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Elvio Di Salvo

Publications and source records attributed to Elvio Di Salvo.

15 recordsLinked to original sources
arXiv2025

Predictions and Tests for Semi-leptonic Decays of Beauty Hadrons,

We make predictions and propose tests for the semi-leptonic decays $B\to D^{(*)} l \barν_l$ and $Λ_b\to Λ_c l \barν_l$, with $l$ = $e$, $μ$ and $τ$. Preliminarily, we study such processes according to the $\ell-s$ scheme, which helps interpreting the results of previous analyses. Then, we write the angular distributions of those decays in the helicity formalism and suggest for each decay some tests for distinguishing among the various Dirac operators which may explain the tensions with the standard model. In particular, we propose, for $l$ = $e$ and $μ$, to study the fully differential distribution. In the cases of the $B\to D^*$ and $Λ_b$ decays, important physical information can be extracted even after integrating over the momentum of the charged lepton, which is quite suitable for $l$ = $τ$. Especially, the decay angular distribution of polarized $Λ_b$ depends on five observables, which assume different values according to the various new-physics interactions; one of these observables is related to the longitudinal polarization of the $τ-\barν_τ$ system, which can be compared with the $D^*$ longitudinal polarization in the $B\to D^*$ decay. Moreover, we analyze the $τ$ anomaly in the branching ratios of the semi-leptonic decays; to this end, we make some assumptions, which have a remarkable predictive power and lead to results in agreement with previous theoretical calculations. Thanks to these assumptions, we combine our present analysis of the $Λ_b$ decay with the previous ones of the $B\to D^{(*)}$ decays, getting strong constraints on the parameters which characterize the different new-physics operators and an indication on the most likely one.

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arXiv2020

Parametrizations of the Spin Density Matrix

We propose for the spin density matrix two parametrizations which automatically fulfil the non-negativity conditions, without setting any bound on the parameters. The first one relies on a theorem, that we prove, and it is rather simple and easily adaptable to some specific reactions, where, for example, parity is conserved or angular momentum conservation entails selection rules. Moreover, in the case when the rank is less than the order of the density matrix, we show how to improve the fits to the data, either by implementing previous suggestions, or by elaborating an alternative method, for which we prove a second theorem. Our second parametrization is a variant of previous treatments, it appears suitable for some particular processes. Moreover, we discuss about the possibility of inferring the elements of the density matrix from the differential decay width. Last, we illustrate various examples of current interest, both in strong and weak decays; some of them may be helpful in the investigation of physics beyond the standard model.

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arXiv2017

Searching for New Physics in Semi-Leptonic Baryon Decays

We propose two different and complementary observables for singling out possible signals of physics beyond the standard model in the semi-leptonic decays $Λ_b \to Λ_c \ell{\bar ν}_{\ell}$, both with the $τ$ lepton and with a light lepton. The two observables are the partial decay width and a T-odd asymmetry, whose respective sensitivities to scalar and/or pseudo-scalar coupling are calculated as functions of the parameters characterizing new physics. Two different form factors are used. Three particular cases are discussed and analyzed in detail.

hep-ph↗
arXiv2015

Phenomenological Analysis of the Decay $B^{\pm}\to K^{\pm} p {\bar p}

We propose a parametrization for intepreting some of the presently available data of the $B^{\pm} \to K^{\pm} p {\bar p}$ decay, in particular those by LHCb and Belle collaborations. The model is inspired by the well-known current and transition contributions, usually assumed in this kind of decay. However, in the light of considerations about the dominant graphs and about final state interactions, we modify some parameters of the model, determining them by means of a best fit to data. We show the results, which we discuss in some detail. Moreover we give some predictions on other observables relative to the decays.

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arXiv2014

T-odd Observables in Elastic Scattering, in Deep Inelastic Processes and in Weak Decays

We give a unitary description of T-odd effects in various sectors of atomic, nuclear and particle physics, like elastic scattering, deep inelastic processes and weak decays. This we get thanks to a particular transformation, which leads us to defining two T-odd observables. This approach allows to revise in quite a natural way various azimuthal asymmetries and normal polarizations already predicted or even observed in deep inelastic processes and in weak decays. In the latter case, useful suggestions for phenomenological analyses and an interesting result for normal and transverse polarization are derived.

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arXiv2007

Q^2 dependence of azimuthal asymmetries in semi-inclusive deep inelastic scattering and in Drell-Yan

We study several azimuthal asymmetries in semi-inclusive deep inelastic scattering and in Drell-Yan, interpreting them in the framework of the formalism of the quark correlator, with a particular reference to T-odd functions. The correlator contains an undetermined energy scale, which we fix on the basis of a simple and rather general argument. We find a different value than the one assumed in previous treatments of T-odd functions. This implies different predictions on the Q^2 dependence of the above mentioned asymmetries. Our result about the azimuthal asymmetry of unpolarized Drell-Yan agrees with presently available data, contrary to the alternative assumption on the scale. Predictions on other azimuthal asymmetries could be tested against data of planned experiments on Drell-Yan and semi-inclusive deep inelastic scattering.

