Higgs decay into photons through a spin-2 loop

A new particle with proprieties similar to those of the Higgs boson in the Standard Model (SM) has been recently discovered. The biggest discrepancy is related to its diphoton decay, whose branching ratio seems to be around two times larger with respect to the correspondent SM value; this evidence, even if still affected by large uncertainties, suggests that clues of new physics related to the spontaneous breaking of the electroweak symmetry could be hidden under this loop-induced process. A new strongly-coupled sector responsible for this breaking, for instance, could produce in analogy with QCD a charged massive spin-2 state. In light of these arguments we calculate and discuss the role of such a resonance in the diphoton decay width of the Higgs.Comment: 12 pages + appendices, 5 figures. v2: minor changes, references adde

Natural minimal dark matter

We show how the Higgs boson mass is protected from the potentially large corrections due to the introduction of minimal dark matter if the new physics sector is made supersymmetric. The fermionic dark matter candidate (a 5-plet of $SU(2)_L$) is accompanied by a scalar state. The weak gauge sector is made supersymmetric and the Higgs boson is embedded in a supersymmetric multiplet. The remaining standard model states are non-supersymmetric. Non vanishing corrections to the Higgs boson mass only appear at three-loop level and the model is natural for dark matter masses up to 15 TeV--a value larger than the one required by the cosmological relic density. The construction presented stands as an example of a general approach to naturalness that solves the little hierarchy problem which arises when new physics is added beyond the standard model at an energy scale around 10 TeV.Comment: 6 pages, 4 figures. v2: Discussion on the mass splitting extended and improved. References adde

The breaking of the SU(2)L×U(1)YSU(2)_L\times U(1)_Y symmetry: The 750 GeV resonance at the LHC and perturbative unitarity

If the di-photon excess at 750 GeV hinted by the 2015 data at the LHC is explained in terms of a scalar resonance participating in the breaking of the electro-weak symmetry, this resonance must be accompanied by other scalar states for perturbative unitarity in vector boson scattering to be preserved. The simplest set-up consistent with perturbative unitarity and with the data of the di-photon excess is the Georgi-Machacek model.Comment: 9 pages, 5 figures. v2: Minor changes, bibliography updated. v3: Minor change

Wormholes and masses for Goldstone bosons

There exist non-trivial stationary points of the Euclidean action for an axion particle minimally coupled to Einstein gravity, dubbed wormholes. They explicitly break the continuos global shift symmetry of the axion in a non-perturbative way, and generate an effective potential that may compete with QCD depending on the value of the axion decay constant. In this paper, we explore both theoretical and phenomenological aspects of this issue. On the theory side, we address the problem of stability of the wormhole solutions, and we show that the spectrum of the quadratic action features only positive eigenvalues. On the phenomenological side, we discuss, beside the obvious application to the QCD axion, relevant consequences for models with ultralight dark matter, black hole superradiance, and the relaxation of the electroweak scale. We conclude discussing wormhole solutions for a generic coset and the potential they generate.Comment: 50 pages, 15 figures. v2: minor changes, refs adde

Constraining the Higgs portal with antiprotons

The scalar Higgs portal is a compelling model of dark matter (DM) in which a renormalizable coupling with the Higgs boson provides the connection between the visible world and the dark sector. In this paper we investigate the constraint placed on the parameter space of this model by the antiproton data. Due to the fact that the antiproton-to-proton ratio has relative less systematic uncertainties than the antiproton absolute flux, we propose and explore the possibility to combine all the available $\bar{p}/p$ data. Following this approach, we are able to obtain stronger limits if compared with the existing literature. In particular, we show that most of the parameter space close to the Higgs resonance is ruled out by our analysis. Furthermore, by studying the reach of the future AMS-02 antiproton and antideuteron data, we argue that a DM mass of $\mathcal{O}(150)$ GeV offers a promising discovery potential. The method of combining all the antiproton-to-proton ratio data proposed in this paper is quite general, and can be straightforwardly applied to other models.Comment: 31 pages, 12 figures, 2 table

