We present an holographic approach to strongly-coupled theories close to the conformal to non-conformal transition, trying to understand the presence of light scalars as recent lattice simulations seem to suggest. We find that the dilaton is always the lightest resonance, although not parametrically lighter than the others. We provide a simple analytic formula for the dilaton mass that allows us to understand this behavior. The pattern of the meson mass spectrum, as we get close to the conformal transition, is found to be quite similar to that in lattice simulations. We provide further predictions from holography that can be checked in the future. These five-dimensional models can also implement new solutions to the hierarchy problem, having implications for searches at the LHC and cosmology.

Some anomalies in the processes b → s ( = µ, e) and b → c ν ( = τ, µ, e), in particular in the observables R K and R D ( * ) , have been found by the BaBar, LHCb and Belle collaborations, leading to a possible lepton flavor universality violation. If these anomalies were confirmed they would inevitably lead to physics beyond the Standard Model. In this paper we try to accommodate the present anomalies in an extra dimensional theory, solving the naturalness problem of the Standard Model by means of a warped metric with a strong conformality violation near the infra-red brane. The R K anomaly can be accommodated provided that the left-handed bottom quark and muon lepton have some degree of compositeness in the dual theory. The theory is consistent with all electroweak and flavor observables, and with all direct searches of Kaluza-Klein electroweak gauge bosons and gluons. The fermion spectrum, and fermion mixing angles, can be reproduced by mostly elementary right-handed bottom quarks, and tau and muon leptons. Moreover the R D ( * ) anomaly requires a strong degree of compositeness for the left-handed tau leptons, which turns out to be in tension with experimental data on the g Z τ L coupling, possibly unless some degree of fine-tuning is introduced in the fixing of the CKM matrix.

Abstract:The experimental value of the anomalous magnetic moment of the muon, as well as the LHCb anomalies, point towards new physics coupled non-universally to muons and electrons. Working in extra dimensional theories, which solve the electroweak hierarchy problem with a warped metric, strongly deformed with respect to the AdS 5 geometry at the infra-red brane, the LHCb anomalies can be solved by imposing that the bottom and the muon have a sizable amount of compositeness, while the electron is mainly elementary. Using this set-up as starting point we have proven that extra physics has to be introduced to describe the anomalous magnetic moment of the muon. We have proven that this job is done by a set of vector-like leptons, mixed with the physical muon through Yukawa interactions, and with a high degree of compositeness. The theory is consistent with all electroweak indirect, direct and theoretical constraints, the most sensitive ones being the modification of the Zμµ coupling, oblique observables and constraints on the stability of the electroweak minimum. They impose lower bounds on the compositeness (c 0.37) and on the mass of the lightest vector-like lepton ( 270 GeV). Vector-like leptons could be easily produced in Drell-Yan processes at the LHC and detected at √ s = 13 TeV.

We explore the limits on lepton-flavor universality (LFU) violation in theories where the hierarchy problem is solved by means of a warped extra dimension. In those theories LFU violation, in fermion interaction with Kaluza-Klein modes of gauge bosons, is provided ab initio when different flavor of fermions are differently localized along the extra dimension. As this fact arises from the mass pattern of quarks and leptons, LFU violation is natural in this class of theories. We analyze the experimental data pointing towards LFU violation, as well as the most relevant electroweak and flavor observables, and the LFU tests in the µ/e and τ /µ sectors. We find agreement with R K ( * ) and R D ( * ) data at 95% CL, provided the third generation left-handed fermions are composite (0.14 < c b L < 0.28 and 0.27 < cτ L < 0.33), and find the absolute limits R K ( * ) > ∼ 0.79 and< ∼ 1.13. Moreover we predict B(B → Kνν) > ∼ 1.14 × 10 −5 at 95% CL, smaller than the present experimental upper bound but a few times larger than the Standard Model prediction.