The baryon number transfer is studied in elementary and complex hadronic
interactions at the CERN experiment NA49 at the SPS, at 158 AGeV beam energy
(\sqrt{s}=17.2 GeV). A two component picture is proposed, which builds up the
net proton distribution from a target and a projectile component. Using pion
beam, the projectile component is experimentally determined for p+p and p+A
interactions. A similar stopping behaviour of the projectile component is found
for p+A and A+A interactions. Based on these observations, the baryon transfer
is assumed to provide a common scale of inelasticity in p+p, p+A and A+A
interactions. A model-independent way is proposed to predict the pion
multiplicity in A+A.Comment: 10 pages, 12 figures. To appear in the proceedings of International
  Workshop on Quark and Hadron Dynamics in Relativistic Heavy Ion Collisions
  (BP 2002), Budapest, Hungary, 3-7 Mar 200

Collaboration NA49

D. Varga

English

arXiv

The baryon number transfer is studied in elementary and complex hadronic interactions at the CERN experiment NA49 at the SPS, at 158 AGeV beam energy (√s=17.2 GeV) A two-component picture is proposed, which builds up the net proton distribution from a target and a projectile component. Using pion beam, the projectile component is experimentally determined for p + p and p + A interactions. A similar stopping behaviour of the projectile component is found for p + A and A + A interactions. Based on these observations, the baryon transfer is assumed to provide a common scale of inelasticity in p + p, p + A and A + A interactions. A model-independent way is proposed to predict the pion multiplicity in A + A

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Baryon number transfer in hadron + nucleus and nucleus + nucleus colllisions: a link between elementary and complex interactions

The baryon number transfer is studied in elementary and complex hadronic interactions at the CERN experiment NA49 at the SPS, at 158 AGeV beam energy $(\sqrt{s}=17.2$ GeV). A two component picture is proposed, which builds up the net proton distribution from a target and a projectile component. Using pion beam, the projectile component is experimentally determined for p+p and p+A interactions. A similar stopping behaviour of the projectile component is found for p+A and A+A interactions. Based on these observations, the baryon transfer is assumed to provide a common scale of inelasticity in p+p, p+A and A+A interactions. A model-independent way is proposed to predict the pion multiplicity in A+A

