arXiv subjects

Stephen Godfrey

Publications and source records attributed to Stephen Godfrey.

At least 19 recordsLinked to original sources

arXiv2022

Probing feebly interacting dark matter with monojet searches

Dark matter may consist of feebly interacting massive particles (FIMPs) that never thermalized with the cosmic plasma. Their relic density is achieved via freeze-in for a wide range of masses, significantly expanding the model space that can be tested compared to other production mechanisms. However, testing the tiny couplings required by freeze-in is challenging. We show that FIMPs can be probed by LHC searches for new physics in mono-jet events with large missing energy. We study a $Z'$ portal model in which gluon annihilation produces FIMPs in the early universe and today at colliders. Monojet searches by LHC Run 3 might discover new physics accounted for by FIMPs with mass in the MeV-TeV range.

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arXiv2021

Testing freeze-in with axial and vector $Z'$ bosons

The freeze-in production of Feebly Interacting Massive Particle (FIMP) dark matter in the early universe is an appealing alternative to the well-known - and constrained - Weakly Interacting Massive Particle (WIMP) paradigm. Although challenging, the phenomenology of FIMP dark matter has been receiving growing attention and is possible in a few scenarios. In this work, we contribute to this endeavor by considering a $Z^\prime$ portal to fermionic dark matter, with the $Z^\prime$ having both vector and axial couplings and a mass ranging from MeV up to PeV. We evaluate the bounds on both freeze-in and freeze-out from direct detection, atomic parity violation, leptonic anomalous magnetic moments, neutrino-electron scattering, collider, and beam dump experiments. We show that FIMPs can already be tested by most of these experiments in a complementary way, whereas WIMPs are especially viable in the $Z^\prime$ low mass regime, in addition to the $Z^\prime$ resonance region. We also discuss the role of the axial couplings of $Z^\prime$ in our results. We therefore hope to motivate specific realizations of this model in the context of FIMPs, as well as searches for these elusive dark matter candidates.

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arXiv2021

Exploring Direct Detection Suppressed Regions in a Simple 2-Scalar Mediator Model of Scalar Dark Matter

We explore regions of parameter space that give rise to suppressed direct detection cross sections in a simple model of scalar dark matter with a scalar portal that mixes with the standard model Higgs. We found that even this simple model allows considerable room in the parameter space that has not been excluded by direct detection limits. A number of effects leading to this result have been previously noted. Our main new result explores interference effects between different contributions to DM annihilation when the DM mass is larger than the scalar portal mass. New annihilation channels open up and the parameters of the model need to compensate to give the correct DM relic abundance, resulting in smaller direct detection cross sections. We find that even in a very simple model of DM there are still sizeable regions of parameter space that are not ruled out by experiment.

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arXiv2020

Implications of the observation of dark matter self-interactions for singlet scalar dark matter

Evidence for dark matter self-interactions has recently been reported based on the observation of a spatial offset between the dark matter halo and the stars in a galaxy in the cluster Abell 3827. Interpreting the offset as due to dark matter self-interactions leads to a cross section measurement of sigma_DM/m ~ (1-1.5) cm^2/g, where m is the mass of the dark matter particle. We use this observation to constrain singlet scalar dark matter coupled to the Standard Model and to two-Higgs-doublet models. We show that the most natural scenario in this class of models is very light dark matter, below about 0.1 GeV, whose relic abundance is set by freeze-in, i.e., by slow production of dark matter in the early universe via extremely tiny interactions with the Higgs boson, never reaching thermal equilibrium. We also show that the dark matter abundance can be established through the usual thermal freeze-out mechanism in the singlet scalar extension of the Yukawa-aligned two-Higgs-doublet model, but that it requires rather severe fine tuning of the singlet scalar mass.

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arXiv2019

Multi-component dark matter from a hidden gauged SU(3)

We studied Dark Matter (DM) phenomenology with multiple DM species consisting of both scalar and vector DM particles in the Hidden Gauged SU(3) model of Arcadi et al. Because of the large parameter space in the Hidden Gauged SU(3) model we restrict ourselves to three representative benchmark points, each with multiple DM species. The relic densities for the benchmark points were found using a program developed to solve the coupled Boltzmann equations for an arbitrary number of interacting DM species with two particles in the final state. For each case, we varied the mass of the DM particles and then found the value of the dark SU(3) gauge coupling that gave the correct relic density. We found that in some regions of parameter space, DM would be difficult to observe in direct detection experiments while it would be easier to observe in indirect detection experiments while for other regions of parameter space the situation was reversed. Thus, measurements from both types of experiments complement each other and could help pinpoint the details of the hidden SU(3) model.

