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Anne M. Green

Publications and source records attributed to Anne M. Green.

At least 55 records · Page 3Linked to original sources
arXiv2011

Coupled Quintessence and the Halo Mass Function

A sufficiently light scalar field slowly evolving in a potential can account for the dark energy that presently dominates the universe. This quintessence field is expected to couple directly to matter components, unless some symmetry of a more fundamental theory protects or suppresses it. Such a coupling would leave distinctive signatures in the background expansion history of the universe and on cosmic structure formation, particularly at galaxy cluster scales. Using semi--analytic expressions for the CDM halo mass function, we make predictions for halo abundance in models where the quintessence scalar field is coupled to cold dark matter, for a variety of quintessence potentials. We evaluate the linearly extrapolated density contrast at the redshift of collapse using the spherical collapse model and we compare this result to the corresponding prediction obtained from the non--linear perturbation equations in the Newtonian limit. For all the models considered in this work, if there is a continuous flow of energy from the quintessence scalar field to the CDM component, then the predicted number of CDM haloes can only lie below that of $Λ$CDM, when each model shares the same cosmological parameters today. In the last stage of our analysis we perform a global MCMC fit to data to find the best fit values for the cosmological model parameters. We find that for some forms of the quintessence potential, coupled dark energy models can offer a viable alternative to $Λ$CDM in light of the recent detections of massive high--$z$ galaxy clusters, while other models of coupled quintessence predict a smaller number of massive clusters at high redshift compared to $Λ$CDM.

astro-ph.CO↗
arXiv2011

Ultra-fine dark matter structure in the Solar neighbourhood

The direct detection of dark matter on Earth depends crucially on its density and its velocity distribution on a milliparsec scale. Conventional N-body simulations are unable to access this scale, making the development of other approaches necessary. In this paper, we apply the method developed in Fantin et al. 2008 to a cosmologically-based merger tree, transforming it into a useful instrument to reproduce and analyse the merger history of a Milky Way-like system. The aim of the model is to investigate the implications of any ultra-fine structure for the current and next generation of directional dark matter detectors. We find that the velocity distribution of a Milky Way-like Galaxy is almost smooth, due to the overlap of many streams of particles generated by multiple mergers. Only the merger of a 10^10 Msun analyse can generate significant features in the ultra-local velocity distribution, detectable at the resolution attainable by current experiments.

astro-ph.GA↗
arXiv2010

Dependence of direct detection signals on the WIMP velocity distribution

The signals expected in WIMP direct detection experiments depend on the ultra-local dark matter distribution. Observations probe the local density, circular speed and escape speed, while simulations find velocity distributions that deviate significantly from the standard Maxwellian distribution. We calculate the energy, time and direction dependence of the event rate for a range of velocity distributions motivated by recent observations and simulations, and also investigate the uncertainty in the determination of WIMP parameters. The dominant uncertainties are the systematic error in the local circular speed and whether or not the MW has a high density dark disc. In both cases there are substantial changes in the mean differential event rate and the annual modulation signal, and hence exclusion limits and determinations of the WIMP mass. The uncertainty in the shape of the halo velocity distribution is less important, however it leads to a 5% systematic error in the WIMP mass. The detailed direction dependence of the event rate is sensitive to the velocity distribution. However the numbers of events required to detect anisotropy and confirm the median recoil direction do not change substantially.

astro-ph.CO↗
arXiv2010

Gamma-rays from ultracompact minihalos: potential constraints on the primordial curvature perturbation

Ultracompact minihalos (UCMHs) are dense dark matter structures which can form from large density perturbations shortly after matter-radiation equality. If dark matter is in the form of Weakly Interacting Massive Particles (WIMPs), then UCMHs may be detected via their gamma-ray emission. We investigate how the {\em{Fermi}} satellite could constrain the abundance of UCMHs and place limits on the power spectrum of the primordial curvature perturbation. Detection by {\em Fermi} would put a lower limit on the UCMH halo fraction. The smallest detectable halo fraction, $f_{\rm UCMH} \gtrsim 10^{-7}$, is for $M_{\rm UCMH} \sim 10^{3} M_{\odot}$. If gamma-ray emission from UCMHs is not detected, an upper limit can be placed on the halo fraction. The bound is tightest, $f_{\rm UCMH} \lesssim 10^{-5}$, for $M_{\rm UCMH} \sim 10^{5} M_{\odot}$. The resulting upper limit on the power spectrum of the primordial curvature perturbation in the event of non-detection is in the range $\mathcal{P_R} \lesssim 10^{-6.5}- 10^{-6}$ on scales $k \sim 10^{1}-10^{6} \, {\rm Mpc}^{-1}$. This is substantially tighter than the existing constraints from primordial black hole formation on these scales, however it assumes that dark matter is in the form of WIMPs and UCMHs are not disrupted during the formation of the Milky Way halo.

