arXiv subjects
Publications and source records attributed to S. Moretti.
The calculation of the tree-level QCD processes e+e- -> qqgggg, qqqqgg and qqqqqq has recently been accomplished. We highlight here the relevance of such reactions for some of the physics at future electron-positron linear accelerators.
The process e+e- -> Htt can be exploited at future Linear Colliders to measure the Higgs-top Yukawa coupling. In this note, we estimate the size of the irreducible backgrounds in the channel Htt -> bbbb WW -> bbbb ln qq, for the case of a Standard Model Higgs boson with mass between 100 and 140 GeV.
We study the decays into six quarks of off-shell pairs of W bosons produced in electron-positron annihilations, through the O(as^2) order in the strong coupling constant. We give explicit helicity amplitude formulae. We present numerical results in the context of phenomenological analyses of relevance at LEP2 and future Linear Colliders: such as MW determinations, New Physics and Higgs searches.
The experimental reconstruction of the Higgs self-energy potential is essential to a verification of the Higgs boson's role in spontaneous electroweak symmetry breaking. The first step towards this goal, the measurement of the triple Higgs self-coupling, can possibly be accomplished at the next generation of linear colliders. Here we present a background study of the most promising channel, double Higgs-strahlung off a Z boson, e+e- -> HHZ, with the subsequent decay H -> b bbar, and evaluate the feasibility of its measurement.
A new version of the Monte Carlo program HERWIG (version 6.1) is now available. This note gives brief details of the new features relative to the previous version (5.9). A fully updated manual will follow shortly. The main new features are: supersymmetric processes (both R-parity conserving and R-parity violating) in hadron-hadron collisions; the new process e+e- to 4 jets; matrix element corrections to top decay and Drell-Yan processes; new soft underlying event options; updates to particle data tables; new LaTeX and html printout options.
If the soft Supersymmetry (SUSY) breaking masses and couplings are complex and cancelations do take place in the SUSY induced contributions to the fermionic Electric Dipole Moments (EDMs), then the CP-violating soft phases can drastically modify much of the known phenomenological pattern of the Minimal Supersymmetric Standard Model (MSSM). In particular, the squark loop content of the dominant Higgs production mechanism at the Large Hadron Collider (LHC), the gluon-gluon fusion mode, could be responsible of large corrections to the known cross sections.
We investigate the signature of a heavy charged Higgs boson of the Minimal Supersymmetric Standard Model in the lepton plus multi-jet channel at the Large Hadron Collider with four $b$-tags. The signal is the gluon-gluon fusion process $gg\to t\bar b H^-$, followed by the $H^-\to \bar t b$ decay, while the main background is from $gg\to t\bar t b\bar b$. We find that the two can be separated effectively by kinematic cuts and mass reconstruction, but the signal size is not very large in the end. Nonetheless, with a good $b$-tagging efficiency, $ε_b\sim50%$, this channel can provide a viable signature over a limited but interesting range of the parameter space.
We study neutral and charged Higgs boson production in association with stop and sbottom squarks at the Large Hadron Collider (LHC), within the so-called M-SUGRA scenario, i.e., the Supergravity (SUGRA) inspired Minimal Supersymmetric Standard Model (MSSM). For low values of \tanβonly the cases \tilde{t}_1\tilde{t}_1^* H, \tilde{t}_1\tilde{t}_1^* h and \tilde{t}_1\tilde{t}_2^* h give detectable rates while for \tanβgreater than 30 a variety of signals involving all Higgs bosons can be accessed, at high collider luminosity. The dependence of these reactions on the M-SUGRA parameters might further allow one to pin down the actual structure of the underlying Supersymmetric (SUSY) model.
We investigate the charged Higgs boson signal at the LHC using its dominant production and decay modes with triple b-tagging, i.e. $tH^- \to t\bar tb \to b\bar bb W^+ W^-$, followed by leptonic decay of one W and hadronic decay of the other. We consider the continuum background from the associated production of $t\bar t$ with a b- or a light quark or gluon jet, which can be mis-tagged as b-jet. We reconstruct the top quark masses to identify the 3rd b-jet accompanying the $t\bar t$ pair, and use its p_T distribution to distinguish the signal from the background. Combining this with the reconstruction of the $H^\pm$ mass gives a viable signature over two interesting regions of the parameter space - i.e. $\tanβ\sim 1$ and $\sim m_t/m_b$.
We assess the possibility of measuring the trilinear self-coupling of the Standard Model Higgs boson at future electron-positron linear colliders with centre-of-mass energies between 500 and 1500 GeV. We consider rather light Higgs scalars, with mass below the W+W- threshold, produced in the double Higgs-strahlung channel e+e- -> HHZ and decaying via the dominant mode H -> bbbar. Assuming ideal vertex tagging of the heavy-quark jets and mass reconstruction of the Z boson produced in association with the Higgs pair, we compare the yield of the above process to those of all irreducible electroweak and strong backgrounds proceeding through a bbbar bbbar Z intermediate state. Total cross sections and differential spectra of phenomenological relevance to the selection and analysis of the signal are given and discussed.
