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 $\alpha_s$ 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