Forward Physics Prospects using FCAL in HighEnergy Collisions pp

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Forward Physics Prospects using FCAL in
High-Energy Collisions (pp pA) at LHC
Bedanga Mohanty VECC, Kolkata
Two important physics issues could be addressed

• Test of pQCD predictions pp collisions
• Gluon distribution function in proton and
  Nuclei pA collisions

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pp Collisions Test pQCD
How partons fragment into hadrons
How partons are distributed in hadrons we collide
What is the probability that the partons will
interact
Parton Distribution Functions dominantly from
deep-inelastic Scattering experiments
Parton Fragmentation Functions determined from
ee- annihilations
Parton-Parton Cross-Section (pQCD)
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Test of pQCD at RHIC
Midrapidity Measurements

• Jet production and high pT identified particle
• production well explained by NLO pQCD
  calculations
• at midrapidity

STAR PLB 637 (2006) 161
PHENIX PRL 91 (2003) 241803
STAR PRL 97 (2006) 252001
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Test of pQCD at Forward Rapidity
Forward rapidity Measurements
vs23.3 GeV
vs52.8 GeV
vs200 GeV
q10o
q6o
Ed3s/dp3mb/GeV3
q15o
q53o
q22o

• ?data / ?pQCD depends on ? in addition to CM
  energy and pT

STAR PRL 97 (2006) 152302
Bourrely and Soffer hep-ph/0311110
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Test of pQCD Role of Forward Rapidity
Deep inelastic scattering
Hard scattering hadroproduction
Simple Kinematics

• What are the Bjorken x dependence on??

• Assumtions
• Initial partons are collinear
• Partonic interaction is elastic ? pT,1 ? pT,2

?
Studying pseudorapidity, ?-ln(tan?/2),
dependence of particle production probes parton
distributions at different Bjorken x values and
involves different admixtures of gg, qg and qq
subprocesses.
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Test of pQCD Role of Forward Rapidity - An
Example

• Mid-rapidity particle detection
• h1?0 and h2?0
• ? xq ? xg ? xT 2 pT / ?s
• Large-rapidity particle detection
• h1gtgth2
• xq ? xT eh1 ? xF (Feynman x)
• xg ? xF e-(h1h2)

Large rapidity different x for quarks and gluons
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Gluon Distribution Proton
?
Low-x gluon density is large and continues to
increase as x?0 It
cannot grow forever Fundamental question - Where
does saturation set in ?
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Gluon Distribution Nuclei
World data on nuclear DIS constrains nuclear
modifications to gluon density only for xgluon gt
0.02 Crucial for knowing the initial conditions
in Nucleus-Nucleus Collisions
M. Hirai, S. Kumano, T.-H. Nagai, Phys. Rev. C70
(2004) 044905
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Forward Rapidity Saturation
Mid Rapidity
CTEQ6M
Forward Rapidity
Nuclear amplification xGA(x) A1/3xG(x), i.e.
gluon density is 6x higher in Gold than the
nucleon
Saturation may set in at forward rapidity when
gluons start to overlap.
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Experimental Signature in p(d)A Collisions - I
Mid Rapidity
In CGC picture, 2 soft gluons can merge to form a
harder gluon. This will lead to a suppression of
low pT hadrons in p(d)A collisions compared to
pp collisions. The effect should be stronger
at forward rapidities where x is smaller but
gluon densities are higher
Forward Rapidity
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Experimental Results dA Collisions at RHIC
PHOBOS
PHENIX
BRAHMS
STAR
Hadron production suppressed at forward rapidity
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Explanation Not Unique
Forward rapidity at RHIC Mid rapidity at LHC
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Experimental Signature in p(d)A Collisions - II
pp Di-jet
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CGC and Forward Rapidity
With assumptions
Taking high pT ?0 in forward rapidity - allows
probing high-x valence quark correlations with
low -x gluons A probe of low-x gluons
Kharzeev, Levin, McLerran NPA748, 627
STAR PRL 97 (2006) 152302
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Summary Why FCAL
Motivations

• Allows us to test pQCD predictions
• ??????data / ?pQCD depends on ? in addition to CM
  energy and pT

• Allows us to understand the gluon distribution
  function in p and A
• Forward rapidity probes low-x regime
• Knowledge of initial conditions in heavy ion
  collisions

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Thanks