Prouve46, V. Pugatch44, A. Puig Navarro39, G. Punzi23,t, W.
Qian4, B. Rachwal26, J.H. Rademacker46, B. Rakotomiaramanana39, M. Rama18,
M.S. Rangel2, I. Raniuk43, N. Rauschmayr38, G. Raven42, S. Reichert54, M.M.
Reid48, A.C. dos Reis1, S. Ricciardi49, S. Richards46, M. Rihl38, K.
Rinnert52, V. Rives Molina36, D.A. Roa Romero5, P. Robbe7, A.B. Rodrigues1, E.
Rodrigues54, P. Rodriguez Perez54, S. Roiser38, V. Romanovsky35, A. Romero
Vidal37, M. Rotondo22, J. Rouvinet39, T. Ruf38, F. Ruffini23, H. Ruiz36, P.
Ruiz Valls64, G. Sabatino25,l, J.J. Saborido Silva37, N. Sagidova30, P.
Sail51, B. Saitta15,e, V. Salustino Guimaraes2, C. Sanchez Mayordomo64, B.
Sanmartin Sedes37, R. Santacesaria25, C. Santamarina Rios37, E.
Santovetti24,l, M. Sapunov6, A. Sarti18,m, C. Satriano25,n, A. Satta24, D.M.
Saunders46, M. Savrie16,f, D. Savrina31,32, M. Schiller42, H. Schindler38, M.
Schlupp9, M. Schmelling10, B. Schmidt38, O. Schneider39, A. Schopper38, M.-H.
Schune7, R. Schwemmer38, B. Sciascia18, A. Sciubba25, M. Seco37, A.
Semennikov31, I. Sepp53, N. Serra40, J. Serrano6, L. Sestini22, P. Seyfert11,
M. Shapkin35, I. Shapoval16,43,f, Y. Shcheglov30, T. Shears52, L. Shekhtman34,
V. Shevchenko63, A. Shires9, R. Silva Coutinho48, G. Simi22, M. Sirendi47, N.
Skidmore46, T. Skwarnicki59, N.A. Smith52, E. Smith55,49, E. Smith53, J.
Smith47, M. Smith54, H. Snoek41, M.D. Sokoloff57, F.J.P. Soler51, F. Soomro39,
D. Souza46, B. Souza De Paula2, B. Spaan9, A. Sparkes50, P. Spradlin51, F.
Stagni38, M. Stahl11, S. Stahl11, O. Steinkamp40, O. Stenyakin35, S.
Stevenson55, S. Stoica29, S. Stone59, B. Storaci40, S. Stracka23,38, M.
Straticiuc29, U. Straumann40, R. Stroili22, V.K. Subbiah38, L. Sun57, W.
Sutcliffe53, K. Swientek27, S. Swientek9, V. Syropoulos42, M. Szczekowski28,
P. Szczypka39,38, D. Szilard2, T. Szumlak27, S. T’Jampens4, M. Teklishyn7, G.
Tellarini16,f, F. Teubert38, C. Thomas55, E. Thomas38, J. van Tilburg41, V.
Tisserand4, M. Tobin39, S. Tolk42, L. Tomassetti16,f, D. Tonelli38, S. Topp-
Joergensen55, N. Torr55, E. Tournefier4, S. Tourneur39, M.T. Tran39, M.
Tresch40, A. Tsaregorodtsev6, P. Tsopelas41, N. Tuning41, M. Ubeda Garcia38,
A. Ukleja28, A. Ustyuzhanin63, U. Uwer11, V. Vagnoni14, G. Valenti14, A.
Vallier7, R. Vazquez Gomez18, P. Vazquez Regueiro37, C. Vázquez Sierra37, S.
Vecchi16, J.J. Velthuis46, M. Veltri17,h, G. Veneziano39, M. Vesterinen11, B.
Viaud7, D. Vieira2, M. Vieites Diaz37, X. Vilasis-Cardona36,p, A. Vollhardt40,
D. Volyanskyy10, D. Voong46, A. Vorobyev30, V. Vorobyev34, C. Voß62, H.
Voss10, J.A. de Vries41, R. Waldi62, C. Wallace48, R. Wallace12, J. Walsh23,
S. Wandernoth11, J. Wang59, D.R. Ward47, N.K. Watson45, D. Websdale53, M.
Whitehead48, J. Wicht38, D. Wiedner11, G. Wilkinson55, M.P. Williams45, M.
Williams56, F.F. Wilson49, J. Wimberley58, J. Wishahi9, W. Wislicki28, M.
Witek26, G. Wormser7, S.A. Wotton47, S. Wright47, S. Wu3, K. Wyllie38, Y.
Xie61, Z. Xing59, Z. Xu39, Z. Yang3, X. Yuan3, O. Yushchenko35, M. Zangoli14,
M. Zavertyaev10,b, L. Zhang59, W.C. Zhang12, Y. Zhang3, A. Zhelezov11, A.
Zhokhov31, L. Zhong3, A. Zvyagin38.

