Transversity via DrellYan processes

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
form factors and
vs.
SSA in
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
chiral-odd
related to
positivity bound
4














in helicity basis


decouples from DIS


5
h1 must couple to another chiral-odd function.
For example D-Y, pp ? µµ- X, and SIDIS, l p ?
l p 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-? plane, µµ- ? plane
Plenty of single and double spin effects
7
h1 from
at RHIC
RHIC energies
small x1 and/or x2
h1q (x, Q2) evolution much slower than ?q(x, Q2)
and q(x, Q2) at small x
Barone, Calarco, Drago
ATT at RHIC is very small smaller s would help
Martin, Schäfer, Stratmann, Vogelsang
8
h1 from
at GSI
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 E866 800 GeV/c
H. Shimizu, G. Sterman, W. Vogelsang and H.
Yokoya, hep-ph/0503270
V. Barone et al., in preparation
10
s30 GeV2
s45 GeV2
s210 GeV2
s900 GeV2
11
s30 GeV2
s45 GeV2
s210 GeV2
s900 GeV2
12
data from CERN WA39, p N processes, s 80 GeV2
13
l
l
q
q
J/?
l
l
q
q
all vector couplings, same spinor structure
and, at large x1, x2
measure ATT also in J/? resonance region
M. A., V. Barone, A. Drago and N. Nikolaev
14
Single Spin Asymmetries (and their partonic
origin)
p
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-Mulde
rs chiral-odd
These effects may generate SSA
15
BNL-AGS vs 6.6 GeV 0.6 lt pT lt 1.2 p?p
E704 vs 20 GeV 0.7 lt pT lt 2.0 p?p
STAR-RHIC vs 200 GeV 1.1 lt pT lt 2.5 p?p
E704 vs 20 GeV 0.7 lt pT lt 2.0 p?p
SSA, pp ? pX
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
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 ? 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
phases
N
S
l
P
?
p
L
p

l-
sN
- s N
Ay Py
there may be a SSA
sN
s 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?