arXiv subjects

Marco Radici

Publications and source records attributed to Marco Radici.

At least 73 records · Page 4Linked to original sources

arXiv2012

Quark angular momentum and the Sivers asymmetry

The determination of quark angular momentum requires the knowledge of the generalized parton distribution $E$ in the forward limit. We assume a connection between this function and the Sivers transverse-momentum distribution, based on model calculations and theoretical considerations. Using this assumption, we show that it is possible to fit at the same time nucleon anomalous magnetic moments and semi-inclusive single-spin asymmetries. This imposes additional constraints on the Sivers function and opens a plausible way to quantifying the quark angular momentum.

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arXiv2012

Status on the transversity parton distribution: the dihadron fragmentation functions way

We report on the extraction of dihadron fragmentation functions (DiFF) from the semiinclusive production of two hadron pairs in back-to-back jets in e+e- annihilation. A nonzero asymmetry in the correlation of azimuthal orientations of opposite $π^+π^-$ pairs is related to the transverse polarization of fragmenting quarks through a significant polarized DiFF. A combined analysis of this asymmetry and the spin asymmetry in the SIDIS process $ep^{\uparrow}\to e'(π^+π^-)X$ has led to the first extraction of the u and d-flavor transversity parton distribution function in the framework of collinear factorization.

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arXiv2012

Unpolarized and Polarized Fragmentation Functions

I give an overview of the present knowledge about nonperturbative functions parametrizing the fragmentation into one or two hadrons of (un)polarized light quarks in vacuum, including information on their transverse momentum dependence.

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arXiv2012

Constraining quark angular momentum through semi-inclusive measurements

The determination of quark angular momentum requires the knowledge of the generalized parton distribution E in the forward limit. We assume a connection between this function and the Sivers transverse-momentum distribution, based on model calculations and theoretical considerations. Using this assumption, we show that it is possible to fit at the same time nucleon magnetic moments and semi-inclusive single-spin asymmetries. This imposes additional constraints on the Sivers function and opens a plausible way to quantifying quark angular momentum.

hep-ph↗

arXiv2011

First glances at the transversity parton distribution through dihadron fragmentation functions

We present first observations of the transversity parton distribution based on an analysis of pion-pair production in deep inelastic scattering off transversely polarized targets. The extraction of transversity relies on the knowledge of dihadron fragmentation functions, which we take from electron-positron annihilation measurements. This is the first attempt to determine the transversity distribution in the framework of collinear factorization.

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arXiv2011

The Dihadron fragmentation functions way to Transversity

Observations of the transversity parton distribution based on an analysis of pion-pair production in deep inelastic scattering off transversely polarized targets are presented. This extraction relies on the knowledge of dihadron fragmentation functions, which are obtained from electron-positron annihilation measurements. This is the first attempt to determine the transversity distribution in the framework of collinear factorization.

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arXiv2010

Weighted azimuthal asymmetries in a diquark spectator model

We analytically calculate weighted azimuthal asymmetries in semi-inclusive lepton-nucleon deep-inelastic scattering and Drell-Yan processes, using transverse-momentum-dependent partonic densities obtained in a diquark spectator model. We compare the asymmetries with available preliminary experimental data, in particular for the Collins and the Sivers effect. We make predictions for other cases of interest in running and planned experiments.

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arXiv2008

Asymmetries involving dihadron fragmentation functions: from DIS to e+e- annihilation

Using a model calculation of dihadron fragmentation functions, we fit the spin asymmetry recently extracted by HERMES for the semi-inclusive pion pair production in deep-inelastic scattering on a transversely polarized proton target. By evolving the obtained dihadron fragmentation functions, we make predictions for the correlation of the angular distributions of two pion pairs produced in electron-positron annihilations at BELLE kinematics. Our study shows that the combination of two-hadron inclusive deep-inelastic scattering and electron-positron annihilation measurements can provide a valid alternative to Collins effect for the extraction of the quark transversity distribution in the nucleon.

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arXiv2007

Nucleon Spin Structure with hadronic collisions at COMPASS

In order to illustrate the capabilities of COMPASS using a hadronic beam, I review some of the azimuthal asymmetries in hadronic collisions, that allow for the extraction of transversity, Sivers and Boer-Mulders functions, necessary to explore the partonic spin structure of the nucleon. I also report on some Monte Carlo simulations of such asymmetries for the production of Drell-Yan lepton pairs from the collision of high-energy pions on a transversely polarized proton target.

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arXiv2007

Evolution equations for extended dihadron fragmentation functions

We consider dihadron fragmentation functions, describing the fragmentation of a parton in two unpolarized hadrons, and in particular extended dihadron fragmentation functions, explicitly dependent on the invariant mass, $M_h$, of the hadron pair. We first rederive the known results on $M_h$-integrated functions using Jet Calculus techniques, and then we present the evolution equations for extended dihadron fragmentation functions. Our results are relevant for the analysis of experimental measurements of two-particle-inclusive processes at different energies.

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arXiv2007

Transversity and inclusive two-pion production

A model for dihadron fragmentation functions is briefly outlined, that describes the fragmentation of a quark in two unpolarized hadrons. The parameters are tuned to the output of the PYTHIA event generator for two-hadron semi-inclusive production in deep inelastic scattering at HERMES. Then, predictions are made for the unknown polarized fragmentation function and the related single-spin asymmetry in the azimuthal distribution of $π^+ π^-$ pairs in semi-inclusive deep inelastic scattering on transversely polarized targets at HERMES and COMPASS. This asymmetry can be used to extract the quark transversity distribution.

