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Experimental Review of Hard Processes (mostly at RHIC) W.A. Zajc Columbia University

Experimental Review of Hard Processes (mostly at RHIC) W.A. Zajc Columbia University. Summary. Hard processes: One in which there exists some scale >> L QCD Examples: Large momentum transfer (jets) Heavy flavor production Direct photons Preview as summary:

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Experimental Review of Hard Processes (mostly at RHIC) W.A. Zajc Columbia University

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  1. Experimental Review of Hard Processes (mostly at RHIC) W.A. Zajc Columbia University

  2. Summary • Hard processes: • One in which there exists some scale >> LQCD • Examples: • Large momentum transfer (jets) • Heavy flavor production • Direct photons • Preview as summary: • RHIC is an ideal machine for using hard processes to probe • Deep interior of heavy ion collisions (quark-gluon plasma?) • Gluon, sea-quark contributions to proton spin (G. Bunce) • Measurements to date: • Large momentum transfer (jets): calibrated, basis for discovery • Heavy flavor production: first measurements • Direct photons: approaching first measurement

  3. RHIC Specifications • 3.83 km circumference • Two independent rings • 120 bunches/ring • 106 ns crossing time • Capable of colliding ~any nuclear species on ~any other species • Energy: • 500 GeV for p-p • 200 GeV for Au-Au(per N-N collision) • Luminosity • Au-Au: 2 x 1026 cm-2 s-1 • p-p : 2 x 1032 cm-2 s-1(polarized)

  4. RHIC’s Experiments STAR

  5. RHIC Achievements to Date • Machine : • Runs 1-4: • Au+Au: operation at 4 energies (19, 62, 130, 200 GeV) • d+Au comparison run (200 GeV) • p+p baseline (200 GeV) • Routine operation in excess of twice design luminosity ! • First polarized hadron collider ! • Experimental Operations: • Routine collection, analysis of 100 Tb datasets • >50 publications in Physical Review Letters • Excellent control of systematics and inter-experiment comparisons • Experimental Results: • Record densities created ~100 times normal nuclear density • New phenomena clearly observed (“jet” quenching) • Strong suggestions of a new state of matter

  6. Run-1 to Run-4 Capsule History Run Year Species s1/2 [GeV ] Ldt Ntot p-p Equivalent Data Size 01 2000 Au+Au 130 1 mb-1 10M 0.04 pb-13 TB 02 2001/2002 Au+Au 200 24 mb-1 170M 1.0 pb-110 TB p+p 200 0.15 pb-1 3.7G 0.15 pb-1 20 TB 03 2002/2003 d+Au 200 2.74 nb-1 5.5G 1.1 pb-146 TB p+p 200 0.35 pb-1 6.6G 0.35 pb-1 35 TB 04 2003/2004 Au+Au 200 241 mb-1 1.5G 10.0 pb-1 270 TB Au+Au 62 9 mb-1 58M 0.36 pb-1 10 TB RHIC Successes (to date) based on ability to deliver physics at ~all scales: barn : Multiplicity (Entropy) millibarn: Flavor yields (temperature) microbarn: Charm (transport) nanobarn: Jets (density) picobarn: J/Psi (deconfinement ?) }This Talk

  7. Why RHIC ? • Different from p-p, e-p colliders Atomic number A introduces new scale Q2 ~ A1/3 Q02 • Different from previous (fixed target) heavy ion facilities • ECM increased by order-of-magnitude • Accessible x (parton momentum fraction)decreases by ~ same factor • Access to perturbative phenomena • Jets • Non-linear dE/dx • Its detectors are comprehensive • ~All final state species measured with a suite of detectors that nonetheless have significant overlap for comparisons • It’s adedicated facility • Able to • Perform required baseline and control measurements • Respond rapidly to new opportunities (e.g., 62 GeV Run) g, p, p0, K, K*0(892), Ks0, h, p, d, r0, f, D, L, S*(1385), L*(1520),X± , W,D0, D±, J/Y’s, (+ anti-particles) …

