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S. Moretti

Publications and source records attributed to S. Moretti.

At least 271 records · Page 15Linked to original sources
arXiv1997

Single-top production in the $tW^\pm$ channel and Higgs signals via $H\to W^+W^-$ at the Large Hadron Collider

In a recent study by Dittmar and Dreiner it was shown that with appropriate selection cuts the signature of events containing two charged leptons and missing energy represents the best chance of detecting the Standard Model Higgs scalar in the mass range between 155 and 180 GeV, the primary decay of the Higgs being into pairs of charged gauge bosons. The largest background to this channel is due to irreducible $W^+W^-X$ production. In the present paper we calculate the contribution of events of the type $bg\to tW^\pm\to b W^+W^-\to b \ell^{+}\ell^{'-}ν_\ell\barν_{\ell'}$, which have not been considered yet within the new selection strategy. We show that the yield of this background is rather large, at the level of that produced by $W^+W^-$, $t\bar t$ or $tb W^\pm$ events and thus needs to be incorporated in future experimental analyses. However, we find that its inclusion will not spoil the possibilities of Higgs detection in the above mentioned channel at the Large Hadron Collider.

hep-ph↗
arXiv1997

Better Jet Clustering Algorithms

We investigate modifications to the $k_\perp$-clustering jet algorithm which preserve the advantages of the original Durham algorithm while reducing non-perturbative corrections and providing better resolution of jet substructure. We find that a simple change in the sequence of clustering (combining smaller-angle pairs first), together with the `freezing' of soft resolved jets, has beneficial effects.

hep-ph↗
arXiv1997

The processes $e^+e^-\to b\bar bW^\pm H^\mp$ and $e^+e^-\to b\bar bH^+H^-$ at the Next Linear Collider in the Minimal Supersymmetric Standard Model

The complete matrix elements for $e^+e^-\to b\bar bW^\pm H^\mp$ and $e^+e^-\to b\bar bH^+H^-$ are computed at tree-level within the Minimal Supersymmetric Standard Model. Rates of interest to phenomenological analyses at the Next Linear Collider are given. In particular, we analyse: (i) $t\bar t$ pair production followed by the top decays into $b\bar bW^\pm H^\mp$ and $b\bar b H^+H^-$ final states; (ii) $H^+H^-$ signals in which $H^\pm\to h W^\pm \to b\bar b W^\pm$ and $H^\pm\to tb\to b\bar b W^\pm$. Top and Higgs finite width effects are included as well as all those due to the irreducible backgrounds.

hep-ph↗
arXiv1997

Higgs Production at the LHC: an Update on Cross Sections and Branching Ratios

New theoretical and experimental information motivates a re-examination of the Standard Model Higgs production rates at the LHC pp collider. We present calculations of the relevant cross sections and branching ratios, including recently calculated QCD next-to-leading order corrections, new parton distributions fitted to recent HERA structure function data, and new values for electroweak input parameters, in particular for the top quark mass. Cross sections are calculated at two collider energies, $\sqrt{s} = 10$ TeV and 14 TeV.

hep-ph↗
arXiv1996

$e^+e^-\to H^+H^-$ signals at LEP2 energies in the Minimal Supersymmetric Standard Model

In this paper we compare $H^+H^-$ and $W^+W^-$ into four-fermion production at centre-of-mass energies typical of LEP2 and somewhat larger. The theoretical framework considered is the Minimal Supersymmetric Standard Model. The interest in exploiting the $e^+e^-$ CERN collider at values of $\sqrt s$ greater than 192 GeV could come from the discovery of Supersymmetric signals during runs at lower energy. If these indicate that a charged Higgs boson exists in the mass range $\MH\approx95-105$ GeV, then a few years of running at $\sqrt s=205-215$ GeV and nominal luminosity could make the detection of such scalars feasible, in the purely leptonic channel $τν_ττν_τ$ and, for small $\tb$'s, also in the semi-hadronic(leptonic) one ${jj}τν_τ$. Charged Higgs bosons of the above nature cannot be produced by the beam energies approved at present for LEP2. However, if runs beyond the so-called `192 GeV cryogenic limit' will be approved by the CERN Council, our selection procedure will enable us to establish the presence, or otherwise, of charged Higgs bosons in the mentioned mass range

