Experimental Studies of QCD in p/d/e-A Collisions at RHIC, the LHC, and e-RHIC

1. Experimental Studies of QCD in p/d/e-A Collisions at RHIC, the LHC, and e-RHIC Prof. B.A. Cole Columbia University

2. p-A Physics Goals • Nuclear effects (hard) • Shadowing / saturation @ low xA. • Jet structure / mono-jets @ low xA. • pT broadening / energy loss. • Modifications of baryon production. • Tests of pQCD: factorization / universality. • Nucleus as a filter (soft) • Diffraction. • Proton break-up, color transparency. • Baryon junction excitation. • Soft phenomenology. • In this talk: focus on “hard” effects (?)

3. Why p-A (d-A) Collisions ? • Probe Initial-State Effects at RHIC • Shadowing of Nuclear PDF’s • Parton saturation • Cronin effect • p broadening of hard processes • It’s becoming clear that these are all due to or reflect the same underlying physics • Unique feature of RHIC measurements • Ability to constrain “centrality” – i.e. impact parameter range of d-A collisions. peripheral central

4. Hard Scattering in p-p Collisions p-p di-jet Event STAR • Factorization: separation of  into • Short-distance physics: • Long-distance physics: ’s From Collins, Soper, StermanPhys. Lett. B438:184-192, 1998

5. Single High-pt Hadron Production data vs pQCD KKP Kretzer • NLO calculation agrees well with PHENIX 0 spectrum (!?) • BUT, FF dependence ? • Lore: KKP better for gluons • Includes soft-gluon resummation! Phys. Rev. Lett. 91, 241803 (2003)

6. Initial and final state radiation leads to QCD evolution Parton distributions Fragmentation func’s Well-controlled (infrared safe) evolution depends on cancellation of real and virtual radiation. Why does this matter? Radiation  broadening of transverse momenta Phase space restrictions inhibit the real/virtual cancellation. High pT hadron production at large xT (low s). Heavy quark production at low transverse momenta. “Re-summation” of large logarithms needed. But QCD is not Nearly So Simple …

7. Application of pQCD vs s Soffer and Bourrely, Eur. Phys. J. C36:371-374,2004 • How well does NLO pQCD work as we go down in energy from RHIC ? • Clearly describes data more poorly for decreasing s. • And for more forward production. • Also, sensitivity to factorization scale also grows.

8. Threshold re-summation • Threshold & soft gluon resummation (NLL) improves agreement with data at lower s. • Much smaller effect for RHIC at mid-rapidity. • But still a factor of ~2! • pT dependence ?? • What about at forward rapidity??

9. Forward  Production at RHIC Soffer and Bourrely, Eur. Phys. J. C36:371-374,2004 • NLO pQCD works at RHIC @ large xF • But ~40% scale error (=pT vs =pT/2) • Re-summed NLO (Vogelsang) also agrees with data. • But, scale error in non-resummed NLO: • Strong sensitivity to “nuclear” effects???

10. PHENIX: 200 GeV p-p Prompt  • Background removed via combination of: • (Jet) isolation cuts • 0 decay tag • Statistical subtraction • Spectrum and yield well-described by NLO pQCD (w/ threshold & recoil resummation). • ~ 15% scale error above 5 GeV/c. • More work needed to go below 5 GeV/c.

11. A-A Hard Scattering Rates • Parton flux density  “thickness” • For point-like interactions: • dNhard / dA  product of nuclear T’s • Integrate over transverse area • Then • Nbinary (also known as Ncoll) is fiction • no successive nucleon-nucleon scattering ! • Just a convenience (pure number not fm-2)

12. View in nucleus rest frame For mid-rapidity jet with MT Relative to nucleus, y=5.4 E  pL = MT cosh(y)  100 MT Lorentz boost:  = cosh(y)  100 Also, Jet formation time:  ~1/ mT Giving (jet) formation length (LF ) LF = 20 GeV fm / mT From this simple analysis we can conclude: All for the “action” for mid-rapidity particle production (and forward) occurs along the straight path of the incoming nucleon. Even high-pT and heavy quark production processes may be affected by coherence in the multiple scattering process. New at RHIC: Ability to select on “centrality” (poor man’s impact parameter) Coherence in p/d-A @ RHIC

13. d-Au “Centrality” • # soft scatters of n/p: • Parameterize multiplicity at large  vs n, p. • Cut data according to fraction of total dA. • For each, determine TdAu • e.g for PHENIX (in %) • 0-20, 20-40, 40-60, 60-88 • Define:

14. STAR d-Au @ High-pT • Beware: • Top plot is RdA • Bottom plot is Rcp • Strong enhancement in charged hadron production at =0. • Enhancement larger for baryons than for mesons. • Ks similar to  •  similar to 

15. PHENIX: d-Au Neutral Mesons • Now evaluate consistency with pQCD: • TAB scaling (factorization)  production vs centrality 0 production vs centrality

16. PHENIX d-Au 0 vs Centrality • Small Cronin effect (not expected to be large) • It is now known that preliminary data suffer from small trigger bias (central will go peripheral ).

