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Systematic Measurement of Proton-Antiproton Production at RHIC

This study presents the systematic measurement of proton-antiproton production in collisions at the Relativistic Heavy Ion Collider (RHIC) and investigates the dependence on collision centrality and energy. The findings provide insights into the particle production mechanisms and shed light on the origin of baryon/meson differences at intermediate transverse momentum.

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Systematic Measurement of Proton-Antiproton Production at RHIC

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  1. RHICにおける陽子反陽子生成の系統的測定 金野正裕（筑波大学）

  2. RHIC Findings (1) Jet Quenching • In central Au+Au collisions, hadrons are suppressed at high pT. • - The suppression is a final state effect (parton energy loss). • Away-side jet peak disappeared in central Au+Au collisions.

  3. 1.0 0.5 0 RHIC Findings (2) Particles & Medium Effects Baryon enhanced B/M Splitting of v2 - Suppression/Enhancement has particle-type dependence. => Baryon/Meson difference in yields and emission patterns at intermediate pT(2-5 GeV/c).

  4. Hadron Production in RHI Collisions - There are multiple hadronization mechanisms at intermediate pT. - The relative contributions and particle-type dependence are not yet fully understood. Understanding Baryon/Meson difference at intermediate pT. => What is the origin?

  5. PHENIX detector EM Calorimeter (PID) TOF-E (PID) Aerogel Cherenkov,TOF-W (PID) Pad Chambers (tracking) Drift Chamber (momentum meas.) Global detectors: - Beam Beam Counter - Zero Degree Calorimeter

  6. Particle Identification detector upgrade Time of Flight ( ~120 ps) “New” Time of Flight ( ~90 ps) Scint.+PMT type MRPC type Aerogel Cherenkov (n=1.011) Veto for proton ID

  7. Proton and Antiproton pT spectra Au+Au sNN = 200 GeV Cu+Cu sNN = 200 GeV p+p sNN = 200 GeV NOTE: No weak decay feed-down correction applied. pT reach extended up to 6 GeV/c for p(p) with fine centrality bins. (1) Aerogel Cherenkov (2) High statistics

  8. Freeze-out Properties

  9. Particle Yield dN/dy at mid rapidity  K p Au+Au/Cu+Cu/p+p (sNN = 200 GeV) Au+Au/Cu+Cu/p+p (sNN = 62.4 GeV) • Particle yields are (roughly) scaled with Npart btw. Au+Au and Cu+Cu. • dN/dy(Cu+Cu) >~ dN/dy(Au+Au) at smaller Npart. • Statistical model describes their ratios with few parameters (T,).

  10. Mean Transverse Momentum p K  Au+Au/Cu+Cu/p+p (sNN = 200 GeV) Au+Au/Cu+Cu/p+p (sNN = 62.4 GeV) • Clear hadron mass dependence: larger <pT> for heavier particles. • => Consistent with radial flow picture. • <pT> increases with Npart. it is clearly seen for (anti)proton.

  11. Blast-wave Model Fit Blast-wave model is a parameterization within a simple boost-invariant model with transverse collective flow. pT spectra reflecting thermal freeze-out temperature and transverse flow at final state. 2 map * Ref: PRC48(1993)2462 Tfo ~120 MeV, T ~0.7 (* Resonance decay feed-down correction not applied. Instead, tighter pT fitting range used. ; 0.6-1.2 GeV/c K; 0.4-1.4 GeV/c, p/pbar; 0.6-1.7 GeV/c) Spectra for heavier particles has a convex shape due to radial flow.

  12. <T> ~0.5 Transverse Flow Velocity Au+Au/Cu+Cu/p+p (sNN = 200 GeV) Au+Au/Cu+Cu/p+p (sNN = 62.4 GeV) • <T>: increasing with Npart. • - Npart scaling of <T> between Au+Au and Cu+Cu. • Almost same <T> at √sNN = 62.4, 200 GeV.

  13. Tfo ~120 MeV Kinetic Freeze-out Temperature Au+Au/Cu+Cu/p+p (sNN = 200 GeV) Au+Au/Cu+Cu/p+p (sNN = 62.4 GeV) • Tfo: decreasing with Npart. • Npart scaling of Tfo between Au+Au and Cu+Cu. • Almost same Tfo at √sNN = 62.4, 200 GeV.

