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d+Au and p+Au Collisions at RHIC

d+Au and p+Au Collisions at RHIC. Carl A. Gagliardi Texas A&M University. Outline Jet quenching and d+Au as the control experiment Small-x physics and saturation Upgrade plans for the future. Hard scattering at RHIC and NLO pQCD. PRL 91, 241803. PHENIX π 0. STAR (h + +h - )/2

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d+Au and p+Au Collisions at RHIC

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  1. d+Au and p+AuCollisions at RHIC Carl A. Gagliardi Texas A&M University • Outline • Jet quenching and d+Au as the control experiment • Small-x physics and saturation • Upgrade plans for the future

  2. Hard scattering at RHIC and NLO pQCD PRL 91, 241803 PHENIXπ0 STAR (h++h-)/2 BRAHMS (h++h-)/2 Calculations by W. Vogelsang At 200 GeV, pQCD does a very good job describing mid-rapidity yields

  3. Binary scaling Factor of 4-5 Suppression of high-pT inclusive hadron yields Au+Au relative to p+p RAA PRL 91, 072301 PRL 91, 172302 • Central Au+Au collisions: factor of 4-5 suppression • pT > 5 GeV/c: suppression ~ independent of pT

  4. trigger Jets and two-particle azimuthal distributions • Trigger: track with pT>4 GeV/c • Dfdistribution: 2 GeV/c<pT<pTtrigger • Normalize to number of triggers p+p  di-jet PRL 90, 082302 At 200 GeV, p+p collisionsshow conventional di-jet structure expected for 2  2 partonic collisions in pQCD

  5. Azimuthal distributions in Au+Au Au+Au central Au+Au peripheral pedestal and flow subtracted PRL 90, 082302 Near side: peripheral and central Au+Au similar to p+p Away side: strong suppression of back-to-back correlations in central Au+Au

  6. Theory vs. data pQCD-I: Wang, nucl-th/0305010 pQCD-II: Vitev and Gyulassy, PRL 89, 252301 Saturation: KLM, Phys Lett B561, 93 PRL 91, 172302 RCP Final state Initial state pT>5 GeV/c: well described by pQCD+jet quenching, but also by gluon saturation model (up to 60% central)

  7. partonic energy loss Is suppression an initial or final state effect? Initial state? Final state? gluon saturation How to discriminate? Turn off final state d+Au collisions!

  8. STAR Pedestal&flow subtracted What RHIC found – Jet Quenching PRL 91, 072304 Inclusive yields and back-to-back di-hadron correlations are very similar in p+p and d+Au collisions Both are strongly suppressed in central Au+Au collisions at 200 GeV

  9. Au+Au 10% Dφ2=φ2-φtrig dN2/dΔφ1dΔφ2/Ntrig Dφ1=φ1-φtrig More Recent Focus – Correlations PHENIX, nucl-ex/0507004 STAR Preliminary Obtain more detailed information regarding the underlying dynamics.

  10. Mid-rapidity vs. forward rapidity Mid Rapidity Forward Rapidity CTEQ6M Gluon density can’t grow forever. Saturation may set in at forward rapidity when gluons start to overlap.

  11. BRAHMS Forward particle production in d+Au collisions BRAHMS, PRL 93, 242303 Sizable suppression in charged hadron production in d+Au collisions relative to p+p collisions at forward rapidity

  12. PHENIX and PHOBOS report similar effects PRL 94, 082302 PRC 70, 061901(R) Charged particles are suppressed in the forward direction in d+Au collsions

  13. Expectations for a color glass condensate t related to rapidity of produced hadrons. D. Kharzeev, hep-ph/0307037 As y grows Iancu and Venugopalan, hep-ph/0303204 Are the BRAHMS data evidence for gluon saturation at RHIC energies?

  14. One calculation within the saturation picture RdAu RCP Saturation model calculation with additional valence quark contribution (Kharzeev, Kovchegov, and Tuchin, PL B599, 23)

  15. Another recent calculation Very good description of the pT dependence of the BRAHMS d+Au → h- + X cross section at η= 3.2 (Dumitru, Hayashigaki, and Jalilian-Marian, hep-ph/0506308)

  16. STAR Pseudo-rapidity yield asymmetry vs pT Au direction / d direction PRC 70, 064907 Back/front asymmetry in 200 GeV d+Au consistent with general expectations of saturation or coalescence; doesn’t match pQCD prediction.

