1. Physics with polarized antiprotons at GSI-PAX
Transversity via Drell-Yan processes
direct access to transversity
Transverse Single Spin Asymmetries
QCD “theorem”: (Sivers)D-Y = – (Sivers)DIS
Time-like e.l.m. form factors
SSA in
form factors and
vs.
Elastic processes
spin misteries like in pp ?
M. Anselmino, Milano, May 4, 2005

2. Polarization data has often been the
graveyard of fashionable theories.
If theorists had their way, they might
just ban such measurements altogether
out of self-protection.
J.D. Bjorken
St. Croix, 1987

3. Parton distributions and (or
are fundamental leading-twist quark distributions
quark distribution – well known
all equally important
quark helicity distribution – known
transversity distribution – unknown
gluon helicity distribution – poorly known
related to
chiral-even
related to
chiral-odd
positivity bound

4. + = – + = – + + + + + – + + – – + – in helicity basis
decouples from DIS

5. – + + – + – – + – + – + – + h1 x h1 h1 x Collins function
h1 must couple to another chiral-odd function. For example: D-Y, pp → μ+μ- X, and SIDIS, l p → l π X, processes – +
h1 x h1 – +
J. Ralston and D.Soper, 1979 J. Cortes, B. Pire, J. Ralston, 1992 + – + –
h1 x Collins function + – – + + –
J. Collins, 1993

6. Elementary LO interaction:
3 planes: plane
polarization vectors, ┴
plenty of spin effects
p-γ* plane, μ+μ- γ* plane
Plenty of single and double spin effects

7. h1q (x, Q2) evolution much slower than
h1 from
at RHIC
RHIC energies:
small x1 and/or x2
h1q (x, Q2) evolution much slower than
Δq(x, Q2) and q(x, Q2) at small x
ATT at RHIC is very small
smaller s would help
Barone, Calarco, Drago
Martin, Schäfer, Stratmann, Vogelsang

8. h1 from at GSI GSI energies:
large x1,x2
GSI energies:
one measures h1 in the quark valence region: ATT is estimated to be large, between 0.2 and 0.4

9. Energy for Drell-Yan processes
"safe region":
QCD corrections might be very large at smaller values of M:
yes, for cross-sections, not for ATT K-factor almost spin-independent
Fermilab E GeV/c
H. Shimizu, G. Sterman, W. Vogelsang and H. Yokoya, hep-ph/
V. Barone et al., in preparation

10. s=30 GeV2
s=45 GeV2
s=210 GeV2
s=900 GeV2

11. s=30 GeV2
s=45 GeV2
s=210 GeV2
s=900 GeV2

12. data from CERN WA39, π N processes, s = 80 GeV2

13. measure ATT also in J/ψ resonance regionq l+ q l+
J/ψ q l– q l–
all vector couplings, same spinor structure
and, at large x1, x2
measure ATT also in J/ψ resonance region
M. A., V. Barone, A. Drago and N. Nikolaev

14. Single Spin Asymmetries (and their partonic origin)π Pq k┴
Collins effect = fragmentation of polarized quark depends on Pq· (pq x k┴) pq q P k┴
Sivers effect = number of partons in polarized proton depends on P · (p x k┴) p Pq q
Boer-Mulders effect = polarization of partons in unpolarized proton depends on Pq · (p x k┴) k┴ p
Collins: chiral-odd
Sivers: chiral-even
Boer-Mulders: chiral-odd
These effects may generate SSA

15. SSA, pp → πX BNL-AGS √s = 6.6 GeV 0.6 < pT < 1.2 p↑p
E704 √s = 20 GeV < pT < p↑p
STAR-RHIC √s = 200 GeV 1.1 < pT < p↑p
E704 √s = 20 GeV 0.7 < pT < p↑p
SSA, pp → πX

16. SSA, SIDIS

17. Transversity and SSA in Drell-Yan processes
extract these unknown chiral-odd functions from unpolarized cross-section
Boer-Mulders functions
combine above measurement with measurement of AN to obtain information on h1
D. Boer, 1999
This AN expected of the order of a few percents
A. Bianconi, M. Radici

18. Direct access to Sivers function
usual parton distribution
test QCD basic result:
J. Collins
process dominated by no Collins contribution
usual fragmentation function
same process at RHIC is dominated by

19. Electromagnetic form factors
Jμem p p '
F1(0) = F2(0) = 1
Κ = 1.79
In pQCD
difficult to separate F1,F2 or GE,GM
JLAB: F2/F1 ~ 1.25 GeV/Q

20. In time-like region GE and GM may have relative phasesθ p p L l- σN
- σ –N
there may be a SSA:
= Ay = Py σN
+ σ –N

21. Py can be very large: predictions depend strongly on model assumptions for F2/F1

22. Unexpected spin effects in pp elastic scattering
larger t region can be explored in

23. Unexpected large polarization in
What about ?

24. Conclusions
ATT in D-Y processes at GSI energies: highway to transversity
…the transverse-spin asymmetries whose measurement is suggested for these experiments, are remarkably insensitive to shifts in the overall normalization. In summary, perturbative corrections appear to make the cross sections larger independently of spin. They would therefore make easier the study of spin asymmetries, and ultimately transversity distributions. (G. Sterman et al.)
AN and SSA: many effects expected and test of QCD theorem: (Sivers)D-Y = – (Sivers)DIS
Exploration and separation of proton form factors, spin results from new time-like region
Spin asymmetries in proton-antiproton elastic processes: as mysterious as in proton-proton?