Angular Correlations at High pt: Craig Ogilvie for the Phenix Collaboration

1. hard-scattered parton from e.g. p+p cone of hadrons gluon radiation p p Angular Correlations at High pt:Craig Ogilvie for the Phenix Collaboration hard-scattered parton during Au+Au hadron distribution softened, broadened? Energy-loss: increased medium-induced gluon-radiation Craig Ogilvie

2. Correlations • 2-particle angular correlation functions • Medium-induced gluon emission within QGP • predicted to be broad angles (>10 deg shown later in talk) • fragmentation angular-width may be broader • correlations at small Df may be broadened • study correlations from peripheral => central reactions • complementary to single-particle pt spectra and g+hadron back-to-back correlations. Craig Ogilvie

3. Outline • Examples of angular correlation data from p+p, e+p • Angle correlations from Au+Au at s1/2 = 130 AGeV • pt dependence • centrality dependence • Simulations of possible sources of angular correlations • work in progress • Measurements for this year-2 • Which observable makes a link between experiment and theory Craig Ogilvie

4. ISR Data p+p s1/2 = 62 GeV • CCOR Collaboration (M. J. Tannenbaum) • Trigger particle (neutral) with pt > 7.0 GeV/c • azimuthal distribution of charged particles • strong back-to-back and near-side emission near-side back-to-back Craig Ogilvie

5. Trigger PT jT PT jT Jet Jet kT Pout Transverse Momentum Within Jet • jT transverse momentum with respect to “jet” axis CCOR Collaboration Phys. Lett. 97B, 163 (1980)) <| jT|> = 400 MeV/c, use as one check for what we observe in HI Craig Ogilvie

6. HERA e+p Angular Distribution Within a Jet • Within a jet • Yield of two particles separated by angle q12 • Transformed to e Yield peaked atsmall q12 , e ~ 1 More complicated variable chosen to match expt. with what can be calculated. Craig Ogilvie

7. Au+Au s1/2 = 130 AGeV • 1.5M events, summer 2000 Phenix data • -20 < collision vertex < 20 cm • Central arm tracks • momenta from drift chamber tracks • 1 < pt < 2.5 GeV/c • Centrality cuts expressed as a % of sint=7.2b • (zero degree energy) vs (charge in beam counters) • Correlation functions • mixed events from similar beam-vertex, centrality • 2-track acceptance cuts on both real, mixed pairs Craig Ogilvie

8. Correlations Presented Today • Both hadrons between 1< pt < 2.5 GeV/c • Two correlations formed • both hadrons in west arm of PHENIX • one hadron in east, west arm of PHENIX • Studied as a function of pt, centrality Craig Ogilvie

9. Possible Causes of Angular Correlation • Elliptic flow, jet fragmentation produce azimuthal correlations • Analysis challenge to extract both • jet fragmentation extends to narrow angles • near-side ~ 0-30 deg • flow extends over full range with a harmonic oscillation Craig Ogilvie

10. Both hadrons in west arm 40-92% Centrality, 1.0 < pt < 2.5 GeV/c Df Near-angle correlation falls more steeply than back-to-back correlation Add correlations by ensuring symmetry near 90o One hadron in west arm, other in east arm Df Craig Ogilvie

11. phenix preliminary 1<pt<2.5 GeV/c (40-92%) symmetric (elliptic flow) fit (poor) stronger near-angle correlation than back-to-back Df phenix preliminary Df Craig Ogilvie

12. Centrality Dependence phenix preliminary 1< pt < 2.5 GeV/c 40 to 92% Df 0 to 5% phenix preliminary • near-angle correlation in central reactions: broader, smaller amplitude • elliptic flow v.small in central reactions Df Craig Ogilvie

13. npart Au+Au Centrality Dependence 1< pt < 2.5 GeV/c phenix preliminary width of correlation broadens for more central reactions systematic errors: how fit changes for different normalization criteria Craig Ogilvie

14. npart First-Order Comparison to p+p for peripheral data, on average both hadrons comparable pt phenix preliminary pp running this year impt. Craig Ogilvie

15. Pt Dependence of Correlation • Fit to full function, display only central 40 –92% 0.2 < pt < 0.5 GeV/c 0.5 < pt < 1.0 GeV/c 1.0 < pt < 2.5 GeV/c near-angle correlation strongest for high-pt Craig Ogilvie

16. Feasible Causes of Near-Angle Correlation(next slides) • Resonance decay leading to correlated particles • Decay of K0s L (in progress, not shown today) • Fragmentation of high-pt parton • Other….? Craig Ogilvie

17. Resonance Study 1) UrQMD Au+Au 2) tag all resonances 3) decay, apply pt cut correlation strength 0.001 due to decay of resonances 5-10 times smaller than data 1.0 < pt < 2.5 GeV/c |h| <0.35 Craig Ogilvie

18. C Hard-Processes PYTHIA 6 p+p at s1/2 = 130 GeV 1.0 < pt < 2.5 GeV/c, |h| <0.35 near-angle correlation stronger than back-to-back small h acceptance reduces back-to-back acceptance for different x1, x2 Df C width of 0.35 rad = 20 deg comparable to periph. Au+Au <pt> scattered parton ~ 3GeV/c Df Craig Ogilvie

19. Possible Reach With Year-2 Data • p+p baseline data • 30-100 times more Au+Au statistics • Higher-pt reach, pt > 5 GeV/c • Asymmetric pt cuts, pt1 > 5 GeV/c, pt2 > 2 GeV/c • better match to transverse momentum within jet • Tag PID of leading hadron, correlate with all others • p0 correlated with others • heading towards g correlated with others • leading p or p • speculative possible sensitivity to gluon vs quark jets Craig Ogilvie

20. Making Connection With E-Loss Theory • Recent calculations, e.g. Baier, Schiff, Zakharov, calculate how much energy is radiated to outside a given cone angle 250 GeV jet 1/3 of DE is radiated > 20o Ann. Rev. Nucl. Sci 2000, 50, p37 Can this formalism calc C(Df)? Do we need new observable that expt and theory can both use Craig Ogilvie

21. Summary • High-pt near-angle correlations slightly stronger than back-angle correlations • well reproduced by Gaussian superimposed on oscillation • width of correlation broadens for more central reactions • Possible causes of near-angle correlation • decay of resonances • factor of 5-10 smaller than observed signal • weak decay of K0s L , in progress • fragmenting hard-physics • needs higher pt reach to be convincing • in this scenario, increasing width, broader fragmentation • open question: medium-induced gluon emission? Craig Ogilvie