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Stefano Moretti

Publications and source records attributed to Stefano Moretti.

At least 289 records · Page 16Linked to original sources
arXiv2003

Slepton production from gauge boson fusion

We emphasise that charged slepton pairs produced via vector-boson fusion along with two high-mass, high-$p_T$ forward/backward jets (in two opposite hemispheres) can have a higher production cross-section for heavy slepton masses than that from conventional Drell-Yan production at a hadronic collider like the LHC. We analyse the signal and leading backgrounds in detail in the minimal supersymmetric standard model with conserved baryon and lepton numbers. Our investigation reveals that the mass reach of the vector-boson fusion channel is certainly an improvement over the scope of the Drell-Yan mode.

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arXiv2003

Towards a No-Lose Theorem for NMSSM Higgs Discovery at the LHC

We scan the parameter space of the NMSSM for the observability of a Higgs boson at the LHC with $300 {\rm fb}^{-1}$ integrated luminosity per detector, taking the present LEP constraints into account. We focus on the regions of parameter space for which none of the usually considered LHC detection modes are viable due to the fact that the only light non-singlet (and, therefore, potentially visible) Higgs boson, $h$, decays mainly to two CP-odd light Higgs bosons, $h\to a a$. We simulate the $WW\to h \to aa$ detection mode. We find that this signal may be detectable at the LHC as a signal/background $\sim 600/600$ bump in the tail of a rapidly falling mass distribution. If further study gives us confidence that the shape of the background tail is predictable, then we can conclude that NMSSM Higgs detection at the LHC will be possible throughout all of parameter space by combining this signal with the usual detection modes previously simulated by ATLAS and CMS. We also show that this $WW\to h\to aa$ signal will be highly visible at the LC due to its cleaner environment and high luminosity. We present a study of the production modes and decay channels of interest at the LC.

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arXiv2002

Improving the discovery potential of charged Higgs bosons at Tevatron Run 2

By exploiting the full process $p\bar p\to t\bar b H^-$, in place of the standard Monte Carlo procedure of factorising production and decay, $p\bar p\to t\bar t$ followed by $\bar t\to\bar b H^-$, we show how to improve the discovery reach of the Tevatron (Run 2) in charged Higgs boson searches, in the large $\tanβ$ region. This is achieved in conjunction with dedicated cuts on a `transverse mass' distribution sensitive to the Higgs boson mass and to `polarisation' effects in the $H^-\toτ^-\barν_τ$ decay channel.

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arXiv2001

Signatures of MSSM charged Higgs bosons via chargino-neutralino decay channels at the LHC

We assess the potential of detecting a charged Higgs boson of the MSSM at the LHC via its decays into a chargino and a neutralino. We focus our attention on the region of parameter space with $m_{H^{\pm}} > m_t$ and $3 \lsim tan(beta) \lsim 10$, where identification of the $H^{\pm}$ via other decay modes has proven to be ineffective. Searching for means to plug this hole, we simulate the decays $H^{\pm} --> {\tildeχ}_1^{\pm} {\tildeχ}_1^0$ and $H^{\pm} --> {\tildeχ}_1^{\pm} {\tildeχ}_2^0, {\tildeχ}_1^{\pm} {\tildeχ}_3^0$ --- the former can yield a single hard lepton (from the chargino decay) while the latter can yield three leptons (from the chargino and neutralino decays). Coupled with the dominant top quark + charged Higgs boson production mode, the resulting signature is one or three hard, isolated leptons, substantial missing transverse momentum and a reconstructed (via a 3-jet invariant mass) top quark. The single lepton channel is swamped by background processes; however, with suitable cuts, a trilepton signal emerges. While such a signal suffers from a low number of surviving events (after cuts) and is dependent on several MSSM input parameters (notably $M_2$, $μ$, and slepton masses), it does fill at least some of the void left by previous investigations.

hep-ph↗
arXiv2001

Gluon Contributions to Parity-Violating Asymmetries in Polarized Proton-Proton Scattering

We report on a calculation of one-loop weak corrections to polarized quark-gluon scattering and the corresponding crossed channels. Such contributions are suppressed formally by one power of alpha_s relative to W- or Z-mediated quark-quark scattering, but would enable the spin asymmetry of the gluon distribution to contribute to parity-violating asymmetries that will soon be investigated in polarized proton-proton scattering experiments at RHIC. In certain kinematic regions, gluon contributions to parity-violating asymmetries can be as large as 10% of the tree-level W- and Z-exchanges in quark-quark scattering, but usually only where the parity-violating asymmetries are already small.

