arXiv subjects

Lorenzo Basso

Publications and source records attributed to Lorenzo Basso.

At least 19 recordsLinked to original sources

arXiv2016

Precision tools and models to narrow in on the 750 GeV diphoton resonance

The hints for a new resonance at 750 GeV from ATLAS and CMS have triggered a significant amount of attention. Since the simplest extensions of the standard model cannot accommodate the observation, many alternatives have been considered to explain the excess. Here we focus on several proposed renormalisable weakly-coupled models and revisit results given in the literature. We point out that physically important subtleties are often missed or neglected. To facilitate the study of the excess we have created a collection of 40 model files, selected from recent literature, for the Mathematica package SARAH. With SARAH one can generate files to perform numerical studies using the tailor-made spectrum generators FlexibleSUSY and SPheno. These have been extended to automatically include crucial higher order corrections to the diphoton and digluon decay rates for both CP-even and CP-odd scalars. Additionally, we have extended the UFO and CalcHep interfaces of SARAH, to pass the precise information about the effective vertices from the spectrum generator to a Monte-Carlo tool. Finally, as an example to demonstrate the power of the entire setup, we present a new supersymmetric model that accommodates the diphoton excess, explicitly demonstrating how a large width can be obtained. We explicitly show several steps in detail to elucidate the use of these public tools in the precision study of this model.

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arXiv2016

Resonant mono Higgs at the LHC

In recent years, the production of a SM particle with large missing transverse momentum, dubbed mono-X searches, have gained increasing attention. After the discovery of the Higgs boson in 2012, the run-II of the LHC will now scrutinise its properties, looking for BSM physics. In particular, one could search for mono-Higgs signals, that are typically studied in models addressing dark matter. However, this signal can appear also in models addressing the neutrino masses, if additional heavier neutrinos with masses at the electroweak scale are present. The latter will couple to the SM neutrinos and the Higgs boson, yielding a type of mono-Higgs signal not considered for dark matter: the resonant production of a Higgs boson and missing energy. In this paper, we address the LHC exclusion power of the latter with dedicated detector simulations, and reinterpret it in a benchmark scenario for neutrino mass generation.

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arXiv2015

Discovery potential for $T' \to tZ$ in the trilepton channel at the LHC

The LHC discovery potential of heavy top partners decaying into a top quark and a $Z$ boson is studied in the trilepton channel at 13 TeV in the single production mode. The clean multilepton final state allows to strongly reduce the background contaminations and to reconstruct the $T'$ mass. We show that a simple cut-and-count analysis probes the parameter space of a simplified model as efficiently as a dedicated multivariate analysis. The trilepton signature finally turns out to be able to probe $T'$ masses up to $2.0$ TeV, when mixing with first generation quarks is included. The reinterpretation in terms of the top-$Z$-quark anomalous coupling is shown.

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arXiv2015

Charged-Higgs production in the Two-Higgs-Doublet Model --- the τνchannel

We update the allowed parameter space of the CP-violating 2HDM with Type II Yukawa couplings, that survives the current experimental and theoretical constraints on the model. For a representative set of allowed parameter points, we study the production of charged Higgs bosons, both at the LHC at 14 TeV and at a possible future hadronic collider at 30 TeV. Two classes of production mechanisms are considered, "bosonic" (pp\to H^\pm W^\mp X) and "fermionic" (pp \to H^+ \bar t (b) X). After commenting on our previous H^\pm\to W^\pm H_1 investigation, we focus on the tauonic decay mode, H^\pm \to τν, performing a detailed signal-over-background analysis at the parton level. The increased features provided when considering CP violation, i.e., the extension of the parameter space and the mixing of the would-be CP-odd scalar boson, only marginally increase the discovery prospects, which remain very challenging both when increased luminosities and higher energies are considered.

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arXiv2015

Doubly-charged Higgs and vacuum stability in left-right supersymmetry

We present an analysis of supersymmetric left-right symmetric models with Higgs fields lying in the adjoint representation of $SU(2)_R$. These models feature a doubly-charged Higgs boson which gets its mass only at the loop level. We present, for the first time, a complete one-loop calculation of this mass and show that contributions that have been neglected so far can shift it by a few hundreds of GeV. We then combine this observation with LHC bounds deduced from extra charged gauge boson and doubly-charged Higgs boson searches. In particular, we point out that existing limits get substantially modified by the presence of singly-charged Higgs bosons that are also predicted in these models. In addition, we address constraints stemming from vacuum stability considerations and show how the considered class of models could be ruled out at the next LHC run in the absence of any signal.

