Results of the production of Xi and Xi-bar hyperons in central Pb+Pb interactions at 158 GeV/c per nucleon are presented. This analysis utilises a global reconstruction procedure, which allows a measurement of 4pi integrated yields to be made for the first time. Inverse slope paramters, which are determined from an exponential fit to the transverse mass spectra, are shown. Central rapidity densities are found to be 1.49 +- 0.08 and 0.33 +- 0.04 per event per unit of rapidity for Xi and Xi-bar respectively. Yields integrated to full phase space are 4.12 +- 0.02 and 0.77 +- 0.04 for Xi and Xi-bar. The ratio of Xi-bar/Xi at mid-rapidity is 0.22 +- 0.03

Afanasiev, S. V.

Collaboration, NA49

English

Afanasiev, Sergey V.

Hochschulschriftenserver - Universität Frankfurt am Main

arXiv:hep-ex/0202037 v1   21 Feb 2002Ξ− and Ξ+ Production in Central Pb+PbCollisions at 158 GeV/c per NucleonS.V. Afanasiev9,T. Anticic20, D. Barna5, J. Bartke7, R.A. Barton3, L. Betev10,H. Bia  lkowska17, A. Billmeier10, C. Blume8, C.O. Blyth3, B. Boimska17,M. Botje1, J. Bracinik4, R. Bramm10, R. Brun11, P. Bunˇ ci´ c10,11, V. Cerny4,J.G. Cramer19, P. Csat´ o5, P. Dinkelaker10, V. Eckardt16, P. Filip16,H.G. Fischer11, Z. Fodor5, P. Foka8, P. Freund16, V. Friese15, J. G´ al5,M. Ga´ zdzicki10, G. Georgopoulos2, E. G  ladysz7, S. Hegyi5, C. H¨ ohne15, G. Igo14,P.G. Jones3, K. Kadija11,20, A. Karev16, V.I. Kolesnikov9, T. Kollegger10,M. Kowalski7, I. Kraus8, M. Kreps4, M. van Leeuwen1, P. L´ evai5,A.I. Malakhov9, S. Margetis13, C. Markert8, B.W. Mayes12, G.L. Melkumov9,A. Mischke8,J. Moln´ ar5, J.M. Nelson3, G. P´ alla5, A.D. Panagiotou2, K. Perl18,A. Petridis2, M. Pikna4, L. Pinsky12, F. P¨ uhlhofer15, J.G. Reid19, R. Renfordt10,W. Retyk18, C. Roland6, G. Roland6, A. Rybicki7, T. Sammer16, A. Sandoval8,H. Sann8, N. Schmitz16, P. Seyboth16, F. Sikl´ er5, B. Sitar4, E. Skrzypczak18,G.T.A. Squier3, R. Stock10, H. Str¨ obele10, T. Susa20, I. Szentp´ etery5, J. Sziklai5,T.A. Trainor19, D. Varga5, M. Vassiliou2, G.I. Veres5, G. Vesztergombi5,D. Vrani´ c8, S. Wenig11, A. Wetzler10, C. Whitten14, I.K. Yoo15, J. Zaranek10,J. Zim´ anyi51NIKHEF, Amsterdam, Netherlands.2Department of Physics, University of Athens, Athens, Greece.3Birmingham University, Birmingham, England.4Comenius University, Bratislava, Slovakia.5KFKI Research Institute for Particle and Nuclear Physics, Budapest, Hungary.6MIT, Cambridge, USA.7Institute of Nuclear Physics, Cracow, Poland.8Gesellschaft f¨ ur Schwerionenforschung (GSI), Darmstadt, Germany.9Joint Institute for Nuclear Research, Dubna, Russia.10Fachbereich Physik der Universit¨ at, Frankfurt, Germany.11CERN, Geneva, Switzerland.12University of Houston, Houston, TX, USA.13Kent State University, Kent, OH, USA.14University of California at Los Angeles, Los Angeles, USA.15Fachbereich Physik der Universit¨ at, Marburg, Germany.16Max Planck Institut f¨ ur Physik, Munich, Germany.17Institute for Nuclear Studies, Warsaw, Poland.18Institute for Experimental Physics, University of Warsaw, Warsaw, Poland.19Nuclear Physics Laboratory, University of Washington, Seattle, WA, USA.20Rudjer Boskovic Institute, Zagreb, Croatia.Preprint submitted to Elsevier Science 7 September 2005AbstractResults of the production of Ξ− and Ξ+ hyperons in central Pb+Pb interactions at158 GeV/c per nucleon are presented. This analysis utilises a global reconstructionprocedure, which allows a measurement of 4π integrated yields to be made for theﬁrst time. Inverse slope parameters, which are determined from an exponential ﬁtto the transverse mass spectra, are found to be 267 ± 9 MeV and 296 ± 17 MeVfor Ξ− and Ξ+ hyperons respectively. Central rapidity densities (dN/dy) are foundto be 1.49 ± 0.08 and 0.33 ± 0.04 particles per event per unit rapidity for Ξ− andΞ+ respectively. The ratio R(Ξ+/Ξ−) at midrapidity, R(y = ycm), is found to be0.22 ± 0.03. Yields integrated to full phase space are found to be 4.12 ± 0.20 and0.77 ± 0.04 particles per event for Ξ− and Ξ+ hyperons respectively.Key words: cascade, anticascade, strangeness enhancement, multi–strangehyperons, integrated yields, heavy–ion collisions, NA49PACS:21 IntroductionHigh energy heavy–ion physics provides a unique opportunity to study nu-clear matter under extreme conditions of temperature and pressure. QuantumChromodynamics (QCD) predicts that at high energy density, nuclear matterwill melt into a deconﬁned state of partonic matter known as the Quark–GluonPlasma (QGP). Experimental eﬀorts to detect this novel phase rely on mea-surements of ﬁnal state particles emitted from the ﬁreball created by collidingnuclear matter. There are three key stages in the collision process in which aQGP is formed: the initial partonic interactions which occur as two nuclei in-terpenetrate, the subsequent partonic interactions in a thermalised QGP andﬁnally hadronic interactions once the temperature of the system has droppedbelow the critical temperature. In order to probe the QGP, signals must besought which are produced dominantly in the equilibrated deconﬁned stateand survive the hadronisation process. (For a general overview, we refer to theproceedings of the Quark Matter ’99 conference [1].)For almost 20 years, the production of strangeness has been proposed as a sen-sitive signal to QGP formation [2]. (For a recent overview see the proceedingsof the Strangeness 2000 conference [3].) Strange particles are of particularinterest in hadronic collisions since they carry a new quantum number notpresent in the colliding nucleons or nuclei. In a QGP, ss pairs are readilycreated because of the newly available gluonic degrees of freedom which con-tribute approximately 80% to the total strangeness produced [4]. Strangenessin a partonic system where chiral symmetry is partially restored, is also ex-pected to be readily produced due to the lower energy threshold for ss quarkpair production. At ﬁnite baryon density, where the chemical potential asso-ciated with the production of further light quarks is raised, the productionof strangeness may be further increased. These considerations lead to theexpectation that the QGP phase should be characterised by a high strange(anti)quark density [5]. By virtue of their strange quark content, multi–strangebaryons and anti–baryons are expected to be a more sensitive probe to thisphase than hadrons which contain only one strange valence quark. The pro-duction of multi–strange baryons, and particularly multi–strange anti–baryonsis otherwise suppressed by high hadronic energy thresholds as well as by longtimescales for multi–step processes. The NA49 [6] and WA97 [7] collaborationshave previously reported an enhancement of multi–strange hyperons from cen-tral Pb+Pb collisions at 158 GeV/c per nucleon at midrapidity compared toa convolution of nucleon-nucleon collisions. Total yields of strange hadronsprovide additional information, since they are an essential ingredient in ther-mal models [8]. Remarkably, these models have described a wide range ofexperimental data with much success [9–11]. Notable exceptions are in themulti–strange sector [12]. This paper reports on the rapidity distributions ofΞ− and Ξ+ measured by the NA49 collaboration over a wider range than has3previously been published.2 The NA49 Experiment and Global AnalysisThe NA49 large acceptance hadron spectrometer has been used to recordthe charged ﬁnal state particles produced in central Pb+Pb collisions at 158GeV per nucleon at the CERN SPS. The main tracking devices are four largevolume time projection chambers (TPC) which are capable