  14. Summary - Freeze-out properties Characterizing bulk properties: - Chemical freeze-out - Kinetic freeze-out => Hadron production at low pT : “Thermal emission + Radial flow” Scaling properties between different systems: - Chemical/kinetic freeze-out properties show similarities between different collision systems. - Npart scaling of freeze-out properties (Au+Au, Cu+Cu), * even though the overlapped region has a different shape. => System volume Npart is a control parameter. * Particle yield: (Cu+Cu) > (Au+Au) at smaller Npart - Similarity at sNN = 200 and 62.4 GeV.

  15. Baryon Enhancement

  16. Baryon enhancement at sNN = 200 GeV p/ p/ • (Anti-)proton enhancement observed/confirmed in 200 GeV Au+Au/Cu+Cu. • Larger than expected from jet fragmentation (measured in pp, e+e-). • p/ (p/) ratios turn over at 2~3 GeV/c ,and fall towards the ratio in p+p.

  17. Strange Baryon enhancement Au+Au sNN = 200 GeV /K0s STAR, nucl-ex/0601042 Baryon enhancement seen in strange.

  18. Baryon enhancement at sNN = 62.4 GeV p/ p/ • (Anti-)proton enhancement observed/confirmed in 62.4 GeV Au+Au/Cu+Cu. • Similar pT dependence as at 200 GeV.

  19. p/ ratio vs. Npart1/3 Cu+Cu vs. Au+Au (62.4 GeV) Cu+Cu vs. Au+Au (200 GeV) • Npart scaling of p/ (p/) at same √sNN. • The ratios are controlled by the initial overlap size of colliding nuclei, • even though overlap region has a different geometrical shape.

  20. Nuclear Modification Factor RAA • Comparison with p+p spectra (reference) in binary collision scaling. • Proton, antiproton are enhanced at 1.5 - 4 GeV/c for all centralities. • - Suppression is seen for , K.

  21. Comparison of RAA in Au+Au/Cu+Cu Pion RAA (pT=2.25 GeV/c) Proton RAA (pT=2.25 GeV/c) RAA (Cu+Cu) > RAA (Au+Au) - Proton is enhanced for all centralities, while /K are suppressed.

  22. RAA (Cu+Cu) > RAA (Au+Au) • - Geometrical shape : Au+Au more deformed • - No. of N-N scatterings per N : narrow peak in Cu+Cu Comparison of Au+Au and Cu+Cu Glauber model calculation (Cu+Cu: b=0.0 fm, Au+Au: b=8.6 fm) <Npart> ~117 Cu+Cu: good resolution at smaller Npart Even though Ncoll-Npart relation is almost same between Au+Au and Cu+Cu, the geometrical overlap shape is different. <Npart> ~100

  23. Beam energy dependence

  24. Beam energy dependence of enhancement p/ p/ * No weak decay feed-down correction applied. - p/+ ratio:decreasing as a function of sNN. - p/-ratio: increasing as a function sNN. • Antiproton is a good probe to study the baryon enhancement.

  25. p/ ratio vs. (dET/d)1/3 • No Npart scaling of p/ (p/) in Au+Au between 62.4 and 200 GeV. • Transverse energy density dET/d scaling of p/ is favored. • - dET/d is a connection key between different √sNN. Proton production at 62.4 GeV is partly from baryon number transport, not only proton-antiproton pair production.

  26. Net proton distribution BRAHMS, PRL 93 (2004) 102301 it drastically changes with beam energy. Energy loss per nucleon: 73±6 GeV

  27. Chemical Potential Au+Au/Cu+Cu/p+p (sNN = 200 GeV) Au+Au/Cu+Cu/p+p (sNN = 62.4 GeV) • q (200 GeV) : ~8 MeV, independent of Npart • q (62.4 GeV) : increasing with Npart • => more baryon stopping at central

  28. Summary - Baryon enhancement Baryon enhancement: - Proton and antiproton enhancement confirmed at intermediate pT (2-5 GeV/c) in Au+Au/Cu+Cu. A turnover of p/ ratio seen at pT = 2-3 GeV/c. - In terms of binary collision scaling, (anti)protons are enhanced while pions/kaons are suppressed. Low energy 62.4 GeV data: - At lower energy 62.4 GeV, proton production seems to be more affected by baryon number transport process. => Antiproton is a good indicator of the baryon enhancement. Scaling properties between different systems: - Npart scaling of p/ (p/) - dET/d scaling of p/

  29. Two-component model(Soft+Hard)

  30. Particle production in expanding matter time high-pT particles low-pT particles z-axis time x-axis

  31. Two-component Model (Soft+Hard) Soft component :Thermal emission + Radial flow - Described by Blast-wave model - Npart scaling seen - Thermal distribution extrapolated up to high pT Hard component : Jet fragmentation + Jet suppression - Measured p+p spectra - Ncoll scaling - Constant suppression factor (power-law distribution & fractional energy loss)

  32. 200 GeV 62.4 GeV Hard component in p+p and Au+Au p+p sNN = 200 GeV Au+Au 200 GeV (pi0: diamond, h+h-: circle) • Hard component (in p+p) at high pT depends on s. • In Au+Au, suppression effect should be taken into account.

