114 research outputs found
Single spin asymmetries in inclusive hadron production from SIDIS to hadronic collisions: universality and phenomenology
In a perturbative QCD approach, with inclusion of spin and transverse
momentum effects, experimental data on azimuthal asymmetries observed in
polarized semi-inclusive deeply inelastic scattering and e+ e- annihilations
can be used to determine the Sivers, transversity and Collins soft functions.
By using these functions, within the same scheme, we predict p(transv.
polarized) p -> h + X single spin asymmetries in remarkable agreement with RHIC
experimental data.Comment: 5 pages, 6 ps figure
General partonic structure for hadronic spin asymmetries
The high energy and large p_T inclusive polarized process, (A, S_A) + (B,
S_B) --> C + X, is considered under the assumption of a generalized QCD
factorization scheme. For the first time all transverse motions, of partons in
hadrons and of hadrons in fragmenting partons, are explicitly taken into
account; the elementary interactions are computed at leading order with
noncollinear exact kinematics, which introduces many phases in the expressions
of their helicity amplitudes. Several new spin and k_T dependent soft functions
appear and contribute to the cross sections and to spin asymmetries; we put
emphasis on their partonic interpretation, in terms of quark and gluon
polarizations inside polarized hadrons. Connections with other notations and
further information are given in some Appendices. The formal expressions for
single and double spin asymmetries are derived. The transverse single spin
asymmetry A_N, for p(transv. polarized) p --> pion + X processes is considered
in more detail, and all contributions are evaluated numerically by saturating
unknown functions with their upper positivity bounds. It is shown that the
integration of the phases arising from the noncollinear kinematics strongly
suppresses most contributions to the single spin asymmetry, leaving at work
predominantly the Sivers effect and, to a lesser extent, the Collins mechanism.Comment: RevTeX, 46 pages, 5 ps figures. v2: some clarifying comments and
appendix on kinematics added, references updated, published versio
The role of Cahn and Sivers effects in Deep Inelastic Scattering
The role of intrinsic \bfk_\perp in inclusive and semi-inclusive Deep
Inelastic Scattering processes () is studied with exact
kinematics within QCD parton model at leading order; the dependence of the
unpolarized cross section on the azimuthal angle between the leptonic and the
hadron production planes (Cahn effect) is compared with data and used to
estimate the average values of both in quark distribution and
fragmentation functions. The resulting picture is applied to the description of
the weighted single spin asymmetry recently
measured by the HERMES collaboration at DESY; this allows to extract some
simple models for the quark Sivers functions. These are compared with the
Sivers functions which succeed in describing the data on transverse single spin
asymmetries in \pup p \to \pi X processes; the two sets of functions are not
inconsistent. The extracted Sivers functions give predictions for the COMPASS
measurement of in agreement with recent
preliminary data, while their contribution to HERMES is
computed and found to be small. Predictions for for kaon production at HERMES are also given.Comment: 21 pages, 12 figures, revtex, version published in PRD, one figure,
comments and references adde
Constraints on the gluon Sivers distribution via transverse single spin asymmetries at midrapidity in p(transv. polarized) p -> pi^0 X processes at BNL RHIC
We consider the recent RHIC data on the transverse single spin asymmetry
(SSA) A_N, measured in p(transv. polarized) p -> pi^0 X processes at
mid-rapidity by the PHENIX collaboration. The measurement is consistent with a
vanishing SSA. We analyze this experimental information within a hard
scattering approach based on a generalized QCD factorization scheme, with
unintegrated, transverse momentum dependent (TMD), parton distribution and
fragmentation functions. It turns out that, in the kinematical region of the
data, only the gluon Sivers effect could give a large contribution to A_N; its
vanishing value is thus an indication about the possible size of the gluon
Sivers function (GSF). Approximate upper limits on its magnitude are derived.
Additional constraints obtained combining available parameterizations of the
quark Sivers function and the Burkardt sum rule (BSR) for the Sivers
distributions are also discussed.Comment: RevTeX, 8 pages, 2 ps figures; v2: few clarifying comments, a "note
added in proof" and some references added; version published in Phys. Rev.
General Helicity Formalism for Polarized Semi-Inclusive Deep Inelastic Scattering
We study polarized Semi-Inclusive Deep Inelastic Scattering (SIDIS)
processes, within the QCD parton model and a factorization scheme, taking into
account all transverse motions, of partons inside the initial proton and of
hadrons inside the fragmenting partons. We use the helicity formalism. The
elementary interactions are computed at LO with non collinear exact kinematics,
which introduces phases in the expressions of their helicity amplitudes.
Several Transverse Momentum Dependent (TMD) distribution and fragmentation
functions appear and contribute to the cross sections and to spin asymmetries.
Our results agree with those obtained with different formalisms, showing the
consistency of our approach. The full expression for single and double spin
asymmetries is derived. Simplified, explicit analytical expressions, convenient
for phenomenological studies, are obtained assuming a factorized Gaussian
dependence on intrinsic momenta for the TMDs.Comment: 29 pages, 2 figure
On the role of Collins effect in the single spin asymmetry A_N in p(transv. polarized) p --> h X processes
The much debated issue of the transverse single spin asymmetry A_N observed
in the inclusive large P_T production of a single hadron in pp interactions,
p(transv. polarized) p --> pion X, is considered in a TMD factorization scheme.
A previous result [1,2] stating that the maximum contribution of the Collins
effect is strongly suppressed, is revisited, correcting a numerical error. New
estimates are given, adopting the Collins functions recently extracted from
SIDIS and e+e- data, and phenomenological consequences are discussed.Comment: RevTeX4, 11 pages, 6 eps figure
Transversity and Collins Fragmentation Functions: Towards a New Global Analysis
A new, preliminary global analysis of the experimental data on azimuthal
asymmetries in SIDIS from HERMES and COMPASS collaborations, and in e+e- -->
h_1 h_2 X processes from the BELLE collaboration, is performed. The new data
allow for a more precise determination of the Collins fragmentation function
and of the transversity distribution function for and quarks, in
comparison with the results of our previous analysis. Estimates for the single
spin asymmetry A_UT^{\sin(\phi_h + \phi_S)} at JLab and COMPASS, operating on a
transversely polarized proton target, are presented.Comment: 4 pages, 4 figures. To appear in the proceedings of the XVI
International Workshop on Deep Inelastic Scattering and Related Subjects, DIS
2008, London, U.K., 7-11 April, 200
Transverse QCD Dynamics Near the Light Cone
Starting from the QCD Hamiltonian in near-light cone coordinates, we study
the dynamics of the gluonic zero modes. Euclidean 2+1 dimensional lattice
simulations show that the gap at strong coupling vanishes at intermediate
coupling. This result opens the possibility to synchronize the continuum limit
with the approach to the light cone.Comment: 15 pages, LaTeX, 3 figures (7 PS files
Towards a first estimate of the gluon Sivers function from A N data in pp collisions at RHIC
Single Spin Asymmetry in Lepton Angular Distribution of Drell-Yan Processes
We study the single spin asymmetry in the lepton angular distribution of
Drell-Yan processes in the frame work of collinear factorization. The asymmetry
has been studied in the past and different results have been obtained. In our
study we take an approach different than that used in the existing study. We
explicitly calculate the transverse-spin dependent part of the differential
cross-section with suitable parton states. Because the spin is transverse, one
has to take multi-parton states for the purpose. Our result agrees with one of
the existing results. A possible reason for the disagreement with others is
discussed.Comment: Typos corrected. Conclusions unchange
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