of detecting 80%of some 1500 charged particles created in a central Pb+Pb collision. TwoTPCs, VT1 and VT2, are located in strong magnetic ﬁelds of 1.5 and 1.1T respectively, which allows charge and momentum to be assigned to thedetected particles. A further two TPCs are situated downstream from themagnets in a ﬁeld free region. The results presented here were analysed with aglobal tracking scheme [13], which combines track segments that belong to thesame physical particle but are detected in diﬀerent TPCs. Generally, globaltracks have better momentum resolution. Utilising a global tracking schemeincreases detector acceptance and improves the track reconstruction eﬃciencyin the TPCs, because tracks measured in the lower density regions far fromthe interaction vertex can be used to identify track segments in high densityregions. Full details of the NA49 detector set–up and performance of trackingare available in [14].The results contained in this paper were obtained from the analysis of 400,000central Pb+Pb events. The events were selected according to the energy de-posited in a forward calorimeter and correspond to the 10% most centralcollisions with an average number of participants Npart ≈ 335.3 Ξ− and Ξ+ from Central Pb+Pb CollisionsThe doubly–strange Ξ− hyperon decays via the channel Ξ− → Λ + π− (100%branching ratio) with the subsequent decay Λ → p+π− (63.9% branching ra-tio). Neutral particle decay candidates were formed by considering all possiblepairs of positively and negatively charged tracks reconstructed in the TPCs.Each pair of tracks was extrapolated from their most upstream point backtowards the target. If the distance of closest approach between the two trackswas less than 1 cm at any point along their trajectory the pair was saved asa candidate Λ (Λ). A large fraction of the combinatorial background arisingfrom random pairs of tracks was eliminated by requiring that the tracks wereseparated by at least 2 cm at the target plane and that the decay vertex was atleast 100 cm downstream of the target. If the invariant mass of the candidatesfell within ±15 MeV/c2 of the Λ (Λ) mass, they were selected as candidate4daughters of Ξ−(Ξ+) decays. Each Λ candidate was paired with each nega-tively charged track in the event, and both were extrapolated back towardthe target. As in the case of reconstructing the neutral decay vertices, if thedistance of closest approach between trajectories of the neutral Λ candidateand the negatively charged track fell within 1 cm, the pair was saved as apossible Ξ− candidate. Equivalently, each Λ candidate was paired with eachpositive track in the event. Once again, straight-forward geometrical cuts wereapplied to eliminate most of the combinatorial background. In this case, themost important cuts were that the trajectories of the charged decay daugh-ters were separated by between 2.5 cm and 7 cm from the primary vertex inthe target plane depending on the position of the decay vertex. An additionalrequirement was that the decay vertex was at least 35 cm downstream of thetarget and that the Ξ−(Ξ+) candidate pointed back to the primary vertex.The resulting invariant mass distributions of Ξ− and Ξ+ candidates are shownin ﬁgure 1.Fig. 1. Invariant mass distributions for Ξ− (left) and Ξ+ (right) from 400000 centralPb+Pb collisions. In the bin centred on the Ξ− mass, the signal–to–noise ratio isbetter than 10:1.The signal was obtained by selecting those particles with invariant mass within±20 MeV/c2 of the Ξ− mass and then subtracting background counts fromregions outside the peak. Speciﬁcally, the background regions were deﬁnedbetween the ranges 1267 < M < 1284 MeV/c2 and 1358 < M < 1381 MeV/c2.The background subtracted data set used consists of approximately 4800 Ξ−and 900 Ξ+ reconstructed particles [15].Those Ξ− and Ξ+ which were excluded by the geometric selection criteria, ornot found due to reconstruction losses, were later accounted for by applyinga correction for the overall reconstruction ineﬃciency.53.1 Transverse Mass DistributionsThe data were evaluated in bins of rapidity, transverse momentum and life-time. Corrections for geometrical acceptance, branching ratio and reconstruc-tion eﬃciency were then applied on a bin–by–bin basis. The GEANT simu-lation package has been adapted for NA49 and was used for calculating thegeometrical acceptance. For the determination of the reconstruction eﬃciency,a single Monte Carlo Ξ− (or Ξ+) generated within the spectrometer acceptancewas embedded into the raw data structures in which no real Ξ− (or Ξ+) wasdetected. These events were processed with the reconstruction software whichtried to reconstruct the embedded particle. The reconstruction eﬃciency ofthe embedded Ξ− (or Ξ+) was determined statistically for each bin of rapid-ity, transverse momentum and lifetime. The kinematic phase space populatedby detectable Ξ− and Ξ+ hyperons was found to be 0.6 < pT < 2.8 GeV/c intransverse momentum and 1.7 < y < 4.5 in rapidity. Note that the rapidityrange covers both sides of midrapidity (ycm = 2.9). Fully corrected transversemass (mT =√pT2 + m02) distributions are shown in ﬁgure 2 and are ﬁttedwith the function given in equation (1) to obtain the inverse slope parameters,T.1mTdNdmT∝ exp￿−mTT￿(1)Fig. 2. Transverse mass distributions for Ξ− (left) and Ξ+ (right) from centralPb+Pb collisions. Inverse slope parameters, T, are from ﬁts to equation (1).The measured transverse mass spectra span the detectable rapidity range ineach pT bin. The ﬁtted temperature parameters are found to be 267 ± 9 MeVand 296 ± 17 MeV for Ξ− and Ξ+ hyperons respectively.63.2 Rapidity Distributions, 4π Yields and Particle RatiosThe large acceptance coverage of the NA49 spectrometer allows the exper-imental determination of fully integrated particle yields. However, physicalmeasurements are not possible for all values of transverse momentum becauseof ﬁnite geometrical acceptance. Consequently, the rapidity spectra for Ξ− andΞ+ are formed by extrapolating the transverse momentum distributions overthe full range by means of eq. (1) using the ﬁtted slope parameters, and assum-ing factorisation in pT and y. The resulting distributions are shown in ﬁgure 3.The extrapolation factors are 1.51 and 1.45 for the Ξ− and Ξ+ distributions,respectively. At midrapidity, the yields per event per unit of rapidity (or cen-tral rapidity densities, dN/dy) are found to be 1.49±0.08 and 0.33±0.04 forΞ− and Ξ+ hyperons respectively.Fig. 3. Rapidity distributions for Ξ− (left), Ξ+ (centre) and the Ξ+/Ξ− ratio (right)from Pb+Pb collisions. Closed circles are measured data points and open circles arereﬂected about midrapidity (y=2.9). The extrapolation factors are 1.51 and 1.45 forthe Ξ− and Ξ+ distributions, respectively. The solid line ﬁts are discussed in thetext.Integration of the measured part of the rapidity distribution and taking intoaccount the reﬂected data, averaging where necessary, provides a lower limitfor the cascade multiplicities of 3.76±0.12 and 0.72±0.05 particles per eventfor Ξ− and Ξ+ respectively. In order to estimate the 4π yield, a ﬁt to a doublegaussian has been made in the case of the Ξ− rapidity spectrum. Each gaussianhas a width σ = 0.75 ± 0.12 centred at ycm ± 0.66. The integral of this ﬁtover the full rapidity range provides an estimated 4π yield of 4.12 ± 0.20 Ξ−per event. A single Gaussian centred at midrapidity with a width of σ =0.87 ± 0.07 is shown in ﬁgure 3 for the Ξ+ distribution. Integrating this ﬁtover the full rapidity range gives an estimated 4π yield of 0.77 ± 0.04 