379 research outputs found
Partonic Transverse Motion in Unpolarized Semi-Inclusive Deep Inelastic Scattering
We analyse the role of partonic transverse motion in unpolarized
Semi-Inclusive Deep Inelastic Scattering (SIDIS) processes. Imposing
appropriate kinematical conditions, we find some constraints which fix an upper
limit to the range of allowed kt values and lead to interesting results,
particularly for some observables like the azimuthal modulation of
the unpolarized SIDIS cross section and the average transverse momentum of the
final, detected hadron.Comment: To appear in the Proceedings of the XIX International Workshop on
Deep-Inelastic Scattering and Related Subjects (DIS 2011), April 11-15, 2011,
Newport News, VA US
A study on the interplay between perturbative QCD and CSS/TMD formalism in SIDIS processes
We study the Semi-Inclusive Deep Inelastic Scattering (SIDIS) cross section
as a function of the transverse momentum, . In order to describe it over a
wide region of , soft gluon resummation has to be performed. Here we will
use the original Collins-Soper-Sterman (CSS) formalism; however, the same
procedure would hold within the improved Transverse Momentum Dependent (TMD)
framework. We study the matching between the region where fixed order
perturbative QCD can successfully be applied and the region where soft gluon
resummation is necessary. We find that the commonly used prescription of
matching through the so-called Y-factor cannot be applied in the SIDIS
kinematical configurations we examine. In particular, the non-perturbative
component of the resummed cross section turns out to play a crucial role and
should not be overlooked even at relatively high energies. Moreover, the
perturbative expansion of the resummed cross section in the matching region is
not as reliable as it is usually believed and its treatment requires special
attention.Comment: Two references and an appendix added, 22 pages, 11 figure
Role of transverse momentum dependence of unpolarised parton distribution and fragmentation functions in the analysis of azimuthal spin asymmetries
Information on the Sivers distribution and the Collins fragmentation
functions and their transverse momentum dependence is mainly based on fitting
single spin asymmetry data from semi-inclusive deep inelastic scattering
(SIDIS). Independent information, respectively on the Sivers distribution and
the Collins fragmentation, can be obtained from Drell-Yan and
annihilation processes. In the SIDIS case, the transverse momentum of the final
observed hadron, which is the quantity measured, is generated both by the
average transverse momentum in the distribution and in the fragmentation
functions. As a consequence, these are strongly correlated and a separate
extraction is made difficult. In this paper we investigate, in a simple
kinematical Gaussian configuration, this correlation, its role on the
transverse single spin asymmetries in SIDIS and the consequences for
predictions of the Sivers asymmetry in Drell-Yan processes and for the Collins
asymmetry in annihilation. We find that, in some cases, these effects
can be relevant and must be carefully taken into account.Comment: 21 pages, 9 figures (18 eps plots
Study of the sign change of the Sivers function from STAR Collaboration W/Z production data
Recent data on the transverse single spin asymmetry measured by the
STAR Collaboration for reactions at RHIC
allow the first investigation of the Sivers function in Drell-Yan processes and
of its expected sign change with respect to SIDIS processes. A new extraction
of the Sivers functions from the latest SIDIS data is performed and a critical
assessment of the significance of the STAR data is attempted.Comment: 17 pages, 16 pdf figures, uses jheppub.st
Extracting the Sivers function from polarized SIDIS data and making predictions
The most recent data on the weighted transverse single spin asymmetry
from HERMES and COMPASS collaborations are
analysed within LO parton model with unintegrated parton distribution and
fragmentation functions; all transverse motions are taken into account, with
exact kinematics, in the elementary interactions. The overall quality of the
data is such that, for the first time, a rather well constrained extraction of
the Sivers function for and quarks is possible and is performed.
Comparisons with models are made. Based on the extracted Sivers functions,
predictions for asymmetries at JLab are given;
suggestions for further measurements at COMPASS, with a transversely polarized
hydrogen target and selecting favourable kinematical ranges, are discussed.
Predictions are also presented for Single Spin Asymmetries (SSA) in Drell-Yan
processes at RHIC and GSI.Comment: 18 pages, 10 figures, version published in PRD, one figure and
comments adde
Kinematic regions in the e+e− → hX factorized cross section in a 2-jet topology with thrust
Factorization theorems allow to separate out the universal, non-perturbative
content of the hadronic cross section from its perturbative part, which can be
computed in perturbative QCD, up to the desired order. In this paper, we derive
a rigorous proof of factorization of the cross section,
sensitive to the transverse momentum of the detected hadron with respect to the
thrust axis, in a completely general framework, based on the
Collins-Soper-Sterman approach. The results are explicitly computed to NLO-NLL
accuracy and subsequently generalized to all orders in perturbation theory.
This procedure naturally leads to a partition of the
kinematics into three different regions, each associated to a different
factorization theorem. In one of these regions, which covers the central and
widest range, the factorization theorem has a new structure, which shares the
features of both TMD and collinear factorization schemes. In the corresponding
cross section, the role of the rapidity cut-off is investigated, as its
physical meaning becomes increasingly evident. An algorithm to identify these
three kinematic regions, based on ratios of observable quantities, is provided.Comment: 78 pages, 23 figure
Universality-breaking effects in hadronic production processes
Recent BELLE measurements provide the cross section for single hadron
production in annihilations, differential in thrust and in the hadron
transverse momentum with respect to the thrust axis. Universality breaking
effects due to process-dependent soft factors make it very difficult to relate
this cross sections to those corresponding to hadron-pair production in
annihilations, where transverse momentum dependent (TMD) factorization
can be applied. The correspondence between these two cross sections is examined
in the framework of the Collins-Soper-Sterman factorization, in the collinear
as well as in the TMD approach. We propose a scheme that allows to relate the
TMD parton densities defined in 1-hadron and in 2-hadron processes, neatly
separating, within the soft and collinear parts, the non-perturbative terms
from the contributions that can be calculated perturbatively. The
regularization of rapidity divergences introduces cut-offs, the arbitrariness
of which will be properly reabsorbed by means of a mechanism closely
reminiscent of a gauge transformation. In this way, we restore the possibility
to perform global phenomenological studies of TMD physics, simultaneously
analyzing data belonging to different hadron classes.Comment: 60 pages, 15 figures, revised argument in section 6 - results
unchanged, 1 figure added in section 3, references added, typos correcte
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