arXiv subjects

Stefano Moretti

Publications and source records attributed to Stefano Moretti.

At least 127 records · Page 7Linked to original sources

arXiv2021

Family Non-Universal U(1)$^\prime$ Model with Minimal Number of Exotics

We have studied phenomenological implications of several family non-universal U(1)$^\prime$ sub-models in the U(1)$^\prime$-extended Minimal Supersysmmetric Standard Model (UMSSM) possesing an extra down quark type exotic field. In doing this, we have started by enforcing anomaly cancellation criteria to generate a number of solutions in which the extra U(1)$^\prime$ charges of the particles are treated as free parameters. We have then imposed existing bounds coming from colliders and astrophysical observations on the assumed sub-models and observed that current limits dictate certain charge orientations, for instance, $Q_{H_u}\sim Q_{H_d}$ is preferred in general and the charge of the singlet $Q_S$ cannot be very small ($|Q_S|>$ 0.4) even if any of the charges is allowed to take any value within the $[-1, 1]$ range. We have finally studied the potential impact of such non-universal charges on $Z'$ mediated processes and made predictions for existing and future experiments. It has turned out that UMSSMs with or without the presence of light exotic quarks can yield distinguisable signatures if non-universal charges are realised in the leptonic sector of such models.

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arXiv2021

Probing tqZ anomalous couplings in the trilepton signal at the HL-LHC, HE-LHC and FCC-hh

We investigate the prospects for discovering the Flavour Changing Neutral Current (FCNC) $tqZ$ couplings via two production processes yielding trilepton signals: top quark pair production $pp\to t\bar{t}$ with one top decaying to the $Z$ boson and one light jet and the anomalous single top plus $Z$ boson production process $pp\to tZ$. We study these channels at various successors of the Large Hadron Collider~(LHC), i.e., the approved High-Luminosity LHC (HL-LHC) as well as the proposed High-Energy LHC~(HE-LHC) and Future Circular Collider in hadron-hadron mode (FCC-hh). We perform a full simulation for the signals and the relevant Standard Model (SM) backgrounds and obtain limits on the Branching Ratios (BRs) of $t\to qZ~(q=u,c)$, eventually yielding a trilepton final state through the decay modes $t\to b W^{+}\to b\ell^{+}ν_{\ell}$ and $Z\to \ell^{+}\ell^{-}$. The upper limits on these FCNC BRs at 95\% Confidence Level (CL) are obtained at the HL-LHC with $\sqrt s=14$ TeV and 3 ab$^{-1}$, at the HE-LHC with $\sqrt s=27$ TeV and 15 ab$^{-1}$ as well as at the FCC-hh with $\sqrt s=100$ TeV and 30 ab$^{-1}$.

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arXiv2020

Explaining electron and muon $g-2$ anomalies in an Aligned 2-Higgs Doublet Model with Right-Handed Neutrinos

We explain anomalies currently present in various data samples used for the measurement of the anomalous magnetic moment of electron ($a_e$) and muon ($a_μ$) in terms of an Aligned 2-Higgs Doublet Model with right-handed neutrinos. The explanation is driven by one and two-loop topologies wherein a very light CP-odd neutral Higgs state ($A$) contributes significantly to $a_μ$ but negligibly to $a_e$, so as to revert the sign of the new physics corrections in the former case with respect to the latter, wherein the dominant contribution is due to a charged Higgs boson ($H^\pm$) and heavy neutrinos with mass at the electroweak scale. For the region of parameter space of our new physics model which explains the aforementioned anomalies we also predict an almost background-free smoking-gun signature of it, consisting of $H^\pm A$ production followed by Higgs boson decays yielding multi-$τ$ final states, which can be pursued at the Large Hadron Collider.

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arXiv2020

Di-photon decay of a light Higgs state in the BLSSM

In the context of the $B-L$ Supersymmetric Standard Model (BLSSM), we investigate the consistency of a light Higgs boson, with mass around $90-95$ GeV, with the results of a search performed by the CMS collaboration in the di-photon channel at the integrated luminosity of 35.9 fb$^{-1}$ and $\sqrt s$ = 13 TeV.

