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. 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 December 6, 2012Zhongbo Kang, LANL2

3. Interesting experimental result - I  A modest nuclear modification in d+Au  Similar modest nuclear modification in Au+Au (at high pt) December 6, 2012Zhongbo Kang, LANL3 arXiv:1208.1234 arXiv:1205.5759

4. Interesting experimental results - II  However, large excess at low pt in Au+Au December 6, 2012Zhongbo Kang, LANL4

5. Interesting experimental results - III  Large excess at low mass (0.2 < M < 0.7 GeV)  Similar results at SPS December 6, 2012Zhongbo Kang, LANL5

6. Hadron production in usual pQCD factorization  Usual hadron production December 6, 2012Zhongbo Kang, LANL6

7. Prompt photon production in p+p collisions  Direct production  Fragmentation component December 6, 2012Zhongbo Kang, LANL7

8. Baseline: works perfect fine with p+p collisions  Comparing to RHIC and LHC experiments December 6, 2012Zhongbo Kang, LANL8

9. How to understand these interesting nuclear modification  Different approaches to incorporate nuclear effects  Nuclear parton distribution functions (nPDFs) December 6, 2012Zhongbo Kang, LANL9

10. Predictions based on nPDFs December 6, 2012Zhongbo Kang, LANL10  Predictions for prompt photon production  in d+Au collisions, isospin effect dominates at high pt  roughly consistent with the data arXiv: 1211.2130

11. 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 December 6, 2012Zhongbo Kang, LANL11

12. 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 December 6, 2012Zhongbo Kang, LANL12 arXiv: 1204.1319

13. 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 December 6, 2012Zhongbo Kang, LANL13

14. Some predictions based on CGC  CGC predictions for RHIC kinematics at forward rapidity December 6, 2012Zhongbo Kang, LANL14 arXiv: 1204.1319

15. Our approach: all kinds of nuclear effects  Cronin effect December 6, 2012Zhongbo Kang, LANL15 J. Cronin, 1975 Cronin ratio: Smaller than one in small pT Larger than one in moderate pT Approach to one in large pT Z. Kang, I. Vitev and H. Xing, 2012

16. Incorporate Cronin effect  Initial-state multiple scattering  Total momentum = pp baseline + nuclear broadening December 6, 2012Zhongbo Kang, LANL16 Z. Kang, I. Vitev and H. Xing, 1209.6030

17. Nuclear shadowing effect  Dynamic shadowing from power correction December 6, 2012Zhongbo Kang, LANL17 Qiu, Vitev, PRL, 2004

18. Generalize to p+A collisions  Power corrections in p+A collisions  At forward rapidity region t-channel dominates (t is small) December 6, 2012Zhongbo Kang, LANL18 Qiu, Vitev, PLB, 2006

19. Parton energy loss in cold nuclear matter  This has been computed in both a GLV-type and higher-twist type formalisms December 6, 2012Zhongbo Kang, LANL19 Xing, Wang, et.al., NPA,2012; Ivan, 2007

20. 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 December 6, 2012Zhongbo Kang, LANL20

21. Incorporate all the cold nuclear matter effects - I  Incorporate these cold nuclear matter effects  Cronin effect:  Shadowing effect: December 6, 2012Zhongbo Kang, LANL21

22. Incorporate all the cold nuclear matter effects - II  Continue … December 6, 2012Zhongbo Kang, LANL22 Energy loss:Isospin:

23. CNM effect: isospin December 6, 2012Zhongbo Kang, LANL23 Strong isospin effect

24. CNM effect: Cronin December 6, 2012Zhongbo Kang, LANL24 Cronin enhancement

25. CNM effect: shadowing December 6, 2012Zhongbo Kang, LANL25 Shadowing suppression

26. CNM effect: energy loss December 6, 2012Zhongbo Kang, LANL26 energy loss suppression

27. Nuclear modification at RHIC December 6, 2012Zhongbo Kang, LANL27 Work well at central and forward rapidities for both photon and hadron. Z. Kang, I. Vitev and H. Xing, 2012

28. Nuclear modification at LHC December 6, 2012Zhongbo Kang, LANL28

29. 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 December 6, 2012Zhongbo Kang, LANL29

30. 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 December 6, 2012Zhongbo Kang, LANL30 Qiu, Zhang, PLB, 2002

31. 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%) December 6, 2012Zhongbo Kang, LANL31 Kang-Qiu-Vogelsang, PRD, NPA, 2009

32. 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? December 6, 2012Zhongbo Kang, LANL32

33. 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 December 6, 2012Zhongbo Kang, LANL33