Composite Higgs theory

I review the idea of realizing the Higgs as a composite pseudo-Nambu-Goldstone boson of a new strongly-interacting sector and collect major constraints on the parameter space of minimal models. Besides limits from electroweak precision tests, LHC searches for resonances and bounds due to Higgs-coupling modifications will be discussed in detail. Finally, the issue of light top partners in these models will be explored, including ways to avoid them which lead to interesting implications for flavor observables.Comment: 12 pages, 5 figures, Proceedings of ALPS2018: An Alpine LHC Physics Summi

Lepton Flavor and Non-Universality from Minimal Composite Higgs Setups

We present a new class of models of lepton flavor in the composite Higgs framework. Following the concept of minimality, they lead to a rich phenomenology in good agreement with the current experimental picture. Because of a unification of the right-handed leptons, our scenario is very predictive and can naturally lead to a violation of lepton-flavor universality in neutral current interactions. We will show that, in particular, the anomaly in $R_K= {\cal B}(B \to K \mu^+ \mu^-)/{\cal B}(B \to K e^+ e^-)$, found by LHCb, can be addressed, while other constraints from quark- and lepton-flavor physics are met. In fact, the minimal structure of the setup allows for the implementation of a very powerful flavor protection, which avoids the appearance of new sources of flavor-changing neutral currents to very good approximation. Finally, the new lepton sector provides a parametrically enhanced correction to the Higgs mass, such that the need for ultra-light top partners is weakened considerably, linking the mass of the latter with the size of the neutrino masses.Comment: 5 pages, 3 figures; v2: matches version published in Physical Review Letters, for some slightly extended discussions see v

Custodial Leptons and Higgs Decays

We study the effects of extended fermion sectors, respecting custodial symmetry, on Higgs production and decay. The resulting protection for the Z->b_L b_L and Z->\tau_R\tau_R decays allows for potentially interesting signals in Higgs physics, while maintaining the good agreement of the Standard Model with precision tests, without significant fine-tuning. Although being viable setups on their own, the models we study can particularly be motivated as the low energy effective theories of the composite Higgs models MCHM_5 and MCHM_10 or the corresponding gauge-Higgs unification models. The spectra can be identified with the light custodians present in these theories. These have the potential to describe the relevant physics in their fermion sectors in a simplified and transparent way. In contrast to previous studies of composite models, we consider the impact of a realistic lepton sector on the Higgs decays. We find significant modifications in the decays to \tau leptons and photons due to the new leptonic resonances. While from a pure low energy perspective an enhancement of the channel pp->h->\gamma\gamma turns out to be possible, if one considers constraints on the parameters from the full structure of the composite models, the decay mode into photons is always reduced. We also demonstrate that taking into account the non-linearity of the Higgs sector does not change the qualitative picture for the decays into \tau-leptons or photons in the case of the dominant Higgs production mechanism.Comment: 33 pages, 12 figures; v2: typos corrected, references added, minor clarifying comments and discussion of new Moriond results added, version published in JHE

Recent B\boldsymbol{B} Physics Anomalies - a First Hint for Compositeness?

We scrutinize the recently further strengthened hints for new physics in semileptonic $B$-meson decays, focusing on the 'clean' ratios of branching fractions $R_K$ and $R_{K^\ast}$ and examining to which pattern of new effects they point to. We explore in particular the hardly considered, yet fully viable, option of new physics in the right-handed electron sector and demonstrate how a recently proposed framework of leptons in composite Higgs setups naturally solves both the $R_K$ and $R_{K^\ast}$ anomalies via a peculiar structure of new physics effects, predicted by minimality of the model and the scale of neutrino masses. Finally, we also take into account further observables, such as ${\cal B}(B_s \to \mu^+\mu^-)$, $\Delta M_{B_s}$, and angular observables in $B \to K^{\ast} \mu^+ \mu^-$ decays, to arrive at a comprehensive picture of the model concerning (semileptonic) $B$ decays. We conclude that -- since it is in good agreement with the experimental situation in flavor physics and also allows to avoid ultra-light top partners -- the model furnishes a very promising scenarios of Higgs compositeness in the light of LHC data.Comment: 21 pages, 6 figure

Uncovering the relation of a scalar resonance to the Higgs boson

We consider the associated production of a scalar resonance with the standard model Higgs boson. We demonstrate via a realistic phenomenological analysis that couplings of such a resonance to the Higgs boson can be constrained in a meaningful way in future runs of the LHC, providing insights on its origin and its relation to the electroweak symmetry breaking sector. Moreover, the final state can provide a direct way to determine whether the new resonance is produced predominantly in gluon fusion or quark-anti-quark annihilation. The analysis focusses on a resonance coming from a scalar field with vanishing vacuum expectation value and its decay to a photon pair. It can however be straightforwardly generalised to other scenarios.Comment: 17 pages, 29 figures. Version matches published versio

An explicit Z'-boson explanation of the B->K*mu+mu- anomaly

A global fit to the recent B->K*mu+mu- data shows indications for a large new-physics contribution to the Wilson coefficient of the semi-leptonic vector operator. In this article we consider a simple Z'-boson model of 3-3-1 type that can accommodate such an effect without violating any other constraint from quark-flavour physics. Implications for yet unobserved decay modes such as B->Xsnunubar and longstanding puzzles like B->piK are also discussed. The Z'-boson masses required to address the observed anomaly lie in the range of 7 TeV. Such heavy Z' bosons evade the existing bounds from precision data and direct searches, and will remain difficult to discover even at a high-luminosity LHC. The potential of an ILC as well as the next generation of low-energy parity-violation experiments in constraining the Z'-boson parameter space is also examined.Comment: 20 pages, 5 figures; v2: Eqs. (9.2), (9.4) and (9.5) corrected, typos fixed and references added; matches version published in JHE