arXiv subjects

Stefano Moretti

Publications and source records attributed to Stefano Moretti.

At least 163 records · Page 9Linked to original sources

arXiv2018

Atomki Anomaly in Family-Dependent $U(1)'$ Extension of the Standard Model

In the context of a gauge invariant, non-anomalous and family-dependent (non-universal) $U(1)'$ extension of the Standard Model, wherein a new high scale mechanism generates masses and couplings for the first two fermion generations and the standard Higgs mechanism does so for the third one, we find solutions to the anomaly observed by the Atomki collaboration in the decay of excited states of Beryllium, in the form of a very light $Z'$ state, stemming from the $U(1)'$ symmetry breaking, with significant axial couplings so as to evade a variety of low scale experimental constraints.

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arXiv2018

Neutral and Charged Higgs boson phenomenology at the LHeC and FCC-eh

We have analyzed the prospects of observing the lightest CP-even neutral Higgs boson ($h_1$) via decays into $b \bar b$ pairs in the neutral and charged current production processes $e^- q \to e^- h_1 q$ and $e^- q\to ν_e h_1 q'$, respectively, at the planneded large Hadron electron Collider (LHeC), with an $e^-$ beam energy of 60 GeV and a $p$ beam energy of 7 TeV. We also focus on observing a relatively light charged Higgs boson ($h^{-}$) via its production mode $e^- b \to ν_e h^{-} b$ followed by the decays $h^{-} \to s \bar c + s \bar u$ at the upcoming Future Circular Collider in hadron-electron mode (FCC-eh) with $\sqrt s \approx 3.5$ TeV. We have performed our analysis in the framework of the Next-to-Minimal Supersymmetric Standard Model (NMSSM) wherein the intermediate Higgs boson ($h_2$) is Standard Model (SM)-like. We have considered constraints from Dark Matter (DM), superparticle and the Higgs boson data. In both analyses, we have carried out signal and background computations with selections optimized to the model at hand and found that in both cases it is possible to get large significances for low mass Higgs bosons.

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arXiv2018

A Concrete Composite 2-Higgs Doublet Model

We consider a Composite Higgs Model (CHM) with two isospin doublet Higgs fields arising as pseudo Nambu-Goldstone bosons from a ${\rm SO}(6)\to {\rm SO}(4)\times {\rm SO}(2)$ breaking. The main focus of this work is to explicitly compute the properties of these Higgses in terms of the fundamental parameters of the composite sector such as masses, Yukawa and gauge couplings of the new spin-1 and spin-1/2 resonances. Concretely, we calculate the Higgs potential at one-loop level through the Coleman-Weinberg mechanism from the explicit breaking of the ${\rm SO(6)}$ global symmetry by the partial compositeness of fermions and gauge bosons. We derive then the phenomenological properties of the Higgs states and highlight the main signatures of this Composite 2-Higgs Doublet Model at the Large Hadron Collider, including modifications to the SM-like Higgs couplings as well as production and decay channels of heavier Higgs bosons. We also consider flavour bounds that are typical of CHMs with more than one Higgs doublet.

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arXiv2018

Supersymmetry versus Compositeness: 2HDMs tell the story

Supersymmetry and Compositeness are two prevalent paradigms providing both a solution to the hierarchy problem and a motivation for a light Higgs boson state. As the latter has now been found, its dynamics can hold the key to disentangle the two theories. An open door towards the solution is found in the context of 2-Higgs Doublet Models (2HDMs), which are necessary to Supersymmetry and natural within Compositeness in order to enable Electro-Weak Symmetry Breaking. We show how 2HDM spectra of masses and couplings accessible at the Large Hadron Collider may allow one to separate the two scenarios.

