Vacua and instantons of ghost-free massive gravity
Recently discovered models of ghost-free massive gravity and bigravity are characterized by a nontrivial potential that gives rise to a rich vacuum structure. We review maximally symmetric vacua of the de Rham-Gabadadze-Tolley (dRGT) massive gravity and of the Hassan-Rosen (HR) bigravity and discuss their perturbative stability. In particular, we discuss perturbations about self-accelerating vacua in HR bigravity and argue that, analogously to what was found in the case of dRGT gravity, some of them contain strongly coupled modes. We then show that it is impossible to construct regular instantons connecting different classically stable vacua of dRGT gravity without violating energy conservation or the null energy condition.

title: Self-accelerating Warped Braneworlds
abstract: Braneworld models with induced gravity have the potential to replace dark energy as the explanation for the current accelerating expansion of the Universe. The original model of Dvali, Gabadadze, and Porrati (DGP) demonstrated the existence of a 'self-accelerating' branch of background solutions, but suffered from the presence of ghosts. We present a new large class of braneworld models which generalize the DGP model. Our models have negative curvature in the bulk, allow a second brane, and have general brane tensions and localized curvature terms. We exhibit three different kinds of ghosts, associated to the graviton zero mode, the radion, and the longitudinal components of massive graviton modes. The latter two species occur in the DGP model, for negative and positive brane tension, respectively. In our models, we find that the two kinds of DGP ghosts are tightly correlated with each other, but are not always linked to the feature of self-acceleration. Our models are a promising laboratory for understanding the origins and physical meaning of braneworld ghosts, and perhaps for eliminating them altogether.

title: Phenomenology of braneworld gravity

title: Sudden variations in the speed of sound during inflation: features in the power spectrum and bispectrum
abstract: We employ the formalism of the effective field theory of inflation to study the effects of a sudden change in the speed of sound of the inflationary perturbations. Such an event generates a feature with high frequency oscillations both in the two- and in the three-point functions of the curvature fluctuations. We study, at first order in the magnitude of the change of the speed of sound, the dependence of the power spectrum and of the bispectrum on the duration of the change. In the limit of a very short duration, the oscillations in the power spectrum persist up to very large momenta and the amplitude of the feature in the bispectrum diverges while its location moves to increasing momenta.

title: Unified approach to cosmic acceleration
abstract: We present a unified framework for the study of late time cosmic acceleration. Using methods of effective field theory, we show that existing proposals for late time acceleration can be subsumed in a single framework, rather than many compartmentalized theories. We construct the most general action consistent with symmetry principles, derive the background and perturbation evolution equations, and demonstrate that for special choices of our parameters we can reproduce results already existing in the literature. Lastly, we lay the foundation for future work placing phenomenological constraints on the parameters of the effective theory. Although in this paper we focus on late time acceleration, our construction also generalizes the effective field theory of inflation to the scalar-tensor and multifield case for perturbatively constructed backgrounds.

title: Quantum aspects of massive gravity
abstract: We consider the effect of quantum interactions on Pauli–Fierz massive gravity. With generic graviton cubic interactions, we observe that the 1-loop counterterms do not conform to the tree level structure of Pauli–Fierz action, resulting in the reappearance of the sixth mode ghost. Then to explore the quantum effects to the full extent, we calculate the resummed graviton propagator with an arbitrary interaction and analyze its complete structure, from which a minimal condition for the absence of the ghost is obtained.

title: The role of interference in unraveling the ZZ-couplings of the newly discovered boson at the LHC
abstract: We present a general procedure for measuring the tensor structure of the coupling of the scalar Higgs-like boson recently discovered at the LHC to two Z bosons, including the effects of interference among different operators. To motivate our concern with this interference, we explore the parameter space of the couplings in the effective theory describing these interactions and illustrate the effects of interference on the differential dilepton mass distributions. Kinematic discriminants for performing coupling measurements that utilize the effects of interference are developed and described. We present projections for the sensitivity of coupling measurements that use these discriminants in future LHC operation in a variety of physics scenarios.

