Stephen M. West

Stephen M. West
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Stephen M. West

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High Energy Physics - Phenomenology (23)
Cosmology and Nongalactic Astrophysics (7)
High Energy Physics - Theory (3)
Astrophysics (3)
General Relativity and Quantum Cosmology (2)
High Energy Astrophysical Phenomena (1)
Solar and Stellar Astrophysics (1)

Publications Authored By Stephen M. West

Authors: Jalal Abdallah, Henrique Araujo, Alexandre Arbey, Adi Ashkenazi, Alexander Belyaev, Joshua Berger, Celine Boehm, Antonio Boveia, Amelia Brennan, Jim Brooke, Oliver Buchmueller, Matthew Buckley, Giorgio Busoni, Lorenzo Calibbi, Sushil Chauhan, Nadir Daci, Gavin Davies, Isabelle De Bruyn, Paul De Jong, Albert De Roeck, Kees de Vries, Daniele Del Re, Andrea De Simone, Andrea Di Simone, Caterina Doglioni, Matthew Dolan, Herbi K. Dreiner, John Ellis, Sarah Eno, Erez Etzion, Malcolm Fairbairn, Brian Feldstein, Henning Flaecher, Eric Feng, Patrick Fox, Marie-Hélène Genest, Loukas Gouskos, Johanna Gramling, Ulrich Haisch, Roni Harnik, Anthony Hibbs, Siewyan Hoh, Walter Hopkins, Valerio Ippolito, Thomas Jacques, Felix Kahlhoefer, Valentin V. Khoze, Russell Kirk, Andreas Korn, Khristian Kotov, Shuichi Kunori, Greg Landsberg, Sebastian Liem, Tongyan Lin, Steven Lowette, Robyn Lucas, Luca Malgeri, Sarah Malik, Christopher McCabe, Alaettin Serhan Mete, Enrico Morgante, Stephen Mrenna, Yu Nakahama, Dave Newbold, Karl Nordstrom, Priscilla Pani, Michele Papucci, Sophio Pataraia, Bjoern Penning, Deborah Pinna, Giacomo Polesello, Davide Racco, Emanuele Re, Antonio Walter Riotto, Thomas Rizzo, David Salek, Subir Sarkar, Steven Schramm, Patrick Skubic, Oren Slone, Juri Smirnov, Yotam Soreq, Timothy Sumner, Tim M. P. Tait, Marc Thomas, Ian Tomalin, Christopher Tunnell, Alessandro Vichi, Tomer Volansky, Neal Weiner, Stephen M. West, Monika Wielers, Steven Worm, Itay Yavin, Bryan Zaldivar, Ning Zhou, Kathryn Zurek

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. Read More

We investigate the interactions of large composite dark matter (DM) states with the Standard Model (SM) sector. Elastic scattering with SM nuclei can be coherently enhanced by factors as large as A^2, where A is the number of constituents in the composite state (there exist models in which DM states of very large A > 10^8 may be realised). This enhancement, for a given direct detection event rate, weakens the expected signals at colliders by up to 1/A. Read More

We examine the production of dark matter by decaying topological defects in the high mass region $m_{\mathrm{DM}} \gg m_W$ of the Inert Doublet Model, extended with an extra U(1) gauge symmetry. The density of dark matter states (the neutral Higgs states of the inert doublet) is determined by the interplay of the freeze-out mechanism and the additional production of dark matter states from the decays of topological defects, in this case cosmic strings. These decays increase the predicted relic abundance compared to the standard freeze-out only case, and as a consequence the viable parameter space of the Inert Doublet Model can be widened substantially. Read More

We investigate the physics of dark matter models featuring composite bound states carrying a large conserved dark "nucleon" number. The properties of sufficiently large dark nuclei may obey simple scaling laws, and we find that this scaling can determine the number distribution of nuclei resulting from Big Bang Dark Nucleosynthesis. For plausible models of asymmetric dark matter, dark nuclei of large nucleon number, e. Read More

We study dark matter production by decaying topological defects, in particular cosmic strings. In topological defect or "top-down" (TD) scenarios, the dark matter injection rate varies as a power law with time with exponent $p-4$. We find a formula in closed form for the yield for all $p < 3/2$, which accurately reproduces the solution of the Boltzmann equation. Read More