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arXiv2020

Leptophobic $Z^\prime$ bosons in the secluded UMSSM

We perform a comprehensive analysis of the secluded UMSSM model, consistent with present experimental constraints. We find that in this model the additional $Z^\prime$ gauge boson can be leptophobic without resorting to gauge kinetic mixing and, consequently, also $d$-quark-phobic, thus lowering the LHC bounds on its mass. The model can accommodate very light singlinos as DM candidates, consistent with present day cosmological and collider constraints. Light charginos and neutralinos are responsible for muon anomalous magnetic predictions within 1$σ$ of the measured experimental value. Finally, we look at the possibility that a lighter $Z^\prime$, expected to decay mainly into chargino pairs and followed by the decay into lepton pairs, could be observed at 27 TeV.

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arXiv2020

Two-Higgs-Doublet Model with Soft CP-violation Confronting Electric Dipole Moments and Colliders

We analyze CP-violating effects in both Electric Dipole Moment (EDM) measurements and future analyses at the Large Hadron Collider (LHC) assuming a Two-Higgs-Doublet Model (2HDM) with "soft" CP-violation. Our analysis of EDMs and current LHC constraints shows that, in the case of a 2HDM Type II and Type III, an $\mathcal{O}(0.1)$ CP-violating phase in the Yukawa interaction between $H_1$ (the $125~\textrm{GeV}$ Higgs boson) and fermions is still allowed. For these scenarios, we study CP-violating effects in the neutron EDM and $t\bar{t}H_1$ production at the LHC. Our analysis shows that such an $\mathcal{O}(0.1)$ CP-violating phase can be easily confirmed or excluded by future neutron EDM tests, with LHC data providing a complementary cross-check.

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arXiv2020

Signal versus Background Interference in $H^+\to t\bar b$ Signals for MSSM Benchmark Scenarios

In this paper, we investigate sizeable interference effects between a heavy charged Higgs boson signal produced via $gg\to t\bar b H^-$ (+ c.c.) followed by the decay $H^-\to b\bar t$ (+ c.c.) and the irreducible background given by $gg\to t\bar t b \bar b$ topologies at the Large Hadron Collider (LHC). We show how such effects could spoil current $H^\pm$ searches where signal and background are normally treated separately. The reason for this is that a heavy charged Higgs boson can have a large total width, in turn enabling such interferences, altogether leading to very significant alterations, both at the inclusive and exclusive level, of the yield induced by the signal alone. This therefore implies that currently established LHC searches for such wide charged Higgs bosons require modifications. We show such effects quantitatively using two different benchmark configurations of the minimal realisation of Supersymmetry, wherein such $H^\pm$ states naturally exist.

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arXiv2020

Multi-component Dark Matter in a Simplified E$_6$SSM Model

We study Dark Matter (DM) in the Exceptional Supersymmetric Standard Model (E$_6$SSM). The model has both active and inert Higgs superfields and by imposing discrete symmetries one can generate two DM candidates. We show that the lightest higgsinos of the active and inert sectors give a viable setup for two-component DM. We also illustrate the scope of both direct and indirect detection experiments in extracting such a DM sector. Future experiments of the former kind have a good chance of finding the active component while the inert higgsino will be very hard to detect while those of the latter kind will have no sensitivity to either candidate.

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arXiv2020

CP asymmetries of ${\overline B}\to X_s/X_dγ$ in models with three Higgs doublets

Direct CP asymmetries (${\cal A}_{CP}$) in the inclusive decays of ${\overline B}\to X_sγ$ and ${\overline B}\to X_{s+d}γ$ of the order of $1\%$ will be probed at the BELLE II experiment. In this work, three such asymmetries are studied in the context of a three-Higgs-doublet model (3HDM), and it is shown that all three ${\cal A}_{CP}$ can be as large as the current experimental limits. Of particular interest is ${\cal A}_{CP}$ for ${\overline B}\to X_{s+d}γ$, which is predicted to be effectively zero in the Standard Model (SM). A measurement of $2.5\%$ or more for this observable with the full BELLE II data would give $5σ$ evidence for physics beyond the SM. We display parameter space in the 3HDM for which such a clear signal is possible.

