arXiv subjects

Gabe Shaughnessy

Publications and source records attributed to Gabe Shaughnessy.

At least 19 recordsLinked to original sources

arXiv2021

Have We Observed the Higgs (Imposter)?

We interpret the new particle at the Large Hadron Collider as a CP-even scalar and investigate its electroweak quantum number. Assuming an unbroken custodial invariance as suggested by precision electroweak measurements, only four possibilities are allowed if the scalar decays to pairs of gauge bosons, as exemplified by a dilaton/radion, a non-dilatonic electroweak singlet scalar, an electroweak doublet scalar, and electroweak triplet scalars. We show that current LHC data already strongly disfavor both the dilatonic and non-dilatonic singlet imposters. On the other hand, a generic Higgs doublet give excellent fits to the measured event rates of the newly observed scalar resonance, while the Standard Model Higgs boson gives a slightly worse overall fit due to the lack signal in the tau tau channel. The triplet imposter exhibits some tension with the data. The global fit indicates the enhancement in the diphoton channel could be attributed to an enhanced partial decay width, while the production rates are consistent with the Standard Model expectations. We emphasize that more precise measurements of the ratio of event rates in the WW over ZZ channels, as well as the event rates in b bbar and tau tau channels, are needed to further distinguish the Higgs doublet from the triplet imposter.

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arXiv2014

Measuring the 2HDM Scalar Potential at LHC14

After the extraordinary discovery of the Higgs boson at the LHC, the next goal is to pin down its underlying dynamics by measuring the Higgs self-couplings, along with its couplings to gauge and matter particles. As a prototype model of new physics in the scalar sector, we consider the Two Higgs Doublet Model (2HDM) with CP-conservation, and evaluate the prospects for measuring the trilinear scalar couplings among the CP-even Higgs bosons $h$ and $H$ ($λ^{hhh}$, $λ^{hhH}$, $λ^{hHH}$) at LHC14. The continuum and resonant production of CP-even Higgs boson pairs, $hh$ and $hH$, offer complementary probes of the scalar potential away from the light-Higgs decoupling limit. We identify the viable search channels at LHC14 and estimate their expected discovery sensitivities.

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arXiv2014

New physics in resonant production of Higgs boson pairs

We advocate a search for an extended scalar sector at the LHC via $hh$ production, where $h$ is the 125 GeV Higgs boson. A resonance feature in the $hh$ invariant mass is a smoking gun of an $s$-channel heavy Higgs resonance, $H$. With one $h$ decaying to two photons and the other decaying to $b$-quarks, the resonant signal may be discoverable above the $hh$ continuum background for $M_H<$ 1 TeV. The product of the scalar and top Yukawa couplings can be measured to better than $10-20\%$ accuracy, and its sign can be inferred from the $hh$ lineshape via interference effects.

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arXiv2013

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

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 bb gamma gamma. 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 40% uncertainty on this coupling in the SM and a range of 25-80% uncertainties for non-SM values.

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arXiv2013

Gamma Rays from Top-Mediated Dark Matter Annihilations

Lines in the energy spectrum of gamma rays are a fascinating experimental signal, which are often considered "smoking gun" evidence of dark matter annihilation. The current generation of gamma ray observatories are currently closing in on parameter space of great interest in the context of dark matter which is a thermal relic. We consider theories in which the dark matter's primary connection to the Standard Model is via the top quark, realizing strong gamma ray lines consistent with a thermal relic through the forbidden channel mechanism proposed in the Higgs in Space Model. We consider realistic UV-completions of the Higgs in Space and related theories, and show that a rich structure of observable gamma ray lines is consistent with a thermal relic as well as constraints from dark matter searches and the LHC. Particular attention is paid to the one loop contributions to the continuum gamma rays, which can easily swamp the line signals in some cases, and have been largely overlooked in previous literature.

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arXiv2013

Gamma-ray lines and One-Loop Continuum from s-channel Dark Matter Annihilations

The era of indirect detection searches for dark matter has begun, with the sensitivities of gamma-ray detectors now approaching the parameter space relevant for weakly interacting massive particles. In particular, gamma ray lines would be smoking gun signatures of dark matter annihilation, although they are typically suppressed compared to the continuum. In this paper, we pay particular attention to the 1-loop continuum generated together with the gamma-ray lines and investigate under which conditions a dark matter model can naturally lead to a line signal that is relatively enhanced. We study generic classes of models in which DM is a fermion that annihilates through an s-channel mediator which is either a vector or scalar and identify the coupling and mass conditions under which large line signals occur. We focus on the "forbidden channel mechanism" advocated a few years ago in the "Higgs in space" scenario for which tree level annihilation is kinematically forbidden today. Detailed calculations of all 1-loop annihilation channels are provided. We single out very simple models with a large line over continuum ratio and present general predictions for a large range of WIMP masses that are relevant not only for Fermi and Hess II but also for the next generation of telescopes such as CTA and Gamma-400. Constraints from the relic abundance, direct detection and collider bounds are also discussed.

