-
[show abstract]
[hide abstract]
ABSTRACT: We propose a novel antiparticle detector. The gaseous antiparticle spectrometer (GAPS) effects particle identification through the characteristic X-rays emitted by antiparticles when they form exotic atoms in gases. GAPS obtains particularly high grasp (effective area-solid angle product) at lower particle energies, where conventional schemes are most limited in their utility. The concept is simple and lightweight, so it can be readily employed on balloon- and space-based missions. An extremely powerful potential application of GAPS is a space-based search for the neutralino through the detection of a neutralino annihilation by-product—the antideuteron. Paradoxically, this space-based search for the neutralino is capable of achieving comparable sensitivity to as yet unrealized third-generation, underground dark matter experiments. And GAPS can obtain this performance in a very modest satellite experiment. GAPS can also provide superior performance in searches for primary antiprotons produced via neutralino annihilation and black hole evaporation and in probing subdominant contributions to the antiproton flux at low energies. In a deep space mission, GAPS will obtain higher sensitivity for a given weight and power than BGO calorimeters.
The Astrophysical Journal 12/2008; 566(1):604. · 6.02 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We present the luminosity function to very faint magnitudes for the globular clusters in M87, based on a 30 orbit Hubble Space Telescope (HST) WFPC2 imaging program. The very deep images and corresponding improved false source rejection allow us to probe the mass function further beyond the turnover than has been done before. We compare our luminosity function to those that have been observed in the past, and confirm the similarity of the turnover luminosity between M87 and the Milky Way. We also find with high statistical significance that the M87 luminosity function is broader than that of the Milky Way. We discuss how determining the mass function of the cluster system to low masses can constrain theoretical models of the dynamical evolution of globular cluster systems. Our mass function is consistent with the dependence of mass loss on the initial cluster mass given by classical evaporation, and somewhat inconsistent with newer proposals that have a shallower mass dependence. In addition, the rate of mass loss is consistent with standard evaporation models, and not with the much higher rates proposed by some recent studies of very young cluster systems. We also find that the mass-size relation has very little slope, indicating that there is almost no increase in the size of a cluster with increasing mass.
The Astrophysical Journal 12/2008; 650(2):885. · 6.02 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Only one certain classical nova eruption has ever been detected inside a globular cluster—nova T Sco (observed in A.D. 1860) in M80. During a survey of M87 we have detected an erupting star coincident (to within 0.08 pixels) with a globular cluster of that giant elliptical galaxy. We are able to discount variables in the foreground or background of M87. The light curve and color of the erupting star match those expected for a nova at the distance of M87. The chance superposition of an M87 field nova on the globular cluster is very unlikely but cannot be completely ruled out. Our detection hints at a globular cluster nova frequency f ~ 0.004 nova per cluster per year, much higher than previous observations have suggested.
The Astrophysical Journal 12/2008; 605(2):L117. · 6.02 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The position of the giant elliptical galaxy M87 at the center of the Virgo Cluster means that the inferred column density of dark matter associated with both the cluster halo and the galaxy halo is quite large. This system is thus an important laboratory for studying massive dark objects in elliptical galaxies and galaxy clusters by gravitational microlensing, strongly complementing the studies of spiral galaxy halos performed in the Local Group. We have performed a microlensing survey of M87 with the WFPC2 instrument on the Hubble Space Telescope. Over a period of 30 days, with images taken once daily, we discover seven variable sources. Four are variable stars of some sort, two are consistent with classical novae, and one exhibits an excellent microlensing light curve, although with a very blue color, implying the somewhat disfavored possibility of a horizontal-branch source being lensed. On the basis of sensitivity calculations from artificial stars and from artificial light curves, we estimate the expected microlensing rate. We find that the detection of one event is consistent with a dark halo with a 20% contribution of microlensing objects for both M87 and the Virgo Cluster, similar to the value found from observations in the Local Group. Further work is required to test the hypothesized microlensing component to the cluster.
The Astrophysical Journal 12/2008; 610(2):691. · 6.02 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We present an analysis of a 50 orbit HST ACS observation of the M87 globular
cluster system. We use the extraordinary depth of this dataset to test whether
the colors and magnitudes show evidence for a mass-metallicity relation in
globular cluster populations. We find only a weak or absent relation between
the colors and magnitudes of the metal poor subpopulation of globular clusters.
The weakness or absence of a color-magnitude relation is established over a
wide range in luminosity from $M_V=-11$ to $M_V=-6$, encompassing most of the
M87 globular clusters. The constancy of the colors of the metal-poor
subpopulation seen in our 50 orbit observation is in contrast to suggestions
from single orbit ACS data that the metal-poor globular clusters in M87 and
several other galaxies show a "blue tilt." The formal best fit for the
mass-metallicity relation for the metal-poor subpopulation in our much deeper
data is $Z\propto M^{0.08\pm0.05}$. Our analysis of these data also shows a
possible small "red tilt" in the metal-rich globular cluster subpopulation.
