1. ETD-HIC July 16-19, 2007 Jet quenching: what's next?1 Jets quenching: what’s next? Peter Jacobs Lawrence Berkeley National Laboratory

2. ETD-HIC July 16-19, 2007 Jet quenching: what's next?2 LRP Phases of QCD: Recommendation #1 1. Our central goal is a dramatic advance in our understanding of QCD Matter, through quantitative comparison of theory and experiment to determine the properties of the strongly coupled Quark-Gluon Plasma discovered in the initial phase of RHIC operations, and through further exploration of the QCD phase diagram at non-zero baryon density where a critical point has been predicted. The essential requirements for the success of this scientific program are therefore our highest priority: Effective utilization of the RHIC facility and completion of the ongoing detector upgrade program; The RHIC II luminosity upgrade, which will enable quantitative study of rare processes; Strong support for the ongoing theoretical studies of QCD matter, including finite temperature and finite baryon density lattice QCD studies and phenomenological modeling, and an increase of funding to support new initiatives enabled by experimental and theoretical breakthroughs. What does this mean?

3. ETD-HIC July 16-19, 2007 Jet quenching: what's next?3 GLV ASW χ 2 fit to R AA 1-pvalue Towards precision: measuring PHENIX QM06 B. Sahlmueller et al, nucl-ex/0701060 R AA constrains theory parameters to ~factor 2

4. ETD-HIC July 16-19, 2007 Jet quenching: what's next?4 Quantitative jet quenching: dihadron correlations at higher p T … Recoil jet clearly seen above background but at suppressed rate differential measurement of`  E  upper bound on qhat trigger recoil ? p T trigger >8 GeV/c Yield per trigger STAR, nucl-ex/0604018

5. ETD-HIC July 16-19, 2007 Jet quenching: what's next?5 Zhang, Owens, Wang and Wang nucl-th/0701045 from inclusive and di-hadon suppression  Consistent minima for two independent measurements

6. ETD-HIC July 16-19, 2007 Jet quenching: what's next?6 RHIC Performance: Run 7 slope Run7 ~ 2 X slope Run4

7. ETD-HIC July 16-19, 2007 Jet quenching: what's next?7 Transport Coefficients RHIC I AuAu 2nb -1 (Recorded?) Systematics dominated. Run-4 AuAu 0.2 nb -1 RHIC I ++

8. ETD-HIC July 16-19, 2007 Jet quenching: what's next?8 Jet in LHC Heavy Ions Pb+Pb at 5.5 TeV: huge kinematic reach P. Jacobs and M. van Leeuwen Nucl. Phys A774, 237 (2006)

9. ETD-HIC July 16-19, 2007 Jet quenching: what's next?9 Jets at RHIC in the fb era (from Jamie Dunlop’s talk on Monday) Jet reco under optimization Data in hand: p+p, Cu+Cu, Au+Au From LHC studies should work for E t >~20-30 GeV N bin projection from p+p # Jets in bin at 40 GeV Run 6 Cu+Cu: ~50 Run 7 Au+Au: ~1000 600 ub -1, 23 pb -1 p+p equiv. RHIC II: ~50, 000 Precise D(z) 2 pb/GeV 40% precision with 0.3 pb -1, half barrel STAR PRL 97 (2006) 252001

10. ETD-HIC July 16-19, 2007 Jet quenching: what's next?10 Fully reconstructed single jets –much reduced geometric bias –Jet shape and fragmentation modified by the medium Observables –“RAA” of jets –Fragmentation function –Acoplanarity of dijets –Jet-  –Jet-Z 0 –Multi jets –… c d a b ATLAS Jet observables

11. ETD-HIC July 16-19, 2007 Jet quenching: what's next?11 Jet reconstruction: generic issues Two broad classes of algorithms: “k T /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 colinear safety: finite calorimeter threshold misses jet on left? infrared safety: one or two jets?

12. ETD-HIC July 16-19, 2007 Jet quenching: what's next?12 Jet reconstruction in heavy ion collisions 100 GeV jet in central Pb+Pb Energy (GeV) Large backgrounds  optimal resolution using small jet cones R~0.3? Complex underlying event fluctuations in heavy ion events: full jet reconstruction is difficult jet trigger is tricky (large background fluctuations)

13. ETD-HIC July 16-19, 2007 Jet quenching: what's next?13 Soft Background in Jet Cones Cone radius R=√(Δη 2 +ΔΦ 2 ) Large cone radius  large background Radius cut of.4 + p T cut  lowers background  > 80% of E jet R Energy in cone R: background and jets Central Pb+Pb Preserve “most” of jet while strongly suppressing bkgd: p T >2 GeV/c R~0.4

14. ETD-HIC July 16-19, 2007 Jet quenching: what's next?14 Small cones and jet splitting But why this huge tail for a mono-energetic jet sample with E jet T =100 GeV?

