---
abstract: 'In this paper we review the recent results on strangeness production measured by HADES in the Ar+KCl system at a beam energy of 1.756 AGeV. A detailed comparison of the measured hadron yields with the statistical model is also discussed.'
address:
- |
    GSI Helmholtz Centre for Heavy Ion Research GmbH Planckstrasse 1,\
    D-64291 Darmstadt, GERMANY
- 'Excellence Cluster ’Universe’, Technische Universität München, Boltzmannstr. 2, D-85748 Garching Germany.'
author:
- 'J. Pietraszko [^1] L. Fabbietti (for the HADES collaboration)'
title: Strangeness Production at SIS measured with HADES
---

Hadron production in Ar+KCl collisions
======================================

The study of nuclear matter properties at high densities and temperatures is one of the main objectives in relativistic heavy-ion physics. In this context, the paramount aim of measuring the particle yields emitted from heavy-ion collisions is to learn about the K-N potential, production mechanism of strange particles or the nuclear equation of state. Strangeness production in relativistic heavy-ion collisions at SIS/Bevelac energy range has been extensively studied by various groups including experiments at the Bevelac[@bevelac_1] and at SIS, KaoS[@kaos_1], FOPI[@fopi_1], and in recent years also HADES.\
Recently, the HADES[@hades_nim] collaboration measured charged particle production in the Ar+KCl system[@hades_kaons] at 1.756 AGeV. Although HADES was designed primarily for di-electron measurements, it has also shown an excellent capability for the identification of a wide range of hadrons like $K^-$, $K^+$, K$^0$$\rightarrow$$\pi^+\pi^-$, $\Lambda$$\rightarrow$p$\pi^-$, $\phi$$\rightarrow$$K$$^+$$K$$^-$ and even $\Xi$$\rightarrow$$\Lambda$$\pi^-$. The measured yields and slopes of the transverse mass of kaons and $\Lambda$ particles have been found to be in good agreement with the results obtained by KaoS[@kaos_0] and FOPI[@fopi_0]. The dependencies of the measured $K^-/K^+$ ratio on the centrality and on the collision energy follow the systematics measured by KaoS [@hades_kaons] too.\
A combined and inclusive identification of $K^+K^-$ and $\phi$ mesons was performed for the first time in the same experimental setup at subthreshold beam energy for $K^-$ and $\phi$ production. The obtained $\phi/K^-$ ratio of 0.37$\pm$0.13$\%$ indicates that 18$\pm$7$\%$ of kaons stem from $\phi$ decays. Since the $\phi$ mesons reconstructed via $K^+K^-$ channel are those coming mainly from decays happening outside the nuclear medium, this value should be considered as a lower limit. In addition the non-resonant $K^+K^-$ production can contribute to the measured $K^-$ yield. Unfortunately, this part is not known in heavy-ion collisions, but it has been measured to be about 50$\%$ of the overall $K^+K^-$ yield in elementary p+p collisions[@anke_1]. In this view, the $K^-$ production in heavy-ion collisions at SIS energies can not be explained exclusively by the strangeness exchange mechanism and the processes mentioned above must also be taken into account to achieve a complete description.

Comparison to statistical models
================================

The yields of reconstructed hadron species have been extrapolated to the full solid angle and compared to the result from a fit with the statistical model THERMUS[@thermus_1] as shown in Fig. \[fig:thermus\]. The measured yields nicely agree with the results of the model, except for the $\Xi^-$. One should note that in this approach a good description of the $\phi$ meson yield is obtained without assuming any strangeness suppression (net strangeness content of the $\phi$ is S=0). This is very different as compared to higher energies where $\phi$ meson does not behave as a strangeness neutral object but rather as an object with net strangeness between 1 and 2 [@strgns_phi].\

(200,180)(0,0) (-32,-2.6)[ ![Chemical freeze-out parameters obtained in the statistical thermal model (for details see [@cleymans_1]). The HADES point corresponds to Ar+KCl collisions at 1.756 AGeV.[]{data-label="fig:statmodel"}](thermus_fits1.10_publ.eps "fig:"){width="110.00000%" height=".915\textwidth"}]{}

