The Large High Altitude Air Shower Observatory (LHAASO), located high up on the edge of the Tibetan Plateau, in China’s Sichuan province, searches for gamma rays from the Milky Way galaxy and beyond. Even the clearest mountain air is opaque to these high-energy photons, so astronomers infer their presence by measuring the bursts of secondary particles that gamma rays create when they collide with the atmosphere.
The LHAASO team sifted such measurements for signs of gamma rays produced by the decay of heavy dark matter within the galaxy. Not finding any signs of rays, the team puts an upper limit on the decay rate of dark matter particles with masses in the petaelectronvolt (PeV, or 1015 eV) range. (Dark matter is a hypothetical form of matter thought to account for approximately 85 per cent of the matter in the universe. It is called “dark” because it does not appear to interact with the electromagnetic field, which means it does not absorb, reflect, or emit electromagnetic radiation and is, therefore, difficult to detect.) The results of the LHAASO search were published in a recent issue of Physical Review Letters.
Some models predict that dark matter could exist in the form of particles with masses of 100 teraelectronvolt (TeV, or 1012 eV) or greater. If those dark matter particles have a finite lifetime and if they decay into standard-model particles, they should generate gamma rays with energies above 10 TeV. However, for a long time, the observation of ultra-high-energy gamma rays produced by heavy dark matter has been challenging mainly because of the presence of background radiation.
The team analysed data collected by LHAASO’s Kilometer Square Array (KM2A) during its first 570 days of operation. Thanks to its unprecedentedly high detection sensitivity to ultra-high-energy gamma rays (>100 TeV), LHAASO has the unique potential to significantly reduce background interference and improve the ability to capture gamma rays. The researchers searched for gamma rays in five areas of the sky away from the galactic plane.
According to mass models of the Milky Way, dark matter density should be the greatest near the galactic centre. Thus, if decaying dark matter produces high-energy gamma rays, the measured flux should vary between survey areas. As no such difference was detected, the researchers concluded that PeV-mass dark matter has a lifetime of at least a billion trillion years (1021 years).