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Alessandro Ballestrero

Publications and source records attributed to Alessandro Ballestrero.

At least 19 recordsLinked to original sources
arXiv2023

Precise predictions for same-sign W-boson scattering at the LHC

Vector-boson scattering processes are of great importance for the current run-II and future runs of the Large Hadron Collider. The presence of triple and quartic gauge couplings in the process gives access to the gauge sector of the Standard Model (SM) and possible new-physics contributions there. To test any new-physics hypothesis, sound knowledge of the SM contributions is necessary, with a precision which at least matches the experimental uncertainties of existing and forthcoming measurements. In this article we present a detailed study of the vector-boson scattering process with two positively-charged leptons and missing transverse momentum in the final state. In particular, we first carry out a systematic comparison of the various approximations that are usually performed for this kind of process against the complete calculation, at LO and NLO QCD accuracy. Such a study is performed both in the usual fiducial region used by experimental collaborations and in a more inclusive phase space, where the differences among the various approximations lead to more sizeable effects. Afterwards, we turn to predictions matched to parton showers, at LO and NLO: we show that on the one hand, the inclusion of NLO QCD corrections leads to more stable predictions, but on the other hand the details of the matching and of the parton-shower programs cause differences which are considerably larger than those observed at fixed order, even in the experimental fiducial region. We conclude with recommendations for experimental studies of vector-boson scattering processes.

hep-ph↗
arXiv2020

Different polarization definitions in same-sign $WW$ scattering at the LHC

We study the polarization of positively charged $W$'s in the scattering of massive electroweak bosons at hadron colliders. We rely on the separation of weak boson polarizations in the gauge-invariant, doubly-resonant part of the amplitude in Monte Carlo simulations. Polarizations depend on the reference frame in which they are defined. We discuss the change in polarization fractions and in kinematic distributions arising from defining polarization vectors in two different reference frames which have been employed in recent experimental analyses.

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arXiv2020

VBSCan Mid-Term Scientific Meeting

This document summarises the talks and discussions happened during the VBSCan Mid-Term Scientific Meeting workshop. The VBSCan COST action is dedicated to the coordinated study of vector boson scattering (VBS) from the phenomenological and experimental point of view, for the best exploitation of the data that will be delivered by existing and future particle colliders.

hep-ph↗
arXiv2019

Polarized vector boson scattering in the fully leptonic WZ and ZZ channels at the LHC

Isolating the scattering of longitudinal weak bosons at the LHC is an important tool to probe the ElectroWeak Symmetry Breaking mechanism. Separating polarizations of $W$ and $Z$ bosons is complicated, because of non resonant contributions and interference effects. Additional care is necessary when considering $Z$ bosons, due to the $γ/Z$ mixing in the coupling to charged leptons. We propose a method to define polarized signals in $Z\!Z$ and $W^+\!Z$ scattering at the LHC, which relies on the separation of weak boson polarizations at the amplitude level in Monte Carlo simulations. After validation in the absence of lepton cuts, we investigate how polarized distributions are affected by a realistic set of kinematic cuts (and neutrino reconstruction, when needed). The total and differential polarized cross sections computed at the amplitude level are well defined, and their sum reproduces the full results, up to non negligible but computable interference effects which should be included in experimental analyses. We show that polarized cross sections computed using the reweighting method are inaccurate, particularly at large energies. We also present two procedures which address the model independent extraction of polarized components from LHC data, using Standard Model angular distribution templates.

hep-ph↗
arXiv2018

$W$ boson polarization in vector boson scattering at the LHC

Measuring the scattering of longitudinally-polarized vector bosons represents a fundamental test of ElectroWeak Symmetry Breaking. In addition to the challenges provided by low rates and large backgrounds, there are conceptual issues which need to be clarified for the definition of a suitable signal. Since vector bosons are unstable and can only be observed through their decay products, the polarization states interfere among themselves. Moreover, already at tree level, there are diagrams which cannot be interpreted as production times decay of EW bosons but are necessary for gauge invariance. We discuss a possible way to define a cross section for polarized $W$'s, dropping all non resonant diagrams, and projecting on shell the resonant ones, thus, preserving gauge invariance. In most cases, the sum of polarized distributions reproduces the full results. In the absence of cuts, the ratios of the polarized cross sections to the full one agree with the results of a standard projection on Legendre polynomials. While the latter cannot be employed in a realistic environment, a comparison of the data with the shapes of the angular distributions for polarized vector bosons allows the extraction of the polarization fractions in the presence of selection cuts on the charged leptons.

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arXiv2015

Interference Effects in Higgs production through Vector Boson Fusion in the Standard Model and its Singlet Extension

Interference effects play an important role in Electroweak Physics. They are responsible for the restoration of unitarity al large energies. When, as is often the case, higher order corrections are only available for some particular subamplitude, interferences need to be carefully computed in order to obtain the best theoretical prediction. In the new proposal to estimate the total Higgs width from the off shell cross section, the interference between the Higgs signal and the background is essential. It has been recently pointed out in gluon fusion that whenever more than one neutral, CP even, scalars are present in the spectrum large cancellations can occur. We extend these studies to Vector Boson Scattering, examining interference effects in the Higgs sector in the Standard Model and its one Higgs Singlet extension.

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arXiv2012

Exploring alternative symmetry breaking mechanisms at the LHC with 7, 8 and 10 TeV total energy

In view of the annnouncement that in 2012 the LHC will run at 8 TeV, we study the possibility of detecting signals of alternative mechanisms of ElectroWeak Symmetry Breaking, described phenomenologically by unitarized models, at energies lower than 14 TeV. A complete calculation with six fermions in the final state is performed using the PHANTOM event generator. Our results indicate that at 8 TeV some of the scenarios with TeV scale resonances are likely to be identified while models with no resonances or with very heavy ones will be inaccessible, unless the available luminosity will be much higher than expected.

