What is dark matter made of? The latest data of the Pierre Auger Observatory constrains scenarios of super-heavy particle relics.

Dark matter could have been created in the primordial universe from gravitational interactions between all forms of matter. The absence, to date, of ultra-high energy photons in the Pierre Auger Observatory data allows us to constrain this minimal scenario for the production of dark matter. 

It is often thought that dark matter, the mysterious substance that constitutes 27% of the mass of the Universe, should look like "heavy protons" behaving like neutrinos. However, dark matter could be much, much heavier than a "heavy proton" and even more elusive than a neutrino. This would be the case if, beyond their own dynamics in a so-called hidden sector, dark matter particles interact with visible matter through the gravitational force alone. These particles, which could weigh up to one microgram (for comparison, a human cell typically weighs 3.5 micrograms), would then have emerged from the primordial universe during the reheating period following the inflation era. Until now, the detection of a large number of primordial gravitational waves in the next observations of the cosmic microwave background was considered as the only indirect signature of this scenario.



Model of the sky map (in galactic coordinates) sought for in the data of the Pierre Auger Observatory to uncover superheavy dark matter particles decay after about 10^23 years


Another way, more direct, to uncover this scenario has just been reported in articles jointly published in the journals Physical Review Letters and Physical Review D. The existence of a new quantum number related to the internal dynamics of the hidden sector would guarantee a priori the stability of one or several superheavy particles. However, even particles stabilized by a symmetry will eventually disintegrate under the effect of pseudo-particles in imaginary time, known as "instantons", which destabilize particles in quantum field theory. For sufficiently strong "instanton effects", the decay by-products should lead to copious fluxes of ultra-high energy photons. The non-observation, until now, of such fluxes of photons of energy higher than 10^8 GeV in the Pierre Auger Observatory data allows us to set upper limits to the coupling constant of the dark sector (an analogue of the fine structure constant, but which concerns only dark matter). These limits are, to date, the best ever obtained from instanton processes, which are invoked in several fields of theoretical physics (string theory and particle physics); they are also complementary to those obtained on the cosmological parameters governing the thermal history and the geometry of the universe from observations of the cosmic microwave background in a superheavy dark matter scenario.


Related papers:

Limits to gauge coupling in the dark sector set by the non-observation of instanton-induced decay of Super-Heavy Dark Matter in the Pierre Auger Observatory data
The Pierre Auger Collaboration, Phys. Rev. Lett. 130 (2023) 061001
[arxiv.org/abs/2203.08854] [doi: 10.1103/PhysRevLett.130.061001

Cosmological implications of photon-flux upper limits at ultra-high energies in scenarios of Planckian-interacting massive particles for dark matter
The Pierre Auger Collaboration, Phys. Rev. D 107 (2023) 042002
[arxiv.org/abs/2208.02353] [doi: 10.1103/PhysRevD.107.042002]

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