High-mass Dilepton Physics at RHIC-II
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Transcript of High-mass Dilepton Physics at RHIC-II
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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.