  17. Saturation physics at RHIC? • Fundamental question regarding saturation – Where does it set in? • Forward hadron production at RHIC samples similar x values as mid-rapidity production at the LHC • Complex interplay at the LHC • Will probably need p+p, p+Pb, and Pb+Pb – all at the same √s– to unravel it fully

  18. x values in saturation calculations In CGC calculations, the BRAHMS kinematics corresponds to <xg> <~ 0.001(Dumitru, Hayashigaki, and Jalilian-Marian, hep-ph/0506308)

  19. Is saturation really the explanation? Difficult to explain BRAHMS results with standard shadowing, but in NLO pQCD calculations <xg> ~ 0.02 is not that small (Guzey, Strikman, and Vogelsang, PL B603, 173)

  20. Comparing d+Au dN/dηto p+emulsion nucl-ex/0409021 PHOBOS attributes effects to limiting fragmentation

  21. Many recent descriptions of low-x suppression A short list (probably incomplete) Saturation (color glass condensate) Shadowing • R. Vogt, PRC 70 (2004) 064902. • Guzey, Strikman, and Vogelsang, PLB 603 (2004) 173. • Jalilian-Marian, NPA 748 (2005) 664. • Kharzeev, Kovchegov, and Tuchin, PLB 599 (2004) 23; PRD 68 (2003) 094013. • Armesto, Salgado, and Wiedemann, PRL 94 (2005) 022002. Parton recombination • Hwa, Yang, and Fries, PRC 71 (2005) 024902. Multiple scattering • Qiu and Vitev, PRL 93 (2004) 262301; hep-ph/0410218. Others? • ... Factorization breaking • Kopeliovich, et al., hep-ph/0501260. • Nikolaev and Schaefer, PRD 71 (2005) 014023.

  22. <z> <xq> <xg> Forward π0 production at a hadron collider Ep p0 EN qq qp N xgp N xqp qg EN (collinear approx.) • Large rapidity π production (η~4) probes asymmetric partonic collisions • Mostly high-x quark + low-x gluon • 0.3 < xq< 0.7 • 0.001< xg < 0.1 • <z> nearly constant and high ~ 0.7-0.8 • A probe of low-x gluons NLO pQCD S. Kretzer

  23. √s=23.3GeV √s=52.8GeV Data-pQCD differences at pT=1.5GeV NLO calculations with different scales: pT and pT/2 Ed3s/dp3[mb/GeV3] Ed3s/dp3[mb/GeV3] q=5o q=10o q=15o q=53o q=22o xF xF Do we understand forward π0 production in p + p? Bourrely and Soffer, EPJ C36, 371: NLO pQCD calculations underpredict the data at low s from ISR Ratio appears to be a function of angle and √s, in addition to pT

  24. p+p  p0+X at 200 GeV • The error bars are statistical plus point-to-point systematic • Consistent with NLO pQCD calculations at 3.3 < η < 4.0 • Data at low pT trend from KKP fragmentation functions toward Kretzer. PHENIX observed similar behavior at mid-rapidity.

  25. d+Au  p0+X at 200 GeV d+Au π0 cross section at η= 4.0 is well described by a LO CGC calculation with a K-factor of 0.8 (Dumitru, Hayashigaki, and Jalilian-Marian, hep-ph/0506308)

  26.  dependence of RdAu Observe significant rapidity dependence, similar to BRAHMS measurements and expectations from saturation framework.

