Research

Xenon TPC, dark matter, neutrinos, weak decays

Insert corresponding author name

# Dual-phase xenon time projection chambers for rare-event searches

Laura Baudis  Physik-Institut, University of Zürich, Winterthurerstrasse 190,
8057 Zürich, Switzerland<EMAIL_ADDRESS>
###### Abstract

In the past decade, dual-phase xenon time projection chambers (Xe-TPCs) have
emerged as some of the most powerful detectors in the fields of astroparticle
physics and rare-event searches. Developed primarily towards the direct
detection of dark matter particles, experiments presently operating deep
underground have reached target masses at the multi-tonne scale, energy
thresholds around 1 keV and radioactivity-induced background rates similar to
those from solar neutrinos. These unique properties, together with
demonstrated stable operation over several years, allow for the exploration of
new territory via high-sensitivity searches for a plethora of ultra-rare
interactions. These include searches for particle dark matter, for second
order weak decays, and the observation of astrophysical neutrinos. We first
review some properties of xenon as a radiation detection medium and the
operation principles of dual-phase Xe-TPCs together with their energy
calibration and resolution. We then discuss the status of currently running
experiments and of proposed next-generation projects, describing some of the
technological challenges. We end by looking at their sensitivity to dark
matter candidates, to second order weak decays and to solar and supernova
neutrinos. Experiments based on dual-phase Xe-TPCs are difficult, and, like
all good experiments, they are constantly pushed to their limits. Together
with many other endeavours in astroparticle physics and cosmology they will
continue to push at the borders of the unknown, hopefully to reveal profound
new knowledge about our cosmos.

###### keywords:

dark matter direct detection, astrophysical neutrinos, neutrinoless double
beta decay

##  1 Introduction

The idea of a Time Projection Chamber (TPC), "born on a waning winter
afternoon" in February 1974 [1] was first realised and successfully applied in
particle physics experiments at accelerators. In the last decades, the concept
evolved and was tailored to a large range of applications, most notably to
experiments in astroparticle and nuclear physics physics searching for rare
interactions deep underground. Present and future experiments designed to
observe interactions of dark matter and other exotic particles, second order
weak nuclear decays, as well as neutrinos from a variety sources are based on
TPCs operated with pure noble elements, either in gaseous or liquid form. As
in the original idea by David Nygren [2], modern TPCs capture the $x-y$
information of events, along with their drift time, allowing for a three-
dimensional position reconstruction if the start time is known.

The two most common noble fluids employed as detector media in present TPCs
for rare event searches are argon and xenon. Here we will focus on dual-phase
(liquid and gas) xenon TPCs (Xe-TPCs). For general reviews of experiments
based on noble elements, we refer the reader to [3, 4, 5]. Historical
introductions, the physics and technological details, as well as applications
of noble gas and electron emission detectors are detailed in [6, 7].

This article is structured as follows. In Section 2 the properties of xenon as
a radiation detection medium will be briefly reviewed. The working principle
of two-phase Xe-TPCs and their three-dimensional imaging capabilities are
introduced in Section 3, while their energy calibration and resolution is
discussed in Section 4. Section 5 looks at past, current and future detectors,
and at some technological challenges. Section 6 and 7 will discuss the search
for dark matter and other exotic particles, and for second order weak decays,
respectively. Section 8 will scrutinise the potential of dual-phase Xe-TPCs to
detect neutrinos from the Sun and from core-collapse supernovae, while Section
9 will conclude the brief review.

##  2 Properties of xenon as radiation detection medium

Xenon is an excellent scintillator and good ioniser in response to the passage
of radiation. In a TPC, the simultaneous detection of ionisation and
scintillation allows for the identification of the primary particle
interacting in the medium based on the linear energy transfer, $dE/dx$, and
for the determination of the three-dimensional position of an interaction with
sub-mm (in the $z$-coordinate) to mm (in the $x-y$-coordinate) precision.
Table 1 lists some of the physical properties of xenon, relevant for practical
aspects of a detector. The high atomic mass number and high liquid density
allows for compact, large and homogeneous detector geometries with efficient
self-shielding against external radiation, given that the cross sections for
the photoelectric effect, Compton scattering and pair production scale as
$Z^{5}/E_{\gamma}^{7/2}$, $Z/E_{\gamma}$ and $Z^{2}\ln{(2E_{\gamma})}$,
respectively, for incoming X-rays and gammas with energy $E_{\gamma}$. The
radioactive isotopes 124Xe, 126Xe, 134Xe and 136Xe have very long half-lives,
and their second-order weak decay modes are subject to investigation, as we
will show in Section 7.

Table 1: Physical properties, volume fraction in the atmosphere, radioactive isotopes of the noble element xenon. Property [unit] | Xe
---|---
Atomic number | 54
Mean atomic mass [g/mol] | 131.29
Boiling point $T_{b}$ at 1 atm [K] | 165.0
Melting point $T_{m}$ at 1 atm [K] | 161.4
Gas density at 1 atm & 298 K [g/l] | 5.40
Gas density at 1 atm & $T_{b}$ [g/l] | 9.99
Liquid density at $T_{b}$ [g/cm3] | 2.94
Volume ratio | 526
Dielectric constant of liquid | 1.95
Volume fraction in Earth’s atmosphere [ppm] | 0.087
Isotopes with spin, abundance [%] |  129Xe, 26.44; 131Xe, 21.18
Radioactive isotopes, |  136Xe, 8.87; 2.2$\times$1021 [8, 9]
abundance [%] and T1/2 [y] |  124Xe, 0.095; 1.1$\times$1022 [10, 11]
|  134Xe, 10.4; $>$8.7$\times$1020 (90% C.L.) [12]
|  126Xe, 0.089; $>$1.9$\times$1022 (90% C.L.) [13]

In a xenon detector, the energy loss of an incident particle is shared between
ionisation, excitation and sub-excitation electrons liberated in the
ionisation process. The average energy loss in ionisation is slightly larger
than the ionisation potential, as it includes multiple ionisation processes.
The energy $E_{0}$ transferred to the medium, for light particles such as
electrons, is [14]:

$E_{0}=N_{i}E_{i}+N_{ex}E_{ex}+N_{i}\epsilon,$ (1)

where $N_{i}$ and $N_{ex}$ are the mean number of ionised and excited atoms,
$E_{i}$ and $E_{ex}$ are the mean energies to ionise or excite the atoms and
$\epsilon$ is the average kinetic energy of sub-excitation electrons, the
energy of which goes into heat. For liquid xenon, the value of $\epsilon$ is
in the range $4.65-5.25$ eV. In its condensed phase, xenon exhibits a band
structure of electronic states, and if we divide all terms in Eq. 1 by the
band gap energy $E_{g}$ and uses the $W_{i}$-value, defined as the energy
required to produce one electron-ion pair, $W_{i}=E_{0}/N_{i}$, we obtain:

$\frac{W_{i}}{E_{g}}=\frac{E_{i}}{E_{g}}+\frac{N_{ex}}{N_{i}}\times\frac{E_{ex}}{E_{g}}+\frac{\epsilon}{E_{g}},$
(2)

The band gap energy is 9.22 eV and 9.28 eV for liquid and solid xenon,
respectively, while the ratio $\alpha=N_{ex}/{N_{i}}$ is between 0.06 and 0.2.
For nuclear recoils, the ratio $N_{ex}/{N_{i}}$ is 1, and thus much larger
than for electronic recoils. This property is employed in dark matter searches
via nuclear recoils, discussed in Section 6. The ratio $W_{i}/E_{g}$ has been
calculated as 1.65, in good agreement with measurements, which yield 1.6 [3].

Scintillation arises from excited xenon atoms $\mathrm{Xe}^{*}$ (excitons) and
from ions $\mathrm{Xe}^{+}$, as follows:

  1. 1.

${\mathrm{Xe}}^{*}+{\mathrm{Xe}}+{\mathrm{Xe}}\rightarrow{\mathrm{Xe}}^{*}_{2}+{\mathrm{Xe}}$
${\mathrm{Xe}}^{*}_{2}\rightarrow 2{\mathrm{Xe}}+{\mathrm{h}\nu}$

  2. 2.

