Diapositiva 1

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Spin physics in Drell-Yan processes past and
future experiments

Michela Chiosso University of Torino Dip.
Fisica Generale I.N.F.N - Torino
IWHSS 2008 Torino April 2nd 2008
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Drell-Yan dilepton production
d
d
u
u
Target p
This process is electromagnetic and exactly
calculable Propagator of the virtual photon in
the amplitude ? factor M-2 in the cross section
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Past Drell-Yan experiments

• This process was first described by Sydney Drell
  and Tung-Mow Yan in 1970.
• First quantitative Drell-Yan experiments late
  1970s ? waiting for spectrometers able to measure
  pbarn cross-sections in the presence of much
  larger background.

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Present and future Drell-Yan facilities
RHIC collider S(200)2 GeV2
J-PARC fixed target S
60-100 GeV2 FAIR collider
S200 GeV2 fixed target
S30-80 GeV2 COMPASS
fixed target S100-400 GeV2 (CERN-SPS)
NICA
S400 GeV2 (JINR) E906
fixed target S230 GeV2 (FMI) CMS
collider S196
TeV2 (CERN-LHC)
Les Bland
Yuji Goto
Klaus Peters
Alexander Sorin
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Investigating PDFs with Drell-Yan processes
Since many years Drell-Yan process has been
playing a key role in the study of parton
distribution functions (PDFs)

• Flavor asymmetry of nucleon parton distribution
  functions
• E772, E866, NA51, E906
• p, k mesons parton distribution functions
• NA10, E615, NA3
• Boer-Mulders transverse momentum dependent parton
  distribution
• Function (TMD PDFs)
• E615, NA10, E866

Recently it is drawing back attention as a unique
tool to directly access spin dependent parton
distribution functions

• Transversity
• Sivers function

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Flavor asymmetry in the nucleon sea
E772, E866, NA51, E906
Light sea-quarks flavor asymmetry
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Pion quarks distribution function
P.J. Sutton, A.D. Martin, Phys.Rev.D, 45 (7) 1992

• No available experimental
• data at small x (x0.2) to
• determine sea-quark distr.
• unambiguously
• Performed various fits with
• sea carrying an increasing
• fraction of pion momentum
• pp- valence distr. are the
• same, but different contr. to
• DY through quark charge
• squared

gluon distribution Prompt photon production
valence distribution Fit to Drell-Yan data
sea-quark distribution Fit to Drell-Yan data
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Light mesons parton distribution functions
p, k mesons parton distribution functions
NA10, E615, NA3
pion structure function
NA3
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Angular distribution Boer-Mulders PDF
Unpolarized angular distribution in Collin-Soper
frame

?1,µ?0 at LO, in collinear approximation ?
transversely polarized g, no transverse
momenta Lam-Tung relation 1-l-n0 NLO
prediction small deviation from
for Ptlt3GeV/c
n ? 0
Boer-Mulders effect if PT2M2 NLO corrections,
at large qt
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Angular distribution Boer-Mulders PDF
1.There is a sizable cos2f asymmetry (n up to
0.3) in the unpolarized pion-induced Drell-Yan
the Lam-Tung sum rule is violated beyond the
QCD-improved parton model.
NA10 , E615
2. l -1 at large xf
E615

3. No azimuthal asymmetry in proton-induced
Drell-Yan
E866
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Angular distribution Boer-Mulders PDF
Violation of Lam-Tung sum rule
Boer-Mulders function can lead to
azimuthal dependence

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Angular distribution Boer-Mulders PDF
E866 at Fermilab
800 GeV/c pd
800 GeV/c pp
No noticeable flavour asymmetry between
Sea-quark Boer-Mulders function is relatively
small
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Spin dependent parton distribution functions
Polarized Drell-Yan Transversity ? direct access
to without convolution
with fragmentation function , like
in SIDIS
Sivers function

• Boer-Mulders function

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Spin dependent parton distribution functions

Sivers
Boer-Mulders
transversity
No polarization ( )
Boer-Mulders
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Spin dependent PDFs with Drell-Yan processes
What do we need to access spin dependent PDFs
through DY?
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Spin physics with Drell-Yan processes in
COMPASS
Measures of single-spin DY asymmetries Polarised
DY SSA sin(f-fs) sin(ffs)
Beam
p - p?
S100400 GeV2
Polarized target NH3 / 6LiD Polarization gt80
/ gt40 Dilution factor 0.15 / 0.35 Luminosity
Unpolarised DY ds cos(2f) ?
Using p- beam it is necessary to make an
assumption connecting pion and proton PDFs
p - p
1031 cm-2 s-1
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COMPASS spectrometer layout
ECAL1
HCAL1
Ibeam 1?108 p/s
25 of current PT
Polarized target 180 mrad acceptance 2 cells, 15
cm each
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Kinematic range
MltJ/y ? dominated by semileptonic decay of
charmed hadrons MgtJ/y ?Drell-Yan dilepton
production major contribution M must be large
enough to apply pQCD But production rate falls
off rapidly with M
Safe region
Out from resonances regions dominated by strong
production mechanism
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Kinematic range
x1, x2
Sizeable single and double spin asymmetries in
valence quark region
COMPASS

• s100-400 GeV2 M4-9 GeV/c2
• t0.16-0.8 _at_ s100 GeV2
• t0.08-0.4 _at_ s200 GeV2
• t0.05-0.3 _at_ s300 GeV2
• t0.04-0.2 _at_ s400 GeV2

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Kinematic range
COMPASS
S300GeV2
S100GeV2
S200GeV2
x2
x2
x2
x1
x1
x1
S100GeV2
S300GeV2
Q2
Q2
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Kinematic range COMPASS acceptance
x1vs x2
??in valence region 0.1 ? x1/2 ? 0.5
t
Sensitive to Sivers effect at low PT PT ltlt Q
PT
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Kinematic range COMPASS vs other experiments
COMPASS
E866
E615
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Spin physics in Drell-Yan processes at COMPASS
SUMMARY