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arXiv2015

Techniques for the treatment of IR divergences in decay processes at NLO and application to the top-quark decay

We present the extension of two general algorithms for the treatment of infrared singularities arising in electroweak corrections to decay processes at next-to-leading order: the dipole subtraction formalism and the one-cutoff slicing method. The former is extended to the case of decay kinematics which has not been considered in the literature so far. The latter is generalized to production and decay processes with more than two charged particles, where new "surface" terms arise. Arbitrary patterns of massive and massless external particles are considered, including the treatment of infrared singularities in dimensional or mass regularization. As an application of the two techniques we present the calculation of the next-to-leading-order QCD and electroweak corrections to the top-quark decay width including all off-shell and decay effects of intermediate W bosons. The result, e.g., represents a building block of a future calculation of NLO electroweak effects to off-shell top-quark pair (WWbb) production. Moreover, this calculation can serve as the first step towards an event generator for top-quark decays at next-to-leading order accuracy, which can be used to attach top-quark decays to complicated many-particle top-quark processes, such as for tt+H or tt+jets.

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arXiv2015

The Higgs sector of the minimal SUSY $B-L$ model

I review the Higgs sector of the $U(1)_{B-L}$ extension of the minimal supersymmetric standard model (MSSM). I will show that the gauge kinetic mixing plays a crucial role in the Higgs phenomenology. Two light bosons are present, a MSSM-like one and a $B-L$-like one, that mix at one loop solely due to the gauge mixing. After briefly looking at constraints from flavour observables, new decay channels involving right-handed (s)neutrinos are presented. Finally, it will be reviewed how model features pertaining to the gauge extension affect the model phenomenology, concerning the existence of R-Parity-conserving minima at loop level and the Higgs-to-diphoton coupling.

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arXiv2015

Discovery potential for $T' \to tZ$ in the trilepton channel at the LHC

The LHC discovery potential of heavy top partners decaying into a top quark and a $Z$ boson is studied in the trilepton channel at $\sqrt{s}=13$ TeV. The clean multilepton final state allows to strongly reduce the background contaminations and to reconstruct the $T'$ mass. We show that a simple cut-and-count analysis probes the parameter space of a simplified model as efficiently as a dedicated multivariate analysis. The trilepton signature finally turns out to be as sensitive in the low $T'$ mass region as the complementary channel with a fully hadronic top quark, and more sensitive in the large mass domain. The reinterpretation in terms of the top-$Z$-up anomalous coupling is shown.

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arXiv2014

From realistic 2HDM-II CPV benchmarks to the $H^\pm \toτν$ decay at the LHC

Phenomenological studies of a CP-violating two-Higgs-doublet Model with type-II Yukawa couplings are presented. In the light of recent LHC data, an update on the viable parameter space that survives both the experimental and theoretical constraints is provided. In addition, the scope of the LHC in exploring this model through the discovery of a charged Higgs boson that decays in the tauonic mode is analysed. For this, various production channels were investigated, with emphasis on the boson-associated channel $gg\to H_i\to H^\pm W^\mp$ and the fermion-associated channels $gb\to H^\pm t$ and $gg\to H^\pm bt$. For the latter, a signal-over-background analysis is performed.

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arXiv2014

A renormalization group analysis of the Hill model and its HEIDI extension

The parameter space of the simplest extension of the standard model, is studied in the light of the 125 GeV Higgs boson discovery. The Hill model extends the scalar sector of the standard model with a real singlet, that mixes with the SM Higgs boson. The two-loop SM renormalization group equations are completed with the one-loop Hill equations. Stability up to the Planck scale is possible without tension with the other parameters. An extension with more singlet fields, in particular a higher dimensional (HEIDI) field is presented.

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arXiv2014

Precision tests of unitarity in leptonic mixing

In the light of the recent LHC data, we study precision tests sensitive to the violation of lepton universality, in particular the violation of unitarity in neutrino mixing. Keeping all data we find no satisfatory fit, even allowing for violations of unitarity in neutrino mixing. Leaving out sin$^2 θ_{\scriptsize \mbox{eff}}$ from the hadronic forward-backward asymmetry at LEP, we find a good fit to the data with some evidence of lepton universality violation at the $\mathcal{O}(10^{-3})$ level.

