arXiv subjects

Stefano Moretti

Publications and source records attributed to Stefano Moretti.

At least 217 records · Page 12Linked to original sources

arXiv2016

Possible Explanation of the Electron Positron Anomaly at 17 MeV in $^8Be$ Transitions Through a Light Pseudoscalar

We estimate the values of Yukawa couplings of a light pseudoscalar A with a mass of about 17 MeV, which would explain the $^8Be$ anomaly observed in the Atomki pair spectrometer experiment. The resulting couplings of A to up and down type quarks are about 0.3 times the coupling of the standard Higgs boson. Then constraints from K and B decays require that loop contributions to flavour changing vertices cancel at least at the 10% level. Constraints from beam dump experiments require the coupling of A to electrons to be larger than about 4 times the coupling of the standard Higgs boson, leading to a short enough A life time consistent with an explanation of the anomaly.

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arXiv2016

Using asymmetry observables to discover and distinguish Z' signals in top pair production with the lepton-plus-jets final state at the LHC

We study the sensitivity of top pair production with six-fermion decay at the LHC to the presence and nature of an underlying Z' boson, accounting for full tree-level Standard Model ttbar interference, with all intermediate particles allowed off-shell. We concentrate on the lepton-plus-jets final state and simulate experimental conditions, including kinematic requirements and top quark pair reconstruction in the presence of missing transverse energy and combinatorial ambiguity in jet-top assignment. We focus on the differential mass spectra of the cross section and asymmetry observables, especially demonstrating the use of the latter in probing the coupling structure of a new neutral resonance, in addition to cases in which the asymmetry forms a complementary discovery observable.

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arXiv2016

Impact of the Photon-Initiated process on Z'-boson searches in di-lepton final states at the LHC

We discuss the effect of the Photon Initiated (PI) process on the dilepton channel at the LHC. Adopting various QED PDF sets, we evaluate the contribution produced by two resolved photons which is not included in the Equivalent Photon Approximation (EPA). We compare the PI central value as predicted by the CTEQ, MRST and NNPDF collaborations. With the NNPDF2.3QED set of replicas we also estimate the PDF uncertainties on the PI central value. We show the effect of the inclusion of the PI contribution and its PDF uncertainties on neutral heavy Z'-boson searches. We explore the two scenarios of narrow and broad resonances, including in the analysis the reconstructed Forward-Backward Asymmetry observable, the latter being less affected by systematics effects.

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arXiv2016

Search for $Z'$, vacuum (in)stability and hints of high-energy structures

We study the high-energy behaviour of a class of anomaly-free abelian extensions of the Standard Model. We focus on the interplay among the phenomenological characterisation of the model and the use of precise renormalisation group methods. Using as boundary conditions regions of the parameter space at the verge of current LHC probe, interesting unification patterns emerge linked to thresholds belonging to a SO(10) grand unification theory (GUT). We stress how the evolution of the mixing between the two abelian factors may provide a valuable tool to address the candidate high-energy embedding. The emerging unification scenarios are then challenged to be perturbative and to allow for a stable vacuum.

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arXiv2016

Perturbative Unitarity Bounds in Composite 2-Higgs Doublet Models

We study bounds from perturbative unitarity in a Composite 2-Higgs Doublet Model (C2HDM) based on the spontaneous breakdown of a global symmetry $SO(6)\to SO(4)\times SO(2)$ at the compositeness scale $f$. The eight pseudo Nambu-Goldstone Bosons (pNGBs) emerging from such a dynamics are identified as two isospin doublet Higgs fields. We calculate the $S$-wave amplitude for all possible 2-to-2-body elastic (pseudo)scalar boson scatterings at energy scales $\sqrt{s}$ reachable at the Large Hadron Collider (LHC) and beyond it, including the longitudinal components of weak gauge boson states as the corresponding pNGB states. In our calculation, the Higgs potential is assumed to have the same form as that in the Elementary 2-Higgs Doublet Model (E2HDM) with a discrete $Z_2$ symmetry, which is expected to be generated at the one-loop level via the Coleman-Weinberg (CW) mechanism. We find that the $S$-wave amplitude matrix can be block-diagonalized with maximally $2\times 2$ submatrices in a way similar to the E2HDM case as long as we only keep the contributions from ${\cal O }(ξs)$ and ${\cal O }(ξ^0 s^0)$ in the amplitudes, where $ξ=v_{\text{SM}}^2/f^2$ and $v_{\text{SM}}^{}\simeq 246$ GeV, which is an appropriate approximation for our analysis. By requiring the C2HDM to satisfy perturbative unitarity at energies reachable by the LHC, we derive bounds on its parameters such as $ξ$ and the masses of extra Higgs bosons present in the scenario alongside the Standard Model (SM)-like Higgs state discovered in 2012.