15. ETD-HIC July 16-19, 2007 Jet quenching: what's next?15 Input Jet Energy [GeV] fraction of events with N reconstructed >1 Jet splitting/Sub-jet summing Jet Energy [GeV] # Jets all particles, R=0.3, p t >2GeV - input - N jets,rec =1 - N jets,rec >=1 highest jet - N jets,rec >=1 mid-cone - N jets,rec >=1 sum Small cone radius splits jets - need a correction pass to sum sub- jets and recover jet energy accurately Internal structure of jet and its modification may also be of great interest… Work in progress

16. ETD-HIC July 16-19, 2007 Jet quenching: what's next?16 k T at a hadron collider: CDF inclusive jets …same performance as modern cone algorithms

17. ETD-HIC July 16-19, 2007 Jet quenching: what's next?17 Infrared safe  measure area using zero-energy ghost particles Potential advantage over cone: smaller effective area, lower integrated background PhysLett B641, 57 (2006) Fast k T

18. ETD-HIC July 16-19, 2007 Jet quenching: what's next?18 Fast k T for LHC Heavy Ions: ATLAS

19. ETD-HIC July 16-19, 2007 Jet quenching: what's next?19 Benchmark measurement: modified fragmentation function MLLA: good description of vacuum fragmentation (basis of PYTHIA) introduce medium effects at parton splitting Borghini and Wiedemann, hep-ph/0506218 Jet quenching  fragmentation strongly modified at p T hadron ~1-5 GeV  =ln( E Jet / p hadron ) p T hadron ~2 GeV Jet quenching

20. ETD-HIC July 16-19, 2007 Jet quenching: what's next?20 Measuremement of modified fragmentation Ratio of purple/red Kinematic reach beyond ~200 GeV 175 GeV jets in ALICE acceptance Jet quenching Dashed line = no jet quenching

21. ETD-HIC July 16-19, 2007 Jet quenching: what's next?21 So what? What is learned by probing it with ~200 GeV jets? is a transport property of a medium at T=200 MeV

22. ETD-HIC July 16-19, 2007 Jet quenching: what's next?22 Evolution of qhat High energy jet  small x Large momentum transfer  large scale (DLA) k=E p q Casalderrey-Solana and Wang, arXiv 0705.1352

23. ETD-HIC July 16-19, 2007 Jet quenching: what's next?23 Evolution of qhat cont’d Casalderrey-Solana and Wang arXiv:0705.1352 Consider jet quenching similar to DIS  jet energy variation probes QGP structure at small and varying x…

24. ETD-HIC July 16-19, 2007 Jet quenching: what's next?24 Frag Fn modification via elastic scattering H. Pirner et al (LHC Final Predictions workshop): Frag Fn modified by scattering in a screened gluon gas (n g ~T 3 ) “evolution” is kinematic in origin ln(1/x) dN/d ln(1/x) Virtuality Q 2 Jet multiplicity

25. ETD-HIC July 16-19, 2007 Jet quenching: what's next?25 Direct measurement of qhat: dijet acoplanarity X.-N. Wang jet,  

26. ETD-HIC July 16-19, 2007 Jet quenching: what's next?26 q,g   + jet : photon E T = parton E T at LO Detailed measurement of modified fragmentation  needs RHIC II luminosity Phys.Rev.C74:034906,2006. Phys.Rev.Lett.77:231-234,1996. Run-4 AuAu 0.2 nb -1 RHIC I AuAu 2 nb -1 RHIC II AuAu 20 nb -1 Direct Photons at RHIC II

27. ETD-HIC July 16-19, 2007 Jet quenching: what's next?27 p+p Pb+Pb // hep-ph/0311131 Direct Photons in LHC Heavy Ions 10 4 /year This is a difficult and limited measurement  cannot be the flagship of the LHC heavy ion program

28. ETD-HIC July 16-19, 2007 Jet quenching: what's next?28 Final remarks Jet quenching is well-established multiple high-p T signatures with large experimental effects  enables detailed quantitative study theory: qualitative successes but quantitative gaps New provocations and speculations: AdS/CFT, Mach cones/whatever,... Qualitatively new opportunities at RHIC II and LHC But the Gee-Whiz Era of RHIC Physics is over now vital to turn our qualitative successes into solid quantitative measurements of hot QCD matter with credible error bars  ongoing, intensive collaboration of experiment and theory