astro-ph.CO↗
arXiv2010

Constraints from primordial black hole formation at the end of inflation

Primordial black hole (PBH) abundance limits constrain the primordial power spectrum, and hence models of inflation, on scales far smaller than those probed by cosmological observations. Single field inflation models which are compatible with all cosmological data can have large enough perturbations on small scales to overproduce PBHs, and hence be excluded. The standard formulae for the amplitude of perturbations do not hold for modes that exit the horizon close to the end of inflation however. We use a modified flow analysis to identify models of inflation where the amplitude of perturbations on small scales is large. For these models we then carry out a numerical evolution of the perturbations and use the PBH constraints on the power spectrum to eliminate models which overproduce PBHs. Significant PBH formation can occur in models in which inflation can continue indefinitely and is ended via a secondary mechanism. We demonstrate that PBHs constrain these types of inflation models and show that a numerical evaluation of the power spectrum decreases the number of otherwise viable models of inflation.

hep-ph↗
arXiv2010

The median recoil direction as a WIMP directional detection signal

Direct detection experiments have reached the sensitivity required to detect dark matter WIMPs. Demonstrating that a putative signal is due to WIMPs, and not backgrounds, is a major challenge however. The direction dependence of the WIMP scattering rate provides a potential WIMP `smoking gun'. If the WIMP distribution is predominantly smooth, the Galactic recoil distribution is peaked in the direction opposite to the direction of Solar motion. Previous studies have found that, for an ideal detector, of order 10 WIMP events would be sufficient to reject isotropy, and rule out an isotropic background. We examine how the median recoil direction could be used to confirm the WIMP origin of an anisotropic recoil signal. Specifically we determine the number of events required to reject the null hypothesis that the median direction is random (corresponding to an isotropic Galactic recoil distribution) at 95% confidence. We find that for zero background 31 events are required, a factor of roughly 2 more than are required to simply reject isotropy. We also investigate the effect of a non-zero isotropic background. As the background rate is increased the number of events required increases, initially fairly gradually and then more rapidly, once the signal becomes subdominant. We also discuss the effect of features in the speed distribution at large speeds, as found in recent high resolution simulations, on the median recoil direction.

astro-ph.CO↗
arXiv2010

Direct detection of WIMPs

A generic weakly interacting massive particle (WIMP) is one of the most attractive candidates to account for the cold dark matter in our Universe, since it would be thermally produced with the correct abundance to account for the observed dark matter density. WIMPs can be searched for directly through their elastic scattering with a target material, and a variety of experiments are currently operating or planned with this aim. In these notes we overview the theoretical calculation of the direct detection rate of WIMPs as well as the different detection signals. We discuss the various ingredients (from particle physics and astrophysics) that enter the calculation and review the theoretical predictions for the direct detection of WIMPs in particle physics models.

astro-ph.CO↗
arXiv2009

Generalised constraints on the curvature perturbation from primordial black holes

Primordial black holes (PBHs) can form in the early Universe via the collapse of large density perturbations. There are tight constraints on the abundance of PBHs formed due to their gravitational effects and the consequences of their evaporation. These abundance constraints can be used to constrain the primordial power spectrum, and hence models of inflation, on scales far smaller than those probed by cosmological observations. We compile, and where relevant update, the constraints on the abundance of PBHs before calculating the constraints on the curvature perturbation, taking into account the growth of density perturbations prior to horizon entry. We consider two simple parameterizations of the curvature perturbation spectrum on the scale of interest: constant and power-law. The constraints from PBHs on the amplitude of the power spectrum are typically in the range 10^{-2}-10^{-1} with some scale dependence.