1Centro Brasileiro de Pesquisas Físicas (CBPF), Rio de Janeiro, Brazil
2Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil
3Center for High Energy Physics, Tsinghua University, Beijing, China
4LAPP, Université de Savoie, CNRS/IN2P3, Annecy-Le-Vieux, France
5Clermont Université, Université Blaise Pascal, CNRS/IN2P3, LPC, Clermont-
Ferrand, France
6CPPM, Aix-Marseille Université, CNRS/IN2P3, Marseille, France
7LAL, Université Paris-Sud, CNRS/IN2P3, Orsay, France
8LPNHE, Université Pierre et Marie Curie, Université Paris Diderot,
CNRS/IN2P3, Paris, France
9Fakultät Physik, Technische Universität Dortmund, Dortmund, Germany
10Max-Planck-Institut für Kernphysik (MPIK), Heidelberg, Germany
11Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg,
Germany
12School of Physics, University College Dublin, Dublin, Ireland
13Sezione INFN di Bari, Bari, Italy
14Sezione INFN di Bologna, Bologna, Italy
15Sezione INFN di Cagliari, Cagliari, Italy
16Sezione INFN di Ferrara, Ferrara, Italy
17Sezione INFN di Firenze, Firenze, Italy
18Laboratori Nazionali dell’INFN di Frascati, Frascati, Italy
19Sezione INFN di Genova, Genova, Italy
20Sezione INFN di Milano Bicocca, Milano, Italy
21Sezione INFN di Milano, Milano, Italy
22Sezione INFN di Padova, Padova, Italy
23Sezione INFN di Pisa, Pisa, Italy
24Sezione INFN di Roma Tor Vergata, Roma, Italy
25Sezione INFN di Roma La Sapienza, Roma, Italy
26Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of
Sciences, Kraków, Poland
27AGH - University of Science and Technology, Faculty of Physics and Applied
Computer Science, Kraków, Poland
28National Center for Nuclear Research (NCBJ), Warsaw, Poland
29Horia Hulubei National Institute of Physics and Nuclear Engineering,
Bucharest-Magurele, Romania
30Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia
31Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia
32Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow,
Russia
33Institute for Nuclear Research of the Russian Academy of Sciences (INR RAN),
Moscow, Russia
34Budker Institute of Nuclear Physics (SB RAS) and Novosibirsk State
University, Novosibirsk, Russia
35Institute for High Energy Physics (IHEP), Protvino, Russia
36Universitat de Barcelona, Barcelona, Spain
37Universidad de Santiago de Compostela, Santiago de Compostela, Spain
38European Organization for Nuclear Research (CERN), Geneva, Switzerland
39Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
40Physik-Institut, Universität Zürich, Zürich, Switzerland
41Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands
42Nikhef National Institute for Subatomic Physics and VU University Amsterdam,
Amsterdam, The Netherlands
43NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine
44Institute for Nuclear Research of the National Academy of Sciences (KINR),
Kyiv, Ukraine
45University of Birmingham, Birmingham, United Kingdom
46H.H. Wills Physics Laboratory, University of Bristol, Bristol, United
Kingdom
47Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom
48Department of Physics, University of Warwick, Coventry, United Kingdom
49STFC Rutherford Appleton Laboratory, Didcot, United Kingdom
50School of Physics and Astronomy, University of Edinburgh, Edinburgh, United
Kingdom
51School of Physics and Astronomy, University of Glasgow, Glasgow, United
Kingdom
52Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom
53Imperial College London, London, United Kingdom
54School of Physics and Astronomy, University of Manchester, Manchester,
United Kingdom
55Department of Physics, University of Oxford, Oxford, United Kingdom
56Massachusetts Institute of Technology, Cambridge, MA, United States
57University of Cincinnati, Cincinnati, OH, United States
58University of Maryland, College Park, MD, United States
59Syracuse University, Syracuse, NY, United States
60Pontifícia Universidade Católica do Rio de Janeiro (PUC-Rio), Rio de
Janeiro, Brazil, associated to 2
61Institute of Particle Physics, Central China Normal University, Wuhan,
Hubei, China, associated to 3
62Institut für Physik, Universität Rostock, Rostock, Germany, associated to 11
63National Research Centre Kurchatov Institute, Moscow, Russia, associated to
31
64Instituto de Fisica Corpuscular (IFIC), Universitat de Valencia-CSIC,
Valencia, Spain, associated to 36
65KVI - University of Groningen, Groningen, The Netherlands, associated to 41
66Celal Bayar University, Manisa, Turkey, associated to 38

aUniversidade Federal do Triângulo Mineiro (UFTM), Uberaba-MG, Brazil
bP.N. Lebedev Physical Institute, Russian Academy of Science (LPI RAS),
Moscow, Russia
cUniversità di Bari, Bari, Italy
dUniversità di Bologna, Bologna, Italy
eUniversità di Cagliari, Cagliari, Italy
fUniversità di Ferrara, Ferrara, Italy
gUniversità di Firenze, Firenze, Italy
hUniversità di Urbino, Urbino, Italy
iUniversità di Modena e Reggio Emilia, Modena, Italy
jUniversità di Genova, Genova, Italy
kUniversità di Milano Bicocca, Milano, Italy
lUniversità di Roma Tor Vergata, Roma, Italy
mUniversità di Roma La Sapienza, Roma, Italy
nUniversità della Basilicata, Potenza, Italy
oAGH - University of Science and Technology, Faculty of Computer Science,
Electronics and Telecommunications, Kraków, Poland
pLIFAELS, La Salle, Universitat Ramon Llull, Barcelona, Spain
qUniversity of Utrecht, Utrecht, The Netherlands
rHanoi University of Science, Hanoi, Viet Nam
sUniversità di Padova, Padova, Italy
tUniversità di Pisa, Pisa, Italy
uScuola Normale Superiore, Pisa, Italy
vUniversità degli Studi di Milano, Milano, Italy

##  1 Introduction

The study of $b$-baryon decays is of considerable interest both to probe the
dynamics of heavy flavour decay processes and to search for the effects of
physics beyond the Standard Model. Owing to their non-zero spin, $b$ baryons
provide the potential to improve the limited understanding of the helicity
structure of the underlying Hamiltonian [1, *Hiller:2007ur].

Beauty baryons are copiously produced at the LHC, where the ${\mathchar
28931\relax}^{0}_{b}$ baryon cross-section is about half of the size of the
${B}^{0}$ meson production in the forward region [3, *LHCb-PAPER-2011-018].
The ATLAS, CMS and LHCb collaborations measured the ${\mathchar
28931\relax}^{0}_{b}$ lifetime [5, 6, *Chatrchyan:2013sxa, 8], and the masses
of the ground [9, 8] and first excited states [10]. The ${\mathchar
28931\relax}^{0}_{b}$ polarisation has been measured and found to be
compatible with zero [11]. The LHCb collaboration has studied ${\mathchar
28931\relax}^{0}_{b}$ decays to charmonium [11, 12], open charm [13, 14],
charmless states [15] and final states induced by electroweak penguins [16].
No evidence for $C\\!P$ violation has been reported in decays of baryons.
Searches with $b$-baryon decays have been performed with the decay channels
${{\mathchar 28931\relax}^{0}_{b}}\\!\rightarrow{p}{}{{\pi}^{-}}$,
$p$${K}^{-}$ [17] and ${K}^{0}_{\rm\scriptscriptstyle S}$$p$${\pi}^{-}$ [15].
The corresponding theoretical literature is still limited compared to that on
$B$ meson decays.

The study of ${b}\\!\rightarrow{c}{}{\overline{c}}{}{q}$ decays can be used to
constrain penguin pollution in the determination of the $C\\!P$-violating
phases in ${B}^{0}$ and ${B}^{0}_{s}$ mixing [18, *Faller:2008gt]. While
decays originating from the ${b}\\!\rightarrow{c}{}{\overline{c}}{}{s}$
transitions, such as ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow{{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{\mathchar 28931\relax}$ or ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$, are largely dominated by the tree amplitudes, penguins
amplitudes are enhanced in Cabibbo-suppressed
${b}\\!\rightarrow{c}{}{\overline{c}}{}{d}$ transitions, such as the
${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ decay.

This article reports the first observation of the ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ decay and the determination of its branching fraction
relative to the Cabibbo-favoured mode ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$. The latter, which was recently observed, has been used
to obtain a precise measurement of the ratio of ${\mathchar
28931\relax}^{0}_{b}$ to ${B}^{0}$ lifetimes [12, 5]. Its absolute branching
ratio is yet to be determined. A measurement of the $C\\!P$ asymmetry
difference between the ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ and ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ decays is also reported. The analysis is based on a
data sample of proton-proton collisions, corresponding to an integrated
luminosity of 1 $\mbox{\,fb}^{-1}$ at a centre-of-mass energy of 7
$\mathrm{\,Te\kern-1.00006ptV}$ and 2 $\mbox{\,fb}^{-1}$ at 8
$\mathrm{\,Te\kern-1.00006ptV}$, collected with the LHCb detector.

