9,369 research outputs found
Asymmetries in t tbar production: LHC versus Tevatron
The measurement of a charge asymmetry in t tbar production at LHC constitutes
more than an independent confirmation of the forward-backward asymmetry found
at Tevatron. Indeed, both measurements together can be used to identify the
source of the asymmetry. This is demonstrated for the case of new Z', W' vector
bosons and colour-sextet and triplet scalars, exchanged in t, u channels
respectively, and a very heavy axigluon in the s channel.Comment: LaTeX 5 pages. Updated with discussion on implications of summer
results. Final version to appear in PR
Looking for signals beyond the neutrino Standard Model
Any new neutrino physics at the TeV scale must include a suppression
mechanism to keep its contribution to light neutrino masses small enough. We
review some seesaw model examples with weakly broken lepton number, and comment
on the expected effects at large colliders and in neutrino oscillations.Comment: LaTeX 10 pages, 9 PS figures. Contribution to the Proceedings of the
XXXI International School of Theoretical Physics "Matter To The Deepest"
Ustron, Poland, September 5-11, 2007. Typos correcte
Production of Single Heavy Charged Leptons at a Linear Collider
A sequential fourth generation of quarks and leptons is allowed by precision
electroweak constraints if the mass splitting between the heavy quarks is
between 50 and 80 GeV. Although heavy quarks can be easily detected at the LHC,
it is very difficult to detect a sequential heavy charged lepton, L, due to
large backgrounds. Should the L mass be above 250 GeV, it can not be
pair-produced at a 500 GeV ILC. We calculate the cross section for the one-loop
process e+e- -> L tau. Although the cross section is small, it may be
detectable. We also consider contributions from the two Higgs doublet model and
the Randall-Sundrum model, in which case the cross section can be substantially
higher.Comment: 14 pages, 7 figure
Triboson interpretations of the ATLAS diboson excess
The ATLAS excess in fat jet pair production is kinematically compatible with
the decay of a heavy resonance into two gauge bosons plus an extra particle.
This hypothesis would explain the absence of such a large excess in the
analogous CMS analysis of fat dijet final states, as well as the negative
results of diboson resonance searches in the semi-leptonic decay modes. If the
extra particle is the Higgs boson, this hypothesis might also explain
-statistical fluctuations aside- why the CMS search for WH resonances in the
semi-leptonic channel finds some excess while in the fully hadronic one it does
not have a significant deviation.Comment: LaTeX 17 pages. v2: Enlarged discussion to address CMS WH excess. v3:
Added discussion of diboson helicities. Final version to appear in JHE
Asymmetries in top quark pair production at hadron colliders
We review the asymmetries in top quark pair production at the Tevatron and
the LHC. We summarize the experimental measurements and the interpretations of
a possible excess in terms of new physics. We also review other top quark
properties-emphasizing effects related to the asymmetries-as well as
other collider signals.Comment: RevTeX 40 pages. Final version to be published in Reviews of Modern
Physics, with several addition
Strong flavour changing effective operator contributions to single top quark production
We study the effects of dimension six effective operators on the production
of single top quarks at the LHC. The operator set considered includes terms
with effective gluon interactions and four-fermion terms. Analytic expressions
for the several partonic cross sections of single top production will be
presented, as well as the results of their integration on the parton density
functions.Comment: 20 pages, 7 fig
Single Vectorlike Quark Production at the LHC
A gluon resonance G of mass below 1 TeV could be the origin of the t\bar{t}
forward-backward asymmetry observed at the Tevatron provided that new decay
modes G->\bar{q}Q, with q a standard quark and Q its massive excitation, make G
broad enough. We consider all the different cases, with q the top, the bottom
or a light quark and dominant decay modes Q->Wq' or Q->Zq. We show that current
experimental searches are unable to probe the model, but that minimal
departures from these analyses can explore a large region of its parameter
space for the current LHC luminosity. This includes the challenging case with
the new quarks decaying mostly into light quark flavors. In some channels not
only the heavy quark but also the massive gluon can be reconstructed, which
would stablish the origin of the t\bar{t} asymmetry. Similar analyses can be
applied to more general models with new massive gluons and vectorlike quarks.Comment: 17 pages, 8 figures. Version 2: references adde
Constraints on Mass Spectrum of Fourth Generation Fermions and Higgs Bosons
We reanalyze constraints on the mass spectrum of the chiral fourth generation
fermions and the Higgs bosons for the standard model (SM4) and the two Higgs
doublet model (THDM). We find that the Higgs mass in the SM4 should be larger
than roughly the fourth generation up-type quark mass, while the light CP even
Higgs mass in the THDM can be smaller. Various mass spectra of the fourth
generation fermions and the Higgs bosons are allowed. The phenomenology of the
fourth generation models is still rich.Comment: 15 pages, 16 figures; some points clarified, references added, to
appear in Phys.Rev.
Measure of the size of CP violation in extended models
In this letter we introduce a possible measure of the size of CP violation in
the Standard Model and its extensions, based on quantities invariant under the
change of weak quark basis. We also introduce a measure of the ``average size''
of CP violation in a model, which can be used to compare the size of CP
violation in models involving extra sequential or vector-like quarks, or
left-right symmetry.Comment: LaTeX, 7 pages, no figure
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