Angular Correlation Studies in PHENIX

1. Wolf G. Holzmann for the Collaboration Angular Correlation Studies in PHENIX

2. Why study azimuthal angular correlations ? Flow: Jets: CGC: • Primarily from pressure build-up • Produced early • Reflect conditions in collision zone (energy density etc.) • Provides insights on Saturation Physics • Primarily from gluons at RHIC • Produced early in the collision • Probe hot and dense media that they traverse Correlation studies can provide information On the particle production mechanism, EOS, QGP formation …

3. Features of Angular Correlation Functions (Distributions) Correlation functions or distributions can exhibit asymmetry & anisotropy The latter is mainly determined by the away side (> 90 deg) if you have an asymmetric shape. Asymmetry ( ) Anisotropy ( )

4. How to extract relevant information from correlation Hydro or Transport With large Opacity HIJING Saturation Model Mini-Jets, lead to strong anisotropy and an asymmetry Flow leads to strong anisotropy Jets & Mini-Jets, lead to strong anisotropy and an asymmetry Very different mechanisms can account for similar observables Experimental challenge to disentangle them …

5. Some of the approaches undertaken in PHENIX • Various methods currently exploited in PHENIX: • Angular Distributions with respect to a leading photon • Two Particle Azimuthal Correlations with • A) 2 particles from the same pT range • (fixed pT correlations) • B) 1 particle in a given reference pT range, • the other particle outside in a different • pT bin (assorted pT correlations) • Three Particle Angular Correlations • Cumulants • Reaction Plane (not covered here, please see ShinIchi Esumi’s presentation) Different methods reveal different aspects of the correl. fctns. What can one learn from combining all of these analyses ? Well, relax, lean back and enjoy the ride …

6. raw differential yields trigger  PHENIX Preliminary 2-4 GeV 0 incoming partons associated h Leading Photon Correlations • Select events with a photon of • pt > 2.5 GeV/c. • Assumption: • Mostly ’s from decay of a high pt  (leading particle) Build distributions in delta  -space of the charged hadrons relative to the trigger photons. • black = pair distribution • green = mixed event pair distribution • purple = bkg subtracted distribution Task: Look at the relatively “clean” environment of p-p where we Think we know there are jets and relate this to Au-Au data …

7. Leading Correlations in p-p and Au-Au P-P Au-Au (*) (*) • Au-Au also shows jet like features • strong near angle correlation • broader and reduced away side • P-P shows jet like features: • strong near angle correlation • broader and reduced away side Interestingly, the Au-Au shows many of the same features of pp. Thus being indicative of the presence of jets.

8. (*) (M. Chiu for the PHENIX Collaboration, QM2002 proceedings, nucl-ex/0211008) Per Trigger quantification … In order to quantify the effect one can retrieve a “per trigger yield” (since one is actually looking at an angular distribution). Fit with pythia + cos(2phi) and look at the result. Fit results of the jet-like amplitude (*) (*) Observed effect can be well described by a fit to PYTHIA expect. and a harmonic ( cos(2phi) ) term. Within the errors (statistical only) no strong centrality dependence

9. Preliminary Preliminary Preliminary Preliminary Preliminary Preliminary One observes asymmetries and anisotropies and can probe them The latter behave consistent with the idea of jet-like correlations What do angular correlation functions reveal in addition opp. charge small delta eta same charge large delta eta “fixed” pT correlations 2 particles within the same bin pT pT One can further tighten the case by looking at the angular correlation functions and their characteristics (P. Constantin, QM2002 poster)

10. jT PT jT Jet Jet kT Pout Near angle width vs pT Jet topologies … j=400 MeV PHENIX PRELIMINARY (P. Constantin, QM2002 poster) Again, the Au-Au data seems to follow the trend of the p-p, i.e. within the errors no apparent broadening as compared to p-p is observed.

11. Df13 Df12 p3 Df23 p2 Jets and flow correlations look entirely different in this variable. Good way to disentangle the relative contributions of jets and flow Another Approach: Three Particle Correlations p1 Since jets cluster in phase space and if the average jet multiplicity > 2 correlating three particles will enhance the jet signal over the background in regions were the signal is small. Flow: Jets: geometric mean “bump” Y3=(Df12 x Df23 xDf13)1/3 large near angle enhancement “dip”

12. HIJING HIJING FLOW FLOW fit to data fit to data Provide additional information by comparing to 2 particle correlation Next step: Quantify the observed effect and compare to other methods used. Data exhibits the same features PHENIX PRELIMINARY For lower pT the data is fairly well described by a pure flow case, while at higher pT the data lies in between the cases of pure flow and HIJING (jet-dominated) thus indicating an admixture of both mechanisms. Centrality 30-60%

13. pT (ref) pT Asymmetries vs Anisotropies “assorted” pT correlations 1 part within ref. bin the 2nd part. out- side illustrative sketch illustrative sketch PHENIX PRELIMINARY assorted correlation functions Asymmetry sensitive to the choice of reference pT range Anisotropy largely insensitive … Q: Can the anisotropies reveal additional information on jets?

14. Strategy • Need to establish a method of comparison between the v2 values • of Au-Au and p-p (d-A). • One way of testing this: • Establish a scaling relationship in Au-Au first, then compare to pp,dA • Use v2(pT)/v2(int) (scale the differential by the integral v2, • Idea: account for centrality) • -Compare to the other methods currently exploited

15. Scaling in Au-Au not scaled v2(pT) scaled v2(pT)/v2_int Interestingly enough, the differential v2(pT) scales with the integral v2! Promising point to start from. Comparison with pp currently underway …

16. Conclusion and Outlook • PHENIX has a wide variety of correlation measurements. • While each of these measurements targets a specific aspect of angular correlations, when combined reveal a wealth of information about the transverse dynamics at RHIC • The correlation methods show asymmetry and anisotropy • The asymmetries are consistent with the presence of jets and show striking similarities to pp • A scaling relationship for v2 has been established and the differential v2 in AuAu scales with the integral v2. The next step: • The scaling might give a good • handle to probe jet effects in the data and compare pp to AA • Comparisons are currently underway … still many more to come, so stay tuned