arXiv subjects

Alessandro Bacchetta

Publications and source records attributed to Alessandro Bacchetta.

At least 37 records · Page 2Linked to original sources

arXiv2022

Unpolarized Transverse Momentum Distributions from a global fit of Drell-Yan and Semi-Inclusive Deep-Inelastic Scattering data

We present an extraction of unpolarized transverse-momentum-dependent parton distribution and fragmentation functions based on more than two thousands data points from several experiments for two different processes: semi-inclusive deep-inelastic scattering and Drell-Yan production of lepton pairs. The baseline analysis is performed with a Monte Carlo replica method resumming large logarithms at N3LL accuracy. The resulting description of the data is very good ($χ^2/N_{\rm dat} = 1.06$). For semi-inclusive deep-inelastic scattering, predictions for multiplicities are normalized by factors that cure the discrepancy with data introduced by higher-order perturbative corrections.

hep-ph↗

arXiv2022

Extraction of Pion Transverse Momentum Distributions from Drell-Yan data

We map the distribution of unpolarized quarks inside a unpolarized pion as a function of the quark's transverse momentum, encoded in unpolarized Transverse Momentum Distributions (TMDs). We extract the pion TMDs from available data of unpolarized pion-nucleus Drell-Yan processes, where the cross section is differential in the lepton-pair transverse momentum. In the cross section, pion TMDs are convoluted with nucleon TMDs that we consistently take from our previous studies. We obtain a fairly good agreement with data. We present also predictions for pion-nucleus scattering that is being measured by the COMPASS Collaboration.

hep-ph↗

arXiv2022

Phenomenology of gluon TMDs from $η_{b,c}$ production

We present the potential of $η_{b,c}$ production in proton collisions to access the gluon transverse-momentum dependent parton distributions (TMDs). In particular, we explore the phenomenology of the unpolarized and linearly-polarized gluon TMDs in unpolarized collisions for different kinematic settings, and the potential of a fixed-target experiment at the LHC.

hep-ph↗

arXiv2022

A spectator-model way to transverse-momentum-dependent gluon distribution functions

We present exploratory analyses of the 3D gluon content of the proton via a study of unpolarized and polarized gluon TMDs at twist-2, calculated in a spectator model for the parent nucleon. Our approach embodies a flexible parametrization for the spectator-mass function, suited to describe both moderate and small-$x$ effects. All these studies can serve as a useful guidance in the investigation of the gluon dynamics inside nucleons and nuclei, which constitutes one of the major goals of new-generation colliding machines, as the EIC, the HL-LHC, NICA, and the FPF.

hep-ph↗

arXiv2022

Unveiling the proton structure via transverse-momentum-dependent gluon distributions

We present exploratory studies of the proton content via unpolarized and polarized transverse-momentum-dependent gluon distributions in the proton at leading twist. We make use of an enhanced spectator-model approach to encode in our densities both small- and moderate-$x$ effects. These studies are relevant to the investigation of the inner structure of hadrons via tomographic analyses at new-generation colliding machines.

hep-ph↗

arXiv2022

Toward twist-2 $T$-odd transverse-momentum-dependent gluon distributions: the $f$-type Sivers function

We report progresses on leading-twist transverse-momentum-dependent (TMD) gluon distributions, calculated in a spectator-model framework for the parent nucleon. We encode in our formalism a fit-based parameterization for the spectator-mass density, suited to describe the gluon content of the proton in a wide range of $x$. We present preliminary results on the $f$-type Sivers function, a key ingredient to access relevant spin-asymmetries emerging when the nucleon is transversely polarized. These studies are helpful to shed light on the gluon dynamics inside nucleons and nuclei, which is one of the primary goals of new-generation colliding facilities, as the Electron-Ion Collider, the High-Luminosity LHC, NICA-SPD, and the Forward Physics Facility.

hep-ph↗

arXiv2022

Toward twist-2 $T$-odd transverse-momentum-dependent gluon distributions: the $f$-type linearity function

We present preliminary results on the leading-twist transverse-momentum-dependent (TMD) gluon linearity function, which is directly connected to nucleon transverse-spin asymmetries originating from the density of linearly-polarized gluons. The function is calculated in a spectator-model framework for the parent proton and for the $f$-type gauge-link structure. Our work represents a further step toward the definition of a complete set of gluon TMD distributions at twist-2 that can be used to access the gluon dynamics inside nucleons and nuclei at new-generation colliders.