17. PHENIX d-Au  Production • PHENIX sees small Cronin effect • Approx. consistent within errors with STAR Ks result • Enhancement seen in charged (baryons) all the more striking!

18. PHOBOS: d-Au hRdA • Clearly the “enhancement” of charged hadron production in d-Au depends on rapidity (). •  dependence suggests suppression for >1 nucl-ex/0406017, PRC in press nucl-ex/0406017, PRC in press

19. PHENIX d-Au Forward/Backward h • PHENIX observes similar trend in hadron spectra • Suppression relative to “expected” TAB scaling • Suppression greater for more central collisions • Suppression NOT confined to large  only!

20. BRAHMS: d-Au RdAor Rcp vs  • BRAHMS also sees suppression of (h-) yields at larger  (beware “isospin” effect for =2.2, =3.2) • Suppression increases for more central collisions.

21. BRAHMS – A closer look =3 • Rcp shows suppression increases with TAu • Clearer than RdA (pp data?) • Suppression smooth in  • But see h+/h- difference ! • Reflects Z=+1 of d ?? • Rcp with (h++h-)/2 still shows suppression. Rcp

22. Kharzeev, Kovchegov, Tuchin(Phys.Lett.B599:23-31,2004) Evolution from enhancement (Cronin effect) at mid-rapidity to suppression at forward rapidity. h-RdA modified by charge bias in p-p coll’s. Rcp less sensitive. Forward Suppression (CGC ??)

23. Model Comparisons (I) • Vitev(nucl-th/0302002) • pQCD w/ shadowing • Include self-consistent p broadening, dE/dx • Both elastic & radiative •  correct enhancement at mid-rapidity • But EKS anti-shadowing overestimates RdA • Predict RdA >1at y = 3 • dE/dx small effect. • But significant dE/dx effects at y = -3.

24. Vitev and Qiu: Higher Twist • “Higher Twist”: • multiple exchanges between projectile & target. • Vitev & Qiu: coherent multiple scattering • Effective rescaling of x of parton from deuteron.

25. Model Comparisons (II) A. Accardi nucl-th/0402101 • Describe hard scattering in nuclear rest frame. • “Cronin effect” from multiple semi-hard scattering • With unitarity corrections: • Fit to p-p + Fermilab p-A • Reproduces y=0 0 Rcp • But not y=3 *** • Even if opacity increased x3 • BRAHMS data changed • But p dependence wrong …

26. We don’t have to look very hard to see the effects of coherence. Effects near mid- disappear by pT ~ 6 (?) @  = 3.2 kinematic limit: pT  8 GeV/c. Limited phase space for truly high-pT physics (Semi) Hard Scattering in d-A @ RHIC Brahms

27. d-A J/ Production (from M. Leitch) E866: PRL 84, 3256 (2000)NA3: ZP C20, 101 (1983)  compared to lower s RdA Low x2 ~ 0.003 (shadowing region) • Not universal versus X2 : not shadowing !?? • BUT does scale with xF ! - why? • Initial-state gluon energy loss depends on x1~xF - weak at RHIC energy? • But Kopeliovich: • Effect can be due to “energy loss” (in gold) xF = xd - xAu Klein,Vogt, PRL 91:142301,2003 Kopeliovich, NP A696:669,2001 • Data favors (weak) shadowing + (weak) absorption ( > 0.92) • With current statistics hard to separate different nuclear effects • Will need more d-Au data!

28. Summary of d-A @ RHIC • Observe clear suppression of forward hadron production at pT >~ 4 GeV/c. • Continuation of trend over large y range. • Does not fit within pQCD calculations • Issue: EMC (0.2 < x < 0.9) suppression of gluons typically included in calculations, valid??? • weak Cronin effect at mid-y for mesons. • But, also clearly depends on rapidity. • Some crucial aspect of physics is missing in “pQCD” calculations. • Kopeliovich: factorization breaking? • “Sudakov suppression” – but at low/negative xF ?

29. The ability to select on centrality in d(p)-A collisions is NEW and very important. Potentially the first opportunity to measure the impact parameter dependence of: Initial-state broadening, Shadowing, … Observations of centrality dependence have already been important. But, there are some limitations: Rely on Glauber model to indirectly relate “centrality” observables to impact parameter. Kopeliovich: Flaw in Glauber models due to neglect of diffraction – which I think is a real issue. May be important for understanding RCP. Centrality in d(p)-A

30. Di-hadron Azimuthal () Correlations • jT represents hadron pT relative to jet • kT represents the di-jet momentum imbalance • “y” implies projection onto transverse plane. Jet

31. PHENIX d-Au/p-p,  - h,  Correlations PHENIX preliminary 1-2 GeV/c 0.4-1 GeV/c 2-3 GeV/c 3-5 GeV/c • “Trigger” pion pT > 5 GeV/c • Four different associated hadron pT bins • Clearly see role of constant jT, contribution from kT d-Au p-p

32. PHENIX: Di-jet KT • No jet reconstruction in PHENIX (yet) • But can measure KT via two-hadron  correlations. • Additional broadening from fragmentation. • But can be measured in single jet. • Then: • Study vs ph1 • KT Same in p-p, d-Au? • Sensitivity ?? • More work needed.

