Simona Murgia: Dark Matter searches with GLAST May 23, 2008

Posted by dorigo in astronomy, cosmology, physics, science.
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Now linked by Peter Woit’s blog with appreciative words, I cannot escape my obligation to continue blogging on the talks I have been listening at PPC2008. So please find below some notes from Simona’s talk on the GLAST mission and its relevance for dark matter (DM) searches.

Glast will observe gamma rays in the energy range 20 MeV to 300 GeV. A better flux sensitivity than earlier experiments such as EGRET and AGILE. It is a 5-year mission, with a final goal of 10 years. It will orbit at 565 km of altitude, with a 25.6° inclination with the terrestrial equator. It has a large area telescope (LAT), for pair conversions. Features a precision Silicon strip tracker, endowed with 18 xy tracking planes with tungsten layers interleaved. The tracker is followed by a small calorimeter with CsI crystals. Tracker surrounded by anti-coincidence detector, 89 plastic scintillator tiles. The segmented design avoids self-veto problems. The total payload of GLAST is 2000 kg.

GLAST has four times the field of view of EGRET, and it covers the whole sky in two orbits (3 hours). The broad energy range has never been explored at this sensitivity. The energy resolution is about 10%, and the point-spread function is 7.2 arcminutes above 10 GeV. More than 30x better sensitivity than previous searches below 10 GeV, x100 at higher energy.

EGRET cataloged 271 sources of gamma rays, GLAST expects to do thousands. Active galactic nuclei, gamma ray bursts, supernova remnants, pulsars, galaxies, clusters, x-ray binaries. There is very small gamma ray attenuation below 10 GeV, so GLAST can probe cosmological distances.

Simona asked herself, what is the nature of DM? There are several models out. GLAST will investigate the existence of weakly interacting massive particles (WIMPS) through two-photon annihilation. Not an easy task, for there are large uncertainties in the signal and in the background. The detection of a DM signal from GLAST would be complementary to others.

Gamma rays may come from neutral pions emitted in $\chi \chi$ annihilation. These give a continuum spectrum. Instead, direct annihilation to two photons is expected to have a branching ratio of $10^{-3}$ or less, but the latter would provide a line in the spectrum, a spectacular signal.

Other models provide an even more distinctive gamma spectrum. With the gravitino as a lightest supersymmetric particle, it would have a very long lifetime, and it could decay into photon and neutrino: this yields a enhanced line, and then a continuum spectrum at lower energy.

Instrumental backgrounds mostly come from charged particles (protons, electrons, positrons). Also neutrons, and earth albedo photons. These dominate the flux from cosmic photons. But less than one in hundred thousand survives the photon selection. Above a few GeV, background contamination is required to be less than 10% of the isotropic photons measured by EGRET.

Searches for WIMP annihilations can be done in the galactic center or complementary in the galactic halo. In the latter case there is no source crowding, but significant uncertainties in the astrophysical backgrounds. The 3-sigma signal on $<\sigma v>$ as a function of mass of the WIMP goes below $10^{-26} cm^2 s^{-1}$ with 5 years of exposure.

Simona then mentioned that one can also search for DM satellites: simulations predict a substructure of DM in the galactic halo. The annihilation spectrum predicted is different from a power law. The emission is expected to be constant in time. Considering a 100 GeV WIMP, with $sigma v = 2.3 \times 10^{-26}$, annihilating into a b-quark pair, with extragalactic background and diffuse galactic, it is generically observable at 5-sigma level in one year. To search for these, you first scan the sky, and then when you have found something you can concentrate on observing it.

Also, dwarf galaxies can be studied. The mass to light ratio there is high, and it is thus a promising place to look for a annihilation signal. The 3-sigma sensitivity of GLAST for 5 years data goes down to 10^-26 and below for WIMP mass in the tens of GeV range.

To search for lines in the spectrum, you search in a annulus between 20 and 35 degrees in galactic latitude, removing a 15° band from the galactic disk. It is a very distinctive spectral signature. A better sensitivity is achieved if the location of the line is known beforehand (if discovered by the LHC, for instance). A 200 GeV line can be seen at 5-sigma in 5 years.

GLAST can also look for cosmological WIMPs at all redshifts. There is a spectral distorsion caused by integration over redshift. The reach of GLAST is a bit higher here, 10^-25. One can do better if there is a high concentration of DM in substructures.

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