  8. Access to Perturbative Phenomena? • Consider measurement of p0’s in p+p collisions at RHIC. • Compare to pQCD calculation • Phys. Rev. Lett. 91, 241803 (2003) • parton distribution functions, for partons a and b • measured in DIS, universality • perturbative cross-section (NLO) • requires hard scale • factorization between pdf and cross section • fragmentation function • measured in e+e-

  9. RHIC Energy Reduces Scale Dependence • The high √s of RHIC • makes contact with rigorous pQCD calculations • minimizes “scale dependence” • A huge advantage in • Spin program • Providing calibrated probes in A+A PHENIX p+p p0 + X • NLO pQCD F. Aversa et al. Nucl. Phys. B327, 105 (1989) • CTEQ5M pdf/PKK frag • Scalesm=pT/2, pT, 2pT m=pT/2 m=2pT

  10. Transverse Dynamics • The ability to access “jet” physics also clearly anticipated in RHIC design manual • (vintage: ISAJET) • a new perturbative probe of the colliding matter • Most studies to date have focused on single-particle“high pT” spectra • Please keep in mind: “High pT” is lower than you think

  11. ‘Jets’ at RHIC Jet Axis R • Tremendous interest in hard scattering (and subsequent energy loss in QGP) at RHIC • Production rate calculable in pQCD  a superb probe of density • But strong reduction predicted due to dE/dx ~ path-length (due to non-Abelian nature of medium) • However: • “Traditional” jet methodology very difficult at RHIC • Dominated by the soft background • Investigate by (systematics of) high-pT single particles

  12. Predicting pT Distributions at RHIC • Focus on some slice of collision: • Assume 3 nucleons struck in A, and 5 in B • Do we weight this contribution as • Npart ( = 3 + 5) ? • Ncoll ( = 3 x 5 ) ? • Answer is a function of pT : • Low pT  large cross sections  yield ~Npart • Soft, non-perturbative, “wounded nucleons”, ... • High pT small cross sectionsyield ~Ncoll • Hard, perturbative, “binary scaling”, point-like, A*B, ...

  13. Systematizing our Knowledge Binary Collisions Spectators Participants Participants Spectators b (fm) • All four RHIC experiments have carefully developed techniques for determining • the number of participating nucleons NPARTin each collision(and thus the impact parameter) • The number of binary nucleon-nucleon collisions NCOLL as a function of impact parameter • This effort has been essential in making the QCD connection • Soft physics ~ NPART • Hard physics ~ NCOLL • Often express impact parameter b in terms of “centrality”, e.g., 10-20% most central collisions

  14. Luminosity • Consider collision of ‘A’ ions per bunchwith ‘B’ ions per bunch: • Luminosity Cross-sectional area ‘S’ A B

  15. Change scale by ~ 109 • Consider collision of ‘A’ nucleons per nucleuswith ‘B’ nucleons per nucleus: • ‘Luminosity’ Cross-sectional area ‘S’ A B Provided: No shadowing Small cross-sections

  16. An example of Ncoll ~ A*B scaling Small cross section processes scale as though scattering occurs incoherently off nucleons in nucleus scale as A1.0 in m+A scale as Ncoll ~A*B in A+B 7.2 GeV muons on various targets. M. May et al., Phys. Rev. Lett. 35, 407, (1975)

  17. Ncoll Scaling in d+Au PHENIX PRELIMINARY PHENIX PRELIMINARY PHENIX PRELIMINARY 1/TABEdN/dp3 [mb GeV-2] 1/TABEdN/dp3 [mb GeV-2] 1/TAB 1/TAB 1/TABEdN/dp3 [mb GeV-2] PHENIX PRELIMINARY PHENIX PRELIMINARY 1/TABEdN/dp3 [mb GeV-2] 1/TABEdN/dp3 [mb GeV-2] 1/TAB 1/TAB • single electrons from non-photonic sources agree well with pp fit and binary scaling

