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Cold nuclear matter effects on dilepton and photon production
Zhong-Bo KangLos Alamos National Laboratory
Thermal Radiation WorkshopRBRC, Brookhaven National LaboratoryDecember 5-7, 2012
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Zhongbo Kang, LANL 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, 2012
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Zhongbo Kang, LANL 3
Interesting experimental result - I A modest nuclear modification in d+Au
Similar modest nuclear modification in Au+Au (at high pt)
December 6, 2012
arXiv:1208.1234
arXiv:1205.5759
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Zhongbo Kang, LANL 4
Interesting experimental results - II However, large excess at low pt in Au+Au
December 6, 2012
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Zhongbo Kang, LANL 5
Interesting experimental results - III Large excess at low mass (0.2 < M < 0.7 GeV)
Similar results at SPS
December 6, 2012
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Zhongbo Kang, LANL 6
Hadron production in usual pQCD factorization Usual hadron production
December 6, 2012
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Zhongbo Kang, LANL 7
Prompt photon production in p+p collisions
Direct production
Fragmentation component
December 6, 2012
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Zhongbo Kang, LANL 8
Baseline: works perfect fine with p+p collisions Comparing to RHIC and LHC experiments
December 6, 2012
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Zhongbo Kang, LANL 9
How to understand these interesting nuclear modification
Different approaches to incorporate nuclear effects Nuclear parton distribution functions (nPDFs)
December 6, 2012
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Zhongbo Kang, LANL 10
Predictions based on nPDFsDecember 6, 2012
Predictions for prompt photon production
in d+Au collisions, isospin effect dominates at high pt roughly consistent with the data
arXiv: 1211.2130
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Zhongbo Kang, LANL 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, 2012
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Zhongbo Kang, LANL 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, 2012
arXiv: 1204.1319
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Zhongbo Kang, LANL 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, 2012
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Zhongbo Kang, LANL 14
Some predictions based on CGC CGC predictions for RHIC kinematics at forward rapidity
December 6, 2012
arXiv: 1204.1319
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Zhongbo Kang, LANL 15
Our approach: all kinds of nuclear effects
Cronin effect
December 6, 2012
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
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Zhongbo Kang, LANL 16
Incorporate Cronin effect Initial-state multiple scattering
Total momentum = pp baseline + nuclear broadening
December 6, 2012
Z. Kang, I. Vitev and H. Xing, 1209.6030
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Zhongbo Kang, LANL 17
Nuclear shadowing effect Dynamic shadowing from power correction
December 6, 2012
Qiu, Vitev , PRL, 2004
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Zhongbo Kang, LANL 18
Generalize to p+A collisions Power corrections in p+A collisions
At forward rapidity region t-channel dominates (t is small)
December 6, 2012
Qiu, Vitev, PLB, 2006
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Zhongbo Kang, LANL 19
Parton energy loss in cold nuclear matter This has been computed in both a GLV-type and higher-twist
type formalisms
December 6, 2012
Xing, Wang, et.al., NPA,2012; Ivan, 2007
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Zhongbo Kang, LANL 20
Recap: all the cold nuclear matter effects All cold nuclear matter effects are centered around the idea of
multiple parton scatteringParton energy lossCronin effectDynamic shadowing
Take a lood again at the p+p baseline
December 6, 2012
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Zhongbo Kang, LANL 21
Incorporate all the cold nuclear matter effects - I Incorporate these cold nuclear matter effects
Cronin effect:
Shadowing effect:
December 6, 2012
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Zhongbo Kang, LANL 22
Incorporate all the cold nuclear matter effects - II Continue …
December 6, 2012
Energy loss:
Isospin:
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Zhongbo Kang, LANL 23
CNM effect: isospinDecember 6, 2012
23
Strong isospin effect
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Zhongbo Kang, LANL 24
CNM effect: CroninDecember 6, 2012
Cronin enhancement
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Zhongbo Kang, LANL 25
CNM effect: shadowingDecember 6, 2012
Shadowing suppression
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Zhongbo Kang, LANL 26
CNM effect: energy lossDecember 6, 2012
energy loss suppression
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Zhongbo Kang, LANL 27
Nuclear modification at RHICDecember 6, 2012
Work well at central and forward rapidities for both photon and hadron.
Z. Kang, I. Vitev and H. Xing, 2012
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Zhongbo Kang, LANL 28
Nuclear modification at LHCDecember 6, 2012
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Zhongbo Kang, LANL 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, 2012
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Zhongbo Kang, LANL 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, 2012
Qiu, Zhang, PLB, 2002
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Zhongbo Kang, LANL 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, 2012
Kang-Qiu-Vogelsang, PRD, NPA, 2009
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Zhongbo Kang, LANL 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, 2012
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Zhongbo Kang, LANL 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, 2012