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Dive into the research topics where Stephen M. West is active.

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Featured researches published by Stephen M. West.


Journal of High Energy Physics | 2010

Freeze-In Production of FIMP Dark Matter

Lawrence J. Hall; Karsten Jedamzik; John March-Russell; Stephen M. West

We propose an alternate, calculable mechanism of dark matter genesis, “thermal freeze-in”, involving a Feebly Interacting Massive Particle (FIMP) interacting so feebly with the thermal bath that it never attains thermal equilibrium. As with the conventional “thermal freeze-out” production mechanism, the relic abundance reflects a combination of initial thermal distributions together with particle masses and couplings that can be measured in the laboratory or astrophysically. The freeze-in yield is IR dominated by low temperatures near the FIMP mass and is independent of unknown UV physics, such as the reheat temperature after inflation. Moduli and modulinos of string theory compactifications that receive mass from weak-scale supersymmetry breaking provide implementations of the freeze-in mechanism, as do models that employ Dirac neutrino masses or GUT-scale-suppressed interactions. Experimental signals of freeze-in and FIMPs can be spectacular, including the production of new metastable coloured or charged particles at the LHC as well as the alteration of big bang nucleosynthesis.


Physical Review Letters | 2009

Kerr Black Holes as Particle Accelerators to Arbitrarily High Energy

Maximo Banados; Joseph Silk; Stephen M. West

We show that intermediate mass black holes conjectured to be the early precursors of supermassive black holes and surrounded by relic cold dark matter density spikes can act as particle accelerators with collisions, in principle, at arbitrarily high center-of-mass energies in the case of Kerr black holes. While the ejecta from such interactions will be highly redshifted, we may anticipate the possibility of a unique probe of Planck-scale physics.


Physics of the Dark Universe | 2015

Simplified models for dark matter searches at the LHC

J. Abdallah; H.M. Araújo; Alexandre Arbey; A. Ashkenazi; Alexander Belyaev; J. Berger; Celine Boehm; A. Boveia; A. J. Brennan; Jim J Brooke; O. L. Buchmueller; Matthew S. Buckley; Giorgio Busoni; Lorenzo Calibbi; S. Chauhan; Nadir Daci; Gavin Davies; Isabelle De Bruyn; Paul de Jong; Albert De Roeck; Kees de Vries; D. Del Re; Andrea De Simone; Andrea Di Simone; C. Doglioni; Matthew J. Dolan; Herbi K. Dreiner; John Ellis; Sarah Catherine Eno; E. Etzion

This document outlines a set of simplified models for dark matter and its interactions with Standard Model particles. It is intended to summarize the main characteristics that these simplified models have when applied to dark matter searches at the LHC, and to provide a number of useful expressions for reference. The list of models includes both s-channel and t-channel scenarios. For s-channel, spin-0 and spin-1 mediation is discussed, and also realizations where the Higgs particle provides a portal between the dark and visible sectors. The guiding principles underpinning the proposed simplified models are spelled out, and some suggestions for implementation are presented.


Physics Letters B | 2005

Asymmetric sneutrino dark matter and the Ωb/ΩDM puzzle

Dan Hooper; John March-Russell; Stephen M. West

Abstract The inferred values of the cosmological baryon and dark matter densities are strikingly similar, but in most theories of the early universe there is no true explanation of this fact; in particular, the baryon asymmetry and thus density depends upon unknown, and a priori unknown and possibly small, CP-violating phases which are independent of all parameters determining the dark matter density. We consider models of dark matter possessing a particle–antiparticle asymmetry where this asymmetry determines both the baryon asymmetry and strongly effects the dark matter density, thus naturally linking Ω b and Ω dm . We show that sneutrinos can play the role of such dark matter in a previously studied variant of the MSSM in which the light neutrino masses result from higher-dimensional supersymmetry-breaking terms.


Physics Letters B | 2009

WIMPonium and Boost Factors for Indirect Dark Matter Detection

John March-Russell; Stephen M. West

Abstract We argue that WIMP dark matter can annihilate via long-lived “WIMPonium” bound states in reasonable particle physics models of dark matter (DM). WIMPonium bound states can occur at or near threshold leading to substantial enhancements in the DM annihilation rate, closely related to the Sommerfeld effect. Large “boost factor” amplifications in the annihilation rate can thus occur without large density enhancements, possibly preferring colder less dense objects such as dwarf galaxies as locations for indirect DM searches. The radiative capture to and transitions among the WIMPonium states generically lead to a rich energy spectrum of annihilation products, with many distinct lines possible in the case of 2-body decays to γγ or γZ final states. The existence of multiple radiative capture modes further enhances the total annihilation rate, and the detection of the lines would give direct over-determined information on the nature and self-interactions of the DM particles.