##  2 Detector and software

The LHCb detector [20] is a single-arm forward spectrometer covering the
pseudorapidity range $2<\eta<5$, designed for the study of particles
containing $b$ or $c$ quarks. The detector includes a high-precision tracking
system consisting of a silicon-strip vertex detector surrounding the proton-
proton interaction region, a large-area silicon-strip detector located
upstream of a dipole magnet with a bending power of about $4{\rm\,Tm}$, and
three stations of silicon-strip detectors and straw drift tubes [21] placed
downstream of the magnet. The combined tracking system provides a momentum
measurement with a relative uncertainty that varies from 0.4% at low momentum
to 0.6% at 100${\mathrm{\,Ge\kern-1.00006ptV\\!/}c}$, and an impact parameter
measurement with a resolution of 20${\,\upmu\rm m}$ for charged particles with
large transverse momentum, $p_{\rm T}$. Different types of charged hadrons are
distinguished using information from two ring-imaging Cherenkov (RICH)
detectors [22]. Photon, electron and hadron candidates are identified by a
calorimeter system consisting of scintillating-pad and preshower detectors, an
electromagnetic calorimeter and a hadronic calorimeter. Muons are identified
by a system composed of alternating layers of iron and multiwire proportional
chambers [23]. The trigger [24] consists of a hardware stage, based on
information from the calorimeter and muon systems, followed by a software
stage, which applies a full event reconstruction.

Candidate events are first required to pass the hardware trigger, which
selects muons with $\mbox{$p_{\rm
T}$}>1.48{\mathrm{\,Ge\kern-1.00006ptV\\!/}c}$. In the subsequent software
trigger, at least one of the candidate muons is required to be inconsistent
with originating from any primary interaction. Finally, the muon pair is
required to form a vertex that is significantly displaced from all primary
vertices (PV) and to have a mass within 120
${\mathrm{\,Me\kern-1.00006ptV\\!/}c^{2}}$ of the known
${J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}$ mass.

In the simulation, proton-proton collisions are generated using Pythia [25,
*Sjostrand:2007gs] with a specific LHCb configuration [27]. Decays of hadronic
particles are described by EvtGen [28], in which final state radiation is
generated using Photos [29]. The interaction of the generated particles with
the detector and its response are implemented using the Geant4 toolkit [30,
*Agostinelli:2002hh] as described in Ref. [32].

##  3 Event Selection

The ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ and ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ decays are reconstructed with the
${J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}$ decaying to two muons. Charge
conjugation is implied throughout except in the definition of the $C\\!P$
asymmetry.

Candidate ${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}\\!\rightarrow{\mu^{+}\mu^{-}}$ decays are reconstructed from
oppositely charged particles passing loose muon-identification requirements
and with $\mbox{$p_{\rm T}$}>500\>{\mathrm{\,Me\kern-1.00006ptV\\!/}c}$. They
are required to form a good quality vertex and have a mass in the range
$[3030,3150]\>{\mathrm{\,Me\kern-1.00006ptV\\!/}c^{2}}$. This interval
corresponds to about eight times the ${\mu^{+}\mu^{-}}$ mass resolution at the
${J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}$ mass and covers part of the
${J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}$ meson radiative tail.

Candidate ${\mathchar 28931\relax}^{0}_{b}$ baryons are selected from
combinations of ${J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}$ candidates and
two oppositely charged particles, one of which must be compatible with the
proton hypothesis. The proton candidate is required to have a momentum, $p$,
larger than 5 ${\mathrm{\,Ge\kern-1.00006ptV\\!/}c}$, while the second charged
particle must have $p>3\>{\mathrm{\,Ge\kern-1.00006ptV\\!/}c}$. Both particles
must have $\mbox{$p_{\rm T}$}>500\>{\mathrm{\,Me\kern-1.00006ptV\\!/}c}$ and
be inconsistent with coming from any PV. All four charged particles are
required to be consistent with coming from a common vertex.

The reconstructed mass and decay time of the ${\mathchar 28931\relax}^{0}_{b}$
candidates are obtained from a kinematic fit [33] that constrains the mass of
the $\mu^{+}\mu^{-}$ pairs to the known ${J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}$ mass and the ${\mathchar 28931\relax}^{0}_{b}$ candidate to originate
from the PV. If the event has multiple PVs, all combinations are considered.
Candidates are required to have a reconstructed decay time larger than
$0.2\>{\rm\,ps}$.

To remove backgrounds from ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow{{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{\mathchar 28931\relax}$ decays, candidates that have a $p$${\pi}^{-}$
mass within 5 ${\mathrm{\,Me\kern-1.00006ptV\\!/}c^{2}}$ of the $\mathchar
28931\relax$ baryon mass are vetoed. To remove reflections from
${{B}^{0}_{s}}\\!\rightarrow{{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{}\phi$ decays, candidates are also vetoed if the hadron-pair mass is
less than 1035 ${\mathrm{\,Me\kern-1.00006ptV\\!/}c^{2}}$ when applying a
${K}^{+}$ mass hypothesis to both particles.

The remaining candidates are split into samples of ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ and ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ according to the estimated probabilities that the
charged meson candidate is a kaon or a pion. These probabilities are
determined using a neural network (NN) exploiting information from the RICH
detectors, calorimeter and muon systems, as well as track quality. Particles
with a larger pion probability are treated as ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ candidates, otherwise they are treated as
${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ candidates. In addition, the larger of these two
probabilities is required to be in excess of 5%. The ${\mathchar
28931\relax}^{0}_{b}$ candidates are required to be in the mass range
$[4900,6100]\,{\mathrm{\,Me\kern-1.00006ptV\\!/}c^{2}}$. After this selection
$4.3\times 10^{5}$ ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ and $1.9\times 10^{5}$ ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ candidates remain.

The selection described above is not sufficient to isolate the small
${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ signal from the combinatorial background. The initial
selection is therefore followed by a multivariate analysis, based on another
NN [34]. The NN classifier’s output is used as the final selection variable.

The NN is trained entirely on data, using the ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ signal as a proxy for the ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ decay. The training is performed using half of the
${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ candidates chosen at random. The other half is used to
define the normalisation sample, allowing an unbiased measurement of the
${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ yield. The training uses signal and background weights
determined using the sPlot technique [35] and obtained by performing a maximum
likelihood fit to the unbinned mass distribution of the candidates meeting the
loose selection criteria.

