8 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
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
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
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
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
Can the Collins mechanism explain the large transverse single spin asymmetries observed in p(transv. polarized) p --> pion X?
We present a calculation of inclusive polarised and unpolarised cross
sections within pQCD and the factorisation scheme, taking into account the
parton intrinsic motion, k_T, in distribution and fragmentation functions, as
well as in the elementary dynamics. We show, in contradiction with earlier
claims, that the Collins mechanism is suppressed and unable to explain the
large asymmetries found in p(transv. polarized) p --> pion X at moderate to
large Feynman x_F. The Sivers effect is not suppressed
Parton Densities and Fragmentation Functions from Polarized Lambda Production in Semi-Inclusive DIS
We consider the longitudinal polarization of Lambda and Lambda-bar produced
in the current fragmentation region of polarized deep inelastic scattering. We
show how the various cross sections can be used to test the underlying parton
dynamics, and how one can extract information about certain parton densities
which are poorly known, in particular the polarized strange density sum
Delta-s(x) + Delta-s-bar(x), and about fragmentation functions which are
totally unknown and which are difficult to access by other means. We show also
how one can obtain information concerning the intriguing question as to whether
s(x) = s-bar(x) and whether Delta-s(x) = Delta-s-bar(x)