In the spirit of establishing a fair reference for nucleus-nucleus
collisions, results on stopping and baryon transfer, correlations of the p+p
interaction and their consequences are shown. In the discussion of new results
from nucleus-nucleus collisions the emphasis is on strange meson and baryon
production at different energies - for the first time at 40 GeV A - with the
study of light nuclei.Comment: 9 pages, 21 figures, proceedings of ISMD200

Sikler, Ferenc

English

arXiv

In the spirit of establishing a fair reference for nucleus-nucleus collisions, results on stopping and baryon transfer, correlations of the p+p interaction and their consequences are shown. In the discussion of new results from nucleus-nucleus collisions the emphasis is on strange meson and baryon production at different energies - for the first time at 40 GeV A - with the study of light nuclei

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RECENT NA49 RESULTS ON PB+PB COLLISIONSAT CERN SPSFERENC SIKLE´R FOR THE NA49 COLLABORATIONKFKI Research Institute for Particle and Nuclear Physics, Budapest, HungaryE-mail: sikler@rmki.kfki.huIn the spirit of establishing a fair reference for nucleus-nucleus collisions, resultson stopping and baryon transfer, correlations of the p+p interaction and theirconsequences are shown. In the discussion of new results from nucleus-nucleuscollisions the emphasis is on strange meson and baryon production at dierentenergies { for the rst time at 40 GeVA { with the study of light nuclei.1 MotivationWe study nuclear reactions in order to elucidate the strong interaction processand study its development in time and space with the hope to learn thecharacteristics of hot and dense matter, moreover to establish experimentallinks between elementary and complex interactions.2 The experimentThe NA49 experiment is a large acceptance detector for charged hadrons1.The tracking of particles is based on four large volume time projection cham-bers, the particle identification is possible via the measurement of their specificenergy loss (dE/dx).One of the key parameters in the systematics of hadron production is thecentrality of the collision: the number of collisions per participant nucleon.In case of p+A reactions the centrality is known to be correlated with thenumber of ”grey” particles – slow protons and deuterons – which are measuredby a centrality detector surrounding the target and by the tracking system.In Pb+Pb reactions the deposited energy in the zero degree calorimeter wasused to select events with given centrality. Both measures were then correlatedwith the average number of collisions a nucleon undergoes, using simulation.The data presented here are preliminary results from p+p, p+Al, p+Pband C+C, Si+Si, Pb+Pb reactions at 158 GeVA and from Pb+Pb reactionsat 40 GeVA energy.sikler˙na49: submitted to World Scientific on February 12, 2001 13 Stopping and baryon transferThe longitudinal momentum distribution of net protons from non-single-diffractive p+p interactions has been measured in the rest frame of the colli-sion (Feynman-x distribution, xF). This can be compared to the measure-ments of the net proton distribution in centrality selected p+A reactions(Fig. 1.a). With increasing centrality (increasing average number of colli-sions) the final state protons appear to be more and more stopped with thebiggest effect in the most central sample. The evolution is smooth, connectingp+p via p+Al to p+Pb.aIn A+A collisions a similar gradual evolution of stopping is seen (Fig. 1.b),although not reaching the high level observed in most central p+Pb.4 Correlations in p+p, predictionsA very characteristic systematics of the net proton distribution as functionof ν¯ is observed in p+A and A+A reactions. What about p+p collisions?Although here ν = 1 by definition, the degree of inelasticity of the interactioncan be characterized by the xF of the final state proton. This is exemplifiedin the following by inspecting the correlations of hadronic variables with xpF.4.1 Pion densityThe average number of charged pions hpii strongly correlates with the xFofthe final state proton (Fig. 2.a): a fast proton will be accompanied by a few,a slow one by many pions.Using the hpii – xpFcorrelation measured in p+p, predictions can be madefor other reactions by folding their – above discussed – proton distributionwith this correlation curve. For a reaction that has slower protons one wouldextrapolate bigger pion density. This is what happens in more and morecentral Pb+Pb collisions: the observed increase of pion density with centralityis close to the prediction from p+p interactions (Fig. 2.b).4.2 Strangeness contentAs another example, a similar study can be performed with the φ(1020) mesonthat carries hidden strangeness. The ratio φ/pi− in the forward hemisphereincreases if the event has a slower proton (Fig. 3.a). This leads to a predictionof φ enhancement in