H.E.S.S. tightens limits on heavy dark matter annihilation using 546 hours aimed at the galactic centre

The whole search depends on the shape of a spectrum. Annihilating dark matter could show up as gamma rays concentrated at one particular energy, where you would otherwise see a smooth spread across many energies ^[3]. The centre of the Milky Way is the place to look, because dark matter is thought to be especially dense there ^[4].

H.E.S.S. is an array of imaging atmospheric Cherenkov telescopes, or IACTs, in Namibia. The telescopes detect flashes of light, not the gamma rays themselves ^[1]. "The data we analyzed consist of Cherenkov flashes of light generated by particles traveling faster than the speed of light in Earth's atmosphere," said Alessandro Montanari, a researcher with the collaboration ^[9]. Those particles arrive in showers set off by high-energy photons from space, so each flash records a gamma ray that has already hit the atmosphere ^[10].

The exposure is 546 hours gathered between 2014 and 2020 ^[2]. Added together, that is about 23 days of observing ^[1]. Montanari put part of the result down to location. "This was possible because H.E.S.S. is currently the only active array of IACTs in the Southern Hemisphere," he said ^[8].

The data showed no statistically significant line, so the team set upper limits instead ^[5]. The quantity being limited is the annihilation cross section ^[11]. "Think of the annihilation cross section as the size of the particle when it collides with another one, and they both disappear," Montanari said ^[12]. He called the outcome "the most constraining results on the supposed annihilation cross section of heavy dark matter particles" ^[11].

The thing this doesn't tell you is whether the ranking holds under other assumptions. The strongest-yet claim is made for one model of how dark matter is distributed in the Milky Way, the Einasto model ^[7]. The search relies on dark matter being dense at the centre, so its limits depend on how dense the model assumes it to be ^[4][7]. In my view the result supports a defensible reading: a lower ceiling on one annihilation channel for heavy particles, under one assumed distribution. The published account does not list specific dark matter models that the limits exclude ^[7][11].

That leaves the collaboration's own framing. Emmanuel Moulin, also with H.E.S.S., said WIMPs are "among the most favored candidates to explain dark matter in the universe" ^[14]. After more than two decades of looking toward the galactic centre, he told Phys.org, "the H.E.S.S. observatory acquired enough data to probe the relevant annihilation cross section of WIMPs" ^[13].