astro-ph.CO↗
arXiv2008

Modelling ultra-fine structure in dark matter halos

Various laboratory-based experiments are underway attempting to detect dark matter directly. The event rates and detailed signals expected in these experiments depend on the dark matter phase space distribution on sub-milliparsec scales. These scales are many orders of magnitude smaller than those that can be resolved by conventional N-body simulations, so one cannot hope to use such tools to investigate the effect of mergers in the history of the Milky Way on the detailed phase-space structure probed by the current experiments. In this paper we present an alternative approach to investigating the results of such mergers, by studying a simplified model for a merger of a sub-halo with a larger parent halo. With an appropriate choice of parent halo potential, the evolution of material from the sub-halo can be expressed analytically in action-angle variables, so it is possible to obtain its entire orbit history very rapidly without numerical integration. Furthermore by evolving backwards in time, we can obtain arbitrarily-high spatial resolution for the current velocity distribution at a fixed point. Although this model cannot provide a detailed quantitative comparison with the Milky Way, its properties are sufficiently generic that it offers qualitative insight into the expected structure arising from a merger at a resolution that cannot be approached with full numerical simulations. Preliminary results indicate that the velocity-space distribution of dark matter particles remains characterized by discrete and well-defined peaks over an extended period of time, both for single and multi-merging systems, in contrast to the simple smooth velocity distributions sometimes assumed in predicting laboratory experiment detection rates.

astro-ph↗
arXiv2008

Determining the WIMP mass from a single direct detection experiment, a more detailed study

The energy spectrum of nuclear recoils in Weakly Interacting Massive Particle (WIMP) direct detection experiments depends on the underlying WIMP mass. We study how the accuracy with which the WIMP mass could be determined by a single direct detection experiment depends on the detector configuration and the WIMP properties. We investigate the effects of varying the underlying WIMP mass and cross-section, the detector target nucleus, exposure, energy threshold and maximum energy, the local circular speed and the background event rate and spectrum. The number of events observed is directly proportional to both the exposure and the cross-section, therefore these quantities have the greatest bearing on the accuracy of the WIMP mass determination. The relative capabilities of different detectors to determine the WIMP mass depend not only on the WIMP and target masses, but also on their energy thresholds. We find that the rapid decrease of the nuclear form factor with increasing momentum transfer which occurs for heavy nuclei, means that heavy nuclei will not necessarily be able to measure the mass of heavy WIMPs more accurately. Uncertainty in the local circular speed and non-negligible background would both lead to systematic errors in the WIMP mass determination. With a single detector it will be difficult to disentangle a WIMP signal (and the WIMP mass) from background if the background spectrum has a similar shape to the WIMP spectrum (i.e. exponential background, or flat background and a heavy WIMP).

hep-ph↗
arXiv2007

Consequences of statistical sense determination for WIMP directional detection

We study the consequences of limited recoil sense reconstruction on the number of events required to reject isotropy and detect a WIMP signal using a directional detector. For a constant probability of determining the sense correctly, 3-d read-out and zero background, we find that as the probability is decreased from 1.0 to 0.75 the number of events required to reject isotropy using the mean angle statistic is increased by a factor of a few. As the probability is decreased further the number of events required using this statistic increases sharply, and in fact isotropy can be rejected more easily by discarding the sense information and using axial statistics. This however requires an order of magnitude more events than vectorial data with perfect sense determination. We also consider energy dependent probabilities of correctly measuring the sense, 2-d read-out and non-zero background. Our main conclusion regarding the sense determination is that correctly determining the sense of the abundant, but less anisotropic, low energy recoils is most important for minimising the number of events required.

astro-ph↗
arXiv2007

Constraints on the primordial curvature perturbation from primordial black holes

We calculate the constraints on the primordial curvature perturbation at the end of inflation from the present day abundance of Primordial Black Holes (PBHs), as a function of the reheat temperature T_{\rm RH}. We first extend recent work on the formation of PBHs on scales which remain within the horizon during inflation and calculate the resulting constraints on the curvature perturbation. We then evaluate the constraint from PBHs that form, more conventionally, from super-horizon perturbations. The constraints apply for T_{\rm RH} < 10^{8} GeV and the inclusion of sub-horizon PBHs leads to a limit which is roughly three times tighter than the bound from super-horizon PBHs.

