Session J5: Dwarf Galaxies, Dark Matter and Magnetic Fields

Estimating the GeV Emission of Millisecond Pulsars in Dwarf Spheroidal Galaxies

Dwarf spheroidal satellite galaxies (dSphs) are among the most dark matter dominated systems in the cosmos, which, complemented by a favorable proximity to the Milky Way, makes them extremely important targets in the ongoing search for indirect dark matter detection via gamma rays. While the conventional astrophysical background in dSphs has long been assumed to be negligible, Fermi LAT measurements of a population of luminous gamma-ray emitting galactic millisecond pulsars (MSPs) potentially challenge this assumption. With that in mind, we present an estimate of the conventional astrophysical emission intrinsic to 30 dSphs of the Milky Way, focusing on MSPs, and evaluate the potential for confusion with dark matter annihilation signatures at GeV energies. We predict that MSPs in the highest stellar mass dSphs, Fornax and Sculptor, produce a gamma-ray flux that is approximately a factor of 10 below the current LAT sensitivity. However, for ultra-faint dSphs, typically the most dark matter dominated, we estimate the MSP emission to be several orders of magnitude below both the LAT sensitivity and the flux expected from dark matter annihilation, suggesting that these targets will remain safe for indirect dark matter searches in the foreseeable future.   

Searches for Angular Extension in High Latitude Fermi-LAT Sources

We present a comprehensive search for angular extension in high-latitude gamma-ray sources detected by the Fermi Large Area Telescope (LAT) using the 4-year LAT Point Source Catalog (3FGL). The majority of high-latitude LAT sources are extragalactic blazars that appear point-like within the LAT angular resolution. However, there are physics scenarios that predict populations of spatially extended sources. In one scenario, electron-positron pair cascades from gamma rays produced in blazars are deflected in the Intergalactic Magnetic Field (IGMF) producing extended emission, or "pair halos". The detection of a pair halo component around a LAT-detected blazar would provide a measurement of the strength and coherence length scale of the IGMF. In another scenario, the annihilation or decay of Weakly Interacting Massive Particles, a candidate for dark matter (DM), in Milky Way subhalos would appear as a population of unassociated gamma-ray sources with an angular extension. The detection of spatial extension in nearby sub halos could provide compelling evidence for a DM interpretation and would serve as an independent cross-check against other DM searches. We report on the angular extension catalog based on 7.5 years of Pass 8 data and discuss the implications of these results.   

Intergalactic Magnetic Field Observations and their Fundamental Implications

I will review current observational evidence for helical intergalactic magnetic fields at the ~$10^-14$ G level on ~10 Mpc length scales. The existence of magnetic fields in cosmic voids and their non-trivial helical structure suggest that they might have originated in the early universe due to CP violating fundamental interactions. The large helicity of the magnetic field suggests a possible crucial role for chiral MHD effects in the early universe.   

Zero-Range Effective Field Theory for Resonant Wino Dark Matter

The most dramatic ``Sommerfeld enhancements'' of neutral-wino-pair annihilation occur when the wino mass is tuned to near critical values where there is a zero-energy S-wave resonance at the neutral-wino-pair threshold. If the wino mass is larger than the critical value, the resonance is a wino-pair bound state. If the wino mass is near a critical value, low-energy winos can be described by a zero-range effective field theory in which the winos interact nonperturbatively through a contact interaction. The parameters of the zero-range effective field theory can be determined by matching wino scattering amplitudes calculated by solving the Schr\"odinger equation for a nonrelativistic effective field theory in which the winos interact nonperturbatively through a potential due to the exchange of weak gauge bosons. The power of the zero-range effective field theory is illustrated by calculating the rate for formation of the bound state in the collision of two neutral winos through the emission of two soft photons.   

Elementary particles, dark matter candidate and new extended standard model

Elementary particle decays and reactions are discussed in terms of the three-dimensional quantized space model beyond the standard model. Three generations of the leptons and quarks correspond to the lepton charges. Three heavy leptons and three heavy quarks are introduced. And the bastons (new particles) are proposed as the possible candidate of the dark matters. Dark matter force, weak force and strong force are explained consistently. Possible rest masses of the new particles are, tentatively, proposed for the experimental searches. For more details, see the conference paper at https://www.researchgate.net/publication/308723916 .   

Dark matter candidate with well-defined mass and couplings

There is as yet no confirmed and statistically significant evidence for direct, indirect, or collider-based detection of dark matter. However, several indirect searches, including AMS-02, Fermi-LAT, and PAMELA, have shown an intriguing excess of positrons when compared to expectations. Here we predict a Higgs-related but spin 1/2 dark matter candidate with a mass of 125 GeV. Since an initially reported 130 GeV gamma-ray excess has been abandoned by the full Fermi-LAT collaboration, this is a genuine prediction rather than postdiction. It would be consistent with a prediction of 125 GeV freshly-created positrons and antiprotons, but the complicated propagation of charged particles makes a comparison problematical.   

Constraining Proton-Dark Matter Scattering Using Cosmic Ray Measurements

Dark matter scattering cross sections with protons and electrons are largely unconstrained below a dark matter mass of 1 GeV. By considering propagation of cosmic rays through a region with the dark matter density of the Milky Way, we demonstrate that if electrons and protons scatter with dark matter, this interaction adds an energy loss term to the cosmic ray propagation equation. This energy loss term distorts the calculated cosmic ray spectra from the observed spectra. By fitting the calculated spectra to data, we can set limits on the proton and electron scattering cross sections for dark matter.   

Dark Matter Annihilation and Decay limits with HAWC

The High Altitude Water Cherenkov (HAWC) gamma-ray observatory is a wide field-of-view observatory sensitive to 100 GeV - 100 TeV gamma-rays and cosmic-rays in the state of Puebla, Mexico at an altitude of 4100m. The HAWC observatory performed an indirect search for dark matter via GeV-TeV photons resulting from dark matter annihilation and decay. We considered the HAWC sensitivity to a set of sources, including 15 individual dwarf spheroidal galaxies (dSphs), the M31 galaxy and the Virgo cluster, as well as a combined limit using 15 dSphs. HAWC has not seen statistically significant excess from these sources. Being a survey experiment, HAWC will include any newly found dark matter rich sources, such as recently discovered TriangulumII dwarf galaxy. We explored dark matter masses above 1 TeV, including masses higher than 70 TeV that are currently unconstrained. We will present the annihilation cross-section and decay lifetime limits.