The fit probability density function (PDF) is defined as the sum of the
${\mathchar 28931\relax}^{0}_{b}$ signal component and the combinatorial
background components. The parameterisation of the individual components is
described in the next section.

Several reflections from ${B}^{0}$, ${B}^{0}_{s}$ and ${\mathchar
28931\relax}^{0}_{b}$ decays, reconstructed using misidentified particles,
must be accounted for in the mass spectrum. In order to avoid the training
being biased by these reflections, all candidates that have a mass compatible
with the ${B}^{0}$, ${B}^{0}_{s}$ or ${\mathchar 28931\relax}^{0}_{b}$ mass
after swapping the $p$ and $K$ assignments with any of $\pi$, $K$, or $p$ are
removed from the training.

The NN classifier uses information about the candidate kinematic
distributions, vertex and track quality, impact parameter and particle
identification information on the proton. The most discriminating quantities
are the proton particle identification probability, the kinematic fit quality
and the kaon separation of the PV in this order. The variables that are used
in the NN are chosen to avoid correlations with the reconstructed ${\mathchar
28931\relax}^{0}_{b}$ mass and to have identical distributions in ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ and ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ simulated data.

Final selection requirements of the NN classifier output are chosen to
optimise the expected statistical precision on the ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ signal yield. The expected signal and background
yields entering the sensitivity estimation are obtained from the training
sample by scaling the number of surviving ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ candidates by the expected yield based on an assumed
branching fraction ratio of $0.1$. The expected background is extrapolated
from the number of ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ candidates in the mass range
$[5770,6100]\>{\mathrm{\,Me\kern-1.00006ptV\\!/}c^{2}}$. After applying the
final requirement on the NN classifier output, the multivariate selection
rejects 99% of the background while keeping 75% of the ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ signal, relative to the initial selection.

After applying the full selection, about $0.1\%$ of the selected events have
more than one candidate sharing at least one track, or more than one PV that
can be used to determine the kinematic properties of the candidate. In these
cases one of the candidates or PVs is used at random.

##  4 Signal and background description

For the candidates passing the NN requirements, the yields of ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ and ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ decays are determined from unbinned maximum likelihood
fits to the mass distributions of reconstructed ${\mathchar
28931\relax}^{0}_{b}$ candidates. The PDF is defined as the sum of a
${\mathchar 28931\relax}^{0}_{b}$ signal component, a combinatorial background
and the sum of several reflections.

The signal shape is parametrised by a Gaussian distribution with power-law
tails on both sides, as indicated by simulation. The parameters describing the
tails are taken from simulation, while the mean and width of the Gaussian are
allowed to vary in the fit. The combinatorial background contribution is
described by an exponential function, with yield and slope parameter allowed
to vary freely.

Several peaking backgrounds due to decays of $b$ hadrons to
${J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}$ mesons and two charged hadrons,
where one or both hadrons are misidentified, survive the selection. In this
fit they are not vetoed, unlike in the training of the NN, except for
candidates consistent with the
${{B}^{0}_{s}}\\!\rightarrow{{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}\phi$ hypothesis. Instead, their contribution is modelled by smoothed
non-parametric functions determined from simulated data. The respective yields
are determined by swapping the mass assignments of the $p$, $\pi$ and $K$ in
turn and searching for peaks at the ${B}^{0}$, ${B}^{0}_{s}$ or ${\mathchar
28931\relax}^{0}_{b}$ masses. In the ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ fit, significant backgrounds are found from the
decays ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ (with ${K}^{-}$ identified as ${\pi}^{-}$),
${{B}^{0}}\\!\rightarrow{{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{}{{K}^{+}}{}{{\pi}^{-}}$ (with ${K}^{+}$ identified as $p$), and
${{B}^{0}_{s}}\\!\rightarrow{{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{}{{K}^{+}}{}{{K}^{-}}$ (with one $K$ identified as $p$ and the other
as $\pi$). In the ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ fit, the main contributions are from the decays
${{\kern
1.79993pt\overline{\kern-1.79993ptB}{}}^{0}}\\!\rightarrow{{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{}{{\pi}^{+}}{}{{K}^{-}}$ (with ${\pi}^{+}$ identified as $p$),
${{B}^{0}_{s}}\\!\rightarrow{{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{}{{K}^{+}}{}{{K}^{-}}$ (with ${K}^{+}$ identified as $p$), and
${{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar
28931\relax}}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{\overline{p}}{{K}^{+}}$}$ (with ${K}^{-}$ and $p$ swapped). These
yields are then used as Gaussian constraints on the normalisation of the
reflection background shapes. The results of the fits are shown in Fig. 1. The
contributions of the reflections are summed, except for the large ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ reflection in the ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ fit, which is shown separately.

Figure 1: Distribution of (left) ${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$ and (right) ${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$ masses with fit projections overlaid. For ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ candidates, only the normalisation sample is shown.

Low-mass contributions of partially reconstructed ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$${\pi}^{0}$ and ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$${\pi}^{0}$ decays, where the ${\pi}^{0}$ is not
considered in the combination, are investigated. Adding such a component to
the fit, with a mass shape taken from simulation, results in yields compatible
with zero and does not change the signal yields.

In total $11\,179\pm 109$ ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ and $2102\pm 61$ ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ decays are obtained. The ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ yield, having been determined on the half of the data
not used in the training, is multiplied by two, resulting in a ratio of
${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ to ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ yields of $0.0940\pm 0.0029$.

The shapes used in the mass fit are varied to determine a systematic
uncertainty related to the mass model. No significant differences are found
when trying alternate signal parameterisation that still result in a good fit.
Changing the peaking backgrounds PDF, or letting their yield free in the fit,
change their relative fit fractions, as well as that of the combinatorial
background, but does not affect the signal yields. The combinatorial
background model is changed from an exponential to a second-order polynomial,
which results in a ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ (${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$) yield reduced by 2.1% (0.3%). These variations are
added in quadrature and used to estimate a systematic uncertainty on the ratio
of branching fractions.

##  5 $C\\!P$ asymmetry

The same fit procedure is repeated separately for baryon (tagged by a
positively charged proton) and antibaryon candidates. All parameters of the
fits are determined again separately, except for the signal shape and the
combinatorial background slope, which are taken from the fit to all
candidates. A total of $1131\pm 40$ ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$, $964\pm 38$ ${{\kern
1.00006pt\overline{\kern-1.00006pt\mathchar
28931\relax}}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{\overline{p}}{{\pi}^{+}}$}$, $5655\pm 77$ ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ and $5529\pm 76$ ${{\kern
1.00006pt\overline{\kern-1.00006pt\mathchar
28931\relax}}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{\overline{p}}{{K}^{+}}$}$ decays are found, corresponding to the raw
asymmetries

$\displaystyle{\cal A}_{\text{raw}}({{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$})$ $\displaystyle\equiv\frac{N({{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$})-N({{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar
28931\relax}}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{\overline{p}}{{\pi}^{+}}$})}{N({{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$})+N({{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar
28931\relax}}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{\overline{p}}{{\pi}^{+}}$})}$ (1) $\displaystyle=(+7.9\pm 2.2)\%,$
$\displaystyle{\cal A}_{\text{raw}}({{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$})$ $\displaystyle=(+1.1\pm 0.9)\%.$

The procedure to assess the systematic uncertainties related to the shape of
the mass distribution, described in Sec. 4, is repeated to determine the
sensitivity of the raw asymmetries. A total variation of 0.7% is obtained,
which is dominated by a change in ${\cal A}_{\text{raw}}({{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$})$ when using a second-order polynomial background
model.

