17 research outputs found
Sivers effect and transverse single spin asymmetries in Drell-Yan processes
Sivers asymmetry, adopted to explain transverse single spin asymmetries (SSA)
observed in inclusive pion production, p(transv. polarized) p --> pion + X and
p-bar(transv. polarized) p --> pion + X is used here to compute SSA in
Drell-Yan processes; in this case, by considering the differential cross
section in the lepton-pair invariant mass, rapidity and transverse momentum,
other mechanisms which may originate SSA cannot contribute. Estimates for RHIC
experiments are given
Transverse single spin asymmetries in Drell-Yan processes
Recently, it has been shown, contrary to previous beliefs, that the k_T
distribution of quarks in a transversely polarized proton can be asymmetric.
This ``Sivers effect'' had already been used to explain transverse single spin
asymmetries (SSA) observed in inclusive pion production, p(transv. polarized) p
--> pion + X and p-bar(transv. polarized) p --> pion + X. In such channels,
however, other mechanisms, like the ``Collins effect'' (a k_T asymmetric
fragmentation of a transversely polarized quark into pions), may generate SSA.
The Sivers asymmetry is used here to compute SSA in Drell-Yan processes; in
this case, by considering the differential cross-section in the lepton-pair
invariant mass, rapidity and transverse momentum, other mechanisms which may
originate SSA cannot contribute. Estimates for RHIC experiments are given
Quark fragmentation and off-diagonal helicity density matrix elements for vector meson production
As confirmed by some recent LEP data on phi, K^* and D^*0 production, final
state interactions in quark fragmentation may give origin to non-zero values of
the off-diagonal element rho_{1,-1} of the helicity density matrix of vector
mesons produced in e^+ e^- annihilations: we give estimates for rho_{1,-1} of
vector mesons with a large x_E and collinear with the parent jet, relating its
size and sign to the associated hard constituent dynamics. We mention possible
non-zero values of rho_{1,-1} in several other processes
Sivers effect and transverse single spin asymmetries in Drell-Yan processes
Sivers asymmetry, adopted to explain transverse single spin asymmetries (SSA)
observed in inclusive pion production, p(transv. polarized) p --> pion + X and
p-bar(transv. polarized) p --> pion + X is used here to compute SSA in
Drell-Yan processes; in this case, by considering the differential cross
section in the lepton-pair invariant mass, rapidity and transverse momentum,
other mechanisms which may originate SSA cannot contribute. Estimates for RHIC
experiments are given
Transversity and Collins Functions: from e+ e- --> h1 h2 X to SIDIS Processes
We present [1] the first simultaneous extraction of the transversity
distribution and the Collins fragmentation function, obtained through a
combined analysis of experimental data on azimuthal asymmetries in
semi-inclusive deep inelastic scattering (SIDIS), from the HERMES and COMPASS
Collaborations, and in e+ e- --> h1 h2 X processes, from the Belle
Collaboration
Transverse single spin asymmetries in Drell-Yan processes
Recently, it has been shown, contrary to previous beliefs, that the k_T
distribution of quarks in a transversely polarized proton can be asymmetric.
This ``Sivers effect'' had already been used to explain transverse single spin
asymmetries (SSA) observed in inclusive pion production, p(transv. polarized) p
--> pion + X and p-bar(transv. polarized) p --> pion + X. In such channels,
however, other mechanisms, like the ``Collins effect'' (a k_T asymmetric
fragmentation of a transversely polarized quark into pions), may generate SSA.
The Sivers asymmetry is used here to compute SSA in Drell-Yan processes; in
this case, by considering the differential cross-section in the lepton-pair
invariant mass, rapidity and transverse momentum, other mechanisms which may
originate SSA cannot contribute. Estimates for RHIC experiments are given
Non-Standard Time Reversal and Transverse Single-Spin Asymmetries
A system of quarks interacting with chiral fields is shown to provide a
physical realization of a "non-standard" time reversal for particle multiplets
which mixes the multiplet components. We argue that, if the internal structure
of the nucleon is governed by a chiral dynamics, the so-called T-odd quark
distribution functions are not forbidden by time-reversal invariance and hence
might be non vanishing. This agrees with some other recent results. From a
phenomenological point of view, this would give rise to single-spin asymmetries
in inclusive processes involving a transversely polarized nucleon: in
particular, in pion lepto- and hadro-production and in Drell-Yan processes
General helicity formalism for Single and Double Spin Asymmetries in p p --> pion + X
We consider within a generalized QCD factorization approach, the high energy
inclusive polarized process p p --> pion + X, including all intrinsic partonic
motions. Several new spin and k_T-dependent soft functions appear and
contribute to cross sections and spin asymmetries. We present here formal
expressions for transverse single spin asymmetries and double longitudinal
ones. The transverse single spin asymmetry, A_N, is considered in detail, and
all contributions are evaluated numerically. It is shown that the azimuthal
phase integrations strongly suppress most contributions, leaving at work mainly
the Sivers effect
Sivers and Collins effects in polarized pp scattering processes
We summarize the present phenomenology of Sivers and Collins effects for
transverse single spin asymmetries in polarized proton-proton collisions within
the framework of the generalized parton model (GPM). We will discuss a
reassessment of the Collins effect and some preliminary predictions for SSA's
in p(pol) p -> pi,K + X processes at RHIC obtained using updated information
from SIDIS data and a new set of meson fragmentation functions
Transversity and Collins functions from SIDIS and e+e- data
A global analysis of the experimental data on azimuthal asymmetries in
semi-inclusive deep inelastic scattering (SIDIS), from the HERMES and COMPASS
Collaborations, and in e+e- --> h1 h2 X processes, from the BELLE
Collaboration, is performed. It results in the extraction of the Collins
fragmentation function and, for the first time, of the transversity
distribution function for u and d quarks. These turn out to have opposite signs
and to be sizably smaller than their positivity bounds. Predictions for the
azimuthal asymmetry A_{UT}^{sin(phi_h + phi_S)}, as will soon be measured at
JLab and COMPASS operating on a transversely polarized proton target, are then
presented