arXiv subjects

E. Accomando

Publications and source records attributed to E. Accomando.

At least 19 recordsLinked to original sources

arXiv2022

Constrained Vector-like quark model with perturbative unitarity

We propose a Vector-like quark (VLQ) model constrained by the requirement of perturbative unitarity. In this scenario, the neutral and charged couplings of the new heavy VLQs to SM quarks and bosons are strictly related. We derive the corresponding sum rules and display the definite structure of the couplings. We analyse, qualitatively, the phenomenological consequences of this model in terms of expected width, production mechanism and decay channels. We show that the upcoming LHC run 3 could have sensitivity to such particles but the search strategy should be adapted to cover wide resonances decaying mainly in the charged channel T(B) -> Wq.

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arXiv2021

Below-threshold CP-odd Higgs boson search via A -> Z* h at the LHC

We study the process q q, g g -> A -> Z* h in a 2-Higgs Doublet Model Type-II where the mass of the CP-odd Higgs state A is lower than the rest mass of the Z and h particles (the latter being the Standard Model-like Higgs state), i.e., mA < mZ + mh = 215 GeV. This is a mass range which is not being currently tested by ATLAS and CMS in this channel, yet we show that there can be sensitivity to it already during Runs 2 and 3, assuming leptonic decays of the gauge boson and bottom-antibottom quark ones for the Higgs boson.

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arXiv2019

Production of Z'-Boson Resonances with Large Width at the LHC

Di-lepton searches for Beyond the Standard Model (BSM) Z' bosons that rely on the analysis of the Breit-Wigner (BW) line shape are appropriate in the case of narrow resonances, but likely not sufficient in scenarios featuring Z' states with large widths. Conversely, alternative experimental strategies applicable to wide Z' resonances are much more dependent than the default bump search analyses on the modelling of QCD higher-order corrections to the production processes, for both signal and background. For heavy Z' boson searches in the di-lepton channel at the CERN Large Hadron Collider (LHC), the transverse momentum q_T of the di-lepton system peaks at q_T \ltap 10^{-2} M_{ll}, where M_{ll} is the di-lepton invariant mass. We exploit this to treat the QCD corrections by using the logarithmic resummation methods in M_{ll} / q_T to all orders in the strong coupling constant α_s. We carry out studies of Z' states with large width at the LHC by employing the program {\tt reSolve}, which performs QCD transverse momentum resummation up to Next-to-Next-to-Leading Logarithmic (NNLL) accuracy. We consider two benchmark BSM scenarios, based on the Sequential Standard Model (SSM) and dubbed `SSM wide' and `SSM enhanced'. We present results for the shape and size of Z' boson signals at the differential level, mapped in both cross section (σ) and Forward-Backward Asymmetry (A_{\rm FB}), and perform numerical investigations of the experimental sensitivity at the LHC Run 3 and High-Luminosity LHC (HL-LHC).

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arXiv2019

Constraining Parton Distribution Functions from Neutral Current Drell-Yan Measurements

We study the cross section σand Forward-Backward asymmetry A_{FB} in the process pp \to γ^*,Z \to \ell^+\ell^- (with \ell=e,μ) for determinations of Parton Distribution Functions (PDFs) of the proton. We show that, once mapped in the invariant mass of the di-lepton final state, M({\ell\ell}), both observables, σand A_{FB}, display a statistical error which is presently competitive with that assigned to the existing PDF sets and which will rapidly become smaller than the latter as the luminosity being accumulated at Run-II of the LHC grows. This statement is applicable to both on-peak and off-peak M({\ell\ell}) regions, both (just) below and above it, thereby offering a means of constraining the quark PDFs over a sizeable (x,Q^2) range.

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arXiv2017

Novel SM-like Higgs decay into displaced heavy neutrino pairs in U(1)' models

We examine the observability of heavy neutrino (nu_h) signatures of a U(1)' enlarged Standard Model (SM) encompassing three heavy Majorana neutrinos alongside the known light neutrino states at the the Large Hadron Collider (LHC). We show that heavy neutrinos can be rather long-lived particles producing distinctive displaced vertices that can be accessed in the CERN LHC detectors. We concentrate here on the gluon fusion production mechanism gg -> H_{1,2} -> nu_h nu_h, where H_1 is the discovered SM-like Higgs and H_2 is a heavier state, yielding displaced leptons following nu_h decays into weak gauge bosons. Using data collected by the end of the LHC Run 2, these signatures would prove to be accessible with negligibly small background.

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arXiv2015

$A_{FB}$ as a discovery tool for $Z^\prime$ bosons at the LHC

The Forward-Backward Asymmetry (AFB) in $Z^\prime$ physics is commonly only perceived as the observable which possibly allows one to interpret a $Z^\prime$ signal by distinguishing different models of such (heavy) spin-1 bosons. In this article, we examine the potential of AFB in setting bounds on or even discovering a $Z^\prime$ at the Large Hadron Collider (LHC) and show that it might be a powerful tool for this purpose. We analyze two different scenarios: $Z^\prime$s with a narrow and wide width, respectively. We find that in both cases AFB can complement the cross section in accessing $Z^\prime$ signals.

