eA Physics @ eRHICeA Physics @ eRHIC

Raju VenugopalanRaju Venugopalan

Brookhaven National LaboratoryBrookhaven National Laboratory

eRHIC discussion group, Oct. 18th 2006

eRHIC at BNLeRHIC at BNLTHE PROPOSALTHE PROPOSAL

A high energy, high intensity polarized A high energy, high intensity polarized electron/positron beam facility at BNL to colliding electron/positron beam facility at BNL to colliding with the with the existing heavy ion and polarized proton existing heavy ion and polarized proton

beambeam would significantly enhance RHIC’s ability to would significantly enhance RHIC’s ability to probe probe fundamental, universal aspects of QCDfundamental, universal aspects of QCD

Main Design

Ring Ring Option Linac Ring Option

eRHIC vs. Other DIS eRHIC vs. Other DIS FacilitiesFacilities New kinematic regionNew kinematic region

EEee = 10 GeV (~5-12 GeV = 10 GeV (~5-12 GeV variable)variable)

EEpp = 250 GeV (~50-250 GeV = 250 GeV (~50-250 GeV variable)variable)

EEAA= 100 GeV = 100 GeV Sqrt[SSqrt[Sepep] = 30-100 GeV] = 30-100 GeV Kinematic reach of eRHIC:Kinematic reach of eRHIC:

X = 10X = 10-4-4 --> 0.7 (Q --> 0.7 (Q22 > 1 GeV > 1 GeV22)) QQ22 = 0 --> 10 = 0 --> 1044 GeV GeV22

Polarization of e,p beams Polarization of e,p beams (~70%) and He(~70%) and He33 beams ~30- beams ~30-40% polarized40% polarized

Heavy ions of ALL species Heavy ions of ALL species at RHICat RHIC

Luminosity Goal:Luminosity Goal: L(ep) ~10L(ep) ~1033-3433-34 cm cm-2-2 sec sec-1-1

eRHIC

DIS

CM vs. LuminosityCM vs. Luminosity

eRHICeRHIC Variable beam Variable beam energyenergy

P-U ion beamsP-U ion beams Light ion Light ion polarizationpolarization

Huge Huge luminosityluminosity

eRHIC

ELIC-Jlab

Kinematics:

Cross-section:

g_1,…g_5 - polarized structure functions in pol. e - pol. p scattering

DIS highlights:

Bjorken scaling: the parton model.

Scaling violations: QCD- asymptotic freedom, renormalization group; precision tests of pQCD.

Rapid growth of gluon density at small x, significant hard diffraction.

Measurement of polarized structure functions: scaling violations, the “spin crisis”.

QCD in media: the EMC effect, shadowing, color transparency,..

How can we probe glue with a high luminosity lepton-ioncollider ? Key Measurements

Precision inclusive measurements of structure functions -wide sweep of nuclei from Protons to Uranium

Direct (photon-gluon fusion) semi-inclusive and exclusive probes of final states

Deeply virtual Compton scattering

- observables in blue will be measured for the first time in a wide kinematic domain

- CGC/saturation models predict significant differences

from DGLAP type pQCD models for Q_s > (p_t, M)

Lego plots a la Bjorken and Khoze et al.

Such multi-gap events can also be studied in DIS

Bj, hep-ph/9601363

Bj-scaling - apparent scale invariance of structure functions…

Nobel to Friedman, Kendall, Taylor

Feynman and Bjorken explanation of scaling puzzle…

Parton model

Parton constituents of proton are “quasi-free” on interaction time scale 1/q

Fraction of hadron momentum carried by a parton…

Puzzle resolved in QCD

QCD Parton model

Logarithmic scaling violations

The Hadron at high energies: I

Parton model

QCD-logarithmic corrections

“X”-QCD--RG evolution

# of valence quarks

# quarks…

“sea” quarks

Valence quarks

DGLAP evolution: Linear RG in Q^2Dokshitzer-Gribov-Lipatov-Altarelli-Parisi

# of gluons grows rapidly at small x…

Coefficient functions - C - computed to NNLO for many processes, e.g., gg -> H Harlander, Kilgore; Ravindran,Van Neerven,Smith; …