hep-ph↗
arXiv1996

Higgs Production at the LHC: an Updated Signal-to-Background Analysis

This paper follows hep-ph/9611397, where updated calculations of cross sections and branching ratios relevant for Standard Model Higgs phenomenology at the LHC were presented. Here, we complete that study by carrying out an updated signal-to-background analysis. We present results obtained by using exact matrix element computations at parton level for all processes, by exploiting the most recent parton distributions fitted to HERA structure function data and the most recent values of the electroweak input parameters. Cross sections and distributions are given for two collider energies, $\sqrt s_{pp}=10$ TeV and 14 TeV. Event rates and significances are discussed for two possible values of integrated luminosity, 10 fb$^{-1}$ and 100 fb$^{-1}$.

hep-ph↗
arXiv1996

Production of charged Higgs bosons of the Minimal Supersymmetric Standard Model in b-quark initiated processes at the Large Hadron Collider

We study integrated and differential rates for the production of charged Higgs bosons $H^\pm$ of the Minimal Supersymmetric Standard Model via b-quark initiated subprocesses in pp collisions at the Large Hadron Collider. In detail, we compute cross sections and distributions of the reactions: $bU \to bDH^+ \to bDτ^+ν_τ\oplus C.C.$ and $bU \to bDH^+ \to bD t\bar b \to bDb\bar b {jj}\oplus C.C.$, for a $H^\pm$ scalar in the intermediate (i.e., $M^\pm m_t+m_b$) mass range, respectively (U and D represent generic u- and d-type light quarks). In the first case, a detailed treatment of various possible backgrounds is also given.

hep-ph↗
arXiv1996

Light, long-lived and secluded: can gluinos be driven out from LEP1 data ?

We briefly report about a possible settlement of the still ongoing dispute concerning the existence of SUSY signals in 4jet events at LEP1. We base our arguments on a simple selection strategy exploiting secondary vertex tagging and kinematical constraints, which could allow one to access or exclude gluino events for a broad range of masses and lifetimes.

hep-ph↗
arXiv1996

Heavy flavour tagging and QCD tests in four-jet events at LEP1

We present a theoretical review of the possibilities offered by heavy flavour tagging in testing QCD features in 4-jet events at LEP1. Attention is focused mainly on the properties of the angular variables which are used in the experimental analyses to measure the colour factors of the strong interactions. We also comment on a possible settlement of the ongoing controversy about the existence in LEP1 data of SUSY events involving light gluinos. Integrated and differential rates of interest to phenomenological analyses are given and various tagging procedures are discussed.

hep-ph↗
arXiv1996

Perturbative rates and colour rearrangement effects in four-jet events at LEP2

An important issue in the direct reconstruction method of determining the $W$ mass from $q\bar q Q\bar Q$ events at LEP2 concerns the impact of the relatively unknown QCD interconnection effects. It has been suggested that a study of `short string' states, in which colour singlet states are formed from $q \bar Q $ and $Q\bar q$ pairs with small phase--space separation, could shed important light on this issue. We show that such configurations can also be generated by conventional background $e^+e^-\ar 4$~parton processes, in particular QCD $q \bar q g g$ and $q \bar q Q \bar Q$ and non--resonant electroweak $q \bar q Q \bar Q$ production. We study the colour and kinematic structure of these background contributions, and estimate the event rate to be expected at LEP2. We find that the QCD processes are heavily suppressed, but that non--resonant $q \bar q Q \bar Q$ production may be comparable in rate to the expected `short string' signal from $W^+W^-$ production.

hep-ph↗
arXiv1995

Contributions of below--threshold decays to ${\cal {MSSM}}$ Higgs branching ratios: Erratum

In Ref.~\cite{paper} we calculated all the experimentally relevant branching ratios of the Higgs bosons of the Minimal Supersymmetric Standard Model, paying particular attention to the contributions from below--threshold decays. Unfortunately, an error in one of the subroutines of the {\tt FORTRAN} code we used was affecting the computation of the off--shell partial widths of the decays $A\ar Z^{0*}h^*$ and $H^\pm\ar W^{\pm *}h^*$. This has now been fixed, and the corrected plots are presented here

hep-ph↗
arXiv1995

$e^+e^- \to b\bar b W^+W^-$ events at the Next Linear Collider: colour structure of top signal and irreducible background

