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Hadron Structure with Dilepton Production
• Brief historical review • Recent highlights • Future prospects
Jen-Chieh Peng
Workshop on “High-energy Hadron Physics with Hadron Beams”, KEK, January 6-8, 2010
University of Illinois at Urbana-Champaign
Outline
2
First Dimuon Experiment
29 GeV protonp U X
Lederman et al. PRL 25 (1970) 1523
• Experiment originally designed to search for neutral weak boson (Z0)
• Missed the J/Ψ signal !
4
The Drell-Yan Process
2 2
21 2 1 2
1 2 1 2. .
4( ) ( ) ( ) ( )
9 a a a a aaD Y
de q x q x q x q x
dx dx sx x
5
Lepton-pair production also provides unique information on parton distributions
194 GeV/c
W X
800 GeV/c
p W X
1.8 TeV
p p l l X
Probe antiquark distribution in nucleon Probe antiquark
distribution in pion Probe antiquark
distributions in antiproton
Unique features of D-Y: antiquarks, unstable hadrons…
6
Deep-Inelastic Scattering versus Drell-Yan
Drell-Yan cross sections are well described by NLO calculations
DIS Drell-Yan
(hep-ph/9905409)
7
Quarkonium production provides complementary information
2
1
2 2 2
2 2
1/ 2
1
1
2
2/ ( / ) 2 ( , ; ) /(
where
4 )
/ and
F PT F
F
d dx J F d H x x m x
m s x x x
21 2 1 2
, ,
22
2
1
21( ) ( )
(
{ ( ) ( ) ( ) ( )} ( ; )
,
( ;
; )
)i i i iP T P T
i u
T
s
PT
d
PG x G x
q x q x q x q
gg QQ
x
H x
m
m
q QQ
m
q
x
Gluon-gluon fusion
Quark-antiquark annihilation
Calculations versus data
Schuler, Vogt, PL B 387(1996)181
• Proceed via strong interaction
• Sensitive to gluons
Color evaporation model
F is the “fudge factor”
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Meson East Spectrometer(E605/772/789/866)
Open-aperture Closed-aperture Beam-dump (Cu)
J/ΨJ/Ψ
Ψ’
σ(J/ψ) ~ 15 MeV σ(J/ψ) ~ 150 MeVσ(J/ψ) ~ 300 MeV
800 GeV proton beam
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Physics with High-Mass Dimuon Production
Antiquarks in nuclei and nucleons
Quark energy loss in nuclear medium
Drell-
1) Drell-Yan process:
2) Quarkonium
Yan angular distributions
Pronounced nuclear dependence
produ
Produc
c
t
ti
io
on
n mechn
:
3) Heavy qua
ism and pola
rk productio
rizations
Gluon distributions in the nucleons
Open charm production
B-meson produc
n:
tion
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Is in the proton? u d
The Gottfried Sum Rule 1
2 20
1
0
[( ( ) ( )) / ]
1 2( ( ) ( ))
3 3
( )1
3 p p
p nG
p p
S F x F x x dx
u x d
i
x dx
f u d
New Muon Collaboration (NMC) obtainsSG = 0.235 ± 0.026
( Significantly lower than 1/3 ! )
=Expect if sea quarks
are produced in
d u
g qq
?d u
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/ flavor asymmetry from Drell-Yand u
2 2
1/ 2 (1 ( ) / ( )Dr )
2ell-Yan: pd pp d x u x
2 2
21 2 1 2
1 2 1 2. .
4( ) ( ) ( ) ( )
9 a a a a aaD Y
de q x q x q x q x
dx dx sx x
1 2 :at x x
800 GeV proton beam
on hydrogen and deuterium
mass spectrum
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Gluon distributions in proton versus neutron?