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arXiv2020

Characterisation of Dark Matter in Direct Detection Experiments: Singlino Versus Higgsino

We show how the material used in direct detection experiments of Dark Matter (DM), in the presence of a signal of it, can afford one with the possibility of extracting the nature of the underlying candidate. We do so for the case of a $U(1)'$ Supersymmetric Standard Model (USSM) of E$_6$ origin, by exploiting benchmark points over its parameter space that yield either a Singlino- or Higgsino-like neutralino as DM candidate, the latter being defined in presence of up-to-date constraints, from low to high energy and from collider to non-collider experiments. However, as our method is general, we also introduce a model-independent description of our analysis, for the purpose of aiding similar studies in other Beyond the Standard Model (BSM) scenarios. This has been made possible by adapting a rather simple $χ^2$ analysis normally used for signal extraction in direct detection experiments and the procedure has been applied to Xenon, Germanium and Silicon detectors, those showing maximal and complementary sensitivity to gauge- and Higgs-portal induced interactions of DM with their nuclei.

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arXiv2020

Reinterpretation of LHC Results for New Physics: Status and Recommendations after Run 2

We report on the status of efforts to improve the reinterpretation of searches and measurements at the LHC in terms of models for new physics, in the context of the LHC Reinterpretation Forum. We detail current experimental offerings in direct searches for new particles, measurements, technical implementations and Open Data, and provide a set of recommendations for further improving the presentation of LHC results in order to better enable reinterpretation in the future. We also provide a brief description of existing software reinterpretation frameworks and recent global analyses of new physics that make use of the current data.

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arXiv2020

Mapping $pp\to A\to ZH\to l^+l^-b\bar b$ and $pp\to H\to ZA\to l^+l^-b\bar b$ Current and Future Searches onto 2HDM Parameter Spaces

By borrowing the results from a Large Hadron Collider (LHC) analysis performed with $36.1~\text{fb}^{-1}$ of Run 2 data intended to search for $A$ production followed by $ZH$ decay in turn yielding $l^+l^-b\bar b$ ($l=e,μ$) final states in the context of the standard four Yukawa types of the 2-Higgs Doublet Model (2HDM), we recast it in terms of sensitivity reaches for the similar process $pp\to H\to ZA\to l^+l^-b\bar b$. This simple exercise across the two processes, which is possible because the only kinematic difference between these are different widths for the Higgs bosons, in turn affecting minimally the efficiency of an experimental selection, enables us to expand the region of parameter space that can be tested to the case when $m_H\ge m_A+m_Z$. Furthermore, we extrapolate our results to full Run 3 data samples. We conclude that, while the high energy and luminosity stage of the LHC can afford one with increased sensitivity to the 2HDM in general, the recast analysis does not add anything to what already probed through the actual one.

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arXiv2020

$R_K$ and $R_{K^*}$ in an Aligned 2HDM with Right-Handed Neutrinos

We consider the possibility to explain the recent $R_K$ and $R_{K ^*}$ anomalies in a 2-Higgs Doublet Model, known as Aligned, combined with a low scale seesaw mechanism generating light neutrino masses and mixings. In this class of models, a large Yukawa coupling allows for significant non-universal leptonic contributions, through box diagrams mediated by charged Higgs bosons and right-handed neutrinos, to the $b \to s \ell^+ \ell^-$ transition that can then account for both $R_K$ and $R_{K^*}$ anomalies.

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arXiv2020

Signatures of vector-like top partners decaying into new neutral scalar or pseudoscalar bosons

We explore the phenomenology of models containing one Vector-Like Quark (VLQ), $t'$, which can decay into the Standard Model (SM) top quark, $t$, and a new spin-0 neutral boson, $S$, the latter being either a scalar or pseudoscalar state. We parametrise the underlying interactions in terms of a simplified model which enables us to capture possible Beyond the SM (BSM) scenarios. We discuss in particular three such scenarios: one where the SM state is supplemented by an additional scalar, one which builds upon a 2-Higgs Doublet Model (2HDM) framework and another which realises a Composite Higgs Model (CHM) through partial compositeness. Such exotic decays of the $t'$ can be competitive with decays into SM particles, leading to new possible discovery channels at the Large Hadron Collider (LHC). Assuming $t'$ pair production via strong interactions, we design signal regions optimised for one $t'\rightarrow S t$ transition (while being inclusive on the other \bar{t'} decay, and vice versa), followed by the decay of $S$ into the two very clean experimental signatures $S\rightarrow γγ$ and $S\rightarrow Z(\rightarrow \ell^+\ell^-)γ$. We perform a dedicated signal-to-background analysis in both channels, by using Monte Carlo (MC) event simulations modelling the dynamics from the proton-proton to the detector level. Under the assumption of BR$(t' \rightarrow S t) = 100\%$, we are therefore able to realistically quantify the sensitivity of the LHC to both the $t'$ and $S$ masses, assuming both current and foreseen luminosities. This approach paves the way for the LHC experiments to surpass current VLQ search strategies based solely on $t'$ decays into SM bosons ($W^\pm, Z$, $h$).

