How To Confirm The Existence of Primordial Black Holes? (Cosmology / Physics)

Primordial black holes (PBH’s) are a hypothetical type of black hole that formed soon after the Big Bang. In the early universe, high densities and heterogeneous conditions could have led sufficiently dense regions to undergo gravitational collapse, forming black holes. They are non-baryonic and as such are plausible dark matter (DM) candidates. PBH’s emit Hawking radiation and evaporation process can give rise to observable signals. Now, Antonio Palazzo and colleagues for the first time proposed a possibility that, PBHs with masses in the range of [5 × 1014 − 5 × 1015]g, emit neutrinos during evaporation and these neutrinos can interact through the coherent elastic neutrino nucleus scattering process, producing an observable signals in the dark matter (DM) direct detection experiments (like XENONnT, DARWIN etc.) Their study recently appeared in Arxiv.

Coherent elastic neutrino-nucleus scattering (“CEvNS”) is a process involving the neutral-current scattering of a neutrino with an entire nucleus. It is only recently that, CEvNS process has been successfully observed by COHERENT, where a few kilograms of detector was exposed to an intense neutrino flux of artificial origin. The very same process involving neutrinos of natural origin, such as, from the Sun, diffuse supernovae and Earth’s atmosphere, constitute an irreducible background in DM direct searches. This background gives rise to the so-called “neutrino floor”, which applies only to direct detection experiments. These experiments search for the scattering of a dark matter particle like WIMP’s, off of a nucleus.

Fig 1: Impact of PBHs on the Neutrino floor. The black contour delimiting the yellow region represents the ordinary neutrino floor, while the upper border of the colored bands correspond to the modifications induced by neutrinos from PBHs with masses and DM fractions in the legend. These benchmark values lie on the 90% C.L. exclusion curve obtainable from a liquid xenon experiment with 200 t yr exposure © Antonio Palazzo et al.

Antonio Palazzo and colleagues, showed that, PBHs with masses in the range, I mentioned above, emit neutrinos with peak energy 10 MeV and 100 MeV, which may emerge as a signal on such a familiar background. As a result, it is possible to set prospective bounds on the PBHs fraction of dark matter (DM) in this mass range, by improving the existing neutrino limits obtained with Super-Kamiokande.

“We have shown that with the high exposures envisaged for the next-generation facilities, it will be possible to set bounds on the fraction of dark matter (DM) composed of PBHs, improving the existing neutrino limits obtained with Super-Kamiokande.”

— wrote authors of the study

Finally, they showed how the neutrino floor gets modified by the presence of a hypothetical signal from PBHs. The neutrinos emitted by PBHs would lie on top of an irreducible background. Therefore, the existence of even a minute fraction of PBHs in the DM content would modify the neutrino floor, making it higher.

“In the context of PBHs searches, the direct DM experiments would rather operate as indirect DM observatories. From this perspective, our study lends further support to the emerging role of such underground facilities as multi-purpose low-energy neutrino telescopes complementary to their high-energy “ordinary” counterparts, IceCube and KM3NeT.”

— concluded authors of the study

Reference: Roberta Calabrese, Damiano F.G. Fiorillo, Gennaro Miele, Stefano Morisi, Antonio Palazzo, “Primordial Black Hole Dark Matter evaporating on the Neutrino Floor”, Arxiv, pp. 1-8, 2021.

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