arXiv subjects
Publications and source records attributed to P. Sikivie.
We have performed a high resolution search for galactic halo axions in cold flows using a microwave cavity detector. The analysis procedure and other details of this search are described. No axion signal was found in the mass range 1.98-2.17 micro-eV. We place upper limits on the density of axions in local discrete flows based on this result.
Theoretical arguments predict that the distribution of cold dark matter in spiral galaxies has peaks in velocity space associated with non-thermalized flows of dark matter particles. We searched for the corresponding peaks in the spectrum of microwave photons from axion to photon conversion in the cavity detector of dark matter axions. We found none and place limits on the density of any local flow of axions as a function of the flow velocity dispersion over the axion mass range 1.98 to 2.17 $μ$eV.
The axion is a hypothetical elementary particle and cold dark matter candidate. In this RF cavity experiment, halo axions entering a resonant cavity immersed in a static magnetic field convert into microwave photons, with the resulting photons detected by a low-noise receiver. The ADMX Collaboration presents new limits on the axion-to-photon coupling and local axion dark matter halo mass density from a RF cavity axion search in the axion mass range 1.9-2.3 microeV, broadening the search range to 1.9-3.3 microeV. In addition, we report first results from an improved analysis technique.
Dark matter caustics have specific density profiles and, therefore, precisely calculable gravitational lensing properties. We present a formalism which simplifies the relevant calculations, and apply it to four specific cases. In the first three, the line of sight is tangent to a smooth caustic surface. The curvature of the surface at the tangent point is positive, negative or zero. In the fourth case the line of sight passes near a cusp. For each we derive the map between the image and source planes. In some cases, a point source has multiple images and experiences infinite magnification when the images merge. Unfortunately, for the dark matter caustics expected in realistic galactic halo models, the angular resolution required to resolve the multiple images is not presently achievable. A more promising approach aims to observe the distortions caused by dark matter caustics in the images of extended sources such as radio jets.
Axion physics is briefly reviewed. Constraints from laboratory searches, astrophysics and cosmology require the axion mass to be in the range $10^{-6} \lesssim m_a < 3\cdot 10^{-3}$eV. Near the lower end of this range, axions are all or a major component of the cold dark matter of the universe. The late infall of axions, and of any other cold dark matter particles, onto our galaxy produces streams and caustics in its halo. The outer caustics are topological spheres whereas the inner caustics are rings. The self-similar model of galactic halo formation predicts that the caustic ring radii $a_n$ obey the approximate law $a_n \sim 1/n$. Evidence for this law has been found in a statistical study of 32 extended and well-measured external galactic rotation curves, and in the existence and distribution of sharp rises in the Milky Way rotation curve. Moreover, a triangular feature in the IRAS map of the Galactic plane is consistent with the imprint of a ring caustic upon the baryonic matter. Its position coincides with a rise in the rotation curve, the one nearest to us. These observations imply that the dark matter in our neighborhood is dominated by a single flow. Estimates of that flow's density and velocity vector are given.
Most of the mass of the Milky Way galaxy is contributed by its halo, presumably in the form of non-interacting cold dark matter. The axion is a compelling cold dark matter candidate. We report results from a search which probes the local galactic halo axion density using the Sikivie RF cavity technique. Candidates over the frequency range 550$$ $\le$ f $\le$$$ 810 MHz (2.3$$ $μ$eV $\le$$$ m$_{a}$ $\le$ 3.4 $μ$eV) were investigated. The absence of a signal suggests that KSVZ axions contribute no more than 0.45 GeV/cm$^3$ of mass density to the local dark matter halo over this mass range.
The late infall of cold dark matter onto an isolated galaxy, such as our own, produces streams and caustics in its halo. The outer caustics are topological spheres whereas the inner caustics are rings. The self-similar model of galactic halo formation predicts that the caustic ring radii $a_n$ follow the approximate law $a_n \sim 1/n$. In a study of 32 extended and well-measured external galactic rotation curves evidence was found for this law. In the case of the Milky Way, the locations of eight sharp rises in the rotation curve fit the prediction of the self-similar model at the 3% level. Moreover, a triangular feature in the IRAS map of the galactic plane is consistent with the imprint of a ring caustic upon the baryonic matter. These observations imply that the dark matter in our neighborhood is dominated by a single flow. Estimates of that flow's density and velocity vector are given.
This paper revisits the problem of the string decay contribution to the axion cosmological energy density. We show that this contribution is proportional to the average relative increase when axion strings decay of a certain quantity $N_{\rm ax}$ which we define. We carry out numerical simulations of the evolution and decay of circular and non-circular string loops, of bent strings with ends held fixed, and of vortex-antivortex pairs in two dimensions. In the case of string loops and of vortex-antivortex pairs, $N_{\rm ax}$ decreases by approximately 20%. In the case of bent strings, $N_{\rm ax}$ remains constant or increases slightly. Our results imply that the string decay contribution to the axion energy density is of the same order of magnitude as the well-understood contribution from vacuum realignment.
The late infall of cold dark matter onto an isolated galaxy such as our own produces flows with definite velocity vectors at any physical point in the galactic halo. It also produces caustics which are places where the dark matter density is very large. The outer caustics are topological spheres whereas the inner caustics are rings. The self-similar model of galactic halo formation predicts that the caustic ring radii $a_n$ follow the approximate law $a_n \sim 1/n$. In a recent study of 32 extended and well-measured galactic rotation curves, we found evidence for this law.
