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Mauro Anselmino

Publications and source records attributed to Mauro Anselmino.

At least 19 recordsLinked to original sources
arXiv2021

A new method to extract the valence transversity distributions

A new method is suggested for the extraction of $u$-quark and $d$-quark transversity distributions, using single spin asymmetry (SSA) data in semi-inclusive deep inelastic scattering (SIDIS) processes, where they couple to the Collins or the di-hadron fragmentation functions. We discuss a recent suggestion to extract the transversity distribution using the concept of difference asymmetries and their ratios, which avoids the requirement of Collins function. We suggest new measurements, involving ratios of polarized cross-sections, that would directly probe the ratio $h_1^{d_v}/h_1^{u_v}$. We also show some numerical estimates.

hep-ph↗
arXiv2020

Transverse spin effects in hard semi-inclusive collisions

The nucleons (protons and neutrons) are by far the most abundant form of matter in our visible Universe; they are composite particles made of quarks and gluons, the fundamental quanta of Quantum Chromo Dynamics (QCD). The usual interpretation of the nucleon dynamics in high energy interactions is often limited to a simple one-dimensional picture of a fast moving nucleon as a collection of co-linearly moving quarks and gluons (partons), interacting accordingly to perturbative QCD rules. However, massive experimental evidence shows that, in particular when transverse spin dependent observables are involved, such a simple picture is not adequate. The intrinsic transverse motion of partons has to be taken into account; this opens the way to a new, truly 3-dimensional (3D) study of the nucleon structure. A review of the main experimental data, their interpretation and understanding in terms of new transverse momentum dependent partonic distributions, and the progress in building a 3D imaging of the nucleon is presented.

hep-ph↗
arXiv2019

The Polarising Fragmentation Function and the Lambda polarisation in e+ e- processes

The surprising polarisation of Lambdas and other hyperons measured in many unpolarised hadronic processes, p N --> Lambda X, has been a long standing challenge for QCD phenomenology. One possible explanation was suggested, related to non perturbative properties of the quark hadronisation process, and encoded in the so-called Polarising Fragmentation Function (PFF). Recent Belle data have shown a non zero Lambda polarisation also in unpolarised e+ e- processes, e+ e- --> Lambda X and e+ e- --> Lambda h X. We consider the single inclusive case and the role of the PFFs. Adopting a simplified kinematics it is shown how they can originate a polarisation P_Lambda different from 0 and give explicit expressions for it in terms of the PFFs. Although the Belle data do not allow yet, in our approach, an extraction of the PFFs, some clear predictions are given, suggesting crucial measurements, and estimates of P_Lambda are computed, in qualitative agreement with the Belle data.

hep-ph↗
arXiv2011

A leading-twist beam-spin asymmetry in double-hadron inclusive lepto-production

We show that a new beam-spin asymmetry appears in deep inelastic inclusive lepto-production at low transverse momenta when a hadron in the target fragmentation region is observed in association with another hadron in the current fragmentation region. The beam leptons are longitudinally polarized while the target nucleons are unpolarized. This asymmetry is a leading-twist effect generated by the correlation between the transverse momentum of quarks and the transverse momentum of the hadron emitted by the target. Experimental signatures of this effect are discussed.

hep-ph↗
arXiv2011

Correlations between SIDIS azimuthal asymmetries in target and current fragmentation regions

We shortly describe the leading twist formalism for spin and transverse-momentum dependent fracture functions recently developed and present results for the production of spinless hadrons in the target fragmentation region (TFR) of SIDIS. In this case not all fracture functions can be accessed and only a Sivers-like single spin azimuthal asymmetry shows up at LO cross-section. Then, we show that the process of double hadron production in polarized SIDIS -- with one spinless hadron produced in the current fragmentation region (CFR) and another in the TFR -- would provide access to all 16 leading twist fracture functions. Some particular cases are presented.

hep-ph↗
arXiv2011

Double hadron lepto-production in the current and target fragmentation regions

We study the inclusive production of two hadrons in deep inelastic processes, l N ---> l h_1 h_2 X, with h_1 in the current fragmentation region (CFR) and h_2 in the target fragmentation region (TFR). Assuming a factorized scheme, the recently introduced polarized and transverse momentum dependent fracture functions couple to the transverse momentum dependent fragmentation functions. This allows the full exploration of the fracture functions for transversely polarized quarks. Some particular cases are considered.

hep-ph↗
arXiv2003

Summary of the Working Group on Spin Physics

A summary is given of the experimental and theoretical results presented in the working group on spin physics. New data on inclusive and semi-inclusive deep-inelastic scattering, combined with theoretical studies of the polarized distribution functions of nucleons, were presented. Many talks addressed the relatively new subjects of transversity distributions and generalized parton distributions. These distributions can be studied by measuring single spin asymmetries, while partonic intrinsic motion and models of new spin dependent distribution and fragmentation functions are needed to obtain the corresponding theoretical description. These subjects are not only studied in deep-inelastic lepton scattering, but also in polarized proton-proton collisions at RHIC. A selection of results that have been obtained in these experiments together with several associated theoretical ideas are presented in this paper. In conclusion, a brief sketch is given of the prospects for experimental and theoretical studies of the spin structure of the nucleon in the coming years.

hep-ph↗
arXiv2003

Transversity and Lambda polarization

Two related issues are discussed, which might be easily explored by present and future COMPASS experiments. The first one deals with the new world of transversity, the fundamental polarized parton distribution so far totally unknown. The second issue concerns Lambda production in polarized semi-inclusive processes, with a measurement of the Lambda polarization, which might give novel information on distribution and fragmentation properties of polarized partons. In case of transverse polarization the detection of Lambda's gives access to a new way of measuring transversities. Also the interesting case of Lambda polarization in unpolarized processes is discussed.

hep-ph↗
arXiv2003

What do we know about the proton spin structure?

