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Exploring Dilepton Production and Medium Effects in High-Energy Nuclear Collisions

This study investigates the properties of resonance states in Quark-Gluon Plasma (QGP) and their implications for dilepton production in dense nuclear matter. It discusses the effects of Brown-Rho scaling, spectral modifications, and chiral symmetry restoration on particles such as the rho meson and its decay channels. The work synthesizes historical and recent experimental data to characterize medium effects, focusing on the role of dileptons as probes for understanding the dynamics of heavy-ion collisions at RHIC energies.

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Exploring Dilepton Production and Medium Effects in High-Energy Nuclear Collisions

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  1. Dilepton and medium effect on particle properties Zhangbu Xu • Outline • 共振态在QGP态下的性质 • 双轻子谱 • 探测方法

  2. Medium Effect in Dense Nuclear Matter • Brown-Rho Scaling •  Broadening • Low Mass Dilepton measuring  properties R. Rapp, et al Last Call for RHIC Predictions Nucl.Phys. A661 (1999) 205-260 • Chiral UA(1) symmetry restoration (J. Schaffner, D. Kharzeev, et al.)手征对称

  3. With the establishment of the hydrogen bubble-chamber, entirely new possibilities for research into high-energy physics present themselves. Results have already been apparent in the form of newly-discovered elementary particles. The first, very short-lived, so called, "resonance particle" was found in 1960. The Nobel Prize in Physics 1968 First time Invariant Mass was used (bump hunting) A little bit of history  (1952?) (1385) (1960) (892) (1961) (1405)   1411 (1967)

  4. ISR K*0 from p+p Collisions Measurement Technique 强子道的测量 • Mixed-event, Like-sign Background subtraction • P-Wave Breit-Wigner function or Modified BW

  5. K*  K K* K* measured K* lost  K K*   K* K K K K* measured Kinetic freeze-out Chemical freeze-out All that Matters: X-section作用截面 Different by 5 Rescattering>Regeneration at later stage 强子衰变道: 散射(大于)再产生

  6. 重核子碰撞的时空演化 • Stages in the collision 1. Pre-equilibrium Hard parton scattering processes 2. Equilibration After t≤ 1 fm/c partons materialise and either hadronise or rescatter. 3. Thermal quark-gluon plasma Hydrodynamic expansion 4. Hadronization (phase transition) Quark coalescence + gluon fragmentation … or … String fragmentation 5. Hadron gas Hadrons continue to interact 6. Particle freeze-out • Chemical freeze-out 化学冻结 粒子数不再变化 • Thermal freeze-out热冻结不再有强相互作用 p, K, N, … t p, K, N, … tf tf 6. t(eH) Hadron gas t(eQ) 5. t0 = th 6. Mixed phase Hadron gas t0 = tq 4. QGP 5. 3. 4. 2. 1. 1. Hadron formation. Parton formation and thermalisation. z A A a) Without QGP b) With QGP

  7. 共振态有化学冻结吗? STAR PHENIX Strangeness Enhancement Resonance Suppression Au+Au, p+p, Thermal Fit; Feed-down K. Schweda, O. Barannikova

  8. M. Bleicher et al. J. Phys. G 25 (1999) 1859 σ(Kπ) σ(ππ) Resonance driven by Cross Section K. Schweda, P. Fachini, C. Markert, H. Zhang Statistical and systematic errors added in quadrature • Finite Interaction Cross Section Breakdown of Ideal fluid? • Different Cross Section regeneration or rescattering ? • Different Cross Section Flow at hadron stage

  9. 质量 or 重子/介子? • RCP flattens at intermediate pT • Two groups (2<pt<6GeV/c): - ,K0s, K, K*, f mesons - p,L, X, W baryons • Meson/Baryon Effect!Collective velocity is not the only scale • quark coalescence ? 介子共振态的特征: 质量大

  10. Quark Contents Constituent Quark Scaling? n=20.3 K+K*? s+uK*? C. Nonaka, B. Muller et al. , Phys.Rev.C69:031902,2004

  11. + - Invariant Mass Distribution pp AuAu 40% to 80% STAR Preliminary STAR Preliminary sNN = 200 GeV 0.2  pT  0.8 GeV/c |y|  0.5 0 f0K0S  K*0 0 f0 K0S  K*0 0.2  pT  0.9 GeV/c |y|  0.5 Statistical error only Statistical error only • 2.1106 Au+Au minimum biasevents and 4.7106 pp events • Breit-Wigner  fixed width 0  = 150 MeV and f0  = 75 MeV  fixedf0 masse  0.98 GeV (AuAu) and 0.96 GeV (pp) • 0 mass = 0.698 ± 0.013 GeV (AuAu) • 0 mass = 0.729 ± 0.006 GeV (pp) P. Fachini (STAR)

  12. RHIC The Lost Sword (Fable, 《呂氏春秋》~250B.C.) <qq>, TQGP <qq>, TQGP The Sword A+A Collisions Moving Boat Freeze-out Ferry Hadrons Mark Detecting Signals Jumping into water Chiral Symmetry Restoration e+e-, thermal Radiation g+ge+e-

  13. Medium Effect in Dense Nuclear Matter • Brown-Rho Scaling • Rho Broadening • Low Mass Dilepton measuring  properties R. Rapp, et al Last Call for RHIC Predictions Nucl.Phys. A661 (1999) 205-260 • Chiral UA(1) symmetry restoration (J. Schaffner, D. Kharzeev, et al.)

