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High-mass Dilepton Physics at RHIC-II

• Quark and Gluon Distributions in Nuclei

• Flavor Structure of Nucleon Quark Distributions

• Forward Tagging and Measurement of Meson Cloud

Jen-Chieh Peng

RHIC-II Workshop, April 29, 2005

University of Illinois

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What are the quark and gluon contents of nuclei?

(How do they differ from a free nucleon?)

Spin and flavor decomposition of parton distributions in nuclei?

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Antiquark ( ) distributions in nuclei were

measured in Drell-Yan experiments

u

PRL 83 (1999) 2304

The Drell-Yan p-A measurement can be extended to lower x at RHIC

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Expected kinematic coverage and sensitivites for a two-month run at designed luminosity at PHENIX

EKS98

Shadowing at low x?

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Disappearance of shadowing at small x?

Predicted nuclear dependence of Drell-Yan at small x for dilepton mass of 4.5 GeV

Kopeliovich et al. (hep-ph/0501260)

2 2 2 2

2 (1 )

(1 )q

cT qll

El

k M m

2

Coherence length, , vanishes

at large and small xcl

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Prediction of nuclear modification in the Color Glass Condensate formalism

Jalilian-Marian (nucl-th/0402014)

Rd(p)A shows strong mass dependence at y=2.2

Dilepton production in pA and dA at RHIC energy

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Gluon distributions in nuclei deduced from Q2 – dependences of F2

Sn/F2C

NMC data

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At RHIC, many hard-processes in p-A (d-A) collisions are sensitive to gluon distributions

in nuclei (identical to the tools used to determine ΔG(x) in RHIC-Spin)

• photon - jet• direct photon• dijet/dihadron• open charm• high-pT l+l-

Gluon distributions in nuclei could be probed down to x ~ 10-2

It would be very interesting to reach x ~ 10-3 to search for gluon saturation effect at low x

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J/Ψ production is dominated by gluon-gluon fusion

(x ~ 10-3 could be reached)

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Flavor asymmetry of the nucleon seaComparison of p+d with p+p Drell-Yan

Drell-Yan:

Quarkonium: 1

/ 2 (1 ( ) / ( ))2

1/ 2 (1 ( ) / ( ))

2

pd pp

p

d

d p

p

p

g x g x

d x u x

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Flavor asymmetry of the nucleon sea can be measured at small x at RHIC

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Origins of ?u dMeson-cloud around the nucleon:

Sullivan Process

12 2

T

( , ) ( )

he pion-cloud is a sou

(

rce of

/

( ) : pion-cloud distribution in the proton

:antiquark distributio

anti-quarks in the

proton, and it will lead to

n in the pi

,

o

n

!

)p x

xxq x Q dy f y q x y Q

y

f y

q

d u

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Can one probe the meson cloud directly?

Electron DIS scattering off pomeron or meson was studied at HERA by tagging forward-going proton/neutron

Pion structure functions at 10-4 < x < 10-2 were

measured

Can one study analogous processes at RHIC?

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Tagged Drell-Yan production at RHIC?

One can tag on forward-going proton, neutron, Δ, Λ in

coincidence with lepton-pair

Assuming factorization, then

( )/ )

/ )

(

(

MDY

F

D

B

YFd dy dmdx

d dmdx p

p

y

n

fx

p x

( ) is the probability for , where carries

a fraction of the proton momentumMBf y p n n

y

Tagged D-Y production could provide information on

the antiquark distribution in the mesons and on ( )MBf y

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Can one detect Drell-Yan events tagged by forward-going nucleons at RHIC?

: fraction of the proton beam momentum

carried by the tagged neutron

y

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Zero-Degree Calorimeter for neutron detection

Angular distribution of tagged neutrons

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Tag the forward p (d) using Roman Pots

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Other possible tagged Drell-Yan measurements

probe the component, and the strange quark contents

in the proton

measure helicity asymme

1.

2.

try due to meson cloud (

3.

expect )0

LLLLA

p p l l x

p p n l l x

p A A

A

l x

p K

l

4. Doubly-

"pi

tag

on exc

ged Dr

probe in nucl

ell-Yan:

ei via the A-dependence measurement

annihilati

ess"

m

e onso n- ? meson

p p p p l l x

Tagged J/ production?

Tagged jet

pr

oducti

Other hard-diff

on?

(Gluo

ractive processes at R

n contents of the meson cloud

C

)

HI ?

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Hard diffractive J/Ψ and di-jet production in Tevatron

PRL 87 (2001) 241802

J/Ψ production with a rapidity

gap

Comparison of the J/Ψ and dijet data shows the gluon fraction of proton’s diffractive structure function is 0.59 ± 0.15

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Hard diffractive production of W and Z bosons at the Tevatron

Sensitive to quark contents of the diffractive structure functions

The unique features of RHIC for hard diffractive physics are the availability of polarized protons and p-A

hep-ex/0308032

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Summary

• The quark and gluon structures of protons, nuclei and mesons could be studied at RHIC-II.

• Dilepton production can probe many interesting features of the partonic structure of hadrons.

• Capability of forward-tagging with a Roman Pot in conjuction with di-lepton detection could significantly enhance the physics reach of p-p and p-A programs at RHIC.