1. Jet Measurements at RHIC M.L. Miller, MIT 1.The laboratory 2.Expectations 3.Measurements 4.Missing pieces 5.Conclusions

2. RHIC: A dedicated QCD machine QCD: ~95% of the bandwidth  p+  p, d+Au, Au+Au (Cu+Cu next) Access to perturbative regime Runs I-4: 19, 63, 130, 200 GeV Run 5: commission 0.5 TeV p+p running “Mature performance” demonstrated Run 5 (November?): First “long” polarized p+p run Expect ~14 pb -1 STAR (SVT+TPC+EMC) 0 ≤ Φ ≤ 2π -1 ≤ η ≤ 2 PHENIX (DC+EMC+PID) 2 x |Φ| ≤ π/4 |η| ≤ 0.3 No hadron calorimetery!

3. Jet interests at RHIC (I):  p+  p  PHENIX –Well calibrated for leading π 0  STAR –Tevatron RunII jet algorithms –Ellis/Soper k T cluster –Midpoint cone with split/merge –20-30% jet energy resolution, calibration in progress 5<p T (jet)<25 GeV q+g dominates Expect ~14 pb -1  significant “Δg physics” from  p+  p at RHIC run5

4. In QCD Medium  Additional k T  Significant energy loss?  high p T suppression  Sensitive to color properties of medium Jet Interests at RHIC (II): Au+Au  Hard probe  early time  Calculable: pQCD  Abundant at RHIC, LHC k T : “Radiative Corrections”  pre- and post-scattering  di-jet: 

5. Moment Analysis of QCD Matter I. Vitev, nucl-th/0308028 Induced Gluon Radiation  ~collinear  gluons in cone  “Softened” fragmentation Gyulassy et al., nucl-th/0302077

6. Jets and jet surrogates 1) Integral Distributions:, 2) Single Particle Spectra: d  /dp T  R AA, R dA 3) di-hadron correlations: dN/d(  ) 4) Jet reconstruction: d  /dE T, Frag. Func. (p+p, d+Au and peripheral Au+Au) trigger “Trigger”  = 0 Adler et al., PRL90:082302 (2003), STAR near-side away-side

7. Charged jet evolution at RHIC Thrust axis: direction of leading “charged” jet Study Σ p T, N ch vs. thrust axis Smooth transition to di-jet topology with increasing jet p T Agrees well with CDF charged jet study (see red points/curve) Soft physics of underlying p+p event under study at RHIC (but much to do!) Foundation for jet studies in Au+Au via leading charged particles STAR p T >200 MeV CDF p T >500 MeV MLM, CIPANP03 STAR Preliminary

8. But first, some jargon… peripheral (grazing shot) central (head-on) collision participants Centrality: event classes based on mid-rapidity multiplicity Preliminary  s NN = 200 GeV Uncorrected Particle production scales with increasing “centrality”

9. Adams et al., Phys. Rev. Let. 91 (2003) Adler et al., PRL90:082302 (2003), STAR 1/N trigger dN/d(  ) Background subtracted di-hadron Final state jet quenching  p+p 2-jets  Peripheral Au+Au 2-jets  Central Au+Au 1-jet!  d+Au 2-jets Biased towards high-z hadron pairs  not a sensitive measure of modified fragmentation 2 nd order correlations due to elliptic flow of entire event  competing backgroundBut… Normalization applied potentially sensitive to large k T broadening

10. away toward syst. error Stiffer spectrum in cone indicative of jet fragmentation Similar slope in both systems Not absolutely normalized toward nucl-ex/0404010 STAR Preliminary Towards a frag. function in Au+Au Study h ± p T distributions (p T >150 MeV) in toward, away region. Compare p+p to Au+Au Softened spectrum in both systems Au+Au softer than p+p Effect largest for most violent collisions away

11. hadron Getting quantitative: Jet profiles Reconstructed jetsdi-hadron Jet “width” Jet-coplanarity parton

12. j T, k T from p+p jets, di-jets  STAR di-jet =2.3 ±1.1 GeV/c  PHENIX di-h ± = 1.92 ± 0.1 GeV/c  Reasonable agreement with previous di-jet measures Apanasevich et al.,PRD59, 074007 TPC+EMC reconstructed jets  Good agreement between data and Pythia/Lund+Geant  Mean value depends on p T of hadron  p T >2 GeV/c, STAR di-jet =515±50 MeV/c  p T >1 GeV/c PHENIX di-h ± = 510±10 MeV/c nucl-ex/0403031

13. j T and k T in Au+Au from di-hadrons p+p d+Au pp (2.5  p Ttrigg  4.0)  (1.0  p Tassoc  2.5) j T constant with centrality, consistent with p+p value k T increases ~30% from p+p to peripheral Au+Au  consistent with PHENIX d+Au measure Strong centrality dependent k T broadening  large final state radiation Perhaps most interpretable di-hadron measure at RHIC

14. What’s missing? Yields d 2 σ / dM dy (nb GeV -1 ) p+N  h ± + h ± + X E CM =38.8 GeV -0.4<y<0.2 p Tpair <2 GeV/c normalized dσ / d cos(θ * ) cos(θ * ) p+N  h ± + h ± + X E CM =38.8 GeV -0.25<y<0.1 p Tpair <2 GeV/c Lower energy di-hadron invariant mass spectra: NLO re-summation critical Strong scale dependence Do we understand the yields? di-jet/di-hadron M Inv and cos(θ * ) are critical next measures at RHIC hep-th/0110036

15. Conclusions 2-particle correlations Robust jet probe from p+p to central Au+Au Partonic energy loss in dense QCD matter Evidence of softened fragmentation, increased multiplicity in jet in central Au+Au Large, centrality dependent k T Need to measure, compare di-jet, di-hadron yields to NLO QCD Reduce ambiguities Higher p T jets  access to low-z fragments Possible with Run4 data on tape

16. Backup Slides

17. Some added complexity ΔΦ Δη ΔΦ Δη Central Au+Au Periph. Au+Au Correlation strength STAR Preliminary

18. PHENIX preliminary 1/  x E   -4 to -5 CCOR (ISR)  s = 63 GeV Nucl Phys B209 (1982) 1/  x E   -5.3 Fragmentation from p+p di-hadrons  Simple relation

19. k T, j T from p+p di-hadrons J. Rak, Wed. J. Rak, DNP03 di-hadron Statistical Errors Only near-sideaway-side 

20. k T, j T from p+p di-hadrons Statistical Errors Only PHENIX preliminary  |j Ty |  = 359  11 MeV/c  |k Ty |  = 964  49 MeV/c PHENIX preliminary  |j Ty |  = 359  11 MeV/c  |k Ty |  = 964  49 MeV/c J. Rak, DNP03 Good agreement with previous measurements: PLB97 (1980)163 PRD 59 (1999) 074007 di-hadron

21. Disappearance of the away-side Disappearance (at mid-rapidity) is dominated by final state effect(s)! Jet Quenching Adler et al., PRL90:082302 (2003), STAR 2 nd order correlations due to elliptic flow of entire event  competing background  p+p 2-jets  Peripheral Au+Au 2-jets  Central Au+Au 1-jet!  d+Au 2-jets