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Low-x Physics at RHIC

Low-x Physics at RHIC. Akio Ogawa. Introduction RHIC & Experiments Run3 Results Inclusive Hadron Back-to-back Correlations Run8 and Detector Upgrades Summary. Gluon density at low x. Gluon density can’t grow forever. Nuclear pdf in animatiron?. Mid Rapidity. Forward Rapidity.

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Low-x Physics at RHIC

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  1. Low-x Physics at RHIC Akio Ogawa • Introduction • RHIC & Experiments • Run3 Results • Inclusive Hadron • Back-to-back Correlations • Run8 and Detector Upgrades • Summary

  2. Gluon density at low x Gluon density can’t grow forever. • Nuclear pdf in animatiron? Mid Rapidity Forward Rapidity Saturation must set in at forward rapidity when gluons start to overlap and when recombination becomes important Can we see gluon saturation? Before LHC?

  3. Why is forward rapidity @ hadron collider? • Forward scattering probes asymmetric partonic collisions • Mostly scattering of high-x valence quarks (with known & large polarization) 0.25 < xq < 0.7 on low-x gluons 0.001 < xg < 0.1 Ep p0 qq PN qp N xgpN N =Au xqpN PN qg With heavy nucleus target, gluon density would be even bigger In CGC picture, xg ~ few 10-4

  4. Brhams pp2pp PHENIX STAR The Relativistic Heavy Ion Collider Au+Au Polarized p+p d+Au RHIC is a QCD lab

  5. BRAHMS STAR RHIC Experiments with forward detectors • Run3 d+Au • Run8 d+Au & p+p Forward spectrometer PIDed charged hadrons h= 2.2 - 3.2 Forward Pion Detector (FPD) pi0 h= 3.5 – 4.2 Forward Meson Spectrometer (FMS) pi0 h= 2.5 – 4.2 Muon Arm (penetrated) Hadrons h= 1.2 – 2.2 Muon Piston Calorimeter pi0 h= 3.1 – 3.7

  6. BRAHMS

  7. STAR The STAR Detector (run3) TPC: -1.0 <  < 1.0 FTPC: 2.8 <  < 3.8 FPD: ||  3.8 (p+p) ||  4.0 (p+p, d+Au) • Forward  Detector (FPD) • Pb-glass EM calorimeter • Shower-Maximum Detector (SMD) • Preshower

  8. The PHENIX detector • Central Arms • |η| < 0.35. • hadrons, electrons, photons • Muon arms • 1.2 < |η| < 2.2 • muons, also “light mesons” • Muon Piston Calorimeter • 3.1 < |η| < 3.7 • pi0

  9. BRAHMS STAR Mid-rapidity p+p PRL?????? 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 hadron yields

  10. BRAHMS STAR Forward rapidity p+p nucl-ex/?????? nucl-ex/0602011 At 200 GeV, pQCD does a very good job describing mid-rapidity hadron yields both mid and forward rapidity (0<|h|<4)

  11. STAR Pedestal&flow subtracted Mid-rapidity d+Au PRL 91, 072304 p+p and d+Auvery similar in both inclusive yields back-to-back di-hadron correlations In contrast, Au+Au collisions are very different from p+p and d+Au

  12. Forward rapidity d+Au? Expectations from Color Glass Condensate t related to rapidity of produced hadrons. D. Kharzeev hep-ph/0307037 As y grows Iancu and Venugopalan, hep-ph/0303204 CGC expects suppression of forward hadron production

  13. PHENIX d+Au hadron ratios -2.2<η<-1.4 and 1.4<η<2.2 PRL.94:082302,2005 Hadrons in Muon Arm PanchThrough Hadron Hadron DecayMuon • Backward • (Au direction) • ☐ Forward • (d direction) No suppression at backward rapidity Sizable suppression at forward rapidity

  14. BRAHMS BRAHMS d+Au charged hadron ratios η = 3.2 PRL 93, 242303 Taken from d’Enterria Sizable suppression at forward rapidity pQCD+Shadowing expects suppression, but not enough CGC gives best description

  15. STAR d+Au forward π0 STAR PRL 97, 152302 η = 4.0 (NPA 765, 464) Sizable suppression pQCD+Shadowing expects suppression, but not enough CGC gives best description on pT dependence

  16. BRAHMS RdAu rapidity dependence STAR η = 04 PRL 97, 152302 PRL 93, 242303 η = 4 p0 Observe significant rapidity dependence similar to expectations from the CGC framework