While either of these small tilts may be real, they may also illustrate the
limit to which mass-metallicity relations can be determined, even in such
extraordinarily deep data. We specifically test for a wide range of systematic
effects and find that while small tilts cannot be confirmed or rejected, the
data place a strong upper limit to any tilt of $|0.20|\pm0.05$. This upper
limit is much smaller than some earlier claims from single orbit data, and
strongly limits self-enrichment within globular clusters. This mass-metallicity
relation for globular clusters is also shallower than the relation for
galaxies, suggesting that the formation mechanisms for these two types of
objects are different.
11/2008;
-
[show abstract]
[hide abstract]
ABSTRACT: We explore the full parameter space of Minimal Supergravity (mSUGRA),
allowing all four continuous parameters (the scalar mass m_0, the gaugino mass
m_1/2, the trilinear coupling A_0, and the ratio of Higgs vacuum expectation
values tan beta) to vary freely. We apply current accelerator constraints on
sparticle and Higgs masses, and on the b -> s gamma branching ratio, and
discuss the impact of the constraints on g_mu-2. To study dark matter, we apply
the WMAP constraint on the cold dark matter density. We develop Markov Chain
Monte Carlo (MCMC) techniques to explore the parameter regions consistent with
WMAP, finding them to be considerably superior to previously used methods for
exploring supersymmetric parameter spaces. Finally, we study the reach of
current and future direct detection experiments in light of the WMAP
constraint.
07/2004;
-
[show abstract]
[hide abstract]
ABSTRACT: Using a new instrument, the HEAT collaboration has confirmed the excess of cosmic ray positrons that they first detected in 1994. We explore the possibility that this excess is due to the annihilation of neutralino dark matter in the galactic halo. We confirm that neutralino annihilation can produce enough positrons to make up the measured excess only if there is an additional enhancement to the signal. We quantify the `boost factor' that is required in the signal for various models in the Minimal Supersymmetric Standard Model parameter space, and find that a boost factor >30 provides good fits to the HEAT data. Such an enhancement in the signal could arise if we live in a clumpy halo.
12/2002;
-
[show abstract]
[hide abstract]
ABSTRACT: The question of the nature of dark matter in the Universe remains one of the most outstanding unsolved problems in basic science. One of the best motivated particle physics candidates is the lightest supersymmetric particle, assumed to be the lightest neutralino. We here describe DarkSUSY, an advanced numerical FORTRAN package for supersymmetric dark matter calculations. With DarkSUSY one can: (i) compute masses and compositions of various supersymmetric particles; (ii) compute the relic density of the lightest neutralino, using accurate methods which include the effects of resonances, pair production thresholds and coannihilations; (iii) check accelerator bounds to identify allowed supersymmetric models; and (iv) obtain neutralino detection rates for a variety of detection methods, including direct detection and indirect detection through antiprotons, gamma-rays and positrons from the Galactic halo or neutrinos from the center of the Earth or the Sun.
12/2002;
-
[show abstract]
[hide abstract]
ABSTRACT: Using a new instrument, the HEAT collaboration has confirmed the excess of cosmic ray positrons that they first detected in 1994. We explore the possibility that this excess is due to the annihilation of neutralino dark matter in the galactic halo. We confirm that neutralino annihilation can produce enough positrons to make up the measured excess only if there is an additional enhancement to the signal. We quantify the `boost factor' that is required in the signal for various models in the Minimal Supersymmetric Standard Model parameter space, and study the dependence on various parameters. We find models with a boost factor greater than 30. Such an enhancement in the signal could arise if we live in a clumpy halo. We discuss what part of supersymmetric parameter space is favored (in that it gives the largest positron signal), and the consequences for other direct and indirect searches of supersymmetric dark matter. Comment: 11 pages, 6 figures, matches published version (PRD)
09/2001;
-
[show abstract]
[hide abstract]
ABSTRACT: The question of the nature of the dark matter in the Universe remains one of the most outstanding unsolved problems in basic science. One of the best motivated particle physics candidates is the lightest supersymmetric particle, assumed to be the lightest neutralino. We here describe DarkSUSY, an advanced numerical FORTRAN package for supersymmetric dark matter calculations which we release for public use. With the help of this package, the masses and compositions of various supersymmetric particles can be computed, for given input parameters of the minimal supersymmetric extension of the Standard Model (MSSM). For the lightest neutralino, the relic density is computed, using accurate methods which include the effects of resonances, pair production thresholds and coannihilations. Accelerator bounds are checked to identify viable dark matter candidates. Finally, detection rates are computed for a variety of detection methods, such as direct detection and indirect detection through antiprotons, gamma-rays and positrons from the Galactic halo or neutrinos from the center of the Earth or the Sun.