E866data: ( ) / 2 ( )p d X p p X
/ 2 [1 ( ) / ( )] / 2:
/ , : / 2 [1 ( ) / ( )] / 2
pd pp
pd ppn p
d x uDrell Y x
g
an
J x g x
Lingyan Zhu et al., PRL, 100 (2008) 062301 (arXiv: 0710.2344)
Gluon distributions in proton and neutron are very similar
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Meson Cloud Models Chiral-Quark Soliton Model Instantons
• nucleon = chiral soliton
• expand in 1/Nc
• Quark degrees of freedom in a pion mean-field
Theses models also have implications on
• asymmetry between and ( )s x ( )s x
• flavor structure of the polarized sea
Meson cloud has significant contributions to sea-quark distributions
(For reviews, see Kumano (hep-ph/9702367 ), Garvey and Peng (nucl-ex/0109010))
Theory: Thomas, Miller, Kumano, Londergan, Henley, Speth, Hwang, Liu, Cheng/Li, Ma, etc.
Origins of ( ) ( )?u x d x
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Meson cloud model
Thomas / Brodsky and Ma
Analysis of neutrino DIS data
( )x s s
NuTeV, PRL 99 (2007) 192001
( ) ( ) ?s x s x
p K (( ))us uds
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Predictions for sea-quark polarizations
0 ( ) (
( )
)
( ) ( ) 0
u uu u u K us
u x
s
d x s x
( ) ( ) ( ) ( )u x d x d x u x
• Meson Cloud Model
• Chiral-Quark Soliton Model
First results are obtained from polarized DIS. Remain to be tested by W-production at RHIC
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What's next for / ?d u2
21 2 1 2
1 2 1 2
4[ ( ) ( ) ( ) ( )
1]
3DY
i ii
de q x q
sx q x q x
dx dx x x
J-PARC 50 GeV
Intriguing / behavior at large
can be studied at lower beam energies
d u x DY cross section is 16 times larger
at 50 GeV than at 800 GeV
120 GeV proton bea
Fermilab E-906
(P. Reimer, D. Geesaman et al.)
J-PARC P-04
(J. Peng, S. Sawada et
m
50 GeV proton
al
be
.)
am
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/ from W production at RHICd u production in collisi (Generalized Drell-Yan)o nW p p
p p W x p p W x
( )u d W ( )d u W
No nuclear effects
No assumption of charge-symmetry
Large Q2 scale
21
21
( )( )
(
( )(
( ) ))
( )
FF
F
dpp W X
dx u xR x
d d xpp
d x
uW xXdx
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u d
u d
Using recent PDFs
Yang, Peng, Perdekamp, Phys. Lett. B680, 231 (2009)
/ from W production at RHICd u/ ( )
( ) at 500 GeV/ ( )
FF
F
d dx pp W xR x s
d dx pp W x
A comparison with D-Y could lead to extraction of CSV effect
( ) ( )? ( ) ( )? etc.p n p nIs u x d x u x d x
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Charge Symmetry Violation from MRST Global fits
(Eur. Phys. J. C35, 325 (2004))
4 0.5(
(
) (1 ) ( 0.0
) ( ) ( )
Best fit: 0.2
0.8 0.65 (90%
( ) ( ) ( )
( )
C.L.)
90
(
9
)
)
( )
V V
p nV V V
p nV V V
u x d x f x
d x d x u x
u x u
f x x x
x
x
x d
Best fit: 0.08
(8% of C
0.08 0.18 (90
( ) ( )[
SV
1 ]
( ) ( )[
fo
% C
1 ]
r
.L
.
!
