Jets in Nuclear Collisions: Experimental Aspects

1. Jets in Nuclear Collisions: Experimental Aspects Peter Jacobs CERN and Lawrence Berkeley National Laboratory Lecture 1 Jets in Nuclear Collisions

2. Jets in Nuclear Collisions • Introduction: jets in elementary collisions • what is a jet? • pdfs and fragmentation functions • characteristics of gluon, light quark and heavy quark jets • Hard processes in nuclear collisions • nuclear geometry and scaling rules • experimental issues: collider parameters, luminosity • Partonic energy loss and heavy ion collisions • leading hadrons • correlations • what have we learned? • Open questions and future prospects at RHIC and LHC Lecture 1 Lecture 2 See lectures by Nestor Armesto This is not a complete survey of jet production in nuclear collisions. Several important topics have been omitted; see also other lectures at this meeting. Jets in Nuclear Collisions

3. References • 1. Particle Data Group topical reviews http://pdg.lbl.gov/2004/reviews/contents_sports.html • 2. QCD and jets: CTEQ web page and summer school lectures http://www.phys.psu.edu/~cteq/ • 3. Handbook of Perturbative QCD, Rev. Mod. Phys. 67, 157–248 (1995) • http://www.phys.psu.edu/~cteq/handbook/v1.1/handbook.ps.gz • 3. RHIC overview: • P. Jacobs and X. N. Wang, Prog. Part. Nucl. Phys. 54, 443 (2005) • 4. RHIC experimental white papers • BRAHMS: nucl-ex/0410020 • PHENIX: nucl-ex/0410003 • PHOBOS: nucl-ex/0410022 • STAR: nucl-ex/0501009 • 5. LHC Yellow Reports Jets in Nuclear Collisions

4. Jets in Nuclear Collisions • Introduction: jets in elementary collisions • what is a jet? • pdfs and fragmentation functions • characteristics of gluon, light quark and heavy quark jets • Hard process in nuclear collisions • nuclear geometry and scaling rules • experimental issues: collider parameters, luminosity • Partonic energy loss and heavy ion collisions • leading hadrons • correlations • what have we learned? • Open questions and future prospects at RHIC and LHC This section concerns jet phenomenology in general and is not specific to heavy ion collisions. However, it provides important background for interpreting jet-related measurements in nuclear collisions at RHIC and the LHC… Jets in Nuclear Collisions

5. as(Q2) Q2 high Q2 low Q2 1. Jets in elementary collisions Running of as  no free quarks or gluons The origin of jets: high energy quarks and gluons dress themselves in a spray of hadrons Jets in Nuclear Collisions

6. e+e- jets at LEP Jets in Nuclear Collisions

7. e+p  e+jet at HERA Jets in Nuclear Collisions

8. p+pbar  dijet at Tevatron Jets in Nuclear Collisions

9. p+pbar  (e+4 jets+missing ET) at Tevatron Hadronic collisions are complicated! Jets in Nuclear Collisions

10. Jet and hadron production in hadronic collisions  lectures by Nestor Armesto • pQCD factorization: • parton distribution fn fa/A • + partonic cross section shat • + fragmentation fn Dh/c Jets in Nuclear Collisions

11. Q2 evolution of pdf and fragmentation fns PDFs and fragmentation functions are not calculable ab initio in pQCD They are essentially non-perturbative in origin (soft, long distance physics) and must be extracted from data at some scaleQ02 pQCD then specifies how PDFs and fragmentation functions evolve fromQ02to any other scaleQ2(DGLAP evolution for pdfs) Jets in Nuclear Collisions

12. Q2 evolution Parton distribution functions • PDFs extracted from global fits to wide array of data: • DIS (e,m,n+nucleon); Drell-Yan; W; direct photon production Jets in Nuclear Collisions

13. Nuclear modification of PDFs (shadowing) Eskola, Kolhinen and Salgado, Eur.Phys.J.C6, 61 (1999) Ratio of pdf in Pb relative to pdf in proton (Q2=2.25 GeV2) • based on DIS and Drell-Yan production off nuclear targets • include momentum and baryon number conservation • other approaches may yield larger effects (Wang and Li) x=0.01  pT~1 GeV at RHIC: gluon shadowing ~20% deficit for moderate Q (~moderate pT) in Au+Au (moderate = beyond CGC region of low x and low Q2) Jets in Nuclear Collisions

