CERES and HELIOS-3 have detected a significant enhancement of low--mass dileptons in nuclear collisions at 200 GeV/nucleon with respect to the expected ``conventional'' sources. The onset of the excess, starting at a mass of $\sim2m_{\pi}$, and the possibility of a quadratic dependence on the event multiplicity suggest the opening of the $\pi^+\pi^-\rightarrow e^+e^-(\mu^+\mu^-)$ annihilation channel. This would be the first observation of thermal radiation from dense hadronic matter. Possible interpretations of these results are presented, including the reduction of the $\rho$ mass due to partial restoration of chiral symmetry in the dense fireball formed in the collision

Agakichiev, G

Baur, R

Breskin, Amos

Chechik, R

Drees, A

Faschingbauer, U

Fisher, P H

Fraenkel, Zeev

Fuchs, C

Gatti, E

Glässel, P

Günzel, T F

Hess, F

Irmscher, D

Jacob, C

Lenkeit, B C

Olsen, L H

Panebratsev, Yu A

Pfeiffer, A

Pérez de los Heros, C

Ravinovich, I

Rehak, P

Sampietro, M

Schön, A

Schükraft, Jürgen

Shimansky, S S

Shor, A

Specht, H J

Steiner, V

Tapprogge, Stefan

Tel-Zur, G

Tserruya, Itzhak

Ullrich, T S

Wurm, J P

Yurevich, V I

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN-PPE/96-11131 July 1996LOW MASS DILEPTON PRODUCTION AT THE SPS:PROBING HOT AND DENSE NUCLEAR MATTER.C. P. de los Heros1;), G. Agakichiev4), R. Baur2), A. Breskin1), R. Chechik1), A. Drees2),C. Jacob3), U. Faschingbauer3), P. Fisher2), Z. Fraenkel1), Ch. Fuchs3), E. Gatti5), P. Glassel2),Th. Gunzel2), F. Hess3), D. Irmscher2), B. Lenkeit2), L. H. Olsen2), Y. Panebrattsev4),A. Peifer2), I. Ravinovich1), P. Rehak6), A. Schon2), J. Schukraft7), M. Sampietro5),S. Shimansky4), A. Shor1), H. J. Specht2), V. Steiner1), S. Tapprogge2), G. Tel-Zur1),I. Tserruya1;7), Th. Ullrich2), J. P. Wurm3), V. Yurevich4)AbstractCERES and HELIOS-3 have detected a signicant enhancement of low{mass dilep-tons in nuclear collisions at 200 GeV/nucleon with respect to the expected \conven-tional" sources. The onset of the excess, starting at a mass of  2m, and the pos-sibility of a quadratic dependence on the event multiplicity suggest the opening of the+ ! e+e (+ ) annihilation channel. This would be the rst observation of ther-mal radiation from dense hadronic matter. Possible interpretations of these results arepresented, including the reduction of the  mass due to partial restoration of chiral sym-metry in the dense reball formed in the collision.)Talk given at the XXXIstRencontres de Moriond on QCD and High Energy HadronicInteractions, Les Arcs, France, March 23-30, 1996.1)Weizmann Institute of Science, Rehovot, Israel.2)Physikalisches Institut der Universitat Heidelberg, Heidelberg, Germany.3)Max-Planck-Institut fur Kernphysik, Heidelberg, Germany4)JINR, Dubna, Russia.5)Politecnico di Milano, Milano, Italy.6)Brookhaven National Laboratory, Upton, NY, USA.7)CERN, Geneva, Switzerland.1 Dileptons as a probe of dense nuclear matter.The reason for the spontaneous breaking of chiral symmetry and the origin of connement areprobably the least understood features of QCD and one of the standing unresolved problems inparticle physics. However it seems clear that any conning theory must break chiral symmetry[1] and therefore these are closely related phenomena. Heavy ion collisions at high energies areexpected to produce a hot and dense region, or reball, of hadronic matter where these topicscan be studied.In particular, dileptons provide a clean probe of the dynamics of relativistic heavy ioncollisions since they do not interact strongly and can leave the interaction region without nal-state rescattering [2]. They therefore provide a window to the early stages of the collision,when the system reaches its hottest and most dense stage, with a possible phase transition toQGP [3]. They also provide a means of studying another of the interesting predictions of QCD:the restoration of chiral symmetry at high temperatures or densities [4, 5]. Typical values ofthe temperature, T, and energy density, ", at which these transitions are expected to occur areT  200 MeV or "  15   20"nuclear.2 Chiral symmetry restoration in dense matter.If we neglect the heavy-quark sector, which is not relevant for the following discussions, theQCD Lagrangian almost possesses chiral symmetry due to the small masses of the u and dquarks. However the QCD vacuum is denitely not chirally symmetric as can be seen fromthe vacuum expectation value of