arXiv subjects

Malcolm Fairbairn

Publications and source records attributed to Malcolm Fairbairn.

At least 91 records · Page 5Linked to original sources

arXiv2014

Unifying inflation and dark matter with the Peccei-Quinn field: observable axions and observable tensors

A model of high scale inflation is presented where the radial part of the Peccei-Quinn (PQ) field with a non-minimal coupling to gravity plays the role of the inflaton, and the QCD axion is the dark matter. A quantum fluctuation of $\mathcal{O}(H/2π)$ in the axion field will result in a smaller angular fluctuation if the PQ field is sitting at a larger radius during inflation than in the vacuum. This changes the effective axion decay constant, $f_a$, during inflation and dramatically reduces the production of isocurvature modes. This mechanism opens up a new window in parameter space where an axion decay constant in the range $10^{12}\text{ GeV}\lesssim f_a\lesssim 10^{15}\text{ GeV}$ is compatible with observably large $r$. The exact range allowed for $f_a$ depends on the efficiency of reheating. This model also predicts a minimum possible value of $r=10^{-3}$. The new window can be explored by a measurement of $r$ possible with \textsc{Spider} and the proposed CASPEr experiment search for high $f_a$ axions.

hep-ph↗

arXiv2014

Non-collider searches for stable massive particles

The theoretical motivation for exotic stable massive particles (SMPs) and the results of SMP searches at non-collider facilities are reviewed. SMPs are defined such that they would be sufficiently long-lived so as to still exist in the cosmos either as Big Bang relics or secondary collision products, and sufficiently massive to be beyond the reach of any conceivable accelerator-based experiment. The discovery of SMPs would address a number of important questions in modern physics, such as the origin and composition of dark matter in the Universe and the unification of the fundamental forces. This review outlines the scenarios predicting SMPs and the techniques used at non-collider experiments to look for SMPs, eg in cosmic rays and bound in matter. The limits so far obtained on the fluxes and matter densities of SMPs which possess various detection-relevant properties such as electric and magnetic charge are given.

hep-ph↗

arXiv2014

Directional Dark Matter Detection Beyond the Neutrino Bound

Coherent scattering of solar, atmospheric and diffuse supernovae neutrinos creates an irreducible background for direct dark matter experiments with sensitivities to WIMP-nucleon spin-independent scattering cross-sections of 10^(-46)-10^(-48) cm^2, depending on the WIMP mass. Even if one could eliminate all other backgrounds, this "neutrino floor" will limit future experiments with projected sensitivities to cross-sections as small as 10^(-48) cm^2. Direction-sensitive detectors have the potential to study dark matter beyond the neutrino bound by fitting event distributions in multiple dimensions: recoil kinetic energy, recoil track angle with respect to the sun, and event time. This work quantitatively explores the impact of direction-sensitivity on the neutrino bound in dark matter direct detection.

hep-ph↗

arXiv2014

The CTA Sensitivity to Lorentz-Violating Effects on the Gamma-Ray Horizon

The arrival of TeV-energy photons from distant galaxies is expected to be affected by their QED interaction with intergalactic radiation fields through electron-positron pair production. In theories where high-energy photons violate Lorentz symmetry, the kinematics of the process $γ+ γ\rightarrow e^+ + e^-$ is altered and the cross-section suppressed. Consequently, one would expect more of the highest-energy photons to arrive if QED is modified by Lorentz violation than if it is not. We estimate the sensitivity of Cherenkov Telescope Array (CTA) to changes in the $γ$-ray horizon of the Universe due to Lorentz violation, and find that it should be competitive with other leading constraints.