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arXiv2015

Forward-Backward Asymmetry as a Discovery Tool for Z' Bosons at the LHC

The Forward-Backward Asymmetry (AFB) in Z' physics is commonly only perceived as the observable which possibly allows one to interpret a Z' signal by distinguishing different models of such (heavy) spin-1 bosons. In this paper, we examine the potential of AFB in setting bounds on or even discovering a Z' boson at the Large Hadron Collider (LHC) and show that it might be a powerful tool for this purpose. We analyse two different scenarios: Z' bosons with a narrow and wide width, respectively. We find that, in the first case, the significance of the AFB search can be comparable with that of the bump search usually adopted by the experimental collaborations; however, being a ratio of (differential) cross sections the AFB has the advantage of reducing systematical errors. In the second case, the AFB search can win over the bump search in terms of event shape, as the structure of the AFB distribution as a function of the invariant mass of the reconstructed Z'boson could nail down the new broad resonance much better than the event counting strategy usually adopted in such cases.

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arXiv2013

Uncovering quasi-degenerate Kaluza-Klein Electro-Weak gauge bosons with top asymmetries at the LHC

By exploiting the correlation between charge and spin polarisation asymmetries in t-tbar, we show that combining the two observables could identify the presence of quasi-degenerate states in a resonant signal at the LHC. As an example, we investigate experimental signatures emerging in top-antitop final states in the context of a model where the Standard Model Electro-Weak sector is allowed to propagate in large extra--dimensions of TeV^-1 size while the colour sector is localised. Assuming current experimantal constraints from the 7 and 8 TeV runs and taking into account the estimated top (anti-top) reconstruction efficiencies, we find that the 14 TeV upgraded LHC with the planned integrated luminosity L=100 fb^-1 could access these quasi-degenerate multiple resonances and explore for the first time the rich phenomenology in the asymmetry observables. The main outcome would be having measurable quantities, complementary to the usual total and differential cross sections, capable of distiguishing a quasi-degenerate multiply resonant spectrum from a `standard' single resonance that could present a similar signal in a bump hunt analysis.

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arXiv2012

Charged di-boson production at the LHC in a 4-site model with a composite Higgs boson

We investigate the scope of the LHC in probing the parameter space of a 4-site model supplemented by one composite Higgs state, assuming all past, current and future energy and luminosity stages of the CERN machine. We concentrate on the yield of charged di-boson production giving two opposite-charge different-flavour leptons and missing (transverse) energy, i.e., events induced via the subprocess $q\bar q\to e^+ν_e μ^-\barν_μ$ + ${\rm{c.c.}}$, which enables the production in the intermediate step of all additional neutral and charged gauge bosons belonging to the spectrum of this model, some of which in resonant topologies. We find this channel accessible over the background at all LHC configurations after a dedicated cut-based analysis. We finally compare the yield of the di-boson mode to that of Drell-Yan processes and establish that they have complementary strengths, one covering regions of parameter space precluded to the others and vice versa.

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arXiv2011

Neutral Higgs Boson Decays to Squark Pairs reanalyzed

We analyze neutral Higgs boson decays into squark pairs in the minimal supersymmetric extension of the Standard Model and improve previous analyses. In particular the treatment of potentially large higher-order corrections due to the soft SUSY breaking parameters A_b, the trilinear Higgs coupling to sbottoms, and mu, the Higgsino mass parameter, is investigated. The remaining theoretical uncertainties including the SUSY-QCD corrections are analyzed quantitatively.

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arXiv2009

Drell-Yan production at the LHC in a four site Higgsless model

We consider a four site Higgsless model based on the $SU(2)_L\times SU(2)_1\times SU(2)_2\times U(1)_Y$ gauge symmetry, which predicts two neutral and four charged extra gauge bosons, $Z_{1,2}$ and $W^\pm_{1,2}$. We compute the properties of the new particles, and derive indirect and direct limits on their masses and couplings from LEP and Tevatron data. In contrast to other Higgsless models, characterized by fermiophobic extra gauge bosons, here sizeable fermion-boson couplings are allowed by the electroweak precision data. The prospects of detecting the new predicted particles in the favoured Drell-Yan channel at the LHC are thus investigated. The outcome is that all six extra gauge bosons could be discovered in the early stage of the LHC low-luminosity run.

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arXiv2008

The four site Higgsless model at the LHC

We consider the four-site Higgsless model, which predicts the existence of four charged and two neutral extra gauge bosons, $W_{1,2}^\pm$ and $Z_{1,2}$. In contrast to other Higgsless models, characterized by fermiophobic extra gauge bosons, here sizeable fermion-boson couplings are allowed by the electroweak precision data. We thus analyse the prospects of detecting the new predicted particles in the mostly favored Drell-Yan channel at the LHC.

