Low-x Measurements @ LHC

Prof. Brian A. ColeColumbia University

Overview: Accessible x, Q2 @ LHC

•Kinematically can reach x ~ 10-6 for Q2 > 100 GeV2

•Near mid-rapidity, x < 10-3 for Q2 <~ 100 GeV2

s = 5.5 TeV

Three Experiments: ALICE

Central tracking, particle identification, EM calorimetry, forward muon detection, zero-degree calorimetry

ALICE

Three Experiments: ATLAS

Central tracking, EM and hadronic calorimeters, muon spectrometers, forward calorimeter, zero-degree calorimeter

Three Experiments: CMS

CMS

Central tracking, EM and hadronic calorimeters, muon spectrometers, forward tracking and hadronic calorimeter, zero-degree calorimeters

Experimental Acceptance (e.g.)C

MS

ATLA

S

Overview: Categories of measurements

•Inclusive particle production, multiplicity, dn/d– A+A and p+A collisions

– Test of saturation interpretation of RHIC Au+Au data

•Single moderate pT hadrons, jets, prompt /Zs

– p+A with p+p baseline measurements

– pT spectra sensitive to shadowing / saturation effects

•Di-jet, /Z-jet, di-, Drell-Yan, …– p+p and p+A (all), A+A (di- and Drell-Yan)

– Clean initial-state kinematics (esp. DY and di-)

– Acoplanarity and QT sensitive to BFKL, saturation

+ A in ultra-peripheral A+A (Mark)

Generally: study deviations from collinear pQCD at low x and large g(x, Q2) with many hard processes

Inclusive Particle Production in A+A

•Observe geometric scaling also w/ nuclear targets

•Use kT factorized formula for gluon production and parton-hadron duality

Calculate A+A dn/d

•Compares very well to RHIC (e.g. PHOBOS) data

•Definite(ive?) prediction for A+A dn/d @ LHC– Test in 1st Pb+Pb run

Armesto, Salgado, Wiedemann Phys. Rev. Lett. 94 :022002,2005

Broadening Only Including Quantum Evolution

Proton-Nucleus: Heavy Quark Production

•Modest effects of saturation @ intermediate pT

– but extend over a long range in pT

•Measured by:– Single electron/muon, D meson (ALICE) @ lower pT

– Tagged b jets at higher pT

Forward Jets (e.g. CMS)

•Measure jets using forward hadronic calorimeter in CMS– 3 < || < 5– 15% energy

resolution for jet ET ~ 20 GeV

– Better resolution @ larger due to smaller underlying event (?)

– Even better in p-A

•For jet pT > 20 GeV– Reach x down to 10-4

From Quark Matter 2006 talk by D. d’Enterria

En

ergy

res

olu

tion

%

Q=100 GeVQ=10 GeVQ=2 GeV

Frankfurt, Guzey, Strikman: Phys. Rev. D71:054001, 2005.

p-A: Nuclear ShadowingCompilation by Armetso, hep-ph/0604108

Pb

•Large LT shadowing in Pb– Persisting to high

Q2 (> 104 GeV2)

•But:– Many shadowing

fits/extrapolations

– Producing wide range Rg

A values

•In principle:– Can be resolved

w/ high-pT singles, jets

•In practice:– How well?

Heavy Quark: Acceptance

•Heavy quark measurements will extend to x ~ 10-5 (ALICE & LHCb), few x 10-4 for ATLAS & CMS

Will be sensitive to shadowing or saturation

Beware definition sign of

ALICE: Forward in p-Pb

Heavy quark pairs: Measuring RgA

2, QxRAg

Eskola, Kolhinen and Vogt, Nucl. Phys. A696:729-746, 2001. Comparison of

p+A/p+p cross-section ratio (solid) to

(dashed)

Conclude:

double semi-leptonic decays of heavy quarks give sensitive measurement of

2,QxRAg

LHC: Low-mass Drell-Yan

•From ATLAS std model WG talk by E. Rizvi.