title: Fermi Gamma Ray Line at 130 GeV from Axion-Mediated Dark Matter
abstract: We consider a singlet Dirac fermion with Peccei-Quinn(PQ) symmetry as dark matter. A singlet complex scalar is introduced to mediate between dark matter and the SM through Higgs portal interaction and electroweak PQ anomalies. We show that a resonant annihilation of dark matter with axion mediation can explain the monochromatic photon line of the Fermi LAT data at 130GeV by anomaly interactions while the annihilation cross section with Higgs portal interaction is p-wave suppressed. We discuss the interplay between the direct detection of the fermion dark matter and the collider search of Higgs-like scalars. We also present a ultra-violet completion of the dark matter model into the NMSSM with PQ symmetry.

title: Multi-lepton signals of multiple Higgs bosons
abstract: A bstractWe identify and investigate novel multi-lepton signatures of extended Higgs sectors at the LHC in the guise of CP- and flavor-conserving two-Higgs-doublet models (2HDMs). Rather than designing individual searches tailored to specific 2HDM signals, we employ the combination of many exclusive multi-lepton search channels to probe the collective signal from the totality of production and decay processes. Multi-lepton signals of 2HDMs can arise from a variety of sources, including Standard Model-like production of the CP-even scalars, h and H, through gluon-fusion with h, H → ZZ(∗), or associated production with vector bosons or top quarks, with h, H → WW(∗), ZZ(∗), ττ. Additional sources include gluon-fusion production of the heavy CP-even scalar with cascade decays through the light CP-even scalar, the CP-odd scalar, A, or the charged scalar, H±, such as H → hh, H → AA, H → H+H−, H → ZA, with A → Zh, ττ, H± → Wh, and h → WW∗, ZZ∗, ττ. Altogether, the combined multi-lepton signal may greatly exceed that of the Standard Model Higgs boson and provides a sensitive probe of extended Higgs sectors over a wide range of parameters. As a proof of principle, we use a factorized mapping procedure between model parameters and signatures to determine multi-lepton sensitivities in four different flavor conserving 2HDM parameter spaces by simulating the acceptance times efficiency in 20 exclusive multi-lepton channels for 222 independent production and decay topologies that arise for four benchmark 2HDM spectra within each parameter space. A comparison of these sensitivities with the results of a multi-lepton search conducted by the CMS collaboration using 5 fb−1 of data collected from 7 TeV pp collisions yields new limits in some regions of 2HDM parameter space that have not previously been covered by other types of direct experimental searches.

title: Searching for $t \to c h$ with Multi-Leptons
abstract: The results of a multi-lepton search conducted by the CMS collaboration with 5 fb$^{-1}$ of data collected from 7 TeV $pp$ collisions are used to place the first bound on the rare flavor-changing decay of the top quark to a Higgs boson and charm quark. Combining results from a number of exclusive three- and four-lepton search channels yields an estimated upper limit of ${\rm Br}(t \to ch) < 2.7%$ for a Higgs boson mass of 125 GeV. The sensitivity of future dedicated searches for $t \to c h$ could be improved by adding exclusive same sign di-lepton channels, as well as by sub-dividing channels based on $b$-quark tagging and partial kinematic top quark and Higgs boson tagging. This bound may be interpreted more widely within a range of new physics processes that yield final states with a $W$-boson in association with a Higgs boson. For such processes with kinematics that are similar to top--anti-top production and decay, the estimated limit on cross section times branching ratio corresponds to roughly $\sigma \cdot {\rm Br}(pp \to WhX) < 9$ pb.

title: Interplay between Fermi gamma-ray lines and collider searches
abstract: A bstractWe explore the interplay between lines in the gamma-ray spectrum and LHC searches involving missing energy and photons. As an example, we consider a singlet Dirac fermion dark matter with the mediator for Fermi gamma-ray line at 130 GeV. A new chiral or local U(1) symmetry makes weak-scale dark matter natural and provides the axion or Z′ gauge boson as the mediator connecting between dark matter and electroweak gauge bosons. In these models, the mediator particle can be produced in association with a monophoton at colliders and it produces large missing energy through the decays into a DM pair or ZZ, Zγ with at least one Z decaying into a neutrino pair. We adopt the monophoton searches with large missing energy at the LHC and impose the bounds on the coupling and mass of the mediator field in the models. We show that the parameter space of the Z′ mediation model is already strongly constrained by the LHC 8TeV data, whereas a certain region of the parameter space away from the resonance in axion-like mediator models are bounded. We foresee the monophoton bounds on the Z′ and axion mediation models at the LHC 14 TeV.