Thursday, April 02, 2009

Antimatter Annihilation Anomalies

PAMELA and other experiments are searching for antimatter annihilation, as possible evidence for Dark Matter and other astrophysical phenomenon. These experiments have observed excess positron annihilation, but not antiprotons, which is inconsistent with many popular Dark Matter models. See also Dark Matter Annihilation and the PAMELA and ATIC Anomaly and Discriminate different scenarios to account for the PAMELA and ATIC data by synchrotron and IC radiation and Is the PAMELA anomaly caused by the supernova explosions near the Earth?
See also A hint of dark matter? in Nature.
Cosmic ray positrons are known to be produced in interactions in the interstellar medium. As well as originating from this 'secondary source', positrons might also be generated in primary sources such as pulsars and microquasars — or by dark matter annihilation. A new measurement of the positron fraction in the cosmic radiation for the energy range 1.5–100 GeV has been made using data from the PAMELA satellite experiment. Previous measurements, made predominantly by balloon-borne instruments, yield a positron fraction compatible with 'secondary source' production from interactions between cosmic ray nuclei and interstellar matter. Above 10 GeV the new measurements deviate significantly from this expectation, pointing to the presence of a primary source, either a nearby astrophysical object or dark matter particle annihilations.

The cosmic ray lepton puzzle in the light of cosmological N-body simulations
The PAMELA and ATIC collaborations have recently reported an excess in the cosmic ray positron and electron fluxes. These lepton anomalies might be related to cold dark matter (CDM) particles annihilating within a nearby dark matter clump. We outline regions of the parameter space for both the dark matter subhalo and particle model, where data from the different experiments are reproduced. We then confront this interpretation of the data with the results of the cosmological N-body simulation Via Lactea II. Having a sizable clump (Vmax = 9km/s) at a distance of only 1.2 kpc could explain the PAMELA excess, but such a configuration has a probability of only 0.37 percent. Reproducing also the ATIC bump would require a very large, nearby subhalo, which is extremely unlikely (p~3.10^-5). In either case, we predict Fermi will detect the gamma-ray emission from the subhalo. We conclude that under canonical assumptions, the cosmic ray lepton anomalies are unlikely to originate from a nearby CDM subhalo.

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