arXiv subjects

E. Accomando

Publications and source records attributed to E. Accomando.

At least 37 records · Page 2Linked to original sources

arXiv2006

Les Houches "Physics at TeV Colliders 2005'' Beyond the Standard Model working group: summary report

The work contained herein constitutes a report of the "Beyond the Standard Model'' working group for the Workshop "Physics at TeV Colliders", Les Houches, France, 2-20 May, 2005. We present reviews of current topics as well as original research carried out for the workshop. Supersymmetric and non-supersymmetric models are studied, as well as computational tools designed in order to facilitate their phenomenology.

hep-ph↗

arXiv2005

Kaluza-Klein states at the LHC in models with localized fermions

We give a brief review of some aspects of physics with TeV size extra-dimensions. We focus on a minimal model with matter localized at the boundaries for the study of the production of Kaluza-Klein excitations of gauge bosons. We briefly discuss different ways to depart from this simple analysis.

hep-ph↗

arXiv2005

Boson-boson scattering and Higgs production at the LHC from a six fermion point of view: four jets + l$ν$ processes at $Ø(α_{em}^6)$

Boson-boson scattering and Higgs production in boson-boson fusion hold the key to electroweak symmetry breaking. In order to analyze these essential features of the Standard Model we have performed a partonic level study of all processes $q_1 q_2 \to q_3 q_4 q_5 q_6 l ν$ at the LHC using the exact matrix elements at $Ø(α_{em}^6)$ provided by \Phase, a new MC generator. These processes include also three boson production and the purely electroweak contribution to \toptop production as well as all irreducible backgrounds. Kinematical cuts have been studied in order to enhance the VV scattering signal over background. \Phase has been compared with different Monte Carlo's showing that a complete calculation is necessary for a correct description of the process.

hep-ph↗

arXiv2005

Electroweak corrections and anomalous triple gauge-boson couplings in WW and WZ production at the LHC

We have analysed the production of WW and WZ vector-boson pairs at the LHC. These processes give rise to four-fermion final states, and are particularly sensitive to possible non-standard trilinear gauge-boson couplings. We have studied the interplay between the influence of these anomalous couplings and the effect of the complete logarithmic electroweak O(α) corrections. Radiative corrections to the Standard Model processes in double-pole approximation and non-standard terms due to trilinear couplings are implemented into a Monte Carlo program for p p -> 4f (+γ) with final states involving four or two charged leptons. We numerically investigate purely leptonic final states and find that electroweak corrections can fake new-physics signals, modifying the observables by the same amount and shape, in kinematical regions of statistical significance.

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arXiv2005

Electroweak corrections to Wγand Zγproduction at the LHC

We have calculated the electroweak O(α) corrections to the processes p p -> Z γ+ X -> ν_l \barν_l γ+ X, p p -> Z γ+ X -> l \bar{l} γ+ X, and p p -> W γ+ X -> ν_l \bar{l} γ+ X at the LHC, with l=e,μand ν_l=ν_e,ν_μ,ν_τ. The virtual corrections are evaluated in leading-pole approximation, whereas the real corrections are taken into account exactly. These corrections are implemented into a Monte Carlo generator which includes both phase-space slicing and subtraction to deal with soft and collinear singularities. We present numerical results for total cross sections as well as for experimentally interesting distributions. Applying typical LHC cuts, the electroweak corrections are of the order of -5% for the total cross sections and exceed -20% for observables dominated by high centre-of-mass energies of the partonic processes.

hep-ph↗

arXiv2005

PHASE, a Monte Carlo event generator for six-fermion physics at the LHC

PHASE is a new event generator dedicated to the study of Standard Model processes with six fermions in the final state at the LHC. The code is intended for analyses of vector boson scattering, Higgs search, three gauge boson production, and top physics. This first version of the program describes final states characterized by the presence of one neutrino, $pp\to 4q +lν_l$, at O($α^6$). PHASE is based on a new iterative-adaptive multichannel technique, and employs exact leading order matrix elements. The code can generate unweighted events for any subset of all available final states. The produced parton-level events carry full information on their colour and flavour structure, enabling the evolution of the partons into fully hadronised final states. An interface to hadronization packages is provided via the Les Houches Protocol.

hep-ph↗

arXiv2004

Physics at the CLIC Multi-TeV Linear Collider

This report summarizes a study of the physics potential of the CLIC e+e- linear collider operating at centre-of-mass energies from 1 TeV to 5 TeV with luminosity of the order of 10^35 cm^-2 s^-1. First, the CLIC collider complex is surveyed, with emphasis on aspects related to its physics capabilities, particularly the luminosity and energy, and also possible polarization, γγand e-e- collisions. The next CLIC Test facility, CTF3, and its R&D programme are also reviewed. We then discuss aspects of experimentation at CLIC, including backgrounds and experimental conditions, and present a conceptual detector design used in the physics analyses, most of which use the nominal CLIC centre-of-mass energy of 3 TeV. CLIC contributions to Higgs physics could include completing the profile of a light Higgs boson by measuring rare decays and reconstructing the Higgs potential, or discovering one or more heavy Higgs bosons, or probing CP violation in the Higgs sector. Turning to physics beyond the Standard Model, CLIC might be able to complete the supersymmetric spectrum and make more precise measurements of sparticles detected previously at the LHC or a lower-energy linear e+e- collider: γγcollisions and polarization would be particularly useful for these tasks. CLIC would also have unique capabilities for probing other possible extensions of the Standard Model, such as theories with extra dimensions or new vector resonances, new contact interactions and models with strong WW scattering at high energies. In all the scenarios we have studied, CLIC would provide significant fundamental physics information beyond that available from the LHC and a lower-energy linear e+e- collider, as a result of its unique combination of high energy and experimental precision.