${\mathrm{Xe}}^{+}+{\mathrm{Xe}}\rightarrow{\mathrm{Xe}}^{+}_{2}$
${\mathrm{Xe}}^{+}_{2}+\mathrm{e}^{-}\rightarrow{\mathrm{Xe}}^{**}+{\mathrm{Xe}}$
${\mathrm{Xe}}^{**}\rightarrow{\mathrm{Xe}}^{*}+{\mathrm{heat}}$
${\mathrm{Xe}}^{*}+{\mathrm{Xe}}+{\mathrm{Xe}}\rightarrow{\mathrm{Xe}}^{*}_{2}+{\mathrm{Xe}}$
${\mathrm{Xe}}^{*}_{2}\rightarrow 2{\mathrm{Xe}}+{\mathrm{h}\nu}$

where h$\nu$ denotes the emitted vacuum-ultraviolet (VUV) photon with
wavelength peaked around 175-178 nm.
$\mathrm{Xe}^{**}\rightarrow\mathrm{Xe}^{*}$ \+ heat corresponds to a non-
radiative transition. The excited dimer $\mathrm{Xe}^{*}_{2}$ (excimer), at
its lowest excited level, is de-excited to the dissociative ground state by
the emission of a single VUV photon. This comes from the large energy gap
between the lowest excitation and the ground level, forbidding other decay
channels such as non-radiative transitions. The scintillation light from pure
liquid xenon has two decay components due to de-excitation of spin singlet
(${}^{1}\Sigma_{u}^{+}$) and spin triplet (${}^{3}\Sigma_{u}^{+}$) states of
the excited dimer ${\mathrm{X}e}^{*}_{2}\rightarrow
2{\mathrm{X}e}+{\mathrm{h}\nu}$ to the ground state (${}^{1}\Sigma_{g}^{+}$).
The singlet and triplet states refer to the total spin quantum number (s=0 or
s=1) of the excited Rydberg electron and the angular momentum due to the
molecular orbit, with the shorter and longer decay shapes being produced by
the de-excitation of s=0 states and s=1 states, respectively. The differences
of pulse decay shape between different type of particle interactions can be in
principle be used to discriminate electronic from nuclear recoils. In
practice, this is difficult due to the small time separation, $(2.2\pm 0.3)$
ns versus $(27\pm 1)$ ns, of the two components.

If we denote $W_{ph}$ as the mean energy required for the production of a
single photon, we can express it as:

$W_{ph}=\frac{E_{0}}{N_{ex}+N_{i}}=\frac{W_{i}}{1+N_{ex}/N_{i}}=\frac{W_{i}}{1+\alpha},$
(3)

where $N_{ex},N_{i}$ are the produced number of excitons and electron-ion
pairs, respectively, and we assume for simplicity that the efficiencies for an
exciton or a recombining electron-ion pair to create a detectable photon are
unity:

$N_{ph}=N_{ex}+rN_{i},$ (4)

with $r$ being the recombination fraction. If an electric field is applied,
one can measure the electrons which do not recombine, with the amount of
extracted charge defined as:

$N_{q}=(1-r)N_{i}$ (5)

Using above equations, the recombination independent sum can be defined,
following [15], as:

$E_{0}=(N_{q}+N_{ph})\times W_{ph}$ (6)

The recombination independent energy required to produce a single detectable
quantum, $N_{q}$ or $N_{ph}$, is often called the W-value, and $W_{ph}=W$ is
used in the following. This assumes that each recombining electron-ion pair
produces an exciton, which leads to a photon. The widely-adopted W-value in
liquid xenon has been measured as $W=(13.7\pm 0.2$) eV with a small, 30 g LXe
detector using 122 keV $\gamma$-rays from an external 57Co source [15].
Recently, a lower value of $W=11.5^{+0.2}_{-0.3}$ (syst.) eV was measured with
a small TPC using internal sources at energies between 2.8 keV and 42 keV
[16], which is consistent with the value measured at the MeV-scale, $W=11.5\pm
0.5\mathrm{(syst.)}\pm 0.1\mathrm{(stat.)}$ eV with a larger detector [17].

Figure 1: After a particle interacts in a xenon medium, various processes lead
to ionisation, scintillation and heat. Only scintillation and ionisation (via
electroluminiscence) are observed in a two-phase Xe-TPC. Biexcitonic quenching
[18] is relevant for interaction with high ionisation density.

The partition between excitation and ionisation depends on the density of the
electron-ion pairs produced along the track of a particle, and is thus
different for nuclear and electronic recoils. The recombination fraction $r$
depends on the applied electric field, as well as on the ionisation density in
the track. In particular, for nuclear recoils as generated by interactions of
fast neutrons or hypothetical Weakly Interacting Massive Particles (WIMPs), an
energy-dependent quenching is introduced via the Lindhard factor $\mathcal{L}$
[19] :

$E_{0}=\mathcal{L}^{-1}(N_{q}+N_{ph})W,$ (7)

with $\mathcal{L}$ being around $0.15-0.2$ at nuclear recoil energies in the
range 3-100 keV [20]. A schematic view of the signal production after a
particle interaction in xenon is shown in Figure 1.

##  3 Principles of dual-phase xenon TPCs

Xenon TPCs in astroparticle physics and rare-event searches use either high-
pressure gas, or a liquid phase, or liquid and gas phase (also called two-
phase or dual-phase) as detection medium, and here we focus on the latter.
Table 2 lists some properties relevant for building a TPC.

Table 2: Properties of liquid xenon as a particle detection medium, relevant for building a TPC. Property [unit] | Value
---|---
Scintillation light yield (at 122 keV) [21] | 63 photons/keV
Wavelength (peak centred at) | 175-178 nm
Decay time constants (s=0, s=1) | 2.2 ns, 27 ns
Refractive index | 1.69
Electron mobility [22] | 0.29 mm2/($\mu$s V) ($<$100 V/cm)
| 0.01 mm2/($\mu$s V) ($>$100 V/cm)

The operation principle of a two-phase TPC is easy to grasp, and it is shown
schematically in Figure 2. An interaction within the active volume of a
detector will create ionisation electrons and prompt scintillation photons.
The prompt scintillation signal (S1) is detected with two arrays of
photosensors, one in the liquid phase on the bottom and one in the gas phase
at the top. The electrons drift in the pure liquid under the influence of an
external electric field, are then accelerated by a stronger field and
extracted into the vapour phase above the liquid, where they generate
proportional scintillation, or electroluminiscence. The delayed, proportional
scintillation signal (S2) is observed by the same photosensor arrays. The
array immersed in the liquid collects the majority of the prompt signal, which
is totally reflected at the liquid-gas interface. The ratio of the two signals
is different for nuclear recoils, such as from fast neutron interactions or
hypothetical WIMPs and electronic recoils produced by $\beta$ and
$\gamma$-rays. This provides the basis for background discrimination in dark
matter detectors. Since electron diffusion in the ultra-pure liquid is small
(albeit non-negligible, see Section 5.5), the proportional scintillation
photons carry the $x-y$ information of the interaction site. With the
$z-$information from the drift time measurement, the TPC yields a three-
dimensional event localisation, enabling fiducial volume selections and
differentiation between single- and multiple-scatters in the active volume.

Figure 2: The operation principle of a two-phase xenon TPC. A particle
interaction in liquid xenon gives rise to a prompt scintillation signal (S1)
and a delayed, amplified proportional scintillation signal (S2). The latter is
caused by ionisation electrons, which are drifted in a homogeneous electric
field (of a few 100 V/cm) and extracted into the gas phase above the liquid
with a higher electric field, typically 10 kV/cm. The drift field is produced
between the cathode at negative potential and a grounded gate grid in the
liquid, while the extraction field is obtained by means of the anode placed
above the gate in the gas phase. Both S1 and S2 signals are observed with
photosensor arrays placed on the bottom and top of the TPC.

