(K - +  p)/(K + + p) as a measure of gluon versus quark fragmentation

1. (K- +p)/(K+ + p) as a measure of gluon versus quark fragmentation David Morrison Brookhaven National Laboratory for the PHENIX Collaboration

2. the idea, briefly • neither K- norp contain any initial state valence quarks • gluon fragmentation contributes relatively more to their yields than to K+ orp • we can measure (K- +p) /(K+ +p) in PHENIX • ratio as a function of centrality or reaction plane should show differences between gluons and quarks traversing medium

3. energy loss of high pT partons the observed suppression of high pT hadrons in Au+Au at RHIC energies has been widely interpreted as due to energy loss of light quarks by coherent gluon brehmsstralung x q x x c.f., Phys. Rev. Lett. 88, 022301 (2002), Phys. Lett. B519 (2001) 199 q is it possible to measure the difference in the energy loss suffered by quarks and gluons? g

4. fragmenting quarks, gluons Physics Reports 197 (1990), 263 larger s and larger pT leads to larger differences in the quark and gluon contributions to K+ and K- yields

5. why not just measurep/p? • answer: charged particle identification at high pT often relies on Čerenkov. PHENIX uses a RICH for particle ID • one threshold: can be used to separate p from K, p at ~6 GeV/c, p from K, p at ~17 GeV/c • need a second Čerenkov, with a second threshold, to separate kaons from protons

6. X. N. Wang, Phys. Rev C58, 2321 (1998) expectations for Au+Au • at high pT, contribution from gluons decreases; the relative yields of K-,p and should also decrease • parton energy loss differences exaggerate this effect

7. strategies for determining ratio • least direct: measure h-, h+ and 0 spectra; argue that 0  +  -and look at: • R = (h-- 0)/(h+ - 0) • pros: good statistics; unbroken coverage from low to high pT • cons: different systematics between charged hadrons and p0; pT and centrality dependent corrections c.f., talks/posters by J. Jia, D. d'Enterria, S. Mioduszewski

8. another strategy • more direct: detect charged tracks, look for energy in EM calorimeter above minimum-ionizing peak to reduce background from conversion electrons, demand that the RICH not fire • pros: very clean signal, closely related to technique used to ID high pT pions • cons: introduces EM calorimeter cut which may vary with pT, especially for p andp c.f., high pTp+/p- poster by F. Messer

9. a more direct way • most high pT background comes from low momentum conversion electrons and decays masquerading as high pT tracks • use precision tracking to make geometric cuts against background contamination • Kalman filter for PHENIX developed by J. Lajoie • cons: requires study of significantly new track fitting code

10. vertex pointing benefits from PC2 in west arm 0 using EM calorimeter charged hadrons

11. green shows PHENIX p0 spectrum red and blue are PHENIX non-identified charged hadron spectra local power law fits: yield~pTa, and division of fit results 35% uncertainty on R = (h--0)/(h+-0)

12. no way to turn on and off energy loss as in X-N Wang’sp/p plots instead, alter average distance in medium traveled by partons by varying reaction plane increase sensitivity by varying path c.f., PHENIX reaction plane poster by H. Masui

13. Conclusions • at least three viable techniques for measuring high pT charged “not pion” ratios • should be sensitive to energy loss differences between propagating quarks and gluons • approach based on h   borderline sensitivity to see differences • increase sensitivity by varying reaction plane, using other estimates of (K- +p) /(K+ +p)