)
sea )
n p
n p
u x d x
d x u x
CSV for sea quarks
CSV for valence quarks
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Comparison between MRST and quark-model calculation
Charge symmetry violation for valence quarks
MRST Quark-model
(Rodionov, Thomas, Londergan)Eur. Phys. J. C35, 325 (2004)
x(uVp-dV
n)
x(dVp-uV
n)
Pion-induced D-Y can measure CSV effect(See arXiv:0907.2352 for a recent review on CSV and
possible experimental tests)
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Three parton distributions describing quark’s transverse momentum and/or transverse spin
1) Transversity
2) Sivers function
3) Boer-Mulders function
Correlation between and q Ns S
Correlation between and q qs k
Correlation between and N qS k
Three transverse quantities:
1) Nucleon transverse spin
2) Quark transverse spin
3) Qaurk transverse
mo
Three diff
me
er
ntum
ent correlations
N
q
q
S
s
k
22
4
26 4
Q
sxd
),()(])1(1{[ 211
,
22 h
qqq PzDxfey
22 (1) 2
1 12,
22 (1) 2
1 12,
2 21 1
,
cos(2 )
sin(2 )
(1 ) ( ) ( , )4
| |
s
(1 ) ( ) ( , )4
| | (1 ) ( ) (in( )) ,
q qhq h
q qN h
q qhL q L h
q qN h
q
lh
lh
l lh
qhST q h
q qh
Py e h x H z P
z M M
PS y e h x H z P
z M M
PS y e h x H z P
zM
2 2 (1) 21 1
,
32 (2) 2
1 13 2,
2 21 1
,
1| | (1 ) ( ) ( , )
2
| | (1 ) ( ) ( , )6
1| | (1 ) ( ) ( , )
2
1| | (1 )
sin( )
sin(3 )
co ( )2
s
q qhT q T h
q qN
q qhT q T h
q qN h
q qe L q h
q q
he T
N
l lh S
l lh S
l lh S
PS y y e f x D z P
zM
PS y e h x H z P
z M M
S y y e g x D z P
PS y y e
zM
2 (1) 2
1 1,
( ) ( , )}q qq T h
q q
g x D z P
Unpolarized
Polarized target
Polarzied beam and
target
SL and ST: Target Polarizations; λe: Beam Polarization
Sivers
Transversity
Boer-Mulders
Transversity and Transverse Momentum Dependent PDFs are probed in Semi-Inclusive DIS
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Transversity and Transverse Momentum Dependent PDFs are also probed in Drell-Yan
1 1
1
a) Boer-Mulders functions:
b) Sivers functions
- Unpolarized Drell-Yan:
- Single transverse spin asymmetry in polarized Drell-Yan:
:
c) Transv
( ) ( ) cos(2 )
( ) ( )
e
DY q q
DYN T q q q
d h x h x
A f x f x
1 1
rsity distributions:
Drell-Yan does not require knowledge of the fragmentation functions
T-odd TMDs are pre
- Double transverse spin asymmetry in polarized Drell-Yan:
(
d
) ( )DYTT q qA h x h x
icted to change sign from DIS to DY
(Boer-Mulders and S
Remains to be te
ivers functions)
sted experimenta lly!
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Boer-Mulders function h1┴
1
1
1 represents a correlation between quark's and
transverse spin in an unpolarized hadro
is a time-reversal odd, chiral-odd TMD parton distributio
can lea
n
d
n
to an azimuthal cos(2
T
h
h
h
k
) dependence in Drell-Yan
Boer, PRD 60 (1999) 014012
● Observation of large cos(2Φ) dependence in Drell-Yan with pion beam
●
● How about Drell-Yan with proton beam?
194 GeV/c π + W
2 21 31 cos sin 2 cos sin cos 2
4 2
d
d
1 1 ( ) ( )q qh x h x
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With Boer-Mulders function h1┴:
ν(π-Wµ+µ-X)~ [valence h1┴(π)] * [valence
h1┴(p)]
ν(pdµ+µ-X)~ [valence h1┴(p)] * [sea h1
┴(p)]
Azimuthal cos2Φ Distribution in p+p and p+d Drell-Yan
E866 Collab., Lingyan Zhu et al., PRL 99 (2007) 082301; PRL 102 (2009)
182001
Sea-quark BM functions are much smaller than valence quarks
Smallνis observed for p+d and p+p D-Y
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Polarized Drell-Yan with polarized proton beam?
• Polarized Drell-Yan experiments have never been done before
• Provide unique information on the quark (antiquark) spin
Quark helicity distribution
Quark transversity distribution
Can be measured at RHIC, J-PARC, FAIR etc. (see talk by Goto)
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• Does Sivers function change sign between DIS and Drell-Yan?
• Does Boer-Mulders function change sign between DIS and Drell-Yan?
