J/  in pp, dAu and AuAu

1. J/ in pp, dAu and AuAu Tatia Engelmore Journal Club 5/24

2. Outline • What can be learned from heavy quarks and J/  • J/  production mechanisms • Effects of a medium (dead cone, destruction and recombination) • PHENIX measurements

3. Importance of Heavy Quarks • Probe gluon structure function in nuclei • J/  suppression is important signal of creation of hot and dense matter • Heavy quarks live longer than QGP itself (10^-11 vs. 10^-22 s) and travel far - way to sample plasma • Lose less energy than light quarks due to “dead cone” effect

4. g c g c dNjet(b) = K ∫d^2 r ∑x1fa/A(x1, pT^2, r)x2 fb/B(x2, pT^2, b - r) dab/dt dpT^2dy1dy2 3 Stages of Charm Production • Initial production from hard scattered partons • Quark and gluon structure functions depleted at small x: parton shadowing • After finding gg->cc and qq->cc cross sections, get about 10 charm pairs per 200 GeV AuAu event

5. 3 Stages of Charm Production (Cont’d) 2) Pre-Equilibrium Production 1) high gluon density: rescatter, partial thermalization, produce more cc pairs 2) depends on phase space density of initially produced gluons 3) up to 10% of charm production at RHIC

6. 3 Stages of Charm Production (Cont’d) • Thermal production • Only if temperature > 500 MeV : negligible at RHIC

7. dP = S CF d kp^2 dkP^2   (kP^2 +  ^2 o^2)^2 Dead Cone energy loss • In heavy quarks, the energy loss in a medium depends on o = M/E: • Power distribution spectrum changes by the factor: • To see the effect we plug in the characteristic angle of gluon radiation q = kt/w = (q/ w ^3)^1/4 where q is the typical momentum transfer of scattering gluons. • Get that if the quark energy is greater than M√(q*L^3) then quark mass is irrelevant • This is not the case for RHIC energies though: so heavy quarks lose much less energy than light quarks. (1 + (o/)^2)^-2

8. D0 J/y D- u d r g r r g b g g r b b r r b b r g b b g r g r g b r r g g r b g b g b b J/  Suppression • Debye color screening in medium: if screening radius less than J/  binding radius cc are torn apart into D mesons. • J/  formation prevented just above critical temperature, but still allowed in peripheral collisions • However, J/  also produced by decay of higher mass resonances: this accounts for about 1/3 of total.

9. Other Nuclear Effects • Shadowing: depletion of quarks and gluons at low x - lots in this range, so interact and get kicked up to higher x. Happens when average size of parton exceeds average nucleon separation. • Gluon saturation at small x - color glass condensate, low x depletion • Multiple scattering of gluons before J/ formation (Cronin effect) - pT broadening

10. Channels to look at to understand J/  production • p-p • Provide baseline for J/  production • d-Au • Look at gluon structure function at small x values: shadowing • Study cold nuclear matter effects • Determine initial state for AuAu collisions • AuAu • study energy loss of heavy quarks in medium • Study final state absorption and recombination of J/  : look for suppression factor

11. J/  in dAu • Purpose to look at J/  production in shadowing region and at higher x • Serve as baseline for AuAu measurements: need to understand cold nuclear modification (shadowing and absorption) to factor it out. • Understand primordial distribution of gluons - pre-hadronization conditions • Measurement at √s = 200 GeV

12. Selecting Events • Looked at ee,  channels • For , required 2 tracks in MuID • For e, required track in EMCal and matching hit in RICH • For , subtract combinatoric background from signal using like-sign pairs: 2√N++N-- then fit to Gaussian + exponential to model continuum behavior • For e, take signal - background (sum of like-sign pairs) and look in mass range of 2.6-3.6 GeV

13. Dd/dy = (NJ//dy) ArecBBC(Nevt/(BBC MB* BBC MB)) Ninv = NJ/Cbias(Ncoll) A rec trigNevt(w/w) Obtaining RdA and Differential Cross Sections Where invariant yield is:

14. Finding Efficiencies • For , determine A, reconstruction and trigger efficiencies for pT bins from GEANT simulation of PYTHIA J/  events. MuID and combinatoric background give uncertainty • For e, run GEANT simulation of central arms and trigger response emulation. Also reconstruction efficiency confirmed by studying known photon conversion pairs in data. Uncertainty from run to run efficiencies, yield extraction, and occupancy dependence of efficiency.

