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Production of multi-strange hyperons and strange resonances in the NA49 experiment

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2001 J. Phys. G: Nucl. Part. Phys. 27 367

(http://iopscience.iop.org/0954-3899/27/3/314)

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INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS G: NUCLEAR AND PARTICLE PHYSICS

J. Phys. G: Nucl. Part. Phys. 27 (2001) 367–374 www.iop.org/Journals/jg PII: S0954-3899(01)19434-X

Production of multi-strange hyperons and strangeresonances in the NA49 experiment

R A Barton for the NA49 Collaboration:S V Afanasiev1, T Anticic2, J Bachler3,4, D Barna5, L S Barnby6,J Bartke7, R A Barton6, L Betev8, H Białkowska9, A Billmeier10,C Blume4, C O Blyth6, B Boimska9, M Botje11, J Bracinik12, F P Brady13,R Bramm10, R Brun3, P Buncic3,10, L Carr14, D Cebra13, G E Cooper15,J G Cramer14, P Csato5, V Eckardt16, F Eckhardt17, D Ferenc13,P Filip16, H G Fischer3, Z Fodor5, P Foka10, P Freund16, V Friese17,J Ftacnik12, J Gal5, M Gazdzicki10, G Georgopoulos18, E Gładysz7,S Hegyi5, V Hlinka12, C Hohne17, G Igo8, M Ivanov12, P Jacobs15,R Janik12, P G Jones6, K Kadija2,16, V I Kolesnikov1, T Kollegger10,M Kowalski7, M van Leeuwen11, P Levai5, A I Malakhov1, S Margetis19,C Markert4, B W Mayes20, G L Melkumov1, A Mischke4, J Molnar5,J M Nelson6, G Odyniec15, G Palla5, A D Panagiotou18, A Petridis18,M Pikna12, L Pinsky20, A M Poskanzer15, D J Prindle14, F Puhlhofer17,J G Reid14, R Renfordt10, W Retyk21, H G Ritter15, D Rohrich10,22,C Roland4, G Roland10, A Rybicki7, T Sammer16, A Sandoval4, H Sann4,E Schafer16, N Schmitz16, P Seyboth16, F Sikler3,5, B Sitar12,E Skrzypczak21, R Snellings15, G T A Squier6, R Stock10, P Strmen12,H Strobele10, T Susa2, I Szarka12, I Szentpetery5, J Sziklai5, M Toy8,15,T A Trainor14, S Trentalange8, D Varga5, M Vassiliou18, G I Veres5,G Vesztergombi5, S Voloshin15, D Vranic3, F Wang15,D D Weerasundara14, S Wenig3, A Wetzler10, C Whitten8, N Xu15,T A Yates6, I K Yoo17 and J Zimanyi5

1 Joint Institute for Nuclear Research, Dubna, Russia2 Rudjer Boskovic Institute, Zagreb, Croatia3 CERN, Geneva, Switzerland4 Gesellschaft fur Schwerionenforschung (GSI), Darmstadt, Germany5 KFKI Research Institute for Particle and Nuclear Physics, Budapest, Hungary6 Department of Physics and Astronomy, The University of Birmingham, Edgbaston,Birmingham B15 2TT, UK7 Institute of Nuclear Physics, Cracow, Poland8 University of California at Los Angeles, Los Angeles, CA, USA9 Institute for Nuclear Studies, Warsaw, Poland10 Fachbereich Physik der Universitat, Frankfurt, Germany11 NIKHEF, Amsterdam, Netherlands12 Comenius University, Bratislava, Slovakia13 University of California at Davis, Davis, CA, USA14 Nuclear Physics Laboratory, University of Washington, Seattle, WA, USA15 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA, USA16 Max-Planck-Institut fur Physik, Munich, Germany17 Fachbereich Physik der Universitat, Marburg, Germany18 Department of Physics, University of Athens, Athens, Greece19 Kent State University, Kent, OH, USA20 University of Houston, Houston, TX, USA

0954-3899/01/030367+08$30.00 © 2001 IOP Publishing Ltd Printed in the UK 367

368 R A Barton for the NA49 Collaboration

21 Institute for Experimental Physics, University of Warsaw, Warsaw, Poland22 Present address: University of Bergen, Norway