•DY +- in ATLAS, || < 2.5, pT

> 6 GeV/c – Triggerable

•Very preliminary study, but suggests access to x values down to 10-5

M+- (GeV/c2)

cou

nts

Trigger efficiency does not produce strong x bias

The LHC Schedule (Nominal + Guesses)

•p-p – Commissioning run @ s = 0.9 TeV, late 2007– First full-energy run @ s = 14 TeV, spring 2008– Heavy ion base-line @ s = 5.5 TeV, 2011 or 2012 ?

•Pb+Pb – Short commissioning run, late 2008 (?)– Full (3 weeks) run late 2009

•p-Pb– Full run @ s = 8.8 TeV, likely before 2014

•By 2010-2011: – We should know if LHC Pb+Pb dn/d data are

consistent with production from saturated initial state.

– Have extensive p-p data to test for evidence of BFKL

•Then:– p-Pb run unique studies @ low x in nucleus

Low-x: LHC, e-RHIC•At the LHC

– We will be able to access unprecedented low x values

– With both protons and nuclei

– With a wide variety of probes

– Over a wide range of Q2

But, only for Q2 > 100-1000 GeV2 for observables with clean kinematics

•We will be able to:– Study QCD physics at very low x in both p and A

– Measure/constrain nuclear PDFs

– Measure pre-cursors of saturation (geometric scaling)

– Evaluate whether canonical saturation picture applies

•e-RHIC needed for precision, universality, low Q2

Cou

nts

ATLAS: Low-x Physics w/ ZDC

•Can access x <~ 10-6 @ moderate pT (> 4 GeV/c)– Correlate with jets in ATLAS calorimeters (|| < 5)– Study acoplanarity vs

11 22

π0 acceptanceFrom P. Steinberg Quark Matter 2006 ATLAS heavy ion physics plenary talk

A+A Multiplicity vs Energy

•LHC measurements will provide an essential test of whether we understand the mechanism responsible for bulk particle production.–e.g. does saturation correctly extrapolate?

RHIC200 GeV

Saturation?

Something else?

CGC: Gluon Shadowing

p

A

AF

FR

2

2

•Given a saturation parameterization of proton DIS data and Qs

A prediction of shadowing.

Saturation calculation by Kugeratski et al, hep-ph/058255

Au

Ca

Saturation: proton(deuteron)-A •p/d-A collisions provide an alternative way to study low-x processes in nuclei.–hadron production @ large rapidity (small xA)

Smaller xA

Cen

tral

to

per

iph

eral

rat

io

hadronhadron

Central Peripheral

Brahms Data@ RHIC

CGC w/ Evolution Compared to Data

Larger , Lower x

Effect of evolution

Are we seeing evolved CGC in d-A @ RHIC? Unclear

(Very) low x / Saturation @ LHC

Can reach x < 10-4

•with pT > 20 GeV jets, < -3•with 5 GeV single hadrons ~2.

This does not take into account p-A rapidity shift

2QQ

Minimum accessible x (collinear kinematics) Armesto, Salgado, Wiedemann, Phys. Rev. Lett. 94:022002 (2005)

e.g. ATLAS

Main detector

Low-x Physics: Motivation•CGC + Quantum evolution provides compelling framework for analyzing low-x physics– New perturbative regime of QCD

– Quantum evolution from non-Abelian classical fields a fundamental problem in field theory

–Calculation of heavy ion initial conditions!(?)

•But, we can’t yet be certain that it applies to any system that we will study in the laboratory

p-p, p-A, A-A measurements @ LHC

» Will reach lower x (10-7), but imprecise kinematics

» Test application of BFKL evolution

e-A measurements @ e-RHIC» Will reach less extreme x, but precise kinematics

Low-x Physics: Motivation (2)

•This cannot be allowed to persist …•If not e-RHIC, then how, where ????

K. Itakura (QM 2005)

Single Moderate pT Hadron Measurements

Measurable shadowing even at 100 GeV.

Modest effects at mid-rapidity (but going away slowly)

Q=100 GeVQ=10 GeVQ=2 GeV

Frankfurt, Guzey, Strikman: Leading twist Shadowing

Armesto, Salgado, Wiedemann, Phys. Rev. Lett. 94:022002 (2005)