arXiv subjects
Publications and source records attributed to M. Boglione.
Following the TMD evolution scheme recently proposed for the unpolarized and the Sivers distribution function, we propose a simple strategy to take into account this TMD Q^2 dependence in our phenomenological extraction of the Sivers function from SIDIS data. New results are presented and possible future applications are discussed.
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.
The QCD evolution of the unpolarized Transverse Momentum Dependent (TMD) distribution functions and of the Sivers functions have been discussed in recent papers. Following such results we reconsider previous extractions of the Sivers functions from semi-inclusive deep inelastic scattering data and propose a simple strategy which allows to take into account the Q^2 dependence of the TMDs in comparison with experimental findings. A clear evidence of the phenomenological success of the TMD evolution equations is given, mostly, by the newest COMPASS data off a transversely polarized proton target.
We present an extraction of the Sivers distribution functions from the most recent experimental data of the HERMES and COMPASS experiments, assuming a negligible contribution of sea quark Sivers functions.
Our knowledge on the three-dimensional momentum structure of hadrons is encoded in the Transverse Momentum Dependent partonic distribution and fragmentation functions (TMDs). A brief and updated review of the TMDs and of the processes in which they might play a role is presented.
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.
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 k_perp values. We show that, applying these additional requirements on the partonic kinematics, we obtain different results with respect to the usual phenomenological approach based on the Gaussian smearing with analytical integration over an unlimited range of k_perp values. These variations are particularly interesting for some observables, like the < cos phi_h > azimuthal modulation of the unpolarized SIDIS cross section or the average transverse momentum of the final, detected hadron.
We present a decomposition of the hadronic tensor for a general polarized Drell-Yan process AB --> l^+ l^- X in terms of helicity structure functions using the helicity axes of the dilepton rest frame as a basis. Next, we consider a QCD parton model and in the framework of a generalized QCD factorization scheme, which applies when the dilepton invariant mass M is much larger than the transverse component of the photon momentum, q_T, in the hadronic center of mass frame. In this approximation we compute the angular distribution of the unpolarized Drell Yan cross section and of the Sivers effect, in single polarized Drell-Yan scattering, by taking into account the transverse motion of partons inside the initial hadrons, k_\perp. Interesting and simple results are found in the kinematical region of k_\perp similar to q_T << M, to first order in a q_T / M expansion. Finally, explicit analytical expressions, convenient for phenomenological studies, are obtained assuming a factorized Gaussian dependence on intrinsic momenta for the unpolarized and Sivers distribution functions, in analogy to those obtained for 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.
Polarised Semi-Inclusive Deep Inelastic Scattering (SIDIS) processes allow to study Transverse Momentum Dependent partonic distributions (TMDs), which reveal a non trivial three dimensional internal structure of the hadrons in momentum space. One of the most representative of the TMDs is the so-called Sivers function that describes the distribution of unpolarized quarks inside a transversely polarized proton. We present a novel extraction of the Sivers distribution functions from the most recent experimental data of HERMES and COMPASS experiments. Using suitable parametrizations, within the TMD factorization scheme, and a simple fitting strategy, we also perform a preliminary exploration of the role of the proton sea quarks.
Predictions for the transverse single spin asymmetry (SSA), A_N, are given for the inclusive processes l p(transv. pol.) --> h X and l p(transv. pol.) --> jet + X, which could be measured in operating or future experiments. These estimates are based on the Sivers distributions and the Collins fragmentation functions which fit the azimuthal asymmetries measured in semi-inclusive deep inelastic scattering (SIDIS) processes (l p(transv. pol.) --> l' h X). The factorization in terms of transverse momentum dependent distribution and fragmentation functions (TMD factorization) - which supplies the theoretical framework in which SIDIS azimuthal asymmetries are analyzed - is assumed to hold also for the l p --> h X inclusive process at large P_T. A measurement of A_N would then provide a direct test of the validity of the TMD factorization in this case and would have important consequences for the study and understanding of SSAs in p p(transv. pol.) --> h X processes.
We consider the Sivers, Collins and transversity functions as extracted from SIDIS and e+e- experimental data and investigate to what extent they might explain the large Single Spin Asymmetries (SSA) observed in proton-proton inclusive processes. This phenomenological study is performed within the TMD factorization scheme. As the SIDIS data cover only a limited range of x values (x < 0.3), we allow for different large $x$ behaviours of the SIDIS Sivers functions and transversity distributions. We conclude that, within the available experimental constraints, one cannot observe any clear universality breaking effect for the Sivers functions.
We study the transverse single spin asymmetry (SSA), A_N, for the single inclusive process l p (transv. polarized) --> h + X, in a perturbative QCD factorization scheme with inclusion of spin and transverse momentum dependent (TMD) distributions. By adopting the relevant TMDs (Sivers and Collins functions) as extracted from semi-inclusive deep inelastic scattering (SIDIS) and e+e- data, predictions for these SSAs are given. A measurement of A_N for this process could then provide a direct test of the validity of the TMD factorization.
The Sivers distributions recently extracted from semi-inclusive deep inelastic scattering data [1] are used to compute estimates for Sivers asymmetries in Drell-Yan processes which are being planned at several facilities (RHIC, COMPASS, J-PARC, PAX, PANDA, NICA (JINR) and SPASCHARM (IHEP)). Most of these asymmetries turn out to be large and could allow a clear test of the predicted sign change of the Sivers distributions when active in SIDIS and Drell-Yan processes. This is regarded as a fundamental test of our understanding, within QCD and the factorization scheme, of single spin asymmetries.
We study the Sivers effect in the transverse single spin asymmetries (SSA) for pion and kaon production in semi-inclusive deep inelastic scattering (SIDIS) processes. We perform a fit of A^sin(phi_h-phi_S)_UT which, by including recent high statistics experimental data for pion and kaon production from HERMES and COMPASS Collaborations, allows a new determination of the Sivers distribution functions for quarks and antiquarks with u, d and s flavours. Estimates for forthcoming SIDIS experiments at COMPASS and JLab are given.
We present an update of a previous 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. Compared to the first extraction, a more precise determination of the Collins fragmentation function and the transversity distribution function for u and d quarks is obtained.
The longitudinal double spin asymmetry A_LL in the reaction pp --> pi X has been measured at RHIC with extremely interesting consequences. If the gluon polarization in a proton were as big as needed to resolve the famous "spin crisis" then A_LL would be large and positive. Latest RHIC results indicate that A_LL is small and disfavour large positive values of the gluon polarization. We examine whether the soft mechanisms (Collins, Sivers, Boer-Mulders), essential for generating transverse single spin asymmetries, have any significant influence on A_LL, and whether they could alter the conclusion that the gluon polarization is necessarily small. It turns out that the contribution from these effects is essentially negligible.
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.