E-mail: R.A.Barton@bham.ac.uk

Received 2 November 2000

AbstractThe NA49 large-acceptance hadron spectrometer has measured strange andmulti-strange hadrons from Pb + Pb and p + p collisions at the CERN SPS.Preliminary results for the transverse mass and rapidity distributions for �−

and �+ from central Pb + Pb collisions at 158 GeV c−1/nucleon are presented.Fully integrated yields per event of 4.42 ± 0.31 and 0.74 ± 0.04 are found for�− and �+, respectively, leading to a 4π �+/�− ratio of 0.17±0.02. The ratio�+/�− at mid-rapidity is found to be 0.22 ± 0.04, agreeing with previouslypublished values. In addition, preliminary data on the �(1520) and φ(1020)resonances are presented. The �(1520) multiplicity for p + p collisions isfound to be 0.012 ± 0.003. No signal is observed for Pb + Pb collisionsand a production upper limit of 1.36 �(1520) per event indicates an apparentsuppression when comparing with scaled p + p data. Integrated φ(1020) yieldsper event are found to be 7.6 ± 1.1 for Pb + Pb and 0.012 ± 0.0015 for p +p collisions. No significant shift or broadening of the φ(1020) invariant massdistribution is observed in central Pb + Pb collisions.

(Some figures in this article are in colour only in the electronic version; see www.iop.org)

1. Introduction

Heavy-ion physics provides a unique opportunity to study nuclear matter under extremeconditions of temperature and pressure. Quantum chromodynamics (QCD) predicts that athigh energy density, nuclear matter will melt into a deconfined state of matter known as thequark–gluon plasma (QGP). One of the predicted key signatures for such a phase transitionis the enhancement of strange particles [1]. Furthermore, multi-strange (anti-)hadrons havea higher strangeness content and are expected to be more sensitive to the enhancement. TheNA49 collaboration has measured strange and multi-strange hadrons from Pb + Pb and p + pcollisions at 158 GeV c−1/nucleon.

2. The NA49 large-acceptance spectrometer

Figure 1 shows a schematic diagram of the NA49 experimental apparatus. Full details of theset-up are available in [2]. The main tracking devices are four large-volume time projectionchambers (TPC), capable of detecting 80% of some 1500 charged particles produced in acentral Pb + Pb collision. Two TPCs, VT1 and VT2, are located in strong magnetic fieldsof 1.5 and 1.1 T, respectively. Charge and momentum are assigned to all measured tracksaccording to the direction and curvature of the track in the magnetic field. Two further TPCs(MTL and MTR) are located downstream of VT2 in a field-free area. All the TPCs are capableof particle identification from measurements of specific energy loss. Particle identificationis complemented by time-of-flight walls (TOF) situated behind the TPCs. Downstreamcalorimeters complete the set-up and are used to trigger on centrality.

Multi-strange hyperons and strange resonances in NA49 369

Figure 1. Schematic diagram of the NA49 experimental apparatus. Three target configurationsare shown for (a) Pb + Pb, (b) p + p and (c) p + A collisions. The main tracking devices are fourlarge-volume time projection chambers VT1, VT2, MTL and MTR (see text for further details).

3. Multi-strange hyperon analysis

The work presented uses global tracking, which combines track segments in different TPCsbelonging to the same physical particle [3]. �− hyperons decay via the channel �− → �+π−

(100% branching ratio) with the subsequent decay � → p + π− (63.9% branching ratio)following. They are found by reconstructing the decay vertices starting with the � decay vertex.In one central Pb + Pb collision, all positively charged tracks are combined with all negativetracks in turn and tested to see whether they could have originated from a common decay vertexdownstream of the primary interaction. If so and if the pair pass geometrical cuts designed toreduce background, they are considered to be a � candidate. In a second separate procedure,all � candidates are combined with all negative tracks in the search for decay vertices of �−

and �− hyperons. If a successful �− or �− candidate passes other geometrical selectioncuts, properties such as invariant mass, rapidity and transverse momentum are calculated.Reconstruction for the �+ and �+ hyperons occurs in a similar way.

4. Ξ− and Ξ+ from central Pb + Pb collisions

The doubly strange �− and �+ hyperons reconstructed from 400 000 central Pb + Pb collisionshave been analysed. For this data set, the NA49 event trigger selected the 10% most centralPb + Pb collisions. The resulting invariant mass distributions for �− and �+ hyperons areillustrated in figure 2.