astro-ph.HE↗

arXiv2014

Electroweak Vacuum Stability in light of BICEP2

We consider the effect of a period of inflation with a high energy density upon the stability of the Higgs potential in the early universe. The recent measurement of a large tensor-to-scalar ratio, $r_T \sim 0.16$, by the BICEP-2 experiment possibly implies that the energy density during inflation was very high, comparable with the GUT scale. Given that the standard model Higgs potential is known to develop an instability at $Λ\sim 10^{10}$ GeV this means that the resulting large quantum fluctuations of the Higgs field could destabilize the vacuum during inflation, even if the Higgs field starts at zero expectation value. We estimate the probability of such a catastrophic destabilisation given such an inflationary scenario and calculate that for a Higgs mass of $m_h=125.5$ GeV that the top mass must be less than $m_t\sim 172$ GeV. We present two possible cures: a direct coupling between the Higgs and the inflaton and a non-zero temperature from dissipation during inflation.

hep-ph↗

arXiv2014

On the dark matter profile in Sculptor: Breaking the $β$ degeneracy with Virial shape parameters

We present a new method for studying tracers in gravitational systems where higher moments of the line-of-sight velocity distribution are introduced via Virial equations rather than the Jeans equations. Unlike the fourth order Jeans equations, the fourth order Virial equations can simply be added to the standard second order Jeans equation without introducing a new anisotropy parameter $β^{\prime}$. We introduce two new global shape parameters $ζ_A$ and $ζ_B$ which replace the kurtosis as a more statistically robust measure of the shape of the line of sight velocity distribution. We show that in the case of stars in dwarf spheroidal galaxies these new parameters can significantly reduce the range of density profiles that are otherwise consistent with the observed stellar kinematics (a problem sometimes known as the $β$ degeneracy). Specifically, we find that $ζ_A$ focuses tightly on a subset of solutions where cusped density profiles are degenerate with more concentrated cored dark matter halos. If the number density of stars $ν(r)$ is fixed, then introducing $ζ_B$ can further reduce the space of solutions by constraining the outer slope of the dark matter density profile. Assuming a Plummer profile for $ν(r)$ we recover the surprising result that the dark matter in Sculptor may be cuspy after all, in contrast to the conclusions of other approaches.

astro-ph.GA↗

arXiv2014

The Problem with False Vacuum Higgs Inflation

We investigate the possibility of using the only known fundamental scalar, the Higgs, as an inflaton with minimal coupling to gravity. The peculiar appearance of a plateau or a false vacuum in the renormalised effective scalar potential suggests that the Higgs might drive inflation. For the case of a false vacuum we use an additional singlet scalar field, motivated by the strong CP problem, and its coupling to the Higgs to lift the barrier allowing for a graceful exit from inflation by mimicking hybrid inflation. We find that this scenario is incompatible with current measurements of the Higgs mass and the QCD coupling constant and conclude that the Higgs can only be the inflaton in more complicated scenarios.

hep-ph↗

arXiv2014

Analytical Solutions to the Mass-Anisotropy Degeneracy with Higher Order Jeans Analysis: A General Method

The Jeans analysis is often used to infer the total density of a system by relating the velocity moments of an observable tracer population to the underlying gravitational potential. This technique has recently been applied in the search for Dark Matter in objects such as dwarf spheroidal galaxies where the presence of Dark Matter is inferred via stellar velocities. A precise account of the density is needed to constrain the expected gamma ray flux from DM self-annihilation and to distinguish between cold and warm dark matter models. Unfortunately the traditional method of fitting the second order Jeans equation to the tracer dispersion suffers from an unbreakable degeneracy of solutions due to the unknown velocity anisotropy of the projected system. To tackle this degeneracy one can appeal to higher moments of the Jeans equation. By introducing an analog to the Binney anisotropy parameter at fourth order, beta' we create a framework that encompasses all solutions to the fourth order Jeans equations rather than those in the literature that impose unnecessary correlations between anisotropy of second and fourth order moments. The condition beta' = f(beta) ensures that the degeneracy is lifted and we interpret the separable augmented density system as the order-independent case beta'= beta. For a generic choice of beta' we present the line of sight projection of the fourth moment and how it could be incorporated into a joint likelihood analysis of the dispersion and kurtosis. Having presented the mathematical framework, we then use it to develop a statistical method for the purpose of placing constraints on dark matter density parameters from discrete velocity data. The method is tested on simulated dwarf spheroidal data sets leading to results which motivate study of real dwarf spheroidal data sets.

