605 research outputs found
A New Light Particle in B Decays?
We investigate the possibility whether the tensions with SM expectations
observed in several b -> sll transitions, including hints for lepton flavour
non-universality, could be due to the decay of B into a new light resonance. We
find that qualitative agreement with the data can be obtained with a light
vector resonance dominantly decaying invisibly. This scenario predicts a shift
in the muon anomalous magnetic moment that could explain the long-standing
discrepancy. The most stringent constraint comes from searches for B decays
with missing energy. A striking prediction is a strong q^2 dependence of the
lepton flavour universality ratios R_K and R_K* that should allow to clearly
confirm or rule out this possibility experimentally. We also comment on the
possible connection of the invisible decay product with Dark Matter.Comment: 5 pages, 2 figures. v2: typos corrected, references and
clarifications adde
New physics in transitions after LHC run 1
We present results of global fits of all relevant experimental data on rare
decays. We observe significant tensions between the Standard Model
predictions and the data. After critically reviewing the possible sources of
theoretical uncertainties, we find that within the Standard Model, the tensions
could be explained if there are unaccounted hadronic effects much larger than
our estimates. Assuming hadronic uncertainties are estimated in a sufficiently
conservative way, we discuss the implications of the experimental results on
new physics, both model independently as well as in the context of the minimal
supersymmetric standard model and models with flavour-changing bosons. We
discuss in detail the violation of lepton flavour universality as hinted by the
current data and make predictions for additional lepton flavour universality
tests that can be performed in the future. We find that the ratio of the
forward-backward asymmetries in and
at low dilepton invariant mass is a particularly sensitive probe of lepton
flavour universality and allows to distinguish between different new physics
scenarios that give the best description of the current data.Comment: 49 pages, 12 figures. v4: matches version published in EPJ
Direct and indirect signals of natural composite Higgs models
We present a comprehensive numerical analysis of a four-dimensional model
with the Higgs as a composite pseudo-Nambu-Goldstone boson that features a
calculable Higgs potential and protective custodial and flavour symmetries to
reduce electroweak fine-tuning. We employ a novel numerical technique that
allows us for the first time to study constraints from radiative electroweak
symmetry breaking, Higgs physics, electroweak precision tests, flavour physics,
and direct LHC bounds on fermion and vector boson resonances in a single
framework. We consider four different flavour symmetries in the composite
sector, one of which we show to not be viable anymore in view of strong
precision constraints. In the other cases, all constraints can be passed with a
sub-percent electroweak fine-tuning. The models can explain the excesses
recently observed in , , and resonance searches by
ATLAS and CMS and the anomalies in angular observables and branching ratios of
rare semi-leptonic decays observed by LHCb. Solving the physics
anomalies predicts the presence of a dijet or resonance around 1 TeV
just below the sensitivity of LHC run 1. We discuss the prospects to probe the
models at run 2 of the LHC. As a side product, we identify several gaps in the
searches for vector-like quarks at hadron colliders, that could be closed by
reanalyzing existing LHC data.Comment: 74 pages, 21 figures. v2: references added, discussion in 3.2.6
extende
Dipole operator constraints on composite Higgs models
Flavour- and CP-violating electromagnetic or chromomagnetic dipole operators
in the quark sector are generated in a large class of new physics models and
are strongly constrained by measurements of the neutron electric dipole moment
and observables sensitive to flavour-changing neutral currents, such as the
branching ratio and . After a
model-independent discussion of the relevant constraints, we analyze these
effects in models with partial compositeness, where the quarks get their masses
by mixing with vector-like composite fermions. These scenarios can be seen as
the low-energy limit of composite Higgs or warped extra dimensional models. We
study different choices for the electroweak representations of the composite
fermions motivated by electroweak precision tests as well as different flavour
structures, including flavour anarchy and or flavour
symmetries in the strong sector. In models with "wrong-chirality" Yukawa
couplings, we find a strong bound from the neutron electric dipole moment,
irrespective of the flavour structure. In the case of flavour anarchy, we also
find strong bounds from flavour-violating dipoles, while these constraints are
mild in the flavour-symmetric models.Comment: 30 pages, 2 figures, 11 tables. v3: Misprints in table 8 corrected.
Numerics and conclusions unchange
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