hep-ph↗

arXiv2022

The Forward Physics Facility at the High-Luminosity LHC

High energy collisions at the High-Luminosity Large Hadron Collider (LHC) produce a large number of particles along the beam collision axis, outside of the acceptance of existing LHC experiments. The proposed Forward Physics Facility (FPF), to be located several hundred meters from the ATLAS interaction point and shielded by concrete and rock, will host a suite of experiments to probe Standard Model (SM) processes and search for physics beyond the Standard Model (BSM). In this report, we review the status of the civil engineering plans and the experiments to explore the diverse physics signals that can be uniquely probed in the forward region. FPF experiments will be sensitive to a broad range of BSM physics through searches for new particle scattering or decay signatures and deviations from SM expectations in high statistics analyses with TeV neutrinos in this low-background environment. High statistics neutrino detection will also provide valuable data for fundamental topics in perturbative and non-perturbative QCD and in weak interactions. Experiments at the FPF will enable synergies between forward particle production at the LHC and astroparticle physics to be exploited. We report here on these physics topics, on infrastructure, detector, and simulation studies, and on future directions to realize the FPF's physics potential.

hep-ex↗

arXiv2022

The three-dimensional distribution of quarks in momentum space

We present the distribution of unpolarized quarks in a transversely polarized proton in three-dimensional momentum space. Our results are based on consistent extractions of the unpolarized and Sivers transverse momentum dependent parton distributions (TMDs).

hep-ph↗

arXiv2021

Where do we stand with a 3-D picture of the proton?

We are on the way to obtaining multi-dimensional "pictures" of the proton. The field is bursting with activities, both from the theoretical and experimental side. A brief selection of important achievements of the last years and open challenges for the future is presented. They are already well documented in the various reviews of this Special Issue, but they are gathered here in a more condensed way for convenience, together with some additional remarks. The choice of the items included in this short overview is far from being complete and represents the view of the author.

hep-ph↗

arXiv2020

Transverse-momentum-dependent gluon distribution functions in a spectator model

We present a model calculation of transverse-momentum-dependent distributions (TMDs) of gluons in the nucleon. The model is based on the assumption that a nucleon can emit a gluon, and what remains after the emission is treated as a single spectator particle. This spectator particle is considered to be on-shell, but its mass is allowed to take a continuous range of values, described by a spectral function. The nucleon-gluon-spectator coupling is described by an effective vertex containing two form factors. We fix the model parameters to obtain the best agreement with collinear gluon distributions extracted from global fits. We study the tomography in momentum space of gluons inside nucleons for various combinations of their polarizations. These can be used to make predictions of observables relevant for gluon TMD studies at current and future collider facilities.

hep-ph↗

arXiv2020

Transverse-momentum-dependent parton distributions up to N$^3$LL from Drell-Yan data

We present an extraction of unpolarised Transverse-Momentum-Dependent Parton Distribution Functions based on Drell-Yan production data from different experiments, including those at the LHC, and spanning a wide kinematic range. We deal with experimental uncertainties by properly taking into account correlations. We include resummation of logarithms of the transverse momentum of the vector boson up to N$^3$LL order, and we include non-perturbative contributions. These ingredients allow us to obtain a remarkable agreement with the data.

hep-ph↗

arXiv2020

Probing Nucleons and Nuclei in High Energy Collisions

This volume is a collection of contributions for the 7-week program "Probing Nucleons and Nuclei in High Energy Collisions" that was held at the Institute for Nuclear Theory in Seattle, WA, USA, from October 1 until November 16, 2018. The program was dedicated to the physics of the Electron Ion Collider (EIC), the world's first polarized electron-nucleon (ep) and electron-nucleus (eA) collider to be constructed in the USA. These proceedings are organized by chapters, corresponding to the weeks of the program: Week I, Generalized parton distributions; Week II, Transverse spin and TMDs; Week III, Longitudinal spin; Week IV, Symposium week; Weeks V & VI, eA collisions; Week VII, pA and AA collisions. We hope these proceedings will be useful to readers as a compilation of EIC-related science at the end of the second decade of the XXI century.