  14. q l s  q l How to Probe Early Stage? • Modification in medium • Decay quicklymatter exists 10-23s • Small or no FSIleptons, photons Golden:   J/ Small Branching Ratio(10-4), Low Production Rate

  15. NA49 at SPS A central Pb+Pb Collision 1 cm slice centred on beam-axis B = 1.1 T B = 1.5 T NA49 Pb+Pb 158 GeV/A

  16. f-Production: Hadronic versus Leptonic Decay Channel • Different decay channels: • NA49: F K-K+ • NA50: F -+ • Transverse momentum spectrum • fit: 1/mTdn/dmT ~ exp(-mT/T) (NA49: 3.0<y<3.8) significant differences in slopes and yields Dieter Röhrich, QM02

  17. K l+l- K  K+K-  c=50fm AMPT, STAR Nucl-th/0202086  Vector Meson at RHIC 130GeV • AMPT: (l+l-)/(K+K-)=1.5 • Experiments’ comparison

  18. The NA45 experiment 95% of charged hadrons do not produced Cherenkov light Ring Imaging Cherenkov (RICH) Radiator chosen to select electrons Time Projection Chamber Used for charged particle tracking

  19. Dileptons at SPS • Conventional cocktail of particles • Underpredicts dilepton production • Mass shifted??

  20. The NA50/NA60 experiment NA50: di-muon spectrometer Note: absorber to remove all hadrons absorber muon spectrometer ET E0 event selection

  21. 双轻子的质量和动量谱斜率 Phys. Rev. Lett. 96 (2006) 162302 drop -Teff rises in low mass region  radial flow of a hadronic source - drop at M~1 GeV suggests partonic nature of equilibrated particles~thermal radiation? - note: yield goes like (dN ch /dy)2 S. Damjanovic, Quark Matter 2008 21

  22. PHENIX from Above

  23. West Arm tracking: DC,PC1, PC2, PC3 electron ID: RICH, EMCal Photons EMCal East Arm tracking: DC, PC1, TEC, PC3 hadron& electron ID: RICH,TEC, TOF, EMC photons: EMCal PHENIX Setup • Other Detectors • Vertex & centrality: • ZDC, BBC, • MVD • South Arm • tracking: • MuTr • muon ID: • MuID accumulated ~200 106 Au-Au full heavy ion program and first spin physics including electron & muon pairs

  24. Measured p+p Mee • Cocktail filtered in PHENIX acceptance • Charm, Bottom, DY contributions from PYTHIA • Excellent agreement! arXiv:0802.0050

  25. Thermal contribution perhaps if ccbar mass spectrum softened by rescatt. Au+Au Mee compared to Cocktail • Data and Cocktail absolutely normalized • Cocktail normalized to Au+Au measurements • Except ccbar • Low mass excess • 150 < Mee < 750 • X 3.4±0.2(stat.) ±1.3(syst.)±0.7(model) • Intermediate mass agreement arXiv:0706.3034

  26. 0 < pT < 8 GeV/c 0 < pT < 0.7 GeV/c 0.7 < pT < 1.5 GeV/c 1.5 < pT < 8 GeV/c • Still significant at high pT PT Dependence of Au+Au Mee PHENIX Preliminary QM08 • Low Mass excess is a low pT enhancement • Huge excess at lowest pT • Late Phase info PHENIX Preliminary

  27. STAR Upgrades MTD MRPC ToF barrel EMC barrel EMC End Cap BBCRPSD FMS F0S PMD • DAQ1000 • TPC FEE • MTD • Soft  finished HFT: Si-pixels; IST: Si-strips; FGT: GEM-layers Integrated Tracking Upgrades ongoing

  28. STAR的目标 Measuring the dilepton → To study: 1. vector meson production and in-medium effect 2. heavy Quarkonian production (suppression) Focus on the dilepton spectra at low mass region Electrons may come from the following sources at low mass region: γ conversion π0, η Dalitz decays ρωΦ vector meson decays

  29. What STAR Can Do? Future upgraded detectors: μVertex detector Full TOF Electrons PID Reject electrons not from primary vertex (gamma conversion) Simulation: γconversion and π0, η Dalitz decay background How can μVertex detector deal with γconversion subtraction?

  30. Electron Identification Before, using TPC dE/dx only to separate electrons from hadrons is complicated TOF detector lights up this study A prototype TOF tray (TOFr) installed this year |1/β-1|<0.03 With TOF PID cut, electron band can be separated from others readily!

  31. Dilepton Assume electrons can be identified with full TOF Total bkgd: ~10-4/25MeV (ω) ~2∙10-5/25MeV (Φ) Dalitz decay bkgd: ~5∙10-6/25MeV (ω) ~5∙10-7/25MeV (Φ)

  32. Background Rejection Electrons from gamma conversion L.J.Ruan Require TPC+SVT+μVertex ~98% electrons from gamma conversion rejected Dalitz decays become dominant sources!!! Great thanks to Kai!

  33. Upgraded TOF for dileptons • Background: g  e+e- • HFT discriminates background • Need low mass detector • Statistics comparable to NA60 • Charm background

  34. Vacuum Engineering Big Bang 强作用下的宇称破缺

  35. D. Kharzeev, QM08

  36. Conclusions • Vector Meson Resonances are sensitive tools to study new state of matter • An important physics program STAR will pursue • TOF+TPC+SVT+ μVertexSTAR RHIC II Upgrade

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