14. (or pT, related via ) Partonic cross section: dimensional argument Berman, Bjorken and Kogut, Phys. Rev. D4, 3388 (1971) High energy limit: scattering of massless, structureless particles (partons) Only energy scale is s Then cross sections must then exhibit scaling behavior: Jets in Nuclear Collisions

15. g g  g g q q’  q q’ q g q g q qbar q’ qbar’ g g  q qbar q g q g q q  q q q qbar  g g q qbar  q qbar q qbar  g g q qbar  g g J. F. Owens, Rev. Mod. Phys.59, 465 (1987) Table from B. Mueller, Hard Probes 04 LO-pQCD Cross Sections Jets in Nuclear Collisions

16. Binnewies, Kniehl and Kramer, Z.Phys.C65, 471 Kniehl, Kramer and Pötter, Nucl Phys B582, 514 S. Kretzer, Phys. Rev. D62, 054001 <zhadron> q,gp NLO, Q02=2 GeV2 q,gK NLO, Q02=2 GeV2 <zleading,jet> Dap(z,Q02) DaK(z,Q02) valence, sea refer to quark content of hadron z=Eh/Ejet z Fragmentation functions g g • Distributions are rather “flat”  significant consequences! • fixed ETjet: hardest hadron has <z>~0.3 large fluctuations in jet energy fraction carried by charged particles (= poor energy resolution for “charged jets”) • fixed pThadron: recall that parton pT spectrum ~ 1/pTn (n~6-7 at RHIC) • cheapest way to make hadron at fixed pT: low energy jet which fragments hard • trigger bias for inclusive high pT hadron spectrum: <z>~0.7 ! Jets in Nuclear Collisions

17. Fragmentation function (xE=Ehadron/Ejet) Light Quark vs Gluon Jets uds jet: e+e- Z0hadrons gluon jet = jet recoiling against heavy quark jet pair tagged by secondary vertices G. Abbiendi et al (OPAL), Eur. Phys. J. C11, 217 (1999) ETjet~40 GeV pT wrt jet axis for xE<0.1 Gluons fragment softer and broader than light quarks Jets in Nuclear Collisions

18. Light Quark vs Gluon Jets cont’d • Gluons fragment softer and broader than light quarks •  additional leading hadron trigger bias: prefers quark to gluon jets Jets in Nuclear Collisions

19. Q e+e-B mesons heavy quarks  heavy mesons http://pdg.lbl.gov/2004/reviews/fragrpp.ps • Heavy quark jets fragment hard into leading heavy mesons • qualitatively different than g/uds p • simple kinematic argument (J.D. Bjorken, Phys Rev D17, 171 (1978)): • finite mass suppresses phase space for additional gluon radiation during hadronization (i.e. additional hadron production) pQCD view: interference effects suppress gluon radiation at <mQ/EQ (“dead cone” effect) Jets in Nuclear Collisions

20. Hadrons to partons: jet reconstruction • How to re-associate hadrons to reconstruct the partonic kinematics? • experiment measures fragments of partons: hadrons and calorimeter towers (clusters of hadrons) • pQCD theory calculates partons • Goal: apply “same” jet clustering algorithm to data and theoretical calculations • no unique prescription Jets in Nuclear Collisions

21. Jet reconstruction cont’d Fermilab Run II jet physics hep-ex/0005012 colinear safety: finite calorimeter threshold misses jet on left? infrared safety: one or two jets? • want same jets at parton/particle/detector levels • independence of algorithmic details (ordering of seeds etc) Two broad classes of algorithms: “kT/Durham”: merge all tracks/energy clusters that are nearby in phase space “cone”: fixed shape; stable energy-weighted maxima around seeds; special rules for merging/splitting kT is vacuum cleaner – good for e+e- Cone less sensitive to background, preferred for hadron colliders Jets in Nuclear Collisions

22. Inclusive jet production vs NLO pQCD http://www-cdf.fnal.gov/physics/new/qcd/ktjets/ktjets.html Good agreement over nine orders of magnitude Jets in Nuclear Collisions

23. NLO: W. Vogelsang p0 charged hadrons Hard processes at RHIC: p+p  hadron+X Good agreement over 8 orders of magnitude Jets in Nuclear Collisions

