New results from NA49 on the production of strangeness in elementary and
nuclear reactions are presented. New measurements of charged kaon and pion
production have been obtained from 40 AGeV Pb+Pb collisions. The evolution of
strange meson yields in nucleus-nucleus interactions is studied as a function
of collision energy and system size. Cascade baryon yields are presented for a
wide range of hadronic interactions including first measurements in p+p and
centrality controlled p+Pb collisions.Comment: 4 pages, 4 figures, to be published in the Proceedings of XXXVIth
  Rencontre de Moriond, QCD and High Energy Hadronic Interactions, March 200

Varga, D.

English

arXiv

New results from NA49 on the production of strangeness in elementary and nuclear reactions are presented. New measurements of charged kaon and pion production have been obtained from 40 AGeV Pb+Pb collisions. The evolution of strange meson yields in nucleus-nucleus interactions is studied as a function of collision energy and system size. Cascade baryon yields are presented for a wide range of hadronic interactions including first measurements in p+p and centrality controlled p+Pb collisions

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RECENT RESULTS ON STRANGENESS PRODUCTION FROM CERNEXPERIMENT NA49D. VARGA for the NA49 CollaborationRoland Eo¨tvo¨s University, Department of Atomic Physics,1 Pazmany Peter Setany, H-1117 Budapest, HungaryNew results from NA49 on the production of strangeness in elementary and nuclear reactionsare presented. New measurements of charged kaon and pion production have been obtainedfrom 40 AGeV Pb+Pb collisions. The evolution of strange meson yields in nucleus-nucleusinteractions is studied as a function of collision energy and system size. Cascade baryon yieldsare presented for a wide range of hadronic interactions including first measurements in p+pand centrality controlled p+Pb collisions.1 IntroductionNA49 is a large acceptance hadron spectrometer, which is designed to explore a wide range ofhadronic interactions at dierent beam energies. The tracking system based on large TPC’sallows a systematic study from elementary collisions such as p+p and pi+p through the morecomplex p+A interactions to A+A interactions. A dedicated counter was designed to controlthe centrality of p+A collisions, whereas the system size dependence of A+A collisions can bestudied by measuring C+C and Si+Si collisions from fragmentation beam. The experimentalsetup is described in detail in 1.2 Strange Meson Yields in Nucleus-Nucleus CollisionsThe main motivation to study A+A interactions is the search for the deconned state. Asignature of the onset of deconnement would be the increase of the produced strangeness,measured by the ratio of kaons to pions.2.1 Beam Energy DependenceNew results have been obtained in central Pb+Pb collisions at 40 AGeV beam energy for kaonand pion multiplicities 2. Two independent analysis methods based on time of flight and specicenergy loss (dE/dx) measurement are found to be consistent. The total yield per event measuredin the 7% most central collisions is 56.3  3 for positive and 17.8  0.9 for negative kaons.The energy dependence of the K+/pi+ ratio is shown in Figure 1. The ratio of the totalyields increases at AGS energies 4, shows a maximum around the 40 AGeV point and decreasesslightly towards the top SPS energy. For comparison, the K+/pi+ ratio in p+p collisions is alsoshown. The nonmonotonic behaviour is less pronounced for the yields measured at midrapidity.The analysis of last year’s 80 AGeV beam and next year’s 20 and 30 AGeV datataking willclarify the behaviour.00.050.10.150.20 5 10 15 20K+/pi+, total yield√s (GeV)   A+A40 AGeV160 AGeVPreliminaryp+pp+p, NA49E866/E917NA49 00.050.10.150.20 5 10 15 20K+/pi+√s (GeV)   A+Al Yield at midrapiditym Total yieldPreliminary40 AGeV160 AGeVE866/E917NA49Figure 1: K+/pi+ as a function of collision energy. (Left:) The ratio of total yields in A+A and p+p collisions.(Right:) Comparison of the ratio of total yields and midrapidity yields2.2 System Size DependenceIn the search for the deconned state, the other relevant initial parameter besides the beamenergy is the size of the colliding nuclei. To study this dependence, C+C and Si+Si collisionswere measured: Figure 2 shows the K+/pi+ ratio compared to Pb+Pb and S+S measurements.0.060.080.10.120.140.160.180 50 100 150 200 250 300 350 400K+/〈pi± 〉NpartPreliminaryppCCSiSiPbPbSS 0.060.080.10.120.140.160.18pp 2 3 4 5 6K+/〈pi± 〉R - b/2 [fm]Preliminaryp+pC+CSi+SiPb+PbS+SFigure 2: K+/pi+ ratio at different system sizes, versus Npart (left) and versus R− b/2 (right)The results clearly show that the number of participants (Npart) does not allow a commonscaling for dierent system sizes and centralities. Instead of Npart another scaling variable canbe invented 3: plotting against R− b/2, where R is the radius of the nuclei and b is the impactparameter, the points fall on a uniform curve. This scaling variable corresponds to the width ofthe interaction zone. The analysis of the full available statistics will decrease the errorbars.3 Cascade Baryon ProductionThe cascade baryons (− and +) carry double strangeness, so they are more sensitive tostrangeness production. The NA49 