296 research outputs found
Present Bounds on New Neutral Vector Resonances from Electroweak Gauge Boson Pair Production at the LHC
Several extensions of the Standard Model predict the existence of new neutral
spin-1 resonances associated to the electroweak symmetry breaking sector. Using
the data from ATLAS (with integrated luminosity of L=1.02 fb^{-1}) and CMS
(with integrated luminosity of L=1.55 fb^{-1}) on the production of W+W- pairs
through the process pp -> l^+ l^{\prime -} \sla{E}_T, we place model
independent bounds on these new vector resonances masses, couplings and widths.
Our analyses show that the present data excludes new neutral vector resonances
with masses up to 1-2.3 TeV depending on their couplings and widths. We also
demonstrate how to extend our analysis framework to different models working a
specific example.Comment: 10 pages, 6 figure
Constraining anomalous Higgs interactions
The recently announced Higgs discovery marks the dawn of the direct probing
of the electroweak symmetry breaking sector. Sorting out the dynamics
responsible for electroweak symmetry breaking now requires probing the Higgs
interactions and searching for additional states connected to this sector. In
this work we analyze the constraints on Higgs couplings to the standard model
gauge bosons using the available data from Tevatron and LHC. We work in a
model--independent framework expressing the departure of the Higgs couplings to
gauge bosons by dimension--six operators. This allows for independent
modifications of its couplings to gluons, photons and weak gauge bosons while
still preserving the Standard Model (SM) gauge invariance. Our results indicate
that best overall agreement with data is obtained if the cross section of Higgs
production via gluon fusion is suppressed with respect to its SM value and the
Higgs branching ratio into two photons is enhanced, while keeping the
production and decays associated to couplings to weak gauge bosons close to
their SM prediction.Comment: v3: Added acknowledgment to FP7 ITN INVISIBLES (Marie Curie Actions
PITN-GA-2011-289442). Nothing else changed with respect to v
Disentangling a dynamical Higgs
The pattern of deviations from Standard Model predictions and couplings is
different for theories of new physics based on a non-linear realization of the
gauge symmetry breaking and those assuming a linear
realization. We clarify this issue in a model-independent way via its effective
Lagrangian formulation in the presence of a light Higgs particle, up to first
order in the expansions: dimension-six operators for the linear expansion and
four derivatives for the non-linear one. Complete sets of pure gauge and
gauge-Higgs operators are considered, implementing the renormalization
procedure and deriving the Feynman rules for the non-linear expansion. We
establish the theoretical relation and the differences in physics impact
between the two expansions. Promising discriminating signals include the
decorrelation in the non-linear case of signals correlated in the linear one:
some pure gauge versus gauge-Higgs couplings and also between couplings with
the same number of Higgs legs. Furthermore, anomalous signals expected at first
order in the non-linear realization may appear only at higher orders of the
linear one, and vice versa. We analyze in detail the impact of both type of
discriminating signals on LHC physics.Comment: Version published in JHE
Determination of the Spin of New Resonances in Electroweak Gauge Boson Pair Production at the LHC
The appearance of spin-1 resonances associated to the electroweak symmetry
breaking (EWSB) sector is expected in many extensions of the Standard Model. We
analyze the CERN Large Hadron Collider potential to probe the spin of possible
new charged and neutral vector resonances through the purely leptonic processes
pp --> Z^\prime --> l^+ l^{\prime -} Emiss_T, and pp --> W^\prime --> l^{\prime
\pm} l^+ l^- Emiss_T, with l, l^\prime = e or \mu. We perform a model
independent analysis and demonstrate that the spin of the new states can be
determined with 99% CL in a large fraction of the parameter space where these
resonances can be observed with 100 fb^{-1}. We show that the best sensitivity
to the spin is obtained by directly studying correlations between the final
state leptons, without the need of reconstructing the events in their
center-of-mass frames.Comment: 11 pages, 10 figures, references added, matches published versio
The gauge-Higgs legacy of the LHC Run I
The effective Lagrangian expansion provides a framework to study effects of new physics at the electroweak scale. To make full use of LHC data in constraining higher-dimensional operators we need to include both the Higgs and the electroweak gauge sector in our study. We first present an analysis of the relevant di-boson production LHC results to update constraints on triple gauge boson couplings. Our bounds are several times stronger than those obtained from LEP data. Next, we show how in combination with Higgs measurements the triple gauge vertices lead to a significant improvement in the entire set of operators, including operators describing Higgs couplings
Transport coefficients and resonances for a meson gas in Chiral Perturbation Theory
We present recent results on a systematic method to calculate transport
coefficients for a meson gas (in particular, we analyze a pion gas) at low
temperatures in the context of Chiral Perturbation Theory (ChPT). Our method is
based on the study of Feynman diagrams taking into account collisions in the
plasma by means of the non-zero particle width. This implies a modification of
the standard ChPT power counting scheme. We discuss the importance of
unitarity, which allows for an accurate high energy description of scattering
amplitudes, generating dynamically the and mesons. Our
results are compatible with analyses of kinetic theory, both in the
non-relativistic very low- regime and near the transition. We show the
behavior with temperature of the electrical and thermal conductivities as well
as of the shear and bulk viscosities. We obtain that bulk viscosity is
negligible against shear viscosity, except near the chiral phase transition
where the conformal anomaly might induce larger bulk effects. Different
asymptotic limits for transport coefficients, large- scaling and some
applications to heavy-ion collisions are studied.Comment: Invited talk given at the international workshop Hot Quarks 2008,
Estes Park, Colorado, USA, August 18-23 2008. Accepted as a regular article
in Eur.Phys.J.C. 18 pages EPJC style, 23 figure
Neutrino Masses at LHC: Minimal Lepton Flavour Violation in Type-III See-saw
We study the signatures of minimal lepton flavour violation in a simple
Type-III see - saw model in which the flavour scale is given by the new fermion
triplet mass and it can be naturally light enough to be produced at the LHC. In
this model the flavour structure of the lepton number conserving couplings of
the triplet fermions to the Standard Model leptons can be reconstructed from
the neutrino mass matrix and the smallness of the neutrino mass is associated
with a tiny violation of total lepton number. Characteristic signatures of this
model include suppressed lepton number violation decays of the triplet
fermions, absence of displaced vertices in their decays and predictable lepton
flavour composition of the states produced in their decays. We study the
observability of these signals in the processes pp\rightarrow 3\ell + 2j
+\Sla{E_T} and with or taking into
account the present low energy data on neutrino physics and the corresponding
Standard Model backgrounds. Our results indicate that the new fermionic states
can be observed for masses up to 500 GeV depending on the CP violating Majorana
phase for an integrated luminosity of 30 fb. Moreover, the flavour of
the final state leptons in the above processes can shed light on the neutrino
mass ordering.Comment: 31 pages, 11 Figures, matches published versio
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