Holzmann, R.

Lorenz, M.

Markert, J.

Scheib, T.

Stroth, J.

GSI Repository

Hypertriton reconstruction in Ar+KCl reactions at 1.756A GeV with HADES∗T. Scheib1, M. Lorenz1, R. Holzmann2, J. Markert2, J. Stroth1,2, and the HADES Collaboration1Goethe-Universita¨t Frankfurt am Main; 2GSI, Darmstadt, GermanyIn September 2005 the collision system Ar+KCl at a ki-netic beam energy of 1.756 GeV per nucleon was mea-sured with HADES. Among other observables, in partic-ular several hadron species carrying strangeness have beenreconstructed. Due to the conservation of the strangenessquantum number its production is a highly correlated pro-cess. The relative distribution of strange and anti-strangequarks among the hadrons in the final state, including ex-cited states and resonances, can provide important insightinto the production and propagation of quarks [1].In this contribution, we investigate the production of thehypertriton 3ΛH , the lightest of the so-called hypernuclei.In addition to the nucleons - the proton and the neutron -these nuclei contain at least one hyperon, which is in theexplicit case of the hypertriton a Λ−hyperon.Hypertritons are most easily reconstructed through theirbound decay into a negative pion and a 3He. The branch-ing ratio for this decay is about 35% [2]. Hence, an im-portant issue of the analysis is a good identification of thetwo final-state particles π− and 3He via the Time-of-Flightinformation in the TOF and TOFino walls and the energyloss in the Multiwire Drift Chambers (MDC) as well as inthe TOF and TOFino walls.In order to reduce the background stemming from uncorre-lated pairs of negative pions and helium nuclei, secondary-vertex cuts are applied. The procedure is comparable to theΛ → π−+ p analysis in Ar+KCl, but since the mass of 3ΛHis significantly higher than that of Λ, the cut on the meanflight distance has to be widened. For the optimization ofthe applied cuts, the hypertriton decay is simulated in or-der to observe their effects on the significance of the sig-nal. After generating with Pluto [3] hypertritons producedeither thermally or via coalescence processes, the detectorresponse on the analyzed decay channel is simulated withHGeant [4]. The simulated signal is embedded into realdata to get a realistic estimate of the uncorrelated back-ground. Then, the distributions of the topology variablesfrom simulations are compared to those of data.Additionally, the acceptance and reconstruction efficiencyof the hypertriton are determined via simulations to acc ·effrec = (0, 1031 ± 0, 0054)% for those produced by co-alescence and acc · effrec = (0, 14 ± 0, 013)% for thosecoming from thermal sources.After subtracting the background, which is obtained via themixed-event-method and delivers an invariant mass spec-trum of uncorrelatedπ−-3He pairs, from the invariant massspectrum of the same event, there is no significant hyper-triton signal visible.However, an upper production limit can still be calculated.∗Supported by BMBF (06 FY 9100 I), HIC for FAIR, EMMI and GSIThis can either be done by using the Feldman-Cousinsmethod [5], which only takes underlying statistics of theinvariant mass spectrum into account. Another way is toembed simulated hypertritons into the measured data atsuch a ratio, that a reconstruction becomes just possible.The multiplicities are calculated for the different methodsto MultFC = (7, 493± 3, 371± 0, 465) ·10−4 respectivelyMultembed = (2, 96± 0, 93± 0, 67) · 10−4. The obtainedvalues of the different methods are within errors in agree-ment with each other. Additionally, they agree well withresults from former reconstructions on rare, strange parti-cles in Ar+KCl collisions measured with HADES, namelythe Ξ−, which was observed significantly in this colli-sion system [6]. Considering the upper production limitobtained via the second method, the production ratio ofhypertritons to the successfully reconstructed Λ-hyperonscould be calculated, which provides the limiting value ofN3ΛH/NΛ(embedded) = (7, 24± 3, 94) · 10−3.References[1] G. Agakishiev et al., ”Hyperon Production in ArKCl colli-sions at 1.76A GeV”, Eur. Phys. J. A 47 (2011) 21.[2] H. Kamada, J. Golak, K. Miyagawa, H. Witala, W.Gloeckle, ”Pi-mesonic decay of the hypertriton”, arXiv:nucl-th/9709035v1[3] PLUTO, Pluto++, A Monte Carlo Simu-lation Tool for Hadronic Physics, 2004.http://wwwhades.gsi.de/computing/pluto/html/PlutoIndex.html.[4] GEANT. Detector Description and Simulation Tool, 2004.http://cont.cern.ch/writeup/geant/, Online User Guide.[5] G. J. Feldman and R.D. Cousins, Unified approach to theclassical statistical analysis of small signals. Phys. Rev. D57,3873, 1998.[6] G. Agakishiev et al. (HADES Collaboration), Phys. Rev. Lett.103, 132301, 2009.GSI SCIENTIFIC REPORT 2012 PHN-NQM-EXP-0735

Hypertriton reconstruction in Ar+KCl reactions at 1.756A GeV with HADES

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Hypertriton reconstruction in Ar+KCl reactions at 1.756A GeV with HADES

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