High-p T probes of QCD matter

1. High-pT probes of QCD matter Marco van Leeuwen, Utrecht University

2. Outline of lectures • QCD in p+p collisions • Energy loss in Heavy Ion Collisions at high-pT • Intermediate pTExtracting medium parameters • Jet reconstructionOutlook to LHC

3. Part I: perturbative QCD in p+p collisions • A few words on experiments • What do we know/understand in p+p • Jet production • Direct photons • Hadron production • Heavy flavour • Experimental notes: luminosity and triggering

4. General QCD references • Particle Data Group topical reviews http://pdg.lbl.gov/2004/reviews/contents_sports.html • QCD and jets: CTEQ web page and summer school lectures http://www.phys.psu.edu/~cteq/ • Handbook of Perturbative QCD, Rev. Mod. Phys. 67, 157–248 (1995)http://www.phys.psu.edu/~cteq/handbook/v1.1/handbook.ps.gz • QCD and Collider Physics, R. K. Ellis, W. J. Sterling, D.R. Webber, Cambridge University Press (1996) • An Introduction to Quantum Field Theory, M. Peskin and D. Schroeder, Addison Wesley (1995) • Introduction to High Energy Physics, D. E. Perkins, Cambridge University Press, Fourth Edition (2000)

5. Heavy Ion references • RHIC overviews: • P. Jacobs and X. N. Wang, Prog. Part. Nucl. Phys. 54, 443 (2005) • B. Mueller and J. Nagle, Ann. Rev. Nucl. Part. Sci. 56, 93 (2006) • RHIC experimental white papers • BRAHMS: nucl-ex/0410020 • PHENIX: nucl-ex/0410003 • PHOBOS: nucl-ex/0410022 • STAR: nucl-ex/0501009 • LHC Yellow Reports

6. Experimental facilities: accelerators Centre-of-mass energies √s: SPS < 20 GeV RHIC 200 - 500 GeV TevaTron 1.9 TeV LHC 5.5 - 14 TeV Note also: SppS 630 GeV

7. Example detector for DIS Detectors are designed for specific measurements Here: forward electromagnetic calorimeters to measure scattered electron in Deeply Inelastic Scattering

8. STAR and PHENIX at RHIC STAR PHENIX PHENIX STAR Large acceptance at mid-rapidity: TPC tracking (coarse) EMCal Some forward Calorimeters Central tracking/calo arms (partial coverage, finely segmented calo) Forward muon arms Focus on rare probes (electrons/photons) General purpose detector (PHOBOS, BRAHMS even more specialised)

9. Hard probes in ALICE EMCal for jet reconstruction • Pb-scintillator, 13k towers • Df = 107, |h| < 0.7 • Energy resolution ~10%/√Eg • Trigger capabilities Barrel: tracking + secondary vertices + PID • Charged particles |h| < 0.9 • Excellent momentum resolution up to 100 GeV/c (Dp/p < 6%) • Tracking down to 100 MeV/c • Excellent Particle ID and heavy flavor tagging PHOS: small acceptance, High granularity EMCal • High resolution PbWO4 crystals • |h| < 0.12, 220 < f < 320 • Energy resolution: DEg/Eg = 3%/Eg Forward muon arm ‘STAR+PHENIX in one’ at LHC

10. QCD and quark parton model At low energies, quarks are confined in hadrons At high energies, quarks and gluons are manifest S. Bethke, J Phys G 26, R27 Asymptotic freedom Running coupling: as grows with decreasing Q2 Confinement, asymptotic freedom are unique to QCD Theory only cleanly describes certaint limits Study ‘emergent phenomena’ in QCD

11. Perturbative QCD: a controlled approximation Parton density function Non-perturbative: distribution of partons in proton Extracted from fits to DIS (ep) data Relatively well-known Matrix element Perturbative component Calculated at NLO (as2) or better Often need resummed logs (e.g. FONLL) Fragmentation function Non-perturbative Measured/extracted from e+e- Convolution: most observables integrate over x, Q2

12. Perturbative QCD processes • Hadron production • Heavy flavours • Jet production • e+e-→ jets • p(bar)+p → jets • Direct photon production Theory difficulty Measurement difficulty

13. Resolved kinematics inDeep Inelastic Scattering small x x = partonic momentum fraction large x DIS: Measured electron momentum fixes kinematics

14. Differential kinematics in p+p Example: p0-pairs to probe low-x Forward pion p+p simulation Second pion hep-ex/0502040 Resulting x-range Need at least two hadrons to fix kinematics in p+p