the quark condensate, h0j  j0i   (240 MeV)3. Thereforethe vacuum expectation value of the operator h  i is a good measure of the eects of chiralsymmetry breaking since in a chirally symmetric world h0j  j0i = 0. This spontaneous break-ing of chiral symmetry has important consequences for QCD phenomenology at the hadroniclevel. We expect the appearance of the massless Goldstone bosons associated with the symme-try breaking, and these are indeed found in the  isospin triplet. The small mass of the pioncan be expressed as m2= k (mu+ md) + O(m2).1)The proportionality constant k absorbsthe strength of the symmetry breaking eect through the quark condensate h  i, k / h  i=f2.The behaviour of the quark condensate with density (or temperature) provides then a meansto study the conditions in which the spontaneously broken chiral symmetry is restored. Theeects of the dense medium in hadronic properties can be addressed within several approaches:QCD sum rules, eective Lagrangians or lattice QCD (LQCD). The general result is thath0j  j0i decreases with increasing density or temperature, reaching a symmetry restorationphase transition where it becomes 0. This translates into a reduction of the hadronic masses inthe dense medium, a \melting" eect, as we approach the critical point.2)A well known exam-ple is the reduction of the nucleon mass within the nucleus as calculated by Walecka [6] using ascalar mean-eld model. Calculations by Brown and Rho [7] with an eective Lagrangian andHatsuda and Lee [8] within the sum rule framework, show that a similar eect is expected indense nuclear matter for the vector mesons.Experimentally one would like to have access to the properties of hadrons in a dense medium1)Similar relations hold for the K and  mesons if one includes the s quark in the game. However, for the 'we are faced with the U(1) anomaly and such a simple relation fails.2)The eect is expected to be more pronounced in the density axis than along the temperature axis. In thelimit of zero baryon density and high temperature the reduction of the hadronic masses is due to the presenceof baryon-antibaryon pairs. At lower temperatures, the density eect dominates.2to test these predicted eects. The production of the  meson in heavy ion collisions providesan excellent tool to this end [9]. Due to its much shorter lifetime (= 1:3 fm/c) comparedwith typical reball lifetimes of 10-20 fm/c, several generations of  mesons are produced anddecay during a nuclear collision at SPS energies. The most important aspect is that they decayinside the dense interaction region, while they retain their reduced mass. Thus, their decaythrough the dilepton channel provides a unique experimental window to the dense-mediumeects on the  meson. This eect is not so clear for the other mesons, !,  or J/ becauseof their much longer lifetimes: they will be re-absorbed in the medium or they will decay welloutside the interaction region when they have regained their vacuum masses.3 Dilepton production in relativistic nuclear collisions.There are several processes contributing to dilepton production in nuclear interactions at dif-ferent space-time points of the collision history. I will mention here only the decays of the lighthadrons and dilepton production from the hot hadronic reball, since these are the relevantdilepton sources to the problem I am discussing in this paper.We expect the dilepton sources known from p-p collisions to be an important source ofdileptons also in nucleus-nucleus collisions. These include the decay of the resonances , ! and, the Dalitz decays of the !,  and ' and, in the case of dielectrons, the Dalitz decay of the o.The most dierential aspect of nucleus-nucleus collisions with respect to nucleon-nucleoncollisions is the formation of a (thermalized?) dense reball. This opens new dilepton pro-duction mechanisms through interactions of the reball constituents, which are absent in thenucleon-nucleon case. The most important of them is pion annihilation. According to theVector-Meson Dominance model (VMD), + annihilation proceeds through an intermediate state, + ! o! `+` , and the cross section can be written as+ !`+` =423M2 1 4m2M2!1=2 1  4m2`M2!1=2 1 +2m2`M2! jF(M)j2(1)where M is the dilepton invariant mass and F(M) is the pion form factor, given byjF(M)j2=m4(M2 m2)2+m2 2(2)From the expression of Fwe see that any modication of the  parameters in a densemedium will bear an important change into the shape of + !