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Search Results: 1 - 10 of 409967 matches for " Anne M. Green "
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Determining the WIMP mass using direct detection experiments
Green, Anne M
High Energy Physics - Phenomenology , 2007, DOI: 10.1088/1475-7516/2007/08/022
Abstract: We study the accuracy with which the WIMP mass could be determined by a superCDMS-like direct detection experiment, given optimistic assumptions about the detector set-up and WIMP properties. We consider WIMPs with an interaction cross-section of \sigma_{\rm p} = 10^{-7} {\rm pb} (just below current exclusion limits) and assume, initially, that the local WIMP velocity distribution and density are known and that the experiment has negligible background. For light WIMPs (mass significantly less than that of the target nuclei) small variations in the WIMP mass lead to significant changes in the energy spectrum. Conversely for heavy WIMPs the energy spectrum depends only weakly on the WIMP mass. Consequently it will be far easier to measure the WIMP mass if it is light than if it is heavy. With exposures of {\cal E}= 3 \times 10^{3}, 3 \times 10^{4} and 3 \times 10^{5} {\rm kg day} (corresponding, roughly, to the three proposed phases of SuperCDMS) it will be possible, given the optimistic assumptions mentioned above, to measure the mass of a light WIMP with an accuracy of roughly 25%, 15% and 2.5 % respectively. These numbers increase with increasing WIMP mass, and for heavy WIMPs, m_{\chi} > {\cal O}(500 {\rm GeV}), even with a large exposure it will only be possible to place a lower limit on the mass. Finally we discuss the validity of the various assumptions made, and the consequences if these assumptions are not valid. In particular if the local WIMP distribution is composed of a number of discrete streams it will not be possible to determine the WIMP mass.
Determining the WIMP mass from a single direct detection experiment, a more detailed study
Anne M. Green
Physics , 2008, DOI: 10.1088/1475-7516/2008/07/005
Abstract: 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).
Extracting information about WIMP properties from direct detection experiments: astrophysical uncertainties
Anne M. Green
Physics , 2010,
Abstract: WIMP direct detection experiments probe the ultra-local dark matter density and velocity distribution. We review how uncertainties in these quantities affect the accuracy with which the WIMP mass and cross-section can be constrained or determined.
Effect of realistic astrophysical inputs on the phase and shape of the WIMP annual modulation signal
Anne M Green
Physics , 2003, DOI: 10.1103/PhysRevD.69.109902
Abstract: The orbit of the Earth about the Sun produces an annual modulation in the WIMP direct detection rate. If the local WIMP velocity distribution is isotropic then the modulation is roughly sinusoidal with maximum in June, however if the velocity distribution is anisotropic the phase and shape of the signal can change. Motivated by conflicting claims about the effect of uncertainties in the local velocity distribution on the interpretation of the DAMA annual modulation signal (and the possibility that the form of the modulation could be used to probe the structure of the Milky Way halo), we study the dependence of the annual modulation on various astrophysical inputs. We first examine the approximations used for the Earth's motion about the Sun and the Sun's velocity with respect to the Galactic rest frame. We find that overly simplistic assumptions lead to errors of up to ten days in the phase and up to tens of per-cent in the shape of the signal, even if the velocity distribution is isotropic. Crucially, if the components of the Earth's velocity perpendicular to the motion of the Sun are neglected, then the change in the phase which occurs for anisotropic velocity distributions is missed. We then examine how the annual modulation signal varies for physically and observationally well-motivated velocity distributions. We find that the phase of the signal changes by up to 20 days and the mean value and amplitude change by up to tens of per-cent.
Determining the WIMP mass from a single direct detection experiment
Anne M Green
Physics , 2008,
Abstract: The energy spectrum of nuclear recoils in Weakly Interacting Massive Particle (WIMP) direct detection experiments depends on the underlying WIMP mass (strongly for light WIMPs, weakly for heavy WIMPs). We discuss 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. In particular we examine the effects of varying the underlying WIMP mass, the detector target nucleus, exposure, energy threshold and maximum energy, the local velocity distribution and the background event rate and spectrum.