A new scalar resonance at 750 GeV: Towards a proof of concept in favor of strongly interacting theories

We interpret the recently observed excess in diphoton invariant mass as a new spin-0 resonant particle. On the theoretical ground, an interesting question is whether this new scalar resonance belongs to a strongly coupled sector or a well-defined weakly coupled theory. A possible UV-completion that has been widely considered in literature is based on the existence of new vector-like fermions whose loop contributions---Yukawa-coupled to the new resonance---explain the observed signal rate. The large total width preliminarily suggested by data seems to favor a large Yukawa coupling, at the border of a healthy perturbative definition. This potential problem can be fixed by introducing multiple vector-like fermions or large electric charges, bringing back the theory to a weakly coupled regime. However, this solution risks to be only a low-energy mirage: Large multiplicity or electric charge can dangerously reintroduce the strong regime by modifying the renormalization group running of the dimensionless couplings. This issue is also tightly related to the (in)stability of the scalar potential. First, we study---in the theoretical setup described above---the parametric behavior of the diphoton signal rate, total width, and one-loop $\beta$ functions. Then, we numerically solve the renormalization group equations, taking into account the observed diphoton signal rate and total width, to investigate the fate of the weakly coupled theory. We find that---with the only exception of few fine-tuned directions---weakly coupled interpretations of the excess are brought back to a strongly coupled regime if the running is taken into account.Comment: 32 pages, 38 figures, version appeared in JHEP, Fig.1 and 4 revised, references added, new section V.C adde

On gravitational echoes from ultracompact exotic stars

At the dawn of a golden age for gravitational wave astronomy, we must leave no stone unturned in our quest for new phenomena beyond our current understanding of General Relativity (GR), particle physics and nuclear physics. In this paper we discuss gravitational echoes from ultracompact stars. We restrict our analysis to exact solutions of Einstein field equations in GR that are supported by physically motivated equations of state (EoS), and in particular we impose the constraint of causality. Our main conclusion is that ultracompact objects supported by physical EoS are not able to generate gravitational echoes like those that characterize the relaxation phase of a putative black hole mimicker. Nevertheless, we identify a class of physical exotic objects that are compact enough to accommodate the presence of an external unstable light ring, thus opening the possibility of trapping gravitational radiation and affecting the ringdown phase of a merger event. Most importantly, we show that once rotation is included these stars -- contrary to what usually expected for ultracompact objects -- are not plagued by any ergoregion instability. We extend our analysis for arbitrary values of angular velocity up to the Keplerian limit, and we comment about potential signals relevant for gravitational wave interferometers.Comment: 44 pages, 20 figures, references adde

Composite Dark Matter and LHC Interplay

The actual realization of the electroweak symmetry breaking in the context of a natural extension of the Standard Model (SM) and the nature of Dark Matter (DM) are two of the most compelling questions in high-energy particle physics. Composite Higgs models may provide a unified picture in which both the Higgs boson and the DM particle arise as pseudo Nambu-Goldstone bosons of a spontaneously broken global symmetry at a scale $f\sim$ TeV. In this paper we analyze a general class of these models based on the coset SO(6)/SO(5). Assuming the existence of light and weakly coupled spin-1 and spin-1/2 resonances which mix linearly with the elementary SM particles, we are able to compute the effective potential of the theory by means of some generalized Weinberg sum rules. The properties of the Higgs boson, DM, top quark and the above resonances are thus calculable and tightly connected. We perform a wide phenomenological analysis, considering both collider physics at the LHC and astrophysical observables. We find that these models are tightly constrained by present experimental data, which are able to completely exclude the most natural setup with $f\simeq 800$ GeV. Upon increasing the value of $f$, an allowed region appears. In particular for $f\simeq 1.1$ TeV we find a concrete realization that predicts $m_{DM}\simeq 200$ GeV for the DM mass. This DM candidate lies close to the present sensitivity of direct detection experiments and will be ruled out - or discovered - in the near future.Comment: 36 pages + 2 appendices, 9 figure