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arXiv2016

Drell-Yan production of multi Z'-bosons at the LHC within Non-Universal ED and 4D Composite Higgs Models

The Drell-Yan di-lepton production at hadron colliders is by far the preferred channel to search for new heavy spin-1 particles. Traditionally, such searches have exploited the Narrow Width Approximation (NWA) for the signal, thereby neglecting the effect of the interference between the additional Z'-bosons and the Standard Model Z and γ. Recently, it has been established that both finite width and interference effects can be dealt with in experimental searches while still retaining the model independent approach ensured by the NWA. This assessment has been made for the case of popular single Z'-boson models currently probed at the CERN Large Hadron Collider (LHC). In this paper, we test the scope of the CERN machine in relation to the above issues for some benchmark multi Z'-boson models. In particular, we consider Non-Universal Extra Dimensional (NUED) scenarios and the 4-Dimensional Composite Higgs Model (4DCHM), both predicting a multi-Z' peaking structure. We conclude that in a variety of cases, specifically those in which the leptonic decays modes of one or more of the heavy neutral gauge bosons are suppressed and/or significant interference effects exist between these or with the background, especially present when their decay widths are significant, traditional search approaches based on the assumption of rather narrow and isolated objects might require suitable modifications to extract the underlying dynamics.

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arXiv2016

Optimising Charged Higgs Boson Searches at the Large Hadron Collider Across $b\bar b W^\pm$ Final States

In the light of the most recent data from Higgs boson searches and analyses, we re-assess the scope of the Large Hadron Collider in accessing heavy charged Higgs boson signals in $b\bar b W^\pm$ final states, wherein the contributing channels can be $H^+\to t\bar b$, $hW^\pm, HW^\pm$ and $AW^\pm$. We consider a 2-Higgs Doublet Model Type-II and we assume as production mode $bg\to tH^-$ + c.c., the dominant one over the range $M_{H^\pm}\ge 480$ GeV, as dictated by $b\to sγ$ constraints. Prospects of detection are found to be significant for various Run 2 energy and luminosity options.

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arXiv2016

CP Violating Two-Higgs-Doublet Model: Constraints and LHC Predictions

Two-Higgs-Doublet Models (2HDMs) are amongst the simplest extensions of the Standard Model. Such models allow for tree-level CP Violation (CPV) in the Higgs sector. We analyse a class of CPV 2HDM (of Type-I) in which only one of the two Higgs doublets couples to quarks and leptons, avoiding dangerous Flavour Changing Neutral Currents. We provide an up to date and comprehensive analysis of the constraints and Large Hadron Collider (LHC) predictions of such a model. Of immediate interest to the LHC Run 2 is the golden channel where all three neutral Higgs bosons are observed to decay into gauge boson pairs, $WW$ and $ZZ$, providing a smoking gun signature of the CPV 2HDM.

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arXiv2016

The 750 GeV diphoton excess and its explanation in 2-Higgs Doublet Models with a real inert scalar multiplet

We discuss a possible explanation of the recently observed diphoton excess at around 750 GeV as seen by the ATLAS and CMS experiments at the Large Hadron Collider. We calculate the cross section of the diphoton signature in 2-Higgs Doublet Models with the addition of a real isospin scalar multiplet without a vacuum expectation value, where a neutral component of such a representation can be a dark matter candidate. We find that the branching fraction of an additional CP-even Higgs boson $H$ from the doublet fields into the diphoton mode can be significantly enhanced, by up to a factor of $10^3$, with respect to the case of the simple 2-Higgs Doublet Model. Such a sizable enhancement is realized due to multi-charged inert particle loops entering the $H\to γγ$ decay mode. Through this enhancement, we obtain a suitable cross section of the $gg\to H \to γγ$ process to explain the data with the fixed mass of $H$ being 750 GeV.