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arXiv2001

Single production of charged Higgs bosons at linear colliders

We discuss single charged Higgs boson production from $e^+e^-$ annihilation at next generation linear colliders. They can be important to study the phenomenology of charged Higgs bosons, especially when charged Higgs bosons are too heavy to be produced by the pair production mechanism. Cross sections for various single production processes are evaluated at the leading order. Our analysis shows that in some parameter regions the phenomenology of charged Higgs bosons can be explored even beyond the kinematic limit for pair production by using single production processes.

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arXiv2000

The Metropolis algorithm for on-shell four-momentum phase space

We present several implementations of the Metropolis method, an adaptive Monte Carlo algorithm, which allow for the calculation of multi-dimensional integrals over arbitrary on-shell four-momentum phase space. The Metropolis technique reveals itself very suitable for the treatment of high energy processes in particle physics, particularly when the number of final state objects and of kinematic constraints on the latter gets larger. We compare the performances of the Metropolis algorithm with those of other programs widely used in numerical simulations.

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arXiv1999

The process `e+ e- --> H t t-bar' and its backgrounds at future electron-positron colliders

The process $e^+e^-\to H t\bar t$ can be used at the Next Linear Collider to measure the Higgs-top Yukawa coupling. In this paper, we compute $2\to 8$ processes of the form $e^+e^-\to b\bar b b\bar b W^+W^-\to b\bar b b\bar b \ell^\pmν_\ell q\bar q'$, accounting for the Higgs-top-antitop signal as well as several irreducible backgrounds in the semi-leptonic top-antitop decay channel. We restrict ourselves to the case of a light Higgs boson in the range 100 GeV $\le M_H \le$ 140 GeV. We use helicity amplitude techniques to compute exactly such processes at tree level in the framework of the Standard Model. Total rates and differential spectra of phenomenological interest are given and discussed

hep-ph↗
arXiv1999

Irreducible QCD backgrounds to top searches in semi-leptonic final states at the Next Linear Collider

At future electron-positron colliders, one of the largest irreducible backgrounds to top searches in the channel `4 jets + lepton + missing energy' comes from QCD events of order $α_s^2$. We compute here such processes exactly at the parton level by resorting to 2 -> 6 matrix elements exploiting helicity amplitude techniques. We adopt a typical selection procedure based on the tagging of a high momentum and separated lepton. We finally outline kinematic differences between signal and background events that can be exploited to further reduce such a QCD noise.

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arXiv1999

Spin correlations in $e^+e^-\to 4 jets$

Existence of discrepancies between experimental data and Monte Carlo predictions for angular distributions in four-jet production via $e^+e^-$ annihilation has been known for some time at LEP1. As such QCD processes constitute significant backgrounds to $e^+e^-\to W^+W^-\to 4$-jet production at LEP2, we consider the possibility that an erroneous modelling of the helicity structure of the final-state partons could affect the accuracy of experimental measurements of the $W^\pm$ boson parameters

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arXiv1998

Six-jet production at lepton colliders

We study electron-positron annihilations into six jets at the parton level in perturbative Quantum Chromo-Dynamics (QCD), via the elementary processes $e^+e^-\ar q\bar q gggg$, $e^+e^-\ar q\bar q q'\bar q' gg$ and $e^+e^-\ar q\bar q q'\bar q' q''\bar q''$, for massive quarks $q, q'$ and $q''$ and massless gluons $g$. Several numerical results of phenomenological relevance are given, at three different collider energies and for a representative selection of jet clustering algorithms. We also present helicity amplitudes and colour factors needed for the tree-level calculation

hep-ph↗
arXiv1998

New and Old Jet Clustering Algorithms for Electron-Positron Events

Over the years, many jet clustering algorithms have been proposed for the analysis of hadronic final states in $e^+e^-$ annihilations. These have somewhat different emphasis and are therefore more or less suited for various applications. We here review some of the most used and compare them from a theoretical and experimental point of view.