p+A that is in agreement with the data (Fig. 3.b).aNote the similarity of p+Al and p+Pb in the overlap region at ν  3.sikler˙na49: submitted to World Scientific on February 12, 2001 25 A+A strangenessThe 10% most central Pb+Pb collisions have been used. Ξ− hyperons decayvia the channel Ξ− ! Λpi− with the subsequent decay Λ ! ppi− following.They are found by reconstructing the decay vertices starting with the Λ decayvertex. The analyses of Λ(1520) and φ(1020) employ an alternative method ofsignal extraction. Here, the signal has been extracted from the invariant massspectra after a procedure of mixed event background subtraction. Correctionsfor geometrical acceptance, branching ratio and reconstruction efficiency havebeen applied.5.1 Ξ− analysisTransverse mass (Fig. 4) and rapidity distributions (Fig. 5) are shown here.Integrating the Gaussian fits over the full rapidity range gives total yields of4.42  0.31 and 0.74  0.04 particles per event for Ξ− and Ξ+ respectively.The Ξ−/Ξ+ratio at midrapidity is found to be 0.220.04 in good agreementwith our previous publication and other experiment2. The integrated ratio is0.17 0.02.5.2 Λ(1520) analysisPreliminary invariant mass distribution for the Λ(1520) ! p + K− channelfor p+p and Pb+Pb collisions is obtained. In the case of p+p collisions thecorrected yield amounts to 0.00120.003 per event. Scaling the p+p yield bythe number of participants to central Pb+Pb collisions, the expectation wouldbe 0.012 (350/2)  2.1Λ(1520) per event. At the same time in Pb+Pb onlya weak signal is seen which may hint a possible suppression.5.3 φ(1020) analysisInvariant mass distributions for the φ(1020) ! K+ + K− channel in Pb+Pband p+p collisions are obtained. No shift in the position or significant broad-ening of the mass peak is observed. Inverse slopes of 305 15 and 169 17MeV, integrated yields of 7.6  1.1 and 0.012  0.0015 per event are foundfor central Pb+Pb and inelastic p+p reactions respectively. This indicates afactor of 3 enhancement3.sikler˙na49: submitted to World Scientific on February 12, 2001 36 Energy dependenceIn order to explore the energy range between top AGS and top SPS energiesan energy scan has been started. The importance of this study is supportedby a statistical approach that predicts the appearance of a phase transitionto quark-gluon plasma in the early stage of nucleus-nucleus collisions in thisenergy range. In the following the results of the preliminary analysis of 100kreconstructed central Pb+Pb events at 40 GeVA beam energy are presented.The excitation function of K+ show peak around 40 GeVA (Fig. 7). Theincrease of K−/pi− is continuous. The energy dependence of the ratio of meanpion multiplicity to the number of participating nucleons (Fig. 8.a) and thedependence of strangeness production (Fig. 8.b) are shown with the predictionof the statistical model of the early state4.It is observed that the pion yield per participant ratio follows the estab-lished trend of low energy data. The strangeness content of the event (ES) inPb+Pb collisions at 40 GeVA is higher by about 30% than the correspondingratios at top AGS and SPS energies. This results may be the first indicationof a non-monotonic energy dependence of the strangeness to pion ratio.RQMD shows similar tendency of central yieldsb but fails for total ones5.7 System size dependenceBoth AGS and SPS experiments observe particle ratios that show smoothdependence on the centrality of the nucleus-nucleus collision, that is, on thenumber of participants (Npart). On the other hand, for comparison of inter-actions of nuclei of different size, Npart does not seem to be the right variable,particle ratios do not scale with the volume of the reaction zone (Fig. 9, newpoints for C+C and Si+Si are from our preliminary analysis).Here a new scaling measure, theR-b/2breaction lensFigure 10. Some geometry, the nuclei moveinward and outward.thickness of the reaction zone is pro-posed. Using the cross section andperimeter of the reaction lens, the vol-ume to surface ratio can be well ap-proximated to be V/A  23 (R − b/2)which can be regarded as the aver-age escape pathlength from the reac-tion volume (Fig. 10c). The impactbThe authors state that the strangeness increase is mainly due to rescattering of mesonswith baryons. Compare this to the thickness-scaling found in section 7.cThe central collision of small nuclei (right) produces as much kaons as the large one atmedium impact (left).sikler˙na49: submitted to World Scientific on February 12, 2001 4parameter b can be estimated via the energy deposited in the zero degreecalorimeter. Plotting the above measures against R− b/2 all the four centralvalues (C+C, Si+Si, S+S, Pb+Pb) and even the Pb+Pb centrality selectedpoints are closely on the same curve, a line (Fig. 11). There are indicationsthat the increase of transverse momentum for protons is already present forC+C and Si+Si collisions.8 SummaryThe