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arXiv2019

Spectroscopic Assignments of the Excited $B$-Mesons

Excited $B$-mesons have been observed by the D0, CDF, LHCb and CMS experiments. We use the predictions of the relativized quark model to make quark model spectroscopic assignments for these states. We identify the $B_2^*(5747)$ and $B_1(5721)$ as the $B_2^*[1^3P_2]$ and $B_1[1P_1]$ states and the $B_{s2}^*(5840)$ and $B_{s1}(5830)$ as the $B_{s2}^*[1^3P_2]$ and $B_{s1}[1P_1]$ states. More information is needed to identify the $B_J(5970)$ and $B_J(5840)$ states and we suggest a number of measurements to make this identification: the determination of their $J^P$ quantum numbers and either confirming or ruling out their decays to the $Bπ$ final state. With the current information available we believe it most likely that the $B_J(5970)$ is the $B^*[2^3S_1]$ state, with the $B_J(5840)$ needing confirmation.

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arXiv2019

Multi-component dark matter from a hidden gauged SU(3)

We study Dark Matter (DM) phenomenology with multiple DM species consisting of both scalar and vector DM particles. More specifically, we study the Hidden Gauged SU(3) model of Arcadi {\it et al}. Before proceeding to the Hidden Gauged SU(3) model, we study the relic abundances of simplified multi-species DM scenarios to gain some insights when multiple species and interactions are included. In the Hidden Gauged SU(3) model, because of the large parameter space, we restrict ourselves to three representative benchmark points, each with multiple DM species. The relic densities for the benchmark points were found using a program developed to solve the coupled Boltzmann equations for an arbitrary number of interacting DM species with two particles in the final state. For each case, we varied the mass of the DM particles and then found the value of the dark SU(3) gauge coupling that gives the correct relic density. We found that in some regions of the parameter space, the DM would be difficult to observe in direct detection experiments while easier to observe in indirect detection experiments and vice versa, so that complementary measurements could help pinpoint the details of the Hidden Gauged SU(3) model. Important to this, is that even for moderate changes in input parameter values, the relative relic density of each species can change significantly resulting in large changes in the observability of multi-species DM by direct or indirect detection.

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arXiv2016

Real singlet scalar dark matter extension of the Georgi-Machacek model

The Georgi-Machacek model extends the Standard Model Higgs sector with the addition of isospin-triplet scalar fields in such a way as to preserve the custodial symmetry. The presence of higher-isospin scalars contributing to electroweak symmetry breaking offers the interesting possibility that the couplings of the 125 GeV Higgs boson to both gluons and vector boson pairs could be larger than those of the Standard Model Higgs boson. Constraining this possibility using measurements of Higgs production and decay at the CERN Large Hadron Collider is notoriously problematic if a new, non-Standard Model decay mode of the 125 GeV Higgs boson is present. We study an implementation of this scenario in which the Georgi-Machacek model is extended by a real singlet scalar dark matter candidate, and require that the singlet scalar account for all the dark matter in the universe. The combination of the observed dark matter relic density and direct detection constraints exclude singlet scalar masses below about 57 GeV. Higgs measurements are not yet precise enough to be very sensitive to $h\rightarrow SS$ in the remaining allowed kinematic region, so that constraints from Higgs measurements are so far the same as in the GM model without a singlet scalar. We also find that, above the Higgs pole, a substantial region of parameter space yielding the correct dark matter relic density can escape the near-future direct detection experiments DEAP and XENON~1T for dark matter masses as low as 120 GeV and even have a direct detection cross section below the neutrino floor for $m_S\gtrsim 150$ GeV. This is in contrast to the singlet scalar dark matter extension of the Standard Model, for which these future experiments are expected to exclude dark matter masses above the Higgs pole up to the multi-TeV range.

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arXiv2016

Project X: Physics Opportunities

Part 2 of "Project X: Accelerator Reference Design, Physics Opportunities, Broader Impacts". In this Part, we outline the particle-physics program that can be achieved with Project X, a staged superconducting linac for intensity-frontier particle physics. Topics include neutrino physics, kaon physics, muon physics, electric dipole moments, neutron-antineutron oscillations, new light particles, hadron structure, hadron spectroscopy, and lattice-QCD calculations. Part 1 is available as arXiv:1306.5022 [physics.acc-ph] and Part 3 is available as arXiv:1306.5024 [physics.acc-ph].

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arXiv2016

$B$ and $B_s$ Meson Spectroscopy

Properties of bottom and bottom-strange mesons are computed in two relativized quark models. Model masses and wavefunctions are used to predict radiative transition rates and the $^3P_0$ quark pair creation model is used to compute strong decay widths. A comparison to recently observed bottom and bottom-strange states is made. We find that there are numerous excited $B$ and $B_s$ mesons that have relatively narrow widths and significant branching ratios to simple final states such as $Bπ$, $B^*π$, $BK$, and $B^*K$ that could be observed in the near future.