15. p+pbar  dijet at Tevatron

16. Testing QCD at high energy small x x = partonic momentum fraction large x CDF, PRD75, 092006 Dominant ‘theory’ uncertainty: PDFs DIS to measure PDFs Theory matches data over many orders of magnitude Universality: PDFs from DIS used to calculate jet-production Note: can ignore fragmentation effects

17. Testing QCD at RHIC with jets STAR, hep-ex/0608030 Jets also measured at RHIC NLO pQCD also works at RHIC However: signficant uncertainties in energy scale, both ‘theory’ and experiment Note: kinematic range up to 50 GeV

18. Direct photon basics Production processes Direct (LO): Compton and annihilation Fragmentation (NLO) direct fragment Small Rate: Yield aas Fragmentation: Gordon and Vogelsang, PRD48, 3136 (1993)

19. Experimental challenge: p0gg Below pT=5 GeV: decays dominant at RHIC

20. Direct photons: comparison to theory P. Aurenche et al, PRD73:094007 Good agreement theory-experimentFrom low energy (√s=20 GeV at CERN) to highest energies (1.96 TeV TevaTron) Exception: E706, fixed target FNAL deviates from trend: exp problem?

21. Experimental access to fragmentation g Two Methods in p+p 200GeV Isolation cut ( 0.1*E > Econe(R=0.5) ): identifies non-fragmentation photons Photons associated with high-pT hadron: fragmentation Look at associated photons Triggering leading hadron R Eg g(Isolated)/g(all direct) g(fragment) / g(inclusive) PHENIX, PRL98, 012002 (2007) Only ~10% of g show significant associated hadronic activity

22. QCD NLO resources • PHOX family (Aurenche et al)http://wwwlapp.in2p3.fr/lapth/PHOX_FAMILY/main.html • MC@NLO (Frixione and Webber)http://www.hep.phy.cam.ac.uk/theory/webber/MCatNLO/ You can use these codes yourself to generate the theory curves! And more: test your ideas on how to measure isolated photons or di-jets or...

23. p0 at lower energies C. Bourelly and J. Soffer, hep-ph/0311110 Good description at RHIC energies p0 production at lower energies (ISR, fixed target FNAL) not well described Soft jets at lower √s not well controlled?

24. Light hadron production at RHIC PRL 91, 241803 Star, PRL 91, 172302 Brahms, nucl-ex/0403005 NLO calculations: W. Vogelsang p0 and charged hadrons at RHIC in good agreement with NLO pQCD

25. Baryon production in p+p @ √s=200 GeV Albino, Kniehl, Kramer, Nucl Phys B725, 181 Several sets of fragmentation functions (from e+e-) give large differences for baryon production at RHIC Need to keep track of uncertainties in FF

26. Uncertainty analysis of Fragmentation function Hirai, Kumano, Nagai, Sudo, PRD75:094009 z=pT,h / 2√s z=pT,h / Ejet Full uncertainty analysis being pursuedUncertainties increase at small and large z

27. Global analysis of FF proton anti-proton pions De Florian, Sassot, Stratmann, PRD 76:074033, PRD75:114010 ... or do a global fit, including p+p data Universality still holds

28. Hadron spectra AKK  S. Albino, B. A. Kniehl and G. Kramer, Nucl. Phys. B 725 (2005) 181 KKP  B. A. Kniehl, G. Kramer and B. PotterNucl. Phys. B 597 (2001) 337 DSS  D. de Florian, R. Sassot, and M. Stratmann, Phys.Rev.D75 (2007) 114010 Kretzer  S. Kretzer, Phys. Rev. D 62 (2000) 054001 New data up to pT~14 GeV Latest FF fits describe data reasonably well Extra lesson: don’t ‘just take something’ if you want to do serious physics

29. Non-Photonic Electrons D0 ~factor 2 CDF, PRL 91, 241804 (2003) hep-ex/0609010 Theoretical Uncertainty Band Non-photonic electrons measure charm+bottom Reasonable agreement with theory Note: large scale uncertainties

30. Charm from non-photonic electrons PHENIX electrons from heavy-flavor decays agree with FONLL pQCD calculation STAR electrons, D mesons, muons disagree with FONLL hep-ex/0609010 Theoretical Uncertainty Band

31. Summary: QCD in p+p collisions • Testing universality of PDFs, FFs • Jets, photons mainly depends on PDFs: works well • Baryon fragmentation uncertain • Hadron spectra agree with theory, if you take right FF • Heavy flavour • Large discrepancy between RHIC experiments • Theory also large uncertainty? Spectra measurements integrate over large ranges in x, Q2 Di-hadron measurements can be used to select low-x