##  4 Energy calibration and resolution of xenon TPCs

An advantage of two-phase Xe-TPCs is their relatively good energy resolution,
achieved by taking a linear combination of the two anti-correlated signals, S1
and S2. To calibrate the energy scale, mono-energetic lines from external
(e.g., 57Co, 137Cs, 228Th, etc) and internal (83mKr, 37Ar) calibration sources
are used. Additional signals are provided by neutron-activated xenon (129mXe,
131mXe) and the radioactivity of detector components (e.g., 60Co, 208Tl). The
mean detected S1 and S2 signals per produced photon and ionisation electron
are denoted as g1 and g2, respectively: $g_{1}=S1/N_{ph}$, $g_{2}=S2/N_{q}$,
also called the total photon detection efficiency and the charge amplification
factor. We can then express equation 6 as:

$E_{0}=(N_{ph}+N_{q})\times
W=\left(\frac{S1}{g_{1}}+\frac{S2}{g_{2}}\right)\times W$ (8)

Rearranging this equation yields:

$\frac{S2}{E_{0}}=\frac{g_{2}}{W}-\frac{g_{2}}{g_{1}}\frac{S1}{E_{0}}$ (9)

Since W and E0 (for a given mono-energetic signal) are known, we can determine
g1 and g2 from the measured S1 and S2 signals at several energies, in a so-
called "Doke plot", a schematic of which is shown in Figure 3, left. If we
plot $S2/E_{0}$ (charge yield, Qy) as a function of $S1/E_{0}$ (light yield,
Ly) , we can obtain g2/g1 from the slope, and g2 from the intercept of the
fitted line.

Figure 3: (Left): Schematic of a Doke plot in charge (Qy) versus light (Ly)
yield for an interaction energy E0. Measurements of the charge and light yield
at different electric drift fields or interaction energies yield data along
the solid line. The dashed line, showing the relation to the gain parameters
$g_{1}$ and $g_{2}$, is obtained by scaling with the $W$-value. (Right):
Normalised scintillation and ionisation signals at low energies for different
drift fields in the range 80-968 V/cm for a small TPC, Xurich. Figures from
[16].

Another method to measure g1 and g2 by varying the signal ratio between S1 and
S2 is to use different electric fields with one or several calibration
sources, exemplified in Figure 3, right. The quantities g1 and g2 are thus
detector dependant, for they include the detector’s efficiency to detect the
light and charge signals. Finally, the S2 gain can be determined directly from
the mean size of the S2 signal for single electrons extracted into the gas
phase. Relative energy resolutions ($\sigma/\mu$) at the level of 4-6% at
energies of a few tens of keV and 2-3% at energies of a few 100 keV were
reached in xenon TPCs [23, 24, 10]. At higher energies, relevant for the
neutrinoless double beta decay of 136Xe, relative resolutions of 0.67%, 0.8%
and 1.2% around 2.5 MeV were obtained by LZ [25], XENON1T [26] and EXO-200
[27], respectively111Superior energy resolutions can be achieved in high-
pressure gas TPCs with electroluminiscent amplification, see [28]. As a
concrete example, a relative resolution of 0.4% was obtained in the 5 kg scale
NEXT-White detector [29], a prototype for NEXT [30]..

A comprehensive framework to simulate scintillation and ionisation yields and
resolutions as a function of interaction type, energy and electric field in a
TPC is the Noble Element Simulation Technique (NEST) [31]. The code, based on
phenomenological models informed by a vast array of data, also allows for
simulating detector specific effects, once the primary and secondary
scintillation gains, as well as the drift field, are specified. NEST models
are regularly updated when new data becomes available.

##  5 Past, current and future experiments

The first dual-phase Xe-TPCs that set competitive constraints on WIMP scatters
off nuclei were those of the ZEPLIN and XENON programmes, in particular
ZEPLIN-II and ZEPLIN-III at the Boulby Mine in the UK (2007-2008), and XENON10
at LNGS in Italy (2006). These initial detectors evolved into LUZ and LUX-
ZEPLIN at SURF, USA, and XENON100, XENON1T and XENONnT at LNGS. In parallel,
PandaX-I and PandaX-II were constructed at the China Jinping Underground
Laboratory (CJPL), with first results in 2014, followed by PandaX-4T.

Starting with total masses at the few kilogram and later 100 kg scale, the
detectors evolved and reached target masses at the tonne- and more recently
multi-tonne scale. Concomitantly, the background levels in the most inner
regions constantly decreased, with now unprecedented electronic recoil levels
around 15 events/(t y keV) in the energy region below 100 keV. Figure 4 shows
the remarkable evolution of the sensitivity to WIMPs (left) and of the
background level (right) as a function of target mass. The figures cover a
span of about twenty years.

Figure 4: Evolution of the sensitivity to SI WIMP-nucleon interactions (left)
and electronic recoil background level (right) of dual-phase Xe-TPCs as a
function of target mass. We note the logarithmic scales. Figures from [32].

The current generation of detectors employ several tons of LXe: LZ [33],
PandaX-4T [34] and XENONnT [35] have total (target) LXe masses of 10 t (7 t),
5.6 (3.7 t) and 8.6 t (5.9 t), respectively. While their overall TPC design is
rather similar, with cylindrical, PTFE enclosed target regions viewed by two
arrays of 3-inch diameter Hamamatsu R11410 PMTs, their detailed realisation
differs in many technical aspects, some of which are described in the
following.

###  5.1 The LUX-ZEPLIN experiment

The LUX-ZEPLIN (LZ) experiment is located at the Sanford Underground Research
Facility (SURF) in South Dakota, USA. The inner, xenon detector, is composed
of a TPC and a veto, called Xe Skin, enclosed in a double-walled, low-
background titanium cryostat. The dark matter target in the TPC contains 7
tonnes of LXe in a cylindrical volume which measures 1.5 m in diameter and
height. The vapour phase above the liquid is 8 mm thick, and the site of the
S2 signal production. The TPC is lined with PTFE and instrumented with 494
3-inch Hamamatsu R11410-22 PMTs arranged in two arrays. The Xe Skin detector,
around and underneath the TPC with 2 tonnes of instrumented liquid with 93
1-inch and 38 2-inch PMTs, acts as an anti-coincidence scintillation detector,
especially effective for gamma radiation. The cryostat is surrounded by an
outer detector, a system of acrylic tanks containing 17 t of Gd-loaded liquid
scintillator, placed in a 238 t tank of ultra-pure water. The tank is equipped
with 120 8-inch PMTs which record both outer detector and the water Cherenkov
signals. LZ has presented first results on WIMP dark matter for an exposure of
60 live days with 5.5 t of liquid xenon fiducial mass. The main electronic
recoil background comes from 214Pb $\beta$-decays, with a 222Rn concentration
in LXe of 3.3 $\mu$Bq/kg and from 37Ar decays, with a half-life of 35 days.
The xenon gas was purified of krypton using gas chromatography, with a natKr
rate of $(0.14\pm 0.02)$ ppt. The data was consistent with a background-only
hypothesis and new upper limits on spin-independent and spin-dependent WIMP-
nucleon cross sections for WIMP masses above 9 GeV were set, with the most
stringent constraint at $6.5\times 10^{-48}$cm2 for a mass of 30 GeV/c2 [36].
The LZ experiment continues to take science data at SURF, with a projected
sensitivity for spin-independent (SI) WIMP-nucleon cross sections of
1.5$\times$10-48 cm2 for a 40 GeV/c2 WIMP (at 90% C.L.) and an exposure of 20
t y.

###  5.2 The PandaX-4T experiment

The PandaX-4T experiment is located at CJPL, built deep underground in the
Jinping Mountains of Sichuan. The inner detector is a TPC with 3.7 tonnes of
liquid xenon, contained by 24 PTFE wall panels and observed by 169 and 199
R11410-23 three-inch PMTs in the top and bottom, respectively. The TPC and
xenon are contained in a double-walled, low-background stainless steel
cryostat, placed at the centre of a 10 m diameter and 13 m tall tank filled
with ultra-pure water. The space between the TPC field cage and the cryostat
is instrumented with two rings of one-inch R8520 Hamamatsu PMTs, for an active
xenon veto. The xenon inventory was distilled for krypton through a
distillation tower prior to and during the commissioning run, which included
95 days of stable data taking, for a natKr rate of $(0.33\pm 0.02)$ ppt. The
dominant electronic recoil background was due to $\beta$-decays from 3H, with
an average concentration of $5\times 10^{-24}$ mol/mol in Xe, followed by
$\beta$-decays from 214Pb, with a 222Rn concentration between 4.2 and 5.9
$\mu$Bq/kg. The commissioning data was used for a WIMP search, with an
exposure of 0.63 tonne year. No dark matter candidate events were observed
above the expected background, with the most stringent upper limit on SI WIMP-
nucleon cross sections of 3.8$\times$10-47 cm2 for a 40 GeV/c2 WIMP [34]. Due
to the tritium contamination of the xenon, likely originating from a CH3T
calibration of the previous detector, the collaboration undertook a tritium
removal campaign, before the start of the physics data taking. The goal is to
reach another order in magnitude improvement in sensitivity with an exposure
of 6 t y.