  18. Ncoll Scaling in Au+Au • Again, good agreement of electrons from charm with Ncoll 1/TABEdN/dp3 [mb GeV-2] 1/TABEdN/dp3 [mb GeV-2] 1/TAA 1/TAA 1/TABEdN/dp3 [mb GeV-2] 1/TABEdN/dp3 [mb GeV-2] 1/TABEdN/dp3 [mb GeV-2] 1/TABEdN/dp3 [mb GeV-2] 1/TAA 1/TAA 1/TAA

  19. Ncoll Scaling for Charm 0.906 <  < 1.042 dN/dy = A (Ncoll) • binary collision scaling of pp result works VERY WELL for non-photonic electrons in d+Au, Au+Au open charm is a good CONTROL, similar to direct photons

  20. Ncoll Scaling for Direct Photons PHENIX Preliminary Vogelsang NLO • Ncoll scaling works to describe the direct photon yield in Au+Au, starting from NLO description of measured p+p yields • N.B. This method of analysis (double ratio of g/p0) shows Ncoll scaling after accounting for observed suppression of p0 yields in Au+Au collisions (to be discussed next)

  21. Another Example of Ncoll Scaling • PHENIX (Run-2) data on p0 production in peripheral collisions: • Excellent agreement between PHENIX measured p0’s in p+pandPHENIX measured p0’s in Au-Au peripheralcollisions scaled by the number of collisionsover ~ 5 decades PHENIX Preliminary

  22. Central Collisions Are Profoundly Different Q: Do all processes that should scale like A*B do just that? A: No! Central collisions are different .(Huge deficit at high pT) • This is a cleardiscoveryof new behavior at RHIC • Suppression of low-x gluons in the initial state? • Energy loss in a new state of matter?  PHENIX Preliminary

  23. Systematizing Our Expectations Describe in terms of scaled ratio RAA= 1 for “baseline expectations”> 1 “Cronin” enhancements (as in proton-nucleus)< 1 (at high pT) “anomalous” suppression no effect 

  24. Ratio: Au+Au / ( p+p Expectation )

  25. Is The Suppression Always Seen at RHIC? d+Au results from presented at a press conference at BNL on June, 18th, 2003 • NO! • Run-3: a crucial control measurement via d-Au collisions

  26. Is The Suppression Unique to RHIC? • Yes- all previous nucleus-nucleus measurements see enhancement, not suppression. • Effect at RHIC is qualitatively new physics made accessible by RHIC’s ability to produce • (copious) perturbative probes • (New states of matter?) • Run-2 results show that this effect persists (increases) to the highest available transverse momenta • Describe in terms of scaled ratio RAA= 1 for “baseline expectations” SPS 17 GeV ISR 31 GeV RHIC 200 GeV • Demonstrates importance of in situ measurement of requisite baseline physics!

  27. First Conclusion • The combined data from Runs 1-3 at RHIC on p+p, Au+Au and d+Au collisions establish that a new effect (a new state of matter?) is produced in central Au-Au collisions Au + Au Experiment d + Au Control Experiment Final Data Preliminary Data

  28. High pT Particle Production in A+A Intrinsic kT , Cronin Effect Shadowing, EMC Effect Hard-scattering cross-section c a b d (Slide courtesy of K. Filimonov) Parton Distribution Functions Partonic Energy Loss Fragmentation Function

  29. Energy Loss of Fast Partons • Many approaches • 1983: Bjorken • 1991: Thoma and Gyulassy (1991) • 1993: Brodsky and Hoyer (1993) • 1997: BDMPS- depends on path length(!) • 1998: BDMS • Numerical values range from • ~ 0.1 GeV / fm (Bj, elastic scattering of partons) • ~several GeV / fm (BDMPS, non-linear interactions of gluons)