32. Intermezzo: Luminosity and all that 2007 Au+Au @ sNN = 200 GeV 2006 pp @ s = 200 GeV Integrated delivered luminosity (pb-1) Integrated delivered luminosity (b-1) Time during run From S. Vigdor, QM2008: Improved Collision Luminosity 2006-8 50 40 30 20 10 0 Simple question: what do these plots mean? (in practical terms) sinel = 42 mb L = 45 pb-1 L=3200 mb-1 shadr = 7b 45 1012* 42 10-3 = 1.9 1012 collisions! 2.2 1010 collisions

33. Event rates Examples from RHIC p+p Au+Au L=3200 mb-1 shadr = 7b L = 45 pb-1 sinel = 42 mb 2.2 1010 collisions 45 1012* 42 10-3 = 1.9 1012 collisions Note <Ncoll> ~ 200 Interaction rate500-1000 kHz Interaction rate 5-20 kHz Recording rates: STAR 100 Hz, PHENIX 5 kHz (?) Need to trigger, i.e. select ‘interesting’ events Rate reduction: 1000-10000 for p+p, 10-100 for Au+Au

34. High-pT triggers Fast detectors (measure up to a few MHz) • Obvious choice: (EM) calorimeter • Two strategies: • Small fiducial, trigger photons • Larger fiducial, trigger p0, ‘jet’ energy For example: Keep all events with a photon > 7 GeV, rate few Hz at RHIC Very suitable for high-pT/hard physics Trigger sees all 1012 events

35. End of lecture I

36. 0in p+p, d+Au M. Russcher RdA centrality dependence PHENIX, B. Sahlmüller nucl-ex/0610036 2005 p+p Measures Cronin, initial state effects STAR gearing up g, p0 in p+p, d+Au Good agreement with NLO pQCD and PHENIX

37. Trigger example

38. fiber optic links wave links Simulated effects of beam cooling for full-energy Au+Au “RHIC-II” goal Intensity x 1.5, e-cooling Intensity x 1.5, full stochastic Present intensity, e-cooling Present intensity, full stochastic Present intensity, no cooling Plan to Implement and Test Stochastic Cooling of Heavy Ion Beams at RHIC (submitted to DOE 12/31/07) • Test combined effect of long. & transverse stochastic cooling for one beam in 2009 run. • If results follow detailed simula-tions, full implementation by 2011. • Simulations  long. + trans. stochastic cooling + 56 MHz SRF for both beam goes ~2/3 way (with present bunch intensity) to RHIC-II projected L at order of magnitude less cost, ~5 years quicker than e-cooling.

39. What Are Jets ? Jet colorless states - hadrons - Fragmentation process outgoing parton Really, jets are what jet algorithm defines them to be! Hard scatter Jets are the experimental signatures of quarks and gluons They are expected to reflect kinematics and topology of partons • Colored partons from the hard scatter evolve via soft quark and gluon radiation and hadronization process to form a “spray” of roughly collinear colorless hadrons →JETS • The hadrons in a jet have small transverse momenta relative to their parent parton’s direction and the sum of their longitudinal momenta roughly gives the parent parton momentum • Keep in mind that there are particles in a jet originating from other partons in the event • Jets manifest themselves as localized clusters of energy

40. Probing differential kinematics Example: p0-pairs to for low-x p+p simulation hep-ex/0502040 d+Au simulation FMS plots Measure gluon density at low x in cold nuclear matter Can distinguish shadowing (suppression of yields) and CGC (‘monojets’) Sensitive down to xg ~ 10-3 (few 10-4 in CGC scenario) FMS group

41. Direct g at high-pT T. Isobe 0-10% Au+Au Nuclear effects + E-loss (frag g) p+p year-5 Quark-gin-medium conversions RHIC is accumulating p+p stats Agrees with NLO pQCD No enhancement in Au+Au

42. Particle Ratios • Ratios extended to 15 GeV/c • The effect of the jet trigger on the ratios is negligible(comparison between Pythia minimum bias and Pythia jet triggered data)

43. π and p Spectra • Spectra compared to NLO pQCD calculations AKK  S. Albino, B. A. Kniehl and G. Kramer, Nucl. Phys. B 725 (2005) 181 KKP  B. A. Kniehl, G. Kramer and B. PotterNucl. Phys. B 597 (2001) 337 DSS  D. de Florian, R. Sassot, and M. Stratmann, Phys.Rev.D75 (2007) 114010 Kretzer  S. Kretzer, Phys. Rev. D 62 (2000) 054001