hep-ph↗

arXiv2019

Gluon TMDs and NRQCD matrix elements in $J/ψ$ production at an EIC

In this paper we analyze azimuthal asymmetries in the processes of unpolarized and polarized $J/ψ\,(Υ)$ production at an Electron-Ion Collider. Apart from giving access to various unknown gluon transverse momentum distributions, we suggest to use them as a new method to extract specific color-octet NRQCD long-distance matrix elements, i.e.\ $\langle0|{\cal O}_{8}^{J/ψ}(^{1}S_{0})|0\rangle$ and $\langle0|{\cal O}_{8}^{J/ψ}(^{3}P_{0})|0\rangle$, whose values are still quite uncertain and for which lattice calculations are unavailable. The new method is based on combining measurements of analogous asymmetries in open heavy-quark pair production which can be performed at the same energy. To enhance the gluon contribution one can consider smaller values of $x$ and, in order to assess the impact of small-$x$ evolution, we perform a numerical study using the MV model as a starting input and evolve it with the JIMWLK equations.

hep-ph↗

arXiv2019

Azimuthal asymmetries in unpolarized SIDIS and Drell-Yan processes: a case study towards TMD factorization at subleading twist

We consider the azimuthal distribution of the final observed hadron in semi-inclusive deep-inelastic scattering and the lepton pair in the Drell-Yan process. In particular, we focus on the $\cos ϕ$ modulation of the unpolarized cross section and on its dependence upon transverse momentum. At low transverse momentum, for these observables we propose a factorized expression based on tree-level approach and conjecture that the same formula is valid in transverse-momentum dependent (TMD) factorization when written in terms of subtracted TMD parton distributions. Our formula correctly matches with the collinear factorization results at high transverse momentum, solves a long-standing problem and is a necessary step towards the extension of the TMD factorization theorems up to the subleading twist.

hep-ph↗

arXiv2019

Difficulties in the description of Drell-Yan processes at moderate invariant mass and high transverse momentum

We study the Drell-Yan cross section differential with respect to the transverse momentum of the produced lepton pair. We consider data with moderate invariant mass Q of the lepton pair, between 4.5 GeV and 13.5 GeV, and similar (although slightly smaller) values of the transverse momentum q_T. We approach the problem by deriving predictions based on standard collinear factorization, which are expected to be valid toward the high-q_T end of the spectrum and to which any description of the spectrum at lower q_T using transverse-momentum dependent parton distributions ultimately needs to be matched. We find that the collinear framework predicts cross sections that in most cases are significantly below available data at high q_T. We discuss additional perturbative and possible non-perturbative effects that increase the predicted cross section, but not by a sufficient amount.

hep-ph↗

arXiv2018

QCD$\times$QED evolution of TMDs

We consider for the first time the QED corrections to the evolution of (un)polarized quark and gluon transverse-momentum-dependent distribution and fragmentation functions (TMDs in general). By extending their operator definition to QCD$\times$QED, we provide the mixed new anomalous dimensions up to ${\cal O}(α_sα)$ and the pure QED ones up to ${\cal O}(α^2)$. These new corrections are universal for all TMDs up to the flavor of the considered parton, i.e., the full flavor universality of TMD evolution found in pure QCD is broken in QCD$\times$QED by the presence of the electric charge. In addition, we provide the leading-order QED corrections to the matching coefficients of the unpolarized quark TMD parton distribution function onto its integrated counterparts at ${\cal O}(α_s^0α)$.

hep-ph↗

arXiv2018

Effect of flavor-dependent partonic transverse momentum on the determination of the $W$ boson mass in hadronic collisions

Within the framework of transverse-momentum-dependent factorization, we investigate for the first time the impact of a flavor-dependent intrinsic transverse momentum of quarks on the production of $W^{\pm}$ bosons in proton-proton collisions at $\sqrt{s}$ = 7 TeV. We estimate the shift in the extracted value of the $W$ boson mass $M_W$ induced by different choices of flavor-dependent parameters for the intrinsic quark transverse momentum by means of a template fit to the transverse-mass and the lepton transverse-momentum distributions of the $W$-decay products. We obtain $-6\leq ΔM_{W^+} \leq 9$ MeV and $-4\leq ΔM_{W^-} \leq 3$ MeV with a statistical uncertainty of $\pm 2.5$ MeV. Our findings call for more detailed investigations of flavor-dependent nonperturbative effects linked to the proton structure at hadron colliders.

hep-ph↗