H.E.S.S. detects no gamma-ray line from Galactic Centre, but reaches thermal Higgsino benchmark

The H.E.S.S. Collaboration’s deepest search yet for a telltale gamma-ray line from dark matter near the center of the Milky Way came up empty. But the null result still marks a milestone: using 546 hours of observations, the team reported what it says are its strongest limits so far on this kind of signal and, under its baseline assumptions about the Milky Way’s dark-matter halo, reached the long-sought thermal Higgsino benchmark.

The result appears in the paper, “Search for gamma-ray spectral lines from dark matter annihilation with the H.E.S.S. Inner Galaxy Survey,” posted Aug. 7 on arXiv as arXiv:2608.07234. The manuscript is listed as accepted in Physical Review Letters, with the publisher’s accepted record stating it was accepted July 1, 2026. The analysis used H.E.S.S. observations collected from 2014 to 2020 as part of the Inner Galaxy Survey, targeting the inner few degrees around the Galactic Centre, a prime region for indirect dark-matter searches.

The collaboration reported “No significant signal is detected.” The search covered dark-matter masses from 300 gigaelectronvolts to 70 teraelectronvolts and looked for a narrow spectral feature — effectively a sharp line in the gamma-ray spectrum — that some dark-matter annihilation scenarios could produce. To set its limits, the team used a two-dimensional likelihood analysis that combined both spectral and spatial information.

Under the paper’s baseline assumption of an Einasto dark-matter density profile for the Milky Way, the reported 95% confidence upper limits on the annihilation line cross section reach 2.3 × 10^-28 cubic centimeters per second at 1 TeV and 2.4 × 10^-27 at 10 TeV. The strongest limit in the study is 9.8 × 10^-29 cubic centimeters per second at a dark-matter mass of about 440 GeV. The paper says the new bounds significantly improve on previous H.E.S.S. line searches across roughly 300 GeV to 60 TeV.

That matters because a gamma-ray line is widely considered one of the cleanest possible signatures of dark matter. Ordinary astrophysical sources can produce broad gamma-ray emission, but they rarely generate a truly line-like feature at a single energy. H.E.S.S., a system of atmospheric Cherenkov telescopes designed to study very high-energy gamma rays, is especially well suited to this kind of search at the TeV scale. The Galactic Centre is a key target because it is expected to host a dense concentration of dark matter, which could make any annihilation signal stronger.

The authors also compared their limits with predictions for several leading dark-matter candidates: Wino, Higgsino and Quintuplet models. Under the study’s baseline Milky Way halo assumptions, the paper says thermal Wino and thermal Quintuplet dark matter are excluded, while “For the first time, thermal Higgsino DM is probed for DM Milky Way models.” That point is especially notable because a thermal Higgsino around the 1.0-1.2 TeV scale has long been a major benchmark that is difficult to test with collider or direct-detection experiments, making indirect gamma-ray searches unusually important.

But the model claims come with a major caveat close to the heart of the analysis: they depend strongly on how concentrated dark matter is assumed to be near the Galactic Centre. The paper says switching from the baseline Einasto profile to other Milky Way density profiles changes the limits by factors of about 1.3 or about 6.5, depending on the model used.

So the headline is still a non-detection. Even so, the new H.E.S.S. analysis tightens the constraints on some of the most sought-after TeV-scale dark-matter scenarios and sets a tougher benchmark for future line searches.