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arXiv2020

$E_6$ Motivated UMSSM Confronts Experimental Data

We test $E_6$ realisations of a generic $U(1)'$ extended Minimal Supersymmetric Standard Model (UMSSM), parametrised in terms of the mixing angle pertaining to the new $U(1)'$ sector, $θ_{E_6}$, against all currently available data, from space to ground experiments, from low to high energies. We find that experimental constraints are very restrictive and indicate that large gauge kinetic mixing and $θ_{E_6}\approx -π/3$ are required within this theoretical construct to achieve compliance with current data. The consequences are twofold. On the one hand, large gauge kinetic mixing implies that the $Z'$ boson emerging from the breaking of the additional $U(1)'$ symmetry is rather wide since it decays mainly into $WW$ pairs. On the other hand, the preferred $θ_{E_6}$ value calls for a rather specific $E_6$ breaking pattern different from those commonly studied. We finally delineate potential signatures of the emerging UMSSM scenario in both Large Hadron Collider (LHC) and in Dark Matter (DM) experiments.

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arXiv2020

Probing the top-Higgs boson FCNC couplings via the $h\to γγ$ channel at the HE-LHC and FCC-hh

We investigate the sensitivity of future searches for the top-Higgs boson Flavour Changing Neutral Current (FCNC) couplings $tqh$~($q= u, c$) at the proposed High Energy Large Hadron Collider~(HE-LHC) and Future Circular Collider in hadron-hadron mode (FCC-hh). We perform a full simulation for two processes in the $h\to γγ$ decay channel (where $h$ is the discovered Higgs state): single top quark FCNC production in association with the Higgs boson (plus a jet) and top quark pair production with FCNC decays $t\to qh$. All the relevant backgrounds are considered in a cut based analysis to obtain the limits on the Branching Ratios (BRs) of $t\to uh$ and $t\to ch$. It is shown that, at the HE-LHC with an integrated luminosity of 15 ab$^{-1}$ and at the FCC-hh with an integrated luminosity of 30 ab$^{-1}$, {{the BR($t\to uh$) (BR($t\to ch$)) can be probed, respectively, to $7.0~(8.5)\times 10^{-5}$ and $2.3~(3.0) \times 10^{-5}$ at the 95\% Confidence Level (CL) (assuming a 10\% systematic uncertainty on the background), which is almost two orders of magnitude better than the current 13 TeV LHC experimental results.

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arXiv2020

Explaining B decays anomalies in SUSY models

Recent measurements of certain B decays indicate deviations from Standard Model (SM) predictions. We show that Supersymmetric effects can increase the Branching Ratios (BRs) of both $B\to D τ ν_τ$ and $B\to D^\ast τ ν_τ$ with respect to the SM rates, thereby approaching their newest experimentally measured values.

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arXiv2020

Implications of light charged Higgs boson at the LHC Run III in the 2HDM

In this study, we focus on the bosonic decays of light charged Higgs boson (i.e., with $M_{H^\pm}<m_t$) in the 2-Higgs Doublet Model (2HDM) Type-I. To study the signal of such a charged Higgs state at the Large Hadron Collider (LHC), in a scenario where the $H^0$ boson is the Standard Model (SM)-like one already discovered, we assume that it decays mainly via $h^0W^{\pm *}$ and/or $H^\pm\to A^0W^{\pm *}$ (i.e., via an off-shell $W^{\pm}$ boson), which can reach a sizable Branching Ratio (BR) for $\tanβ\geq4$, when the exclusion bounds from $H^\pm\toτν$ and $c{s}$ searches get weaker. By using six Benchmark Points (BPs), which are consistent with current LHC constraints, we perform a Monte Carlo (MC) study and examine the sensitivity of the LHC to light charged Higgs boson decaying via the above bosonic modes and produced in top decay following both single top and top pair production processes. Our findings demonstrate that, when the integrated luminosity can reach 100 fb$^{-1}$, the LHC has the potential to either discover or rule out most of these BPs via either of these two production and decay channels or both.

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