1

Forward Physics Prospects using FCAL in High-Energy Collisions (pp & pA) at

LHC

Bedanga MohantyVECC, Kolkata

Test of pQCD predictions : p+p collisions Gluon distribution function in proton and Nuclei : p+A collisions

Two important physics issues could be addressed :

2

p+p Collisions : Test pQCD

How partons are distributed in hadrons we collide

Parton Distribution Functionsdominantly from deep-inelastic

Scattering experiments

How partons fragment into hadrons

Parton Fragmentation Functionsdetermined from e+e- annihilations

b-quark

c-quark

uds-quarkall

s=91.2 GeVOPAL

cabccbb

cbaaacba dzDxfxfdzdxdxd σσ π

π ˆ)()()( ,,

∑∫ ∫ ∫=

What is the probability that the partons will interact

Parton-Parton Cross-Section (pQCD)

3

Test of pQCD at RHIC

Jet production and high pT identified particle production well explained by NLO pQCD calculationsat midrapidity

STAR : PRL 97 (2006) 252001

Midrapidity Measurements

STAR : PLB 637 (2006) 161 PHENIX : PRL 91 (2003) 241803

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Test of pQCD at Forward Rapidity

σdata / σpQCD depends on in addition to CM energy and pT

STAR : PRL 97 (2006) 152302

Forward rapidity Measurements

Bourrely and Soffer : hep-ph/0311110

√s=23.3 GeV √s=52.8 GeV

Ed

3 σd

p3 [b

/GeV

3 ]

√s=200 GeV

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Test of pQCD : Role of Forward Rapidity

What are the Bjorken x dependence on

Simple Kinematics

Deep inelastic scattering

Hard scattering hadroproduction

Assumtions :

1. Initial partons are collinear2. Partonic interaction is elastic

pT, 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

0 and 0

xq xg xT = 2 pT / s

Large-rapidity particle detection

>>

xq xT e xF (Feynman x)

xg xF e(

Large rapidity : different x for quarks and gluons

p+p π+X, s = 200 GeV, =0

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Gluon Distribution : Proton

Low-x gluon density is large and continues to increase as x0

It cannot grow forever

Fundamental question - Where does saturation set in ?

Deep inelastic scattering

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Gluon Distribution : Nuclei

World data on nuclear DIS constrains nuclear modifications to gluon density only for xgluon > 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

MidRapidity

ForwardRapidity

CTEQ6M

Saturation may set in at forward rapidity when gluons start to overlap.

yT es

px −~

Nuclear amplification: xGA(x) ~ A1/3xG(x), i.e.

gluon density is ~6x higher in Gold than the nucleon

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Experimental Signature in p(d)+A Collisions - I

Mid Rapidity

Forward 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 p+p collisions.

The effect should be stronger at forward rapidities where x is smaller but gluon densities are higher

yT es

px −~

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Experimental Results : d+A Collisions at RHIC

Hadron production suppressed at forward rapidity

dAu: forward suppression & backward enhancement

PHOBOSBRAHMS

STAR

PHENIX

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Explanation : Not Unique

Forward rapidity at RHIC ~ Mid rapidity at LHC

SATURATION (CGC)

Jalilian-Marian, NPA 748 (2005) 664.Kharzeev, Kovchegov, and Tuchin, PLB 599 (2004) 23; PRD 68 (2003) 094013.Armesto, Salgado, and Wiedemann, PRL 94 (2005) 022002.

MULTIPLE SCATTERING &Eloss IN COLD NUCLEAR MATTER

Qiu and Vitev, PRL 93 (2004) 262301; hep-ph/0410218.

BREAKDOWN OF FACTORIZATIONOr SUDAKOV SUPPRESSION

Kopeliovich, et al., hep-ph/0501260.Nikolaev and Schaefer, PRD 71 (2005) 014023.

PARTON RECOMBINATION

Hwa, Yang, and Fries, PRC 71 (2005) 024902.

SHADOWINGR. Vogt, PRC 70 (2004) 064902.Guzey, Strikman, and Vogelsang, PLB 603 (2004) 173.

Armesto & Salgado, hep-ph/0308248

Shaowing : Models differ by factor fo 3

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Experimental Signature in p(d)+A Collisions - II

p+p: Di-jet

Experimental Signature…….

d+Au: Mono-jet

PT is balanced by many gluons

Dilute parton system

(deuteron)

Dense gluon

field (Au)

Kharzeev, Levin, McLerran NPA748, 627

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CGC and Forward Rapidity

Taking high pT π0 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

pd

pAu

π

q

g EN

xqpxgp

π

π

EN

€

Q2 ~ pT2

s = 2EN

η = −ln(tan(θ

2))

xq ≈ xF / z

€

xF ≈2Eπ

s

z =Eπ

Eq

xg ≈pT

se−η g

With assumptions

STAR : PRL 97 (2006) 152302

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Summary : Why FCAL

Allows us to test pQCD predictions

σdata / σpQCD 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

Motivations

Measurements

π0 transverse momentum spectra Nuclear Modification factor for π0

Forward π0 triggered correlations

Unique Opportunity

To find where Saturation sets in To map a new phase diagram ?

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Thanks