###  5.3 The XENONnT experiment

The XENONnT experiment is located at the INFN Laboratori Nazionali del Gran
Sasso (LNGS) in Italy. The TPC contains 5.9 t of LXe, enclosed by a PTFE
cylinder with a diameter of 1.33 m and a height of 1.49 m. A total of 494
Hamamatsu R11410-21 3-inch PMTs distributed in a top and a bottom array view
the sensitive volume. A double-walled, low-radioactivity stainless steel
cryostat houses the TPC and is filled with 8.5 t of LXe. The cryostat is
placed in a 700 t water Cherenkov muon veto, 10.2 m high and 9.6 m in
diameter, equipped with 84 8-inch PMTs. A volume of 33 m3 of water surrounding
the cryostat and delimited by octagonal caps and side reflectors made of high-
reflectivity expanded PTFE is used as a neutron veto, seen by 120 8-inch PMTs.
In the first physics run, the vetos contained ultra-pure water, while later
the water will be doped with Gd to increase the neutron veto efficiency.
XENONnT had accumulated 95.1 days of data during its first science run. A
high-throughput radon distillation column was continuously operated to remove
radon from the gaseous xenon in the TPC, while a krypton distillation column
allowed to achieve a natKr concentration of $<50$ ppq. In XENONnT, a novel LXe
purification system was installed, allowing to reach a very high removal rate
of electronegative impurities on a short time scale, with an electron drift
lifetime above 10 ms during the science run. The dominant electronic recoil
background in the first science run was due to 214Pb $\beta$-decays, with a
222Rn concentration of 1.8 $\mu$Bq/kg. A blind analysis revealed no
significant excess from WIMPs in an exposure of 1.09 t y, with the lowest
upper limit on SI WIMP-nucleon cross sections of 2.58$\times$10-47 cm2 at 28
GeV/c2 (90% CL) [37]. The ER background of $(15.8\pm 1.3)$ events/(t y keV)
was the lowest in any dark matter detector, and in particular 5 times lower
than in XENON1T. XENONnT continues to take data at LNGS, with a further
reduced 222Rn concentration of 0.8 $\mu$Bq/kg, using the radon distillation
system with combined gaseous and liquid xenon flow.

###  5.4 Next-generation projects

DARWIN, first proposed around 2011 [38], is a next-generation observatory with
40 t of LXe in the TPC (50 t total). In the baseline design, the cylindrical
TPC, with 2.6 m diameter and 2.6 m height, is placed in a low-background,
double-walled titanium cryostat, surrounded by active neutron and muon vetos.
Two photosensor arrays with a total of 1910 3-inch PMTs will be located at the
top and bottom of the TPC, which is lined with high-reflectivity PTFE,
surrounded by 92 copper field shaping rings [39]. The R&D for new type of
photosensors, which could potentially replace the 3-inch PMTs, is ongoing. The
studied sensors includes VUV-sensitive silicon photomultipliers,
2-inch$\times$2-inch flat panel PMTs (R12699), and hybrid photosensors.
Recently, the LZ, XENON and DARWIN collaboration joined forces to form the
XLZD consortium [40], with the goal of constructing and operating the next-
generation observatory together. The size and scope of the detector might be
enlarged, compared to DARWIN, with a 3 m$\times$3 m TPC containing 60 t of LXe
(75 t in total). The larger mass would allow for a 3-$\sigma$ WIMP discovery
at a SI cross section of 3$\times$10-49 cm2 at 40 GeV/c2 mass (90% CL), and
would also increase the sensitivity to the neutrinoless double-beta decay of
136Xe. The science potential of a large, dual-phase xenon detector is detailed
in [32].

PandaX-xT is the next step in the PandaX programme at CJPL, with $>$30 t of
LXe target in the TPC. Two arrays of Hamamatsu R12699 2-inch PMTs will view
the Xe volume. Compared to the 3-inch tubes employed in current TPCs, these
new sensors have the advantage of lower radioactivity, faster time response
and the possibility of multi-anode readout, with four independent channels per
unit. The inner cryostat vessel will be made of ultra-pure copper, and the
space between the inner and outer vessel will contain an active veto. PandaX-
xT aims for a 200 t y exposure for WIMPs, and, similar to DARWIN/XLZD, for a
broad science reach [41].

###  5.5 Technological challenges

As we have seen, dual-phase Xe-TPCs were successful in gradually scaling up
their target mass from a few kg to multi-tons, and concomitantly reducing the
background level for each iteration, while maintaining a low-energy threshold
of $\sim$1 keV for electronic recoils. Notwithstanding, the construction of
next-generation detectors with multi-ten-tons target masses poses multiple
technological challenges. The background goals are such that electronic and
nuclear recoils rates are below the ones from irreducible astrophysical
neutrino interactions. This requirement then also sets the goals for the
intrinsic 222Rn and 85Kr concentrations: the background from the decay of
these isotopes should be significantly lower than the solar pp-neutrino
elastic scattering rate, as shown in Figure 5, left. This translates into
0.1$\mu$Bq/kg for 222Rn222A 222Rn concentration of 0.1$\mu$Bq/kg corresponds
to less than one radon atom per 100 mol of xenon. The main background is due
to 214Bi $\beta$-decays which are not accompanied by an $\alpha$-decay and
thus cannot be tagged in the TPC. and 0.1 ppt for natKr, assuming a 85Kr/natKr
ratio of 2$\times$10-11mol/mol. natKr concentrations of $<50$ ppq were already
achieved by cryogenic distillation [42], while for 222Rn a factor of about 10
reduction compared to the current value of 0.8 $\mu$Bq/kg in XENONnT is still
needed, see Figure 5, right. This calls for a high liquid xenon throughput
(close to 1 t/hour) with efficient cooling power based on cryogenic heat pumps
and radon-free heat exchangers. Cryogenic distillation alone is not
sufficient: it must go hand in hand with selection of low radon emanation
materials and new coating techniques to prevent radon emanation from surfaces.

Figure 5: (Left): Differential energy spectra of electronic recoil background
sources in DARWIN. The dominating contribution is from pp- and 7Be solar
neutrinos. Figure from [43]. (Right): The evolution of 222Rn concentration in
two-phase Xe-TPCs (measured values, blue dots), together with the expected
decrease from the surface-to-volume ratio (dashed line, $x^{-1/3}$). The goal
for next-generation TPCs (open circle) is also shown.

Other challenges related to the liquid target are the continuous purification
for electronegative impurities and water, to maintain high charge and light
yields, as well as new solutions for reliable xenon storage and recuperation
at large scales. Liquid phase purification powered by a liquid xenon pump, as
demonstrated in [44], was employed to achieve an electron drift lifetime above
10 ms in about 8.6 t of xenon in XENONnT. A system capable of handling 30 t of
xenon in liquid phase, including long-term storage and and transfer of the
cryogenic liquid between storage module and detectors was constructed for
PandaX-xT [41].

Regarding the detector design, electrodes with large ($>2.5$ m) diameters,
with high transparency, minimal sagging and low spurious electron emission, as
well as high-voltage feed-throughs that can safely deliver 50 kV or more to
the cathode must be developed. The LZ collaboration successfully built custom-
woven wire-mesh grids with 1.5 m diameter, produced with an in-house built
loom to weave the wire meshes [45]. Finally the cryostat design must be such
as to ensure stability, while reducing as much as possible the amount of
material, and thus gamma and neutron emitters in proximity to the TPC.

To address a series of challenges related to constructing and operating next-
generation Xe-TPCs, several large-scale demonstrators have been built, in
particular also Xenoscope, which includes a 2.6 m tall TPC [46, 22] and
Pancake, to deploy a 2.6 diameter TPC [47].

