Lecture I: Introduction and Experimental tests of perturbative QCD

1. Lecture I: Introduction and Experimental tests of perturbative QCD Marco van Leeuwen, Utrecht University Lectures for Helmholtz School Feb/March 2011

2. General QCD references • Particle Data Group topical reviewshttp://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)

3. 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) • Jet quenching reviews:J. Caselderrey-Solana and C. Salgado, arXiv:0712.3443U. Wiedemann, arXiv:0908.2306A. Majumder and M. v. L. arXiv:1002.2206Accardi, Arleo et al, arXiv:0907.3534 • RHIC experimental white papersBRAHMS: nucl-ex/0410020PHENIX: nucl-ex/0410003PHOBOS: nucl-ex/0410022STAR: nucl-ex/0501009 • LHC Yellow Reports

4. What is QCD? From: T. Schaefer, QM08 student talk

5. QCD and hadrons Quarks and gluons are the fundamental particles of QCD (feature in the Lagrangian) However, in nature, we observe hadrons: Color-neutral combinations of quarks, anti-quarks Baryon multiplet Meson multiplet S strangeness I3 (u,d content) I3 (u,d content) Mesons: quark-anti-quark Baryons: 3 quarks

6. Seeing quarks and gluons In high-energy collisions, observe traces of quarks, gluons (‘jets’)

7. How does it fit together? S. Bethke, J Phys G 26, R27 Running coupling: as decreases with Q2 Pole at m = L LQCD ~ 200 MeV ~ 1 fm-1 Hadronic scale

8. Asymptotic freedom and pQCD At high energies, quarks and gluons are manifest At large Q2, hard processes: calculate ‘free parton scattering’ + more subprocesses But need to add hadronisation (+initial state PDFs)

9. Low Q2: confinement a large, perturbative techniques not suitable Bali, hep-lat/9311009 Lattice QCD: solve equations of motion (of the fields) on a space-time lattice by MC Lattice QCD potential String breaks, generate qq pair to reduce field energy

10. Singularities in pQCD (massless case) Soft divergence Collinear divergence Closely related to hadronisation effects

11. Singularities in phase space

12. How to picture a QCD event (As implemented in event generators) Initial hard scattering high virtuality Q2generates high-pT partons Followed by angle-ordered gluonemissions: fragmentation At hadronic scale: hadronisation prescription (e.g. clustering in HERWIG) MC event generators use this picture

13. QCD matter Energy density from Lattice QCD g: deg of freedom Nuclear matter Quark Gluon Plasma Bernard et al. hep-lat/0610017 Tc ~ 170 -190 MeV ec ~ 1 GeV/fm3 Deconfinement transition: sharp rise of energy density at Tc Increase in degrees of freedom: hadrons (3 pions) -> quarks+gluons (37)

14. QCD phase diagram Quark Gluon Plasma (Quasi-)free quarks and gluons Temperature Critical Point Early universe Confined hadronic matter High-density phases? Elementary collisions (accelerator physics) Neutron stars Nuclear matter Bulk QCD matter: T and mB drive phases

15. Heavy quarks Definition: heavy quarks, m >> LQCD Charm: m ~ 1.5 GeV Bottom: m ~ 4.5 GeV Top: m ~ 170 GeV ‘Perturbative’ hadronisation M. Cacciari, CTEQ-MCNet summer school 2008 Complications exist: QCD, EW corrections; quark mass defined in different ways

16. Regimes of QCD Heavy ion physics Asymptotic freedom Dilute, hard scattering Deconfined matter Bulk matter, hot Bound states Hadrons/hadronic matter Baryon-dense matter (neutron stars) Bulk matter, cold

17. Accelerators and colliders • p+p colliders (fixed target+ISR, SPPS, TevaTron, LHC) • Low-density QCD • Broad set of production mechanisms • Electron-positron colliders (SLC, LEP) • Electroweak physics • Clean, exclusive processes • Measure fragmentation functions • ep, mp accelerators (SLC, SPS, HERA) • Deeply Inelastic Scattering, proton structure • Parton density functions • Heavy ion accelerators/colliders (AGS, SPS, RHIC, LHC) • Bulk QCD and Quark Gluon Plasma Many decisive QCD measurements done

18. 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

19. The HERA Collider Zeus H1 The first and only ep collider in the world e± p 27.5 GeV 920 GeV Located in Hamburg √s = 318 GeV Equivalent to fixed target experiment with 50 TeV e±

