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

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

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

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

– + + – + – – + – + – + – + 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

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

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

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

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 E866 800 GeV/c H. Shimizu, G. Sterman, W. Vogelsang and H. Yokoya, hep-ph/0503270 V. Barone et al., in preparation

s=30 GeV2 s=45 GeV2 s=210 GeV2 s=900 GeV2

s=30 GeV2 s=45 GeV2 s=210 GeV2 s=900 GeV2

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

measure ATT also in J/ψ resonance region q 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

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

SSA, pp → πX BNL-AGS √s = 6.6 GeV 0.6 < pT < 1.2 p↑p E704 √s = 20 GeV 0.7 < pT < 2.0 p↑p STAR-RHIC √s = 200 GeV 1.1 < pT < 2.5 p↑p E704 √s = 20 GeV 0.7 < pT < 2.0 p↑p SSA, pp → πX

SSA, SIDIS

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

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

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

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

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

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

Unexpected large polarization in What about ?

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?