Jet Measurements at RHIC

1. Jet Measurements at RHIC M.L. Miller, MIT • The laboratory • Expectations • Measurements • Missing pieces • Conclusions

2. 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 RHIC: A dedicated QCD machine Run 5 (November?): First “long” polarized p+p run • 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! Expect ~14 pb-1

3. Jet interests at RHIC (I): p+p Expect ~14 pb-1 significant “Δg physics” from p+pat RHIC run5 • PHENIX • Well calibrated for leading π0 • STAR • Tevatron RunII jet algorithms • Ellis/Soper kT cluster • Midpoint cone with split/merge • 20-30% jet energy resolution, calibration in progress 5<pT(jet)<25 GeVq+g dominates

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

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

6. Jets and jet surrogates • 1) Integral Distributions: • <pT>, <Nch> • 2) Single Particle Spectra: • d/dpT  RAA, RdA “Trigger”  = 0 • 3)di-hadron correlations: • dN/d() Adler et al., PRL90:082302 (2003), STAR • 4) Jet reconstruction: • d/dET, Frag. Func. • (p+p, d+Au and peripheral Au+Au) away-side trigger near-side

7. Charged jet evolution at RHIC • Thrust axis: direction of leading “charged” jet • Study ΣpT, Nch vs. thrust axis STAR Preliminary MLM, CIPANP03 STAR pT>200 MeV CDF pT>500 MeV • Smooth transition to di-jet topology with increasing jet pT • 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

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

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

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

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

12. jT, kTfrom p+p jets, di-jets • STAR di-jet <kT>=2.3 ±1.1 GeV/c • PHENIX di-h± <kT>= 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 pT of hadron • pT>2 GeV/c, STAR di-jet <jT>=515±50 MeV/c • pT>1 GeV/c PHENIX di-h± <jT>= 510±10 MeV/c nucl-ex/0403031

13. jT and kT in Au+Au from di-hadrons (2.5pTtrigg4.0)(1.0pTassoc2.5) p+p d+Au pp <z><|kTy|> • jT constant with centrality, consistent with p+p value • kT increases ~30% from p+p to peripheral Au+Au  consistent with PHENIX d+Au measure • Strong centrality dependent kT broadening  large final state radiation pp <|jTy|> Perhaps most interpretable di-hadron measure at RHIC

14. What’s missing? Yields d2σ / dM dy (nb GeV-1) normalized dσ / d cos(θ*) p+N  h±+ h ± + X ECM=38.8 GeV -0.25<y<0.1 pTpair<2 GeV/c p+N  h±+ h ± + X ECM=38.8 GeV -0.4<y<0.2 pTpair<2 GeV/c cos(θ*) hep-th/0110036 hep-th/0110036 • Lower energy di-hadron invariant mass spectra: NLO re-summation critical • Strong scale dependence • Do we understand the yields? • di-jet/di-hadron MInv and cos(θ*) are critical next measures at RHIC

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 kT • Need to measure, compare di-jet, di-hadron yields to NLO QCD • Reduce ambiguities • Higher pT jets  access to low-z fragments • Possible with Run4 data on tape

16. Backup Slides

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

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

19. kT, jTfrom p+p di-hadrons Statistical Errors Only di-hadron J. Rak, Wed. J. Rak, DNP03 PHENIX preliminary Df near-side away-side

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

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