  17. BRAHMS Identified Particle RdAu η = 3 RdAu for p+,(p0 from STAR), K+,p all shows RdA <=1 for pT<3 GeV/c The protons may exhibit less suppression

  18. Back-to-back Angular Correlations pQCD 22 process =back-to back Di-jet (Works well for p+p) Forward jet p p Mid-rapidity jet Kharzeev, Levin, McLerran(NPA748, 627) With high gluon density 21 (or 2many) process = Mono-jet ? Mono-jet Dilute parton system (deuteron) PT is balanced by many gluons Dense gluonfield (Au) CGC predicts suppression of back-to-back correlation

  19. STAR p+p Forward-Midrapidity Correlations Forward pi0 (FPD) + Mid-rapidity Leading Charged Particle (TPC) • PYTHIA + detector predict • As <xF> and <pT,p> grows • “S” grows with • “ss” decrease • PYTHIA prediction agrees with data 25<Ep<35GeV ηtrig = 4.0 ηasso = 0.0 45<Ep<55GeV

  20. PYTHIA (p+p) vs HIJING (d+Au) • HIJING predicts similar correlations in d+Au as PYTHIA predicts for p+p. • Only significant difference is combinatorial background level 25<Ep<35GeV ηtrig = 4.0 ηasso = 0.0 35<Ep<45GeV

  21. STAR An initial glimpse: correlations in d+Au (Run3) • suppressed at small <xF> and <pT,π> • consistent with • CGC picture • are similar in d+Au and p+p at larger <xF> and <pT,π> • as expected by HIJING PRL 97, 152302 (2006) <pT,π> ~ 1.0 GeV/c 25<Ep<35GeV Fixed h, as E & pT grows <pT,π> ~ 1.3 GeV/c ηtrig = 4.0 ηasso = 0.0 π0:|<η>| = 4.0 h±: |η| < 0.75; pT > 0.5 GeV/c

  22. Back-to-back Angular Correlations Associated Trigger Phys. Rev. Lett. 96, 222301 -2.2<ηtrig <2.2 ηasso = 0.0 Conditional yields and width are the same for (triggers in 3 spectrometers) x (pp and dAu)

  23. Run8 d+Au Max expectation PHENIX STAR Min expectation Run3 ~x30 luminosity Great success! (polarizaed) pp run  reference data

  24. SOUTH Muon Piston Calorimeter (MPC) 2.22.2 18 cm3 192 PbWO4 crystals with APD readout 37.4cm

  25. Muon Piston Calorimeter (MPC) Foreground  Pairs Mixed Events • E > 8 GeV • Asymmetry z< 0.6 Eta peak above bkg Both north and south side working in Run8

  26. PHENIX Run8 Plans, measurements, etc

  27. STAR STAR Setup 2008 • Particle ID: • MRPC ToF (parts) • Calorimetry: • Photon Multiplicity Detector (PMD) • Barrel EMC • Endcap EMC • Forward Meson Spectrometer • Charged Particle Tracking: • Main TPC • 1/24 with DAQ1000 • Forward TPC (FTPC) • Event Characterization & Trigger: • Beam-Beam Counter (BBC) • Zero Degree Calorimeter (ZDC) • Forward Pion Detectors (FPD)

  28. STAR FPD to FMS Run3-5 FPD Inclusive p0 cross sections AN for inclusive p0 production RUN3 dAu =only one module (South) At deuteron side (west) Inclusive p0 cross sections in dAu and RdA Forward-mid rapidity particle correlations

  29. STAR FPD to FMS • Run8 and beyond: FMS • FMS will provide full azimuthal coverage for range 2.5  h  4.0 • broad acceptance in xF-pT plane for inclusive g,p0,w,K0,… production in p+p and d(p)+Au • broad acceptance for g-p0 and p0-p0 from forward jet pairs to probe low-x gluon density in p+p and d(p)+Au collisions 47 x more area > Order of magnitude more luminosity d Au

  30. STAR Forward Meson Spectrometer for Run8 First pi0 reconstruction of FMS events in Run8 (Calibration is underway)

  31. Ongoing FMS calibration Adjust gain for each detector by high tower sorted Mgg (1264 Mggplots) STAR Need multiple iteration through the data since pion and photon energy get spread over several towers. Run 6 resolution of d(Mp)/Mp~10% should be possible.