##  6 Sensitivity to dark matter candidates

The current generation of detectors did not find evidence for WIMP dark
matter, but were able to set the world’s most stringent limits on WIMP-nucleon
interactions over a wide range of particle masses and thus to exclude a large
range of possibilities. These results were from very first data and the
experiments are yet to reach their design exposures of 20 t y, see Figure 6.
The search for WIMP dark matter is thus ongoing, with the goal of reaching a
sensitivity for the SI WIMP-nucleon cross section around 1.5$\times$10-48 cm2
at 40-50 GeV/c2 mass [33, 35]. While dark matter particles are thought to be
electrically neutral, they could posses tiny electromagnetic couplings to
photons, allowed by all the data. The PandaX collaboration looked for nuclear
recoils generated by potential dark matter-photon interactions, setting tight
constraints on the magnitudes of millicharge, magnetic dipole, electric dipole
and anapole moments, as well as first constraints on the charge radius of dark
matter [48].

Figure 6: Exclusion limits (solid) on the spin-independent WIMP-nucleon cross
section from dual-phase Xe-TPCs as well as from other technologies.
Projections for LZ and XENONnT (dashed) and for DARWIN/XLZD (dashed-dotted)
are also shown. The region where a distinction between a dark matter signal
and astrophysical neutrinos will be challenging, albeit not impossible [49],
is shown in blue. Figure updated from [50].

Due to unprecedented low electronic-recoil backgrounds and low energy
thresholds, high-sensitivity electronic recoil searches also became feasible.
These include searches for dark matter electron scattering with particle
candidates from a hidden sector, searches for keV-scale axion-like-particles
(ALPs) and dark photons via absorption in LXe, and searches for solar axions.
In 2020, XENON1T reported a surprising excess of events in the energy region
1-7 keV, which could have been due to solar axions (with a statistical
significance of 3.4 $\sigma$), galactic ALPs, or a neutrino magnetic moment.
The excess, however, could also have been caused by minute amounts of tritium
in the xenon, at the level of 6$\times$10-25mol/mol, or less than three
tritium atoms per kilogram of LXe [51]. With the accumulated number of events,
it was not possible to confirm or reject the tritium hypothesis with XENON1T,
and a larger detector with lower backgrounds was urgently needed. With data
from its first science run, and an electronic recoil rate five times below the
one of its predecessor, XENONnT did not see any excess above background. The
rate and shape agreed well with predictions and, between (1-10) keV, was
dominated by 214Pb $\beta$-decays from 222Rn dissolved in the LXe, followed by
interactions of pp solar neutrinos [52]333Most likely the XENON1T low-energy
excess was indeed caused by tritium decays, since in XENONnT the fluid
handling and purification is different and measures to reduce any initial
tritium concentration were taken.. A comparison between low-energy event rates
in XENON1T, showing the excess, XENONnT, PandaX-4T and LZ is shown in Figure
7, left. Among other results, XENONnT set new constraints on the coupling of
solar axions to electrons and photons, via the axio-electric and inverse
Primakoff effects, respectively, shown in Figure 7, right [52]. Sensitivity
projections for LZ to new physics via low-energy electronic recoils, including
mirror and leptophilic dark matter, are detailed in Ref. [53].

Figure 7: (Left): Observed event rates at low energies in XENONnT (grey),
compared with XENON1T (blue), as well as with LZ (orange) and PandaX-4T
(green). The shaded areas indicate the estimated backgrounds. Figure by J. Ye,
Columbia University. (Right): Constraints on solar axion couplings to photons
and electrons from XENONnT (black), the XENON1T result (blue), as well as
other direct and indirect constraints [52].

Looking at ionisation-only signals allows for a further reduction of the
energy threshold, given the much higher efficiency to detect an ionisation
electron compared to a primary scintillation photon (typically 90% versus
10%). While this gain comes at the expense of higher backgrounds, it allows
nonetheless to set the most stringent limits on light dark matter electron
interactions at masses from a few tens of MeV to a few GeV. This is
illustrated in Figure 8, which shows recent result from PandaX-4T [54] as well
as from previous searches. Dual-phase Xe-TPCs thus start probing theoretical
predictions for different dark matter production scenarios in hidden sectors.

Figure 8: Upper limits on the cross section for dark matter electron scatters,
assuming a heavy (left, with a dark matter form factor = 1) and light (right,
with a dark matter form factor $\sim 1/q^{2}$) mediator, from the PandaX-4T
experiment (red), as well as other constraints. Dual-phase Xe-TPCs thus start
testing theoretical predictions, shown as grey bands for different dark matter
production scenarios. Figures from [54].

##  7 Second order weak interactions

Large TPCs using liquid xenon in its natural isotopic abundance are sensitive
to double weak decays, such as the double beta decay of 134Xe and 136Xe, with
Q-values of 825.8 keV and 2457.8 keV, respectively, and the double electron
capture process in 124Xe and 126Xe, at Q-values of 2857 keV and 920 keV. In
particular, the observation of the neutrinoless decay modes:

$(Z,A)\rightarrow(Z+2,A)+2e^{-},$ (10) $(Z,A)+2e^{-}\rightarrow(Z-2,A),$ (11)

would be evidence for lepton number violation involving Majorana neutrinos.
The observed standard model processes with two neutrinos (so far,
$2\nu\beta\beta$ in 136Xe and $2\nu$ECEC in 124Xe) allow for comparison to
theoretical half-life predictions from nuclear structure calculations in
different nuclear physics models, as well as for new physics searches (see,
e.g., [55] for a recent overview).

In its electronic recoil channel, the XENON1T experiment observed for the
first time the 2$\nu$ECEC process in 124Xe by detecting the simultaneously
emitted K-shell X-rays/Auger electrons of the daughter atom 124Te with a
combined energy of 64.33 keV, which is twice the K-shell binding energy. With
a half-life of $(1.1\pm 0.2_{\mathrm{stat}}\pm 0.1_{\mathrm{sys}})\times$1022
y, this is the slowest process ever measured directly [10, 11]. LZ, PandaX-4T
and XENONnT will improve upon these results and will also search for the
$2\nu\beta^{+}$EC decay. This channel has a distinct signature due to the two
511 keV gammas emitted after the positron annihilates with an electron in the
medium. With a predicted half-life around $1.6\times 10^{23}$ y [56] its first
observation is within reach of these running experiments, which will also
measure the half-life and in particular also the shape of the
$2\nu\beta\beta$-decay of 136Xe with high statistics and at low energies not
previously accessed.

Finally, the current generation of detectors will set constraints on the
$0\nu\beta\beta$-decays of 136Xe and 134Xe, as predicted or shown in [57, 11,
58, 59], see Figure 9, left. In spite of relative energy resolutions
($\sigma/\mu$) of 0.8% and 0.67% [26, 25] at high energies, the attainable
half-lives are not competitive to dedicated experiments such as KamLAND-Zen
[60] and the proposed nEXO [61]. Notwithstanding, the analyses will deliver
proof-of-principle methods towards higher sensitivity searches in the
DARWIN/XLZD and PandaX-xT detectors. As an example DARWIN, with 40 t of
natural xenon in the TPC, is predicted to achieve a sensitivity of $3\times
10^{27}$ y (90% C.L.) after ten years of operation [62]. The predicted
background spectrum around the Q-value of the decay, together with a
hypothetical signal, is shown in see Figure 9, right. With an enlarged xenon
mass of 60 t in the TPC, as advocated by XLZD, a half-life sensitivity of
$\sim$$5\times 10^{27}$ y (90% C.L.) could be reached after ten years,
allowing to fully probe the inverted mass ordering scenario for neutrinos.

Figure 9: (Left): Projected sensitivity of LZ (green band) to the effective
Majorana neutrino mass, as a function of the lightest neutrino mass
eigenstate. The red and blue contours show the allowed parameter space at
1-sigma for the inverted and normal neutrino mass ordering scenarios,
respectively. Figure from [58]. (Right): Predicted background spectrum in
DARWIN around the region-of-interest for $0\nu\beta\beta$-decay for 5 t LXe
fiducial mass. A hypothetical signal of 0.5 events/y, corresponding to a half-
life of $2\times 10^{27}$ y, is shown for comparison. Figure updated from
[62].