Ξ+per event. The Ξ+/Ξ− ratio, R, can be calculated over the full measuredrapidity range. At midrapidity, R(y=ycm) is found to be 0.22 ± 0.03 in goodagreement with our previous publication [6] and with WA97 [16]. Note that inreference [6], the event selection was based on the 5% most central collisions(10% in this paper), where Npart was approximately 362 compared to 335 inthis analysis. Additionally, the large acceptance of the NA49 spectrometer7allows a determination of the 4π integrated ratio, which is found to be R(4π)= 0.17 ± 0.01.3.3 Lifetime DistributionsIn addition to dividing the data into y and pT bins, the data were also evaluatedin discrete bins of lifetime, τ, allowing lifetime distributions to be measured.Corrections for geometrical acceptance and reconstruction eﬃciency were ap-plied in each of the lifetime bins and the resulting corrected distributions areshown in ﬁgure 4. The distributions were ﬁtted with exponentials to deter-mine the mean lifetimes, which were found to be 5.2±0.2 cm/c and 5.0±0.3cm/c for Ξ− and Ξ+ hyperons respectively. These values are consistent withthe data book mean lifetime value of τ0 = 4.91 cm/c [17].Fig. 4. Lifetime distributions for Ξ− (left) and Ξ+ (right) hyperons corrected foracceptance and eﬃciency.3.4 Discussion and ConclusionsThis new analysis uses a completely revised reconstruction procedure andcovers a wide range in rapidity. The resulting total integrated yields of Ξ−are 30% lower than previous NA49 estimates [6]. This is due in part to thediﬀerent centrality selection. The current measurement beneﬁts from a smallererror in extrapolation since we have been able to measure for the ﬁrst time, therapidity distributions of both Ξ− and Ξ+ over a wider range than previously.The central rapidity densities are in agreement with those found by WA97[18]. Errors on the new results are statistical only, with the systematic errorsestimated to be about 15% derived from diﬀerences between simulation anddata.In summary, Ξ− and Ξ+ hyperons have been measured at 158 GeV/c per8nucleon using the NA49 large acceptance spectrometer. For the Ξ− hyperons,the slope parameter determined from a ﬁt to the transverse mass spectrum is267 ± 9 MeV, the central rapidity density is found to be 1.49 ± 0.08 and thetotal yield is found to be 4.51 ± 0.20 particles per event integrated over allphase space. The Ξ+ hyperons are found to have a slope parameter of 296 ±17 MeV, with a central rapidity density of 0.33 ± 0.04 and a fully integratedyield of 0.77 ± 0.04 particles per event. At midrapidity, the Ξ+/Ξ− ratio isfound to be R(y=ycm) = 0.22 ± 0.03. A full phase space integrated Ξ+/Ξ−ratio of R(4π) = 0.19 ± 0.01 is obtained.3.5 AcknowledgementsThis work was supported by the Director, Oﬃce of Energy Research, Divisionof Nuclear Physics of the Oﬃce of High Energy and Nuclear Physics of the USDepartment of Energy (DE-ACO3-76SFOOO98 and DE-FG02-91ER40609),the US National Science Foundation, the Bundesministerium fur Bildung undForschung, Germany, the Alexander von Humboldt Foundation, the UK Engi-neering and Physical Sciences Research Council, the Polish State Committeefor Scientiﬁc Research (2 P03B 13820, 02615, 01716, and 09916), the Hungar-ian Scientiﬁc Research Foundation (T14920 and T23790), the EC Marie CurieFoundation, and the Polish-German Foundation.91References[1] Proc. of Quark Matter ’99 Conference 1999 Nucl. Phys. A 661[2] Rafelski J 1982 Phys. Rep. 88 331[3] Proc. of Strangeness 2000 Conference 2001 J. Phys. G 27 Vol 3[4] Koch P, M¨ uller B and Rafelski J 1986 Phys. Rep. 142 168[5] Rafelski J 1991 Phys. Lett. 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Xi- and Xi+ production in central Pb+Pb collisions at 158 GeV/c per nucleon

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