hep-ph↗

arXiv2004

Logarithmic electroweak corrections to gauge-boson pair production at the LHC

We have studied the effects of the complete logarithmic electroweak O(alpha) corrections on the production of vector-boson pairs WZ, ZZ, and WW at the LHC. These corrections are implemented into a Monte Carlo program for pp -> 4f (+γ) with final states involving four or two leptons using the double-pole approximation. We numerically investigate purely leptonic final states and find that electroweak corrections lower the predictions by 5-30% in the physically interesting region of large di-boson invariant mass and large angle of the produced vector bosons.

hep-ph↗

arXiv2004

PHASE - An event generator for six fermion physics at the LHC

PHASE is a Monte Carlo event generator, under construction, for all Standard Model processes with six fermions in the final state at the LHC. It employs the full set of tree level Feynman diagrams, taking into account fermion masses for b quarks. The program can generate unweighted events for any subset of all six fermion final states in a single run, by making use of dedicated pre-samples. An interface to hadronization is provided.

hep-ph↗

arXiv2002

WPHACT 2.0: A fully massive Monte Carlo generator for four fermion physics at e^+ e^- colliders

WPHACT 2.0 is the new fully massive version of a MC program and unweighted event generator which computes all Standard Model processes with four fermions in the final state at e^+ e^- colliders. The program can now generate unweighted events for any subset of all four fermion final states in a single run, by making use of dedicated pre-samples which can cover the entire phase space. Improvements with respect to WPHACT 1.0 include the Imaginary Fermion Loop gauge restoring scheme, new phase space mappings, a new input system, the possibility to compute subsets of Feynman diagrams and options for including ISR via QEDPS, running alpha_{QED}, CKM mixing, resonances in q bar(q) channels.

hep-ph↗

arXiv2000

Four-Fermion Production in Electron-Positron Collisions

This report summarises the results of the four-fermion working group of the LEP2-MC workshop, held at CERN from 1999 to 2000. Recent developments in the calculation of four-fermion processes in electron-positron collisions at LEP-2 centre-of-mass energies are presented, concentrating on predictions for four main reactions: W-pair production, visible photons in four-fermion events, single-W production and Z-pair production. Based on a comparison of results derived within different approaches, theoretical uncertainties on these predictions are established.

hep-ph↗

arXiv2000

CP Violaing Phases In SUSY GUT Models

Supersymmetric CP violating phases are examined within the framework of gravity mediated supergravity grand unified models with R parity invariance for models with a light ($\stackrel{<}{\sim} 1$ TeV) particle spectrum. In the minimal model, the nearness of the t quark Landau pole naturally suppresses the t-quark cubic soft breaking parameter at the electroweak scale allowing the electron and neutron experimental electric dipole moment (EDM) constraints to be satisfied with a large GUT scale phase. However, the EDM constraints require that $θ_B$, the quadratic soft breaking parameter phase be small at the electroweak scale unless tan$β\stackrel{<}{\sim}3$, which then implies that at the GUT scale this phase must be large and highly fine tuned to satisfy radiative breaking of $SU(2)\times U(1)$. Similar results hold for non minimal models, and a possible GUT model is discussed where all GUT scale CP violating phases are naturally small (i.e. O($10^{-2})$). An interesting D-brane model is examined which enhances the size of the phases over much of the parameter space at the electroweak sector for tan$β\stackrel{<}{\sim} 5$, but still possesses the fine tuning problem at the GUT scale.

hep-ph↗

arXiv2000

Neutralino Proton Cross Sections In Supergravity Models

The neutralino-proton cross section is examined for supergravity models with R-parity invariance with universal and non-universal soft breaking. The region of parameter space that dark matter detectors are currently (or will be shortly) sensitive i.e. $(0.1-10)\times 10^{-6}$ pb, is examined. For universal soft breaking (mSUGRA), detectors with sensitivity $σ_{\tildeχ_{1}^{0}-p} \geq 1 \times 10^{-6}$ pb will be able to sample parts of the parameter space for $\tan β\stackrel{>}{\sim} 25$. Current relic density bounds restrict $m_{\tildeχ_{1}^{0}} \leq 120$ GeV for the maximum cross sections, which is below where astronomical uncertainties about the Milky Way are relevant. Nonuniversal soft breaking models can allow much larger cross sections and can sample the parameter space for $\tan β\stackrel{>}{\sim} 4$. In such models, $m_0$ can be quite large reducing the tension between proton decay bounds and dark matter analysis. We note the existance of two new domains where coannihilation effects can enter, i.e. for mSUGRA at large $\tan β$, and for nonuniversal models with small $\tan β$.

