arXiv subjects
Publications and source records attributed to S. Moretti.
We illustrate preliminary results obtained through Monte Carlo (HERWIG) and detector (ATLFAST) simulations of the $H^\pm\toτ^\pmν_τ$ signature of charged Higgs bosons with masses comparable to that of the top quark.
Heavy charged Higgs bosons ($H^\pm$) of a Type II 2-Higgs Doublet Model (2HDM) can be detected at future electron-positron Linear Colliders (LCs) even when their mass is larger than half the collider energy. The single Higgs mode $e^+e^-\to t\bar b H^- + ~{\rm{c.c.}} \to 4b +{\rm{j}}{\rm{j}} + \ell + p_T^{\rm{miss}}$ (where j represents a jet and with $\ell=e,μ$) contributes to extend the discovery reach of $H^\pm$ states into the mass region $M_{H^\pm}\gsim \sqrt s/2$, where the well studied pair production channel $e^+e^-\to H^-H^+$ is no longer available. With a technique that allows one to reconstruct the neutrino four-momentum in the decay $t\to b W^+\to b \ell^+ν$, one can suppress the initially overwhelming main irreducible background due to $e^+e^-\to t\bar t b\bar b$ (via a gluon splitting into $b\bar b$ pairs) to a negligible level. However, for currently foreseen luminosities, one can establish a statistically significant $H^\pm$ signal only over a rather limited mass region, of 20 GeV or so, beyond $M_{H^\pm}\approx \sqrt s/2$, for very large or very small values of $\tanβ$ and provided high $b$-tagging efficiency can be achieved.
We demonstrate that Higgs discovery at the LHC is possible in the context of the NMSSM even for those scenarios such that the only strongly produced Higgs boson is a very SM-like CP-even scalar which decays almost entirely to a pair of relatvely light CP-odd states. In combination with other search channels, we are on the verge of demonstrating that detection of at least one of the NMSSM Higgs bosons is guaranteed at the LHC for accumulated luminosity of $300 {\rm fb}^{-1}$.
We illustrate the impact of full one-loop weak corrections onto b-jet-, prompt-photon- and Z-production at Tevatron and Large Hadron Collider (LHC)
We study the process gg -> b bbar H+ H- at large tan(beta), where it represents the dominant production mode of charged Higgs boson pairs in a Type II 2-Higgs Doublet Model, including the Minimal Supersymmetric Standard Model. The ability to select this signal would in principle enable the measurements of some triple-Higgs couplings, which in turn would help understanding the structure of the extended Higgs sector. We outline a selection procedure that should aid in disentangling the Higgs signal from the main irreducible background. This exploits a signature made up by `four b-quark jets, two light-quark jets, a tau-lepton and missing energy'. While, for tan(beta) \gsim 30 and over a significant M_H+/- range above the top mass, a small signal emerges already at the Large Hadron Collider after 100 fb^{-1}, ten times as much luminosity would be needed to perform accurate measurements of Higgs parameters in the above final state, rendering this channel a primary candidate to benefit from the so-called `Super' Large Hadron Collider option, for which a tenfold increase in instantaneous luminosity is currently being considered.
We perform for the Large Hadron Collider (LHC) a detailed study of charged Higgs boson production via the top-bottom quark associated mode followed by decays into a chargino and a neutralino, with masses and couplings as given by the general Minimal Supersymmetric Standard Model (MSSM). We focus our attention on the region of parameter space where the charged Higgs is heavier than the top quark, with intermediate values of tan beta. In this region, the identification of charged Higgs bosons via decays into Standard Model (SM) particles has proven to be ineffective. Modelling the CMS detector, we find that a signature consisting of three hard leptons accompanied by a hadronically reconstructed top quark plus substantial missing transverse energy, which may result from charged Higgs decays into neutralinos and charginos, can be made viable over a large variety of initially overwhelming SM and MSSM backgrounds, provided MSSM input parameters are favourable: notably, a small mu parameter and light sleptons are important prerequisites. We quantify these statements by performing a fairly extensive scan of the parameter space, including realistic hadron-level simulations, and delineate some potential discovery regions.
A light Higgs boson with substantial branching ratio into invisible channels can occur in a variety of models with: light neutralinos, spontaneously broken lepton number, radiatively generated neutrino masses, additional singlet scalar(s) and/or right handed neutrinos in the extra dimensions of TeV scale gravity. We study the observability of the WH and ZH modes at LHC with H decaying invisibly, by carrying out a detailed simulation with two event generators HERWIG and PYTHIA and realistic detector simulations (GETJET and CMSJET). We find that the signal with `single lepton plus missing $E_T$' resulting from WH production suffers from a very large background due to the (off-shell) $W^{*}$ production via the Drell-Yan process. In contrast, the ZH mode provides a clean signal in the `dilepton plus missing $E_T$' channel. By exploiting this second signature, we show that invisible branching ratios of Higgs bosons, ${BR}_{inv}$, larger than $\sim 0.42(0.70)$ can be probed at 5$σ$ level for $M_H = 120$(160) GeV respectively, assuming an accumulated luminosity of L=100 fb$^{-1}$.
