162 research outputs found
Multilepton production via top flavour-changing neutral couplings at the CERN LHC
Zt and gamma t production with Z -> l+ l- and t -> Wb -> l nu b provides the
best determination of top flavour-changing neutral couplings at the LHC. The
bounds on tc couplings eventually derived from these processes are similar to
those expected from top decays, while the limits on tu couplings are better by
a factor of two. The other significant Z and W decay modes are also
investigated.Comment: 30 pages, 23 PS figures. Uses epsfig.sty and elsart.sty. Added some
references and corrected some typos. Added more comments about statistics.
Using elsart.sty reduces the size to 30 pages. Published in Nucl. Phys.
Top effective operators at the ILC
We investigate the effect of top trilinear operators in t tbar production at
the ILC. We find that the sensitivity to these operators largely surpasses the
one achievable by the LHC either in neutral or charged current processes,
allowing to probe new physics scales up to 4.5 TeV for a centre of mass energy
of 500 GeV. We show how the use of beam polarisation and an eventual energy
upgrade to 1 TeV allow to disentangle all effective operator contributions to
the Ztt and gamma tt vertices.Comment: LaTeX 13 pages. Typos corrected. Final version in JHE
Top effective operators at the ILC
We investigate the effect of top trilinear operators in t tbar production at
the ILC. We find that the sensitivity to these operators largely surpasses the
one achievable by the LHC either in neutral or charged current processes,
allowing to probe new physics scales up to 4.5 TeV for a centre of mass energy
of 500 GeV. We show how the use of beam polarisation and an eventual energy
upgrade to 1 TeV allow to disentangle all effective operator contributions to
the Ztt and gamma tt vertices.Comment: LaTeX 13 pages. Typos corrected. Final version in JHE
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
Electroweak scale seesaw and heavy Dirac neutrino signals at LHC
Models of type I seesaw can be implemented at the electroweak scale in a
natural way provided that the heavy neutrino singlets are quasi-Dirac
particles. In such case, their contribution to light neutrino masses has the
suppression of a small lepton number violating parameter, so that light
neutrino masses can arise naturally even if the seesaw scale is low and the
heavy neutrino mixing is large. We implement the same mechanism with fermionic
triplets in type III seesaw, deriving the interactions of the new quasi-Dirac
neutrinos and heavy charged leptons with the SM fermions. We then study the
observability of heavy Dirac neutrino singlets (seesaw I) and triplets (seesaw
III) at LHC. Contrarily to common wisdom, we find that heavy Dirac neutrino
singlets with a mass around 100 GeV are observable at the 5 sigma level with a
luminosity of 13 fb^-1. Indeed, in the final state with three charged leptons
l+- l+- l-+, not previously considered, Dirac neutrino signals can be
relatively large and backgrounds are small. In the triplet case, heavy
neutrinos can be discovered with a luminosity of 1.5 fb^-1 for a mass of 300
GeV in the same channel.Comment: LaTeX 19 pages, 22 PS figures. Enlarged discussion and added a
reference. Final version to appear in PL
Top couplings and top partners
5th International Workshop on Top Quark Physics (TOP2012). Winchester (U.K.) september 16-21, 2012.We review the model-independent description of the couplings of the top quark to
the Higgs and gauge bosons in theories beyond the Standard Model. Then we examine these
couplings in the case of arbitrary heavy vector-like quarks mixing with the third family. We also
discuss the couplings of these top partners, and comment on implications for LHC searches.This work
has been supported by the MICINN project FPA2010-17915
Mixing with vector-like quarks: constraints and expectations
We argue why vector-like quarks are usually expected to mix predominantly with the third generation, and discuss about the expected size of this mixing and its naturalness
W polarisation beyond helicity fractions in top quark decays
We calculate the density matrix for the decay of a polarised top quark into a
polarised W boson and a massive b quark, for the most general Wtb vertex
arising from dimension-six gauge-invariant effective operators. We show that,
in addition to the well-known W helicity fractions, for polarised top decays it
is worth to define and study the transverse and normal W polarisation
fractions, that is, the W polarisation along two directions orthogonal to its
momentum. In particular, a rather simple forward-backward asymmetry in the
normal direction is found to be very sensitive to complex phases in one of the
Wtb anomalous couplings. This asymmetry, which indicates a normal W
polarisation, can be generated for example by a P-odd, T-odd transition
electric dipole moment. We also investigate the angular distribution of decay
products in the top quark rest frame, calculating the spin analysing powers for
a general Wtb vertex. Finally we show that, using a combined fit to top decay
observables and the tW cross section, at LHC it will be possible to obtain
model-independent measurements of all the (complex) Wtb couplings as well as
the single top polarisation. Implications for spin correlations in top pair
production are also discussed.Comment: LaTeX 36 pages, 51 EPS figures. Typos corrected. To appear in NP
Electroweak constraints on see-saw messengers and their implications for LHC
We review the present electroweak precision data constraints on the mediators
of the three types of see-saw mechanisms. Except in the see-saw mechanism of
type I, with the heavy neutrino singlets being mainly produced through their
mixing with the Standard Model leptons, LHC will be able to discover or put
limits on new scalar (see-saw of type II) and lepton (see-saw of type III)
triplets near the TeV. If discovered, it may be possible in the simplest models
to measure the light neutrino mass and mixing properties that neutrino
oscillation experiments are insensitive to.Comment: 8 pages, 4 figures, To appear in the Proceedings of the Rencontres de
Moriond 2008 EW Session, La Thuile (Italy), March 1-8, 200
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