##  8 Neutrino detection

Recently, dual-phase Xe-TPCs have reached target scales which allow them to
explore a variety of astrophysical neutrino sources. The main detection
channels are elastic neutrino-electron scattering and coherent elastic
neutrino-nucleus scattering:

$\nu_{x}+e^{-}\rightarrow\nu_{x}+e^{-},$ (12)
$\nu_{x}+(Z,A)\rightarrow\nu_{x}+(Z,A),$ (13)

where $\nu_{x}$ stands for all three neutrino flavours,
$\nu_{e},\nu_{\mu},\nu_{\tau}$. The latter process, in which the neutrino
scatters off a nucleus via Z-boson exchange and the nucleus recoils a a whole,
is coherent up to neutrino energies around 50 MeV.

The observation of elastic neutrino-electron scattering of solar $pp$ and 7Be
neutrinos will allow to measure the $pp$ and 7Be fluxes with 0.15% and 1%
statistical precision for a 300 t y exposure, with rates of 365 events/(t y)
and 140 events/(t y), respectively. This will imply a first measurement of the
electron neutrino survival probability with 4% relative uncertainty, as well
as of the weak mixing angle, with 5% relative uncertainty, at energies below
200 keV [63]. The main backgrounds for these measurements are 222Rn decays,
with a required concentration of 0.1 $\mu$Bq/kg, and the $2\nu\beta\beta$
decay of 136Xe. A first detection of solar $pp$ neutrinos will already be
achievable with the current generation of detectors. As we have seen, the
solar $pp$ rate was almost half the rate from 214Bi $\beta$-decays in
XENONnT’s first science run, in the energy range $(1-10)$ keV. It is also
interesting to note that, for the first time in a dark matter detector, the
neutrino-induced rate is a factor of $\sim$1.6 above the one from the
radioactivity of detector materials [52].

Coherent elastic neutrino-nucleus scattering (CE$\nu$NS) was proposed fifty
years ago by Daniel Z. Freedman [64], who wrote: "The experiments are very
difficult, although the estimated cross sections (about 10-38 cm2 on carbon)
are favorable."444The manuscript was received in October 1973, only a few
months after the Gargamelle collaboration had presented the first evidence of
the weak neutral current in July of the same year [65]. The experiments were
difficult indeed, and the process was only observed four decades later by the
COHERENT collaboration with a CsI(Na) scintillating crystal [66], and later
also with a liquid argon detector [67]. COHERENT uses neutrinos from pion
decays at rest, produced at the Spallation Neutron Source at Oak Ridge
National Laboratory. Thus, to this day, CE$\nu$NS was never observed on xenon
and never with "naturally occurring" neutrinos.

The cross section is proportional to the number of neutrons squared, hence
heavy nuclei are ideal to observe astrophysical neutrinos via this reaction.
For 8B solar neutrinos, about 99% of events are at nuclear recoil energies
below 3 keV in a xenon detector, which poses one of the main difficulties in
observing CE$\nu$NS-induced events. Firstly, the detection efficiencies in Xe-
TPCs are relatively low at these energies (in particular if both S1 and S2
signals are required) and secondly, the light and charge yields have large
associated uncertainties555We refer to Ref. [68] for a recent study of
uncertainties of low-energy LXe responses to nuclear recoils and their impact
on solar 8B neutrino searches.. In addition, the combinatorial background
(random pairing of S1 and S2 events) is largest at low energies [69], and the
problem is more severe for larger detectors with increased electron drift time
(and possibly higher rates of spurious events). Nonetheless, the current
generation of multi-ton xenon TPCs aim to observe 8B solar neutrinos for the
first time. Figure 10 show constraints on the 8B neutrino flux from XENON1T
[69] and PandaX-4T [70], as well as constraints on non-standard neutrino
interactions from XENON1T and COHERENT. Next-generation experiments will carry
out precision measurements of 8B neutrinos induced CE$\nu$NS, and, with an
event rate of about 90 events per tonne and year [71] they will probe non-
standard neutrino interactions and provide an independent measurement of the
solar 8B neutrino flux [32].

Figure 10: (Left): Constraints on non-standard vector couplings between
electron neutrinos and up/down quarks from XENON1T (blue), COHERENT (light and
dark red) and CHARM (green). Shown is the ratio with respect to the SM
expectation: the presence of NSI would result in an enhancement or a
suppression of the CE$\nu$NS rate. Figure from [69]. (Right): Constraints on
the 8B solar neutrino flux via CE$\nu$NS from XENON1T (black), PandaX-4T (red)
as well as the standard solar model (B16-GS98) prediction (blue). Figure from
[70].

Apart from solar neutrinos, two-phase Xe-TPCs will be able to detect
CE$\nu$NS-induced events from supernova (SN) neutrinos. During the collapse of
a star, about 99% of the gravitational binding energies of the proto-neutron
star goes into neutrinos of all flavours, with energies up to tens of MeV.
Given the nature of the interaction process, xenon detectors are sensitive to
all neutrino flavours with few events per tonne expected for a core-collapse
SN at a distance of 10 kpc. Assuming a SN with 27 M⊙ progenitor at a distance
of 10 kpc, 700 events are expected in the DARWIN detector with 40 t of LXe
[72]. This number increases for a larger mass, as does the significance of a
SN detection, for a given distance, as illustrated in Figure 11.

Figure 11: Number of SN $\nu$ interactions in a Xe detector for a range of
target masses (left) and detection significance (right) as a function of
distance to the SN, for a 27 M⊙ progenitor. The solid (dashed) curves in the
right plot, also showing the sensitivity of XENONnT/LZ, assume current
(decreased by a factor of 10) level of backgrounds. Figures from [73].

Current xenon experiments participate in the supernova early warning system
(SNEWS) network [74], which prepares and provides an early warning system for
galactic SN to facilitate the observation of optical counterparts. Finally,
next-generation Xe-TPCs will start constraining the diffuse supernova
background (DSNB). Understanding core-collapse SN depends on probing the DSNB
with all neutrino flavours, and currently only upper limits on the
$\nu_{e},\bar{\nu}_{e}$ flux the SNO and Super-Kamiokande experiments exist,
around 19 cm-2s-1 and 2.7 cm-2s-1, respectively. Limits on the fluxes of
$\nu_{\mu,\tau},\bar{\nu}_{\mu,\tau}$ are much weaker, around 103 cm-2s-1, and
a large xenon detector could improve these by about a factor of 100. While
this would help in constraining SN models, the actual detection of the DSNB
remains challenging, even with a 1000 t y exposure [75].

##  9 Conclusion

Dual-phase Xe-TPCs are relatively new detectors in the landscape of
astroparticle physics experiments. Developed in the early twenty first century
primarily to search for dark matter in the form of WIMPs, they soon surpassed
other technologies in terms of their sensitivity to WIMP-nucleon interactions
over a large range of WIMP masses [76]. Almost twenty years later, detectors
using several tonnes of liquid xenon can observe signals down to a few quanta
(photons and electrons) with unprecedented low background rates. These
detectors will soon observe for the first time solar neutrinos at energies not
previously explored, both via neutrino-electron elastic scattering and via
CE$\nu$NS processes. While solar and other astrophysical neutrinos will
ultimately limit the sensitivity to dark matter, they bring forth exciting
signatures in their own right. Other compelling searches are those for ultra-
rare, second order weak processes, for these provide unique probes of beyond
standard model physics. While the current generation of detectors (LZ,
PandaX-4T and XENONnT) continue to acquire data in different deep underground
laboratories, next-generation experiments at the multi-ten-ton scale
(DARWIN/XLZD and PandaX-xT) are being planned, with large-scale R&D projects
and prototyping ongoing. With foreseen first data in the early 2030s, these
detectors might discover dark matter particles, and thus solve a problem which
is almost a century old. At the same time, they will break new grounds in
other areas of astroparticle physics, in particular in neutrino physics and
rare nuclear decays. Certainly this will not be the final generation of Xe-
TPCs. To make sense of future observations, including those which are
unexpected, likely a series of new, larger and more sophisticated experiments
will need to be designed. To paraphrase the authors of Ref. [6], we believe
that the best pages in the history of dual-phase Xe-TPCs are yet to be
written.