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arXiv2018

Probing anomalous top-Higgs couplings at the HL-LHC via $H\to WW^{\ast}$ decay channels

We study the prospects of probing the anomalous $tHq$~($q= u, c$) couplings via SS2L or 3L signatures at the High Luminosity (HL-LHC) run of the 14 TeV CERN collider. We focus on signals of the $tH$ associated production followed by the decay modes $t\to b\ell^{+}ν_{\ell}$ and $H\to WW^{\ast}$, and $t\bar{t}$ production followed by the decay modes $t\to b\ell^{+}ν_{\ell}$ and $\bar{t}\to H(\to WW^{\ast})\bar{q}$, where $\ell=e, μ$. Based on two types of $H\to WW^{\ast}$ decay topologies, one assuming the semileptonic decay mode $H\to WW^{\ast}\to \ell^{+}νjj $ and the other the fully leptonic decay mode $H\to WW^{\ast}\to \ell^{+}ν\ell^{-}\barν$, we perform a full simulation for signals and backgrounds. It is shown that, at the future HL-LHC, the branching ratio $Br(t\to uh)~(Br(t\to ch))$ can be probed to $1.17~(1.56)\times 10^{-3}$ for the same-sign di-lepton channel and to $7.1~\times 10^{-4}~(1.39~\times 10^{-3})$ for the 3L channel at $3σ$ sensitivity.

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arXiv2018

Light charged Higgs boson with dominant decay to quarks and its search at LHC and future colliders

The possibility of a light charged Higgs boson $H^\pm$ that decays predominantly to quarks ($cs$ and/or $cb$) and with a mass in the range 80 GeV $\le m_{H^\pm} \le 90$ GeV is studied in the context of Three-Higgs-Doublet Models (3HDMs). At present the Large Hadron Collider (LHC) has little sensitivity to this scenario, and currently the best constraints are from LEP2 and Tevatron searches. The branching ratio of $H^\pm\to cb$ can be dominant in two of the five types of 3HDM, and we determine the parameter space where this occurs. The decay $H^\pm\to cb$ has recently been searched for at the LHC for the first time, and with increased integrated luminosity one would expect sensitivity to the region 80 GeV $\le m_{H^\pm} \le 90$ GeV due to the smaller backgrounds with respect to $H^\pm\to cs$ decays.

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arXiv2018

Quantum interference among heavy NMSSM Higgs bosons

In the Next-to-Minimal Supersymmetric Standard Model (NMSSM), it is possible to have strong mass degeneracies between the new singlet-like scalar and the heavy doublet-like scalar, as well as between the singlet-like and doublet-like pseudoscalar Higgs states. When the difference in the masses of such states is comparable with the sum of their widths, the quantum mechanical interference between their propagators can become significant. We study these effects by taking into account the full Higgs boson propagator matrix in the calculation of the production process of $τ^+τ^-$ pairs in gluon fusion at the Large Hadron Collider (LHC). We find that, while these interference effects are sizeable, they are not resolvable in terms of the distributions of differential cross sections, owing to the poor detector resolution of the $τ^+τ^-$ invariant mass. They are, however, identifiable via the inclusive cross sections, which are subject to significant variations with respect to the standard approaches, wherein the propagating Higgs bosons are treated independently from one another. We quantify these effects for several representative benchmark points, extracted from a large set of points, obtained by numerical scanning of the NMSSM parameter space, that satisfy the most important experimental constraints currently available.

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arXiv2018

Top quark polarization as a probe of charged Higgs bosons

We study the production and decay of a heavy charged Higgs boson in the $bg\to tH^-$ and $H^-\to b\bar t$ processes at the Large Hadron Collider (LHC). We show that the chiral structure of the $H^-t\bar b$ vertex entering both stages is sensitive to the underlying Higgs mechanism of Electro-Weak Symmetry Breaking (EWSB) and we specifically demonstrate that one could distinguish between two popular realizations of a 2-Higgs Double Model (2HDM) embedding a new $H^\pm$ state, i.e., those with Type-I and -Y Yukawa couplings. The chiral structure of such a vertex, which is different in the two cases, in turn triggers a particular spin state of the top quark which is then transmitted to its decay products. Hence, both inclusive rates and exclusive observables can be used to extract the presence of such a charged Higgs boson state in LHC data.

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arXiv2018

${SO(10)}$ inspired $Z'$ models at the LHC

We study and compare various $Z'$ models arising from $SO(10)$, focussing in particular on the Abelian subgroup $U(1)_{R} \times U(1)_{B-L}$, broken at the TeV scale to Standard Model hypercharge $U(1)_{Y}$. The gauge group $U(1)_{R} \times U(1)_{B-L}$, which is equivalent to the $U(1)_{Y}\times U(1)_χ$ in a different basis, is well motivated from $SO(10)$ breaking and allows neutrino mass via the linear seesaw mechanism. Assuming supersymmetry, we consider single step gauge unification to predict the gauge couplings, then consider the detection and characterisation prospects of the resulting $Z'$ at the LHC by studying its possible decay modes into di-leptons as well as into Higgs bosons. The main new result here is to analyse in detail the expected leptonic forward-backward asymmetry at the high luminosity LHC and show that it may be used to discriminate the $U(1)_{R} \times U(1)_{B-L}$ model from the usual $B-L$ model based on $U(1)_{Y}\times U(1)_{B-L}$.