We investigate one-loop weak corrections to the production cross section of two $b$-jets at Tevatron and Large Hadron Collider (LHC). We establish that they are small at inclusive level but dominant in exclusive observables that have a non-trivial dependence on the helicity structure of the hard subprocesses. Such effects can serve as a test of the Standard Model (SM) and, conversely, they should be taken into account in future experimental analyses aiming at extracting possible signals of new physics.
We report on the calculation of the factorisable one-loop weak-interaction corrections to the initial and final states for three-jet observables in electron-positron annihilations. We show that such corrections are of a few percent at $\sqrt s=M_Z$. Hence, while their impact is not dramatic in the context of LEP1 and SLC, where the total error on the measured value of $α_{\mathrm{S}}$ is larger, at a future Linear Collider, running at the $Z$ mass peak (e.g., GigaZ), they ought to be taken into account in the experimental fits, as here the uncertainty on the value of the strong coupling constant is expected to be at the 0.1% level or even smaller. The calculation has been performed using helicity amplitudes so that it can be applied to the case of polarised beams.
We show how the well studied $γγ$ inclusive Higgs signal can be used at the Large Hadron Collider to test Gauge Mediated Supersymmetry Breaking scenarios in which a rather heavy Higgs boson decays into two light neutralinos, the latter yielding two photons and missing (transverse) energy
We investigate the scope of all relevant production modes of charged Higgs bosons in the MSSM, with mass larger than the one of the top quark, at future Linear Colliders operating in $γγ$ mode at the TeV energy scale. Final states with one or two $H^\pm$ bosons are considered, as produced by both tree- and loop-level interactions
HERWIG is a general-purpose Monte Carlo event generator, which includes the simulation of hard lepton-lepton, lepton-hadron and hadron-hadron scattering and soft hadron-hadron collisions in one package. It uses the parton-shower approach for initial- and final-state QCD radiation, including colour coherence effects and azimuthal correlations both within and between jets. This article updates the description of HERWIG published in 1992, emphasising the new features incorporated since then. These include, in particular, the matching of first-order matrix elements with parton showers, a more correct treatment of heavy quark decays, and a wide range of new processes, including many predicted by the Minimal Supersymmetric Standard Model, with the option of R-parity violation. At the same time we offer a brief review of the physics underlying HERWIG together with details of the input and control parameters and the output data, to provide a self-contained guide for prospective users of the program. This version of the manual (version 3) is updated to HERWIG version 6.5, which is expected to be the last major release of Fortran HERWIG. Future developments will be implemented in a new C++ event generator, HERWIG++.
We show how a statistically significant signal of heavy charged Higgs bosons of a Type II Two-Higgs Doublet (2HDM) Model produced in association with tau-neutrino pairs can be established at future $e^+e^-$ Linear Colliders (LCs) in the $H^+\to t\bar b\to 4$ jet decay channel for large $\tanβ$ in the $M_{H^\pm}\gsim \sqrt s/2$ mass region
I review all the new features of the HERWIG event generator which are relevant to Linear Collider (LC) physics starting from version 6.1
The correlation between the two $τ$-polarisations is predicted to be opposite $(+-/-+)$ for the $H/A \to τ^+τ^-$ signal, while it corresponds to the same sign combinations for the Drell-Yan $(++/--)$ and $t \bar t$ $(--)$ backgrounds. We show that this correlation can serve as a distinctive test to confirm the presence of the MSSM Higgs bosons $H$ and $A$ in their hadronic $τ^+τ^-$ decay channels at the LHC.
We compute the contribution to charged Higgs boson pair production at the Large Hadron Collider (LHC) due to the scattering of two electroweak (EW) gauge bosons, these being in turn generated via bremsstrahlung off incoming quarks: q q --> q q V^*V^* --> q q H^+H^- (V=gamma,Z,W^{+/-}). We verify that the production cross section of this mode is tan beta independent and show that it is smaller than that of H^+H^- production via q q-initiated processes but generally larger than that of the loop-induced channel gg --> H^+H^-. Pair production of charged Higgs bosons is crucial in order to test EW symmetry breaking scenarios beyond the Standard Model (SM). We show that the detection of these kind of processes at the standard LHC is however problematic, because of their poor production rates and the large backgrounds.
We show how a statistically significant signal of heavy charged Higgs bosons of a general Two-Higgs Doublet Model produced in association with tau-neutrino pairs can be established at future Linear Colliders in the $H^+\to t\bar b\to 4$ jet decay channel. This signature is particularly relevant in the kinematic configuration $\sqrt s\lsim2 M_{H^\pm}$, when the pair production channel $e^+e^-\to H^-H^+$ is no longer available. Here, the initially overwhelming background, constituted by top quark pair production and decay, can vigorously be reduced thanks to a dedicated selection procedure that allows one to extract a signal in a region of several tens of GeV around $M_{H^\pm}\approx \sqrt s/2$, for $\tanβ\gsim40$.
The HERWIG event generator was widely used throughout the workshop, particularly in the emulation of Supersymmetric and Higgs processes in the context of the Minimal Supersymmetric Standard Model. We briefly review here its main features in this respect