Both invariant mass peaks are well described by a Lorentzian superimposed on a third-order polynomial to fit the low-level background. The signal is obtained by selecting thoseparticles with invariant mass within ±20 MeV c−2 of the �− mass (corresponding to aboutfour times the width parameter of the Lorentzian fit) and then subtracting the background. Thedata used consist of 4804 �− and 907 �+ reconstructed particles.

370 R A Barton for the NA49 Collaboration

Figure 2. Invariant mass distributions for �− (left) and �+ (right) from 400 000 central Pb + Pbcollisions. In the bin centred on the �− mass, the signal-to-noise ratio is better than 10:1 (see textfor details).

4.1. Transverse mass distributions

The data are evaluated in bins of rapidity, transverse momentum and lifetime. Corrections forgeometrical acceptance, branching ratio and reconstruction efficiency are then applied on a bin-by-bin basis. The GEANT simulation has been adapted for NA49 and is used for calculatingthe geometrical acceptance. For the reconstruction efficiency, one simulated Monte Carlo �−

(or �+) that is within the spectrometer acceptance is embedded into a raw data event that doesnot contain a real �− (or �+). The event is run through the reconstruction software which triesto reconstruct the embedded particle. In this way, the reconstruction efficiency is determinedstatistically for each bin of rapidity, transverse momentum and lifetime.

Fully corrected transverse mass (mT =√

pT2 + M0

2) distributions are shown in figure 3and are fitted with the function given in equation (1) to obtain the inverse slope parameter, T .The measured transverse mass spectra span the rapidity range (1.7 < y < 4.5),

1

mT

dN

dmT

∝ exp

(−mT

T

). (1)

4.2. Rapidity distributions, 4π yields and particle ratios

Physical measurements are not possible for all values of transverse momentum because offinite geometrical acceptance. Consequently, the rapidity spectra for �− and �+ are formedby extrapolating the transverse momentum distributions over the full range, by means of thefitted slope parameters. The resulting distributions and extrapolation factors are given infigure 4. At mid-rapidity, the yields per event per unit of rapidity are found to be 1.67 ± 0.18and 0.35 ± 0.05 for �− and �+, respectively.

The extrapolated rapidity spectra are well described by Gaussians fixed at mid-rapidity(also shown in figure 4) with widths of σ = 1.07 ± 0.10 for the �− and σ = 0.74 ± 0.05for the �+. Integrating these fits over the full rapidity range gives 4π yields of 4.42 ± 0.31and 0.74 ± 0.04 particles per event for �− and �+, respectively. The �+/�− ratio, R, canbe calculated over the full measured rapidity range. At mid-rapidity, R(y = ycm) is foundto be 0.22 ± 0.04, in good agreement with our previous publication [4] and with WA97 [5].Additionally, NA49 has measured the 4π integrated ratio, R(4π) = 0.17 ± 0.02.

Multi-strange hyperons and strange resonances in NA49 371

Figure 3. Transverse mass distributions for �− (left) and �+ (right) from central Pb + Pb collisions.Inverse slope parameters, T , are from fits to equation (1) (see text for details).

Figure 4. Rapidity distributions for �− (top left), �+ (top right) and the �+/�− ratio (bottom)from Pb + Pb collisions. Full circles (•) are measured data points and open circles (◦) are reflectedabout mid-rapidity (y = 2.9) (see text for details).

372 R A Barton for the NA49 Collaboration

This new analysis, which uses a completely revised reconstruction procedure, results intotal integrated yields which are 30% lower than previous NA49 estimates [4, 6]. However,the presented central rapidity densities are now in much better agreement with that foundby WA97. Errors shown on the new analysis are statistical only and a detailed study of thesystematic errors is currently in progress.

5. Analysis of strange resonances

The analyses of �(1520) [7] and φ(1020) [8, 9] employ an alternative method of signalextraction. Here, the signal has been extracted from the invariant mass spectra after a procedureof mixed event background subtraction. Event mixing combines positively charged tracks fromone event and negatively charged tracks from different events with a similar multiplicity. Theresulting invariant mass distribution is normalized to and then subtracted from the data. Boththese analyses use particle identification from measurements of specific energy loss in theTPCs in the selection of tracks.

