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

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

Supergravity and matrix theory do not disagree on multi-graviton scattering

We compare the amplitudes for the long-distance scattering of three gravitons in eleven dimensional supergravity and matrix theory at finite N. We show that the leading supergravity term arises from loop contributions to the matrix theory effective action that are not required to vanish by supersymmetry. We evaluate in detail one type of diagram---the setting sun with only massive propagators---reproducing the supergravity behavior.Comment: 10 pages, 1 eps figure, it requires JHEP.cl

Limits on anomalous top quark gauge couplings from Tevatron and LHC data

AbstractWe review and update current limits on possible anomalous couplings of the top quark to W gauge bosons. We consider data from top quark decay (as encoded in the W-boson helicity fractions) and single-top production (in the t-, s- and Wt-channels). We find improved limits with respect to previous results (in most cases of almost one order of magnitude) and extend the analysis to include four-quark operators. We find that new physics is constrained to live above an energy scale between 430 GeV and 3.2 TeV, depending on the form of its contribution

Mono-chromatic single photon events at the muon collider

The cross section for lepton pair annihilation into a photon and a dark photon or an axion-like particle is constant for large center-of-mass energies because some of the portal operators coupling Standard Model and dark sector are proportional to the energy. Feebly coupled though they are, these portal operators will be enhanced by the large center-of-mass energy made available by a muon collider and thus provide the ideal example of possible physics beyond the Standard Model to be studied with such a machine. We discuss the characteristic signature of the presence of these operators: mono-chromatic single photon events for the two benchmarks of having center-of-mass energies of 3 and 10 TeV and integrated luminosity of, respectively, 1 and 10 ab$^{-1}$. We find that an effective scale of the portal operator as large as $\Lambda=112$ TeV for an axion-like particle and $\Lambda=141$ TeV for a dark photon can be separated from the background with a confidence level of 95% in the first benchmark; these interaction scales can be raised to $\Lambda=375$ TeV and $\Lambda=459$ TeV in the case of the second benchmark. The signal for the pseudo-scalar particle can be distinguished from that of the spin-1 with about 200 events. The response of the detector to high-energy photons is examined.Comment: 12 pages, 9 figure