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arXiv2014

Minimal Z' models and the 125 GeV Higgs boson

The 1-loop renormalization group equations for the minimal Z' models encompassing a type-I seesaw mechanism are studied in the light of the 125 GeV Higgs boson discovery. This model is taken as a benchmark for the general case of singlet extensions of the standard model. The most important result is that negative scalar mixing angles are favoured with respect to positive values. Further, a minimum value for the latter exists, as well as a maximum value for the masses of the heavy neutrinos, depending on the vacuum expectation value of the singlet scalar.

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arXiv2013

The CP-violating type-II 2HDM and Charged Higgs boson benchmarks

We review and update the interpretation of the 125 GeV scalar as the lightest Higgs boson of the Two-Higgs-Doublet Model, allowing for CP violation in the potential. The detection of a charged Higgs boson would exclude the Standard Model. Proposed benchmarks for charged-Higgs searches in the channel pp\to H^+W^-X\to W^+W^-H_1X are reviewed and updated.

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arXiv2013

Natural $Z'$ model with an inverse seesaw and leptonic dark matter

We consider a model for a Z'-boson coupled only to baryon minus lepton number and hypercharge. Besides the usual right-handed neutrinos, we add a pair of fermions with a fractional lepton charge, which we therefore call leptinos. One of the leptinos is taken to be odd under an additional Z_2 charge, the other even. This allows for a natural (inverse) seesaw mechanism for neutrino masses. The odd leptino is a candidate for dark matter, but has to be resonantly annihilated by the Z'-boson or the Higgs-boson responsible for giving mass to the former. Considering collider and cosmological bounds on the model, we find that the Z'-boson and/or the extra Higgs-boson can be seen at the LHC. With more pairs of leptinos leptogenesis is possible.

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arXiv2013

Charged Higgs boson benchmarks in the CP-violating type-II 2HDM

We discuss prospects for discovering a charged Higgs boson in the CP-violating type-II two-Higgs-doublet Model. In the light of recent LHC data, we focus on the parameter space that survives experimental as well as the theoretical constraints on the model. Once the phenomenological scenario is set, we analyse the scope for the LHC to discover a charged Higgs in association with the lightest neutral Higgs and a charged vector boson.

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arXiv2012

Enhancing $h \to γγ$ with staus in SUSY models with extended gauge sector

We discuss the impact on the stau masses of the additional D-terms in U(1)-extended MSSM models. We show, explicitly for the B-L-SSM, that these contributions can play a crucial role in the explanation of the enhanced diphoton decay rate of a SM-like Higgs particle around 125 GeV. Even in the most constrained scenario with universal scalar and gaugino masses, it is possible to obtain a sizable enhancement and, in addition, the correct relic density for the LSP. Furthermore, a lighter CP-even scalar that could fit the LEP excess at 98 GeV is viable.

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arXiv2012

Dark matter scenarios in the minimal SUSY B-L model

We perform a study of the dark matter candidates of a constrained version of the minimal R-parity-conserving supersymmetric model with a gauged $U(1)_{B-L}$. It turns out that there are four additional candidates for dark matter in comparison to the MSSM: two kinds of neutralino, which either correspond to the gaugino of the $U(1)_{B-L}$ or to a fermionic bilepton, as well as "right-handed" CP-even and -odd sneutrinos. The correct dark matter relic density of the neutralinos can be obtained due to different mechanisms including new co-annihilation regions and resonances. The large additional Yukawa couplings required to break the $U(1)_{B-L}$ radiatively often lead to large annihilation cross sections for the sneutrinos. The correct treatment of gauge kinetic mixing is crucial to the success of some scenarios. All candidates are consistent with the exclusion limits of Xenon100.

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arXiv2012

The pure $B-L$ model and future linear colliders: the Higgs sector

We summarise the phenomenology of the Higgs sector of the minimal $B-L$ extension of the Standard Model at an $e^+e^-$ Linear Collider. Within such a scenario, we show that (in comparison with the Large Hadron Collider) several novel production and decay channels involving the two physical Higgs states could experimentally be accessed at such a machine. In particular, we present the scope of the $Z'$ strahlung process for single and double Higgs production, the only suitable mechanism for accessing an almost decoupled heavy scalar state.

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