hep-ph↗
arXiv1997

e+e- -> 6 jets in parton level QCD at LEP1, LEP2 and NLC

We study electron-positron annihilations into six jets at the parton level in perturbative Quantum Chromo-Dynamics. We use helicity amplitude methods. Results are presented for the case of the Durham and Cambridge jet clustering algorithms at three different collider energies.

hep-ph↗
arXiv1997

Top-antitop pair production via $b$-quark initiated processes at the Large Hadron Collider

We study $bt\bar t$ production via subprocesses initiated by $b$-quarks at the Large Hadron Collider. Both QCD and electroweak interactions are included in the elementary scattering amplitudes for $bg\to b t\bar t$. Since the additional jet in the final state (arising from the bottom quark) is in most cases at very low transverse momentum and very high pseudorapidity, it tends to escape detection. Therefore, such a process can act as a background to double-top as well as to single-top channels exploited in top quark phenomenology at present and future hadron colliders. Furthermore, if the additional $b$-jet can be tagged then $bt\bar t$ samples can be exploited in constraining possible effects of New Physics. The relevance of this reaction in such contexts is discussed and various total and differential rates of phenomenological interest are given

hep-ph↗
arXiv1996

W+4jet irreducible background events from QCD in top and Higgs searches at the Next Linear Collider

Irreducible background effects due to $e^+e^-\to W^\pm + bbjj$ and $e^+e^-\to W^\pm + jjjj$ events produced via QCD in top-antitop production and heavy Higgs searches in the bremsstrahlung channel are studied at the Next Linear Collider. Various distributions relevant to phenomenological analyses are given and compared to those expected from top and Higgs signals in the decay channel $b\bar bW^+W^-\to b\bar b W^\pm jj$. This analysis follows similar ones previously carried out for the case of the irreducible background proceeding via electroweak interactions

hep-ph↗
arXiv1996

Higgs signals and hard photons at the Next Linear Collider: the $ZZ$-fusion channel in the Standard Model

In this paper, we extend the analyses carried out in a previous article for $WW$-fusion to the case of Higgs production via $ZZ$-fusion within the Standard Model at the Next Linear Collider, in presence of electromagnetic radiation due real photon emission. Calculations are carried out at tree-level and rates of the leading order (LO) processes $e^+e^-\rightarrow e^+e^- H \ar e^+e^- b\bar b $ and $e^+e^-\rightarrow e^+e^- H \ar e^+e^- WW \ar e^+e^- \mathrm{jjjj}$ are compared to those of the next-to-leading order (NLO) reactions $e^+e^-\rightarrow e^+e^- H (γ)\ar e^+e^- b\bar b γ$ and $e^+e^-\rightarrow e^+e^- H (γ)\ar e^+e^- WW (γ) \ar e^+e^- \mathrm{jjjj}γ$, in the case of energetic and isolated photons.

hep-ph↗
arXiv1996

Higgs signals and hard photons via $WW$-fusion in the Standard Model at the Next Linear Collider

Within the framework of the Standard Model, integrated and differential distributions are given for Higgs production via the $WW$-fusion mechanism and decay via the channels $H\ar b\bar b$ and $H\ar WW\ar {jjjj}$, with and without photon radiation, at Next Linear Collider energies. Calculations are carried out at tree-level and rates of the leading processes $e^+e^-\ar\barν_eν_e H \ar \barν_eν_e b\bar b $ and $e^+e^-\ar\barν_eν_e H \ar \barν_eν_e WW \ar \barν_eν_e {jjjj}$ are compared to those of the next-to-leading reactions $e^+e^-\ar\barν_eν_e H (γ)\ar \barν_eν_e b\bar b γ$ and $e^+e^-\ar\barν_eν_e H (γ)\ar \barν_eν_e WW (γ) \ar \barν_eν_e {jjjj}γ$, in the case of hard and detectable photons. Finally, a brief discussion concerning the case of $H\ar ZZ\ar{jjjj} (γ)$ decays is also given.

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