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arXiv2015

Hunting for neutral, long-lived exotica at the LHC using a missing transverse energy signature

Searches at the Large Hadron Collider (LHC) for neutral, long-lived particles have historically relied on the detection of displaced particles produced by their decay $\textit{within}$ the detector volume. In this paper we study the potential of the complementary signature comprising of the missing transverse energy ($E_T^{miss}$) signal, traditionally used to look for dark matter, e.g., the lightest supersymmetric particle (LSP), to extend the LHC coverage to models with long-lived (LL) particles when they decay $\textit{outside}$ the detector. Using CMS and ATLAS analyses at the 8 TeV LHC, we set an upper limit at the 95% confidence level (CL) on the production cross sections for two specific scenarios: (i) a model with a heavy non-standard model Higgs boson decaying to a LL scalar and (ii) an R-parity violating RPV SUSY model with a LL neutralino. We show that this method can significantly extend the LHC sensitivity to neutral, LL particles with arbitrary large lifetimes and that the limits obtained from a $E_T^{miss}$ signal are comparable to those from displaced particle searches for decay distances above a few meters. Results obtained in this study do not not depend on the specific decay channel of the LL particle and therefore are model-independent in this sense. We provide limits for the whole two-dimensional plane in terms of the mass of the LL particle and the mass of the mediator up to masses of 2 TeV including particular benchmarks studied in the original experimental papers. We have made these limits available in the form of a grid which can be used for the interpretation of various other new physics models.

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arXiv2015

Complementarity of Forward-Backward Asymmetry for discovery of Z' bosons at the Large Hadron Collider

The Forward-Backward Asymmetry (AFB) in Z' physics is commonly only thought of as an observable which possibly allows one to profiling a Z' signal by distinguishing different models embedding such (heavy) spin-1 bosons. In this brief review, we examine the potential of AFB in setting bounds on or even discovering a Z' at the Large Hadron Collider (LHC) and proof that it might be a powerful tool for this purpose. We analyse two different scenarios: Z's with a narrow and wide width, respectively. We find that, in both cases, AFB can complement the conventional searches in accessing Z' signals traditionally based on cross section measurements only.

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arXiv2015

Exclusion and discovery via Drell-Yan in the 4DCHM

Searches for Z' bosons are most sensitive in the dilepton channels at hadron colliders. Whilst finite width and interference effects do affect the modifications the presence of BSM physics makes to Standard Model (SM) contributions, generic searches are often designed to minimize these. The experimental approach adopted works well in the case of popular models that predict a single and narrow Z' boson allowing these effects to effectively be neglected. Conversely, finite width and interference effects may have to be taken into account in experimental analyses when such Z' states are wide or where several states are predicted. We explore the consequences of these effects in the 4-Dimensional Composite Higgs Model (4DCHM) which includes multiple new Z' bosons and where the decays of these resonances to non-SM fermions can result in large widths.

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arXiv2015

Top pair production at a future $e^+e^-$ machine in a composite Higgs scenario

The top quark plays a central role in many New Physics scenarios and in understanding the details of Electro-Weak Symmetry Breaking. In the short- and mid-term future, top-quark studies will mainly be driven by the experiments at the Large Hadron Collider. Exploration of top quarks will, however, be an integral part of particle physics studies at any future facility and an $e^+ e^-$ collider will have a very comprehensive top-quark physics program. We discuss the possibilities of testing NP in the top-quark sector within a composite Higgs scenario through deviations from the Standard Model in top pair production for different Centre-of-Mass energy options of a future $e^+e^-$ machine. In particular, we focus on precision studies of the top-quark sector at a CM energy ranging from 370 GeV up to 3 TeV.