`+` . There are other pro-cesses producing dileptons in the reball, though their contribution to the nal spectrum is notdominant. A complete treatment can be found in [10] (see also the discussion in section 5).4 Results on dilepton production in relativistic nuclear collisions.We turn now to the current experimental understanding of the dilepton spectrum from nuclearcollisions. Figure 1 shows the measured dilepton spectrum both from CERES [11] on dielectronsin S+Au collisions at 200 GeV/nucleon and from HELIOS-3 [12] on dimuons in S+W collisionsat the same energy. The results of CERES are compared with the expected dielectron yieldfrom the p-p sources, normalized to the charged-particle rapidity density. The shaded area inthe plot represents the uncertainty in the theoretical calculation of the dilepton yield, whilethe vertical bars in each bin represent the statistical error. The small brackets represent the3statistical and systematic errors added quadratically. The results from HELIOS-3 are comparedwith their measured p+W spectrum, normalized to the charged particle multiplicity.3)CERES/NA45S-Au 200 GeV/up⊥ > 200 MeV/cΘee > 35 mrad2.1 < η < 2.65〈dnch /dη〉 = 125mµµ[GeV/c2]µµ/charged (50 MeV/c2)-1 low mass regionHELIOS-3p-W   200 GeVS-W   200 GeV/u10-1010-910-810-710-610-50 1 2 3 410-60 1Figure 1: Dilepton spectra from CERES (dielectrons) and HELIOS-3 (dimuons) from S+Auand S+W collisions at 200 GeV/nucleon respectively.The message from these results is clear. There is a signicant enhancement of dileptonsover the expected scaled p-p yield in the invariant mass range 0:2 < m`+` < 1:5 GeV/c2,extending up to m+  2:5 GeV/c2in the case of dimuons due to the wider mass coverage ofHELIOS-3. Moreover, the enhancement sets in at masses m`+`  2mwhich would indicatethat we are observing + annihilation from a dense reball. The shape of the spectrum fromboth experiments is also revealing since it does not simply follow that from the p-p sources. Iwill discuss possible interpretations of this feature in the next section.First note that the global excess is somewhat dierent in both experiments. If we concentratein the invariant mass region common to both experiments, 0:2 < m`+` < 1:5 GeV/c2, CERESnds an enhancement of a factor of 5.00:8sta2sys, while HELIOS-3 obtains a factor of  1:6.This discrepancy could be a consequence of the dierent rapidity windows accessible to eachexperiment. CERES measures at mid-rapidity, 2:1 <  < 2:6, while HELIOS-3 covers theforward region, 3:7 <  < 5:5. Therefore the average particle multiplicities seen by the twoexperiments dier by at least a factor of two. If the dilepton yield scales non-linearly withmultiplicity this discrepancy could be accounted for. Such an eect would provide a test of thetype of emitting source, since dierent dilepton production mechanisms are expected to scaledierently with multiplicity [13]. Though CERES sees an increase of the dielectron yield withmultiplicity compatible with a quadratic dependence, the present errors do not allow us to ruleout a conventional linear dependence [14].3)Results from CERES on p+Au collisions at 450 GeV/c show that the dielectron yield in p-nucleus collisionsis well reproduced by the p-p yield [11]. We can then assume that the results from HELIOS-3 on p+W representthe p-p data with some degree of condence.45 Theoretical interpretations.Several models have been proposed to explain the data presented in the previous section. I willsummarize here three approaches which reect the current theoretical eorts on this topic.There are two calculations which use a microscopic approach to the nuclear collision. Li etal. [15] use the RQMD model while Cassing et al. [16] use the HSD model. These are similarmodels where the colliding nuclei are described in terms of their constituent nucleons and par-ticle production is described from the collisions of such nucleons using a string fragmentationapproach. Reinteractions among the produced particles are also taken into account in bothcases. The main contribution to dilepton production in these calculations comes from + annihilation and from the decays of produced resonances.A more unconventional approach is that proposed by Srivastava et al. [17]. In this modelit is assumed that QGP is initially formed in the collision at a typical time of  = 1 fm. Theinitial temperature is calculated to be Ti= 199 MeV. The system expands and cools throughhydrodynamical