We examine the colour structure and charged particle yield for both the $t \bar t$ signal and the irreducible background processes contributing to \eebbww\ production close to the \ttb\ threshold. The charged particle multiplicity for the various components of the cross section is computed as a function of several kinematic variables. Our study may have important implications for recently proposed studies of interconnection phenomena in \ttb\ production at high--energy $e^+e^-$ colliders

hep-ph↗
arXiv1995

QCD effects and $b$-tagging at LEP I

We analyze the impact of using $b$-tagged samples in studying non-Abelian effects due to QCD in $e^+e^-\ar 4$jet events at $\sqrt s=M_{Z^0}$, using angular variable analyses and comparisons with $e^+e^-\ar 3 \mbox{jet}γ$ events. We find that QCD effects are largely enhanced in $b$-quark samples with respect to `unflavoured' ones, where energy-ordering is used to distinguish between gluon and quark jets. We show that the $b$-quark mass influences the angular distributions significantly and should not be neglected

hep-ph↗
arXiv1994

Contributions of below--threshold decays to ${\cal {MSSM}}$ Higgs branching ratios

We calculate all the experimentally relevant branching ratios of the Higgs bosons of the Minimal Supersymmetric Standard Model, paying particular attention to the contributions from below--threshold decays. We show that in some cases these can significantly change the pattern of branching ratios calculated without taking off--shell effects into account

hep-ph↗
arXiv1994

Higgs Production and Decay via $e^+e^-\rightarrow Z^0 H^0 \rightarrow b\bar bW^+W^-$ and Irreducible Backgrounds at the Next Linear Collider

The complete matrix element for $e^+e^-\ar b\bar bW^+W^-$ has been computed at tree--level and applied to $Z^0H^0$ production followed by $Z^0\ar b\bar b$ and $H^0\ar W^+W^-$, keeping into account all irreducible backgrounds, which are dominated by \ttb production, at the Next Linear Colliders. We find that, depending on the center of mass energies and on the search strategies, this channel can be useful for the study of the parameters of the Standard Model Higgs boson over the most part of the heavy mass range.

hep-ph↗
arXiv1993

Trying to Catch the Elusive $a^0$ and More. Reactions Initiated by $b$--Quarks the Higgs Sector of ${\cal MSSM}$

We study the cross sections for the production of a neutral, intermediate mass Higgs boson in the processes $pp\rightarrow tq'Φ$, $pp\rightarrow tW^-Φ$ and $pp\rightarrow bZ^0Φ$ in the Minimal Supersymmetric Standard Model ($Φ=H^0,h^0$ and $A^0$) at Supercollider energies. The additional heavy particles ($t$, $W$, $Z$) in the final state can be used for tagging purposes, increasing the signal to background ratio. These reactions are dominated by $bq$ and $bg$ fusion. Their relevance for Higgs particle searches is discussed taking into account the expected efficiencies and purities for $b$--tagging. We find that, for tan$β= 30$, the cross sections for $pp\rightarrow bZ^0Φ$ are larger than 14 $pb$, over the whole intemediate range of $M_{A^0}$, for $A^0$ and at least one of the other two Higgses. Therefore this reaction is an excellent candidate for the discovery of one or more $\cal MSSM$ Higgs particles.}

hep-ph↗
arXiv1993

Hard Photon Pair Production at LEP

The production of photon pairs in $e^+e^-\rightarrow f \bar f γγ$ processes is studied using exact helicity amplitudes at tree level. Total cross sections, including initial state radiation effects, are given. They are presented in the case of quarks as a function of $y_{\rm cut}$ in the JADE algorithm. In the case of leptons we use cuts similar to the ones employed in a recent L3 analysis. Masses of the final state fermions are taken into account when appropriate. The cross section $e^+e^-\rightarrow τ^+ τ^- γγ$ is about 10\% larger when the $τ$ mass is neglected. We obtain, with a different method, results which are in good agreement with L3 Montecarlo simulation.

hep-ph↗