Milagro Limits on the Rate-Density of Evaporating Primordial Black Holes

Author: Tilan Ukwatta

Abstract: Primordial Black Holes (PBHs) are gravitationally collapsed objects that may have been created by initial density fluctuations in the early universe or other mechanisms and could have arbitrarily small masses down to the Planck scale. Hawking showed that due to quantum-gravitational effects, Black Holes (BHs) possess a temperature inversely proportional to their mass and emit all species of fundamental particles with a thermal spectrum. PBHs created with an initial mass of ∼ 5.0 × 1014 g should be expiring now with bursts of high-energy particles, including gamma radiation in the MeV - TeV energy range, making them candidate gamma ray burst (GRB) progenitors. A number of detectors have searched for these events and reported their upper limits. The Milagro high energy observatory, which operated from 2000 to 2008, is sensitive to the high end of the PBH evaporation gamma ray spectrum. Due to its large field-of-view, more than 90% duty cycle and sensitivity up to 100 TeV gamma rays, the Milagro observatory is ideally suited for the direct search of PBH bursts. Based on published work on the Milagro observatory, we report PBH upper limits which significantly improve the previous limits on PBH bursts based on the standard model.