arXiv subjects

Marc Kamionkowski

Publications and source records attributed to Marc Kamionkowski.

360 records · Page 20Linked to original sources

arXiv1994

Model-Independent Comparison of Direct vs. Indirect Detection of Supersymmetric Dark Matter

We compare the rate for elastic scattering of neutralinos from various nuclei with the flux of upward muons induced by energetic neutrinos from neutralino annihilation in the Sun and Earth. We consider both scalar and axial-vector interactions of neutralinos with nuclei. We find that the event rate in a kg of germanium is roughly equivalent to that in a $10^5$- to $10^7$-m$^2$ muon detector for a neutralino with primarily scalar coupling to nuclei. For an axially coupled neutralino, the event rate in a 50-gram hydrogen detector is roughly the same as that in a 10- to 500-m$^2$ muon detector. Expected experimental backgrounds favor forthcoming elastic-scattering detectors for scalar couplings while the neutrino detectors have the advantage for axial-vector couplings.

hep-ph↗

arXiv1994

Microlensing by Stars

If stars at the lower end of the main sequence are responsible for the microlensing events observed in the Galactic bulge, then light from the lensing star contributes to the observed brightness. The background and lensing stars generally have different colors, and the relative brightness changes during the microlensing event. Therefore, microlensing light curves are not perfectly achromatic if hydrogen-burning stars are the lenses. In most cases, the color shift will be too small to be observable, but we argue that given the current microlensing rates, it is plausible that a few color-shifted microlensing events could be observed in the near future, especially if strategies are optimized to search for them. Although rare, such an event could potentially provide a wealth of information: Light curves in two bands can be used to determine the masses and distances of the two stars as well as the transverse speed of the lensing star. Light curves in additional wavebands could make the determination more precise.

astro-ph↗

arXiv1994

Neutrinos From Particle Decay in the Sun and Earth

Weakly interacting massive particles (WIMPs) may be indirectly detected by observation of upward muons induced by energetic neutrinos from annihilation of WIMPs that have accumulated in the Sun and/or Earth. Energetic muon neutrinos come from the decays of $τ$ leptons, $c$, $b$, and $t$ quarks, gauge bosons, and Higgs bosons produced by WIMP annihilation. We provide analytic expressions, suitable for computing the flux of upward muons, for the neutrino energy spectra from decays of all these particles in the center of the Sun and Earth. These analytic expressions should obviate the need for Monte Carlo calculations of the upward-muon flux. We investigate the effects of polarization of the gauge bosons on the neutrino spectra and find that they are small. We also present simple expressions for the second moments of the neutrino distributions which can be used to estimate the rates for observation of neutrino-induced muons from WIMP annihilation.

hep-ph↗

arXiv1994

Cosmic Microwave Background Anisotropies and the Geometry of the Universe

In this talk, I review some recent work on cosmic microwave background (CMB) anisotropies in an open universe. I emphasize that the observed CMB anisotropies are still consistent with a low value of $Ω$, and I address the question of whether future CMB measurements will be able to provide information on the geometry of the Universe.

astro-ph↗

arXiv1994

CBR Anisotropy in an Open Inflation, CDM Cosmogony

We compute the cosmic background radiation anisotropy, produced by energy-density fluctuations generated during an early epoch of inflation, in an open cosmological model based on the cold dark matter scenario. At $Ω_0 \sim$ 0.3 -- 0.4, the COBE normalized open model appears to be consistent with most observations.

astro-ph↗

arXiv1994

Implications of Recent Nucleon Spin Structure Measurements for Neutralino Dark Matter Detection

Predicted rates for direct and indirect detection of dark-matter neutralinos depend in general on the spin content of the nucleon. Neutralinos that are predominantly $B$-ino are the likeliest candidates for detection via spin-dependent interactions. Uncertainties in the measured spin content of the nucleon may lead to dramatic uncertainties in the rates for detection of $B$-inos by scattering off of nuclei with unpaired neutrons. Rates for spin-dependent scattering of $B$-inos off of nuclei with unpaired protons are far more robust, as are rates for capture of $B$-inos in the Sun.