20. Example DIS events NC: CC: DIS: Measured electron/jet momentum fixes kinematics

21. Proton structure F2 Q2: virtuality of the g x = Q2 / 2 p q ‘momentum fraction of the struck quark’

22. Factorisation in DIS Integral over x is DGLAP evolution with splitting kernel Pqq

23. Parton density distribution Low Q2: valence structure Q2 evolution (gluons) Gluon content of proton risesquickly with Q2 Soft gluons Valence quarks (p = uud) x ~ 1/3

24. p+p  dijet at Tevatron Tevatron: p + p at √s = 1.9 TeV Jets produced with several 100 GeV

25. 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

26. Testing QCD at RHIC with jets STAR, hep-ex/0608030 RHIC: p+p at √s = 200 GeV (recent run 500 GeV) Jets also measured at RHIC NLO pQCD also works at RHIC However: signficant uncertainties in energy scale, both ‘theory’ and experiment

27. e+e-→ qq → jets Direct measurement of fragmentation functions

28. pQCD illustrated fragmentation jet spectrum ~ parton spectrum CDF, PRD75, 092006

29. Note: difference p+p, e++e- e+ + e- QCD events: jetshave p=1/2 √s Directly measure frag function p+p: steeply falling jet spectrum Hadron spectrum convolution of jet spectrum with fragmentation

30. Fragmentation function uncertainties 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

31. 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

32. Heavy quark fragmentation Heavy quarks Light quarks Heavy quark fragmentation: leading heavy meson carries large momentum fraction Less gluon radiation than for light quarks, due to ‘dead cone’

33. Dead cone effect Radiated wave front cannot out-run source quark Heavy quark: b < 1 Result: minimum angle for radiation  Mass regulates collinear divergence

34. Heavy Quark Fragmentation II Significant non-perturbative effects seen even in heavy quark fragmentation

35. Factorisation in perturbative QCD Parton density function Non-perturbative: distribution of partons in proton Extracted from fits to DIS (ep) data Matrix element Perturbative component Fragmentation function Non-perturbative Measured/extracted from e+e- Factorisation: non-perturbative parts (long-distance physics) can be factored out in universal distributions (PDF, FF)

36. Parton kinematics ep DIS: e+e- Know: incoming electron 4-mom Measure: scattered electon 4-mom Reconstruct: exchanged g 4-mom momentum fraction of struck quark Know: incoming electrons 4-mom Measure: scattered quark (jet) directions Reconstruct: exchanged g 4-mom = parton momenta • p+p: direct access to underlying kinematics only via • g, jet reconstruction • Exclusive measurements (e.g. di-leptons, di-hadrons)

37. 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

38. Direct photon basics direct fragment Small Rate: Yield aas LO: g does not fragment,direct measure of partonic kinematics Gordon and Vogelsang, PRD48, 3136 NLO: quarks radiate photons Direct and fragmentation contributionsame order of magnitude ‘fragmentation photons’

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

40. 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?

41. Direct photons LHC (Isolated prompt photons) CMS, arXiv:1012.0799 Good agreement data-theory also in p+p at LHC

42. 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 R Eg Recoil hadron spectra g(Isolated)/g(all direct) PHENIX, PRL98, 012002 (2007) Recoil: pTjet = pTg

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

44. 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...

45. Relativistic Heavy Ion Collider Au+Au sNN= 200 GeV PHENIX STAR RHIC: variety of beams: p+p, d+Au, Au+Au, Cu+Cu Two large experiments: STAR and PHENIX Smaller experiments: PHOBOS, BRAHMS decomissioned Recent years: Large data samples, reach to high pT

46. STAR and PHENIX at RHIC STAR PHENIX PHENIX STAR 2p coverage, -1 < h < 1 for tracking + (coarse) EMCal Partial coverage 2 x 0.5p, -0.35 < h < 0.35 Finely segmented calorimeter + forward muon arm PID by TOF, dE/dx (STAR), RICH (PHENIX) Optimised for acceptance (correlations, jet-finding) Optimised for high-pt p0, g, e, J/y (EMCal, high trigger rates) (PHOBOS, BRAHMS more specialised)

47. Hadron production in p+p and pQCD 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 Perturbative QCD ‘works’ at RHIC energies

48. 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

49. 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 (1kHz), PHENIX 5 kHz (?) Need to trigger, i.e. select ‘interesting’ events Rate reduction: 1000-10000 for p+p, 10-100 for Au+Au

50. 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