cold nuclear matter effects on dilepton and photon production zhong-bo kang los alamos national...
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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
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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Interesting experimental result - I A modest nuclear modification in d+Au
Similar modest nuclear modification in Au+Au (at high pt)
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arXiv:1208.1234
arXiv:1205.5759
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Interesting experimental results - II However, large excess at low pt in Au+Au
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Interesting experimental results - III Large excess at low mass (0.2 < M < 0.7 GeV)
Similar results at SPS
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Hadron production in usual pQCD factorization Usual hadron production
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Prompt photon production in p+p collisions
Direct production
Fragmentation component
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Baseline: works perfect fine with p+p collisions Comparing to RHIC and LHC experiments
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How to understand these interesting nuclear modification
Different approaches to incorporate nuclear effects Nuclear parton distribution functions (nPDFs)
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Predictions based on nPDFs
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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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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
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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
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arXiv: 1204.1319
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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
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Some predictions based on CGC CGC predictions for RHIC kinematics at forward rapidity
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arXiv: 1204.1319
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Our approach: all kinds of nuclear effects
Cronin effect
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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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Incorporate Cronin effect Initial-state multiple scattering
Total momentum = pp baseline + nuclear broadening
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Z. Kang, I. Vitev and H. Xing, 1209.6030
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Nuclear shadowing effect Dynamic shadowing from power correction
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Qiu, Vitev , PRL, 2004
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Generalize to p+A collisions Power corrections in p+A collisions
At forward rapidity region t-channel dominates (t is small)
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Qiu, Vitev, PLB, 2006
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Parton energy loss in cold nuclear matter This has been computed in both a GLV-type and higher-twist
type formalisms
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Xing, Wang, et.al., NPA,2012; Ivan, 2007
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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
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Incorporate all the cold nuclear matter effects - I Incorporate these cold nuclear matter effects
Cronin effect:
Shadowing effect:
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Incorporate all the cold nuclear matter effects - II Continue …
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Energy loss:
Isospin:
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CNM effect: isospin
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Strong isospin effect
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CNM effect: Cronin
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Cronin enhancement
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CNM effect: shadowing
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Shadowing suppression
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CNM effect: energy loss
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energy loss suppression
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Nuclear modification at RHIC
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Work well at central and forward rapidities for both photon and hadron.
Z. Kang, I. Vitev and H. Xing, 2012
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Nuclear modification at LHC
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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
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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
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Qiu, Zhang, PLB, 2002
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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%)
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Kang-Qiu-Vogelsang, PRD, NPA, 2009
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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?
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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