tracking system allows the measurement of these weaklydecaying particles via their decay products 5 6. The acceptance extends over a wide rapidityrange (1 unit around midrapidity) and full transverse momentum range, from pT = 0 up to2 GeV. The centrality of p+A collisions is determined with a dedicated counter which detectsthe \grey protons", the recoil protons from the target. From that, using the VENUS model,the number of collisions (ν) can be determined. The centrality counter is also used for onlinetriggering to enrich the more central sample.00.20.40.60.81-1 0 1dN/dy (*10-3)yΞ−Ξ+Preliminaryp+p024681012-1 0 1dN/dy (*10-3)yΞ−Preliminaryp+Pbν=3.7ν=5.701234-1 0 1dN/dy (*10-3)yΞ+Preliminaryp+Pbν=3.7ν=5.7Figure 3: Cascade baryon yields in p+p (left) and in centrality controlled p+Pb collisions (middle and right)3.1 Cascade Baryon Yields in Proton-Proton and Proton-Lead CollisionsResults on cascade baryon production 7 are shown in Figure 3. The p+Pb sample is dividedinto two subsets of dierent centrality, in which the mean number of collisions are found to be3.7 and 5.7. The minimum bias p+Pb collision corresponds to ν = 3.75. The baryon stoppingeect is visible resulting in the decrease of the forward yield in more central p+A collisions.3.2 Enhancement of Strange Baryons in Proton-Nucleus and Nucleus-Nucleus InteractionsThe new measurements in p+p collision provide a new reference of elementary interaction forextrapolation to A+A collisions. Figure 4 compares the hyperon yields in p+p, p+A and Pb+Pbcollisions at midrapidity, using the WA97 8 and NA49 results.The solid line on the gure corresponds to the simple Wounded Nucleon Model (WNM)prediction. An enhancement is visible in p+A, which means that the predicted scaling breaksalready for p+A interactions.3.3 Dierent Interpretations of the EnhancementThe WNM is a two component picture: both the target and the projectile sides deliver particlesto midrapidity. The simplest assumption is that the yield is proportional to Npart. In case ofp+A, when the projectile has undergone ν collisions, Npart = ν + 1. This predicted scaling isindicated on Figure 4 with the solid line, using p+p as a reference. This scaling apparentlybreaks for p+A interactions, so it can not be used for extrapolation to A+A.One can consider a simple modication 7 of the WNM: let’s assume that in case of p+A, thetarget side delivers ν times half of the p+p yield (as does the target side of p+p), all rest of theobserved yield comes from projectile fragmentation. From the measurement, the contributionof the projectile side can be determined in p+A. This gives a prediction for A+A at the same νassuming that both sides of A+A behave as the projectile side of p+A. This prediction is shownon Figure 4 with the dashed line.The dierence between the conclusions of two oversimplied assumptions asks for a betterunderstanding of p+A and A+A collisions. As the huge predicted increase of the yields comparedto the WNM relied on a modest increase in p+A compared to p+p, precise measurements arealso needed. Amongst others, one has to take into account the isospin-eect, arising from theneutron content of the nuclei; the question can be investigated by the analysis of last year’sdeuteron beam dataset. The other important eect is rescattering in the target, which canincrease the target contribution.0.0010.010.11101 10 100 1000dN/dy y=0Number of participants (wounded nucleons)NA49WA97ΛΞ−pp pBe pPbPbPbPreliminary0.0010.010.111 10 100 1000dN/dy y=0Number of participants (wounded nucleons)NA49WA97ΛΞ+pp pBe pPbPbPbPreliminaryFigure 4: Midrapidity yields of hyperons in p+p, p+A and A+A collisions as a function of Npart4 ConclusionsThe new measurement at 40 AGeV beam energy indicates a non-monotonic behaviour of theK+/pi+ yield in A+A collisions as a function of the beam energy. The upcoming results at 20,30 and 80 AGeV will provide additional information. The study of the system size dependencerevealed that the variable R− b/2 allows a common scaling unlike Npart.The rst results on cascade production in p+p collisions give a new reference for p+A andA+A collisions. The results show an enhancement in p+Pb compared to the simple WNMmodel, which makes the extrapolation to A+A collisions within this framework questionable.AcknowledgementsI’d like to thank the Organisers for the pleasant and useful meeting. The work was partiallysupported by the Hungarian Scientic Research Foundation (F034707 and T32293).References1. S. Afanasiev et al, Nucl. Instrum. Methods A430, 210 (1999)2. C. Blume et al, NEW RESULTS FROM NA49. Proc. QM2001, Stony Brook, USA, 15-20Jan 2001.3. F. Sikler et al, RECENT NA49 RESULTS ON PB + PB COLLISIONS AT CERN SPS.Proc. ISMD 2000, Tihany, Hungary, 9-15 Oct 2000. e-Print Archive: hep-ex/01020044. L. Ahle et al, Phys. Lett. B476, 1-8 (2000) e-Print Archive: nucl-ex/99100085. L. S. Barnby et al, J. Phys. G 25, 469-472 (1999)6. R. A. Barton et al, J. Phys. G 27, 367-374 (2001)7. T. Susa et al, CASCADE PRODUCTION IN P+P, P+A AND A+A INTERACTIONS.Proc. QM2001, Stony Brook, USA, 15-20 Jan 2001.8. F. Antinori et al, Nucl. Phys. A611, 130-139 (1999)

Recent Results on Strangeness Production from CERN Experiment NA49

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