Gabe Shaughnessy

CERN LHC phenomenology of an extended standard model with a real scalar singlet

Gauge singlet extensions of the standard model (SM) scalar sector may help remedy its theoretical and phenomenological shortcomings while solving outstanding problems in cosmology. Depending on the symmetries of the scalar potential, such extensions may provide a viable candidate for the observed relic density of cold dark matter or a strong first order electroweak phase transition needed for electroweak baryogenesis. Using the simplest extension of the SM scalar sector with one real singlet field, we analyze the generic implications of a singlet-extended scalar sector for Higgs boson phenomenology at the Large Hadron Collider (LHC). We consider two broad scenarios: one in which the neutral SM Higgs and singlet mix and the other in which no mixing occurs and the singlet can be a dark matter particle. For the first scenario, we analyze constraints from electroweak precision observables and their implications for LHC Higgs phenomenology. For models in which the singlet is stable, we determine the conditions under which it can yield the observed relic density, compute the cross sections for direct detection in recoil experiments, and discuss the corresponding signatures at the LHC.

Singlet Higgs phenomenology and the electroweak phase transition

We study the phenomenology of gauge singlet extensions of the Standard Model scalar sector and their implications for the electroweak phase transition. We determine the conditions on the scalar potential parameters that lead to a strong first order phase transition as needed to produce the observed baryon asymmetry of the universe. We analyze the constraints on the potential parameters derived from Higgs boson searches at LEP and electroweak precision observables. For models that satisfy these constraints and that produce a strong first order phase transition, we discuss the prospective signatures in future Higgs studies at the Large Hadron Collider and a Linear Collider. We argue that such studies will provide powerful probes of phase transition dynamics in models with an extended scalar sector.

Complex singlet extension of the standard model

We analyze a simple extension of the standard model (SM) obtained by adding a complex singlet to the scalar sector (cxSM). We show that the cxSM can contain one or two viable cold dark matter candidates and analyze the conditions on the parameters of the scalar potential that yield the observed relic density. When the cxSM potential contains a global U(1) symmetry that is both softly and spontaneously broken, it contains both a viable dark matter candidate and the ingredients necessary for a strong first order electroweak phase transition as needed for electroweak baryogenesis. We also study the implications of the model for discovery of a Higgs boson at the Large Hadron Collider.

PAMELA and dark matter

Assuming that the positron excess in PAMELA satellite data is a consequence of annihilations of cold dark matter, we consider from a model-independent perspective if the data show a preference for the spin of dark matter, and find that they do not. We then perform a general analysis of annihilations into two-body states to determine what weighted combination of channels best describes the data.

Dark matter: The leptonic connection

Abstract Recent observations of high-energy positrons and electrons by the PAMELA and ATIC experiments may be an indication of the annihilation of dark matter into leptons and not quarks. This leptonic connection was foreseen already some years ago in two different models of radiative neutrino mass. We discuss here the generic interactions ( ν η 0 − l η + ) χ and l c ζ − χ c which allow this to happen, where χ and/or χ c are fermionic dark-matter candidates. We point out in particular the importance of χ χ → l + l − γ to both positron and gamma-ray signals within this framework.

Higgs-Pair Production and Measurement of the Triscalar Coupling at LHC(8,14)

Abstract We simulate the measurement of the triscalar Higgs coupling at LHC ( 8 , 14 ) via pair production of h (125 GeV). We find that the most promising hh final state is b b ¯ γ γ . We account for deviations of the triscalar coupling from its SM value and study the effects of this coupling on the hh cross-section and distributions with cut-based and multivariate methods. Our fit to the hh production matrix element at LHC(14) with 3 ab − 1 yields a 30% uncertainty on this coupling in the SM and a range of 20–60% uncertainties for non-SM values.

Dark side of the Higgs boson

Current limits from the Large Hadron Collider exclude a standard model-like Higgs mass above 150 GeV, by placing an upper bound on the Higgs production rate. We emphasize that, alternatively, the limit could be interpreted as a lower bound on the total decay width of the Higgs boson. If the invisible decay width of the Higgs is of the same order as the visible decay width, a heavy Higgs boson could be consistent with null results from current searches. We propose a method to infer the invisible decay of the Higgs by using the width of the measured h {yields} ZZ {yields} 4 {ell} line shape, and study the effect on the width extraction due to a reduced signal strength. Assuming the invisible decay product is the dark matter, we show that minimal models are tightly constrained by limits from Higgs searches at the LHC and direct detection experiments of dark matter, unless the relic density constraint is relaxed.

Characteristics and Estimates of Double Parton Scattering at the Large Hadron Collider

We evaluate the kinematic distributions in phase space of 4-parton final-state subprocesses produced by double parton scattering, and we contrast these with the final-state distributions that originate from conventional single parton scattering. Our goal is to establish the distinct topologies of events that arise from these two sources and to provide a methodology for experimental determination of the relative magnitude of the double parton and single parton contributions at Large Hadron Collider energies. We examine two cases in detail, the bb{sup -} jet-jet and the 4 jet final states. After full parton-level simulations, we identify a few variables that separate the two contributions remarkably well, and we suggest their use experimentally for an empirical measurement of the relative cross section. We show that the double parton contribution falls off significantly more rapidly with the transverse momentum p{sub T}{sup j1} of the leading jet, but, up to issues of the relative normalization, may be dominant at modest values of p{sub T}{sup j1}.

The WIMP Forest: Indirect Detection of a Chiral Square

We consider the signals of indirect dark matter detection resulting from a theory of two universal extra dimensions compactified on a chiral square. Aside from the continuum emission, which is a generic prediction of most dark matter candidates, we find a series of prominent annihilation lines (a WIMP forest) that, after convolution with the angular resolution of current experiments, leads to a distinctive (2-bump plus continuum) spectrum, which may be visible in the near future with the Fermi Gamma-Ray Space Telescope (formerly known as GLAST).

Complex scalar dark matter vis-a-vis CoGeNT, DAMA/LIBRA, and XENON100.

The CoGeNT and DAMA/LIBRA experiments have found evidence for the spin-independent scattering from nuclei of a light dark matter (DM) particle, 7-12 GeV, which is not excluded by the XENON DM experiments. We show that this putative DM signal can be explained by a complex scalar singlet extension of the standard model (CSM), with a thermal cosmological DM density, and a Higgs sector that is consistent with LEP constraints. We make predictions for the masses, production, and decays of the two Higgs mass eigenstates and describe how the Higgs and DM particles can be discovered at the LHC.