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U. D'Alesio

Publications and source records attributed to U. D'Alesio.

At least 73 records · Page 4Linked to original sources
arXiv2005

Understanding the role of Cahn and Sivers effects in Deep Inelastic Scattering

The role of intrinsic $k_\perp$ in semi-inclusive Deep Inelastic Scattering (SIDIS) processes ($l p \to l h X$) is studied with exact kinematics within QCD parton model at leading order; the dependence of the unpolarized cross section on the azimuthal angle between the leptonic and the hadron production planes (Cahn effect) is compared with data and used to estimate the average values of $k_\perp$ both in quark distribution and fragmentation functions. The resulting picture is applied to the description of the weighted single spin asymmetry $A_{UT}^{\sin(ϕ_π- ϕ_S)}$ recently measured by the HERMES collaboration at DESY; this allows to extract parameters for the quark Sivers functions. The extracted Sivers functions give predictions for the COMPASS measurement of $A_{UT}^{\sin(ϕ_π- ϕ_S)}$ in agreement with recent data, while their contribution to HERMES $A_{UL}^{\sinϕ_π}$ is computed and found to be small. Predictions for $A_{UT}^{\sin(ϕ_K - ϕ_S)}$ for kaon production at HERMES are also given.

hep-ph↗
arXiv2005

The role of Cahn and Sivers effects in Deep Inelastic Scattering

The role of intrinsic $\bfk_\perp$ in inclusive and semi-inclusive Deep Inelastic Scattering processes ($\ell p \to \ell h X$) is studied with exact kinematics within QCD parton model at leading order; the dependence of the unpolarized cross section on the azimuthal angle between the leptonic and the hadron production planes (Cahn effect) is compared with data and used to estimate the average values of $k_\perp$ both in quark distribution and fragmentation functions. The resulting picture is applied to the description of the weighted single spin asymmetry $A_{UT}^{\sin(ϕ_π- ϕ_S)}$ recently measured by the HERMES collaboration at DESY; this allows to extract some simple models for the quark Sivers functions. These are compared with the Sivers functions which succeed in describing the data on transverse single spin asymmetries in $\pup p \to πX$ processes; the two sets of functions are not inconsistent. The extracted Sivers functions give predictions for the COMPASS measurement of $A_{UT}^{\sin(ϕ_π- ϕ_S)}$ in agreement with recent preliminary data, while their contribution to HERMES $A_{UL}^{\sinϕ_π}$ is computed and found to be small. Predictions for $A_{UT}^{\sin(ϕ_K - ϕ_S)}$ for kaon production at HERMES are also given.

hep-ph↗
arXiv2004

Parton intrinsic motion: unpolarized cross sections and the Sivers effect in inclusive particle production

We present a detailed study, performed in the framework of LO perturbative QCD with the inclusion of spin and k_T effects, of unpolarized cross sections for the Drell-Yan process and for inclusive pion and photon production in hadronic collisions, in different kinematical situations. We find a satisfactory agreement between theoretical predictions and experimental data. This supports the study of spin effects and transverse single spin asymmetries (SSA) within the same scheme. We then present results for SSA, generated by the so-called Sivers effect, in inclusive pion production in proton-proton collisions.

hep-ph↗
arXiv2004

Comments on Cahn and Sivers effects in SIDIS

The role of intrinsic $k_\perp$ in semi-inclusive Deep Inelastic Scattering (SIDIS) processes ($\ell p \to \ell h X$) is studied within QCD parton model at leading order. The resulting picture is applied to the description of the weighted single spin asymmetry $A_{UL}^{\sinϕ_h}$ measured by HERMES. It is shown that these data could be described by the Sivers mechanism alone. However, the extracted Sivers functions fail to describe the HERMES data on $A_{UT}^{\sin(ϕ_h-ϕ_S)}$.

hep-ph↗
arXiv2004

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.

hep-ph↗
arXiv2004

A general formalism for single and double spin asymmetries in inclusive hadron production

We present a calculation of single and double spin asymmetries for inclusive hadron production in hadronic collisions. Our approach is based on Leading Order (LO) perturbative QCD and generalized factorization theorems, with full account of intrinsic parton momentum, k_T, effects. This leads to a new class of spin and k_T-dependent distribution and fragmentation functions. Limiting ourselves to consider leading twist functions, we show how they could play a relevant role in producing non-vanishing spin asymmetries.