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arXiv2016

The Dilaton-like Higgs boson with scalar singlet dark matter

We study a model with a Higgs-like dilaton and a Standard Model gauge-singlet scalar dark matter candidate. We begin by updating the status of identifying the observed 125 GeV Higgs-like boson with the pseudo Nambu-Goldstone boson that arises from the spontaneous breaking of scale invariance using recent Higgs boson signal strength measurements by the ATLAS and CMS collaborations. We then constrain the extended model with recent constraints on the Higgs invisible width, the observed dark matter relic abundance and the latest dark matter direct detection limits. We found that the magnitude of the dilaton-$γγ$ and dilaton-glue-glue coupling is constrained to be close to the standard model values. The mass of the dark matter candidate is contrained to be greater than half the dilaton mass by relic abundance limits and Higgs invisible width limits. Dark matter direct detection limits allow only small mass regions which will be further constrained by upcoming DEAP measurements.

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arXiv2016

Probe of new light Higgs bosons from bottomonium chi_{b0} decay

We calculate the branching ratios of chi_{b0} --> tau+ tau- via an s-channel Higgs boson and estimate the sensitivity to this process from Upsilon --> gamma chi_{b0} --> gamma tau+ tau-. We show that future running at the Upsilon(3S) at a very high luminosity super B factory can put significant constraints on the Type-II two-Higgs-doublet model when the discovered 125 GeV Higgs boson is the heavier of the two CP-even scalars.

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arXiv2016

Properties of Excited Charm and Charm-Strange Mesons

We calculate the properties of excited charm and charm-strange mesons. We use the relativized quark model to calculate their masses and wavefunctions that are used to calculate radiative transition partial widths and the $^3P_0$ quark-pair-creation model to calculate their strong decay widths. We use these results to make quark model spectroscopic assignments for recently observed charm and charm-strange mesons. In particular we find that the properties of the $D_J(2550)^0$ and $D_J^*(2600)^0$ are consistent with those of the $2^1S_0(c\bar{u})$ and the $2^3S_1(c\bar{u})$ states respectively, the $D_1^*(2760)^0$, $D_3^*(2760)^-$, and $D_J(2750)^0$ with those of the $1^3D_1(c\bar{u})$, $1^3D_3(d\bar{c})$, and $1D_2(c\bar{u})$ states respectively. We tentatively identify the $D^*_J(3000)^0$ as the $1^3F_4(c\bar{u})$ and favour the $D_J(3000)^0$ to be the $3^1S_0(c\bar{u})$ although we do not rule out the $1F_3$ and $1F_3'$ assignment. For the recently observed charm-strange mesons we identify the $D_{s1}^*(2709)^\pm$, $D_{s1}^*(2860)^-$, and $D_{s3}^*(2860)^-$ as the $2^3S_1(c\bar{s})$, $1^3D_1(s\bar{c})$, and $1^3D_3(s\bar{c})$ states respectively and suggest that the $D_{sJ}(3044)^\pm$ is most likely the $D_{s1}(2P_1')$ or $D_{s1}(2P_1)$ states although it might be the $D_{s2}^*(2^3P_2)$ with the $DK$ final state too small to be observed with current statistics. Based on the predicted properties of excited states, that they not have too large a total width and they have a reasonable branching ratio to simple final states, we suggest states that should be able to be found in the near future. We expect that the tables of properties summarizing our results will be useful for interpreting future observations of charm and charm-strange mesons.

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arXiv2015

The $D_{sJ}^*(2860)$ Mesons as Excited D-wave $c\bar{s}$ States

A new charm-strange meson, the $D_{s1}^*(2863)$, has recently been observed by the LHCb collaboration which also determined the $D_{sJ}^*(2860)$ to have spin 3. One of the speculations about the previously observed $D_{s1}^*(2710)$ is that it is the $1^3D_1(c\bar{s})$ state. In this paper we reexamine the quark model properties and assignments of these three states in light of these new measurements. We conclude that the $D_{s1}^*(2863)$ and $D_{s3}^*(2863)$ are the $1^3D_1 (c\bar{s})$ and $1^3D_3 (c\bar{s})$ states respectively and the $D_{s1}^*(2710)$ is the $2^3S_1(c\bar{s})$ state. In addition to these three states there are another three excited $D_s$ states in this mass region still to be found; the $2^1S_0$ and two $1D_2$ states. We calculate the properties of these states and expect that LHCb has the capability of observing these states in the near future.