A brief summary of the theoretical and experimental knowledge of the spin structure of the proton is presented. The helicity distributions of quark and gluons are discussed, together with their related sum rules. The transversity distribution is also introduced with possible strategies for its measurement. Novel spin dependent and $\bfk_\perp$ unintegrated distribution and fragmentation functions are discussed, in connection with a new and rich phenomenology of transverse single spin asymmetries.

hep-ph↗
arXiv2002

Single spin asymmetries in QCD

Measurements of single transverse spin asymmetries in high energy inclusive processes have always shown unexpected and challenging results. Several cases are considered and discussed within a QCD approach which couples perturbative dynamics to new non perturbative partonic information; the aim is that of developing a consistent phenomenological description of these unusual single spin phenomena, based on a generalized QCD factorization scheme.

hep-ph↗
arXiv2001

Status of Spin Physics

Fundamental spin physics has made striking progresses in the last years; new ideas, experiments and data interpretations have been proposed and keep emerging. A review of some of the most important issues in the spin structure of nucleons is made and prospects for the future are discussed.

hep-ph↗
arXiv1999

Phenomenology of single spin asymmetries in p(transv. polarized)-p -> pion + X

A phenomenological description of single transverse spin effects in hadron-hadron inclusive processes is proposed, assuming a generalized factorization scheme and pQCD hard interactions. The transverse momentum, k_T, of the quarks inside the hadrons and of the hadrons relatively to the fragmenting quark, is taken into account in distribution and fragmentation functions, and leads to possible non zero single spin asymmetries. The role of k_T and spin dependent quark fragmentations -- the so-called Collins effect -- is investigated in details in p(transv. polarized)-p -> pion + X processes: it is shown how the experimental data could be described, obtaining an explicit expression for the spin asymmetry of a polarized fragmenting quark, on which some comments are made. Predictions for other processes, possible further applications and experimental tests are discussed.

hep-ph↗
arXiv1998

Single spin asymmetries in transversely polarized proton(antiproton) - proton inclusive processes

We consider several single spin asymmetries in inclusive, transversely polarized proton(antiproton) - proton processes as higher twist QCD contributions, taking into account spin and intrinsic transverse momentum effects in the quark distribution functions. This approach has been previously applied to the description of the single spin asymmetries observed in transversely polarized proton - proton -> pion + X reactions and all its parameters fixed: we give here predictions for new processes, which agree with experiments for which data are available, and suggest further possible measurements.

hep-ph↗
arXiv1998

Single spin asymmetries and vector meson production in DIS

We discuss possible measurements and origins of single spin asymmetries in DIS and of some unusual spin properties of vector mesons produced in lepton- nucleon, photon-nucleon and photon-photon interactions. Such effects have already been observed in other processes.

hep-ph↗
arXiv1998

Spin effects in vector meson production at LEP

Spin observables may reveal much deeper properties of non perturbative hadronic physics than unpolarized quantities. We discuss the polarization of hadrons produced in $e^+e^-$ annihilation at LEP. We show how final state $q \bar q$ interactions may give origin to non zero values of the off-diagonal element $ρ^{\,}_{1,-1}$ of the helicity density matrix of vector mesons: some predictions are given for $K^*, ϕ, D^*$ and $B^*$ in agreement with recent OPAL data. We also discuss the relative amount of vector and pseudovector meson states and the probability of helicity zero vector states. Similar measurements in other processes are suggested.

hep-ph↗
arXiv1997

Single spin asymmetries in $\ell p^\uparrow \to hX$ and $e^-e^+ \to q\bar q \to h^\uparrow X$

Spin observables may reveal much deeper properties of non perturbative hadronic physics than unpolarized quantities. We discuss here possible origins of single spin asymmetries in DIS, absent in the elementary lepton-quark interactions, and suggest strategies to isolate and understand some non perturbative spin dependence of distribution and fragmentation functions. We also discuss the polarization of hadrons produced in $e^+e^-$ annihilation at LEP and show how final state $\qq$ interactions may give origin to non zero values of the off-diagonal element $ρ_{1,-1}$ of the helicity density matrix of vector mesons: some predictions are given for $K^{*0}, ϕ$ and $D^*$ in agreement with recent OPAL data. Possible analogous effects in DIS and other processes are suggested.

hep-ph↗
arXiv1997

Off-diagonal Helicity Density Matrix Elements for Vector Mesons Produced at LEP

Final state quark-antiquark interactions may give origin to non zero values of the off-diagonal element rho_{1,-1} of the helicity density matrix of vector mesons produced in e+e- annihilations, as confirmed by recent OPAL data on phi and D^*'s. Predictions are given for rho_{1,-1} of several mesons produced at large z and small p_T, i.e. collinear with the parent jet; the values obtained for phi and D^* are in agreement with data.

hep-ph↗