15. pp Results Get pp = 2.61  0.2(fit) 0.26(abs) barns

16. Low x2 ~ 0.003 (shadowing region) 0 mb 3 mb dAu Results • Some suppression in the forward direction (deuteron beam direction) • Model by Eskola-Kolhinen-Salgado involving moderate shadowing and some absorption most favored

17. xA More dAu Results • No universal x2 scaling - expect from shadowing (other things going on?) • Definition of : dA = pp*(2A)^ • xF = x1 - x2 • Observe xF scaling but only measured in small range - caused by initial gluon energy loss or energy conservation effect?

18. x2~ 0.1 x2~ 0.01 x2~ 0.003 More Results • pT broadening caused by multiple scattering effects • Found RdA about 1 for negative rapidity, slight suppression at large centrality for midrapidity, and moderate suppression for positive rapidity

19. More Results • EKS model(solid) and FGS model (dashed) • Consistent with Shadowing

20. J/  from AuAu • Important to study because confusion over which effects dominate • Dynamic screening caused by long range confining QCD potential should cause J/  suppression: quarks break apart into DD pairs • But there could also be enhancement from decay of higher energy resonances or D+D->J/  + X • Also account for shadowing and parton saturation • This paper is from Run 2 at √s = 200 GeV

21. Selecting Events • Look at e+e- pairs from J/  decay • Level 2 J/  algorithm: look for electron rings in RICH, matching showers in EMCal. • E/p = 1 and invariant mass higher than 2.2 GeV • Left with 25.9*10^6 minbias AuAu events and 23.4*10^6 passed Level 2 cuts.

22. Signal Counting • Background = like sign pairs • But still combinatoric background in unlike sign pairs • Take unlike sign pairs - like sign pairs for total (signal - background) • Fit to Poisson distribution • Most dielectrons from Dalitz decays, photon conversion, open charm/beauty decays, etc. • Can’t determine if J/  primordial or from feed-down

23. Bd/dy = (N J//dy) (Nmb-evt + (lvl2-effNlvl2-evt)) (acc-effcent) Yield Calculation • Level 2 efficiency determined by counting fraction of reconstructed J/  events found by trigger • Efficiency confirmed with real data: confirmed electron pairs in correct mass range fired trigger, and pp J/  events made it through • Biggest difficulty comes from unknown pt distribution • Less efficiency in central collisions due to overlapping hits

24. Results • Binary scaling disfavored - some absorption • Consistent with sum of production, absorption, and re-creation • Thermal equilibrium also consistent • J/ enhancement due to increased cc coalescence in medium in central collisions not favored by data • Hard to distinguish between different effects

25. Coming Soon - Silicon Upgrades • FVTX will be able to resolve displaced vertices, separate initial J/ from decay product J/ • It can look in forward and backward rapidity regions, can spot open charm better, and better mass resolution for J/ • Can look at smaller x values to understand shadowing and gluon structure function

26. Conclusions • Suppression is evident, but determining causes of it is difficult - hard to distinguish between models • We still have a lot to learn!

27. Extra

29. AuAu

30. 1.2 1.0 0.8 RdA 0.6 0.4 0.2 0 Rapidity dAu

31. x2~ 0.1 Low x2 ~ 0.003 (shadowing region) 0 mb x2~ 0.01 3 mb xA x2~ 0.003 dAu