01/2001;
-
[show abstract]
[hide abstract]
ABSTRACT: Are there other planetary systems in our Universe? Indirect evidence has been found for planets orbiting other stars in our galaxy: the gravity of orbiting planets makes the star wobble, and the resulting periodic Doppler shifts have been detected for about a dozen stars. But are there planets in other galaxies, millions of light years away? Here we suggest a method to search for extragalactic planetary systems: gravitational microlensing of unresolved stars. This technique may allow us to discover planets in elliptical galaxies, known to be far older than our own Milky Way, with broad implications for life in the Universe.
10/1999;
-
[show abstract]
[hide abstract]
ABSTRACT: Pixel microlensing, i.e. gravitational microlensing of unresolved stars, can be used to explore distant stellar systems, and as a bonus may be able to detect extragalactic planets. In these studies, binary-lens events with multiple high-magnification peaks are crucial. Considering only those events which exhibit caustic crossings, we estimate the fraction of binary events in several example pixel microlensing surveys and compare them to the fraction of binary events in a classical survey with resolved stars. We find a considerable enhancement of the relative rate of binary events in pixel microlensing surveys, relative to surveys with resolved sources. We calculate the rate distribution of binary events with respect to the time between caustic crossings. We consider possible surveys of M31 with ground-based telescopes and of M87 with HST and NGST. For the latter, a pixel microlensing survey taking one image a day may observe of order one dozen binary events per month. Comment: 11 pages, 8 figures, version to appear in ApJ combined with astro-ph/9909510
09/1999;
-
[show abstract]
[hide abstract]
ABSTRACT: It has been shown that a flux--weighted full width at half maximum timescale of a microlensing event can be used in an unbiased estimator of the optical depth. For the first time, this allows a physical parameter to be easily estimated from pixel microlensing data. We derive analytic expressions for the observed rate of pixel lensing events as a function of the full width at half maximum timescale. This contrasts work in the literature which express rates in terms of an ``event duration'' or Einstein time, which require knowledge of the magnification, which is difficult to determine in a pixel event. The full width at half maximum is the most directly measured timescale. We apply these results to possible pixel lensing surveys, using HST for M87, and CFHT for M31. We predict M87 microlensing rates for the HST Advanced Camera and for NGST, and demonstrate that one will be able to probe the stellar IMF. Next, we describe a new method by which a crude measurement of the magnification can be made in the regime of magnifications A~10-100. This in turn gives a crude measurement of the Einstein time. This program requires good photometry and sampling in the low magnification tails of an event, but is feasible with today's technology. Comment: 18 pages, 30 figures, uses emulateapj
01/1999;
-
[show abstract]
[hide abstract]
ABSTRACT: Supersymmetric neutralinos are one of the most promising candidates for the dark matter in the Universe. If they exist, they should make up some fraction of the Milky Way halo. We investigate the fluxes of positrons expected at the Earth from neutralino annihilation in the halo. Positron propagation is treated in a diffusion model including energy loss. The positron source function includes contributions from both continuum and monochromatic positrons. We find that, for a "canonical" halo model and propagation parameters, the fluxes are generally too low to be visible. Given the large uncertainties in both propagation and halo structure, it is however possible to obtain observable fluxes. We also investigate the shapes of the positron spectra, including fits to a feature indicated by the results of the HEAT experiment. Comment: 16 pages, 19 figures, uses revtex
08/1998;
-
[show abstract]
[hide abstract]
ABSTRACT: We compute the neutralino decay rate in the minimal supersymmetric standard model with the addition of explicit R-parity violation. We include the complete squark and slepton mixing matrices, previously neglected, and we improve and correct published formulas. These decays are relevant to accelerator and nonaccelerator searches for R-parity violation, and are especially interesting in light of the reported high Q2 anomaly at DESY HERA. Journal Article
-
[show abstract]
[hide abstract]
ABSTRACT: We constrain the hadronic R-parity violating couplings in extensions to the minimal supersymmetric standard model. These interactions violate baryon and lepton number, and allow the lightest superpartner (LSP) to decay into standard model particles. The observed flux of cosmic ray antiprotons places a strong bound on the lifetime of the LSP in models where the lifetime is longer than the age of the universe. We exclude 10-18 ≤|λ"l≤&10-15 and 2x10-18≤|λ'|≤2x10-15 except in the case of a top quark, where we can only exclude 4x10-19≤|λ'|≤4x10-16. Journal Article