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arXiv2007

The process gg->WW as a probe into the EWSB mechanism

We present the first estimate of the gluon-gluon fusion process, gg->4f, in the high energy domain as a probe of the electroweak symmetry breaking mechanism (EWSB). We consider the exact matrix element at $O(α_sα_{em}^2)$, and we include all irreducible background coming from qq->4f. Purely leptonic final states, pp->llvv, are numerically investigated. We find that this channel is extremely sensitive to the regime of the interaction between gauge bosons. It can thus be associated to the traditionally used vector boson scattering (VBS) to improve the analysis of the EWSB physics.

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arXiv2007

Boson Fusion and Higgs production at the LHC in six fermion final states with one charged lepton pair

Boson boson scattering and Higgs production in boson boson fusion will be actively investigated at the LHC. We have performed a parton level study of all processes of the type $q_1 q_2 \to q_3 q_4 q_5 q_6 l^+l^-$ using for the first time a full fledged six fermion Monte Carlo event generator which employs exact matrix elements at $Ø(α_{em}^6)$. We have examined Higgs production in vector boson fusion followed by the decay chain $H\to ZZ\to l^+l^-jj$, including exactly all electroweak irreducible backgrounds. In the high mass region we have compared the case of a relatively light Higgs with the no-Higgs case. The integrated cross section for the latter case is more than twice that in the former for a minimum invariant mass of the $ZV$ pair of about 800 \GeV. We find, in a preliminary anlysis at parton level that, summing up the muon and the electron channels, about 30 events are expected in the light Higgs case for L=100 $fb^{-1}$.

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arXiv2006

Logarithmic electroweak corrections to e+ e- -> nue anti-nue W+ W-

We consider W-boson scattering at high-energy e+ e- colliders and study one-loop logarithmic electroweak corrections within the Standard Model assuming a light Higgs boson. We present explicit analytical results for W+ W- -> W+ W-. Using the equivalent vector-boson approximation, we have implemented these corrections into a Monte Carlo program for the process e+ e- -> nue anti-nue W+ W-. The quality of the equivalent vector-boson approximation and of the logarithmic high-energy approximation for the electroweak corrections is discussed in detail. The impact of the radiative effects is quantitatively analysed. The corrections are negative and their size, typically of the order of 10%, increases with energy reaching up to -20% and -50% at the ILC and CLIC, respectively.

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arXiv2006

CP Studies and Non-Standard Higgs Physics

There are many possibilities for new physics beyond the Standard Model that feature non-standard Higgs sectors. These may introduce new sources of CP violation, and there may be mixing between multiple Higgs bosons or other new scalar bosons. Alternatively, the Higgs may be a composite state, or there may even be no Higgs at all. These non-standard Higgs scenarios have important implications for collider physics as well as for cosmology, and understanding their phenomenology is essential for a full comprehension of electroweak symmetry breaking. This report discusses the most relevant theories which go beyond the Standard Model and its minimal, CP-conserving supersymmetric extension: two-Higgs-doublet models and minimal supersymmetric models with CP violation, supersymmetric models with an extra singlet, models with extra gauge groups or Higgs triplets, Little Higgs models, models in extra dimensions, and models with technicolour or other new strong dynamics. For each of these scenarios, this report presents an introduction to the phenomenology, followed by contributions on more detailed theoretical aspects and studies of possible experimental signatures at the LHC and other colliders.

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arXiv2006

Pseudo-observables in Axial gauge

We have given a first application of the Axial gauge a` la Dams and Kleiss to the Standard Model (SM) physics at the LHC. We have focused on the issue of providing a well-behaved signal definition in presence of potentially strong gauge cancellations at high energies. As a first illustration, we have analysed the production of WZ vector-boson pairs, which gives rise to four final-state fermions. Purely leptonic final states, pp->lvll, have been numerically investigated in the region of high center-of-mass energies and large scattering angles, particularly sensitive to gauge dependences. We have found that the Axial gauge is the appropriate framework to recover a meaningful separation of signal and irreducible background over the full energy domain.

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arXiv2006

Les Houches Physics at TeV Colliders 2005, Standard Model and Higgs working group: Summary report

This Report summarises the activities of the "SM and Higgs" working group for the Workshop "Physics at TeV Colliders", Les Houches, France, 2-20 May, 2005. On the one hand, we performed a variety of experimental and theoretical studies on standard candles (such as W, Z, and ttbar production), treating them either as proper signals of known physics, or as backgrounds to unknown physics; we also addressed issues relevant to those non-perturbative or semi-perturbative ingredients, such as Parton Density Functions and Underlying Events, whose understanding will be crucial for a proper simulation of the actual events taking place in the detectors. On the other hand, several channels for the production of the Higgs, or involving the Higgs, have been considered in some detail. The report is structured into four main parts. The first one deals with Standard Model physics, except the Higgs. A variety of arguments are treated here, from full simulation of processes constituting a background to Higgs production, to studies of uncertainties due to PDFs and to extrapolations of models for underlying events, from small-$x$ issues to electroweak corrections which may play a role in vector boson physics. The second part of the report treats Higgs physics from the point of view of the signal. In the third part, reviews are presented on the current status of multi-leg, next-to-leading order and of next-to-next-to-leading order QCD computations. Finally, the fourth part deals with the use of Monte Carlos for simulation of LHC physics.

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