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arXiv2015

Bottomonium Mesons and Strategies for Their Observation

The $B$-factories and Large Hadron Collider experiments have demonstrated the ability to observe and measure the properties of bottomonium mesons. In order to discover missing states it is useful to know their properties to develop a successful search strategy. To this end we calculate the masses and decay properties of excited bottomonium states. We use the relativized quark model to calculate the masses and wavefunctions and the $^3P_0$ quark-pair creation model to calculate decay widths to open bottom. We also summarize results for radiative transitions, annihilation decays, hadronic transitions and production cross sections which are used to develop strategies to find these states. We find that the $b\bar{b}$ system has a rich spectroscopy that we expect to be substantially extended by the LHC and $e^+e^-$ experiments in the near future. Some of the most promising possibilities at the LHC are observing the $χ_{b(1,2)}(3P)$, $χ_{b(1,2)}(4P)$ and $η_b(3S)$ states in $γμ^+μ^-$ final states that proceed via radiative transitions through $Υ(nS)$ intermediate states and $1^3D_J$ and $2^3D_J$ into $γγμ^+μ^-$ final states proceeding via $1^3P_J\to 1^3S_1$ and $2^3P_J\to 2^3S_1$ intermediate states respectively. Some of the most interesting possibilities in $e^+e^-$ collisions are studying the $1^3D_J$ states via $4γ$ cascades starting with the $Υ(3S)$ and the $3^3P_J$ states in $γγμ^+ μ^-$ final states starting with the $Υ(4S)$ and proceeding via $Υ(nS)$ intermediate states. Completing the bottomonium spectrum is an important validation of lattice QCD calculations and a test of our understanding of bottomonium states in the context of the quark model.

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arXiv2014

Comment on the Nature of the $D_{s1}^(2710)$ and $D_{sJ}^(2860)$ Mesons

Two charm-strange mesons, the $D_{s1}^*(2710)$ and the $D_{sJ}^*(2860)$, have recently been observed by several experiments. There has been speculation in the literature that the $D_{s1}^*(2710)$ is the $2^3S_1(c\bar{s})$ state and the $D_{sJ}^*(2860)$ is the $1^3D_1(c\bar{s})$ state. In this paper we explore this and other explanations in the context of the relativized quark model and the pseudoscalar emission decay model. We conclude that the $D_{s1}^*(2710)$ is most likely the $1^3D_1 (c\bar{s})$ state and the $D_{sJ}^*(2860)$ is most likely the $1^3D_3 (c\bar{s})$ state with the $1D_2$ resonances also contributing to the observed signals and explaining the observed ratios of branching ratios to $D^*K$ and $DK$ final states. We point out that measuring the $D_{sJ}^*(2860)$ spin can support or eliminate this explanation and that there are six excited $D_s$ states in this mass region; the $2^3S_1$, $2^1S_0$, $1^3D_1$, $1^3D_3$ and two $1D_2$ states. Observing some of the missing states would help confirm the nature of the $D_{s1}^*(2710)$ and the $D_{sJ}^*(2860)$ states.

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arXiv2013

Z' Discovery Reach at Future Hadron Colliders: A Snowmass White Paper

Extra neutral gauge bosons are a feature of many models of physics beyond the standard model (BSM) and their discovery could possibly be the first evidence for new physics. In this Snowmass white paper we compare the discovery reach of the high energy hadron colliders considered by the Snowmass study for a broad range of BSM models. It is expected that the LHC should be able to see evidence for a Z' arising from a large variety of BSM models up to a mass of ~5 TeV when the LHC reaches its design energy and luminosity, and up to ~6 TeV with the high luminosity upgrade. Further into the future, the high energy LHC would substantially extend this reach to ~11 TeV, while the ~100 TeV VHE-LHC could see evidence for Z' 's up to ~30 TeV.

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arXiv2012

Constraining Extra Neutral Gauge Bosons with Atomic Parity Violation Measurements

The discovery of a new neutral gauge boson, $Z'$, could provide the first concrete evidence of physics beyond the standard model. We explore how nuclear weak charge measurements in atomic parity violation (APV) experiments can be used to constrain $Z'$ bosons. We use the recent measurement of the $^{133}$Cs nuclear weak charge to estimate lower bounds on the mass of $Z'$ bosons for a number of representative models and to put constraints on the couplings of a newly discovered $Z'$ boson. We also consider how these constraints might be improved by future APV experiments that will measure nuclear weak charges of multiple isotopes. We show how measurements of a single isotope, and combining measurements into ratios and differences, can be used to constrain the couplings of a $Z'$ and discriminate between models. We find that current and future APV experiments could potentially play an important role in unravelling new physics if a $Z'$ were discovered.

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