Open questions in QCD at high parton density (e+A, p+A, …)

Cyrille Marquet

University of Santiago de Compostelaand

CERN - Theory Division

Contents

• gluon saturation: status and open questions- what we know- what we would like to know- what questions can(not) be answered with e+A (p+A)- why QGP physicists should care

• e+A measurements: highlights- structure functions F2 and FL

- hard diffraction- di-hadron production- exclusive vector meson production- cold nuclear matter effects

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Gluon saturation:status and open questions

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What we know for sure• fundamental consequence of QCD dynamics:

at asymptotically small x:- QCD evolution becomes non-linear- particle production becomes non-linear- QCD stays weakly coupled

• the energy dependence of the saturation scale, and more generally of observables, can be computed from first principles

although in practice, the predictivity will depend on the level of accuracy of thecalculation (LO vs NLO, amount of non-perturbative inputs needed, …) 4

both in terms of practical applicabilityand phenomenological success

• the Color Glass Condensate (CGC) has emerged as the best candidateto approximate QCD in the saturationregime

see student lecture by Albacete

A big open question• is this relevant at today’s colliders ?

- for each of these observables, there are alternatives explanations

- the applicability of the theory can be questioned when values of QS start to drop below 1 GeV (e.g. p+p and peripheral d+Au at RHIC)

• the CGC is not widely accepted because

in other words: can we get away with using sucha gluon distribution (with ad hoc cutoff if necessary) ?

or do we need to properly take into account

the QCD dynamics at kT ~ QS and below ?

the CGC phenomenology is successfulfor every collider process that involves

small-x partons and kT ~ QS , i.e. for abroad range for high-energy observables:

multiplicities in p+p, d+Au, Au+Au and Pb+Pb; forward spectra and correlationsin p+p and d+Au; total, diffractive and exclusive cross sections in e+p and e+A, …

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What EICs can do• provide golden measurements

EICs = EIC stage 1, EIC stage 2, LHeC

modification of particle production at forward rapidities in p+A versus p+p

• twice one thought one had found such observables

the kind that will prove non-linear QCD evolution to be indispensable, or irrelevant

see parallel talks by Jalilian-Marian, Lappi, Li

EICs would provide smoking guns for saturation, somethingthat very likely cannot be done with p+A (let alone A+A)

single inclusive di-hadron correlations

see parallel talk by Xiao

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see plenary talk by Deshpande

Bigger open questions (I)

• the impact parameter dependence of the gluon density and of QS

EICs would also provide data that can help us address the following questions

what is done in the most advanced CGC phenomenologicalstudies, is to treat the nucleus as a collection of Woods-Saxondistributed CGCs, and to evolve (down in x) the resulting gluon

density at different impact parameters independently

but is this good enough ? (in principle not)7

this has always been the main non-perturbative input in CGC calculations

modeling

in the case of a proton, using an impact-parameter averaged saturationscale is enough most of the time, but in the case of a nucleus it is not

Bigger open questions (II)

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• the transition from the saturation to the high-pT (leading-twist) regime

rcBK evolution (down in x) does not containthe DGLAP limit, hence after some evolution

(at forward rapidities), RpA predictions reach

unity only at unrealistically large values of pT

how RpA goes back towards unity at high-pT ?

Albacete, Dumitru

in this talk, I focus on what is unique to e+A, p+A provides great possibilities aswell, already presented see student lecture by Armesto, plenary talk by Dumitru

• the transition from the saturation regime to confinement

how does it happen ? does the coupling run with Qs ?are classical fields still the right degrees of freedom ?

p+A and e+A collisions offer special opportunities to explorethis many-body system of strongly-correlated gluons

• universality properties of the saturation regime

Why QGP physicists should care• bulk observables in heavy-ion collisions reflect the properties of the initial state as much as those of the hydro evolution of the QGP

see plenary talks by Dusling, Song, Wiedemann

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new sources of uncertainties keepemerging, for instance even two CGCmodels predict different eccentricities

