Aspect(s) of “Jet” Production with PHENIX

1. Aspect(s) of “Jet” Production with PHENIX OR Can we really learn about QCD from heavy-ion collisions? Matthew Nguyen Moriond 2009

2. PHENIX @ RHIC Colliding Au+Au, p+p, etc. @ a modest of 200 GeV PHENIX Central Arms |h| < 0.35 PHENIX specializes in rare probes: Photons and Leptons Matt Nguyen -- Moriond 2009

3. Jet Tomography “The medium”: A dense, thermalized, effectively deconfined, strongly coupled Quark Gluon Plasma formed in high energy nuclear collisions Parton beam QGP Brick q Matt Nguyen -- Moriond 2009 By modeling energy loss of hard partons can we determine properties of medium and understand QCD in the soft, collective phase? Can also turn the question around. Can we learn something about hard scattered partons by investigating how they interact with dense matter?

4. The Hard Probe Paradigm In practice we have neither parton beams nor QGP bricks Lifetime, System size order 10 fm Instead use hard scattered partons “Medium modified FF”: Dmed(z) The question of energy loss of fast partons led to the consideration of destructive interference, the QCD LPM effect, previously unsolved QGP A fundamental test of QCD radiation Path-length through the medium Scattering strength [GeV2/fm] Matt Nguyen -- Moriond 2009

5. The Baseline: NLO vs. p+p Collisions 200 GeV Inclusive Direct g Pions Inclusive Isolated PHENIX can identify p0 by direct reconstruction out to > 20 GeV Measure both inclusive and isolated direct photon cross sections NLO pQCD works at RHIC! Matt Nguyen -- Moriond 2009

6. Nuclear Modification RAA = Observed yield over expected yield = Yield in A+A / yield in p+p scaled by the number of binary collisions Head-on Collisions Direct g p0 Strong jet quenching, yield of high pT fragments reduced by factor of 5! To first approx. direct g are unmodified as expected for color neutral objects Matt Nguyen -- Moriond 2009

7. Jet Reconstruction in HI Collisions ~ 21 GeV STAR preliminary pt per grid cell [GeV] η ϕ PHENIX Reconstructed Jet Out-of-cone area Jet Reconstruction in Heavy-Ion Collisions is an extremely active topic Backgrounds from soft collisions are non-trivial even at the LHC, particularly at small z Out-of-cone area Matt Nguyen -- Moriond 2009 ϕ

8. Two-Particle Correlations • Essential features of jet production are evident via azimuthal correlations between particle pairs • In p+p collisions: • Approximate UE as a flat pedestal • Fit double Gaussian + constant to remove pedestal – may be taken as definition • In A+A collsions: • Two-source model: Jet + combinatorial background from soft collisions • Estimate background by event-mixing • Background has it’s own azimuthal correlations due to collective behavior of medium : It flows! Per-Trigger (Conditional) Yield: Yield of Associated Particles Per Trigger Per-Trigger Yield Underlying Event = Combinatorial Bknd + Elliptic Flow

9. Modified Jet Shapes: The Cone Centrality • At intermediate pT, away-side peaks are displaced in central collisions • Collective phenomenon, e.g., shock wave or modification to QCD bremsstrahlung in medium? Di-hadron Correlations Matt Nguyen -- Moriond 2009

10. Modified Jet Shapes: The Ridge Jet shape also modified on the near-side Dh Limited to pT < 4 GeV, same region as away-side cone structure PHOBOS: Ridge correlation extend out to Dh > 4! Matt Nguyen -- Moriond 2009

11. Modified Jet Shapes from Modified pQCD Polosa and Salgado, PRC75, 041901 (2007) • Uses standard perturbative methods (Sudakov Form Factors) calculates shower evolution • Introduce modified splitting functions to account for multiple scattering in medium • Large angle scattering is enhanced, reproducing conical emission Matt Nguyen -- Moriond 2009