The raw decay-rate asymmetry can be decomposed as

$\displaystyle{\cal A}_{\text{raw}}({{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow{{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}h^{-})$ $\displaystyle={\cal A}_{{C\\!P}}({{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow{{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}h^{-})+{\cal A}_{\text{prod}}({{\mathchar
28931\relax}^{0}_{b}})-{\cal A}_{\text{reco}}(h^{+})+{\cal
A}_{\text{reco}}({p}),$ (2)

where the terms on the right hand side of the equation are the
$C\\!P$-violating, ${\mathchar 28931\relax}^{0}_{b}$ production and
reconstruction asymmetries of the hadron $h^{\pm}={{\pi}^{\pm}},{{K}^{\pm}}$
and the proton, respectively. Reconstruction asymmetries are defined following
the convention ${\cal
A}_{\text{reco}}(h^{+})\equiv\frac{\epsilon(h^{+})-\epsilon(h^{-})}{\epsilon(h^{+})+\epsilon(h^{-})}$
throughout, where $\epsilon$ is the reconstruction efficiency. The production
asymmetry ${\cal A}_{\text{prod}}$ and the proton reconstruction asymmetry
cancel in the difference of the two asymmetries

$\displaystyle\Delta{\cal A}_{{C\\!P}}$ $\displaystyle\equiv{\cal
A}_{{C\\!P}}({{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$})-{\cal A}_{{C\\!P}}({{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$})$ $\displaystyle={\cal A}_{\text{raw}}({{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$})-{\cal A}_{\text{raw}}({{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$})+{\cal A}_{\text{reco}}({{\pi}^{+}})-{\cal
A}_{\text{reco}}({{K}^{+}}).$ (3)

The kaon and pion asymmetries can be determined from the raw asymmetry of the
${{\kern
1.79993pt\overline{\kern-1.79993ptB}{}}^{0}}\rightarrow{{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{{\kern 1.99997pt\overline{\kern-1.99997ptK}{}}^{*}(892)^{0}}$ decay
with ${{\kern
1.99997pt\overline{\kern-1.99997ptK}{}}^{*}(892)^{0}}\\!\rightarrow{{K}^{-}}{}{{\pi}^{+}}$.
It has been measured [36] as

${\cal A}_{\text{raw}}(\mbox{${{\kern
1.79993pt\overline{\kern-1.79993ptB}{}}^{0}}\rightarrow{{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{{\kern
1.99997pt\overline{\kern-1.99997ptK}{}}^{*}(892)^{0}}$})\equiv\frac{N({{\kern
1.79993pt\overline{\kern-1.79993ptB}{}}^{0}})-N({{B}^{0}})}{N({{\kern
1.79993pt\overline{\kern-1.79993ptB}{}}^{0}})+N({{B}^{0}})}=(-1.10\pm 0.32\pm
0.06)\%,$ (4)

where the first uncertainty is statistical and the second systematic. It can
be decomposed as

$\displaystyle{\cal A}_{\text{raw}}(\mbox{${{\kern
1.79993pt\overline{\kern-1.79993ptB}{}}^{0}}\rightarrow{{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{{\kern 1.99997pt\overline{\kern-1.99997ptK}{}}^{*}(892)^{0}}$})$
$\displaystyle={\cal A}_{{C\\!P}}(\mbox{${{\kern
1.79993pt\overline{\kern-1.79993ptB}{}}^{0}}\rightarrow{{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{{\kern
1.99997pt\overline{\kern-1.99997ptK}{}}^{*}(892)^{0}}$})-\kappa{\cal
A}_{\text{prod}}({{B}^{0}})$ (5) $\displaystyle+{\cal
A}_{\text{reco}}({{\pi}^{+}})-{\cal A}_{\text{reco}}({{K}^{+}})$
$\displaystyle\approx{\cal A}_{\text{reco}}({{\pi}^{+}})-{\cal
A}_{\text{reco}}({{K}^{+}}),$ (6)

where $\kappa$ is a dilution factor due to ${B}^{0}$ mixing and ${\cal
A}_{\text{prod}}({{B}^{0}})$ is the ${B}^{0}$ production asymmetry, which is
compatible with zero [37]. Under the assumption of no $C\\!P$ asymmetry in the
${{\kern
1.79993pt\overline{\kern-1.79993ptB}{}}^{0}}\rightarrow{{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{{\kern 1.99997pt\overline{\kern-1.99997ptK}{}}^{*}(892)^{0}}$ decay
and negligible production asymmetry, this value can thus be taken as the
combined kaon and pion reconstruction asymmetry, and is consistent with
measurements in other decay modes to kaon and pions [37, 38].

The difference of $C\\!P$ asymmetries in the ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ and ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ decays can then be rewritten as

$\displaystyle\Delta{\cal A}_{C\\!P}$ $\displaystyle={\cal
A}_{\text{raw}}({{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$})-{\cal A}_{\text{raw}}({{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$})+{\cal A}_{\text{raw}}(\mbox{${{\kern
1.79993pt\overline{\kern-1.79993ptB}{}}^{0}}\rightarrow{{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{{\kern 1.99997pt\overline{\kern-1.99997ptK}{}}^{*}(892)^{0}}$})$ (7)
$\displaystyle=(+5.7\pm 2.4)\%,$

where the uncertainty is statistical only.

The kaon and pion momenta in ${\mathchar 28931\relax}^{0}_{b}$ decays are not
identical to those in ${{\kern
1.79993pt\overline{\kern-1.79993ptB}{}}^{0}}\rightarrow{{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{{\kern 1.99997pt\overline{\kern-1.99997ptK}{}}^{*}(892)^{0}}$ decays,
which could induce different detector asymmetries in the ${\mathchar
28931\relax}^{0}_{b}$ and ${B}^{0}$ modes. This is investigated by weighting
the ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ and ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ data to match the pion and kaon momentum distributions
observed in ${{\kern
1.79993pt\overline{\kern-1.79993ptB}{}}^{0}}\rightarrow{{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{{\kern 1.99997pt\overline{\kern-1.99997ptK}{}}^{*}(892)^{0}}$ decays.
The value of $\Delta{\cal A}_{C\\!P}$ changes by 0.8%, which is assigned as
the systematic uncertainty related to reconstruction asymmetries.