astro-ph.CO↗

arXiv2013

Rescuing Quadratic Inflation

Inflationary models based on a single scalar field $ϕ$ with a quadratic potential $V = \frac{1}{2} m^2 ϕ^2$ are disfavoured by the recent Planck constraints on the scalar index, $n_s$, and the tensor-to-scalar ratio for cosmological density perturbations, $r_T$. In this paper we study how such a quadratic inflationary model can be rescued by postulating additional fields with quadratic potentials, such as might occur in sneutrino models, which might serve as either curvatons or supplementary inflatons. Introducing a second scalar field reduces but does not remove the pressure on quadratic inflation, but we find a sample of three-field models that are highly compatible with the Planck data on $n_s$ and $r_T$. We exhibit a specific three-sneutrino example that is also compatible with the data on neutrino mass difference and mixing angles.

astro-ph.CO↗

arXiv2013

Baryogenesis and Dark Matter with Vector-like Fermions

We show that vector-like fermions can act as the dark matter candidate in the universe whilst also playing a crucial role in electroweak baryogenesis through contributing to the barrier in the one-loop thermal scalar potential. In order for the new fermions to give rise to a strong first order phase transition, we show that one requires rather large Yukawa couplings in the new sector, which are strongly constrained by electroweak precision tests and perturbativity. Strong couplings between the dark matter candidate and the Higgs boson intuitively lead to small values of the relic density and problems with dark matter direct detection bounds. Nevertheless, when considering the most general realisation of the model, we find regions in the parameter space that respect all current constraints and may explain both mysteries simultaneously.

hep-ph↗

arXiv2013

Axionic Dark Radiation and the Milky Way's Magnetic Field

Recently it has been suggested that dark radiation in the form of axions produced during the decay of string theory moduli fields could be responsible for the soft x-ray excess in galaxy clusters. These soft X-ray photons come about due to the conversion of these axions into photons in the magnetic fields of the clusters. In this work we calculate the conversion of axionic dark radiation into X-ray photons in the magnetic field of our own Galaxy. We consider Delta N ~ 0.5 worth of dark radiation made up of axions with energy of order 0.1-1 keV. We show that it is possible, if a little optimistic, to explain the large regions of X-ray emission located above and below the centre of the Galactic plane detected in the 3/4 keV ROSAT all sky map completely due to the conversion of dark radiation into photons with an inverse axion-photon coupling of M ~ 3e13 GeV and an axion mass of m < 1e-12 eV. Different parameter values could explain both these features and the 3/4 keV X-ray background. More conservatively, these X-ray observations are a good way to constrain such models of axionic dark radiation.

astro-ph.CO↗

arXiv2013

Singlet Fermionic Dark Matter and the Electroweak Phase Transition

We consider a model with a gauge singlet Dirac fermion as a cold dark matter candidate. The dark matter particle communicates with the Standard Model via a gauge singlet scalar mediator that couples to the Higgs. The scalar mediator also serves to create a tree-level barrier in the scalar potential which leads to a strongly first order electroweak phase transition as required for Electroweak Baryogenesis. We find a large number of models that can account for all the dark matter and provide a strong phase transition while avoiding constraints from dark matter direct detection, electroweak precision data, and the latest Higgs data from the LHC. The next generation of direct detection experiments could rule out a large region of the parameter space but can be evaded in some regions when the Higgs-singlet mixing is very small.

hep-ph↗

arXiv2013

Cores in Classical Dwarf Spheroidal Galaxies? A Dispersion-Kurtosis Jeans Analysis Without Restricted Anisotropy

We attempt to measure the density of dark matter in the two Dwarf Spheroidal Galaxies Fornax and Sculptor using a new method which employs Jeans equations based on both the second and fourth moment of the Collisionless Boltzmann Equation (i.e. variance and kurtosis of line of sight stellar velocities). Unlike previous related efforts, we allow the anisotropy of the radial and tangential second and fourth order moments to vary independently of each other. We apply the method to simulated data and establish that to some degree it appears to be able to break the degeneracies associated with second order only Jeans analyses. When we apply the technique to real data, we see no huge improvement in our understanding of the inner density of Fornax, which can still be fit by either a quite cuspy or cored density profile. For Sculptor however we find that the technique suggests that the data is incompatible with a steep profile and a cored profile seems more consistent. As well as presenting these new results and comparing them to other estimates in the literature, we try to understand why the technique is more effective in one system than the other.