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arXiv2006

Effective role of unpolarized nonvalence partons in Drell-Yan single spin asymmetries

We perform numerical simulations of the Sivers effect from single spin asymmetries in Drell-Yan processes on transversely polarized protons. We consider colliding antiprotons and pions at different kinematic conditions of interest for the future planned experiments. We conventionally name "framework I" the results obtained when properly accounting for the various flavor dependent polarized valence contributions in the numerator of the asymmetry, and for the unpolarized nonvalence contribution in its denominator. We name "framework II" the results obtained when taking a suitable flavor average of the valence contributions and neglecting the nonvalence ones. We compare the two methods, also with respect to the input parametrization of the Sivers function which is extracted from data with approximations sometimes intermediate between frameworks I and II. Deviations between the two approaches are found to be small except for dilepton masses below 3 GeV. The Sivers effect is used as a test case; the arguments can be generalized to other interesting azimuthal asymmetries in Drell-Yan processes, such as the Boer-Mulders effect.

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arXiv2006

Extraction of proton form factors in the timelike region from single-polarized e+ e- -> \vec p anti-p events

We have performed numerical simulations of the single-polarized e+ e- --> \vec p anti p process in kinematic conditions under discussion for a possible upgrade of the existing DAFNE facility. By fitting the cross section and spin asymmetry angular distributions with typical Born expressions, we can study the conditions for extracting information on moduli and phases of the proton electromagnetic form factors in the timelike region, which are poorly known and whose preliminary data show puzzling features. We have explored also non-Born contributions by introducing a further component in the angular fit, which is related to two-photon exchange diagrams. Using a dipole parametrization, we show that these corrections can be identified if larger than 5% of the Born contribution; we also explore the conditions for extracting information on the phase and, consequently, on the relative weight between their real and imaginary parts, which are presently unknown.

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arXiv2006

Modeling dihadron fragmentation functions

We present a model for dihadron fragmentation functions, describing the fragmentation of a quark into two unpolarized hadrons. We tune the parameters of our model to the output of the PYTHIA event generator for two-hadron semi-inclusive production in deep inelastic scattering at HERMES. Once the parameters of the model are fixed, we make predictions for other unknown fragmentation functions and for a single-spin asymmetry in the azimuthal distribution of pi+ pi- pairs in semi-inclusive deep inelastic scattering on a transversely polarized target at HERMES and COMPASS. Such asymmetry could be used to measure the quark transversity distribution function.

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arXiv2006

Monte Carlo simulation of single spin asymmetries in pion-proton collisions

We present Monte Carlo simulations of both the Sivers and the Boer-Mulders effects in the polarized Drell-Yan $π^\pm p^\uparrow \to μ^+ μ^- X$ process at the center-of-mass energy $\sqrt{s} \sim 14$ GeV reachable at COMPASS with pion beams of energy 100 GeV. For the Sivers effect, we adopt two different parametrizations for the Sivers function to explore the statistical accuracy required to extract unambiguous information on this parton density. In particular, we verify the possibility of checking its predicted sign change between Semi-Inclusive Deep-Inelastic Scattering (SIDIS) and Drell-Yan processes, a crucial test of nonperturbative QCD. For the Boer-Mulders effect, because of the lack of parametrizations we can make only guesses. The goal is to explore the possibility of extracting information on the transversity distribution, the missing piece necessary to complete the knowledge of the nucleon spin structure at leading twist, and the Boer-Mulders function, which is related to the long-standing problem of the violation of the Lam-Tung sum rule in the unpolarized Drell-Yan cross section.

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arXiv2005

Alternative approaches to transversity: how convenient and feasible are they?

The complete knowledge of the nucleon spin structure at leading twist requires also addressing the transverse spin distribution of quarks, or transversity, which is yet unexplored because of its chiral-odd nature. Elaborating strategies to extract it from (spin) asymmetry data represents a unique opportunity to explore more generally the transverse spin structure of the nucleon and the transverse momentum dynamics of partons inside it. Here, we critically review some of the most promising approaches.

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arXiv2005

Monte Carlo simulation of the Sivers effect in high-energy proton-proton collisions

We present Monte Carlo simulations of the Sivers effect in the polarized Drell-Yan $pp^\uparrow \to μ^+ μ^- X$ process at the center-of-mass energy $\sqrt{s}=200$ GeV reachable at the Relativistic Heavy-Ion Collider (RHIC) of BNL. We use two different parametrizations for the Sivers function, one deduced from the analysis of Semi-Inclusive Deep-Inelastic Scattering (SIDIS) data at much lower energies, and another one constrained by the RHIC data for the $pp^\uparrow \to πX$ process at the same energy. For a given luminosity of $10^{32}$ cm$^{-2}$ sec$^{-1}$, we explore the necessary conditions to reach a statistical accuracy that allows to extract unambiguous information on the structure of the Sivers function. In particular, we consider the feasibility of the test on its predicted universality property of changing sign when switching from SIDIS to Drell-Yan processes.

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arXiv2005

Monte Carlo simulation of events with Drell-Yan lepton pairs from antiproton-proton collisions: the fully polarized case

In this paper, we extend the study of Drell-Yan processes with antiproton beams already presented in a previous work. We consider the fully polarized $\bar{p}^\uparrow p^\uparrow \to μ^+ μ^- X$ process, because this is the simplest scenario for extracting the transverse spin distribution of quarks, or transversity, which is the missing piece to complete the knowledge of the nucleon spin structure at leading twist. We perform Monte Carlo simulations for transversely polarized antiproton and proton beams colliding at a center-of-mass energy of interest for the future HESR at GSI. The goal is to possibly establish feasibility conditions for an unambiguous extraction of the transversity from data on double spin asymmetries.

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