The strong CP problem and its resolution through the existence of an axion are briefly reviewed. The constraints on the axion from accelerator searches, from the evolution of red giants and from supernova SN1987a combine to require $m_a < 3 \cdot 10^{-3}$ eV, where $m_a$ is the axion mass. On the other hand, the constraint that axions do not overclose the universe implies $m_a \gtwid 10^{-6}$ eV. If $m_a \sim 10^{-5}$ eV, axions contribute significantly to the cosmological energy density in the form of cold dark matter. Dark matter axions can be detected by resonant conversion to microwave photons in a cavity permeated by a static magnetic field and tuned to the axion mass. Experiments using this effect are described, as well as several other types of axion searches.
I investigate the caustics produced by the fall of collisionless dark matter in and out of a galaxy in the limit of negligible velocity dispersion. The outer caustics are spherical shells enveloping the galaxy. The inner caustics are rings. These are located near where the particles with the most angular momentum are at their distance of closest approach to the galactic center. The surface of a caustic ring is a closed tube whose cross-section is a $D_{-4}$ catastrophe. It has three cusps amongst which exists a discrete $Z_3$ symmetry. A detailed analysis is given in the limit where the flow of particles is axially and reflection symmetric and where the transverse dimensions of the ring are small compared to the ring radius. Five parameters describe the caustic in that limit. The relations between these parameters and the initial velocity distribution of the particles are derived. The structure of the caustic ring is used to predict the shape of the bump produced in a galactic rotation curve by a caustic ring lying in the galactic plane.
We give a systematic discussion of the contributions to the cosmological energy density in axions from vacuum realignment, string decay and wall decay. We call these the cold axion populations because their kinetic energy per particle is at all times much less than the ambient temperature. In case there is no inflation after the Peccei-Quinn phase transition, the value of the axion mass for which axions contribute the critical energy density for closure is estimated to be of order $6 \cdot 10^{-6}$ eV, with large uncertainties. It is emphasized that there are two groups of cold axions differing in velocity dispersion by a factor of order $10^3$.
The late infall of cold dark matter onto an isolated galaxy produces flows with definite local velocity vectors throughout the galactic halo. It also produces caustic rings, which are places in the halo where the dark matter density is very large. The self-similar model of halo formation predicts that the caustic ring radii $a_n$ follow the approximate law $a_n \simeq 1/n$. I interpret bumps in the rotation curves of NGC 3198 and of our own galaxy as due to caustic rings of dark matter. In this model of our halo the annual modulation effect in direct searches for WIMPs has the opposite sign from that predicted by the isothermal sphere model.
We discuss the appearance at the QCD phase transition, and the subsequent decay, of axion walls bounded by strings in N=1 axion models. We argue on intuitive grounds that the main decay mechanism is into barely relativistic axions. We present numerical simulations of the decay process. In these simulations, the decay happens immediately, in a time scale of order the light travel time, and the average energy of the radiated axions is $<ω_a > \simeq 7 m_a$ for $v_a/m_a \simeq 500$. $<ω_a>$ is found to increase approximately linearly with $\ln(v_a/m_a)$. Extrapolation of this behaviour yields $<ω_a> \sim 60 m_a$ in axion models of interest. We find that the contribution to the cosmological energy density of axions from wall decay is of the same order of magnitude as that from vacuum realignment, with however large uncertainties. The velocity dispersion of axions from wall decay is found to be larger, by a factor $10^3$ or so, than that of axions from vacuum realignment and string decay. We discuss the implications of this for the formation and evolution of axion miniclusters and for the direct detection of axion dark matter on Earth. Finally we discuss the cosmology of axion models with $N>1$ in which the domain wall problem is solved by introducing a small U$_{PQ}$(1) breaking interaction. We find that in this case the walls decay into gravitational waves.
We discuss the decay of axion walls bounded by strings and present numerical simulations of the decay process. In these simulations, the decay happens immediately, in a time scale of order the light travel time, and the average energy of the radiated axions is $<ω_a > \simeq 7 m_a$ for $v_a/m_a\simeq 500$. $<ω_a>$ is found to increase approximately linearly with $\ln(v_a/m_a)$. Extrapolation of this behaviour yields $<ω_a> \simeq 60 m_a$ in axion models of interest.
We have studied numerically the evolution and decay of axion strings. These global defects decay mainly by axion emission and thus contribute to the cosmological axion density. The relative importance of this source relative to misalignment production of axions depends on the spectrum. Radiation spectra for various string loop configurations are presented. They support the contention that the string decay contribution is of the same order of magnitude as the contribution from misalignment.
We report the first results of a high-sensitivity (10^{-23} Watt) search for light halo axions through their conversion to microwave photons. At 90 percent confidence we exclude a KSVZ axion of mass 2.9 x 10^{-6} eV to 3.3 x 10^{-6} eV as the dark matter in the halo of our Galaxy.
The spectrum of cold dark matter particles on Earth is expected to have peaks in velocity space associated with particles which are falling onto the Galaxy for the first time and with particles which have fallen in and out of the Galaxy only a small number of times in the past. We obtain estimates for the velocity magnitudes and the local densities of the particles in these peaks. To this end we use the secondary infall model of galactic halo formation which we have generalized to take account of the angular momentum of the dark matter particles. The new model is still spherically symmetric and it admits self-similar solutions. In the absence of angular momentum, the model produces flat rotation curves for a large range of values of a parameter $ε$ which is related to the spectrum of primordial density perturbations. We find that the presence of angular momentum produces an effective core radius, i.e. it makes the contribution of the halo to the rotation curve go to zero at zero radius. The model provides a detailed description of the large scale properties of galactic halos including their density profiles, their extent and total mass. We obtain predictions for the kinetic energies of the particles in the velocity peaks and estimates for their local densities as functions of the amount of angular momentum, the age of the universe and $ε$.