The production of four hadronic jets in e+e- collisions above the Z pole is dominated by the QCD e+e- --> qqqq,qqgg processes and, for sufficiently high energy, the electroweak e+e- --> W+W- --> qqqq process. However there is another mechanism for producing four jets, e+e- --> Z gg --> qqgg, which can be regarded as a `hadronic return' to the Z pole. We investigate this new process in detail
We study neutral and charged Higgs boson production in association with stop and sbottom squarks at the Large Hadron Collider, within the Supergravity inspired Minimal Supersymmetric Standard Model. The phenomenological relevance of such reactions is twofold. Firstly, they constitute a novel production mechanism of Higgs particles, either through a decay of a heavier (anti)squark into a lighter one or via a Higgs bremsstrahlung process. Secondly, their production rates are extremely sensitive to the values assumed by the five input parameters of the model, this possibly allowing one to put stringent constraints on the latter. After an exhaustive scan of the parameter space, we find that the majority of such processes could be detectable at high luminosity, provided tan(beta) is large, tan(beta) > 30 (except in the case of \tilde{t}_1\tilde{t}_1^* h and \tilde{t}_1\tilde{t}_2^* h final states, whose detection is also possible for smaller values), that the universal soft Supersymmetric breaking masses are in the ranges M_0 < 500 GeV and M_{1/2} < 220 GeV, and that the trilinear couplings are negative, A_0 < 0. We also point out some sizable decay signatures and discuss their Standard Model (SM) backgrounds. Finally, we derive compact analytical formulae of the corresponding scattering matrix elements.
We study the processes `gluon gluon -> squark antisquark CP-odd Higgs' within the theoretical framework of the Supergravity (SUGRA) inspired Minimal Supersymmetric Standard Model (MSSM) at the leading order (LO) in perturbative Quantum Chromo-Dynamics (QCD). Other than constituting a novel production mechanism of the neutral, CP-odd Higgs particle, they also allow one to relate the size of the corresponding production rates to the ratio of the vacuum expectation values of the two Higgs fields tan(beta), to the trilinear couplings A_0 and to the sign of the Higgsino mass term sign(mu). This interplay is made easier by the absence of any mixing in the {tilde{q}}_1{tilde{q}}_2 A vertices, contrary to the case of all other Higgs bosons of the theory.
Barrientos Bendezu' and Kniehl [hep-ph/9807480] recently suggested that $W^\pm H^\mp$ associated production may be a useful channel in the search for the elusive heavy charged Higgs bosons of the 2 Higgs Doublet Model at the Large Hadron Collider. We investigate the phenomenology of this mechanism in the Minimal Supersymmetric Standard Model, with special attention paid to the most likely heavy Higgs decay, $H^\mp\to tb\to b\bar b W^\mp$, and to the irreducible background from top pair production. We find that the semi-leptonic signature `$b\bar b W^+W^-\to b\bar b jj \ell$ + missing momentum' is dominated by top-antitop events, which overwhelm the charged Higgs signal over the heavy mass range that can be probed at the CERN collider
We study charged Higgs boson production at future electron-proton colliders in the framework of the Minimal Supersymmetric Standard Model. We focus our attention to the case of single-top production and decay through the channel $t\to bH^\pm$ and of vector-scalar fusion via $W^{\pm*} Φ^*\to H^\pm$ (where $Φ=H,h$ and $A$). We consider the signature $H^\pm\to τν_τ$ and compare it to the irreducible background from Standard Model interactions. For $M_{H^\pm}<m_t$, the $H^\pm$ signal is accessible through lepton universality breaking if $M_A<100-120$ GeV at both low and large values of $\tanβ$. Furthermore, although the bulk of the production cross section comes from single-top events, a sizable contribution due to vector-scalar-scalar interactions should be observable at large $\tanβ$, this possibly offering some insights into the structure of the scalar sector of the theory. The possibility of the CERN collider running in the LEPXLHC mode is considered in detail
The production of top quarks in single mode at future ep colliders is studied, the attention being mainly focused to the case of the proposed LEPXLHC collider. We are motivated to reanalyse such a process following the discovery of the top quark at Fermilab. Thanks to the measurement of its mass one is now able to establish more accurately the relevance of single top production for itself and for many other processes to which it may act as a background. In addition, the recent improvement of our knowledge of the quark and gluon dynamics inside the proton now allows one to pin down the dependence of single top production on the partonic structure functions. Both the leptonic and hadronic decay channels of the top quark are studied and compared to the yield of the corresponding irreducible background in presence of b-tagging
Heavy flavour tagging provides a broad range of possibilities in testing QCD features at LEP. We present here a study of 4jets events at LEP I where the so-called light gluinos could be directly produced. We show that microvertex techniques offer a unique chance to exploit simple kinematical distributions in order to optimise the signal coming from gluino production with respect to the background of ordinary QCD events. Our results indicate that experimental analyses along the lines suggested here can exclude or reveal the presence of a gluino for masses up to 10 GeV and lifetimes below 10$^{-9}$ sec. We also point out that a large fraction of gluino events could decay in configurations carrying large missing energy, so to escape the usual selection criteria of 4jet samples. In our study, mass effects of quarks and gluinos have been taken into account exactly. Our results are independent from both the jet algorithm and its resolution parameter.}
The W Mass and QCD Working Group discussed a wide variety of topics relating to present and future measurements of M(W) at LEP2, including QCD backgrounds to W+W- production. Particular attention was focused on experimental issues concerning the direct reconstruction and threshold mass measurements, and on theoretical and experimental issues concerning the four jet final state. This report summarises the main conclusions.