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Direct Photon Production in ppcollisions at the LHC
Théorie LHC France 06 April 2010 IPN Lyon
F.M. Liu IOPP/CCNU, Wuhan, China
K. Werner Subatech, Nantes, France
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outline• pp collisions at extremely high energy (i.e. 14TeV)
– Multiple scattering, many Pomerons / strings involved– How to treat the afterward secondary scattering?
• Direct photon production – RHIC Au+Au data are explained with 4 sources– Hydrodynamic description of the hot dense matter in AA– Hydrodynamic treatment of the secondary scattering in pp
• Results and discussion– pQCD + plasma contribution– Is it possible to form a QGP in pp collisions?
PHOS@ALICE will answer. pt 100Mev~100GeV .
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Particle production in pp
Usually, a pair of strings are formed via
a) longitudinal excitation
b) color exchange
hadron production: string fragmentation, i.e.
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pp collisions at high E
Problem: Two-string picture can not explain observables (~100GeV)
1) High multiplicity (multiplicity dis. predicted is too narrow)2) Increase of mean pt3) High pt jets4) Rise of central rapidity density
Solution: Multiple scattering becomes important at high energies More Pomerons/strings are added
Pomeron = a pair of strings number Pomeron )(Prob :,
F.M.Liu et al. PRD 67, 034011 (2003)H.J.Drescher et al, Phys.Rept.350,93(2001).
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pp at extremely high E, ie, E=14TeVMultiple elementary interactions (Pomerons) in NEXUS/EPOS:
Question: How to treat the afterward secondary scattering ? on parton level ? or on hadron level? or string interaction? or Pomeron interaction?
Pomeron number might be very big.
We need a post-collision evolution to treat the many-body system.
Rap
idity
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Present data are generally in good agreement with NLO QCD prediction.
But a tendency for the data to be above (below) the theory for lower (large) pt.
Direct photon data in pp (ppbar)
Latest PDG review
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pp-> gamma with NLO pQCD
)ˆˆˆ()(ˆ
ˆ),(),( 2
/2
/2
)LO(
utscdabtd
dsMxGMxGdxdx
pdyd
dNbpb
abapaba
t
pp
),(1 20
/2t
2,t
2QzD
zpdyd
dNdz
pdyd
dNcc
c
c
gqcc
qqggqq ,
qqgqq ,2. Fragmentation contribution: High order contribution
1. Leading Order contribution
preliminary
D0 and CDF can only measure pt > 10GeV.
Should the secondary interaction be responsible for this deviation?
Saturation makes a decrease of PDF at low x. can not be.
W.Vogelsang & M.R.Walley 1997 JPG: 23,A1-A69
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Secondary scattering in AA
Evolution of core region, or huge number of secondary collisions, can be treated with hydrodynamics.
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Hydrodynamic treatment
fm/c6.00
),,,,...(,,,, zyxBsup
Initial condition: thermalized QCD matter at rest at
Evolution: 3D ideal hydrodynamic equation
described with 3+1D ideal hydrodynamics
0 T
MeV100~or fm/GeV08.0 3 thth T
MeV170cTEoS: 1st order phase transition at
QGP phase: 3 flavor free Q & G gas
HG phase: hadronic gas PCE
Freeze-out:
May followed with cascade treatment
• parameterized based on Glauber model • string overlapping and melting
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4 main sources in AA
),(1 2
/2t
2,t
2QzD
zpdyd
dNdz
pdyd
dNcc
c
c
gqcc
)ˆˆˆ()(ˆ
ˆ),(),( 2
/2
/AB2
)LO(
utscdabtd
dsMxGMxGdxdxT
pdyd
dNbBb
abaAaba
t
AB
Jets lose energy in plasma
1. Leading Order contribution
2. Fragmentation contribution:
upETExd
pdyd
dN
t
**thermal
42
thermal
),,(
),(),(),( ***thermal HQGP TETETE G
3. Thermal contribution
),( *JPC
42
TExdpdyd
dN
t
JPC
4. Jet-photon conversion
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Au+Au -> direct photons at 200AGeV
Direct photon production from AuAu collisions at top RHIC energy is well explained in a large pt range at all centralities.
FML, T.Hirano, K.Werner, Y. Zhu, Phys.Rev.C79:014905,2009
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Plasma effect in AA
The thermal contribution makes an evident increase of the production at low pt region!
And energy loss will reduce fragmentation contribution at high pt region.
So secondary collisions can be responsible for the deviation between direct photon data and NLO pQCD prediction: The data will be above (below) the theory for lower (large) pt for high energy pp collisions!
What we learn from AA:
The enlightenment for pp:
When pp collision energy E is extremely high, ie, at 14TeVThere is a great number of secondary collisions.Let’s treat with hydrodynamics. A QGP might be formed.
1)
2)
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Plasma evolution in pp (EPOS)
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Plasma contribution to direct photon
Effect from secondary scattering:
PHOS@ALICE Pt range: 100Mev~100GeV, detectable!
51035.2 Pt =1GeV/c
61049.3 71009.8 2GeV/c 3GeV/c
pQCD
plasma 41083.5 51001.3 61008.2
25 6.8 6.2plasma pQCD
preliminary
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Conclusion
1. A great number of Pomerons(strings) are involved in pp collisions at very
high energies. Therefore secondary scattering of produced particles shoul
d not be ignored.
2. Hydrodynamic approach is proposed to treat the secondary scattering. Ba
sed on this approach we compare direct photon’s pQCD production and p
lasma production in pp collisions at 14TeV.
3. We find direct photon is a very useful probe if a QGP can be formed in pp
@LHC. PHOS@ALICE is able to test.
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Thank you!