13 research outputs found
Precision measurement constraints on the 4-Site model & the effect of interference in searches for new physics
This work is divided in two parts.The first part presents a careful computation of Electroweak Precision Tests constraints on the 4-Site model, and the resulting limits on fermion couplings. The new heavy W and Zbosons present in this model can couple significantly to Standard Model fermions.Previous computations of these quantities were performed using an approximation that is here shown to have a more restricted validity domain as what was originally thought.The second part of the discussion is about searches of extra W and Z bosons as predicted in some extensions of the Standard Model (such as the 4-Site model) in the Drell-Yanchannels.The interference between the new physics and the Standard Model is commonly neglected in the interpretation of experimental searches. The importance of this effect isinvestigated in detail. The quantitative error in exclusion bounds due to neglecting the interference may be small, but important qualitative features are missed when using thisapproximation.It is important to be aware of the effect of interference in order to make sure wrong statements and bad conclusions are avoided, and to guarantee that analyses do stay within thedomain of validity of the approximations they rely o
Constraining new coloured matter from the ratio of 3- to 2-jets cross sections at the LHC
The Large Hadron Collider experiments are probing the evolution of the strong
coupling up to the TeV scale. We show how the ratio of 3- to 2-jets
cross sections is affected by the presence of new physics and argue that it can
be used to place a model-independent bound on new particles carrying QCD color
charge. The current data potentially constrains such states to be heavier than
a few hundred GeVs.Comment: 9 pages, 7 figure
Improved analysis of the bounds from the electroweak precision tests on the 4-site model
We present a new complete analysis of the electroweak precision observables
within the recently proposed 4-site Higgsless model, which is based on the
SU(2)_L x SU(2)_1 x SU(2)_2 x U(1)_Y gauge symmetry and predicts six extra
gauge bosons, W_{1,2} and Z_{1,2}. Within the epsilon_i (i=1,2,3,b)
parametrization, we compute for the first time the EWPT bounds via a complete
numerical algorithm going beyond commonly used approximations. Both
epsilon_{1,3} impose strong constraints. Hence, it is mandatory to consider
them jointly when extracting EWPT bounds and to fully take in to account the
correlations among the electroweak precison measurements. The phenomenological
consequence is that the extra gauge bosons must be heavier than 250 GeV. Their
couplings to SM fermions, even if bounded, might be of the same order of
magnitude than the SM ones. In contrast to other Higgsless models, the 4-site
model is not fermiophobic. The new gauge bosons could thus be discovered in the
favoured Drell-Yan channel already during the present run of the LHC
experiment.Comment: Latex file, 35 pages, 10 figures, corrected typos, published versio
Interference effects in heavy W'-boson searches at the LHC
Interference effects are widely neglected in searches for new physics. This
is the case in recent publications on searches for W'-bosons using leptonic
final states.
We examine the effects of interference on distributions frequently used to
determine mass limits for possible W'-bosons and show that there are important
qualitative effects on the behaviour of the new physics signal.
There are two main consequences. Firstly, exclusion limits where
interferences effects have not been considered are likely to have been
overestimated. Secondly, presenting experimental results as a function of a
transverse mass cut rather than in terms of the contribution of new physics to
the total cross-section would be more informative.Comment: 31 pages, 8 figures. Published versio
W' production at the LHC in the 4-site Higgsless model
We study the phenomenology of the 4-site Higgsless model, based on the
gauge symmetry, at present
colliders. The model predicts the existence of two neutral and four charged
extra gauge bosons, Z1,Z2,W1,W2. In this paper, we focus on the charged gauge
sector. We first derive limits on W1,W2-boson masses and couplings to SM
fermions from direct searches at the Tevatron. We then estimate at the 7 TeV
LHC the exclusion limits with the actual L=1 fb-1 and the discovery potential
with the expected L=10 fb-1. In contrast to the minimal (or 3-site) Higgsless
model which predicts almost fermiophobic extra gauge bosons, the
next-to-minimal (or 4-site) Higgsless model recovers sizeable W1,W2-boson
couplings to ordinary matter, expressing the non-fermiophobic multiresonance
inner nature of extra-dimensional theories. Owing to this feature, we find that
in one year from now the new heavy gauge bosons, W1 and W2, could be discovered
in the final state with an electron and large missing transverse energy at the
7 TeV LHC for W1,W2-boson masses in the TeV region, depending on model
parameters.Comment: 28 pages, references adde
Custodial Vector Model
We analyze the Large Hadron Collider (LHC) phenomenology of heavy vector
resonances with a spectral global symmetry. This
symmetry partially protects the electroweak S-parameter from large
contributions of the vector resonances. The resulting custodial vector model
spectrum and interactions with the standard model fields lead to distinct
signatures at the LHC in the diboson, dilepton and associated Higgs channels.Comment: 35 pages, 45 figure
AdS/QCD: The Relevance of the Geometry
We investigate the relevance of the metric and of the geometry in
five-dimensional models of hadrons. Generically, the metric does not affect
strongly the results and even flat space agrees reasonably well with the data.
Nevertheless, we observe a preference for a decreasing warp factor, for example
AdS space. The Sakai-Sugimoto model reduces to one of these models and the
level of agreement is similar to the one of flat space. We also consider the
discrete version of the five-dimensional models, obtained by dimensional
deconstruction. We find that essentially all the relevant features of
"holographic" models of QCD can be reproduced with a simple 3-site model
describing only the states below the cut-off of the theory.Comment: 25 pages + appendix. v2 minor changes and Refs. adde