  27. For 22 processes Log10(xGluon) TPC Barrel EMC FTPC FTPC FPD FPD Gluon Constraining the x-values probed in hadronic scattering Guzey, Strikman, and Vogelsang, Phys. Lett. B 603, 173 Log10(xGluon) Collinear partons: • x+ = pT/s (e+h1 + e+h2) • x = pT/s (eh1 + eh2) • FPD: ||  4.0 • TPC and Barrel EMC:|| < 1.0 • Endcap EMC:1.0 <  < 2.0 • FTPC: 2.8 <  < 3.8 Measure two particles in the final state to constrain the x-values probed

  28. STAR FPD-TPC correlations in p+p • PYTHIA (with detector effects) predicts • “S” grows with <xF> and <pT,p> • “ss”decrease with <xF> and <pT,p> • PYTHIA prediction agrees with data • Larger intrinsic kT required to fit data STAR Preliminary 25<Ep<35GeV STAR Preliminary 45<Ep<55GeV Statistical errors only

  29. Any difference between p+p and d+Au? p+p: Di-jet d+Au: Mono-jet? Dilute parton system (deuteron) PT is balanced by many gluons Dense gluon field (Au) Kharzeev, Levin, McLerrangives physics picture (NPA748, 627) Color glass condensate predicts that the back-to-back correlation from p+p should be suppressed

  30. Back-to-back correlations with the color glass The evolution between the jets makes the correlations disappear. (Kharzeev, Levin, and McLerran, NP A748, 627)

  31. Are there “trivial” differences between p+p and d+Au? 25<Ep<35GeV HIJING predicts similar effects in d+Au as seen in p+p 35<Ep<45GeV

  32. STAR Fixed h, as E & pT grows Correlations in d+Au • are suppressed at small <xF> and <pT,p> • Spp-SdAu= (9.0 ± 1.5) % • consistent with CGC picture • are consistent in d+Au and p+p at larger <xF> and <pT,p> • as expected by HIJING STAR Preliminary STAR Preliminary 25<Ep<35GeV STAR Preliminary STAR Preliminary 35<Ep<45GeV Statistical errors only

  33. STAR Forward Meson Spectrometer upgrade • FMS increases areal coverage of forward EMC from 0.2 m2 to 4 m2 • Addition of FMS to STAR provides nearly continuous EMC from -1<h<+4

  34. p+p and d+Au  p0+p0+X correlations with forward p0 p+p in PYTHIA d+Au in HIJING hep-ex/0502040 Conventional shadowing will change yield, but not coincidence structure. Sensitive to xg ~ 10-3 in pQCD scenario; few x 10-4 in CGC scenario.

  35. PHENIX Nosecone Calorimeter upgrade • PHENIX Forward Spectrometer: • Forward Silicon • charged particle tracking • Forward Calorimeter (em and hadronic) • W/Si calorimeter • 0.9 < η < 3 • energy and position measurements • γ/e/jet trigger • Forward muon system • muon tracking • muon trigger NCC Same upgrades on South side

  36. Detection at forward / backward rapidity with Nosecone Calorimeters - • Direct detection of neutral pions (0.9 < || <3.0). • Large acceptance for high pT pions. • High energy photons. • Determines jet direction plus a rough energy measurement. • γ+ jet coincidences and detection. χc EM had Front view Side view

  37. PHENIX Endcap Silicon Vertex upgrade μ barrel endcap endcap B IP Endcaps detect following by displaced vertex of muons: D  μ + X B  μ + X B  J/  + X  μ+ μ- Secondary vertex resolution ~133 mm (endcap)

  38. Gluon shadowing and spin structure function with VTX Extracting gluon structure function • Vertex detector provides broad range in x in the predicted shadowing region (x <= 10-2) and at larger x • Measure gluon shadowing in p+A versus p+p Gluon Shadowing Predictions coverage

  39. Double parton correlations CDF, PRL 79, 584 PRL 88, 031801 A-dependence of 4-jet yields in p+A collisions can be used to measure x1 – x2 momentum correlations within the proton.

  40. Conclusions • d+Au collisions have played a key role in understanding the physics of Au+Au collisions • d+Au results provide hints that saturation effects are becoming important • Both STAR and PHENIX have upgrade plans that will dramatically improve their forward capabilities • RHIC may be the ideal accelerator to explore the onset of saturation

  41. Nuclear Gluon Density e.g., see M. Hirai, S. Kumano, T.-H. Nagai, Phys. Rev. C70 (2004) 044905 and data references therein World data on nuclear DIS constrains nuclear modifications to gluon density only for xgluon > 0.02

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