• Are all Boer-Mulders functions alike (proton versus pion Boer-Mulders functions)
• Flavor dependence of TMD functions• Independent measurement of transversity
with Drell-Yan
Outstanding questions in TMD to be addressed by future Drell-Yan experiments
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Future prospect for Drell-Yan experiments• Fermilab p+p, p+d, p+A
– Unpolarized beam and target
• RHIC– Polarized p+p collision
• COMPASS– π-p and π-d with polarized targets
• FAIR– Polarized antiproton-proton collision
• J-PARC– Possibly polarizied proton beam and target
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Modification of Parton Distributions in NucleiEMC effect observed in DIS
How are the antiquark distributions modified in nuclei?F2 contains contributions from quarks and antiquarks
(Ann. Rev. Nucl. Part. Phys., Geesaman, Sato and Thomas)
Extensive study by Kumano et al. and Strikman et al.
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Drell-Yan on nuclear targets
The x-dependence of can be directly measured( ) / ( )A Nu x u x
( )
( )
pAA
pdN
u x
u x
PRL 64 (1990) 2479 PRL 83 (1999) 2304
No evidence for enhancement of antiquark in niclei !?
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Can there be other nuclear effects in Drell-Yan?
1) DIS 2) Drell-Yan
3) Semi-inclusive DIS
• No initial state rescattering
• No final state rescattering
• No initial state rescattering
• Possible final state rescattering
• Possible initial state rescattering
• No final state rescattering
e
e’
e
e’
• Possible initial state rescattering effects in Drell-Yan need to be identified (and subtracted)
• Drell-Yan is analogous to semi-inclusive DIS (rather than DIS)
h
h
μ+μ-
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How to identify initial-state interaction in Drell-Yan?
• Interaction would degrade the longitudinal momentum of the dimuons
• dσ/dxF would shift to more negative xF for Drell-Yan on nuclei
• Nuclear dependence would drop below 1 as xF -> 1
E772 Drell-Yan data
Nuclear modification of PDF depends on x2, initial-state effect depends on xF
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D-Y measurement at lower energies is importantFractional energy loss is larger at 50 GeV
Garvey and JCP, PRL 90 (2003) 092302
Very sensitive measurement of quark energy loss at J-PARC is possible
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Cloet, Bentz, and Thomas, arXiv:0901.3559
Isovector mean-field generated in Z≠N nuclei can modify nucleon’s u and d PDFs in nucleons
Is / asymmetry modified in nuclei?d u
What are the flavor dependences of the EMC effects?
(first considered by Kumano et al.)
35
The Drell-Yan Process: pN X
2 2 /
1 /
pA
pd
Z N d u
A A d u
Assuming dbar/ubar = 1.5 for the nucleons at x=0.15, then the above ratios are:
1.0 for 40Ca, 1.042 for 208Pb
Drell-Yan ratios for p-A /p-d :
Can be measured at J-PARC
Can one measure / in nuclei?d u
36
Nuclear modification of spin-dependent PDF?
Bentz, Cloet et al., arXiv:0711.0392
EMC effect for g1(x)
Very difficult to measure !
Easier to measure the nuclear modification of Boer-Mulders functions (only unpolarized targets are required)? (See Bianconi and Radici, J. Phys. G31 (2005) 645)
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Can one measure gluon distribution in nuclei with quarkonium production?
p + A at 800 GeV/cE772 data σ(p+A) = Aασ(p+N)
Strong xF - dependence
Nuclear effects scale with xF, not x2 → Effects other than parton distribution modification need to be separated.
See talk by Leitch
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Targets (Z = -24.0”)0 = Empty1 = 0.502 “ Copper2 = 2.036 “ Beryllium3 = 1.004 “ Copper
High-pT Single Muon Trigger
Single muon measurement in E866 p+AThesis of Stephen Klinksiek
High-pT single muon events are dominated by D-meson decays
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Preliminary E866 results on the single-muon (open-charm) nuclear-dependence
Cu / Be Ratios
PT (GeV/C) Rapidity (y)
PT and XF (y) dependences have similar trend as J/Ψ
Can be further studied at J-PARC
Thesis of S. Klinksiek
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Summary• Unique information on hadron structures has been
obtained with dilepton production experiments using hadron beams.
• On-going and future dilepton production experiments at various hadron facilities can address many important unresolved issues in the spin and flavor structures of nucleons and nuclei.