Local $C\\!P$ asymmetries in the Dalitz plane are also searched for using the
technique outlined in Ref. [39, *Bediaga:2009tr]. No significant local
asymmetries are found.

##  6 Efficiency corrections and systematic uncertainties

The raw quantities need to be corrected to determine the physics quantities.
The efficiency of the selection requirements is studied with simulation. Some
quantities are known not to be well reproduced in simulation, namely the
${\mathchar 28931\relax}^{0}_{b}$ transverse momentum and lifetime, the
particle multiplicity, and the ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ and ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ decay kinematic properties. For all these quantities
the simulated data are weighted to match the observed distributions in data.
They are obtained with the sPlot technique using the ${\mathchar
28931\relax}^{0}_{b}$ candidate mass as the control variable.

For three-body $b$-hadron decays, both the signal decays and the dominant
combinatorial backgrounds populate regions close to the kinematic boundaries
of the ${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}}{p}{{\pi}^{-}}$ and
${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}}{p}{{K}^{-}}$ Dalitz plot [41].
For more accurate modelling of these regions, it is convenient to transform
the conventional Dalitz space to a rectangular space (hereafter referred to as
the square Dalitz plot [42]). We follow the procedure described in Ref. [15].

The ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ and ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ decays have different detector acceptance,
reconstruction and selection efficiencies. They are determined from simulated
data, which are weighted to match the experimental data. The main differences
are induced by

  1. i.

the detector acceptance, as the efficiency of ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ is 6% larger than that for the ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ decays due to the lower kinetic energy release in the
former, which causes smaller opening angles;

  2. ii.

the reconstruction and preselection efficiency, which is 4% larger in
${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ decays due to the average total and transverse
momentum of the final state particles being larger than in ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ decays;

  3. iii.

the particle identification requirements on the ${\pi}^{-}$ or ${K}^{-}$,
which are more efficient on ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ decays by 7%;

  4. iv.

the $\phi$ veto, which removes 7% (3%) of the ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ (${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$) signal.

Overall, these effects result in a further correction on the ratio of
branching fractions of ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ to ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ decays of $0.913\pm 0.040$.

The efficiency of particle identification is not perfectly modelled in
simulation. The kaon and pion identification efficiencies are further weighted
after the kinematic weighting described above, using a large sample of
${D}^{*}$-tagged ${D}^{0}$ $\rightarrow$ ${K}^{-}$${\pi}^{+}$ decays. The
uncertainties are determined by varying the weights of the simulated data
within their uncertainties, yielding a correction of $0.960\pm 0.010$. The
kinematic properties of the proton in the ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ and ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ decays are found to be the same. The same applies to
the muons. The efficiencies of proton and muon identification therefore cancel
in the ratio of branching fractions, as well as in the $C\\!P$ asymmetries.

The trigger efficiency is determined using simulation, which is validated
using ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ decays from data. Differences between the ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ and ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ decays efficiencies are at the percent level and are
assigned as systematic uncertainties.

The value of the ${\mathchar 28931\relax}^{0}_{b}$ lifetime used in simulation
is taken from Ref. [43], and is 3% lower than the current most precise
measurement [5]. The simulated data is weighted to account for this and the
difference is assigned as a systematic uncertainty.

Table 1: Corrections and related systematic uncertainties on the ratio of ${{\mathchar 28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}}{p}{{\pi}^{-}}$}$ to ${{\mathchar 28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}}{p}{{K}^{-}}$}$ branching fractions (BF) and on the difference between the $C\\!P$ asymmetries $\Delta{\cal A}_{C\\!P}$. The corrections are multiplicative on the branching fraction and additive for the asymmetry. Source | BF | $\Delta{\cal A}_{C\\!P}$
---|---|---
Simulation-based corrections | $0.913\pm 0.040$ | -
PID | $0.960\pm 0.010$ | -
Trigger | $1.000\pm 0.010$ | -
${\mathchar 28931\relax}^{0}_{b}$ lifetime | $1.000\pm 0.001$ | -
Mass distribution model | $1.000\pm 0.021$ | $0.0\pm 0.7\%$
${{\kern 1.79993pt\overline{\kern-1.79993ptB}{}}^{0}}\rightarrow{{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}}{{\kern 1.99997pt\overline{\kern-1.99997ptK}{}}^{*}(892)^{0}}$ | $-$ | $-1.1\pm 0.3\%$
Detection asymmetries | $-$ | $0.0\pm 0.8\%$
Total | $0.876\pm 0.045$ | $-1.1\pm 1.2\%$

The estimates of the systematic uncertainties described above are summarised
in Table 1, along with the total obtained by summing them in quadrature. The
uncertainty on the ratio of branching fractions is dominated by the
uncertainty on corrections obtained from simulation, mostly due to the unknown
decay kinematic properties of the ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ decay. For $\Delta{\cal A}_{C\\!P}$, the mass model
distribution and the detection asymmetries contribute about equally.

##  7 Results and Conclusions

A signal of the Cabibbo-suppressed ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ decay is observed for the first time using a data
sample of proton-proton collisions at 7 and 8 $\mathrm{\,Te\kern-1.00006ptV}$,
corresponding to an integrated luminosity of 3 $\mbox{\,fb}^{-1}$. Applying
the appropriate corrections detailed in Table 1, the ratio of branching
fractions is measured to be

$\displaystyle\frac{{\cal B}({{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$})}{{\cal B}({{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$})}$ $\displaystyle=0.0824\pm 0.0025\text{\>(stat)}\pm
0.0042\text{\>(syst)}.$

Assuming these decays are dominated by tree
${b}\\!\rightarrow{c}{}{\overline{c}}{}{d}$ and
${b}\\!\rightarrow{c}{}{\overline{c}}{}{s}$ transitions, respectively, the
ratio of Cabibbo-Kobayashi-Maskawa (CKM) matrix elements
$|{V_{{c}{d}}}|^{2}/|{V_{{c}{s}}}|^{2}$ times ratio of phase space factors is
approximately $0.08$, compatible with the measured value. This ratio can also
be compared to that of the decays ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow{{\mathchar 28931\relax}^{+}_{c}}{{D}^{-}}$
and ${{\mathchar 28931\relax}^{0}_{b}}\\!\rightarrow{{\mathchar
28931\relax}^{+}_{c}}{{D}^{+}_{s}}$, which involve the same quark lines as
${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ and ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$, respectively. The ratio of ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow{{\mathchar 28931\relax}^{+}_{c}}{{D}^{-}}$
and ${{\mathchar 28931\relax}^{0}_{b}}\\!\rightarrow{{\mathchar
28931\relax}^{+}_{c}}{{D}^{+}_{s}}$ has been measured as $0.042\pm
0.003\mathrm{\,(stat)}\pm 0.003\mathrm{\,(syst)}$ [14], which is consistent
with this measurement, when taking into account the ${D}^{-}$ and
${D}^{+}_{s}$ meson decay constants [43] and the different ratio of phase
space factors.