hep-ph↗
arXiv2004

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.

hep-ph↗
arXiv2004

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.

hep-ph↗
arXiv2004

Accessing Sivers gluon distribution via transverse single spin asymmetries in p(transv. polarized) p --> D X processes at RHIC

The production of D mesons in the scattering of transversely polarized protons off unpolarized protons at RHIC offers a clear opportunity to gain information on the Sivers gluon distribution function. D production at intermediate rapidity values is dominated by the elementary g g --> c cbar channel; contributions from q qbar --> c cbar s-channel become important only at very large values of x_F. In both processes there is no single spin transfer, so that the final c or cbar quarks are not polarized. Therefore, any transverse single spin asymmetry observed for D's produced in p(transv. polarized) p interactions cannot originate from the Collins fragmentation mechanism, but only from the Sivers effect in the distribution functions. In particular, any sizeable spin asymmetry measured in p(transv. polarized) p --> D X at mid-rapidity values will be a direct indication of a non zero Sivers gluon distribution function. We study the p(transv. polarized) p --> D X process including intrinsic transverse motion in the parton distribution and fragmentation functions and in the elementary dynamics, and show how results from RHIC could allow a measurement of the Sivers gluon distribution function.

hep-ph↗
arXiv2003

An improved analysis of k_T effects for SSA in hadronic collisions

The role of intrinsic transverse momentum both in unpolarized and polarized processes is discussed. We consider inclusive cross sections for pion production in hadronic collisions and for Drell-Yan processes. We reanalyze transverse single spin asymmetries (SSA) in inclusive pion production, p(transv. polarized) p --> pion + X, in terms of Sivers effect and show its contribution to A_{UL} in semi-inclusive DIS (SIDIS).

hep-ph↗
arXiv2002

Cross sections and single spin asymmetries in hadronic collisions with intrinsic transverse momentum effects

The role of intrinsic transverse momentum both in unpolarized and polarized processes is discussed. We consider inclusive cross sections for pion production in hadronic collisions and for Drell-Yan processes; the results are compared with available experimental data in several different kinematical situations. We reanalyze transverse single spin asymmetries (SSA) observed in inclusive pion production, p(transv. polarized) p --> pion + X, in terms of Sivers effect and show how it can be disentangled in polarized Drell-Yan processes by suitably integrating over some final configurations. Estimates for RHIC experiments are given.

hep-ph↗
arXiv2002

Single spin asymmetries, unpolarized cross sections and the role of partonic transverse momentum

Partonic intrinsic transverse momentum can be essential for the explanation of large single spin asymmetries in hadronic reactions in the framework of perturbative QCD. The status of an ongoing program investigating in a consistent way the role of intrinsic transverse momentum both in unpolarized and polarized processes is discussed. We compute inclusive cross sections for hadron and photon production in hadronic collisions and for Drell-Yan processes; the results are compared with available experimental data in several different kinematical situations.

hep-ph↗
arXiv2002

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.

hep-ph↗
arXiv2002

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.

hep-ph↗
arXiv2002

Transverse Lambda polarization in semi-inclusive DIS

Following a previous description of Lambda and Lambda-bar polarization in unpolarized p-p interactions, within a perturbative QCD factorization scheme with new polarizing fragmentation functions, here we investigate the transverse polarization of Lambda's and Lambda-bar's produced in semi-inclusive DIS. Analytical expressions for both neutral and charged current exchange are given. Since quantitative predictions cannot be given at this stage and comparison with existing data are not yet significant, we present the general formalism and a qualitative analysis displaying generic features of the Lambda and Lambda-bar polarization for specific scenarios. Different kinematical situations are considered, corresponding to experiments currently able to study Lambda production in semi-inclusive DIS.

hep-ph↗
arXiv2002

Transverse Lambda polarization in inclusive processes

A formalism proposed to study transverse Lambda polarization in unpolarized hadronic processes, based on a generalized pQCD factorization theorem, is extended to semi-inclusive DIS. Analytical expressions and examples of numerical estimates are given.

hep-ph↗
arXiv2001

Origins of single transverse spin asymmetries

Single transverse spin asymmetries in p-p -> pion + X and l-p -> l-pion + X processes have been observed; their possible origins and connections are investigated. A phenomenological description within a pQCD generalized factorization scheme is discussed.

hep-ph↗