hep-ph↗
arXiv2006

The QCD Parton Model: a Useful Approximation

Approximate relations among transverse momentum dependent quark distribution functions are established in the framework of the QCD parton model. The validity of such results survives QCD evolution effects, owing to the Politzer theorem on equations of motion. Furthermore the model fixes an energy scale, involved in the parametrization of the correlator, which determines the Q^2 dependence of the azimuthal asymmetry in inclusive reactions. Some of the present data - the cos2ϕsymmetry in unpolarized Drell-Yan, the sin2ϕsingle spin asymmetry in semi-inclusive deep inelastic scattering (SIDIS) and the cosϕasymmetry in unpolarized SIDIS - support model predictions. Further measurements of SIDIS and Drell-Yan asymmetries are suggested, in particular the double spin asymmetry in SIDIS, which allows to determine approximately the proton transversity.

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arXiv2005

Inequalities between quark densities

We propose an inequality between the longitudinally polarized density and the transversity of a quark in a nucleon. This inequality, whose validity is limited to very small scales, is based on considerations about Lorentz transformations and on commonly accepted models. Therefore it may be used as a consistecy check with other models. It turns out to agree with most model preedictions. Moreover it allows to establish, thanks to the positivity constraint, another inequality between the longitudinally polarized and the unpolarized valence quark density. We show that this latter inequality may be extended to any Q^2, consistently with commonly used factorization schemes and with some nonperturbative evolution models. This inequality finds nontrivial applications to the valence "d"-quark densities and is compared with data analyses, with model predictions and with parametrizations of quark densities.

hep-ph↗
arXiv2004

Constraints on the Quark Correlation Matrix from Equations of Motion

We show that the Politzer theorem on the equations of motion implies strong constraints on the quark correlation matrix, restricting the number of independent distribution functions that characterize the internal structure of the nucleon. Then we draw some important consequences from this result. First of all, we suggest an alternative method for determining transversity. Secondly, we predict the Q^2-dependence of some azimuthal asymmetries. Lastly, we illustrate the origin of the main contributions to some twist-3 distribution functions.

hep-ph↗
arXiv2003

The Transversity Function and Double Spin Azimuthal Asymmetry in Semi-Inclusive Pion Leptoproduction

We show that the transverse momentum dependent transversity function is proportional to the longitudinal polarization of a quark in a transversely polarized proton. This result suggests an alternative, convenient method for determining transversity, without knowing unusual fragmentation functions. The method consists of measuring the double spin azimuthal asymmetry in semi-inclusive pion leptoproduction by a transversely polarized proton target. The asymmetry, which is twist 3, is estimated to be more than 10% under the most favorable conditions. The experiment we suggest is feasible at facilities like DESY and CERN.

hep-ph↗
arXiv2002

Double Spin Asymmetry in Semi-inclusive Deep Inelastic Scattering

We shortly review the various methods suggested for determining the transversity function. Among such methods, we consider especially those based on semi-inclusive deep inelastic scattering. In the framework of this kind of reactions, we propose to measure a double spin asymmetry, using a transversely polarized proton target and a longitudinally polarized lepton beam, and fixing the direction of the final pion. Under particular conditions, the asymmetry is sensitive to the transversity function.

hep-ph↗
arXiv2001

Transverse Spin Asymmetries in Drell-Yan Processes: $\mathbf p^{\uparrow} p \to μ^+ μ^- X$}

We consider two different spin asymmetries in Drell-Yan processes generated by the collisions of an unpolarized proton beam on transversely polarized protons: the muon helicity asymmetry and the left-right asymmetry. We calculate the asymmetries in the framework of the QCD improved parton model, taking into account the parton transverse momentum and considering the first order QCD corrections. The muon helicity asymmetry is sensitive to the quark transversity distribution and is nonvanishing even at zero order. On the contrary the left-right asymmetry vanishes at zero order but not at first order in the QCD coupling constant, as a result of gluon contribution.

hep-ph↗
arXiv2000

The $η_cγγ^*$ transition form factor in the decay $η_c\to γ\ell^+\ell^-$ and in the crossed channels $γe^-\to η_c e^- $ and $e^+e^-\toη_cγ

The Dalitz distribution and the effective mass spectrum of the lepton pair produced in the decay $η_c\to γ\ell^+ \ell^-$, as well as the differential cross sections for the crossed channels $γe^-\to η_c e^- $ and $e^+ e^-\to η_c γ$, are calculated in terms of the sole electromagnetic transition form factor for the vertex $η_c\toγγ^*$, where $γ^*$ is a virtual photon with a space-like (for $γe^- \to η_c e^- $) or a time-like (for $e^+ e^-\to η_c γ$ and $η_c\toγ\ell^+\ell^-$) four-momentum. Explicit parametrisations of the transition form factor are found in two different approaches, namely in Vector Dominance Model and in a QCD-inspired consideration on the basis of the process $c \bar{c}\to γ\ell^+ \ell^-$, with zero relative three-momentum of the annihilating quarks.

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