Journal of High Energy Physics | 2004

TeV scale resonant leptogenesis from supersymmetry breaking

Thomas Hambye; John March-Russell; Stephen M. West

We propose a model of TeV-scale resonant leptogenesis based upon recent models of the generation of light neutrino masses from supersymmetry-breaking effects with TeV-scale right-handed (rhd) neutrinos, Ni. The model leads to large cosmological lepton asymmetries via the resonant behaviour of the one-loop self-energy contribution to Ni decay. Our model addresses the primary problems of previous phenomenological studies of low-energy leptogenesis: a rational for TeV-scale rhd neutrinos with small Yukawa couplings so that the out-of equilibrium condition for Ni decay is satisfied; the origin of the tiny, but non-zero mass splitting required between at least two Ni masses; and the necessary non-trivial breaking of flavour symmetries in the rhd neutrino sector. The low mass-scale of the rhd neutrinos and their superpartners, and the TeV-scale A-terms automatically contained within the model offer opportunities for partial direct experimental tests of this leptogenesis mechanism at future colliders.


Physical Review D | 2011

Emergent Flux from Particle Collisions Near a Kerr Black Hole

Maximo Banados; Babiker Hassanain; Joseph Silk; Stephen M. West

The escape fraction at infinity is evaluated for massless particles produced in collisions of weakly interacting particles accreted into a density spike near the particle horizon of an extremal Kerr black hole, for the case of equatorial orbits. We compare with the Schwarzschild case, and argue that in the case of extremal black holes, redshifted signatures can be produced that could potentially explore the physics of particle collisions at center of mass energies that extend beyond those of any feasible terrestrial accelerator.


Physical Review D | 2010

Light WIMPs in the Sun: Constraints from Helioseismology

Daniel T. Cumberbatch; A. Guzik; Joseph Silk; L. Scott Watson; Stephen M. West

We calculate solar models including dark matter (DM) weakly-interacting massive particles (WIMPs) of mass 5-50 GeV and test these models against helioseismic constraints on sound speed, convection zone depth, convection zone helium abundance, and small separations of low-degree pmodes. Our main conclusion is that both direct detection experiments and particle accelerators may be complemented by using the Sun as a probe for WIMP DM particles in the 5-50 GeV mass range. The DM most sensitive to this probe has suppressed annihilations and a large spin-dependent elastic scattering cross section. For the WIMP cross-section parameters explored here, the lightest WIMP masses < 10 GeV are ruled out by constraints on core sound speed and low-degree frequency spacings. For WIMP masses 30-50 GeV, the changes to the solar structure are conned to the inner 4% of the solar radius and so do not signicantly aect the solar p-modes. Future helioseismology observations, most notably involving g-modes, and future solar neutrino experiments may be able to constrain the allowable DM parameter space in a mass range that is of current interest for direct detection.


Physics Letters B | 2004

A simple model of neutrino masses from supersymmetry breaking

John March-Russell; Stephen M. West

Abstract We analyze a class of supersymmetric models first introduced by Arkani-Hamed et al. and Borzumati et al. in which the light neutrino masses result from higher-dimensional supersymmetry-breaking terms in the MSSM super-potentials and Kahler potentials. The mechanism is closely related to the Giudice–Masiero mechanism for the MSSM μ parameter, and leads to TeV-scale right-handed neutrino and sneutrino states, that are in principle accessible to direct experimental study. The dominant contribution to the light neutrino (Majorana) mass matrix is a one-loop term induced by a lepton-number violating B-term for the sneutrino states that is naturally present. We focus upon the simplification and analysis of the flavour structure of this general class of models, finding that simple and novel origins for the light neutrino mass matrix are possible. We find that a subdominant tree-level ‘see-saw’ contribution may lead to interesting perturbations of the leading one-loop-induced flavour structure, possibly generating the small ratio Δmsolar2/Δmatm2 dynamically.


Physical Review D | 2012

Regenerating WIMPs in the light of direct and indirect detection

A. J. Williams; Céline Bœhm; Stephen M. West; D. Albornoz Vásquez

There are several ways to explain the dark matter relic density other than by the ordinary freeze-out scenario. For example, the freeze-in mechanism may constitute an alternative for generating the correct relic density for dark matter candidates whose predicted freeze-out abundance is too low due to a large total annihilation cross section. Here we show that although such a mechanism could explain why a dark matter candidate has the correct relic density, some candidates may still be ruled out because they would lead to a large gamma ray flux in dwarf spheroidal galaxies or a large elastic scattering rate in direct detection experiments. To investigate this scenario we examine neutralino dark matter in the MSSM. However our conclusions can be generalised to other types of annihilating DM candidates with a low relic density in the freeze-out scenario but which have their relic densities generated by some other mechanism.

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Joseph Silk

Johns Hopkins University

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Lawrence J. Hall

Lawrence Berkeley National Laboratory

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Edward Hardy

International Centre for Theoretical Physics

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