Effect of halo modelling on WIMP exclusion limits
Anne M Green
Physics , 2002, DOI: 10.1103/PhysRevD.66.083003
Abstract: WIMP direct detection experiments are just reaching the sensitivity required to detect galactic dark matter in the form of neutralinos. Data from these experiments are usually analysed under the simplifying assumption that the Milky Way halo is an isothermal sphere with maxwellian velocity distribution. Observations and numerical simulations indicate that galaxy halos are in fact triaxial and anisotropic. Furthermore, in the cold dark matter paradigm galactic halos form via the merger of smaller subhalos, and at least some residual substructure survives. We examine the effect of halo modelling on WIMP exclusion limits, taking into account the detector response. Triaxial and anisotropic halo models, with parameters motivated by observations and numerical simulations, lead to significant changes which are different for different experiments, while if the local WIMP distribution is dominated by small scale clumps then the exclusion limits are changed dramatically.
Viability of primordial black holes as short period gamma-ray bursts
Anne M. Green
Physics , 2001, DOI: 10.1103/PhysRevD.65.027301
Abstract: It has been proposed that the short period gamma-ray bursts, which occur at a rate of $\sim 10 {\rm yr^{-1}}$, may be evaporating primordial black holes (PBHs). Calculations of the present PBH evaporation rate have traditionally assumed that the PBH mass function varies as $M_{{\rm BH}}^{-5/2}$. This mass function only arises if the density perturbations from which the PBHs form have a scale invariant power spectrum. It is now known that for a scale invariant power spectrum, normalised to COBE on large scales, the PBH density is completely negligible, so that this mass function is cosmologically irrelevant. For non-scale-invariant power spectra, if all PBHs which form at given epoch have a fixed mass then the PBH mass function is sharply peaked around that mass, whilst if the PBH mass depends on the size of the density perturbation from which it forms, as is expected when critical phenomena are taken into account, then the PBH mass function will be far broader than $ M_{{\rm BH}}^{-5/2}$. In this paper we calculate the present day PBH evaporation rate, using constraints from the diffuse gamma-ray background, for both of these mass functions. If the PBH mass function has significant finite width, as recent numerical simulations suggest, then it is not possible to produce a present day PBH evaporation rate comparable with the observed short period gamma-ray burst rate. This could also have implications for other attempts to detect evaporating PBHs.
Interpreting data from WIMP direct detection experiments
Anne M. Green
Physics , 2002,
Abstract: Weakly Interacting Massive Particle (WIMP) direct detection experiments are closing in on the region of parameter space where relic neutralinos may constitute the galactic halo dark matter. We discuss two issues in the interpretation of data, in particular the calculation of exclusion limits, from these experiments. Firstly we show that the technique that has been used for calculating exclusion limits from binned data without background subtraction produces erroneously tight limits, and discuss alternative methods which avoid this problem. We then argue that the standard maxwellian halo model is likely to be a poor approximation to the dark matter distribution and examine how halo models with triaxiality, velocity anisotropy and small scale clumping affect exclusion limits.
Effects of halo triaxiality, anisotropy and small scale clumping on WIMP direct detection exclusion limits
Anne M Green
Physics , 2002,
Abstract: Weakly Interacting Massive Particle (WIMP) direct detection experiments are closing in on the region of parameter space where neutralinos may constitute the Galactic halo dark matter. Numerical simulations and observations of galaxy halos indicate that the standard Maxwellian halo model is likely to be a poor approximation to the dark matter distribution. We examine how halo models with triaxiality and/or velocity anisotropy affect exclusion limits, before discussing the consequences of the possible survival of small scale clumps.
Probing the width of the MACHO mass function
Anne M Green
Physics , 2000,
Abstract: The simplest interpretation of the microlensing events observed towards the Large Magellanic Cloud is that approximately half of the mass of the Milky Way halo is in the form of MAssive Compact Halo Objects with $M \sim 0.5 M_{\odot}$. This poses severe problems for stellar MACHO candidates, and leads to the consideration of more exotic objects such as primordial black holes (PBHs). Constraining the MACHO mass function will shed light on their nature. Using the current data we find, for four halo models, the best fit delta-function, power law and PBH mass functions. The best fit PBH mass functions, despite having significant finite width, have likelihoods which are similar to, and for one particular halo model greater than, those of the best fit delta functions. We also find that if the correct halo model is known then $\sim$ 500 events will be sufficient to determine whether the MACHO mass function has significant width, and will also allow determination of the mass function parameters to $\sim 5%$.
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