12. Modified Leading Log Approach CERN-PH-TH-2005-100 Borghini and Wiedemann MLLA -- Resummation of interference effects in shower evolution, used to calculate D(z) using LPHD z 0.37 0.14 0.05 0.02 0.007 0.002 Very different than “fractional energy loss”, may be Q2 dependent Di-hadron measurements introduce a trigger bias which makes their calculation difficult. For quantitative comparisons we need to look to full jet reconstruction or direct photon correlations where the parton energy is determined Matt Nguyen -- Moriond 2009

13. Direct g Correlations 200 GeV p+p 7 < pTg < 9 GeV, 3 < pTh < 5 GeV Inclusive g-h Decay g-h Direct g-h Inclusive (direct + decay) photon correlations are measured Decay correlations are estimated from measured p0-hadron and h-hadron yields Direct g-h hadrons are obtained by statistical subtraction of subtraction of decay correlations from inclusive:

14. Isolated Direct g Distributions Zhang, Owens, Wang, Wang: arXiv:0902.4000 Yield of hadrons per isolated, direct g Relation to the FF: 1/Ng dNg-h/dzT CTEQ + NLO + KKP To first order p+p baseline well described by NLO: Work being done to quantify scale uncertainties, sensitivity to kT effect, etc. Matt Nguyen -- Moriond 2009

15. Direct g-h in HI Collisions • Expectation in HI depends on model • Surface bias for single hadrons • Tangential Bias for high pT di-hadrons • Diffuse medium/few scattering model: • Punch-through • g-jet surface biased at large zT, but probe progressively further into medium as zT decreases Black Core / Corona vs. Diffuse Medium Single hadron di-hadron g-hadron ZOWW ZOWW  Model of energy loss using effective FF’s arXiv:0902.4000 + ref’s therein Thesis Defense

16. Jet Suppression Opposite Direct g Ratio of Au+Au Yields to p+p expectation At intermediate zTg-h consistent with p0 RAA dominated by surface bias Just getting started, new higher statistics data soon, Will help us push to lower values of zT No isolation cut applied Nuclear Modification Matt Nguyen -- Moriond 2009

17. Conclusions • QCD at high density/temperature accessible at RHIC • Qualitatively new features in jet correlations attest to strong medium modifications to vacuum jet fragmentation • Modeling such effects is challenging and requires better hard probe observables, jet reconstruction and direct photon correlations with high luminosity data from RHIC and large collision energy data from the LHC • g-h data are now available from RHIC and will enable precision studies of jet fragmentation in medium as more statistics are accumulated Matt Nguyen -- Moriond 2009

18. Backup Slides Matt Nguyen -- Moriond 2009

19. Away-side: head vs shoulder Matt Nguyen -- Moriond 2009

20. Di-hadron IAA Df (rad) Matt Nguyen -- Moriond 2009

21. Two Component Fits Matt Nguyen -- Moriond 2009

22. The Ridge and the Cone Matt Nguyen -- Moriond 2009

23. Jet Hadro-chemsitry Matt Nguyen -- Moriond 2009

24. Near and away: ridge vs shoulder Spectra and yields for ridge and shoulder are similar, and show same trend with centrality Matt Nguyen -- Moriond 2009

25. pT Dependence of Di-hadron Corrleations partner pT Dip develops Yield suppressed Yield enhanced trigger pT Jet reemerges Matt Nguyen -- Moriond 2009

26. Photon Sources in Au+Au Matt Nguyen -- Moriond 2009

27. World direct g data (p+p) Aurenche et al. Phys.Rev. D73 094007 (2006) • PHENIX bridges the gap between Tevatron and fixed target data • Highest energy data that removes decay background by direct reconstruction rather than calorimeter response Matt Nguyen -- Moriond 2009

28. Isolated Direct g Cross Section Matt Nguyen -- Moriond 2009

29. Direct g x-sec, Data/Fit Matt Nguyen -- Moriond 2009

30. g-jet in HI Cartoon Matt Nguyen -- Moriond 2009