The Search for the Dark Matter: WIMPs and MACHOs

Review talk presented at the Texas/PASCOS Symposium, Berkeley, CA, Dec 1992. We review the status of experiments and ideas relevant for the detection of the dark matter which is suspected to be the dominant constituent of the Universe. Great progress is being made and the chances are non-negligible that one of the many currently in-progress experiments will discover the nature of the dark matter. We discuss the main dark matter candidates, and review the experiments relevant to each of them.Comment: 17 pages, tex, figures not include

The First Data from the MACHO Experiment

MAssive Compact Halo Objects such as brown dwarfs, Jupiters, and black holes are prime candidates to comprise the dark halo of our galaxy. Paczynski noted that objects (dubbed MACHOs) with masses in the range 10^{-6}M_\odot < M \simlt 100 M_\odot. can be detected via gravitational microlensing of stars in the Magellanic Clouds with the caveat that only about one in $10^6$ stars will be lensed at any given time. Our group has recently begun a search for microlensing using a refurbished 1.27 meter telescope at the Mount Stromlo Observatory in Australia. Since the summer of 1992, we have been imaging up to $10^7$ stars a night in the Large Magellanic Cloud using our large format two-color $3.4\times 10^7$ pixel CCD camera. Here I report on our first results based on an analysis of $\sim 10^6$ of these stars. Although this is not enough data to make definitive statements about the nature of the dark matter, we are able to conclude that the rate of variable star background events is not larger than the expected MACHO signal.Comment: et. al., 7 pages, UCRL-JC-1111360

On the Neutralino as Dark Matter Candidate - I. Relic Abundance

The neutralino relic abundance is evaluated for a wide range of the neutralino mass, ${\rm 20\ GeV} \leq m_\chi \leq {\rm 1\ TeV}$, by taking into account the full set of final states in the neutralino-neutralino annihilation. The analysis is performed in the Minimal SuSy Standard Model; it is not restricted by stringent GUT assumptions but only constrained by present experimental bounds. We also discuss phenomenological aspects which are employed in the companion paper (II. Direct Detection) where the chances for a successful search for dark matter neutralino are investigated.Comment: (10 pages plain TeX, 8 figures not included, available from the authors) DFTT-37/9

Probing fundamental constant evolution with neutral atomic gas lines

We have detected narrow HI 21cm and CI absorption at $z \sim 1.4 - 1.6$ towards Q0458$-$020 and Q2337$-$011, and use these lines to test for possible changes in the fine structure constant $\alpha$, the proton-electron mass ratio $\mu$, and the proton gyromagnetic ratio $g_p$. A comparison between the HI 21cm and CI line redshifts yields $\Delta X/X = [+6.8 \pm 1.0] \times 10^{-6}$ over $0 \le 1.46$, where $X = g_p \alpha^2/\mu$, and the errors are purely statistical, from the gaussian fits. The simple line profiles and the high sensitivity of the spectra imply that statistical errors in this comparison are an order of magnitude lower than in previous studies. Further, the CI lines arise in cold neutral gas that also gives rise to HI 21cm absorption, and both background quasars are core-dominated, reducing the likelihood of systematic errors due to local velocity offsets between the hyperfine and resonance lines. The dominant source of systematic error lies in the absolute wavelength calibration of the optical spectra, which appears uncertain to $\sim 2$ km/s, yielding a maximum error in $\Delta X/X$ of $\sim 6.7 \times 10^{-6}$. Including this, we obtain $\Delta X/X = [+6.8 \pm 1.0 (statistical) \pm 6.7 (max. systematic)] \times 10^{-6}$ over $0 \le 1.46$. Using literature constraints on $\Delta \mu/\mu$, this is inconsistent with claims of a smaller value of $\alpha$ from the many-multiplet method, unless fractional changes in $g_p$ are larger than those in $\alpha$ and $\mu$.Comment: 5 pages, 3 figures, accepted for publication in ApJ

Solitosynthesis: Cosmological evolution of non-topological solitons

The thermal creation, fusion, evaporation, and destruction of non-topological solitons (NTS) after a phase transition in the early universe is considered. By defining and following NTS statistical equilibrium and departures from it, and depending on particle physics parameters, one of three possible scenarios occurs. If reaction rates are high enough, a period of equilibrium occurs and relic abundances are determined by the freeze-out temperature. Equilibrium first drives most NTS's into their constituents (free phi particles) and then causes rapid fusion into large NTS's. If freeze-out occurs during the first phase, the NTS's are almost entirely destroyed, while if it occurs during the second phase, solitosynthesis occurs and NTS's may be cosmically relevant. For slow reaction rates the NTS's are born frozen out and have the abundance determined by the phase transition. Analytic approximations for determining the abundances are developed, and tested by numerically integrating a reaction network in an expanding universe. Unfortunately, for most of the parameter space considered, solito-destruction/evaporation occurs

Rate for annihilation of galactic dark matter into two photons

A calculation of the cross section for neutralino-neutralino annihilation into two photons is performed and applied to dark matter in the galactic halo to find the counting rate in a large gamma ray detector such as EGRET (Energetic Gamma Ray Experiment Telescope) or ASTROGAM. Combining constraints from particle accelerators with the requirement that the neutralinos make up the dark matter, it is found that rates of over a few dozen events per year are unlikely. The assumptions that go into these conclusions are listed. Other particle dark matter candidates which could give larger and perhaps observable signals are suggested

Exact Cross Sections for the Neutralino WIMP Pair-Annihilation

We derive a full set of exact, analytic expressions for the annihilation of the lightest neutralino pairs into all two-body tree-level final states in the framework of minimal supersymmetry. We make no simplifying assumptions about the neutralino nor about sfermion masses and mixings other than the absence of explicit CP--violating terms. The expressions should be particularly useful in computing the neutralino WIMP relic abundance without the usual approximation of partial wave expansion.Comment: LaTeX, 46 pages, no figures. Several minor typographical errors correcte

Experimental Limits on Primordial Black Hole Dark Matter from the First Two Years of Kepler Data

We present the analysis on our new limits of the dark matter (DM) halo consisting of primordial black holes (PBHs) or massive compact halo objects (MACHOs). We present a search of the first two years of publicly available Kepler mission data for potential signatures of gravitational microlensing caused by these objects, as well as an extensive analysis of the astrophysical sources of background error. These include variable stars, flare events, and comets or asteroids which are moving through the Kepler field. We discuss the potential of detecting comets using the Kepler lightcurves, presenting measurements of two known comets and one unidentified object, most likely an asteroid or comet. After removing the background events with statistical cuts, we find no microlensing candidates. We therefore present our Monte Carlo efficiency calculation in order to constrain the PBH DM with masses in the range of 2 x 10^-9 solar masses to 10^-7 solar masses. We find that PBHs in this mass range cannot make up the entirety of the DM, thus closing a full order of magnitude in the allowed mass range for PBH DM.Comment: 12 pages, 6 figure