longitudinal expansion, reaching a mixed phase at a transition temperatureTc= 160 MeV. Further expansion brings the hadron gas to a freeze-out temperature Tf= 140MeV where the hadrons decouple. Dilepton emission from the QGP phase proceeds through qqannihilation, whereas in the hadronic system dilepton production comes from meson annihila-tion and the decays of the resonances. The relatively low initial temperature results in a veryshort lived QGP and, therefore, the main contribution to dilepton production comes from themixed and hadronic phases.CERES S+Au 200 GeV/up⊥ > 200 MeV/c, Θee > 35 mrad. 2.1 < η < 2.65CERES S+Au 200 GeV/up⊥ > 200 MeV/c, Θee > 35 mrad. 2.1 < η < 2.65Figure 2: Comparison of several theoretical calculations with the CERES data. See the textfor details.These three models give remarkably similar results as is shown in the left plot of Figure2.4)They reproduce the observed dilepton excess in the  region due to the + annihilationchannel (eq. 1) but, although they account for a sizeable fraction of the excess at lower masses,they fail to reproduce the data in the mass region 0:2 < me+e < 0:5 GeV/c2.4)Note that Srivastava et al. have not convoluted their prediction with the CERES mass resolution andtherefore the  peak appears as the bare resonance.5The right plot in Figure 2 shows an extension of the models of Li et al. and Cassing et al.where the eect of the reduction of the  mass is included. Li et al. calculate the in-medium mass, m, using an extended Walecka model, where the vector mesons are allowed to coupleto the scalar eld. In this approach m= m  gshSi. The scalar eld S is determinedself-consistently from the energy density of the reball. Cassing et al. calculate the reductionof the  mass from QCD sum rules where, in an obvious notation, m=mo= 1   0:18(=o).In both cases the reduction of the  mass opens pion annihilation channels at lower + invariant masses, giving rise to an enhanced dilepton yield below the nominal  peak. Theagreement with the CERES result when this eect is considered is remarkable. Though I haveconcentrated here on CERES results, recent calculations by Li et al. (unpublished) and Cassinget al. [18] including the in-medium modication of the  mass reproduce also the results ofHELIOS-3 shown in Figure 1.6 SummaryI have presented several models proposed recently to explain the enhancement of dileptons innuclear collisions seen by CERES and HELIOS-3. The dilepton enhancement in the  peakregion seems to be well understood from + annihilation, whereas the enhancement at lowerinvariant masses (0:2 < m`+` < 0:5 GeV/c2) is not reproduced by any conventional eect.The explanation of the enhancement in this region seems to call for partial restoration of chiralsymmetry in the dense reball and in its eects on the vector mesons' masses.Acknowledgments: I would like to thank Y. Nir for his useful comments and G. Li andW. Cassing for providing the theoretical curves and explanations about their models.References1. A. Casher. Phys. Lett. 83B, 395{398, 1979.2. K. Kajantie et al. Phys. Rev. D34, 2746{2754, 1986.P. Koch Z. Phys. C57, 283{303, 1993.3. A. M. Polyakov. Phys. Lett. 72B, 477{480, 1978.4. R. D. Pisarski. Phys. Lett. 110B, 155{158, 1982.5. G. Q. Li, C. M. Ko and G. E. Brown. Nucl. Phys. A582, 731{748, 1995.6. J. D. Walecka, B. D. Serot. Adv. Nucl. Phys. 16, 1, 1986.7. G. E. Brown, C. M. Rho. Phys. Rev. Lett. 66, 2720, 1991.8. T. Hatsuda, S. H. Lee. Phys. Rev. C46, R34, 1992.9. U. Heinz, K. S. Lee. Phys. Lett. B259, 162{168, 1991.10. C. Gale, P. Lichard. Phys. Rev. D49 (7), 3338{3344, 1994.11. G. Agakichev et. al. (CERES collab.) Phys. Rev. Lett. 75, 1272, 1995.P. Wurm et. al. (CERES collab.) Nucl. Phys. A590, 103c, 1995.12. M. Masera et. al. (HELIOS collab.) Nucl. Phys. A590, 93c, 1995.13. V. Cerny, P. Lichard and J. Pisut. Z. Phys. C31, 163{166, 1986.14. C. P. de los Heros. PhD thesis. Weizmann Institute, April 1996.15. G. Q. Li, C. M. Ko and G. E. Brown. Phys. Rev. Lett. 75, 4007{4010, 1995.16. W. Cassing, W. Ehehalt and C. M. Ko. Phys. Lett. B363, 35{40, 1995.17. D. K. Srivastava, B. Sinha and C. Gale. Phys. Rev. C (in press), 1995.18. W. Cassing, W. Ehehalt and I. Kralik. hep-ph/9691214, Jan. 1996.6

Low mass dilepton production at the SPS: probing hot and dense nuclear matter

Low mass dilepton production at the SPS: probing hot and dense nuclear matter

Abstract

Similar works

Full text

Available Versions

CERN Document Server