Two Particle Correlations in PHENIX

1. Ali Hanks July 7th, 2008 Journal Club Two Particle Correlations in PHENIX

2. Journal Club Resources • Journal club page: http://www.nevis.columbia.edu/~ahanks/jclub.html • Previous journal club talks/topics • Links to useful resources • PHENIX focus page: http://www.phenix.bnl.gov/WWW/run/08/focus/ • Very good intro talks on detector/analysis/physics

3. Why Correlations? “jet quenching:” since jets are expected to be the source of high-pT hadrons, depletion of hadrons is presumed to be due to an effect on the high-energy quarks/gluons, or on the hadronization process, or both. But, do the particles really come from jets? Can we see evidence for jets? And are they modified? Difficult Normal jet ID through “energy-in-a-cone” impossible in Au+Au final state. Correlated, close-angle high-pT groups of hadrons are next best proxy, and also reveal details of hadronization.

4. Tracks in PHENIX • Reconstruct tracks using the Drift Chamber: Main purpose: • Precise measurement of the charged particle’s momentum • Gives initial information for the global tracking in PHENIX Acceptance: • 2 arms 90º in f each • ±90 cm in Z • 0.7 units of h Location: • Radial :2.02<R<2.48 m • Angular: • West: -34º < f < 56º • East : 125º < f < 215º More info: Run 4 focus talk

5. Tracks in PHENIX • And the Pad Chambers (PC1 and PC3): • Straight line tracking by space points • Z-coord from PC1 • Pattern in 3D • Verify tracks thru arm for safe part. ID. • Charged particle veto in front of EmCal • Entrance/exit points (RICH,EmCal) for Lvl2 trigger More info: Run 4 focus talk

6. Tracking Basics (the Drift Chamber) Main assumptions: • Track is straight in the detector region • f and a variables defined on the figure • Use hough transform – calculate f and a for all possible combinations of hits and bin those values into hough array – 2D histogram on f and a • Look for local maxima in hough array that surpass the threshold

7. Tracking Basics (the Drift Chamber)

8. The two-source model We assume that all hadrons come from one of two sources: jet fragmentation (prompt) or thermal/flow (multicollisional). Goal: to count same-jet pairs and look at their distribution in relative angle. Particles A from high-pT “trigger” bin Particles B from low-pT “partner” bin Jet 1 Therm Therm Jet 1 Jet 2 Therm Therm Therm Therm The good stuff “Background” Same jet Unrelated jets Jet-thermal Thermal-thermal {

9. Correlation functions From PHENIX AN 263 M. Reuter

10. ( Background expectations º = + x AB A B A B n n n n n 1 To see the same-jet pairs, all we have to do is subtract away the background, i.e. all the other kinds of pairs. • Background pairs distribution should have quadrupole shape • Background pair rate follows from combinatorics ) - Pairs per event Correction for residual multiplicity correlations

11. Methods of background estimation Describing the distribution of background pairs boils down to getting two numbers: the average background rate, and the quadrupole modulation strength. In PHENIX we investigated three independent approaches: • Fitting pairs distribution with background + peaks • Absolute normalization and subtraction • ZYAM: (“Moored-floating”) fix quadrupole strength from other measurements, then fix normalization by match to all pairs

12. Specific case: the ZYAM method In the ZYAM approach we raise the background level until the background meets the data at one point; the remaining jet pairs distribution then have zero yield at minimum. We thus make no assumption about the shape of the jet pairs. This is the method used in PPG032, non-PID high-pT charged hadrons.

13. Summary of method • Getting pairs distribution is not that hard; correlation functions using mixed events provides acceptance correction in fine detail. • Isolating jet pairs is equivalent to identifying non-jet pairs and subtracting them away (duh). Three methods used in PHENIX have different advantages and drawbacks: • Fitting. Can give complete description of near- and away-side peaks, even if they overlap.Requires assumption about peak shape, can confuse wide peaks with backgrounds. • Absolute background normalization. Requires no assumptions about peak shapes or background shapes, and can be used even when full Df acceptance is not available.Requires residual centrality correlation correction. • Moored-floating, PHENIX example is ZYAM approach. Requires no assumption of peak shapes or background rates.Requires v2 measurements, and provides only lower limit on jet pairs.

14. Quick look at results • At high pT consistent with single particle suppression • Low pT reveals medium response to propagating jets • Jet energy shifted to lower energy? • Bulk response to jet? Strong modification to away-side (enhanced yield in region offset from p+p di-jet signal) High pT suppression! (and a little punch through?) Location of away-side shoulder peaks seems independent of pT

15. Closer look • Various methods for trying to quantify medium modification to correlation measurements • Suggest that “shoulder” region is largely bulk-like (meaning similar properties to non-jet particles)

16. Future Journal Clubs • Next few weeks should be decided today • Don’t need to settle on a topic today, just the speakers • Try to limit the focus to one paper (or one relatively focused topic) • Beyond a few weeks: • For new students: no pressure to fully understand any paper you choose • Advanced students (and everybody else): please read the selected papers and be prepared to provide explanations wherever your own expertise overlaps