Varga, D

CERN Document Server

Proc. Budapest’02 Workshop onQuark & Hadron Dynamics (2002) 000–000Budapest’02 Workshop onQuark&HadronDynamicsBudapest, HungaryMarch 3–7, 2002Baryon Number Transfer in Hadron+Nucleus and Nu-cleus+Nucleus Collisions: a Link Between Elementary andComplex InteractionsD. Varga1 for the NA49 Collaboration1 Roland Eo¨tvo¨s University,H1117 Budapest, HungaryAbstract. The baryon number transfer is studied in elementary and complexhadronic interactions at the CERN experiment NA49 at the SPS, at 158 AGeVbeam energy (ps = 17.2GeV ). A two component picture is proposed, whichbuilds up the net proton distribution from a target and a projectile compo-nent. Using pion beam, the projectile component is experimentally determinedfor p+p and p+A interactions. A similar stopping behaviour of the projectilecomponent is found for p+A and A+A interactions. Based on these observa-tions, the baryon transfer is assumed to provide a common scale of inelasticityin p+p, p+A and A+A interactions. A model-independent way is proposed topredict the pion multiplicity in A+A.Keywords: baryon transfer, two component picture, multiplicityPACS: 13.85, 25.751. IntroductionThe NA49 experiment at the CERN SPS is a large acceptance hadron spectrometer,based on Time Projection Chambers. [1]. The detector provides precision trackingand particle identication via the measurement of the ionization energy loss in theTPCs. The system is completed by Time of Flight detectors and calorimeters, thelatter providing neutron detection above xF = 0.2 in proton induced reactions. Adedicated counter is designed to control the centrality in p+A interactions, whichmeasures the multiplicity of the recoil (grey) protons.2 D. Varga for NA492. Net proton distribution in the elementary collisionsThe net proton yield in a hadronic interaction is dened by p− p, so antiprotons aresubtracted from the protons to cancel the pairproduced component. The net protondistribution (see Fig.1) in p+p interaction is forward-backward symmetric, to rstorder flat with a diractive peak. Above xF > 0.2 the Feynman-scaling holds,which predicts the invariant cross-section as a function of xF being independent ofps at high energies. Intuitively, this symmetric distribution has two components:the forward region associated with the projectile, and the backward region with thetarget. In the following sections, the experimental study of this intuitive picturewill be discussed.0.11-0.2 0 0.2 0.4 0.6 0.8 1dN / dxFxFp - p--1/2p+ppi+pFig. 1. Net proton distribution in p+p and pi+p interactions. The prediction ofthe two component picture is that the pi+p yield at xF = 0 is half the p+p value3. Two component picture of the elementary collisionsThe basic idea of the two component picture [2] is shown on a cartoon (Fig.2):the net proton distribution in p+p is built up from two symmetric components.Assuming, that the target component is independent of the projectile type of theinteraction, the two components can be experimentally separated. For the mea-surement, a baryon free projectile was used (pi+p) which allows the measurementof the target component alone (see Fig.2).As pi+ and pi− has non-zero isospin, the produced net proton on the projectileside will also be non-zero (only p− p + n− n would vanish). To cancel this eect,the average of pi+ + p and pi− + p collisions were used for the measurement. TheBaryon Number Transfer: Linking Elementary and Complex Interactions 3_21pi pB−B B−B−−x xF FppdN / dxFFig. 2. In the two component picture of p+p interactions, the net proton yield isthe sum of a target and a projectile component; in the middle, both componentscontribute with half the yield (left). The target component can be measuredseparately using a baryon free projectile (right)net proton distribution in this averaged pi + p is presented in Fig.1One simple prediction of the two component picture is that the yield of the netprotons in pi + p at xF = 0, so at the center of mass of the colliding system, has tobe half of the value measured in p + p. This prediction is reasonably fullled. Inthe forward region, subtracting the pi + p measurement from the p + p values, theprojectile component of the net proton spectrum in p+p can be obtained, shown inFig.3.4. Net proton distribution in hadron+nucleus interactionsThe net proton distribution in p+Pb at dierent centralities is shown in Fig 4,compared to net protons in p+p. The centrality of the p+Pb collision is measuredusing a dedicated detector which determines the multiplicity of the knockout pro-tons from the nucleus. From this multiplicity, applying Glauber-model calculationsand detailed detector simulation, the number of collisions (ν) is estimated. The dis-tributions steepen up at higher centrality due to the baryon stopping phenomenon[3].5. Two component picture of