astro-ph↗
arXiv2006

On mini-halo encounters with stars

We study, analytically and numerically, the energy input into dark matter mini-haloes by interactions with stars. We find that the fractional energy input in simulations of Plummer spheres agrees well with the impulse approximation for small and large impact parameters, with a rapid transition between these two regimes. Using the impulse approximation the fractional energy input at large impact parameters is fairly independent of the mass and density profile of the mini-halo, however low-mass mini-haloes experience a greater fractional energy input in close encounters. We formulate a fitting function which encodes these results and use it to estimate the disruption timescales of mini-haloes, taking into account the stellar velocity dispersion and mass distribution. For mini-haloes with mass M< {\cal O}(10^{-7} M_{\odot} on typical orbits which pass through the disc, we find that the estimated disruption timescales are independent of mini-halo mass, and are of order the age of the Milky Way. For more massive mini-haloes the estimated disruption timescales increase rapidly with increasing mass.

astro-ph↗
arXiv2006

Optimizing WIMP directional detectors

We study the dependence of the exposure required to directly detect a WIMP directional recoil signal on the capabilities of a directional detector. Specifically we consider variations in the nuclear recoil energy threshold, the background rate, whether the detector measures the recoil momentum vector in 2 or 3 dimensions and whether or not the sense of the momentum vector can be determined. We find that the property with the biggest effect on the required exposure is the measurement of the momentum vector sense. If the detector cannot determine the recoil sense, the exposure required is increased by an order of magnitude for 3-d read-out and two orders of magnitude for 2-d read-out. For 2-d read-out the required exposure, in particular if the senses can not be measured, can be significantly reduced by analyzing the reduced angles with the, time dependent, projected direction of solar motion subtracted. The background rate effectively places a lower limit on the WIMP cross-section to which the detector is sensitive; it will be very difficult to detect WIMPs with a signal rate more than an order of magnitude below the background rate. Lowering the energy threshold also reduces the required exposure, but only for thresholds above 20 keV.

astro-ph↗
arXiv2005

Directional Statistics for WIMP direct detection II: 2-d read-out

The direction dependence of the WIMP direct detection rate provides a powerful tool for distinguishing a WIMP signal from possible backgrounds. We study the the number of events required to discriminate a WIMP signal from an isotropic background for a detector with 2-d read-out using non-parametric circular statistics. We also examine the number of events needed to i) detect a deviation from rotational symmetry, due to flattening of the Milky Way halo and ii) detect a deviation in the mean direction due to a tidal stream. If the senses of the recoils are measured then of order 20-70 events (depending on the plane of the 2-d read out) will be sufficient to reject isotropy of the raw recoil angles at 90% confidence. If the senses can not be measured these number increase by roughly two orders of magnitude (compared with an increase of one order of magnitude for the case of full 3-d read-out). The distributions of the reduced angles, with the (time dependent) direction of solar motion subtracted, are far more anisotropic, however, and if the isotropy tests are applied to these angles then the numbers of events required are similar to the case of 3-d read-out. A deviation from rotational symmetry will only be detectable if the Milky Way halo is significantly flattened. The deviation in the mean direction due to a tidal stream is potentially detectable, however, depending on the density and direction of the stream.

astro-ph↗
arXiv2005

Small Scale Structure Formation in Chameleon Cosmology

Chameleon fields are scalar fields whose mass depends on the ambient matter density. We investigate the effects of these fields on the growth of density perturbations on sub-galactic scales and the formation of the first dark matter halos. Density perturbations on comoving scales $R < 1 {\rm pc}$ go non--linear and collapse to form structure much earlier than in standard $Λ$CDM cosmology. The resulting mini-halos are hence more dense and resilient to disruption. We therefore expect (provided that the density perturbations on these scales have not been erased by damping processes) that the dark matter distribution on small scales would be more clumpy in chameleon cosmology than in the $Λ$CDM model.

astro-ph↗
arXiv2005

Small scale WIMP physics

The dark matter distribution on small scales may depend on the the properties of the first generation of dark matter halos to form, which is in turn determined by the microphysics of the dark matter particles. We overview the microphysics of WIMPs and our calculations of the collisional damping and free streaming scales. We then plot the resulting density perturbation power spectrum and red-shift at which typical halos form, taking into account the effect of uncertainties in the WIMP properties (mass and interaction channel) and the primordial power spectrum. Finally we review recent developments regarding the properties and fate of the first WIMPy halos.

astro-ph↗