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arXiv2013

Dark Matter in the Coming Decade: Complementary Paths to Discovery and Beyond

In this Report we discuss the four complementary searches for the identity of dark matter: direct detection experiments that look for dark matter interacting in the lab, indirect detection experiments that connect lab signals to dark matter in our own and other galaxies, collider experiments that elucidate the particle properties of dark matter, and astrophysical probes sensitive to non-gravitational interactions of dark matter. The complementarity among the different dark matter searches is discussed qualitatively and illustrated quantitatively in several theoretical scenarios. Our primary conclusion is that the diversity of possible dark matter candidates requires a balanced program based on all four of those approaches.

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arXiv2013

Scrutinizing h(125) in Two Higgs Doublet Models at the LHC, ILC, and Muon Collider

The discovery at the LHC of a scalar particle with properties that are so far consistent with the SM Higgs boson is one of the most important advances in the history of particle physics. The challenge of future collider experiments is to determine whether its couplings will show deviations from the SM Higgs, as this would indicate new physics at the TeV scale, and also to probe the flavor structure of the Yukawa couplings. As a benchmark alternative to the SM Higgs, we consider a generic two Higgs doublet model (2HDM) and analyze the precision to which the LHC14, an ILC250, 500, 1000 GeV and a 125 GeV Muon Collider (MC) can determine the gauge and Yukawa couplings. We allow for correlations among the couplings. We include the impact of a Higgs total width measurement, indirectly at the LHC and ILC and by a direct scan at the MC. We also discuss pattern relations among the couplings that can test for singlet or doublet Higgs extensions of 2HDMs.

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arXiv2013

Discriminators of 2 Higgs Doublets at the LHC14, ILC and MuonCollider(125): A Snowmasss White Paper

The historic LHC discovery of the 125 GeV particle with properties that closely resemble the Standard Model (SM) Higgs boson verifies our understanding of electroweak symmetry breaking, but solidifies the need for a resolution to the hierarchy problem. Many extensions of the SM that address the hierarchy problem contain a non-minimal Higgs sector. Therefore, as a benchmark alternative to the SM Higgs mechanism, we study a general 2 Higgs doublet model (2HDM-G) framework for evaluating future sensitivity to Higgs couplings. We study how well it can be distinguished from the SM Higgs boson by future measurements at LHC14, ILC (250, 500,1000 GeV) and a Muon Collider (125 GeV). Additionally, our study bears on singlet Higgs extensions of two Higgs doublet models through predicted coupling relationships.

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arXiv2013

The NMSSM at the Cosmic Frontier for Snowmass 2013

We examine the NMSSM at the cosmic frontier in the post Higgs discovery world. For DM relic abundance consistent with measurement, we find the neutralino can either be singlino or bino dominated. Wino and higgsino DM generally yield a lower abundance, but offer opportunities of detection at IceCube. For both cases, future SI direct detection experiments cover a majority of the model, including nearly all of the bino and higgsino scenarios.

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arXiv2012

2HDM Portal Dark Matter: LHC data and the Fermi-LAT 135 GeV Line

We study a two Higgs doublet model augmented by a scalar dark matter particle that provides an excellent fit to the LHC Higgs data and the Fermi-LAT 135 GeV line. The heavy CP-even Higgs boson, which predominantly mediates annihilation and scattering, must have a coupling to weak gauge bosons at or below percent level to suppress the continuum gamma-ray spectrum below the limit from the Fermi-LAT data and the anti-proton spectrum constrained by the PAMELA data. Discovering or excluding this CP-even Higgs boson at the LHC with a mass between 265 and 280 GeV and an enhanced diphoton branching ratio is crucial to test this scenario.

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arXiv2012

The 2HDM-X and Large Hadron Collider Data

We study the consistency of two Higgs doublet models in light of the new bosonic particle discovery at the LHC. We work within a general setup that we call the 2HDM-X, in which the quarks couple to both scalar doublets with aligned couplings such that flavor-changing neutral currents are absent at tree level. The 2HDM-X encompasses the traditional Type I, Type II, lepton specific, and flipped models, but also provides for more general possibilities. The best fit to the current data with a suppressed tau tau signal and a b bbar signal of Standard Model strength is given by the 2HDM-X with specific parameter choices; however, very good fits are also obtained within the lepton-specific model and a democratic model, the 2HDM-D, in which both the up-type and down-type quarks couple to each doublet with equal strengths. The approach provides a general framework in which to interpret future LHC Higgs data within extensions of the Standard Model with two Higgs doublets.