hadron+nucleus interactionsThe proposed two component picture is more complicated for p+A than for p+p.The following assumption can be made: to rst order, on the target side each colli-sion of the projectile produces the target component of the net proton distributionin p+p, i.e. the target component in pA piles up in proportion with the number4 D. Varga for NA490.11-0.4 -0.2 0 0.2 0.4 0.6 0.8 1dN / dxFxFp - p--Fig. 3. The projectile component of the net proton spectrum in p+p interaction0.11-0.2 0 0.2 0.4 0.6 0.8 1dN / dxFxFp - p--p+pp+Pb,intermediatep+Pb, centralν=3.1ν=5.3Fig. 4. Net proton distribution in p+Pb interactions at different centralities(different number of collisions)of collisions. The projectile however, as it suers multiple collisions, develops astopped projectile component. A cartoon describing the picture is shown on Fig.5.In order to determine the projectile component of the net proton distributionin p+A, the idea of baryon free projectile can be used again. According to theprediction of the two component picture, the target component measured by pi + ABaryon Number Transfer: Linking Elementary and Complex Interactions 5maxB−B B−B−−pi Axx FFν=1νν=2pA=3dN / dxFFig. 5. In case of p+A, the target component of the net proton yield may havea similar shape as in p+p, but it is scaled up by the number of collisions (ν); theprojectile component, as the projecile suffered multiple collisions, gets stopped(left). The target component can again be separately determined using pionprojectile (right)scales up from pi + p with the number of collisions; Fig.6 demonstrates this eect.0.11-0.2 0 0.2 0.4 0.6 0.8 1dN / dxFxFp - p--pi+ppi+Pb,intermediatepi+Pb, centralFig. 6. Net proton distribution in pi+Pb interactions at different centralities.Due to the pileup on the target side, the pi+p yield is scaled up in pi+PbSubtracting the net proton distribution measured in pi+Pb from the net protonsin p+Pb at the same centrality, the projectile component of the net proton distri-6 D. Varga for NA49bution can be determined in p+Pb (see Fig.7). A smooth progression of baryonnumber transfer is observed as a function of the number of collisions.0.11-0.2 0 0.2 0.4 0.6 0.8 1dN / dxFxFp - p--p+pp+Pb, ν=3.1p+Pb, ν=5.3Fig. 7. Net proton projectile component in p+Pb interaction6. Two component picture of nucleus+nucleus collisionsIn case of AA, the baryon-free projectile concept is not applicable. The only con-straint which provides an approximation of the projectile component of the netproton spectrum is the following: at xF = 0, the value of the projectile componentis half of the measured spectra, and the target component dies out fast as we moveforward. In Fig.8 the measured net proton distribution and the estimated projectilecomponent is presented.The conclusion of the studies above is that with increasing centrality, the pro-jectile component of the net proton distribution have a similar behaviour in p+Aand A+A interactions, which is a smooth transition of baryon number transfer fromthe elementary to more complex interactions.7. Reference (isospin corrected) net proton for Pb+Pb colli-sionsIn the Pb nucleus, the n to p ratio is about 60 to 40, so the net proton distribution ina reference N+N collision has to be determined to take care of the neutron content;here N=0.4p+0.6n. Within the framework of the two component picture the isospinsymmetry holds separately for the two hemispheres, so the net forward proton inn+p or n+n is equal to the net neutron in p+p. Relying on this assumption, theBaryon Number Transfer: Linking Elementary and Complex Interactions 70.010.110 0.2 0.4 0.6 0.8 1dN/dx F / NparticipantxFν=4.6ν=3.9ν=2.5ppPbPb, centralPbPb, mediumPbPb, peripherial0.010.110 0.2 0.4 0.6 0.8 1dN/dx F / NparticipantxFν=4.6ν=3.9ν=2.5ppPbPb, centralPbPb, mediumPbPb, peripherialFig. 8. Net proton distribution in Pb+Pb interaction (left); net proton projectilecomponent in Pb+Pb (right)net proton distribution in an isospin averaged N+N collision can be obtained; thisis shown in Fig.9.00.10.20.30.40.50.60.70.80.910 0.2 0.4 0.6 0.8 1dN / dxFxFNet protonNet neutronN+N reference curveFig. 9. Net proton distribution in the reference nucleon-nucleon collision8 D. Varga for NA498. Correlation of the pion multiplicity with the final statebaryon in p+p interactionIn case of the p+A and A+A collisions, the baryon number transfer is characterisedby the centrality. In the elementary p+p collision the centrality can not be reliablydened, so the study must be done in the other way round: Fig.10 shows that theposition of the nal state baryon characterises the pion multiplicity p+p events.[4]00.511.522.533.544.50 0.2 0.4 0.6 0.8 1Forward charged pion multiplicityxF of the final state baryonCorrelation with protonCorrelation with neutronFig. 