hep-ph↗

arXiv1994

Diffuse Gamma Rays from WIMP Decay and Annihilation

I discuss contributions to the diffuse gamma-ray background from decay and annihilation of weakly interacting massive particles (WIMPs). I first review the calculation of the cosmological abundance of a WIMP and show that it is simply related to the cross section for annihilation of the WIMP into lighter particles. The diffuse extragalactic background radiation (DEBRA) from WIMP decay is then discussed. I show how observational upper limits to the DEBRA can be used to constrain properties of WIMPs that decay to photons, and I present additional new constraints that unitarity of the annihilation cross section imposes on such particles. I then discuss gamma rays from annihilation of WIMPs in the halo, spheroid, and disk of our galaxy, as well as those from WIMP annihilation in the Large Magellanic Cloud. (To appear in The Gamma Ray Sky with Compton GRO and SIGMA, proceedings of the School, Les Houches, France, January 25-February 4, 1994.)

astro-ph↗

arXiv1994

Indirect Detection of WIMPs

I review several proposed techniques for indirect detection of weakly interacting massive particles (WIMPs) in the halo. I focus on distinctive signatures from cosmic-ray positrons, antiprotons, and gamma rays produced by annihilation of WIMPs in the galactic halo. (To appear in Particle Astrophysics, Atomic Physics, and Gravitation}, proceedings of the the XIVth Moriond Workshop, Villars sur Ollon, Switzerland, Jan. 22-29, 1994.)

hep-ph↗

arXiv1994

Microwave Background Fluctuations in an Open Universe

( to appear in: Proceedings of the Nishonomiya Yukawa Memorial Symposium Edited by M. Sasaki) There are several models for generating fluctuations in an open universe that are compatible with the microwave background fluctuations detected by COBE {\it and} observations of large scale structure. Topological defects, such as strings and textures, appear to be more successful in a low-$Ω$ model than in an $Ω= 1$ model. These models predict that the cosmic microwave background (CMB) fluctuations are non-Gaussian with distinctive signatures that may lead to their confirmation. If there exists a mechanism for generating scale invariant mass fluctuations, $(δM/M)^2 \propto M^{-4/3}$, then these models are also compatible with COBE. Both models predict mass fluctuations on the 8/h Mpc scale, $σ_8$, $\sim 0.5 - 1$ for $Ωh \sim 0.2 - 0.3$. A variety of observations of large-scale structure, clusters and galaxies, as well as the age problem, suggest that the low-$Ω$ models are attractive.

astro-ph↗

arXiv1994

Small-Scale Cosmic Microwave Background Anisotropies as a Probe of the Geometry of the Universe

We perform detailed calculations of cosmic microwave background (CMB) anisotropies in a CDM-dominated open universe with primordial adiabatic density perturbations for a variety of reionization histories. We show that to a great extent, the CMB anisotropies depend only on the geometry of the Universe, which in a matter dominated universe is determined by $Ω$, and the optical depth to the surface of last scattering. In particular, the location of the primary Doppler peak depends primarily on $Ω$ and is fairly insensitive to the other unknown parameters, such as $Ω_b$, $h$, $Λ$, and the shape of the power spectrum. Therefore, measurements of CMB anisotropies on small scales may be used to determine $Ω$.

astro-ph↗

arXiv1993

Large-Angle Cosmic Microwave Background Anisotropies in an Open Universe

If the Universe is open, scales larger than the curvature scale may be probed by large-angle fluctuations in the cosmic microwave background (CMB). We consider primordial adiabatic perturbations and discuss power spectra that are power laws in volume, wavelength, and eigenvalue of the Laplace operator. The resulting large-angle anisotropies of the CMB are computed. The amplitude generally increases as $Ω$ is decreased, but decreases as $h$ is increased. Interestingly enough, for all three ansatzes, anisotropies on angular scales larger than the curvature scale are suppressed relative to the anisotropies on scales smaller than the curvature scale. Models with $0.2<Ωh<0.3$ appear compatible with CMB fluctuations detected by COBE and Tenerife and with the amplitude and spectrum of fluctuations of galaxy counts in galaxy surveys. COBE normalization for these models yields $σ_8\simeq 0.5-0.7$. Models with smaller values of $Ωh$ when normalized to COBE require bias factors in excess of 2 to be compatible with the observed galaxy counts on the $8/h$ Mpc scale. Requiring that the age of the universe exceed 10 Gyr implies that $Ω>0.25$, while requiring that the age exceed 13 Gyr implies that $Ω>0.35$. Unlike in the flat-Universe case where the anisotropy comes only from the last-scattering term in the Sachs-Wolfe formula, large-angle anisotropies come primarily from the decay of potential fluctuations at $z<1/Ω$. Thus, if the Universe is open, COBE has been detecting fluctuations produced at moderate redshift rather than at $z\sim 1300$.