  32. Is saturation really the explanation? Difficult to explain suppression with standard shadowing But in NLO pQCD calculations <xg> ~ 0.02 is not that small (Guzey, Strikman, Vogelsang, PL B603, 173) In contrast, <xg> <~ 0.001 in CGC calculations (Dumitru, Hayashigaki, Jalilian-Marian, NP A765, 464) Basic difference: pQCD: 2  2 CGC: 2  1 Forward-forward di-hadron correlation : <xg>~10-3 with CGC : <xg>~10-4

  33. p+p and d+Au  p0+p0+X correlations with forward p0 d+Au in HIJING hep-ex/0502040 p+p in PYTHIA FMS FTPC EEMC TPC Barrel EMC FTPC FPD FTPC Conventional shadowing will change yield, but not angular correlation CGC evolution will change yield and the angular correlation Sensitive to xg ~ 10-3 in pQCD scenario; few x 10-4 in CGC scenario Forward-forward di-hadron correlation to reach lowest x

  34. Summary and Outlook • Forward rapidity at RHIC access to low-x physics • pp mid-rapidity, pp forward and dAu mid-rapidity under control • Run3 dAu • Hadron production • No suppression at backward • Suppression in forward hadron production • Di-hadron back-to-back correlation • No suppression at eta<2.2 • Suppression at eta=4 & low pt • Run8 dAu • ~x30 more integrated luminosity • STAR FMS (~ x50 acceptance) • PHENIX Muon Arm? • PHENIX MPC?

  35. Possible addition

  36. Calibration is ongoing Minimal run-by-run dependence in mass peak observed MIT (LED optics) UC Berkeley/SSL (flasher boards) Texas / Protovino / BNL (assembly) SULI program (Stony Brook students) / BNL (control electronics) Calorimeter stable at level of ~1%. LED system : critical calibration tool Entire Run 8 data set should become quickly available with final calibration.

  37. Id+Au with combined statistics

  38. Recent saturation model 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, NP A765, 464)

  39. B.Kopeliovich (PRC72(2005)054606) • Sudakov Suppression, not low-x phenomena. • Reproduce pt trend and centrality dependence.

  40. backup

  41. Forward single-spin asymmetries in STAR arXiv:hep-ex/0801.2990 • Large transverse single-spin asymmetries at large xF • xF dependence matches Sivers effect expectations qualitatively (but not quantitatively) • pT dependence at fixed xF not consistent with 1/pT expectation of pQCD-based calculations

  42. (Q2) Access to range of Q2 and x The x-Q2 region accessible is illustrated in the following. Note the region reachable at RHIC. In p(d)Aa the saturation will decrease the effectivex-range by A1/3 At RHIC at y~3 can reach into x2 ~ 10-3 Saturation

  43. √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

  44. STAR Foward Rapidity p+p nucl-ex/0602011

  45. A newly posted paper uses these BRAHMS data in a global fit to e+e- SIDIS, and pp data to extract fragmentation functions and explicitly check factorization. An example of their fit to our K+- data are shown. The RHIC pp data are important to obtain flavor separated fragmentation functions. D.Florian, R.Sassot and M.Stratman, Hep-ph/0703242

  46. Nuclear Modification on Jet Correlation Nuclear Modification on Jet Correlation is measured by comparing the jet strength in p+p collisions and the jet strength in specified event classes of d+Au collisions. It is defined as It is noted that there are two factors convolute into this quantity. The modification on jet production, such as jet multiplicity. The modification on single particle( trigger) production.

  47. Summary and discussions • We do not see any strong nuclear modification on per trigger jet yield. Especially we do not see a depletion in per trigger jet yield when triggering at forward. This contradicts with the predictions from CGC and power corrections. • In fact, at forward rapidity, our data seems to suggest a slight enhancement on per trigger jet yield in d+Au relative to p+p • We noted that per trigger yield are determined by singled particle production and jet production together. From single measurement, we know there is a strong suppression at forward rapidities in d+Au. Thus a slight enhancement on per trigger jet yield may suggest • Jet production is not modified. • Jet production is also suppressed, but is less suppressed than single is.

  48. RdAu rapidity dependence BRAHMS PRL 93, 242303 Sizable suppression as rapidity goes forward

  49. Associated Trigger Back-to-back Angular Correlations Phys. Rev. Lett. 96, 222301 htrig :|ηtrig| = 1.2 – 2.4; 2.0 < pTtrig < 5 GeV/c h±,asso: |ηasso| < 0.35; 1.0 < pTasso< 1.5 GeV/c Ratio of conditional yields

  50. Back-to-back Angular Correlations Compare with power correction (Dr. Ivan Vitev) Phys. Rev. Lett. 96, 222301 No change in d+Au relative to p+p at 1.4<htrig<2.0 Contradicts with the predictions from CGC and power corrections STAR results at <htrig>=4.0

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