NA49 experiment identifies particles in the forward hemisphere with con-trolled centrality. Hadronic reactions with different targets, projectiles andenergies are studied.Using the internal structure of the p+p interaction, predictions can bemade for p+A and A+A collisions. This is to be confronted with the usualmethod of comparing with minimum bias p+p collision.New results from Pb+Pb collisions explore the characteristics of Ξ−, Ξ+,Λ(1520) and φ(1020) production. Our energy scan opens up the excitingrange of lower energy collisions (40 GeV A) which has the highest strangenesscontent observed so far. New studies with light nuclei help to fill the desertbetween elementary p+p interactions and Pb+Pb collisions. Already semi-central C+C collisions shows increase of strangeness. An interesting scalingwith the thickness of the reaction zone is revealed.References1. S.V. Afanasiev et al., [NA49], Nucl. Instrum. Methods A 430, 210 (1999).2. R.A. Barton for the NA49 Collaboration, Proc. of Strangeness 2000.3. S.V. Afanasiev et al., [NA49], Phys. Lett. B 491, 59 (2000).4. M. Gaz´dzicki, M. I. Gorenstein, Acta Phys. Pol. B 30, 2705 (1999).5. F. Wang, H. Liu, H. Sorge, N. Xu and J. Yang, nucl-th/9909001.sikler˙na49: submitted to World Scientific on February 12, 2001 50.10.20.51230 0.1 0.2 0.3 0.4 0.5 0.6dN/dx FxFp+pp+Al −ν=2.2p+Al −ν=3.0p+Pb −ν=3.0p+Pb −ν=4.3p+Pb −ν=5.70.10.20.51230 0.1 0.2 0.3 0.4 0.5 0.6dN/dx FxFN+NC +C −ν≈1.3Si+Si −ν≈1.9Pb+Pb −ν=2.6Pb+Pb −ν=3.2Pb+Pb −ν=3.6Pb+Pb −ν=3.9Pb+Pb −ν=4.3Pb+Pb −ν=4.6Figure 1. Feynman-x distribution of net protons (p − p) with dierent average number ofcollisions (ν) in a) p+A and b) A+A collisions. Lines are to guide the eye.sikler˙na49: submitted to World Scientific on February 12, 2001 60123450 0.2 0.4 0.6 0.8 1〈pi〉 yieldxpFp+pforward hemisphere11.11.21.31 2 3 4 5 672 130188240300360〈pi〉Pb+Pb / Npart·〈pi〉NN−νNpartPb+Pbpion enhancementpredictionFigure 2. a) Average number of charged pions (hpii = (pi+ + pi−)/2) in the forward hemi-sphere in p+p collision if the fastest proton has longitudinal momentum xpF . b) Pion en-hancement in Pb+Pb collisions relative to minimum bias p+p as function of ν. Measuredpoints and predicted line using p+p correlations are shownsikler˙na49: submitted to World Scientific on February 12, 2001 700.511.522.530 0.2 0.4 0.6 0.8 1YieldsxFφ [10-2]pi-p+pforward hemisphere11.521 2 3 4 5 6φ/pi- [10-2 ]−νp+Aφ enhancementpredictionFigure 3. a) Yields of negative pions and φ in the forward hemisphere in p+p collision ifthe fastest proton has longitudinal momentum xpF . b) φ enhancement in p+A collisions inthe forward hemisphere as function of ν. Measured points and predicted line using p+pcorrelations are shown. The enhancement for central Pb+Pb is at 3.sikler˙na49: submitted to World Scientific on February 12, 2001 8Figure 4. Transverse mass distributions for − (left) and + (right) from central Pb+Pbcollisions. Inverse slope parameters are also given.sikler˙na49: submitted to World Scientific on February 12, 2001 9sikler˙na49: submitted to World Scientific on February 12, 2001 1010-210-11100 0.5 1mt-m0 [GeV]∆n / (mt ∆mt ∆y)Pb+Pbp+p *100(a)00.511.522.533.51 2 3 4 5y∆n / ∆y Pb+Pbp+p *460(b)Figure 6. φ mesons in central Pb+Pb and minimum bias p+p collisions: a) transverse massdistributions around midrapidity; b) rapidity distributions (full symbols represent measuredpoints, open ones are reflected about midrapidity).sikler˙na49: submitted to World Scientific on February 12, 2001 1100.050.10.150.20.250.3101 102K+/pi+√¬s [GeV]∆ centraln total40GeV160GeVK+00.020.040.060.080.10.120.140.160.180.2101 102K-/pi−√¬s [GeV]∆ centraln total40GeV160GeVK-Figure 7. Ratios of K+/pi+ and K−/pi− yields as function of collision energy. Closed boxesrepresent total yields, closed triangles are for central yields.sikler˙na49: submitted to World Scientific on February 12, 2001 12Figure 8. The dependence of a) the dierence between pion to participant ratios { b)strangeness to pion ratio { for central A+A collisions and N+N interactions on the collisiond b F i’ Th li i di t th di ti f th t ti ti lsikler˙na49: submitted to World Scientific on February 12, 2001 1300.050.10.150.20 50 100 150 200 250 300 350 400K+/〈pi± 〉NpartK+ppCCSiSiPbPbSS00.020.040.060.080.10.120 50 100 150 200 250 300 350 400K-/〈pi± 〉NpartK-ppCCSiSiPbPbSSFigure 9. Ratios of K+/hpii and K−/hpii total yields as function of number of partici-pants in nuclear collisions.sikler˙na49: submitted to World Scientific on February 12, 2001 1400.050.10.150.2pp 2 3 4 5 6K+/〈pi± 〉R - b/2 [fm]K+p+pC+CSi+SiPb+PbS+S00.020.040.060.080.10.12pp 2 3 4 5 6K-/〈pi± 〉R - b/2 [fm]K-p+pC+CSi+SiPb+PbS+SFigure 11. Ratios of K+/hpii and K−/hpii total yields as function of R-b/2 in nuclearcollisions.sikler˙na49: submitted to World Scientific on February 12, 2001 15

Recent NA49 results on Pb+Pb collisions at CERN SPS

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