Cold nuclear matter effects on dilepton and photon production

1. Cold nuclear matter effects on dilepton and photon production Zhong-Bo Kang Los Alamos National Laboratory Thermal Radiation Workshop RBRC, Brookhaven National Laboratory December 5-7, 2012

2. Zhongbo Kang, LANL Outline • Introduction • Nuclear PDFs • Color Glass Condensate (MS-bar) • Our approach on cold nuclear matter effects • Isospin • Nuclear shadowing • Cronin effect • Parton energy loss • Power corrections at low mass (and low pt) • Summary

3. Zhongbo Kang, LANL Interesting experimental result - I • A modest nuclear modification in d+Au • Similar modest nuclear modification in Au+Au (at high pt) arXiv:1208.1234 arXiv:1205.5759

4. Zhongbo Kang, LANL Interesting experimental results - II • However, large excess at low pt in Au+Au

5. Zhongbo Kang, LANL Interesting experimental results - III • Large excess at low mass (0.2 < M < 0.7 GeV) • Similar results at SPS

6. Zhongbo Kang, LANL Hadron production in usual pQCD factorization • Usual hadron production

7. Zhongbo Kang, LANL Prompt photon production in p+p collisions • Direct production • Fragmentation component

8. Zhongbo Kang, LANL Baseline: works perfect fine with p+p collisions • Comparing to RHIC and LHC experiments

9. Zhongbo Kang, LANL How to understand these interesting nuclear modification • Different approaches to incorporate nuclear effects • Nuclear parton distribution functions (nPDFs)

10. Zhongbo Kang, LANL Predictions based on nPDFs • Predictions for prompt photon production • in d+Au collisions, isospin effect dominates at high pt • roughly consistent with the data arXiv: 1211.2130

11. Zhongbo Kang, LANL Color glass condensate approach • At small-x region, Color Glass Condensate approach takes care of gluon saturation effect • An incoming quark scatters with the classical gluon field of the target nucleus and then radiate a photon • Calculation is straightfoward

12. Zhongbo Kang, LANL Structure of divergence • There is a divergence in this naïve formalism • coming from the phase space when photon is radiated collinearly to the parent quark: collinear divergence • Jalilian-Marian, Rezaeian regularize this divergence by a cut-off: if radiated collinearly, then it is absorbed into a quark-to-photon fragmentation function; if photon is well separated from the quark, it remains as a direct contribution • It is okay, it will lead to mismatch if one wants to use the standard PDFs, which is usually extracted based on MS-bar scheme arXiv: 1204.1319

13. Zhongbo Kang, LANL Prompt photon production in MS-bar schme • Use dimensional regularization to regular and separate the divergence • Expression with divergence explicit • Then one sees to avoid large logarithms, it is better to choose factorization scale • A main feature: factorization scale depends on r, the PDFs need to change accordingly when we integrate over all the coordiates

14. Zhongbo Kang, LANL Some predictions based on CGC • CGC predictions for RHIC kinematics at forward rapidity arXiv: 1204.1319

15. Zhongbo Kang, LANL Our approach: all kinds of nuclear effects • Cronin effect Z. Kang, I. Vitev and H. Xing, 2012 J. Cronin, 1975 Cronin ratio: • Smaller than one in small pT • Larger than one in moderate pT • Approach to one in large pT

16. Zhongbo Kang, LANL Incorporate Cronin effect • Initial-state multiple scattering • Total momentum = pp baseline + nuclear broadening Z. Kang, I. Vitev and H. Xing, 1209.6030

17. Zhongbo Kang, LANL Nuclear shadowing effect • Dynamic shadowing from power correction Qiu, Vitev , PRL, 2004

18. Zhongbo Kang, LANL Generalize to p+A collisions • Power corrections in p+A collisions • At forward rapidity region t-channel dominates (t is small) Qiu, Vitev, PLB, 2006

19. Zhongbo Kang, LANL Parton energy loss in cold nuclear matter • This has been computed in both a GLV-type and higher-twist type formalisms Xing, Wang, et.al., NPA,2012; Ivan, 2007

20. Zhongbo Kang, LANL Recap: all the cold nuclear matter effects • All cold nuclear matter effects are centered around the idea of multiple parton scattering • Parton energy loss • Cronin effect • Dynamic shadowing • Take a lood again at the p+p baseline

21. Zhongbo Kang, LANL Incorporate all the cold nuclear matter effects - I • Incorporate these cold nuclear matter effects • Cronin effect: • Shadowing effect:

22. Zhongbo Kang, LANL Incorporate all the cold nuclear matter effects - II • Continue … Energy loss: Isospin:

23. Zhongbo Kang, LANL CNM effect: isospin Strong isospin effect 23

24. Zhongbo Kang, LANL CNM effect: Cronin Cronin enhancement

25. Zhongbo Kang, LANL CNM effect: shadowing Shadowing suppression

26. Zhongbo Kang, LANL CNM effect: energy loss energy loss suppression

27. Zhongbo Kang, LANL Nuclear modification at RHIC Z. Kang, I. Vitev and H. Xing, 2012 Work well at central and forward rapidities for both photon and hadron.

28. Zhongbo Kang, LANL Nuclear modification at LHC

29. Zhongbo Kang, LANL Comparison with the latest ALICE data • reasonable agreement: larger energy loss effect at high pt • So far we are the only model with energy loss: these new data help us to constrain energy loss much better

30. Zhongbo Kang, LANL Several final comments • What about the low pt and low mass dilepton (photon) data? Is it possible to understand (at least partially) the large excess? • Initial state multiple scattering leads to enhancement for low mass dilepton Qiu, Zhang, PLB, 2002

31. Zhongbo Kang, LANL Also an enhancement for low pt photon • Initial state multiple scattering to direct photon production also leads to an enhancement at low pt (70-90%) Kang-Qiu-Vogelsang, PRD, NPA, 2009

32. Zhongbo Kang, LANL Comments • The old was extracted from transverse momentum broadening Drell-Yan data at Fermilab (very old data, also low energy). This parameter is much smaller than those constrained from RHIC data by 3-4 times • If add this new contribution to the A+A cross section, it leads to about 3 times enhancement at A+A collisions. Certainly not be able to describe the PHENIX data (~30 times enhancement) • The remaining is thermal photons?

33. Zhongbo Kang, LANL Summary • There are different approaches to incorporate various cold nuclear matter effects • We clarified a mis-match in a usual widely used CGC formalism for photon production, by providing a formalism in MS-bar scheme • Based on a pQCD formalism, we incorporate so far the Cronin, shadowing, parton energy loss, which give a good description of RHIC and LHC data: parton energy loss should be further constrained • Initial-state multiple scattering could indeed lead to enhancement at low mass and low pt. This might not be enough to explain the observed ~30 times enhancement • Looking forward to the LHC data on both p+A and A+A for low mass lepton pair