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arXiv2011

Vector-Vector scattering at the LHC with two charged leptons and two neutrinos in the final state

A complete parton level analysis of 2l2v2j and 4l2j, l = mu,e production at the LHC is presented, including all processes at order alpha^6, alpha^4*alpha_s^2. The infinite Higgs mass scenario, which is considered as a benchmark for strong scattering theories and is the limiting case for composite Higgs models, and one example of Strongly Interacting Light Higgs models are confronted with the Standard Model light Higgs predictions. This analysis is combined with the results in the lv4j, the ll4j and the 3lv2j channels presented in previous papers, in order to determine whether these alternative Higgs frameworks can be detected as an excess of events in boson--boson scattering.

hep-ph↗
arXiv2009

How well can the LHC distinguish between the SM light Higgs scenario, a composite Higgs and the Higgsless case using VV scattering channels?

A complete parton level analysis of ll + four jets l = e,mu and 3lv + two jets production at the LHC is presented, including all processes at order $\ordEW$, $\ordQCD$ and $\ordQCDsq$ when appropriate. The infinite Higgs mass scenario, which is considered as a benchmark for strong scattering theories and is the limiting case for composite Higgs models, and one example of a model incorporating a Strongly Interacting Light Higgs are confronted with the Standard Model light Higgs predictions. This analysis is combined with the results in the lv + four jets channel presented in a previous paper, in order to determine whether a composite Higgs signal can be detected as an excess of events in boson--boson scattering.

hep-ph↗
arXiv2009

A complete parton level analysis of boson-boson scattering and ElectroWeak Symmetry Breaking in lv + four jets production at the LHC

A complete parton level analysis of lv + four jets production at the LHC is presented, including all processes at order O(alpha^6), O(alpha^4*alpha_s^2) and O(alpha^2*alpha_s^4). The infinite Higgs mass scenario, which is considered as a benchmark for strong scattering theories and is the limiting case for composite Higgs models, is confronted with the Standard Model light Higgs predictions in order to determine whether a composite Higgs signal can be detected as an excess of events in boson--boson scattering.

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arXiv2008

A new analysis of PP --> bblvjj at the LHC: Higgs and W boson associated production with two tag jets

Higgs production in association with a $W$ boson and two energetic tag jets at the LHC is studied for $M_H = 120$ \GeV, with the Higgs decaying to $b\bar{b}$ and the $W$ to $\ellν$ ($\ell = e, μ$), guaranteeing high trigger efficiency. All parton level backgrounds are analyzed, including the effect of fake $b$--tagging. We discuss two detection strategies: in the first, more traditional, one, two jets are required to be $b$--tagged while in the second, which has not been previously examined in detail, only one tag is required. After all selection cuts about 80 and 200 events are foreseen in the two cases for a standard luminosity of 100 $fb^{-1}$ with a S/B ratio of 1/25 and 1/60 respectively. The corresponding statistical significancies, S/$\sqrt{\mathrm B}$ are 1.81 and 1.82.

hep-ph↗
arXiv2007

Isolating Vector Boson Scattering at the LHC: gauge cancellations and the Equivalent Vector Boson Approximation vs complete calculations

We have studied the possibility of extracting the $W^+W^-\to W^+W^-$ signal using the process $us\to cd W^+W^-$ as a test case. We have investigated numerically the strong gauge cancellations between signal and irreducible background, analysing critically the reliability of the Equivalent Vector Boson Approximation which is commonly used to define the signal. Complete matrix elements are necessary to study Electro--Weak Symmetry Breaking effects at high $WW$ invariant mass.

hep-ph↗
arXiv2005

Boson boson scattering at LHC

We analyse some features of WW scattering processes at LHC. The severe cancellations between fusion diagrams and the other contributions evidence the necessity of complete calculations for studying the high WW invariant mass region and disentangling the standard Higgs case from new physics.

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arXiv2000

Fermion Loops, conserved currents and single-W

The relevance of fermion loop corrections to four fermion processes at e+ e- colliders is reviewed with regard to the recent extension to the case of massive external particles and its application to single-W processes.

hep-ph↗
arXiv1999

Non-conserved currents and gauge-restoring schemes in single W production

We generalize the inclusion of the imaginary parts of the fermionic one-loop corrections for processes with unstable vector bosons to the case of massive external fermions and non conservation of weak currents. We study the effect of initial and final state fermion masses in single W production in connection with the gauge-invariant treatment of the finite-width effects of W and Z bosons, giving numerical comparisons of different gauge-invariance-preserving schemes in the energy range of LEP2 and LC for e+e- -> e- v u d. We do not find significant differences between the results obtained in the imaginary part fermion loop scheme and in other exactly gauge preserving methods.

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arXiv1999

LEP2 4f review

We review some recent results and open problems on four fermion physics at LEP2 and beyond

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arXiv1998

Higgs Production in Charged Current Six Fermion Processes at Future $e^+e^-$ Colliders

We study higgs physics at future $e^+e^-$ colliders taking into account the full set of Feynman diagrams for six fermion final states, which are produced for higgs masses near or above the two $W$'s threshold. In particular we examine events where one isolated lepton or two isolated leptons of different flavours signal the presence of two $\rm W^*$'s. For these charged current processes, a detailed analysis of the relevance of irreducible and QCD background shows that appropriate cuts are generally sufficient to deal with it in the case of reconstructed or missing higgs mass distributions. These latter are however affected by a non negligible distortion and shift of the maximum.

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