hep-ph↗

arXiv1999

Looking For TeV-Scale Strings and Extra-Dimensions

In contrast to the old heterotic string case, the (weakly coupled) type I brane framework allows to have all, part or none of the standard model gauge group factors propagating in large extra--dimensions of TeV$^{-1}$ size. We investigate the main experimental signatures of these possibilities, related to the production of Kaluza-Klein excitations of gluons and electroweak gauge bosons. A discovery through direct observation of resonances is possible only for compactification scales below 6 TeV. However effects due to exchange of virtual Kaluza-Klein excitations could be observed for higher scales. We find that LHC can probe compactification scales as high as 20 TeV for excitations of gluons and 8-15 TeV for excitations of electroweak gauge bosons. Finally, in the case where no gauge boson feels the extra-dimension, we find that effective contact interactions due to massive string mode oscillations dominate those due to the exchange of Kaluza-Klein excitations of gravitons and could be used to obtain bounds on the string scale.

hep-ph↗

arXiv1999

CP Violating Phases, Nonuniversal Soft Breaking And D-brane Models

The question of CP violating phases and electric dipole moments (EDMs)for the electron ($d_e$) and the neutron ($d_n$) for supergravity models with nonuniversal soft breaking is considered for models with a light ($\stackrel{<}{\sim}$1 TeV) mass spectrum and R-parity invariance. As with models with universal soft breaking (mSUGRA) one finds a serious fine tuning problem generally arises for $θ_{0B}$ (the phase of the B soft breaking parameter at the GUT scale), if the experimental EDM constraints are obeyed and radiative breaking of $SU(2)_L\times U(1)_Y$ occurs. A D-brane model where $SU(3)_C\times U(1)_Y$ is associated with one set of 5-branes and $SU(2)_L$ with another intersecting set of 5-branes is examined, and the cancellation phenomena is studied over the parameter space of the model. Large values of $θ_B$ (the phase of B at the electroweak scale) can be accommodated, though again $θ_{0B}$ must be fine tuned. Using the conventional prescription for calculating $d_n$, one finds the region in parameter space where the experimental EDM constraints on both $d_e$ and $d_n$ hold is significantly reduced, and generally requires tan$β\stackrel{<}{\sim}$5 for most of the parameter space, though there are small allowed regions even for tan$β\stackrel{>}{\sim}$10. We find the Weinberg three gluon term generally makes significant contributions, and results are sensitive to the values of quark masses.

hep-ph↗

arXiv1999

The Tevatron Tripler: How to Upgrade the Fermilab Tevatron for the Higgs Boson and Supersymmetry

Recent advances in superconductor properties and superconducting magnet technology have made it possible to build cost-effective, high-performance dipoles with a field of 12 Tesla - 3 times the field strength of the Tevatron. Such magnets could be used to upgrade Fermilab's collider in its existing tunnel to a collision energy $\sqrt s$= 6 TeV and luminosity {$\cal L$}$>$ 10$^{33}$ cm$^{-2}$s$^{-1}$. We have calculated the parton luminosities for quark-antiquark and gluon-gluon scattering for the Tevatron, the Tripler, and LHC. In most models of the Higgs field and supersymmetry, the Tripler would have a high likelihood to discover many of the predicted particle states.

hep-ex↗

arXiv1999

Grand Unification Scale CP Violating Phases And The Electric Dipole Moment

The question of CP violating phases in supersymmetry and electric dipole moments (EDMs) is considered within the framework of supergravity grand unification (GUT) models with a light ($\stackrel{<}{\sim}$1 TeV) mass spectrum. In the minimal model, the nearness of the t-quark Landau pole automatically suppresses the t-quark cubic soft breaking phase at the electroweak scale. However, current EDM data require the quadratic soft breaking phase to be small at the electroweak scale unless tan$β$ is small (tan$β\stackrel{<}{\sim}$3), and the EDM data combined with the requirement of electroweak symmetry breaking require this phase to be both large and highly fine tuned at the GUT scale unless tan$β$ is small. Non minimal models are also examined, and generally show the same behavior.

hep-ph↗

arXiv1998

Trilepton Signal of Grand Unified Models at the Tevatron

At the Tevatron, the most promising channel to detect supersymmetry is three leptons plus missing energy, where the leptons are $e$'s and/or $μ$'s. This final state appears from the production of chargino and second lighetst neutralino. However in grand unified models with universal scalar masses at the grand unified scale, this final state mostly consists of $τ$'s which are hard to detect. We show that for some regions of non universality in the scalar masses at the GUT scale based on unifying groups like SU(5) or SO(10), the final state mostly consists of 3$l$+${\rlap/E}_T$ and $τll$+${\rlap/E}_T$. The first mode has very high detection efficiency and the second one is expected to have high detection efficency as well. We also show that these models can have enough events in these modes to be detected in RUN II.

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