40 research outputs found

### Precision Electroweak Measurements and Constraints on the Standard Model

This note presents constraints on Standard Model parameters using published
and preliminary precision electroweak results measured at the electron-positron
colliders LEP and SLC. The results are compared with precise electroweak
measurements from other experiments, notably CDF and D{\O}at the Tevatron.
Constraints on the input parameters of the Standard Model are derived from the
combined set of results obtained in high-$Q^2$ interactions, and used to
predict results in low-$Q^2$ experiments, such as atomic parity violation,
M{\o}ller scattering, and neutrino-nucleon scattering. The main changes with
respect to the experimental results presented in 2008 are new combinations of
results on the W-boson mass and the mass of the top quark.Comment: 18 page

### General Lepton Mixing in Holographic Composite Higgs Models

We introduce a scenario of lepton mixing in holographic composite Higgs
models based on non-abelian discrete symmetries of the form G_f=X x Z_N, broken
to Z_2 x Z_2 x Z_N in the elementary sector and to Z_N^(D) in the composite
sector with Z_N^(D) being the diagonal subgroup of a Z_N contained in X and the
external Z_N. By choosing X = Delta(96) or Delta(384), a non-vanishing
theta_{13} of order 0.1 is naturally obtained. We apply our considerations to a
5D model in warped space for the particular cases of X = S_4, A_5, Delta(96)
and Delta(384) and N=3 or 5. Lepton flavour violating processes and electric
dipole moments are well below the current bounds, with the exception of mu -> e
gamma that puts a very mild constraint on the parameter space of the model, for
all presented choices of G_f.Comment: 20 pages, 2 figures; v2: clarifications added in section 2 regarding
the form of the flavour group and the size of the flavour violating kinetic
terms, typos corrected, minor improvements, to appear in JHE

### General analysis of signals with two leptons and missing energy at the Large Hadron Collider

A signal of two leptons and missing energy is challenging to analyze at the
Large Hadron Collider (LHC) since it offers only few kinematical handles. This
signature generally arises from pair production of heavy charged particles
which each decay into a lepton and a weakly interacting stable particle. Here
this class of processes is analyzed with minimal model assumptions by
considering all possible combinations of spin 0, 1/2 or 1, and of weak
iso-singlets, -doublets or -triplets for the new particles. Adding to existing
work on mass and spin measurements, two new variables for spin determination
and an asymmetry for the determination of the couplings of the new particles
are introduced. It is shown that these observables allow one to independently
determine the spin and the couplings of the new particles, except for a few
cases that turn out to be indistinguishable at the LHC. These findings are
corroborated by results of an alternative analysis strategy based on an
automated likelihood test.Comment: 18 pages, 3 figures, LaTe

### FCNC Effects in a Minimal Theory of Fermion Masses

As a minimal theory of fermion masses we extend the SM by heavy vectorlike
fermions, with flavor-anarchical Yukawa couplings, that mix with chiral
fermions such that small SM Yukawa couplings arise from small mixing angles.
This model can be regarded as an effective description of the fermionic sector
of a large class of existing flavor models and thus might serve as a useful
reference frame for a further understanding of flavor hierarchies in the SM.
Already such a minimal framework gives rise to FCNC effects through exchange of
massive SM bosons whose couplings to the light fermions get modified by the
mixing. We derive general formulae for these corrections and discuss the bounds
on the heavy fermion masses. Particularly stringent bounds, in a few TeV range,
come from the corrections to the Z couplings.Comment: 19 pages, 1 figur

### T-parity, its problems and their solution

We point out a basic difficulty in the construction of little-Higgs models
with T-parity which is overlooked by large part of the present literature.
Almost all models proposed so far fail to achieve their goal: they either
suffer from sizable electroweak corrections or from a breakdown of collective
breaking. We provide a model building recipe to bypass the above problem and
apply it to build the simplest T-invariant extension of the Littlest Higgs. Our
model predicts additional T-odd pseudo-Goldstone bosons with weak scale masses.Comment: 25 pages, 2 appendice

### Slepton Discovery in Electroweak Cascade Decay

The LHC studies on the MSSM slepton sector have mostly been focused on direct
slepton Drell-Yan pair production. In this paper, we analyze the case when the
sleptons are lighter than heavy neutralinos and can appear in the on-shell
decay of neutralino states. In particular, we have studied the \chi_1^\pm
\chi_2^0 associated production, with the consequent decays of \chi_1^\pm and
\chi_2^0 via on-shell sleptons. The invariant mass of the lepton pairs,
m_{\ell\ell}, from the neutralino decay has a distinctive triangle shape with a
sharp kinematic cutoff. We discuss the utilization of this triangle shape in
m_{\ell\ell} distribution to identify the slepton signal. We studied the
trilepton plus missing E_T signal and obtained the effective cross section,
\sigma \times BR \times acceptance, that is needed for a 5\sigma discovery as a
function of the cutoff mass for the LHC with center of mass energy 14 TeV and
100 fb^{-1} integrated luminosity. Our results are model independent such that
they could be applied to other models with similar decay topology. When applied
to the MSSM under simple assumptions, it is found that with 100 fb^{-1}
integrated luminosity, a discovery reach in the left-handed slepton mass of
about 600 GeV could be reached, which extends far beyond the slepton mass reach
in the usual Drell-Yan studies.Comment: 20 pages, 5 figure