33. Studying Jet Properties @ RHIC • Use hadron pairs to study jet properties • pout dist. has both non-pert. (Gaussian) + hard (power) contributions. Pout Pout Jet PHENIX, From J. Jia, DNP’04 Talk Radiative tails pp PHENIX Preliminary

34. Jet Properties in d-Au • Compare pout dist’s in p-p and d-Au. • Evidence for effects of re-scattering, modified radiation, … ? • Not so far! • But this is just the beginning! • Such measurements w/ one jet @  > 2 would be very interesting!! • But not possible yet

35. Radiative Effects on (di)Jets • Conclude: large radiative component to di-jet kT • Also see Vitev, Qiu : Phys.Lett.B570:161-170,2003. • Without accounting for radiation initial parton intrinsic kT ~ 2 GeV/c (RMS). • After accounting for radiation ~ 1 GeV/c Analysis of STAR di-hadron  distribution by Boer & Vogelsang, Phys. Rev. D69 094025, 2004

36. Radiative contributions from initial & final state Initial state radiation due to parton shower prior to the hard scattering The development of the initial-state shower must be different in nucleus (?). “Quantum evolution” an important part of CGC Treatment of soft radiation in co-linear vs kT factorization? Hard Scattering – IS/FS Radiation • “Model-independent approach” • Measure di-jet acoplanarity • Better: -jet and -  (hard) processes

37. kT broadening and evolution of parton distributions will modify  production. If there are mono-jets, are there mono-photons?? -jet angular correlations more sensitive because less broadening from jet. Di-  production even more interesting – kinematics completely determined. Need good photon/0 separation. Direct Photon Production J. Jalilian-Marian, hep-ph/0501222

38. p-A Collisions @ LHC • Summary of LHC “Yellow Report” on p-A

39. Physics Motivation / Goals • From DOE LHC Heavy Ion Review (2002)

40. p-A @ LHC • p-A @ LHC can reach low x at high Q2 • Rates for high-pT processes are enormous • Concerns • No p-p measurements at same s (?). • Centrality selection will require care. • Little particle (baryon) identification away from mid-rapidity Parameters from LHC Yellow Report Rates for pT > 100 GeV/c

41. Measurable shadowing even at 100 GeV. Modest effects at mid-rapidity (but going away slowly) Low-x Effects @ LHC Q=100 GeV Q=10 GeV Q=2 GeV Armesto, Salgado, Wiedemann, Phys. Rev. Lett. 94:022002 (2005) Frankfurt, Strikman: Shadowing

42. Di-jet / -jet / - Acoplanarity (2) • d-A measurements @ RHIC limited by • Luminosity and Acceptance • Both of these limitations are removed in (e.g.) ATLAS @ LHC • Isolate initial-state radiation effects (modified in p-A) by comparing: • Di-jets, (isolated)  -jets, (hard) di-photon • Prediction from saturation: • “disappearance” of di-jet signal at pT ~ Qs • But, presumably measurable (calculable?) effects at higher pT?? (precision vs “discovery”)

43. Example:  from CDF

44. p-A in ATLAS (CMS) Electromagnetic Calorimeter Muon chambers Hadronic Calorimeter Superconducting Solenoid Inner Detectors Silicon Pixels Silicon Strips Transition Radiation Tracker Superconducting Coils for Toroidal Field for Muon System • p-p detectors @ LHC ideal for studying high-pT physics in p-A collisions. For CMS: EMCal covers ||<5 Had. Cal: ||<5 TOTEM: ||<7

45. ATLAS Calorimeter System (1) Hadronic Tile Calorimeters Silicon Tracker in Inner Detector EM Accordion Calorimeters Forward LAr Calorimeters Hadronic LAr End Cap Calorimeters

46. p-A Collisions: Soft “Background” • Some numerology: • @ LHC energies, p-Pb collisions  ~ 7 • Due to coherence (wounded-nucleon scaling)  ~ 7  4 times soft multiplicity (on average) • In p-p @ high-, ~ 25 collisions/bunch crossing • Typical p-Pb collision has 1/6 the soft background of high-  p-p collision. • Conclusion: for high-pT measurements ATLAS p-Pb performance better than p-p. • Beware: this argument neglects rapidity dependence of soft p-Pb/p-p. • Observe: best performance in low XA direction.

47. Simulated (& Recon) Hijing p-Pb Event

48. Simulated (& Recon) Hijing p-Pb Event #2 • Jet at forward (actually backward) rapidity

49. Detecting Forward jets (from Takai)

50. Event Characteristics dNchg/d ALL Pseudo-rapidity () Fraction of events ATLAS Charged part. multiplicity • Use Hijing to simulate (central) p-Pb events • Apply ideal ATLAS acceptance cuts to particles. • Study what ATLAS “sees” in typical events • e.g. charged multiplicity ATLAS does not measure a large fraction of charged particles 1)  coverage 2) Magnetic bend (minimum pT ~ 0.5)