• the main source of error in the extraction of medium parameters (e.g. η/s) is our insufficient understanding of initial state fluctuations

QGP properties cannot be precisely extracted from data without a properunderstanding of the initial state; e+A collisions: access to a precise picture

see parallel talks by Moreland, Schenke

e+A measurements:highlights

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see also: - student lecture by Armesto- plenary talk by Deshpande- parallel talks by Lee, Stasto- poster by Lamont

Deep inelastic scattering (DIS)

e+A @ EIC e+Pb @ LHeC

NOT all processes require Q2 ~ QS2 in order to probe saturation effects

*A center-of-mass energyW2 = (q+p)2

photon virtualityQ2 = - (k-k’)2 = - q2 > 0

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Inclusive structure functions

can NLO DGLAP simultaneously accommodate F2 and FL

data if saturation sets in according to current models ?

precisely measuring FL is crucial, and this requires an e+A energy ( ) scan

Albacete, Ullrich

measures quark distributions gluon distribution

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Hard diffraction in DIS

observable subject to strong non-lineareffects even with Q2 values

significantly bigger than QS2

at HERA the NLO DGLAP descriptionbreaks down already at Q2 ~ 8 GeV2

clean and unambiguous signal of saturation, already at EIC stage-1

this enhancement is specific to e+A(there is no equivalent in p+A)

Guzey,Lamont, CM

a surprising QCD feature at HERA: a proton in its rest frame hit by a 25TeV electron remains intact 15% of the time

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@ LHeC

Exclusive Vector Meson production

through a Fourier transformation, one canextract the spatial gluon distribution (andcorrelations), this is not feasible in p+A

energydependence

momentum transfer dependence

@ EIC

Toll, Ullrich

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Newman, Watt

Di-hadrons in DIS

at the qualitative level:similar effects as in p+A

• directly sensitive to the kT dependence of the gluon distribution

Lee, Xiao, Zheng

unique access to Weizsacker-Williams gluon distribution(a different operator definition is involved in p+A)

but at the qualitative level, thisprocess involves a different

unintegrated gluon distribution

see paralleltalk by Yuan

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Cold nuclear matter effects• hard probes (esp. jets) in heavy-ion collisions need calibration

what is the effect of cold nuclear matteron parton branching ? on hadronization ?

see plenary talk by Milhano

what is the x,Q2 dependenceof nuclear quarks and gluons?

answering these questions can help “establish the probe”

• the complementarity of e+A with respect to p+A can be of help

especially when coldmatter effects in p+A

collisions are “strangerthan expected” 

see plenary talkby Wysocki

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Nuclear parton distributionsEICs can reveal the nuclear structure throughout the (x,Q2) plane, from

gluon saturation at low x to the gluon EMC effect and its Q2 evolution at high x

uncertainties on nuclear gluons

p+A will already do a lot here, complementarity of EIC/LHeC:better handle on kinematics, systematics, and A coverage 17

Ullrich

kinematical reach of EIC

the EICs have constraining power, they will be to nuclei what HERA is to the proton

advantage of LHeC : kinematical reach; advantage of EIC: the wide range of nuclei

In-medium fragmentation

small v : in-medium hadronization

- dynamics of confinement

- stages of hadronization

and their time scales

- energy loss and pT-broadening

- modifications of jet shapes

• unprecedented ν range large v : in-medium parton propagation

semi-inclusive

DIS

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• first time access to heavy quarks

Brooks, QiuWang

Conclusions• all detailed studies can be found in

- the INT report on the Physics case for the Electron-Ion Collider, arXiv:1108.1713e+A conveners : A. Accardi, M. Lamont and CM

- the upcoming EIC white papere+A conveners: Y. Kovchegov and T. Ullrich

- the LHeC Conceptual Design Report, arXiv:1206.2913small-x conveners: N. Armesto, B. Cole, P. Newman and A. Stasto

• thanks to the EIC task forces at Brookhaven and Jefferson labs• thanks to the LHeC small-x working group

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