astro-ph↗

arXiv1993

Gravitational Radiation from First-Order Phase Transitions

We consider the stochastic background of gravity waves produced by first-order cosmological phase transitions from two types of sources: colliding bubbles and hydrodynamic turbulence. First we discuss the fluid mechanics of relativistic spherical combustion. We then numerically collide many bubbles expanding at a velocity $v$ and calculate the resulting spectrum of gravitational radiation in the linearized gravity approximation. Our results are expressed as simple functions of the mean bubble separation, the bubble expansion velocity, the latent heat, and the efficiency of converting latent heat to kinetic energy of the bubble walls. We also estimate the gravity waves produced by a Kolmogoroff spectrum of turbulence and find that the characteristic amplitude of gravity waves produced is comparable to that from bubble collisions. Finally, we apply these results to the electroweak transition. Using the one-loop effective potential for the minimal electroweak model, the characteristic amplitude of gravity waves produced is $h\simeq 1.5\times 10^{-27}$ at a characteristic frequency of $4.1\times 10^{-3} \,\rm Hz$ corresponding to $Ω\sim10^{-22}$ in gravity waves, far too small for detection. Gravity waves from more strongly first-order phase transitions, including the electroweak transition in non-minimal models, have better prospects for detection, though probably not by LIGO.

astro-ph↗

arXiv1993

Cosmic-Ray Antiprotons from Neutralino Annihilation into Gluons

We estimate the flux of cosmic-ray antiprotons expected from the annihilation of neutralinos in the galactic halo. The antiproton signal may offer an important alternative detection scheme in the case that neutralino annihilation proceeds mainly to the two-gluon final state.

astro-ph↗

arXiv1993

Vacuum-Polarization Corrections to Solar-Fusion Rates

The vacuum-polarization corrections to rates for nuclear-fusion reactions in the $pp$ chain and in the CNO cycle are calculated. For the reactions of particular importance to the solar-neutrino problem, the $\Hethree$, $\Hefour$, $\Be$, and $\Ni$ reactions, we find the magnitude of the effect to be less than 2\%. The effect of VP on all the other reaction rates is expected to be of a similar order of magnitude. We discuss how these results affect the predicted fluxes of solar neutrinos.

astro-ph↗

arXiv1993

The Rate of the Proton-Proton Reaction

We re-evaluate the matrix element for the proton-proton reaction which is important for stellar-evolution calculations and for the solar-neutrino problem. We self-consistently determine the effect of vacuum polarization on the matrix element by first correcting the low-energy scattering data to account for vacuum polarization. We then calculate the proton-proton wave function by integrating the Schrodinger equation with vacuum polarization included. We use improved data for proton-proton scattering and for the deuteron wave function. We evaluate the uncertainties that are due to experimental error and estimate those that are due to theoretical inadequacies. We estimate the theoretical uncertainty by using six different deuteron potentials and five different proton-proton potentials. Vacuum polarization decreases the calculated value by $0.6_{-0.4}^{+0.1}$\%. The complete result is $Λ^2=6.92\times(1\pm0.002^{+0.014}_{-0.009})$ where the first uncertainty is due to experimental errors and the second uncertainty is due to theoretical uncertainties. Our value of $Λ^2$ is 2\% smaller than the value obtained in 1969 by Bahcall and May. The improved calculations of the rate of the $pp$ reaction described here increase slightly the predicted event rates for the chlorine and the Kamiokande solar-neutrino experiments.

astro-ph↗

arXiv1992

Instability and Subsequent Evolution of Electroweak Bubbles

Bubbles in a first-order electroweak phase transition are nucleated with radii $R_0$ and expand with velocity $v$. If $v$ is subsonic, a bubble becomes unstable to non-spherical perturbations when its radius is roughly $10^4\, R_0$. These perturbations accelerate the transition, and the effective velocity of bubble growth rapidly becomes supersonic. The transition should subsequently proceed spherically via detonation. If for some reason the onset of detonation is postponed, the surface area of the bubbles may be enhanced by $10^5$. We discuss consequences for electroweak baryogenesis.

hep-ph↗

arXiv1992

Planck-Scale Physics and the Peccei-Quinn Mechanism

Global-symmetry violating higher-dimension operators, expected to be induced by Planck-scale physics, in general drastically alter the properties of the axion field associated with the Peccei-Quinn solution to the strong-CP problem, and render this solution unnatural. The particle physics and cosmology associated with other global symmetries can also be significantly changed.

hep-th↗

arXiv1992

Are Textures Natural?

We make the simple observation that, because of global symmetry violating higher-dimension operators expected to be induced by Planck-scale physics, textures are generically much too short-lived to be of use for large-scale structure formation.

hep-th↗