astro-ph.GA↗

arXiv2012

Mixed perturbations from an Inflaton and Two Curvatons - towards understanding non-gaussianity in more complicated curvaton scenarios

We analyse the situation where the primordial curvature perturbations are produced by the joint effects of an inflaton field and two curvaton fields. We present general equations which allow the reader to obtain f_NL for several different scenarios which differ in the order in which fields decay into radiation after inflation. In order to investigate the physics of these equations we analyse some simplified situations where the fields are harmonic and both curvatons are frozen at the same expectation value during inflation. We find quite complex behaviour - for a given situation where the inflaton contributes a fixed amount to the total curvature perturbation there are situations where f_NL is maximised if both curvatons share equally in contributing the rest and situations where f_NL is maximised if only one of the two curvatons contributes the rest. We are unable therefore to make any completely general extrapolations about the expected non-gaussianity from N curvatons. We find that as the curvaton contribution to the overall perturbation is gradually increased, f_NL rises to a maximum before falling again and that a given f_NL can correspond to many different parameter sets for the two curvatons.

astro-ph.CO↗

arXiv2012

Quantifying Astrophysical Uncertainties on Dark Matter Direct Detection Results

We attempt to estimate the uncertainty in the constraints on the spin independent dark matter-nucleon cross section due to our lack of knowledge of the dark matter phase space in the galaxy. We fit the density of dark matter before investigating the possible solutions of the Jeans equation compatible with those fits in order to understand what velocity dispersions we might expect at the solar radius. We take into account the possibility of non-Maxwellian velocity distributions and the possible presence of a dark disk. Combining all these effects, we still find that the uncertainty in the interpretation of direct detection experiments for high (>100 GeV) mass dark matter candidates is less than an order of magnitude in cross section.

astro-ph.CO↗

arXiv2012

Constraining the Equation of State of Dark Energy with Gamma Rays

Starlight in the Universe impedes the passage of high energy (e.g. TeV) gamma rays due to positron-electron pair production. The history of this stellar radiation field depends upon observations of star formation rate which themselves can only be interpreted in the context of a particular cosmology. For different equations of state of dark energy, the star formation rate data suggests a different density of stellar photons at a particular redshift and a different probability of arrival of gamma rays from distant sources. In this work we aim to show that this effect can be used to constrain the equation of state of dark energy. The current work is a proof of concept and we outline the steps that would have to be taken to place the method in a rigorous statistical framework which could then be combined with other more mature methods such as fitting supernova luminosity distances.

astro-ph.CO↗

arXiv2011

Vanishing Dimensions and Planar Events at the LHC

We propose that the effective dimensionality of the space we live in depends on the length scale we are probing. As the length scale increases, new dimensions open up. At short scales the space is lower dimensional; at the intermediate scales the space is three-dimensional; and at large scales, the space is effectively higher dimensional. This setup allows for some fundamental problems in cosmology, gravity, and particle physics to be attacked from a new perspective. The proposed framework, among the other things, offers a new approach to the cosmological constant problem and results in striking collider phenomenology and may explain elongated jets observed in cosmic-ray data.

hep-ph↗

arXiv2011

Voids as Alternatives to Dark Energy and the Propagation of Gamma Rays through the Universe

We test the opacity of a void Universe to TeV energy gamma rays having obtained the extra-galactic background light in that Universe using a simple model and the observed constraints on the star formation rate history. We find that the void Universe has significantly more opacity than a Lambda-CDM Universe, putting it at odds with observations of BL-Lac objects. We argue that while this method of distinguishing between the two cosmologies contains uncertainties, it circumvents any debates over fine-tuning.

astro-ph.CO↗