10. Correlation of the forward charged pion multiplicity with the xF positionof the final state baryon. Strong correlation is observed; in those events wherethe final state baryon is close to the center, the pion multiplicity increasesThe measurement conrms the intuitive expectation: in those events, wherethe nal state baryon is fast (especially in the diractive region), the multiplicityis low. If the nal state baryon is more stopped, more pions are produced. Thecorrelation is independent of the baryon isospin.9. Prediction of the pion multiplicity in Pb+Pb collisions fromcorrelation measurements in p+pBased on the phenomenon of baryon stopping in central p+A and A+A collisions,and the strong correlation of p+p observables with the nal state baryon, the follow-ing assumption can be made: the baryon number transfer characterises the inelas-ticity (centrality) in p+p, p+A and A+A interactions. This assumption providesa method to compare minimum bias p+p collisions with central A+A collisions,where the trigger bias is to be taken into account.In a p+p collision, any quantity, e.g. the pion multiplicity measured as a func-tion of xF of the nal state proton denes a correlation function C(x). The pionBaryon Number Transfer: Linking Elementary and Complex Interactions 9multiplicity in minimum bias p+p then can be calculated by weighing C(x) withthe net projectile proton distribution; this trivial statement can be expressed in thefollowing way:< pi >pp=∫C(x)Pppdx∫Pppwhere Ppp is the net projectile proton distribution in p+p as a function of xF .In A+A, the stopped net projectile proton distribution can be viewed as protonsfrom a superposition of p+p (or N+N) interactions, where events with more stoppednal state protons have larger weight; this means that the pion multiplicity in A+A(per participating nucleon) is predicted to be< pi >PbPb=∫C(x)PPbPbdx∫PPbPbdxIn this case, the net proton distribution in Pb+Pb (PPbPb) is to be applied. Theresult of this prediction is presented on Fig.11 in comparison with pion multiplicitymeasurements from PbPb interactions at dierent centralities.0.80.911.11.21.31.41.50.5 1 1.5 2 2.5 3 3.5 4 4.5 5< pi > / N participant relative to p+pNumber of collisions (ν)pp PbPb, different centralitiesPrediction from p+pMeasurementFig. 11. Comparison of the predicted and the measured pion multiplicity inPb+Pb collisions. The prediction, which assumes that the final state baryoncharacterises the inelasticity also in p+p gives a good qualitative agreementThe prediction gives a very good qualitative descriprion of the measurements.If the assumptions are correct, this means that the full increase of pion multiplicityin central Pb+Pb interaction can be explained with a superposition of elementarycollisions, taking proper care of the centrality selection.10 D. Varga for NA4910. ConclusionsAn experimental study of the two component picture of hadronic interactions atps=17.2 GeV was discussed. In this picture, the net proton distribution is builtup by a target and a projectile component; these components can be separatedexperimentally with a baryon free (pion) projectile. The projectile component ofthe net proton distribution in p+A and A+A collisions is found to behave similarlyat increasing centrality, namely that the baryon number moves towards the center.Based on the assumption that the baryon transfer characterises the inelasticity ofp+p, p+A and A+A collisions, a model independent procedure was developed tocompare minimum bias p+p and central Pb+Pb collisions, which showed that theincrease of the pion multiplicity in A+A compared to p+p interactions can be fullypredicted only by taking care of the sizeable baryon stopping in A+A.AcknowledgementThis work was supported by the Director, Oce of Energy Research, Division ofNuclear Physics of the Oce of High Energy and Nuclear Physics of the US De-partment of Energy (DE-ACO3-76SFOOO98 and DE-FG02-91ER40609), the USNational Science Foundation, the Bundesministerium fur Bildung und Forschung,Germany, the Alexander von Humboldt Foundation, the UK Engineering and Phys-ical Sciences Research Council, the Polish State Committee for Scientic Research(5 P03B 13820 and 2 P03B 02418), the Hungarian Scientic Research Foundation(T032648, T14920, T32293 and F034707), the EC Marie Curie Foundation, and thePolish-German Foundation.Notesa. Permanent address: Eo¨tvo¨s Lorand University, Department of Atomic Physics,Budapest, Hungary;E-mail: dezso.varga@cern.chReferences1. S. Afanasiev et al, Nucl. Instr. Meth. A430 (1990) 210.2. G.J Bobbink et al.Nucl. Phys. B204 (1982) 1733. W.Busza and R.Ledoux Ann. Rev. Nucl. Part. Sci. 38 (1988) 1194. J.Whitmore et al. Phys. Rev. D11 (1975) 3124.

Baryon Number Transfer in Hadron+Nucleus and Nucleus+Nucleus Collisions: A Link between Elementary and Complex Interactions

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