28 research outputs found
One light composite Higgs boson facing electroweak precision tests
We study analytically and numerically the bounds imposed by the electroweak
precision tests on a minimal composite Higgs model. The model is based on
spontaneous SO(5)/SO(4) breaking, so that an approximate custodial symmetry is
preserved. The Higgs arises as a pseudo-Goldstone boson at a scale below the
electroweak symmetry breaking scale. We show that one can satisfy the
electroweak precision constraints without much fine-tuning. This is the case if
the left-handed top quark is fully composite, which gives a mass spectrum
within the reach of the LHC. However a composite top quark is strongly
disfavoured by flavour physics. The alternative is to have a singlet top
partner at a scale much lighter than the rest of the composite fermions. In
this case the top partner would be light enough to be produced significantly at
the LHC.Comment: 15 pages, 4 figures; v2: figures and discussion improved, references
added; v3: minor changes, final version to appear in PR
Scale Anomalies, States, and Rates in Conformal Field Theory
This paper presents two methods to compute scale anomaly coefficients in
conformal field theories (CFTs), such as the c anomaly in four dimensions, in
terms of the CFT data. We first use Euclidean position space to show that the
anomaly coefficient of a four-point function can be computed in the form of an
operator product expansion (OPE), namely a weighted sum of OPE coefficients
squared. We compute the weights for scale anomalies associated with scalar
operators and show that they are not positive. We then derive a different sum
rule of the same form in Minkowski momentum space where the weights are
positive. The positivity arises because the scale anomaly is the coefficient of
a logarithm in the momentum space four-point function. This logarithm also
determines the dispersive part, which is a positive sum over states by the
optical theorem. The momentum space sum rule may be invalidated by UV and/or IR
divergences, and we discuss the conditions under which these singularities are
absent. We present a detailed discussion of the formalism required to compute
the weights directly in Minkowski momentum space. A number of explicit checks
are performed, including a complete example in an 8-dimensional free field
theory.Comment: 39 pages, 7 figure
Vector-like Bottom Quarks in Composite Higgs Models
Like many other models, Composite Higgs Models feature the existence of heavy
vector-like quarks. Mixing effects between the Standard Model fields and the
heavy states, which can be quite large in case of the top quark, imply
deviations from the SM. In this work we investigate the possibility of heavy
bottom partners. We show that they can have a significant impact on electroweak
precision observables and the current Higgs results if there is a sizeable
mixing with the bottom quark. We explicitly check that the constraints from the
measurement of the CKM matrix element are fulfilled, and we test the
compatibility with the electroweak precision observables. In particular we
evaluate the constraint from the coupling to left-handed bottom quarks.
General formulae have been derived which include the effects of new bottom
partners in the loop corrections to this coupling and which can be applied to
other models with similar particle content. Furthermore, the constraints from
direct searches for heavy states at the LHC and from the Higgs search results
have been included in our analysis. The best agreement with all the considered
constraints is achieved for medium to large compositeness of the left-handed
top and bottom quarks.Comment: additional figures, extended discussion of numerical result
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