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arXiv2018

Collider Bounds on 2-Higgs Doublet Models with $U(1)_X$ Gauge Symmetries

2-Higgs Doublet Models (2HDMs) typically need to invoke an ad-hoc discrete symmetry to avoid severe flavor bounds and in addition feature massless neutrinos, thus falling short of naturally complying with existing data. However, when augmented by an Abelian gauge symmetry naturally incorporating neutrino masses via a type-I seesaw mechanism while at the same time escaping flavor changing interactions, such enlarged 2HDMs become very attractive phenomenologically. In such frameworks, the distinctive element is the $Z'$ gauge boson generated by the spontaneous breaking of the Abelian group $U(1)_X$. In this work, we derive updated collider bounds on it. Several theoretical setups are possible, each with different and sometimes suppressed couplings to quarks and leptons. Thus, complementary data from dijet and dilepton resonance searches need to be considered to fully probe these objects. We employ the corresponding datasets as obtained at the Large Hadron Collider (LHC) at the 13 TeV CMs energy for $\mathcal{L}=12,36$ and $300$ fb$^{-1}$ of luminosity. Moreover, we present the potential sensitivity to such $Z'$s of the High Luminosity LHC (HL-LHC) and High Energy LHC (HE-LHC).

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arXiv2018

Single production of vector-like quarks with large width at the Large Hadron Collider

Vector-Like Quarks (VLQs) are predicted by several theoretical scenarios of new physics and, having colour quantum numbers, can copiously be produced at the Large Hadron Collider (LHC), so long that their mass is in the testable kinematic regime of such a machine. While it would be convenient to assume that such objects are narrow and can be treated in the so-called Narrow Width Approximation (NWA), this is not always possible, owing to the fact that couplings and particle content of such new physics scenarios are not generally constrained, so that a large value of the former and/or a large variety of VLQ decay channels into the latter can contribute to generate a large decay width for such extra quarks. We have addressed here the issue of how best to tackle in LHC analysis the presence of large (and model-dependent) interference effects between different VLQ production and decay channels as well as between these and the corresponding irreducible background. We have confined ourselves to the case of single production of VLQs, which is rapidly becoming a channel of choice in experimental searches owing to the ever increasing limits on their mass, in turn depleting the yield of the historically well-established double production channel. Indeed, this poses a further challenge, as the former is model-dependent while the latter is essentially not. Despite these conditions, we show here that an efficient approach is possible, which retains to a large extent a degree of model independence in phenomenological studies of such VLQ dynamics at the LHC.

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arXiv2018

Constraining PDFs from neutral current Drell-Yan measurements and effects of resummation in slepton pair production

The high statistics that will be collected during the LHC Run-II (and beyond) open the path to precision measurements at the TeV scale, where the PDFs will play a crucial role in BSM searches. In the di-lepton final state accurate measurements of the Forward-Backward Asymmetry (AFB) will be available, particularly in the invariant mass region around the Z peak. We show that this observable displays a statistical error which is competitive with that assigned to the existing PDF sets and which will rapidly become smaller than the latter as the integrated luminosity grows, thereby offering a means of constraining the (anti)quark PDFs over a sizeable $(x,Q^2)$ range. In the context of SUSY searches we consider the effects of employing threshold-improved PDFs in a consistent calculation at NLO+NLL of slepton pair production cross sections. The calculations featuring a consistent resummation procedure both at PDF and partonic matrix element level are accompanied by PDF and scale uncertainties, and they provide a reliable and updated theoretical estimation for experimental data analyses at the LHC Run-II.

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arXiv2018

Prospects for Sneutrino Dark Matter in the BLSSM

The $(B-L)$ Supersymmetric Standard Model (BLSSM) motivates several Dark Matter (DM) candidates beyond the Minimally Supersymmetric Standard Model (MSSM). We assess the comparative naturalness of the two models and discuss the potential detection properties of a particular candidate, the Right-Handed (RH) sneutrino.

