Diphotons, New Vacuum Angles, and Strong CP

The Standard Model contains a well-understood, natural, spin-0 diphoton resonance: the $\pi^0$. Numerous studies have pointed out that the hint of a new diphoton resonance at 750 GeV could be a pion analog, identified with the pseudo-Nambu-Goldstone boson of a chiral symmetry spontaneously broken by new strong dynamics at the TeV scale. These "hypercolor" models are generically expected to violate parity through a topological angle $\tilde\theta$. We discuss the physics of $\tilde\theta$ and its impact on the phenomenology of the new sector. We also describe some of the theoretical implications of a nonzero $\tilde\theta$. In particular, $\tilde\theta$ can generate an ${\cal O}(1)$ threshold correction to the QCD vacuum angle $\theta$ near the TeV scale, sharply constraining ultraviolet solutions to the strong CP problem. Alternatively, finding that $\tilde\theta$ is small may be interpreted as evidence in favor of UV solutions to strong CP, particularly those based on spontaneously broken P or CP symmetries.Comment: 23 pages, 6 figures. v2: references added, fig 1 update

Diphotons from Tetraphotons in the Decay of a 125 GeV Higgs at the LHC

Recently the ATLAS and CMS experiments have presented data hinting at the presence of a Higgs boson at $m_h\simeq125$ GeV. The best-fit $h\rightarrow\gamma\gamma$ rate averaged over the two experiments is approximately $2.1\pm0.5$ times the Standard Model prediction. We study the possibility that the excess relative to the Standard Model is due to $h\rightarrow aa$ decays, where $a$ is a light pseudoscalar that decays predominantly into $\gamma\gamma$. Although this process yields $4\gamma$ final states, if the pseudoscalar has a mass of the order tens of MeV, the two photons from each $a$ decay can be so highly collimated that they may be identified as a single photon. Some fraction of the events then contribute to an effective $h\rightarrow\gamma\gamma$ signal. We study the constraints on the parameter space where the net $h\rightarrow\gamma\gamma$ rate is enhanced over the Standard Model by this mechanism and describe some simple models that give rise to the pseudoscalar-photon interaction. Further tests and prospects for searches in the near future are discussed.Comment: 14 pages, 7 figures, revtex4-1; v2: references added and rearranged, g-2 limit improved, published version; v3: typos correcte

Muon Capture Constraints on Sterile Neutrino Properties

We show that ordinary and radiative muon capture impose stringent constraints on sterile neutrino properties. In particular, we consider a sterile neutrino with a mass between 40 to $80 {\rm MeV}$ that has a large mixing with the muon neutrino and decays predominantly into a photon and light neutrinos due to a large transition magnetic moment. Such a model was suggested as a possible resolution to the puzzle presented by the results of the LSND, KARMEN, and MiniBooNE experiments. We find that the scenario with the radiative decay to massless neutrinos is ruled out by measurements of the radiative muon capture rates at TRIUMF in the relevant mass range by a factor of a few in the squared mixing angle. These constraints are complementary to those imposed by the process of electromagnetic upscattering and de-excitation of beam neutrinos inside the neutrino detectors induced by a large transition magnetic moment. The latter provide stringent constraints on the size of the transitional magnetic moment between muon, electron neutrinos and $N$. We also show that further extension of the model with another massive neutrino in the final state of the radiative decay may be used to bypass the constraints derived in this work.Comment: 5 pages, 5 figures, revtex4-1. v2: updated to consider anisotropic sterile neutrino decay and a way of relaxing the RMC constraints by introducing another massive sterile neutrino; improved estimate of decay probability in targe