Splitting functions -P - computed to 3-loops recently! Moch, Vermaseren, Vogt

+ higher twist (power suppressed) contributions…

STRUCTURE OF HIGHER ORDER CONTRIBUTIONS IN DIS

increasing

But… the phase space density decreases-the proton becomes more dilute

Resolving the hadron -DGLAP evolution

RG evolution…

- Large x

- Small x

Gluon density saturates at f=

Resolving the hadron-BFKL evolution

Proton

Proton is a dense many body system at high energies

QCD Bremsstrahlung

Non-linear evolution:Gluon recombination

Mechanism for parton saturation:

Competition between “attractive” bremsstrahlungand “repulsive” recombination effects.

Maximal phase space density =>

Saturated for

Need a new organizing principle-beyond the OPE- at small x.

Higher twists (power suppressed-in ) are important when:

Leading twist “shadowing’’ of these contributions can extend up to at small x.

F_L is a positive definite quantity- more sensitive to higher twists than F_2 ?

- clarify comparision with leading twist NLO pQCD at low x and moderate

Golec-Biernat & Wusthoff’s model

where

&

Parameters:

Geometrical scaling at HERA

Scaling seen for all x < 0.01 and

(Golec-Biernat,Kwiecinski,Stasto)

Comparison with DataComparison with Data

FS model with/without saturation and IIM CGC model hp-ph/0411337.

Fit F2 and predictxIPF2

D(3)

F2

F2

FS(nosat)

x

CGC

FS(sat)

Comparison with DataComparison with Data

Exclusive J/Psi production: Kowalski-Teaney

R_{A1,A2} = 1 => Pomeron flux is A -independent = f(A1,A2) - universal form

Diffractive Vector Meson Production:

Very sensitive to small x glue! Brodsky,Gunion,Mueller,Frankfurt,Strikman

III: Hard diffractive processes

“Pomeron” exchange

30 % of eRHIC eA events may be hard diffractive events-Study sizes and distributions of Rapidity Gaps

Viewing the hadron in the transverse plane at high energies…

Y_0 ---> Y+Y_0

Gluon radiation Gluon radiation & recombination

Overlapping gluon clouds… at large impact parameters, interplay between perturbative (hard Pomeron) and non-perturbative (soft Pomeron) physics…

By studying the “t” dependence of diffractive final states , can we learn more about the transition regime ?

S-matrix:

Shadowing and diffraction:

• Is shadowing a non-perturbative leading twist phenomenon, or is generated by weak coupling, high parton density effects?

•What is the relation of shadowing to diffraction? AGK rules relating the two are valid at low parton densities-how do these generalize to large parton densities?

Armesto,Capella,Kaidalov, Salgado

Cartoon of ratio of nuclear

structure functions Uncertainity in ratio of Ca to nucleon gluon distributions-Hirai,Kumano,Nagai, hep-ph/0404093

Armesto

Ratio of Gluon densities in Lead to Proton at

x

in x range

Factor 3 uncertainity in glue => Factor 9 uncertainity in Semi-hard HI-parton cross-sections at LHC!

The nuclear “oomph” factor!

eA at eRHIC same parton density as ep at LHC energies!

?

Reasonable agreement withforward spectra

Hayashigaki et al.

Virtual photon coherence length:

x_Bj << 0.01 : Photon coherence length exceeds nuclear size

0.01 < x_Bj < 0.1: Intermediate length scale between R_p & R_A

x_Bj >> 0.1: Photon localized to longitudinal size smaller than nucleon size

Strong hints from AA multiplicity dists. andD Au forward data of novel

physics of strong color fields

A dependence of saturation scale ?Numbers on “phase” diagram ?Breakdown of linear QCD evolutioneqns. ?

II: Extracting gluon distributions in pA relative to eA

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Direct photons

Open charm

Drell-Yan

As many channels…but more convolutions, kinematic

constraints-limit precision and range.

Drell-Yan

But very difficult to see scaling violations

Impressive reach…