High p_T at LHC Tokaj, March 17, 2008 1

Expected nuclear modifications and pseudorapidity asymmetry at the LHC

George Fai Kent State University

Collaborators: A. Adeluyi, Kent G.G. Barnaföldi, Kent and Budapest P. Lévai, Budapest

U.S. Department of Energy DE-FG02-86ER40251

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Outline

1. Introduction

2. Motivation: why do we insist on pA collisions?

4. Calculations

4a. d+Au results at RHIC

4b. d+Pb prediction for the LHC

3. Pseudorapidity asymmetry and nuclear modifications

5. Summary and conclusions

G. Fai, Workshop on pA physics at RHIC, BNL, Oct. 29, 2000

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The importance of pA measurements

We expect QGP to be formed in AA, but NOT in pA

Miklos Gyulassy

“The deuteron-goldcontrol experiment”

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Centrality Dependence

Dramatically different and opposite centrality evolution of Au+Au experiment from d+Au control.

Au + Au Experiment d + Au Control Experiment

Preliminary DataFinal Data

Tom Hemmick: Quark Matter 04

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The importance of pA measurements

The jump from pp to AA (heavy) is too large to reliably

• provide baseline (enhancement/suppression relative to what?)• separate various nuclear effects (shadowing, multiple scattering, EMC effect,…)

Examples:

J/ψ production, Drell-Yan, strangeness “enhancement”

Direct observation of asymmetry should come from forward-backward difference in pA

In light of no QGP formation and difference from pp, largeforward-backward asymmetry should be expected in pA

?

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The pT distributions soften in a similar way in pp and dAu collisions

BRAHMS

R. Debbe, 2005

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Forward-backward asymmetry in d+Au

PHOBOS, PRC72, 031901(R) (2005)

around 40%effect in the central bin

back/forward asymmetry ratio > 1

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Forward-backward asymmetry in d+Au

BRAHMS, PRL 94, 032301 (2005)

upto 40% effect back/forward asymmetry ratio > 1

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v1,v3!Paul Stankus

The importance of pA measurements: other

reasons PeterSteinberg

2005

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In the past at RHIC: small detectors to go to higher rapidity

PHOBOS, PRC70, 061901 (2004)

BRAHMS,PRL 93, 242303 (2004)

STAR, ALICE, ATLAS, CMS are big barrels …

Notwithstanding some additions of large-rapidity capabilitiesNotwithstanding some additions of large-rapidity capabilities

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Front view of ALICE detector

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… and here are many of them, with content measured (sampled) at this conference …

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Ingredients of the pQCD calculation

Shadowing parameterizations (nPDF-s)

1. Role of multiple scattering

2. Role of the EMC effect

A. Adeluyi and G. Fai, PRC74, 054904 (2006)

EKS, FGS, HKN, HIJING

Particularly interested in

(over and above intrinsic kT )

Fragmentation functions (FF-s)

KKP, BKK, AKK, Kretzer

B.A. Cole & Hungarians, hep-ph/0702101

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Calculated spectra at RHIC -- compared to data

intrinsic kT increases the calculation mostly at low pT

need to include multiplescattering with HIJINGshadowing for same D/T

Central rapidity

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Calculated spectra at RHIC

Nonzero rapidity

Forward andbackwarddata averagedby STAR

multiple scatt.influences d-side(forward) more than Au-side (backward)

Need separated forward/backward data!

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Calculated spectra at Tevatron – compared to data

p + Be @ sqrt(s) ~ 35 GeV

Central rapidity

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Calculated spectra at Tevatron – compared to data

Nonzero rapidity

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Study role of multiple scattering in rapidity distributions using HIJING shadowing

Pseudorapiditydistribution atfixed pT

intrinsic kT turned off

asymmetrydisappears in the absence ofshadowing

d+Au at RHIC

deuteron sidegold side

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deuteron sidegold side

Add intrinsic transverse momentum in the nucleon

No shadowing added symmetry preserved

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Turn on shadowing

and multiple scattering

intrinsic kT

increases theyield, keepingasymmetry

multiple scatt.increases yieldfurther (mostly d side), reverses asymmetry

gold side deuteron side

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Start from full calculation, turn off multiple scatteringand shadowing separately

shadowing suppresses yield, more on deuteronside (forward)

multiple scatt.contribution is large on the deuteron side

deuteron sidegold side

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What happens with increasing transverse momentum?

When pT becomes large compared to (broadened) intrinsic kT, intrinsic transverse momentum effects become less important + we approach antishadowing

deuteron sidegold side

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What happens with increasing transverse momentum?

When pT becomes large compared to (broadened) intrinsic kT, intrinsic transverse momentum effects become less important + we approach antishadowing

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Summary so far:

pseudorapidity asymmetry arises from nuclear effectsin constrained geometry

In an asymmetric geometry(his wine thief), if there are some physical effects (gravity here), and the conditions are carefully controlled, the medium prefers one direction over the other (herethe one leading to the wine glass)

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Nuclear modification factors for deuteron on gold

total chargedhadrons

min.bias

EKS

FGS HKN

intrinsic kT

turned off

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Central-to-peripheral ratio for deuteron on gold

HKN

Without kT and without the possibility to tune multiplescattering, RCP overpredicted at forward rapidities

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Semicentral-to-peripheral ratio for deuteron on gold

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Pseudorapidity asymmetry for several species in |η| < 0.5

with EKS,FGS, HKN,without kT

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Pseudorapidity asymmetry in 0.5 < |η| < 1.0

with EKS,FGS, HKN,without kT

HKN

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Comparing the two kinds of descriptions

no significantdifference atsmall η

differences at high pT and higher pseudorapidity

GO TO LHC !

at high η and pT, shadowing-typedifferences start to show up

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Expected pseudorapidity asymmetry at the LHC

When going to LHC:

HIJING shadowing very strong

much larger < kT > is used 2

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Expected nuclear modification factor at the LHC

central rapidity HKN without kT

possible effects of jet quenching

EMC effect

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Conclusions

In the framework of a pQCD-based model, two options todescribe spectra in d+Au both at y=0 and y≠0:

(tentative)

EITHER use EKS (or FGS or HKN) shadowing

forward RCP not well reproduced

OR use HIJING-type shadowing + multiscattering

pseudorapidity asymmetry not reproduced

unless kT broadening given rapidity dependence

working on it

LHC will hopefully prefer one over the other atsufficiently high rapidity and transverse momentum

NEED SEPARATED FORWARD/BACKWARDDATA FOR THIS KIND OF ANALYSIS!

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Intrinsic transverse momentum width in the proton

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Differences in RAA between Au+Au and d+Au

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Shadowing and EMC summary

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Shadowing parameterizations as functions of x

HKM

EKS98

Armesto

new HIJING

Sarcevic

Frankfurt

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Rapidity evolution of the Cronin peak

Deuteron side

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Rapidity evolution of the Cronin peak

Gold side