24. Hard processes at RHIC: p+p  g+X One more check: direct photons Jets in Nuclear Collisions

25. initial state final state Dq Dg pTp (GEV/c) pTp What makes high pT hadrons at RHIC? (s=200 GeV) S. Kretzer, hep-ph/0410219 p+p  p0 + X at mid-rapidity fraction of total • pT<~9 GeV/c: mainly gluon fragments • pT>~9 GeV/c: mainly quark fragments • Initial state (pdf): <x>~0.1-0.2 • Final state (fragm.): <z>~0.6-0.7 LHC very different: measurable cross section dominated by gluon jets Jets in Nuclear Collisions

26. jet parton nucleon nucleon Underlying event CFD: http://www-cdf.fnal.gov/physics/new/qcd/QCD.html p+p dijet event ≠ two well-collimated jets + p+p minimum bias event Jets in Nuclear Collisions

27. Underlying event (cont’d) Minimum bias • Multiplicity density in transverse region grows rapidly from minimum bias with increasing jet pT: • initial and final state radiation • remnants of projectiles Jets in Nuclear Collisions

28. minimum bias pbar+p, 1.8 TeV Dihadron angular distributions • recoil yield grows with increasing trigger pT (birth of dijets) • low pT(trig): transverse yield undershoots minbias (momentum conservation) • high pT(trig): transverse yield overshoots minbias (underlying event) Jets in Nuclear Collisions

29. Jets in Nuclear Collisions • Introduction: jets in elementary collisions • what is a jet? • pdfs and fragmentation functions • characteristics of gluon, light quark and heavy quark jets • Hard processes in nuclear collisions • nuclear geometry and scaling rules • experimental issues: collider parameters, luminosity • Partonic energy loss and heavy ion collisions • leading hadrons • correlations • what have we learned? • Open questions and future prospects at RHIC and LHC Jets in Nuclear Collisions

30. Normalized nuclear density r(b,z): Nuclear thickness function Inelastic cross section for p+A: Nuclear geometry: Glauber theory for p+A • Hard processes with large momentum transfer: • short coherence length  successive NN collisions independent • p+A is incoherent superposition of N+N collisions Jets in Nuclear Collisions

31. sinel for 7 GeV muons on nuclei M.May et al, Phys Rev Lett 35, 407 (1975) A1.00 NA50 Phys Lett B553, 167 A Glauber scaling of hard process cross sections in p(m)+A Experimental tests of Glauber scaling: sDrell-Yan/A in p+A at SPS Small/hard cross sections in p+A scale as A1.0 Jets in Nuclear Collisions

32. b is suitably averaged over impact parameter distribution of events Glauber Scaling for A+A b-dependent nuclear overlap function: Hard process rate for restricted impact parameter range: Number of binary nucleon-nucleon collisions in A+B: Jets in Nuclear Collisions

33. Centrality dependence in A+A Binary collisions weight towards small impact parameter ds/dNch 200 GeV Au+Au • Rule of thumb for A+A collisions (A>40): • 40% of the hard cross section is contained in • the 10% most central collisions Jets in Nuclear Collisions

34. Figure of merit for colliders: luminosity Jets in Nuclear Collisions

35. sy sx Luminosity Bunched beam with Gaussian transverse density n1, n2= particles per bunch f = frequency of bunch collisions Luminosity = number of interactions per unit area per unit time: Jets in Nuclear Collisions

36. Rate of hard process: Luminosity cont’d • Rough estimates for Au+Au luminosity at RHIC: • f ~1/100 ns~107/s • maximum charge/bunch~1011 n~1011/79~109/bunch • sx~sy~1 mm •  L~ 107 * (109)2 /(4p*10-2) cm-2s-1 ~ 1026 cm-2s-1 (~0.1 mb-1s-1) • Actual RHIC Au+Au design luminosity = 2 * 1026 cm-2s-1 • Actual RHIC performance: currently exceeds design by factor ~4 Jets in Nuclear Collisions

37. pT (GeV) Au+Au luminosity history at RHIC (through ’04) analysis now in progress Au+Au results to date are from here Jets in Nuclear Collisions

38. Collider Design Parameters http://pdg.lbl.gov/2004/reviews/accelrpp.pdf Jets in Nuclear Collisions

39. Summary of Lecture 1 • Review of jet phenomenology • pQCD is a precision tool: detailed agreement over many orders of magnitude with cross sections measured for • jets the Tevatron • inclusive hadron and direct photons at RHIC • Trigger biases are large: make sure you understand what you are measuring •  leading hadrons bias • towards hard fragmentation • towards quark jets and against gluon jets • towards more central collisions in A+A Jets in Nuclear Collisions