Compilation is shown of the measurements of the total extragalactic gamma-ray intensity between 1 keV and 820 GeV, with different components from current models. Credit: The e-ASTROGAM Collaboration.{{free media}}

"Previously, it was suggested that the [dark matter] DM consists of [weakly interacting massive particles] WIMPs that naturally emerge from the super-symmetric extension of the Standard model. Such a WIMP particle was predicted to have a mass of the order of 100 GeV. However, no such particle has been experimentally found and the search for DM candidates is now being broadened into other directions. Recently, the idea of involving a complete hidden sector of new particles was revitalized."[1]

## Dark matter particles

"Even though neither the nature of the DM particle(s) (χ) nor the mechanism that generates it are known, there are indirect experimental evidences suggesting that χ is indeed a weakly interacting particle. Given that, it is natural to assume that the annihilation, and/or its decay, will involve leptons [...] These can be electrons and positrons, but also muons, which can be generated via annihilation χ + χ → μ+ + μ and/or decay χ → μ+ + μ. It should be noted, however, that a similar scenario is not forbidden for the τ particles either, but the cross-section for formation of a two-tauon bound state is negligible, and hence, the observation of a signature of true taonium is considered to be less likely [460]. The advantage of using muonium annihilation lines for the search of DM particles is that the muon mass is much larger than the e± and, hence, the expected signal will be cleaner."[1]

## Leptons

"The other two types of electrically charged leptons in the Standard Model, which can annihilate into photons, are the muons μ and tauons τ with masses Mμ = 105.6 MeV and Mτ =1777 MeV, respectively [466]. It is worth noting that in contrast to the electrons and positrons, the muons and the tauons can not be produced in radioactive decays of atomic nuclei, owing to their superior masses. As such, the maps based on the μ+ + μ and/or τ+ + τ annihilation peaks can provide a cleaner signal and a new information about the sites of enhanced [dark matter] DM concentration which would be complementary to the data obtained from the 511-keV surveys."[1]

## Gamma rays

The "leptons can be created not only via processes involving DM particles such as χ + χ → l+ + l, but in high energy astrophysical environments a significant number of them can also be produced via the γ + γ → l + l+ and e + e+ → l + l+ reactions. However, the muons created in these high-energy environments have energies much higher than the ionization energy (Eion ≈ 1.4 keV) of the true muonium [460] and, hence, only a small fraction of pairs with energies less then Eion will form a bound system. The muonium has two states, depending on the particles spin orientation. These are para- and orto-muonium. The para-muonioum predominantly decays via two-photon annihilation, while the orto-muonium – via electron-positron annihilation. The energy released in the two-photon annihilation is E=105.66 MeV [460]."[1]

## Half-lives

"The advantage of using unstable leptons, rather than using electrons, for tracing DM particles is in their finite lifetime. The tauons have a lifetime of 2.9 × 10−13 s., while the muons have lifetimes of 2.2 μs. Their finite lifetimes provide an unique opportunity for mapping of DM regions with an enhanced precision. Thus, for example, DM particles with masses of the order of Mχ = 100 GeV can either annihilate or decay into muons."[1]

"Since a tauon decays into a light meson (lepton) with neutrino(s), the measurements of angular distribution for tauon on ${\displaystyle {\overline {B}}}$ → D(∗)τ${\displaystyle {\overline {\nu }}_{\tau }}$ are difficult."[2]

## Earth

"For ultrahigh energies the neutrino spectrum at the detector is influenced by neutrino-nucleon interactions and tauon decays during the passage through the interior of the earth."[3]

The "incoming neutrino flux [is disentangled] from the consequent τ air-shower physics. [To] establish the τ production rate we introduce an effective volume and mass for Earth-skimming τ’s, which is independent on any incoming neutrino flux [...]. This volume describes a strip within the Earth where neutrino/antineutrino-nucleon, ντ (${\displaystyle {\overline {\nu }}_{\tau }}$) − N, interactions may produce emerging τ+ leptons which then shower in air. [General] τ upward-going showers [can be compared] to detectors such as the ongoing photo-fluorescence ground-based observatory Auger".[4]

## References

1. G. Vankova-Kirilova, V. Bozhilov, V. Kozhuharov, S. Lalkovski (2017). "All-sky mapping in the 100 MeV region in search for point-like dark matter sources". arxiv: 112. Retrieved 2018-4-04.
2. Yasuhito Sakaki and Hidekazu Tanaka (1 March 2013). "Constraints on the charged scalar effects using the forward-backward asymmetry on ${\displaystyle {\overline {B}}}$ → D(∗)τ${\displaystyle {\overline {\nu }}_{\tau }}$". Physical Review D 87 (5): 054002. Retrieved 2018-4-03.
3. Hettlage, C.; Mannheim, K. (20-25 September 1999). "Tau Sources in the Sky". AG Abstract Services 15 (04). Retrieved 2014-10-02.
4. D. Fargion, P.G. De Sanctis Lucentini, M. De Santis. M.Grossi (10 2004). "Tau Air Showers from Earth". The Astrophysical Journal 613 (2): 1285-1301. doi:10.1086/423124. Retrieved 2018-4-04.