Background-subtracted and efficiency-corrected distributions of kinematic
distributions determined in the ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ decay are shown in Fig. 2. In this case the
${\mathchar 28931\relax}^{0}_{b}$ mass is fixed to its known value and the
kinematic properties recomputed [33]. No attempt is made to fit the decay rate
on the Dalitz plane. The $p$${\pi}^{-}$ mass distribution shows a rich
resonant structure, as expected from fits to fixed-target experiment data [44,
45], and suggests the presence of the narrow $N(1520)$ or $N(1535)$, the
$N(1650)$, as well as the broad $N(1440)$ resonances. No signs of exotic
structures in the ${J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}$${\pi}^{-}$ or
${J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}$$p$ mass distributions are seen.
More data and further studies will be needed to inverstigate the underlying
dynamics of this decay.

Figure 2: Efficiency-corrected and background-subtracted ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ Dalitz plane and projections normalised to unit area
on the $p$${\pi}^{-}$, ${J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}$${\pi}^{-}$ and ${J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}$$p$ axes.

The $C\\!P$ asymmetry difference between the ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$}$ and ${{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$}$ decays is measured to be

$\Delta A_{{C\\!P}}=A_{{C\\!P}}({{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{\pi}^{-}}$})-A_{{C\\!P}}({{\mathchar
28931\relax}^{0}_{b}}\\!\rightarrow\mbox{${{J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}}{p}{{K}^{-}}$})=(+5.7\pm 2.4\text{\>(stat)}\pm 1.2\text{\>(syst)})\%,$

corresponding to a $2.2\sigma$ deviation from zero. No indications of large
local $C\\!P$ asymmetries in the Dalitz plane are observed. The precision of
these measurements illustrate the potential of Cabibbo-suppressed ${\mathchar
28931\relax}^{0}_{b}$ decays in studies of direct $C\\!P$ violation.

## Acknowledgements

We express our gratitude to our colleagues in the CERN accelerator departments
for the excellent performance of the LHC. We thank the technical and
administrative staff at the LHCb institutes. We acknowledge support from CERN
and from the national agencies: CAPES, CNPq, FAPERJ and FINEP (Brazil); NSFC
(China); CNRS/IN2P3 (France); BMBF, DFG, HGF and MPG (Germany); SFI (Ireland);
INFN (Italy); FOM and NWO (The Netherlands); MNiSW and NCN (Poland); MEN/IFA
(Romania); MinES and FANO (Russia); MinECo (Spain); SNSF and SER
(Switzerland); NASU (Ukraine); STFC (United Kingdom); NSF (USA). The Tier1
computing centres are supported by IN2P3 (France), KIT and BMBF (Germany),
INFN (Italy), NWO and SURF (The Netherlands), PIC (Spain), GridPP (United
Kingdom). We are indebted to the communities behind the multiple open source
software packages on which we depend. We are also thankful for the computing
resources and the access to software R&D tools provided by Yandex LLC
(Russia). Individual groups or members have received support from EPLANET,
Marie Skłodowska-Curie Actions and ERC (European Union), Conseil général de
Haute-Savoie, Labex ENIGMASS and OCEVU, Région Auvergne (France), RFBR
(Russia), XuntaGal and GENCAT (Spain), Royal Society and Royal Commission for
the Exhibition of 1851 (United Kingdom).

