arXiv subjects

Francesco Giuli

Publications and source records attributed to Francesco Giuli.

46 records · Page 3Linked to original sources

arXiv2019

PDF Profiling Using the Forward-Backward Asymmetry in Neutral Current Drell-Yan Production

Non-perturbative QCD effects from Parton Distribution Functions (PDFs) may be constrained by using high-statistics Large Hadron Collider (LHC) data. Drell-Yan (DY) measurements in the Charged Current (CC) case provide one of the primary means to do this, in the form of the lepton charge asymmetry. We investigate here the impact of measurements in Neutral Current (NC) DY data mapped onto the Forward-Backward Asymmetry ($A_{\rm FB}$) on PDF determinations, by using the open source fit platform {\tt{xFitter}}. We demonstrate the potential impact of $A_{\rm FB}$ data on PDF determinations and perform a thorough analysis of related uncertainties.

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arXiv2019

Probing the strange content of the proton with charm production in charged current at LHeC

We study charm production in charged-current deep-inelastic scattering (DIS) using the xFitter framework. Recent results from the LHC have focused renewed attention on the determination of the strange-quark parton distribution function (PDF) and the DIS charm process provides important complementary constraints on this quantity. We examine the current PDF uncertainty, and use LHeC pseudodata to estimate the potential improvement from this proposed facility. As xFitter implements both fixed-flavor- and variable-flavor-number schemes, we can compare the impact of these different theoretical choices; this highlights some interesting aspects of multi-scale calculations. We find that the high-statistics LHeC data covering a wide kinematic range could substantially reduce the strange PDF uncertainty.

hep-ph↗

arXiv2019

Determination of proton parton distribution functions using ATLAS data

Fits to determine parton distribution functions using top-antitop, inclusive $W/Z$ boson and $W^{\pm}$ boson production measurements in association with jets from ATLAS, in combination with deep-inelastic scattering data from HERA, are presented. The ATLAS $W/Z$ boson data exhibit sensitivity to the valence quark distributions and the light quark sea composition, whereas the top-quark pair production data have sensitivity to the gluon distribution. The impact of the these data is increased by fitting several distributions simultaneously, with the full information on the systematic and statistical correlations between data points. The parton distribution functions extracted using $W^{\pm}$ + jets data show an improved determination of the high-$x$ sea-quark densities, while confirming the unsuppressed strange-quark density at lower $x$ < 0.02 found by previous ATLAS analyses.

hep-ex↗

arXiv2019

Recent QCD results from the xFitter project: Probing the strange content of the proton with charm production in charged current at LHeC

We investigate charm production in charged-current deep-inelastic scattering (DIS) using the xFitter program. xFitter is an open-source software framework for the determination of PDFs and the analysis of QCD physics, and has been used for a variety of LHC studies. The study of charged current DIS charm production provides an important perspective on the strange quark PDF, s(x). We make use of the xFitter tools to study the present s(x) constraints, and then use LHeC pseudodata to infer how these might improve. Furthermore, as xFitter implements both Fixed Flavor and Variable Flavor number schemes, we can examine the impact of these different theoretical choices; this highlights some interesting aspects of multi-scale calculations. This study provides a practical illustration of the many features of xFitter.

hep-ph↗

arXiv2019

Improved description of the HERA data with a new simple PDF parametrization

A new parametrization for the parton distribution functions with a higher flexibility in the small-$x$ region is presented. It has been implemented in the xFitter open-source PDF fitting tool, and compared to the default xFitter parametrization, used for the determination of the HERAPDF set. It has been found that the combined inclusive HERA I+II data can be described using NNLO theory with a significantly higher quality than HERAPDF2.0: the $χ^2$ is reduced by more than 60 units, having used only four more parameters. Our result highlights a significant parametrization bias in the default xFitter parametrization at small $x$, which would lead to even more dramatic effects when used for higher energy colliders, where the small-$x$ region is more relevant. We also find that the inclusion of small-$x$ resummation leads to a further reduction by approximately 30 extra units in $χ^2$. In this contribution, we review the results of the recent paper "A new simple PDF parametrization: improved description of the HERA data" (arXiv:1902.11125).

hep-ph↗

arXiv2018

Proton PDFs constraints from measurements using the ATLAS experiment

High-precision measurements of Standard Model processes provide information on different aspects contributing to the process, such as parton distribution functions, and comparisons with the current precision reached theoretically on the calculations of the cross sections of such processes. This document describes ATLAS measurements, performed at different centre-of-mass energies, of vector boson (W and Z) cross sections and cross-section ratios. It also discusses measurements of ratios of Z-boson and top-quark pair production cross sections, which provide important information on the proton PDFs. Finally, a measurement involving the di-lepton decay cahnnel of top-quark pairs at 8 TeV is presented, because its direct sensitivity to the gluon PDF.

hep-ex↗

arXiv2018

Impact of low-$x$ resummation on QCD analysis of HERA data

Fits to the final combined HERA deep-inelastic scattering cross-section data within the conventional DGLAP framework of QCD have shown some tension at low $x$ and low $Q^2$. A resolution of this tension incorporating $\ln(1/x)$-resummation terms into the HERAPDF fits is investigated using the xFitter program. The kinematic region where this resummation is important is delineated. Such high-energy resummation not only gives a better description of the data, particularly of the longitudinal structure function $F_L$, it also results in a gluon PDF which is steeply rising at low $x$ for low scales, $Q^2 \simeq 2.5$ GeV$^2$, contrary to the fixed-order NLO and NNLO gluon PDF.

hep-ph↗

arXiv2018

The importance of ln(1/x) resummation: a new QCD analysis of HERA data

In this contribution, I present a new QCD fit to final combined HERA deep-inelastic scattering cross-section data incorporating $\ln(1/x)$-resummation terms. This fit has been performed within the xFitter framework. The main effect of including such resummed terms results not only in a better description of the low-$x$ and low-$Q^{2}$ data, but also in a steeply rising gluon PDF at low x for low scales. Furthermore, the kinematic region where this resummation is important is delineated. This contribution is based on the results presented in arXiv:1802.00064.

hep-ph↗

arXiv2017

Performance of Monte Carlo Event Generators for the Production of Boson and Multi-Boson States ATLAS Analysis

The Monte Carlo (MC) setups used by ATLAS to model boson$+\mathrm{jets}$ and multi-boson processes at $\sqrt{s}$ = 13 TeV in proton-proton collisions are described. Comparisons between data and several event generators are provided for key kinematic distributions. Issues associated with the evaluation of systematic uncertainties are also discussed. This proceeding is a summary of the results collected in two recent ATLAS PUB notes published for the MC workshop jointly-organised by the the ATLAS and CMS Collaborations and held at CERN in May 2017.

hep-ex↗

arXiv2017

The photon PDF from high-mass Drell Yan data at the LHC

Achieving the highest precision for theoretical predictions at the LHC requires the calculation of hard-scattering cross-sections that include perturbative QCD corrections up to (N)NNLO and electroweak (EW) corrections up to NLO. Parton distribution functions (PDFs) need to be provided with matching accuracy, which in the case of QED effects involves introducing the photon parton distribution of the proton, $xγ(x,Q^2)$. In this work a determination of the photon PDF from fits to recent ATLAS measurements of high-mass Drell-Yan dilepton production at $\sqrt{s}=8$ TeV is presented. This analysis is based on the xFitter framework, and has required improvements both in the APFEL program, to account for NLO QED effects, and in the aMCfast interface to account for the photon-initiated contributions in the EW calculations within MadGraph5_aMC@NLO. The results are compared with other recent QED fits and determinations of the photon PDF, consistent results are found.

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