This work was supported by the University of Zurich, by the European Research
Council (ERC) under the European Union’s Horizon 2020 research and innovation
programme, grant agreement No. 742789 (Xenoscope), by the SNF grant
20FL20-201437, as well as by the European Union’s Horizon 2020 research and
innovation programme under the Marie Skłodowska-Curie grant agreement No
860881-HIDDeN. We thank Michael Murra for providing the data for Figure 5,
right.

## References

  * [1] Nygren DR. 2018 Origin and development of the TPC idea. Nucl. Instrum. Meth. A 907, 22–30. (10.1016/j.nima.2018.07.015)
  * [2] Nygren D. 1974 The Time Projection Chamber: A New 4 pi Detector for Charged Particles. eConf C740805, 58.
  * [3] Aprile E, Doke T. 2010 Liquid Xenon Detectors for Particle Physics and Astrophysics. Rev. Mod. Phys. 82, 2053–2097. (10.1103/RevModPhys.82.2053)
  * [4] Chepel V, Araujo H. 2013 Liquid noble gas detectors for low energy particle physics. JINST 8, R04001. (10.1088/1748-0221/8/04/R04001)
  * [5] Gonzalez-Diaz D, Monrabal F, Murphy S. 2018 Gaseous and dual-phase time projection chambers for imaging rare processes. Nucl. Instrum. Meth. A 878, 200–255. (10.1016/j.nima.2017.09.024)
  * [6] Aprile E, Bolotnikov AE, Bolozdynya AL, Doke T. 2008 Noble Gas Detectors. Wiley. (10.1002/9783527610020)
  * [7] Bolozdynya AI. 2010 Emission detectors. World Scientific.
  * [8] Albert J et al.. 2014 Improved measurement of the $2\nu\beta\beta$ half-life of 136Xe with the EXO-200 detector. Phys. Rev. C 89, 015502\. (10.1103/PhysRevC.89.015502)
  * [9] Gando A et al.. 2019 Precision measurement of the 136Xe two-neutrino $\beta\beta$ spectrum in KamLAND-Zen and its impact on the quenching of nuclear matrix elements. Phys. Rev. Lett. 122, 192501. (10.1103/PhysRevLett.122.192501)
  * [10] Aprile E et al.. 2019 Observation of two-neutrino double electron capture in 124Xe with XENON1T. Nature 568, 532–535. (10.1038/s41586-019-1124-4)
  * [11] Aprile E et al.. 2022 Double-Weak Decays of 124Xe and 136Xe in the XENON1T and XENONnT Experiments. Phys. Rev. C 106, 024328. (10.1103/PhysRevC.106.024328)
  * [12] Albert JB et al.. 2017 Searches for double beta decay of 134Xe with EXO-200. Phys. Rev. D 96, 092001. (10.1103/PhysRevD.96.092001)
  * [13] Abe K et al.. 2018 Improved search for two-neutrino double electron capture on 124Xe and 126Xe using particle identification in XMASS-I. PTEP 2018, 053D03. (10.1093/ptep/pty053)
  * [14] Platzman R. 1961 Total ionization in gases by high-energy particles: An appraisal of our understanding. The International Journal of Applied Radiation and Isotopes 10, 116 – 127. (https://doi.org/10.1016/0020-708X(61)90108-9)
  * [15] Dahl CE. 2009 The Physics of Background Discrimination in Liquid Xenon, and First Results from XENON10 in the Hunt for WIMP Dark Matter. PhD thesis Princeton University.
  * [16] Baudis L, Sanchez-Lucas P, Thieme K. 2021 A measurement of the mean electronic excitation energy of liquid xenon. Eur. Phys. J. C 81, 1060. (10.1140/epjc/s10052-021-09834-x)
  * [17] Anton G et al.. 2020 Measurement of the scintillation and ionization response of liquid xenon at MeV energies in the EXO-200 experiment. Phys. Rev. C 101, 065501. (10.1103/PhysRevC.101.065501)
  * [18] Hitachi A. 2005 Properties of liquid xenon scintillation for dark matter searches. Astropart. Phys. 24, 247–256. (10.1016/j.astropartphys.2005.07.002)
  * [19] J. Lindhard, V. Nielsen MS, Thomsen P. 1963 Integral equations governing radiation effects. Kgl. Danske Videnskab., Selskab. Mat. Fys. Medd 33.
  * [20] Sorensen P, Dahl CE. 2011 Nuclear recoil energy scale in liquid xenon with application to the direct detection of dark matter. Phys. Rev. D 83, 063501. (10.1103/PhysRevD.83.063501)
  * [21] Lenardo B, Kazkaz K, Manalaysay A, Mock J, Szydagis M, Tripathi M. 2015 A Global Analysis of Light and Charge Yields in Liquid Xenon. IEEE Trans. Nucl. Sci. 62, 3387–3396. (10.1109/TNS.2015.2481322)
  * [22] Baudis L et al.. 2023 Electron transport measurements in liquid xenon with Xenoscope, a large-scale DARWIN demonstrator. .
  * [23] Aprile E et al.. 2012 The XENON100 Dark Matter Experiment. Astropart. Phys. 35, 573–590. (10.1016/j.astropartphys.2012.01.003)
  * [24] Akerib DS et al.. 2017 Signal yields, energy resolution, and recombination fluctuations in liquid xenon. Phys. Rev. D 95, 012008. (10.1103/PhysRevD.95.012008)
  * [25] Pereira G, Silva C, Solovov VN. 2023 Energy resolution of the LZ detector for high-energy electronic recoils. JINST 18, C04007. (10.1088/1748-0221/18/04/C04007)
  * [26] Aprile E et al.. 2020 Energy resolution and linearity of XENON1T in the MeV energy range. Eur. Phys. J. C 80, 785. (10.1140/epjc/s10052-020-8284-0)
  * [27] Anton G et al.. 2019 Search for Neutrinoless Double-$\beta$ Decay with the Complete EXO-200 Dataset. Phys. Rev. Lett. 123, 161802. (10.1103/PhysRevLett.123.161802)
  * [28] Nygren D. 2009 High-pressure xenon gas electroluminescent TPC for 0nu beta beta-decay search. Nucl. Instrum. Meth. A 603, 337–348. (10.1016/j.nima.2009.01.222)
  * [29] Renner J et al.. 2019 Energy calibration of the NEXT-White detector with 1% resolution near Qββ of136Xe. JHEP 10, 230. (10.1007/JHEP10(2019)230)
  * [30] Adams C et al.. 2020 Sensitivity of a tonne-scale NEXT detector for neutrinoless double beta decay searches. .
  * [31] Szydagis M, Barry N, Kazkaz K, Mock J, Stolp D, Sweany M, Tripathi M, Uvarov S, Walsh N, Woods M. 2011 NEST: A Comprehensive Model for Scintillation Yield in Liquid Xenon. JINST 6, P10002. (10.1088/1748-0221/6/10/P10002)
  * [32] Aalbers J et al.. 2023 A next-generation liquid xenon observatory for dark matter and neutrino physics. J. Phys. G 50, 013001. (10.1088/1361-6471/ac841a)
  * [33] Akerib DS et al.. 2020 The LUX-ZEPLIN (LZ) Experiment. Nucl. Instrum. Meth. A 953, 163047. (10.1016/j.nima.2019.163047)
  * [34] Meng Y et al.. 2021 Dark Matter Search Results from the PandaX-4T Commissioning Run. Phys. Rev. Lett. 127, 261802. (10.1103/PhysRevLett.127.261802)
  * [35] Aprile E et al.. 2020 Projected WIMP sensitivity of the XENONnT dark matter experiment. JCAP 11, 031. (10.1088/1475-7516/2020/11/031)
  * [36] Aalbers J et al.. 2023 First Dark Matter Search Results from the LUX-ZEPLIN (LZ) Experiment. Phys. Rev. Lett. 131, 041002. (https://doi.org/10.1103/PhysRevLett.131.041002)
  * [37] Aprile E et al.. 2023 First Dark Matter Search with Nuclear Recoils from the XENONnT Experiment. Phys. Rev. Lett. 131, 041003. (10.1103/PhysRevLett.131.041003)