  33. Pion pT spectra Au+Au 200 GeV + - Soft Line Hard Line Reproduce the measured pion pT spectra.

  34. Pion fraction vs. pT Au+Au 200 GeV + - Hard Soft Residual

  35. Proton pT spectra Au+Au 200 GeV p p Soft Line Hard Line Reproduce the measured proton pT spectra.

  36. RAA vs. Npart Proton fraction vs. pT Au+Au 200 GeV p p Hard Soft Residual

  37. Proton pT spectra Au+Au 200 GeV p p Soft Line Hard Line Using pion’s RAA for suppression factor.

  38. Proton fraction vs. pT Au+Au 200 GeV p p Hard Soft Residual Need 3rd component ?

  39. p p + - Cross point (S=H) vs. pT - - - Both soft and hard components are necessary to reproduce the hadron spectra at intermediate pT (2-5 GeV/c). - Soft component is extended to higher pT in central. - Intermediate pT: Hard pions vs. Soft protons Fraction of soft and hard components

  40. Summary - Two-component model Two-component model: - Reproduce the measured pT spectra for pions and protons with a consistent way. - Identify crossover region from soft to hard hadron production at intermediate pT (2-5 GeV/c). Baryon/Meson difference: - Intermediate pT: “Hard” pions vs. “Soft” protons - Origin of baryon enhancement is radial flow. It pushes heavier particles to higher pT. Baryon/Meson difference is trivial?

  41. Quark Flow vs. Hadron Flow

  42. Quark recombination Fries, R et al PRC 68 (2003) 044902 Greco, V et al PRL 90 (2003) 202302 Hwa, R et al PRC 70(2004) 024905 • One of the hadronization mechanisms. • Recombination of thermal quarks in local phase space: • qq  Meson, qqq  Baryon • At intermediate pT, (recombination) > (fragmentation) • because quark distribution is thermal: ~exp(-mT/T). • At high pT, fragmentation (power-law shape) would be dominant.

  43. Applicability of quark recombination model p/ vs. pT v2/n vs. KET/n - Baryon enhancement & quark number scaling of v2 explained by “Quark recombination” - v2 at quark level => Collective flow at quark level - In a simple recombination picture, radial flow cannot be distinguished between hadron and quark phases. => Can we separate hadron flow and quark flow ?

  44. y z x 1+1D Adiabatic Expansion • - Ideal gas: P=(1/3) • Entropy conservation • Longitudinal expansion • & Transverse expansion - cooling curves - tfo fixed at 10 fm/c at most central bj vs. Np T scaled with (bj)1/4 at t = 1 fm/c Cooling stopped at Tfo

  45. Freeze-out Time & Temperature Freeze-out time vs. Np Freeze-out temperature vs. Np • More central collisions freeze-out later • at lower temperature. • Consistent with freeze-out condition: • (t)=R(t) • Even if quark phase is created before • hadronization, hadronic scattering • should be taken into account. • As expected, Tfo is lower than Tch. • Different centrality dependences. • Tfo dropping is consistent with • 1+1D adiabatic expansion.

  46. Conclusions • - Systematic measurement of proton and antiproton pT spectra • (Au+Au, Cu+Cu, p+p at sNN = 200/62.4 GeV) • Proton and antiproton enhancement confirmed • at intermediate pT (2-5 GeV/c). • - Antiproton is a good indicator for study of the baryon enhancement. • - p/ ratio & freeze-out properties show Npart scaling • between Au+Au and Cu+Cu at same sNN. • The Initial volume (~Npart) of colliding nuclei is a control parameter. • - Baryon enhancement is caused by transverse radial flow • - pT and centrality dependences are described by two-component model. • - Intermediate pT (2-5 GeV/c): hard pions vs. soft protons • - Chemical/Kinetic Freeze-out temperatures provide a hint • for further expansion at hadronic stage.

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