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arXiv2015

Two Higgs bosons near 125 GeV in the complex NMSSM and the LHC Run-I data

We analyse the impact of explicit CP-violation in the Higgs sector of the Next-to-Minimal Supersymmetric Standard Model (NMSSM) on its consistency with the Higgs boson data from the Large Hadron Collider (LHC). Through detailed scans of the parameter space of the complex NMSSM for certain fixed values of one of its CP-violating (CPV) phases, we obtain a large number of points corresponding to five phenomenologically relevant scenarios containing $\sim 125$ GeV Higgs boson(s). We focus, in particular, on the scenarios where the visible peaks in the experimental samples can actually be explained by two nearly mass-degenerate neutral Higgs boson states. We find that some points corresponding to these scenarios give an overall slightly improved fit to the data, more so for non-zero values of the CPV phase, compared to the scenarios containing a single Higgs boson near 125 GeV.

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arXiv2015

Observable Heavy Higgs Dark Matter

Dark Matter (DM), arising from an Inert Higgs Doublet, may either be light, below the $W$ mass, or heavy, above about 525 GeV. While the light region may soon be excluded, the heavy region is known to be very difficult to probe with either Direct Detection (DD) experiments or the Large Hadron Collider (LHC). We show that adding a second Inert Higgs Doublet helps to make the heavy DM region accessible to both DD and the LHC, by either increasing its couplings to the observed Higgs boson, or lowering its mass to $360 \gev \lesssim m_{DM}$, or both.

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arXiv2015

$H^\pm$ in the $W^\pm h$ channel at the LHC Run 2

We analyse the discovery prospects of the charged Higgs boson, $H^\pm$, via its decay in the $W^\pm h$ channel in the minimal supersymmetric Standard Model (MSSM) as well as several 2-Higgs Doublet Models (2HDMs). $h$, the lightest scalar Higgs boson in these models, is identified with the recently discovered $\sim 125$ GeV state, $H_\text{obs}$, at the Large Hadron Collider (LHC). We find that, while it provides an important input in the kinematic selection of signal events, the measured $H_\text{obs}$ mass renders this channel inaccessible in the MSSM. In the 2HDMs though, through a dedicated signal-to-background analysis for the $pp\to t(\bar{b})H^-\to \ell^\pmν_\ell jj bb\bar{b}(\bar{b})$+h.c process, we establish that some parameter space regions will be testable at the LHC with $\sqrt{s}=14$ TeV.

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arXiv2015

Imprints of Composite Higgs Models at e+e- Colliders

We test the sensitivity of a future e+e- collider to composite Higgs scenarios encompassing partial compositeness. Besides the detailed study of the Higgs properties, such a machine will have a rich top-quark physics programme mainly in two domains: top property accurate determination at the $t \bar t$ production threshold and search for New Physics with top quarks above it. In both domains, a composite Higgs scenario can manifest itself via sizable deviations in both cross-section and asymmetry observables. Herein we discuss such a possibility using a particular realisation, namely the 4-Dimensional Composite Higgs Model.

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arXiv2015

Enhancement of the $H^\pm W^\mp Z$ vertex in the three scalar doublet model

We compute one-loop induced trilinear vertices with physical charged Higgs bosons $H^\pm$ and ordinary gauge bosons, i.e., $H^\pm W^\mp Z$ and $H^\pm W^\mp γ$, in the model with two active plus one inert scalar doublet fields under a $Z_2(\text{unbroken})\times \tilde{Z}_2(\text{softly-broken})$ symmetry. The $Z_2$ and $\tilde{Z}_2$ symmetries are introduced to guarantee the stability of a dark matter candidate and to forbid the flavour changing neutral current at the tree level, respectively. The dominant form factor $F_Z$ of the $H^\pm W^\mp Z$ vertex can be enhanced by non-decoupling effects of extra scalar boson loop contributions. We find that, in such a model, $|F_Z|^2$ can be one order of magnitude larger than that predicted in two Higgs doublet models under the constraints from vacuum stability, perturbative unitarity and the electroweak precision observables. In addition, the branching fraction of the $H^\pm \to W^\pm Z$ $(H^\pm \to W^\pm γ)$ mode can be of order 10~(1)\% level when the mass of $H^\pm$ is below the top quark mass. Such a light $H^\pm$ is allowed by the so-called Type-I and Type-X Yukawa interactions which appear under the classification of the $\tilde{Z}$ charge assignment of the quarks and leptons. We also calculate the cross sections for the processes $H^\pm \to W^\pm Z$ and $H^\pm \to W^\pm γ$ onset by the top quark decay $t\to H^\pm b$ and electroweak $H^\pm$ production at the LHC.

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