  * [38] Baudis L. 2012 DARWIN: dark matter WIMP search with noble liquids. J. Phys. Conf. Ser. 375, 012028. (10.1088/1742-6596/375/1/012028)
  * [39] Aalbers J et al.. 2016 DARWIN: towards the ultimate dark matter detector. JCAP 1611, 017. (10.1088/1475-7516/2016/11/017)
  * [40] Consortium X. 2022 XLZD Dark Matter Detection Consortium. https://xlzd.org. Accessed: 2023-06-23.
  * [41] Wang X, Lei Z, Ju Y, Liu J, Zhou N, Chen Y, Wang Z, Cui X, Meng Y, Zhao L. 2023 Design, construction and commissioning of the PandaX-30T liquid xenon management system. JINST 18, P05028. (10.1088/1748-0221/18/05/P05028)
  * [42] Aprile E et al.. 2022 Application and modeling of an online distillation method to reduce krypton and argon in XENON1T. PTEP 2022, 053H01. (10.1093/ptep/ptac074)
  * [43] Schumann M et al.. 2015 Dark matter sensitivity of multi-ton liquid xenon detectors. JCAP 1510, 016. (10.1088/1475-7516/2015/10/016)
  * [44] Plante G, Aprile E, Howlett J, Zhang Y. 2022 Liquid-phase purification for multi-tonne xenon detectors. Eur. Phys. J. C 82, 860. (10.1140/epjc/s10052-022-10832-w)
  * [45] Stifter K. 2020 Development and performance of high voltage electrodes for the LZ experiment. J. Phys. Conf. Ser. 1468, 012016. (10.1088/1742-6596/1468/1/012016)
  * [46] Baudis L, Biondi Y, Galloway M, Girard F, Manfredini A, McFadden N, Peres R, Sanchez-Lucas P, Thieme K. 2021 Design and construction of Xenoscope — a full-scale vertical demonstrator for the DARWIN observatory. JINST 16, P08052. (10.1088/1748-0221/16/08/P08052)
  * [47] Julia Mueller UoF. 2023 Talk at XeSAT 2023. https://indico.in2p3.fr/event/28661/contributions/125533/attachments/78407/114331/Pancake_Mueller_Xesat2023.pdf. Accessed: 2023-07-28.
  * [48] Ning X et al.. 2023 Limits on the luminance of dark matter from xenon recoil data. Nature 618, 47–50. (10.1038/s41586-023-05982-0)
  * [49] O’Hare CAJ. 2021 New Definition of the Neutrino Floor for Direct Dark Matter Searches. Phys. Rev. Lett. 127, 251802. (10.1103/PhysRevLett.127.251802)
  * [50] Workman RL et al.. 2022 Review of Particle Physics. PTEP 2022, 083C01. (10.1093/ptep/ptac097)
  * [51] Aprile E et al.. 2020 Excess electronic recoil events in XENON1T. Phys. Rev. D 102, 072004. (10.1103/PhysRevD.102.072004)
  * [52] Aprile E et al.. 2022 Search for New Physics in Electronic Recoil Data from XENONnT. Phys. Rev. Lett. 129, 161805. (10.1103/PhysRevLett.129.161805)
  * [53] Akerib DS et al.. 2021 Projected sensitivities of the LUX-ZEPLIN experiment to new physics via low-energy electron recoils. Phys. Rev. D 104, 092009. (10.1103/PhysRevD.104.092009)
  * [54] Li S et al.. 2023 Search for Light Dark Matter with Ionization Signals in the PandaX-4T Experiment. Phys. Rev. Lett. 130, 261001. (10.1103/PhysRevLett.130.261001)
  * [55] Bossio E, Agostini M. 2023 Probing Beyond the Standard Model Physics with Double-beta Decays. .
  * [56] Wittweg C, Lenardo B, Fieguth A, Weinheimer C. 2020 Detection prospects for the second-order weak decays of 124Xe in multi-tonne xenon time projection chambers. Eur. Phys. J. C 80, 1161. (10.1140/epjc/s10052-020-08726-w)
  * [57] Si L et al.. 2022 Determination of Double Beta Decay Half-Life of 136Xe with the PandaX-4T Natural Xenon Detector. Research 2022, 9798721. (10.34133/2022/9798721)
  * [58] Akerib DS et al.. 2020 Projected sensitivity of the LUX-ZEPLIN experiment to the $0\nu\beta\beta$ decay of ${}^{136}Xe$. Phys. Rev. C 102, 014602\. (10.1103/PhysRevC.102.014602)
  * [59] Akerib DS et al.. 2021 Projected sensitivity of the LUX-ZEPLIN experiment to the two-neutrino and neutrinoless double $\beta$ decays of 134Xe. Phys. Rev. C 104, 065501. (10.1103/PhysRevC.104.065501)
  * [60] Abe S et al.. 2023 Search for the Majorana Nature of Neutrinos in the Inverted Mass Ordering Region with KamLAND-Zen. Phys. Rev. Lett. 130, 051801. (10.1103/PhysRevLett.130.051801)
  * [61] Adhikari G et al.. 2022 nEXO: neutrinoless double beta decay search beyond 1028 year half-life sensitivity. J. Phys. G 49, 015104. (10.1088/1361-6471/ac3631)
  * [62] Agostini F et al.. 2020 Sensitivity of the DARWIN observatory to the neutrinoless double beta decay of 136Xe. Eur. Phys. J. C 80, 808. (10.1140/epjc/s10052-020-8196-z)
  * [63] Aalbers J et al.. 2020 Solar neutrino detection sensitivity in DARWIN via electron scattering. Eur. Phys. J. C 80, 1133. (10.1140/epjc/s10052-020-08602-7)
  * [64] Freedman DZ. 1974 Coherent Neutrino Nucleus Scattering as a Probe of the Weak Neutral Current. Phys. Rev. D 9, 1389–1392. (10.1103/PhysRevD.9.1389)
  * [65] Hasert FJ et al.. 1973 Observation of Neutrino Like Interactions Without Muon Or Electron in the Gargamelle Neutrino Experiment. Phys. Lett. B 46, 138–140. (10.1016/0370-2693(73)90499-1)
  * [66] Akimov D et al.. 2017 Observation of Coherent Elastic Neutrino-Nucleus Scattering. Science 357, 1123–1126. (10.1126/science.aao0990)
  * [67] Akimov D et al.. 2021 First Measurement of Coherent Elastic Neutrino-Nucleus Scattering on Argon. Phys. Rev. Lett. 126, 012002. (10.1103/PhysRevLett.126.012002)
  * [68] Xiang X et al.. 2023 Nuclear recoil response of liquid xenon and its impact on solar 8B neutrino and dark matter searches. .
  * [69] Aprile E et al.. 2021 Search for Coherent Elastic Scattering of Solar 8B Neutrinos in the XENON1T Dark Matter Experiment. Phys. Rev. Lett. 126, 091301. (10.1103/PhysRevLett.126.091301)
  * [70] Ma W et al.. 2023 Search for Solar B8 Neutrinos in the PandaX-4T Experiment Using Neutrino-Nucleus Coherent Scattering. Phys. Rev. Lett. 130, 021802. (10.1103/PhysRevLett.130.021802)
  * [71] Baudis L et al.. 2014 Neutrino physics with multi-ton scale liquid xenon detectors. JCAP 01, 044. (10.1088/1475-7516/2014/01/044)
  * [72] Lang RF, McCabe C, Reichard S, Selvi M, Tamborra I. 2016 Supernova neutrino physics with xenon dark matter detectors: A timely perspective. Phys. Rev. D94, 103009. (10.1103/PhysRevD.94.103009)
  * [73] Mota Peres RJ. 2023 Advancing Multi-Messenger Astrophysics and Dark Matter Searches with XENONnT and the Top SiPM Array of Xenoscope. PhD thesis University of Zurich, 2023.
  * [74] Network S. 2023 SNEWS Supernova Early Warning System. https://snews.bnl.gov. Accessed: 2023-07-22.
  * [75] Suliga AM, Beacom JF, Tamborra I. 2022 Towards probing the diffuse supernova neutrino background in all flavors. Phys. Rev. D 105, 043008. (10.1103/PhysRevD.105.043008)
  * [76] Baudis L. 2012 Direct dark matter detection: the next decade. Phys. Dark Univ. 1, 94–108. (10.1016/j.dark.2012.10.006)