1. DOE Review Jan 31, 2004 The Prompt Probes Program at RHIC Paul Stankus Oak Ridge National Laboratory

2. DOE Review Jan 31, 2004 What is really new at RHIC? First Shown in 1996 Early hand-drawn version of a now-ubiquitous cartoon Highly-excited nuclear matter created in RHIC collisions exhibits bulk final- state properties (energy density, P T slopes, etc.) which are not very different from those seen in SPS collisions. What is new at RHIC is the availability of prompt probes: high/mid-Q 2 processes create well-defined QCD objects on very short time scales, which then experience the excited medium from its earliest stages.

3. DOE Review Jan 31, 2004 A Pallet of Prompt Probes q: fast color triplet g: fast color octet Q: slow color triplet QQbar: slow color singlet/octet Virtual photon: colorless Real photon: colorless Unknown Medium Induced gluon radiation? Energy Loss? Dissociation? Controls Version 1997Version 2003 The most general way to classify QCD probes is by speed and color multiplet; different combinations give rise to different classes of high-Q 2 observables:

4. DOE Review Jan 31, 2004 High-P T Singles Phys. Rev. Lett. 88, 022301 (2002) (PWS PPG Chair) Phys. Rev. Lett. 91, 072301 (2003) (TCA PPG Member) The past is prologue: My hand- drawn slide from a 1997 PHENIX collaboration meeting at which the prompt-probes/hard-scattering program was first introduced (”sold”) to the collaboration in full multi-year detail. Done

5. DOE Review Jan 31, 2004 High-P T Pairs AuAu (60-90%) correlation function In Progress Method 1: Correlation Function Fitting (J. Rak, N. Ajitanand QM04) Method 2: Absolute Background Subtraction (M. Reuter QM04) PWS to chair PPG

6. DOE Review Jan 31, 2004 The 1997 Program, Con’t In Progress Importance of p+p and p+A controls appreciated from the beginning Near Future Far Future

7. DOE Review Jan 31, 2004 Direct Photons from A+A 1 + (  pQCD direct x N coll ) / (  phenix pp backgrd x N coll ) 1 + (  pQCD direct x N coll ) /  phenix backgrd Vogelsang NLO 1 + (  pQCD direct x N coll ) /  phenix backgrd Vogelsang, m scale = 0.5, 2.0 Direct photon “figure of merit”: J. Frantz QM04 First results of direct photons in A+A suggest agreement with basic pQCD rate. (TCA to chair PPG) 0-10% Central The blue curve indicates the  Direct (theory)/  Inclusive (expt) ratio from p+p collisions, essentially ~  Direct /  0. The disagreement with the data is a direct indication of jet quenching independent of initial-state parton effects.

8. DOE Review Jan 31, 2004 Search for Thermal Direct Photons Thermal direct photons, while not prompt probes in the strictest sense, are also of great interest, and must be disentangled from the pQCD source. Predictions hadronic gas QG P sum pQCD Turbide Rapp Gale D. Peressounko QM04 WA98 Pb+Pb Recent achievement of WA98: identify direct photons at low P T in SPS Pb+Pb collisions via their HBT correlation. (Work of Dmitri Peressounko, close supervision by WA98 spokesman TCA.) Strength of HBT peak indicates fraction of  Inclusive which are  Direct.

9. DOE Review Jan 31, 2004 Prompt Probes Near Future ORNL Focus 1: Jet-Induced Pairs J. Rak for N. Ajitanand QM04 A very striking PHENIX preliminary result: the yield of associated mid-P T particles (1- 2.5 GeV/c) per high-P T particle (2.5-4 GeV/c) is not strongly affected by centrality, but the away-side associated particles are spread over wider angles. Jet- and dijet-induced pairs can be measured, either as jet-pair rates or as associated yields, through two separate methods: correlation function fitting and normalized subtraction. Full exploration over (P T A,P T B ), sign, flavor, etc. can map out A+A medium modifications to jet and dijet properties, using p+p and d+A results as controls.

10. DOE Review Jan 31, 2004 ORNL Focus 2: Direct Photons ORNL Focus 3:  Direct + Jet Pairs Challenging; need to identify  Direct for individual photons “Holy Grail” of jet quenching measurements: can reveal complete modification/fragmentation behavior of jets Near-side may reveal induced  radiation Basic pQCD process; p+p, p+A and A+A maps out all initial-state effects (and gluon structure functions as well) Search for thermal sources, induced jet radiation, etc. in A+A once pQCD source is calibrated

11. DOE Review Jan 31, 2004 Other Prompt Probes interests: Flavor-dependence of high-P T singles and pairs. Do “excess” baryons seen in PHENIX come from jets? Can we identify gluon vs quark behavior? New approaches include areogel Ck PID, high-P T K 0 S ->  0 +  0 Very low cross-section processes: continuum dileptons measure Drell-Yan and virtual photon processes; double direct photons are “gold standard” for initial-state K T. Low-x processes. Combination of  Direct in central arm with jet in forward detector isolates initial low-x gluon. Speculative ideas (always need some): W +/- production in p+A and A+A, measure nuclear modification to pdf’s at highest Q 2 ever.

12. DOE Review Jan 31, 2004 Wrap-Up 1.By working in the prompt probes program we take best advantage of what RHIC has to offer over previous expt’s. 2.The prompt probe/hard scattering program has produced some of the most exciting results at RHIC, and ORNL group members have been directly involved in the program from its very earliest days. 3.There are many opportunities for important and interesting prompt probes measurements in the next several years. 4.This is only part of the story so far: Heavy-flavor prompt probes @ RHIC – Cianciolo Prompt probes in ALICE – Awes