  30. QCD Analog of the LPM Effect E0 E • When can a gluon be considered radiated? • When it’s about a wavelength away from source (LPM = Landau-Pomeranchuk-Migdal) • Coherent (reduced!) emission when second scatter occurs “before” gluon is radiated • Gyulassy-Wang obtained for a parton crossing a series of static scattering centers • But!Baier-Dokshitzer-Mueller-Peigne-Schiff (BDMPS) noted: • QCD  non-Abelian • Gluon FSI LESS destructive interference • Non-linear energy loss (Slide courtesy of M. Chiu)

  31. Exceedingly High Densities? Both • Au+Au suppression (I. Vitev and M. Gyulassy, hep-ph/0208108) • d+Au enhancement(I. Vitev, nucl-th/0302002) understood in an approach that combines multiple scattering with absorption in a dense partonic medium • Our high pT probeshave been calibrateddNg/dy ~ 1100e > 100 e0 (!) d+Au 25% (50% ?) Au+Au

  32. Current Status (Runs 1-3) time • Established: • p+p: • Quantitative description of perturbative probes • d+Au: Role of • initial state effects • multiple scattering • Au+Au: • (Strong) role of final state effects • Quantitative measure of parton energy loss in a dense (expanding) medium • To do: Develop same quantitative understanding of • Angular correlations of jet partners • Flavor composition of “jets” p+p d+Au Au+Au

  33. Further Evidence GONE GONE Pedestal&flow subtracted Df • STAR azimuthal correlation function shows ~ complete absence of “away-side” jet • Surface emission only (?) • That is, “partner” in hard scatter is absorbed in the dense medium

  34. Path-Length (L) Effects? y x Back-to-back suppression out-of-plane stronger than in-plane Effect of path length on suppression is experimentally accessible pTtrigger=4-6 GeV/c, 2<pTassociated<pTtrigger, ||<1 Out-of-plane 3/4 /4 in-plane -3/4 -/4

  35. Baryons Are Different • Results from • PHENIX (protons and anti-protons) • STAR (lambda’s and lambda-bars) indicate little or no suppression of baryons in the range ~2 < pT < ~5 GeV/c • One explanation: quark recombination (next slide)

  36. Recombination • The (normal) in vacuofragmentationof a high momentum quark to produce hadrons competes with the (new)in mediumrecombinationof lower momentum quarks to produce hadrons • Example: • Fragmentation: Dq→h(z) • produces a 6 GeV/c pfrom a 10 GeV/c quark • Recombination: • produces a 6 GeV/c pfrom two 3 GeV/c quarks • produces a 6 GeV/c protonfrom three 2 GeV/c quarks Fries, et al, nucl-th/0301087 Greco, Ko, Levai, nucl-th/0301093 ...requires the assumption of a thermalized parton phase... (which) may be appropriately called a quark-gluon plasma Fries et al., nucl-th/0301087 Lepez, Parikh, Siemens, PRL 53 (1984) 1216

  37. Recombination Extended The complicated observed flow pattern in v2(pT) for hadronsd2n/dpTdf ~ 1 + 2 v2(pT) cos (2 f) is predicted to be simple at the quark level underpT → pT / n , v2 → v2 / n , n = (2, 3) for (meson, baryon) if the flow pattern is established at the quark level Compilation courtesy of H. Huang

  38. Further Extending Recombination • New PHENIX Run-2 result on v2 of p0’s: • New STAR Run-2 result on v2 for X’s: • ALL hadrons measured to date obey quark recombination systematics(!) PHENIX Preliminary p0 X STAR Preliminary

  39. Recombination Challenged • Successes: • Accounts for pT dependence of baryon/meson yields • Unifies description of v2(pT) for baryons and mesons • Challenged by • “Associated emission” at high pT • Can the simple appeal of Thermal-Thermal correlations survive extension to Jet-Thermal ?

