"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 H.E.S.S. Collaboration researcher. "With this technique, we could observe the most energetic phenomena, such as what we believe could happen when two dark matter particles annihilate promptly into two high-energy photons—what we call a spectral line from dark matter annihilation."

What did the new analysis actually find?

The team found no convincing gamma-ray line. But the absence of a signal, paired with very deep data, lets physicists set upper limits on how often WIMPs could be annihilating.

Their new bounds draw on 546 hours of inner-galaxy observations collected between 2014 and 2020—one of the largest such datasets assembled for an imaging Cherenkov array. Emmanuel Moulin of the H.E.S.S. Collaboration noted:

"After looking for WIMPs for more than two decades in the center of the Milky Way, long acknowledged as the most promising target to detect WIMPs via their self-annihilation in very-high-energy (E>100 GeV) gamma rays, the H.E.S.S. observatory acquired enough data to probe the relevant annihilation cross section of WIMPs."

Which dark matter candidate did the data challenge?

Using the Einasto model—a standard recipe for how dark matter spreads through our galaxy—the team calculated the lowest allowed annihilation rate their telescopes could test. The "cross section" Montanari described is essentially the probability that two particles meet and disappear.

The squeeze matters because it touches the thermal Higgsino, a hypothetical particle and a long-standing WIMP candidate. The new H.E.S.S. results rule out a Higgsino dark-matter interpretation under the Milky Way models the team examined—the first time such exclusion has been achieved.

"Our analysis sets the smallest upper limit for this size across a wide range of possible dark matter particle masses," Montanari said.

What comes next?

The hunt is far from over. The Cherenkov Telescope Array Observatory (CTAO), now under construction at two sites, will soon take over:

• CTAO-South in Paranal, Chile
• CTAO-North at the Roque de los Muchachos Observatory on La Palma, Canary Islands

CTAO will record gamma rays with sharper energy and angular resolution than H.E.S.S., and a much wider field of view. The new instrument should reach lower cross sections and cover the galactic center more efficiently.

"The observation program carried out with H.E.S.S. is an important legacy and paves the way for future planned observations of the galactic center with CTAO, which will provide crucial insights into the TeV WIMP paradigm as a whole," Moulin said.

If CTAO spots a gamma-ray line, the field will know something dramatic. If it sees nothing, the noose tightens further on whole classes of WIMP models—and the next chapter of dark matter physics will have to be written from a much smaller set of options.