1. past and future experiments
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

2. Drell-Yan dilepton production
Target p
This process is electromagnetic and exactly calculable
Propagator of the virtual photon in the amplitude  factor M-2 in the cross section

3. 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.
1970s s
- Omega, CERN (Corden at al, 1978, 1980)
- CIP, FERMILAB (Anderson et al 1976, Hogan et al 1979)
NA3, CERN (Badier et al 1979, 1989)
CFS, FERMILAB (Lederman et al, 1982)
CHFMNP, CERN (Antreasyan et al)
E605, FERMILAB (Moreno et al, 1989) …
1980s – 1990s
- E615, FERMILAB (Conway et al, 1989)
- NA10, CERN (Anderson et al, 1984)
E772, FERMILAB (McGaughey et al, 1994)
NA51, CERN (Abreu et al, 1998)
E866 (E.A. Hawker et al, 1998) …

4. Present and future Drell-Yan facilities
RHIC collider S=(200)2 GeV2
J-PARC fixed target S= GeV2
FAIR collider S=200 GeV2
fixed target S=30-80 GeV2
COMPASS fixed target S= GeV2
(CERN-SPS)
NICA S=400 GeV2
(JINR)
E fixed target S=230 GeV2
(FMI)
CMS collider S=196 TeV2
(CERN-LHC)
Les Bland
Yuji Goto
Klaus Peters
This talk
Alexander Sorin

5. 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

6. Flavor asymmetry in the nucleon sea
E772, E866, NA51, E906
Light sea-quarks flavor asymmetry

7. Pion quarks distribution function
NA10, -----E615
P.J. Sutton, A.D. Martin, Phys.Rev.D, 45 (7) 1992
No available experimental
data at small x (x≤0.2) to
determine sea-quark distr.
unambiguously
Performed various fits with
sea carrying an increasing
fraction of pion momentum
p+p- 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

8. p, k mesons parton distribution functions
Light mesons parton distribution functions
p, k mesons parton distribution functions
NA10, E615, NA3
pion structure function
NA3 :

9. Angular distribution & Boer-Mulders PDF
Unpolarized angular distribution in Collin-Soper frame
λ=1,μ=ν=0 at LO, in collinear approximation  transversely polarized g,
no transverse momenta:
Lam-Tung relation: 1-l-n=0
NLO prediction: small deviation from for Pt<3GeV/c
n ≠ 0:
Boer-Mulders effect if PT2«M2
NLO corrections, at large qt

10. 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

11. Angular distribution & Boer-Mulders PDF
Violation of Lam-Tung sum rule
Boer-Mulders function can lead to
azimuthal dependence

12. Angular distribution & Boer-Mulders PDF
E866 at Fermilab
800 GeV/c p+d
800 GeV/c p+p
No noticeable flavour asymmetry between
Sea-quark Boer-Mulders function is relatively small

13. 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

14. Spin dependent parton distribution functions
Double polarization ( )
transversity
Single polarization ( )
Sivers
Boer-Mulders
transversity
No polarization ( )
Boer-Mulders

15. Spin dependent PDFs with Drell-Yan processes
What do we need to access spin dependent PDFs through DY?
polarized beam + polarized target (beam)
unpolarized beam + polarized target or
Valence
Larger asymmetries (SSA and DSA)
in region with large valence quark
content.
High luminosity:
very small DY cross section

16. Spin physics with Drell-Yan processes
in COMPASS
Measures of single-spin DY asymmetries:
Polarised DY
SSA ~ sin(f-fs) + sin(f+fs)
Beam:
p - p
S=100÷400 GeV2
Polarized target:
NH3 / 6LiD
Polarization: >80% / >40%
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

17. COMPASS spectrometer layout
MW1
Beam PT
ECAL1
HCAL1
Ibeam ~ 1108 p/s
25% of current PT
Polarized target
180 mrad acceptance
2 cells, 15 cm each

18. Kinematic range M<J/y  dominated by semileptonic
decay of charmed hadrons
M>J/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

19. Kinematic range x1, x2 Sizeable single and double spin asymmetries
in valence quark region
COMPASS
s= GeV2 M=4-9 GeV/c2
s=100 GeV2
s=200 GeV2
s=300 GeV2
s=400 GeV2

20. Kinematic range COMPASS S=100GeV2 S=300GeV2 S=200GeV2 x2 x2 x2 x1 x1Q2 Q2

21. Kinematic range: COMPASS acceptance
in “valence” region: 0.1  x1/2  0.5
x1vs x2 t
Sensitive to Sivers effect at low PT:
PT << Q PT

22. Kinematic range: COMPASS vs other experiments
x2 vs x1
COMPASS
E866
E615

23. Spin physics in Drell-Yan processes at COMPASS
SUMMARY
What…
Spin dependent PDFs transversity, Sivers function,
Boer-Mulders function
How…
1·108 p- beam on NH3/6LiD polarized target
expected luminosity: 1031 cm-2 sec-1
expected events rate: ~ in 150 days of run
S=300 GeV2 , M(m+m-): 4-9 GeV/c
When
Beyond 2010
November 11-12, Test beam at CERN SPS:
verified radiation conditions, PT and spectrometer peformances with high intensity hadron beam
EOI in preparation