  40. Does charm flow? PHENIX PRELIMINARY • Is partonic flow realized? • v2 of non-photonic electrons indicates non-zero charm flow in Au+Au collisions • Uncertainties are large • Definite answer: Au+Au Run-04 at RHIC!(on tape)

  41. J/Y Measurements To Date • p+p results: • ~comparable to other hadron facilities (especially at low pT) • Au+Au results: • A limit only • To be addressed in Run-4 (on tape) • An entire program of charmonium physics is just getting underway

  42. Run-3 J/Y’s in d+Au • 2.7 nb-1 d+Au • Providesclear J/Y signals • With modest discrimination power to test shadowing models • A clear indication of the much greater x2 range made availableby RHIC • A clear need for • 20 nb-1 : shadowing • 200 nb-1 : Y’, Drell-Yan • >200 nb-1 : U’s • Measurements beyond ~20 nb-1 require RHIC II luminosities SOUTH ARM NORTH ARM Vogt, PRL 91:142301,2003 Kopeliovich, NP A696:669,2001

  43. Direct photons: centrality dependence PHENIX Preliminary PbGl / PbSc Combined PHENIX Preliminary PbGl / PbSc Combined PHENIX Preliminary PbGl / PbSc Combined PHENIX Preliminary PbGl / PbSc Combined PHENIX Preliminary PbGl / PbSc Combined PHENIX Preliminary PbGl / PbSc PHENIX Preliminary PbGl / PbSc Combined PHENIX Preliminary PbGl / PbSc Combined PHENIX Preliminary PbGl / PbSc Combined 30-40% Central AuAu 200 GeV 1 + (g pQCD x Ncoll) / gphenix backgrd Vogelsang NLO 1 + (g pQCD x Ncoll) / gphenix backgrd Vogelsang 1 + (g pQCD x Ncoll) / gphenix backgrd Vogelsang NLO 1 + (g pQCD x Ncoll) / gphenix backgrd Vogelsang NLO 1 + (g pQCD x Ncoll) / gphenix backgrd Vogelsang NLO 1 + (g pQCD x Ncoll) / gphenix backgrd Vogelsang NLO 1 + (g pQCD x Ncoll) / gphenix backgrd Vogelsang NLO 1 + (g pQCD x Ncoll) / gphenix backgrd Vogelsang NLO 1 + (g pQCD x Ncoll) / gphenix backgrd Vogelsang NLO 20-30% Central AuAu 200 GeV 80-92% Central AuAu 200 GeV 0-10% Central 200 GeV AuAu 50-60% Central AuAu 200 GeV 10-20% Central 200 GeV AuAu 60-70% Central AuAu 200 GeV 70-80% Central AuAu 200 GeV 40-50% Central AuAu 200 GeV direct photons are not inhibited by hot/dense medium rather shine through consistent with pQCD thermal photons: reduction of systematic uncertainties is essential

  44. CGC + Hydro + Jets • A beautiful example of the convergence between • CGC as a description of the initial state • Hydrodynamics as a description of the bulk matter evolution • Jets as a probe of same • T. Hirano and Y. Nara, nucl-th/0404039: • What measurements can we perform at RHIC to test the assumptions of CGC initial state? (p+A measurements)

  45. A Striking Connection • We’ve yet to understand the discrepancy between lattice results and Stefan-Boltzmann limit: • The success of naïve hydrodynamics requires very low viscosities • Both are predicted from ~gravitational phenomena in N=4 supersymmetric theories:

  46. Summary t0 L h h • Evidence for bulk behavior (flow, thermalization): unequivocal • Evidence for high densities (high pT suppression): unequivocal (Control measurement of d+Au essential supporting piece of evidence) • The same • initial state conditions • time evolution of density needed to explain hydrodynamic flow are obtained by measurements with perturbative probes • OUR FIRST TENTATIVE STEPS TOWARD “CONCORDANCE” • Strong suggestions of recombination at the (bulk!) quark level: • scaling of v2 based on quark content • pT dependence of meson/baryon ratios • Again, perturbative probes may prove critical test of the model • What remains? • (Much) more robust quantitative understanding • Quantitative understanding of “failures” (e.g., HBT) • Direct evidence for deconfiment leading to COMPLETE CHARACTERIZATION OF THE NEW STATE OF MATTER FORMED AT RHIC ?

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