5.1. �(1520) analysis

The preliminary invariant mass for the �(1520) → p + K− channel (22.5% branching ratio)for p + p and Pb + Pb collisions is shown in figure 5. In the case of p + p collisions, asignal is observed leading to an integrated yield of 0.012 ± 0.003 per event, corrected foracceptance, efficiency and branching ratio. A simulation was performed for the acceptancecalculation using a thermal phase-space distribution with T = 150 MeV, flat rapidity (p + p)and T = 300 MeV, Gaussian rapidity with σ = 1.0 (Pb + Pb).

For the Pb + Pb data, no signal is present and a production upper limit of 1.36 �(1520)per event (at 95% confidence level) is computed. Scaling the p + p yield by the number ofparticipants in central Pb + Pb collisions, the expected yield in Pb + Pb is estimated to be0.012 × (350/2) ≈ 2.1 �(1520) per event. A Monte Carlo simulation based on 3.5 produced�(1520) per event as predicted by the thermal model [10] was performed and would leadto a reconstructed signal as shown by the broken curve. The absence of a peak indicates asuppression of the �(1520) in Pb + Pb collisions, which may be due to absorption in the nuclearmedium.

5.2. φ(1020) analysis

The invariant mass of the φ(1020), calculated from the φ(1020) → K+ + K− channel (49.1%branching ratio) is shown in figure 6. The data, fitted with a Lorentzian with the naturalresonance width folded in with a Gaussian, peak at 1018.7 ± 0.5 (Pb + Pb) and 1019.4 ± 0.2(p + p) MeV c−2. The additional width of the Gaussian distributions is consistent with themomentum resolution in the TPCs. No shift in the position or significant broadening of themass peak is observed. Furthermore, fits to the transverse mass distributions lead to inverseslopes of 305 ± 15 and 169 ± 17 MeV for central Pb + Pb and inelastic p + p collisions,respectively. Fully integrated yields of 7.6 ± 1.1 (Pb + Pb) and 0.012 ± 0.0015 (p + p) perevent are found, indicating an enhancement of about a factor of 3 when comparing Pb + Pb top + p collisions per participant.

Multi-strange hyperons and strange resonances in NA49 373

Figure 5. �(1520) invariant mass from (top) p + p and (bottom) Pb + Pb collisions. Thesignal-to-background ratio is 1:6 for p + p and 1:1330 for Pb + Pb collisions [7].

Figure 6. Invariant mass of the φ(1020) for (a) Pb + Pb (central rapidity), (b) Pb + Pb (fullacceptance) and (c) p + p (full acceptance). The signal-to-background ratios are, respectively, 1:18(a), 1:92 (b) and 2:7 (c) [9] (see text for further details).

6. Summary and conclusions

The new global analysis within NA49 allows a measurement of �− and �+ from Pb + Pbcollisions over a large rapidity and transverse momentum range. Preliminary results show the

374 R A Barton for the NA49 Collaboration

yield per event per unit rapidity at mid-rapidity to be 1.67 ± 0.18 for �− and 0.35 ± 0.05 for�+, leading to a �+/�− ratio, R(y = ycm), of 0.22 ± 0.04. Integrating over full phase spacegives 4π multiplicities of 4.42 ± 0.31 (�−) and 0.74 ± 0.04 (�+), resulting in an integratedratio, R(4π) = 0.17±0.02. There is an apparent suppression of the �(1520) when comparingp + p and Pb + Pb collisions. The integrated yield in p + p collisions is 0.012±0.003, whereasan upper limit of 1.36 �(1520) at 95% confidence level is found in central Pb + Pb collisions.Analysis of the φ(1020) is complete. The φ(1020) yield per participant increases by abouta factor of three from p + p to Pb + Pb collisions. No significant shift or broadening of theinvariant mass peaks are observed.

Acknowledgments

This work was supported by the Director, Office of Energy Research, Division of NuclearPhysics of the Office of High Energy and Nuclear Physics of the US Department of Energy(DE-ACO3-76SFOOO98 and DE-FG02-91ER40609), the US National Science Foundation,the Bundesministerium fur Bildung und Forschung, Germany, the Alexander von HumboldtFoundation, the UK Engineering and Physical Sciences Research Council, the Polish StateCommittee for Scientific Research (2 P03B 02418, 02615, 01716 and 09916), the HungarianScientific Research Foundation (T14920 and T23790), the EC Marie Curie Foundation, andthe Polish–German Foundation.

References

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[10] Becattini F et al 1998 Eur. Phys. J. C 5 143–53