(200,180)(0,0) (0,-15)[ ![Chemical freeze-out parameters obtained in the statistical thermal model (for details see [@cleymans_1]). The HADES point corresponds to Ar+KCl collisions at 1.756 AGeV.[]{data-label="fig:statmodel"}](eovern_2009_to_be_publ.eps "fig:"){width=".97\textwidth" height=".97\textwidth"}]{}

The $\Xi^-$ baryon yields measured in heavy-ion collisions above the production threshold at RHIC[@rhic_1], SPS[@sps_1] and AGS[@ags_1] nicely agree with the statistical model predictions. On the contrary the result of the first $\Xi^-$ measurement below the production threshold published by HADES[@hades_ksi], shows a deviation of about an order of magnitude from the calculations (Fig. \[fig:thermus\]). Using the measured hadron multiplicities the statistical model predicts that the chemical freeze-out of the Ar+KCl collision at 1.765 AGeV occurs at a temperature of T=73$\pm$6MeV and at chemical baryon potential of $\mu$=770$\pm$43MeV. The strangeness correlation radius[@hades_kaons] of R$_c=$2.4$\pm$0.8fm was used which is significantly smaller than the radius of fireball R$_{fireball}$=4.9$\pm$1.4fm.\
This result nicely follows the striking regularity shown by particle yields at all beam energies [@cleymans_1], as presented in Fig. \[fig:statmodel\]. For all available energies, starting from the highest at RHIC down to the lowest at SIS, the measured particle multiplicities are consistent with the assumption of chemical equilibrium which sets in at the end of the collision phase. Only two parameters (the temperature and baryon chemical potential) are needed within a thermal-statistical model to describe particle yields in a very systematic way at a given collision energy [@cleymans_1]. As one can also see, all experimental results are in good agreement with a fixed-energy-per-particle condition $<E>$$/$$<N>$$\approx1GeV$, which is one of the available freeze-out criteria[@cleymans_1].

The new HADES results on strangeness production shed new light on the understanding of kaon production mechanisms in HI collisions, namely the results have provided compelling evidence that the contribution from the $\phi$ decay to the $K^-$ yield has to be also taken into account. The measured hadron yields have been found in general to be in good agreement with statistical model predictions, besides the $\Xi^-$, which is produced far below the production threshold and shows a considerable deviation from the statistical model. The already performed experiments p+p at 3.5 GeV and p+Nb at 3.5 GeV and further planned HADES experiments with heavier systems, like Au+Au will deliver new valuable data on strangeness production. The on-going upgrade of the HADES spectrometer will increase its performance and capability and the installed Forward Wall detector will allow for reaction plane reconstruction in all upcoming runs, allowing to study kaon flow observables as well.

[9]{} S. Schnetzer et al. Phys. Rev. Lett. 49 (1982) 989; Phys. Rev. C, 40 (1989) 640 C. Sturm et al. (KAOS) Phys. Rev. Lett. 86 (2001) 39 J. L. Ritman et al. (FOPI), Z. Phys. A 352 (1995) 355 G. Agakichiev et al. (HADES), Eur. Phys. J. A 41 (2009) 243 G.Agakichev et al., (HADES), in press Phys. Rev. C and arXiv:0902.3487 A. Förster et al. (KAOS) Phys. Rev. C 75 (2007) 024906. M. Merschmeyer et al. (FOPI) Phys. Rev. C 76 (2007) 024906 Y.Maeda et al. (ANKE), Phys. Rev. C 77 (2008) 015204 S. Wheaton and J. Cleymans, hep-ph/0407174\
S. Wheaton and J. Cleymans, J. Phys. G31 (2005) S1069 I. Kraus et al., Phys. Rev. C76, 064903 (2007) J. Adams et al. (STAR), Phys. Rev. Lett. bold[98]{}, 062301 (2007) F. Antinori et al. (NA57) Phys. Lett. B 595, 68 (2004)\
C. Alt et al. (NA49), Phys. Rev. C 78, 034918 (2008) P. Chung et al. (E895), Phys. Rev. Lett. 91, 202301 (2003) G. Agakishiev et al. (HADES) Phys. Rev. Lett. 1003 (2009) 132301 J. Cleymans at al. Phys. Rev. C 73, 034905 (2006) and J. Cleymans private communication

[^1]: e-mail: j.pietraszko@gsi.de
