1,216 research outputs found
Structural changes at the semiconductor-insulator phase transition in the single layered La0.5Sr1.5MnO4 perovskite
The semiconductor-insulator phase transition of the single-layer manganite
La0.5Sr1.5MnO4 has been studied by means of high resolution synchrotron x-ray
powder diffraction and resonant x-ray scattering at the Mn K edge. We conclude
that a concomitant structural transition from tetragonal I4/mmm to orthorhombic
Cmcm phases drives this electronic transition. A detailed symmetry-mode
analysis reveals that condensation of three soft modes -Delta_2(B2u), X1+(B2u)
and X1+(A)- acting on the oxygen atoms accounts for the structural
transformation. The Delta_2 mode leads to a pseudo Jahn-Teller distortion (in
the orthorhombic bc-plane only) on one Mn site (Mn1) whereas the two X1+ modes
produce an overall contraction of the other Mn site (Mn2) and expansion of the
Mn1 one. The X1+ modes are responsible for the tetragonal superlattice
(1/2,1/2,0)-type reflections in agreement with a checkerboard ordering of two
different Mn sites. A strong enhancement of the scattered intensity has been
observed for these superlattice reflections close to the Mn K edge, which could
be ascribed to some degree of charge disproportion between the two Mn sites of
about 0.15 electrons. We also found that the local geometrical anisotropy of
the Mn1 atoms and its ordering originated by the condensed Delta_2 mode alone
perfectly explains the resonant scattering of forbidden (1/4,1/4,0)-type
reflections without invoking any orbital ordering.Comment: 3 tables and 10 figures; accepted in Phys. Rev.
Can new generations explain neutrino masses?
In this talk we explore the possibility that the smallness of the observed
neutrino masses is naturally understood in a modified version of the standard
model with N extra generations of fermions and N right-handed neutrinos, in
which light neutrino masses are generated at two loops. We find that with N = 1
it is not possible to fit the observed spectrum of masses and mixings while
with N = 2 it is. Within this extension, we analyse the parameters which are
allowed and the possible phenomenological signals of the model in future
experiments. Contribution to the proceedings of Les Rencontres de Moriond EW
2011, Young Scientist Forum
Collider phenomenology of vector resonances in WZ scattering processes
We study the production of vector resonances at the LHC via scattering
processes and explore the sensitivities to these resonances for the expected
future LHC luminosities. The electroweak chiral Lagrangian and the Inverse
Amplitude Method (IAM) are used for analyzing a dynamically generated vector
resonance, whose origin would be the (hypothetically strong) self interactions
of the longitudinal gauge bosons, and . We implement the unitarized
scattering amplitudes into a single model, the IAM-MC, that has been adapted to
MadGraph~5. It is written in terms of the electroweak chiral Lagrangian and an
additional effective Proca Lagrangian for the vector resonances, so that it
reproduces the resonant behavior of the IAM and allows us to perform a
realistic study of signal versus background at the LHC. We focus on the channel, discussing first on the potential of the hadronic and
semileptonic channels of the final , and next exploring in more detail the
clearest signals. These are provided by the leptonic decays of the gauge
bosons, leading to a final state with , ,
having a very distinctive signature, and showing clearly the emergence of the
resonances with masses in the range of -, which we have
explored.Comment: 8 pages, 5 figures, contributed to the XIII Quark Confinement and the
Hadron Spectrum - Confinement2018, 31 July - 6 August 2018, Maynooth
University, Irelan
Production of vector resonances at the LHC via WZ-scattering: a unitarized EChL analysis
In the present work we study the production of vector resonances at the LHC
by means of the vector boson scattering and explore the
sensitivities to these resonances for the expected future LHC luminosities. We
are assuming that these vector resonances are generated dynamically from the
self interactions of the longitudinal gauge bosons, and , and work
under the framework of the electroweak chiral Lagrangian to describe in a model
independent way the supposedly strong dynamics of these modes. The properties
of the vector resonances, mass, width and couplings to the and gauge
bosons are derived from the inverse amplitude method approach. We implement all
these features into a single model, the IAM-MC, adapted for MonteCarlo, built
in a Lagrangian language in terms of the electroweak chiral Lagrangian and a
chiral Lagrangian for the vector resonances, which mimics the resonant behavior
of the IAM and provides unitary amplitudes. The model has been implemented in
MadGraph, allowing us to perform a realistic study of the signal versus
background events at the LHC. In particular, we have focused our study on the
type of events, discussing first on the potential of the hadronic
and semileptonic channels of the final , and next exploring in more detail
the clearest signals. These are provided by the leptonic decays of the gauge
bosons, leading to a final state with ,
, having a very distinctive signature, and showing clearly the
emergence of the resonances with masses in the range of 1.5-2.5 TeV, which we
have explored.Comment: Revised version accepted for publication in JHEP. Enlarged analysis.
References added. 44 pages, 23 figures, 3 table
Radiative corrections to from three generations of Majorana neutrinos and sneutrinos
In this work we study the radiative corrections to the mass of the lightest
Higgs boson of the MSSM from three generations of Majorana neutrinos and
sneutrinos. The spectrum of the MSSM is augmented by three right handed
neutrinos and their supersymmetric partners. A seesaw mechanism of type I is
used to generate the physical neutrino masses and oscillations that we require
to be in agreement with present neutrino data. We present a full one-loop
computation of these Higgs mass corrections, and analyze in full detail their
numerical size in terms of both the MSSM and the new (s)neutrino parameters. A
critical discussion on the different possible renormalization schemes and their
implications is included.Comment: 42 pages, 39 figures, 1 appendix, version published in AHE
Unitarization effects in EFT predictions of WZ scattering at the LHC
Effective field theories are an incredibly powerful tool in order to study
and understand the true nature of the symmetry breaking sector dynamics of the
Standard Model. However, they can suffer from some theoretical problems such as
that of unitarity violation. Nevertheless, in order to interpret experimental
data correctly a fully unitary prescription is needed. To this purpose,
unitarization methods are addressed, but each of them leads to a different
(unitary) prediction. Because of this, there is an inherent theoretical
uncertainty in the determination of the effective field theory parameters due
to the choice of one unitarization scheme. In this work, we quantify this
uncertainty assuming a strongly interacting electroweak symmetry breaking
sector, described by the effective electroweak chiral Lagrangian. We focus on
the bosonic part of this effective Lagrangian and choose in particular the WZ
scattering as our main VBS channel to study the sensitivity to new physics at
the LHC. We study the different predictions of various well known unitarization
methods, considering the full coupled system of helicity amplitudes, and
construct the 95\% confidence level exclusion regions for the most relevant
electroweak chiral Lagrangian parameters, given by the two anomalous quartic
gauge couplings and . This provides a consistent analysis of the
different constraints on EChL parameters that can be achieved by using
different unitarization methods in a combined way.Comment: 25 pages, 8 figures (20 plots), this version matches the published
article in Phys. Rev.
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