1,018 research outputs found
Parton intrinsic motion: suppression of the Collins mechanism for transverse single spin asymmetries in p(transv. polarised) p --> pion + X
We consider a general formalism to compute inclusive polarised and
unpolarised cross sections within pQCD and the factorisation scheme, taking
into account parton intrinsic motion in distribution and fragmentation
functions, as well as in the elementary dynamics. Surprisingly, the intrinsic
partonic motion, with all the correct azimuthal angular dependences, produces a
strong suppression of the transverse single spin asymmetry arising from the
Collins mechanism. As a consequence, and in contradiction with earlier claims,
the Collins mechanism is unable to explain the large asymmetries found in
p(transv. polarised) p --> pion + X at moderate to large Feynman x_F. The
Sivers effect is not suppressed.Comment: LaTeX, 21+1 pages, 1 ps figur
On the importance of Lorentz structure in the parton model: target mass corrections, transverse momentum dependence, positivity bounds
We show that respecting the underlying Lorentz structure in the parton model
has very strong consequences. Failure to insist on the correct Lorentz
covariance is responsible for the existence of contradictory results in the
literature for the polarized structure function g_2(x), whereas with the
correct imposition we are able to derive the Wandzura-Wilczek relation for
g_2(x) and the target-mass corrections for polarized deep inelastic scattering
without recourse to the operator product expansion. We comment briefly on the
problem of threshold behaviour in the presence of target-mass corrections.
Careful attention to the Lorentz structure has also profound implications for
the structure of the transverse momentum dependent parton densities often used
in parton model treatments of hadron production, allowing the k_T dependence to
be derived explicitly. It also leads to stronger positivity and Soffer-type
bounds than usually utilized for the collinear densities.Comment: RevTeX4, 17 pages, 7 eps 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
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.Comment: LaTeX 2e, 5 pages, 1 ps figure, uses ws-procs9x6. Talk delivered by
U. D'Alesio at the ``16th International Spin Physics Symposium'', SPIN2004,
October 10-16, 2004, Trieste, Ital
The Gluon Sivers Distribution in D production at RHIC
The single transverse spin asymmetry in D meson production at RHIC can
provide a clean measure of the gluon Sivers distribution function. At
intermediate rapidity, D production is largely dominated by the elementary gg
-> c c-bar channel, where there cannot be any transverse spin transfer.
Therefore, any transverse single spin asymmetry observed for D's produced in
polarized proton-proton interactions can only originate from the Sivers effect
in the gluon distribution functions. A sizeable transverse single spin
asymmetry measured by PHENIX or STAR experiments would then give direct
information on the size of the gluon Sivers distribution function.Comment: 4 pages, 2 postscript figures, talk given by M. Boglione at SPIN04,
16th International Spin Physics Symposium, Trieste, Oct. 11-16 200
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.Comment: LaTeX, 5 pages, 1 eps figure, uses epsfig, wrapfig. Talk delivered by
S. Melis at the ``XI Workshop on High Energy Spin Physics'', SPIN 05,
September 27 - October 1, 2005, Dubna, Russi
Helicity Formalism and Spin Asymmetries in Hadronic Processes
We present a generalized QCD factorization scheme for the high energy
inclusive polarized process, (A, S_A) + (B, S_B) -> C + X, including all
intrinsic partonic motions. This introduces many non-trivial azimuthal phases
and several new spin and k_T dependent soft functions. The formal expressions
for single and double spin asymmetries are discussed. Numerical results for
A_N[p(transv. polarized) p -> pi + X] are presented.Comment: 8 pages, 4 ps figures, uses ws-procs9x6.cls and rotating_pr.sty
(included). Talk delivered by F. Murgia at the "International Workshop on
Transverse Polarisation Phenomena in Hard Processes" (TRANSVERSITY 2005),
September 7-10 2005, Como, Ital
Generation of maximally entangled states with sub-luminal Lorentz boost
Recent work has studied entanglement between the spin and momentum components
of a single spin-1/2 particle and showed that maximal entanglement is obtained
only when boosts approach the speed of light. Here we extend the boost scenario
to general geometries and show that, intriguingly, maximal entanglement can be
achieved with boosts less than the speed of light. Boosts approaching the speed
of light may even decrease entanglement. We also provide a geometric
explanation for this behavior
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.Comment: 8 pages, 10 eps figures, uses ws-procs9x6.cls; Talk presented by F.
Murgia at the II International Workshop on Transverse Polarisation Phenomena
in Hard Processes (Transversity 2008), May 28-31, 2008, Ferrara (Italy
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