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

T -odd proton-helicity asymmetry in semi-inclusive DIS in perturbative QCD

We compute the single-spin asymmetry AUL in semi-inclusive deep-inelastic scattering of unpolarized leptons and longitudinally polarized protons at a large transverse momentum of the produced hadron. Our calculation is performed in collinear factorization at the lowest order of QCD perturbation theory. For photon exchange, the asymmetry is T-odd and receives contributions from the interference of the tree level and one-loop absorptive amplitudes. We consider the behavior of the spin asymmetry at low transverse momentum where contact to the formalism based on transverse momentum dependent distribution functions can be made. We also present some phenomenological results relevant for the COMPASS and HERMES experiments and the future Electron-Ion Collider

Regimi di QCD perturbativa e non perturbativa in osservabili dipendenti dal momento trasverso

The thesis investigates the role of quantum chromodynamics (QCD) in scattering cross sections at particle accelerators, in cases which include information on the transverse momentum of outgoing particles, with respect to the beam axis. More specifically, I focus on two processes: semi-inclusive Deep Inelastic Scattering (SIDIS) and Drell-Yan. These are key processes for the extraction of the Transverse Momentum Distribution (TMD) of quarks. In Chapt. 2 I summarise the basic aspects of these cross sections and their description in QCD, while in Chapt. 3 I introduce the theory of TMDs, delineating the theoretical framework for Chapt. 4, where I present separate phenomenological studies for SIDIS and Drell-Yan, concentrated on the low transverse-momentum part of the measured spectrum. I discuss for the first time the description of SIDIS data beyond leading order in QCD, which reveals important discrepancies. I present a theoretical argument for the explanation of these discrepancies, related to the separation of TMD and collinear factorisation regimes in transverse momentum observables, and supported by a numerical study. For Drell-Yan, I present a TMD fit of the newest data at the highest available perturbative accuracy. The possible role of non-perturbative transverse momentum at LHC is investigated, by analysing high precision measurements from ATLAS, CMS and LHCb, and a non-perturbative function such to describe, simultaneously, collider and fixed-target data from different experiments is provided. In Chapt. 5 I discuss Drell-Yan in the regime of fixed-target (and low-energy collider) experiments. While different TMD studies in the literature have considered the low transverse momentum region, where TMD factorisation is well established, there have been no studies to date of the high end of the transverse momentum spectrum, where a transition to collinear factorisation is expected. Similar to what was recently observed for SIDIS, I show that predictions from collinear factorisation largely underestimate data. In addiction, I explore possible explanations of the discrepancy, of both perturbative and non-perturbative nature. In Chapt. 6 I present the calculation in perturbative QCD of a single-spin asymmetry in SIDIS, more precisely for the case of a nucleon target polarised along the direction of the exchanged vector boson. This observable has been previously analysed in the low transverse momentum framework of TMDs, but an analysis in the framework of collinear factorisation was lacking. After calculating at 1-loop level the relevant Feynman diagrams, I show numerical results for modulations in the angular distribution of the final hadron, comparing to data from the HERMES experiment, and making predictions for a future Electron-Ion Collider

Difficulties in the description of Drell-Yan processes at low 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 qT . We approach the problem by deriving predictions based on standard collinear factorization, which are expected to be valid toward the high-qT end of the spectrum and to which any description of the spectrum at lower qT based on 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 qT . We discuss additional perturbative and possible non-perturbative effects that increase the predicted cross section, but not by a sufficient amount

Global Fits of Unpolarized TMDs at N3LL Accuracy

International audienceWe review the recent progress on the extraction of unpolarized TMD PDFs and TMD FFs from global data sets of Semi-Inclusive Deep-Inelastic Scattering, Drell–Yan and Z boson production. In particular, we address the tension between the low-energy SIDIS data and the theory predictions, and explore the impact of the very precise LHC data on the fit results

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 ($\chi^2/N_{\rm dat} = 1.06$). 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