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. Read More

It is argued that experimental constraints on theories of asymmetric dark matter (ADM) almost certainly require that the DM be part of a richer hidden sector of interacting states of comparable mass or lighter. A general requisite of models of ADM is that the vast majority of the symmetric component of the DM number density must be removed in order to explain the observed relationship $\Omega_B\sim\Omega_{DM}$ via the DM asymmetry. Demanding the efficient annihilation of the symmetric component leads to a tension with experimental limits if the annihilation is directly to Standard Model (SM) degrees of freedom. Read More

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 centre of mass energies that extend beyond those of any feasible terrestrial accelerator. Read More

We propose a unified theory of dark matter (DM) genesis and baryogenesis. It explains the observed link between the DM density and the baryon density, and is fully testable by a combination of collider experiments and precision tests. Our theory utilises the "thermal freeze-in" mechanism of DM production, generating particle anti-particle asymmetries in decays from visible to hidden sectors. Read More

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 p-modes. 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. Read More

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. Read More

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 centre 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. Read More

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. Read More

Motivated by Higgs Portal and Hidden Valley models, heavy particle dark matter that communicates with the supersymmetric Standard Model via pure Higgs sector interactions is considered. We show that a thermal relic abundance consistent with the measured density of dark matter is possible for masses up to $\sim 30\tev$. For dark matter masses above $\sim 1\tev$, non-perturbative Sommerfeld corrections to the annihilation rate are large, and have the potential to greatly affect indirect detection signals. Read More

We consider, in the effective field theory context, anomalies of gauge field theories on a slice of a five-dimensional, Anti-de Sitter geometry and their four-dimensional, holographic duals. A consistent effective field theory description can always be found, notwithstanding the presence of the anomalies and without modifying the degrees of freedom of the theory. If anomalies do not vanish, the d=4 theory contains additional pseudoscalar states, which are either present in the low-energy theory as physical, light states, or are eaten by (would-be massless) gauge bosons. Read More

We argue that supersymmetric flat direction vacuum expectation values can decay non-perturbatively via preheating. Considering a toy U(1) gauge theory, we explicitly calculate the scalar potential, in the unitary gauge, for excitations around several flat directions. We show that the mass matrix for the excitations has non-diagonal entries which vary with the phase of the flat direction vacuum expectation value. Read More

We present a supersymmetric model of electroweak symmetry-breaking exhibiting improved naturalness, wherein the stop mass can be pushed beyond the reach of the Large Hadron Collider without unnatural fine tuning. This implies that supersymmetry may still solve the hierarchy problem, even if it eludes detection at the LHC. Read More

It is known that the cosmological baryon density ($\Omega_{\rm{b}}$) and dark matter density ($\Omega_{\rm{dm}}$) have strikingly similar values. However, in most theories of the early Universe, each density is explained by separate dynamics and consequently there is no compelling reason for this observation. In this note, I briefly review a model in which the dark matter species possesses a particle-antiparticle asymmetry. Read More

It is known that the cosmological baryon density (Omega(b)) and dark matter density (Omega(dm)) have strikingly similar values. However, in most theories of the early Universe, each density is explained by separate dynamics and consequently there is no compelling reason for this observation. In this note, I briefly review a model in which the dark matter possesses a particle-antiparticle asymmetry. Read More

Many models of electroweak symmetry-breaking with an extended Higgs sector exhibit improved naturalness, wherein the new physics scale, at which quadratic divergences of Higgs mass parameters due to top quark loops are cut off, can be pushed beyond the reach of the Large Hadron Collider without unnatural fine tuning. Such models include examples where the new physics is supersymmetry, implying that supersymmetry may still solve the hierarchy problem, even if it eludes detection at the LHC. Read More

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 {\it 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 $\Omega_{\rm{b}}$ and $\Omega_{\rm{dm}}$. 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. Read More

In the context of supersymmetric theories, a weakly broken gauged SO(3) flavour symmetry is used to produce two highly degenerate right handed (RH) neutrinos. It is then shown that this SO(3) flavour symmetry is compatible with all fermion masses and mixings if it is supplemented with a further SU(3) flavour symmetry. A specific Susy breaking model is used to generate the light neutrino masses as well as a natural model of TeV scale resonant leptogenesis. Read More

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, $N_i$. The model leads to naturally large cosmological lepton asymmetries via the resonant behaviour of the one-loop self-energy contribution to $N_i$ 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 $N_i$ decay is satisfied; the origin of the tiny, but non-zero mass splitting required between at least two $N_i$ masses; and the necessary non-trivial breaking of flavour symmetries in the rhd neutrino sector. Read More

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- and Kahler-potentials. The mechanism is closely related to the Giudice-Masiero mechanism for the MSSM $\mu$ 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. Read More