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arXiv2012

The case for three-body decaying dark matter

Fermi-LAT has confirmed the excess in cosmic positron fraction observed by PAMELA, which could be explained by dark matter annihilating or decaying in the center of the galaxy. Most existing models postulate that the dark matter annihilates or decays into final states with two or four leptons, which would produce diffuse gamma ray emissions that are in tension with data measured by Fermi-LAT. We point out that the tension could be alleviated if the dark matter decays into three-body final states with a pair of leptons and a missing particle. Using the goldstino decay in a certain class of supersymmetric theories as a prime example, we demonstrate that simultaneous fits to the total e+ + e- and the fractional e+/e- fluxes from Fermi-LAT and PAMELA could be achieved for a 2 TeV parent particle and a 1 TeV missing particle, without being constrained by gamma-ray measurements. By studying different effective operators giving rise to the dark matter decay, we show that this feature is generic for three-body decaying dark matter containing a missing particle. Constraints on the hadronic decay widths from the cosmic anti-proton spectra are also discussed.

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arXiv2012

The 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 to ZZ to 4l lineshape, 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.

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arXiv2011

Calculation of W b bbar Production via Double Parton Scattering at the LHC

We investigate the potential to observe double parton scattering at the Large Hadron Collider in p p -> W b bbar X -> l nu b bbar X at 7 TeV. Our analysis tests the efficacy of several kinematic variables in isolating the double parton process of interest from the single parton process and relevant backgrounds for the first 10 inverse fb of integrated luminosity. These variables are constructed to expose the independent nature of the two subprocesses in double parton scattering, pp -> l nu X and pp -> b bbar X. We use next-to-leading order perturbative predictions for the double parton and single parton scattering components of W b bbar and for the pertinent backgrounds. The next-to-leading order contributions are important for a proper description of some of the observables we compute. We find that the double parton process can be identified and measured with significance S/sqrt(B) ~ 10, provided the double parton scattering effective cross section sigma_{eff} ~ 12 mb.

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arXiv2011

LHC Sensitivity to Wbb Production via Double Parton Scattering

We investigate the potential to observe double parton scattering at the Large Hadron Collider in pp -> W bb X -> mu nu bb X at 7 TeV. Our analysis tests the efficacy of several kinematic variables in isolating the double parton process of interest from the single parton process and relevant backgrounds for the first 10/fb of integrated luminosity. These variables are constructed to expose the independent nature of the two subprocesses in double parton scattering, pp -> mu nu and pp -> bb X. We use next-to-leading order perturbative predictions for the double parton and single parton scattering components of W bb and for the pertinent backgrounds. The next-to-leading order contributions are important for a proper description of some of the observables we compute. We find that the double parton process can be identified and measured with significance ~ 10, provided the double parton scattering effective cross section ~ 12 mb.

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arXiv2011

Searching for the Layered Structure of Space at the LHC

Alignment of the main energy fluxes along a straight line in a target plane has been observed in families of cosmic ray particles detected in the Pamir mountains. The fraction of events with alignment is statistically significant for families with superhigh energies and large numbers of hadrons. This can be interpreted as evidence for coplanar hard-scattering of secondary hadrons produced in the early stages of the atmospheric cascade development. This phenomenon can be described within the recently proposed "crystal world," with latticized and anisotropic spatial dimensions. Planar events are expected to dominate particle collisions at a hard-scattering energy exceeding the scale Λ_3 at which space transitions from 3D \rightleftharpoons 2D. We study specific collider signatures that will test this hypothesis. We show that the energy-spectrum of Drell-Yan scattering and the parton momenta sum rule are significantly modified in this framework. At the LHC, two jet and three jet events are necessarily planar, but four jet events can test the hypothesis. Accordingly, we study in a model-independent way the 5σdiscovery reach of the ATLAS and CMS experiments for identifying four jets coplanarities. For the extreme scenario in which all pp \to 4 jet scattering processes become coplanar above Λ_3, we show that with an integrated luminosity of 10(100) fb^{-1} the LHC experiments have the potential to discover correlations between jets if Λ_3 \alt 1.25(1.6) TeV.

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arXiv2011

Singlet scalars as Higgs imposters at the Large Hadron Collider

An electroweak singlet scalar can couple to pairs of vector bosons through loop-induced dimension five operators. Compared to a Standard Model Higgs boson, the singlet decay widths in the diphotons and Z gamma channels are generically enhanced, while decays into massive final states like WW and ZZ are kinematically disfavored. The overall event rates into gamma gamma and Z gamma can exceed the Standard Model expectations by orders of magnitude. Such a singlet may appear as a resonant signal in the gamma gamma and Z gamma channels, even with a mass above the WW kinematic threshold.

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