## References

  * [1] T. Mannel and S. Recksiegel, Probing the helicity structure of $b\\!\rightarrow s\gamma$ in $\Lambda_{b}^{0}\\!\rightarrow\Lambda\gamma$, Acta Phys. Polon. B28 (1997) 2489, arXiv:hep-ph/9710287
  * [2] G. Hiller, M. Knecht, F. Legger, and T. Schietinger, Photon polarization from helicity suppression in radiative decays of polarized $\Lambda_{b}$ to spin-3/2 baryons, Phys. Lett. B649 (2007) 152, arXiv:hep-ph/0702191
  * [3] LHCb collaboration, R. Aaij et al., Study of the kinematic dependences of $\Lambda_{b}^{0}$ production in $pp$ collisions and a measurement of the $\Lambda_{b}^{0}\rightarrow\Lambda_{c}^{+}\pi^{-}$ branching fraction, arXiv:1405.6842, submitted to JHEP
  * [4] LHCb collaboration, R. Aaij et al., Measurement of $b$ hadron production fractions in 7 TeV $pp$ collisions, Phys. Rev. D85 (2012) 032008, arXiv:1111.2357
  * [5] LHCb collaboration, R. Aaij et al., Precision measurement of the ratio of the $\Lambda_{b}^{0}$ to $\bar{B}^{0}$ lifetimes, Phys. Lett. B734 122, arXiv:1402.6242
  * [6] LHCb collaboration, R. Aaij et al., Measurements of the $B^{+}$, $B^{0}$, $B_{s}^{0}$ meson and $\Lambda_{b}^{0}$ baryon lifetimes, JHEP 04 (2014) 114, arXiv:1402.2554
  * [7] CMS collaboration, S. Chatrchyan et al., Measurement of the $\Lambda_{b}^{0}$ lifetime in pp collisions at $\sqrt{s}=7$ TeV, JHEP 07 (2013) 163, arXiv:1304.7495
  * [8] ATLAS collaboration, G. Aad et al., Measurement of the $\Lambda_{b}$ lifetime and mass in the ATLAS experiment, Phys. Rev. D87 (2013) 032002, arXiv:1207.2284
  * [9] LHCb collaboration, R. Aaij et al., Measurements of the $\Lambda_{b}^{0}$, $\Xi_{b}^{-}$ and $\Omega_{b}^{-}$ baryon masses, Phys. Rev. Lett. 110 (2013) 182001, arXiv:1302.1072
  * [10] LHCb collaboration, R. Aaij et al., Observation of excited $\Lambda_{b}^{0}$ baryons, Phys. Rev. Lett. 109 (2012) 172003, arXiv:1205.3452
  * [11] LHCb collaboration, R. Aaij et al., Measurements of the $\Lambda_{b}^{0}\rightarrow J/\psi\Lambda$ decay amplitudes and the $\Lambda_{b}^{0}$ polarisation in $pp$ collisions at $\sqrt{s}=7$ TeV, Phys. Lett. B724 (2013) 27, arXiv:1302.5578
  * [12] LHCb collaboration, R. Aaij et al., Precision measurement of the $\Lambda_{b}^{0}$ baryon lifetime, Phys. Rev. Lett. 111 (2013) 102003, arXiv:1307.2476
  * [13] LHCb collaboration, R. Aaij et al., Study of beauty baryon decays to $D^{0}ph^{-}$ and $\Lambda_{c}^{+}h^{-}$ final states, Phys. Rev. D89 (2014) 032001, arXiv:1311.4823
  * [14] LHCb collaboration, R. Aaij et al., Study of beauty hadron decays into pairs of charm hadrons, Phys. Rev. Lett. 112 (2014) 202001, arXiv:1403.3606
  * [15] LHCb collaboration, R. Aaij et al., Searches for $\Lambda^{0}_{b}$ and $\Xi^{0}_{b}$ decays to $K^{0}_{S}p\pi^{-}$ and $K^{0}_{S}pK^{-}$ final states with first observation of the $\Lambda^{0}_{b}\rightarrow K^{0}_{S}p\pi^{-}$ decay, JHEP 04 (2014) 087, arXiv:1402.0770
  * [16] LHCb collaboration, R. Aaij et al., Measurement of the differential branching fraction of the decay $\Lambda_{b}^{0}\rightarrow\Lambda\mu^{+}\mu^{-}$, Phys. Lett. B725 (2013) 25, arXiv:1306.2577
  * [17] CDF collaboration, T. A. Aaltonen et al., Measurements of Direct CP-Violating Asymmetries in Charmless Decays of Bottom Baryons, arXiv:1403.5586
  * [18] K. De Bruyn, R. Fleischer, and P. Koppenburg, Extracting $\gamma$ and penguin topologies through $C\\!P$ violation in ${B}^{0}_{s}$ $\rightarrow$ ${J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}$${K}^{0}_{\rm\scriptscriptstyle S}$, Eur. Phys. J. C70 (2010) 1025, arXiv:1010.0089
  * [19] S. Faller, R. Fleischer, and T. Mannel, Precision physics with $B^{0}_{s}\rightarrow J/\psi\phi$ at the LHC: the quest for New Physics, Phys. Rev. D79 (2009) 014005, arXiv:0810.4248
  * [20] LHCb collaboration, A. A. Alves Jr. et al., The LHCb detector at the LHC, JINST 3 (2008) S08005
  * [21] R. Arink et al., Performance of the LHCb Outer Tracker, JINST 9 (2014) P01002, arXiv:1311.3893
  * [22] M. Adinolfi et al., Performance of the LHCb RICH detector at the LHC, Eur. Phys. J. C73 (2013) 2431, arXiv:1211.6759
  * [23] A. A. Alves Jr. et al., Performance of the LHCb muon system, JINST 8 (2013) P02022, arXiv:1211.1346
  * [24] R. Aaij et al., The LHCb trigger and its performance in 2011, JINST 8 (2013) P04022, arXiv:1211.3055
  * [25] T. Sjöstrand, S. Mrenna, and P. Skands, PYTHIA 6.4 physics and manual, JHEP 05 (2006) 026, arXiv:hep-ph/0603175
  * [26] T. Sjöstrand, S. Mrenna, and P. Skands, A brief introduction to PYTHIA 8.1, Comput. Phys. Commun. 178 (2008) 852, arXiv:0710.3820
  * [27] I. Belyaev et al., Handling of the generation of primary events in Gauss, the LHCb simulation framework, Nuclear Science Symposium Conference Record (NSS/MIC) IEEE (2010) 1155
  * [28] D. J. Lange, The EvtGen particle decay simulation package, Nucl. Instrum. Meth. A462 (2001) 152
  * [29] P. Golonka and Z. Was, PHOTOS Monte Carlo: a precision tool for QED corrections in $Z$ and $W$ decays, Eur. Phys. J. C45 (2006) 97, arXiv:hep-ph/0506026
  * [30] Geant4 collaboration, J. Allison et al., Geant4 developments and applications, IEEE Trans. Nucl. Sci. 53 (2006) 270
  * [31] Geant4 collaboration, S. Agostinelli et al., Geant4: a simulation toolkit, Nucl. Instrum. Meth. A506 (2003) 250
  * [32] M. Clemencic et al., The LHCb simulation application, Gauss: design, evolution and experience, J. Phys. Conf. Ser. 331 (2011) 032023
  * [33] W. D. Hulsbergen, Decay chain fitting with a Kalman filter, Nucl. Instrum. Meth. A552 (2005) 566, arXiv:physics/0503191
  * [34] M. Feindt, A neural Bayesian estimator for conditional probability densities, arXiv:physics/0402093
  * [35] M. Pivk and F. R. Le Diberder, sPlot: a statistical tool to unfold data distributions, Nucl. Instrum. Meth. A555 (2005) 356, arXiv:physics/0402083
  * [36] LHCb collaboration, R. Aaij et al., Measurement of the $CP$ asymmetry in $B^{0}\rightarrow K^{*0}\mu^{+}\mu^{-}$ decays, Phys. Rev. Lett. 110 (2013) 031801, arXiv:1210.4492
  * [37] LHCb collaboration, R. Aaij et al., First observation of $CP$ violation in the decays of $B_{s}^{0}$ mesons, Phys. Rev. Lett. 110 (2013) 221601, arXiv:1304.6173
  * [38] LHCb collaboration, R. Aaij et al., Measurement of $CP$ asymmetry in $D^{0}\rightarrow K^{-}K^{+}$ and $D^{0}\rightarrow\pi^{-}\pi^{+}$ decays, arXiv:1405.2797, submited to PLB
  * [39] I. Bediaga et al., Second generation of ’Miranda procedure’ for CP violation in Dalitz studies of $B$, ($D$ & $\tau$) decays, Phys. Rev. D86 (2012) 036005, arXiv:1205.3036
  * [40] I. Bediaga et al., On a CP anisotropy measurement in the Dalitz plot, Phys. Rev. D80 (2009) 096006, arXiv:0905.4233
  * [41] R. Dalitz, On the analysis of tau-meson data and the nature of the tau-meson, Phil. Mag. 44 (1953) 1068
  * [42] BaBar collaboration, B. Aubert et al., An amplitude analysis of the decay $B^{\pm}\rightarrow\pi^{\pm}\pi^{\pm}\pi^{\mp}$, Phys. Rev. D72 (2005) 052002, arXiv:hep-ex/0507025
  * [43] Particle Data Group, J. Beringer et al., Review of particle physics, Phys. Rev. D86 (2012) 010001, and 2013 partial update for the 2014 edition
  * [44] A. Anisovich et al., Properties of baryon resonances from a multichannel partial wave analysis, Eur. Phys. J. A48 (2012) 15, arXiv:1112.4937
  * [45] R. Arndt, W. Briscoe, I. Strakovsky, and R. Workman, Extended partial-wave analysis of $\pi N$ scattering data, Phys. Rev. C74 (2006) 045205, arXiv:nucl-th/0605082