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arXiv2018

Sneutrino Dark Matter, Constraints and Perspectives

The prevalent Dark Matter (DM) candidate of the (B-L) Supersymmetric Standard Model (BLSSM) is the Right Handed (RH) sneutrino. In this work we assess the ability of ground and space based experiments to establish the nature of this particle, through indirect and collider detection.

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arXiv2018

Constraining $Z^\prime$ widths from $p_T$ measurements in Drell-Yan processes

We define a Focus Point (FP) Asymmetry, $A_{\rm FP}$, obtained by integrating the normalised transverse momentum distribution of either lepton produced in the Drell-Yan (DY) process below and above a point where a variety of popular $Z^\prime$ models all have the same magnitude. For a given $Z^\prime$ mass the position of this FP is predictable, depending only on the collider energy and on the low transverse momentum cut chosen in the normalisation procedure. The resulting $A_{\rm FP}$ is very sensitive to the $Z^\prime$ width, and can be used to constrain this parameter in experimental fits.

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arXiv2018

Explanation of the 17 MeV Atomki Anomaly in a $U(1)^\prime$-Extended 2-Higgs Doublet Model

Motivated by an anomaly observed in the decay of an excited state of Beryllium by the Atomki collaboration, we study an extension of the Standard Model with a gauged $U(1)'$ symmetry in presence of a 2-Higgs Doublet Model structure of the Higgs sector. We show that this scenario complies with a variety of experimental results and is able to explain the potential presence of a resonant spin-1 gauge boson, $Z'$, with a mass of 17 MeV in the Atomki experimental data, for appropriate choices of $U(1)'$ charges and Yukawa interactions.

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arXiv2018

Closing in on the Wino LSP via trilepton searches at the LHC

The neutralino dark matter (DM) predicted by the Minimal Supersymmetric Standard Model (MSSM) has been probed in several search modes at the Large Hadron Collider (LHC), one of the leading ones among which is the trilepton plus missing transverse momentum channel. The experimental analysis of this mode has, however, been designed to probe mainly a bino-like DM, originating in the decays of a pair of next-to-lightest neutralino and lightest chargino, both of which are assumed to be wino-like. In this study, we analyse how this trilepton channel can be tuned for probing also the wino-like DM. We note that, while the mentioned standard production mode generally leads to a relatively poor sensitivity for the wino-like DM, there are regions in the MSSM parameter space where the net yield in the trilepton final state can be substantially enhanced at the LHC with $\sqrt{s}=14$ TeV. This is achieved by taking into account also an alternative channel, pair-production of the wino-like DM itself in association with the heavier chargino, and optimisation of the kinematical cuts currently employed by the LHC collaborations. In particular, we find that the cut on the transverse mass of the third lepton highly suppresses both the signal channels and should therefore be discarded in this DM scenario. We perform a detailed detector-level study of some selected parameter space points that are consistent with the most important experimental constraints, including the recent ones from the direct and indirect DM detection facilities. Our analysis demonstrates the high complementarity of the two channels, with their combined significance reaching above 4$σ$ for a wino-like DM mass around 100 GeV, with an integrated luminosity as low as 100 fb$^{-1}$.

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arXiv2017

Multi-photon production in the Type-I 2HDM

This paper presents a study of a possible contribution to a Higgs boson signal in the $hh\to γγγγ$ channel due to $H\to hh$ decays, in the framework of the CP-conserving 2-Higgs Doublet Model Type-I (2HDM-I), where the heavier of the two CP-even Higgs bosons defined herein, $H$, is the SM-like Higgs state observed with a mass of 125 GeV at the Large Hadron Collider (LHC). We perform a broad scan of the 2HDM-I parameter space, in presence of both up-to-date theoretical and experimental constraints, in order to extract the interesting regions yielding such a signal. Then, after validating our numerical framework against public experimental analyses carried out at the LHC, we proceed to assess its scope in constraining and/or extracting the $gg\to H\to hh\to γγγγ$ signal in presence of a sophisticated Monte Carlo (MC) simulation. We find that, over a substantial region of the 2HDM-I parameter space presently un-accessible, the LHC will be able to establish such a potential signature in the next 2--3 years.

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