Photons coming from an opaque obstacle as a manifestation of heavy neutrino decays

Within the framework of physics beyond the standard model, we study the possibility that mesons produced in the atmosphere by the cosmic-ray flux decay to heavy Majorana neutrinos and the latter, in turn, decay mostly to photons in the low-mass region. We study the photon flux produced by sterile Majorana neutrinos ($N$) decaying after passing through a massive and opaque object such as a mountain. To model the production of $N$’s in the atmosphere and their decay to photons, we consider the interaction between the Majorana neutrinos and the standard matter as modeled by an effective theory. We then calculate the heavy neutrino flux originated by the decay of mesons in the atmosphere. The surviving photon flux, originated by $N$ decays, is calculated using transport equations that include the effects of Majorana neutrino production and decay.

Figure 1

Branching ratios for the main decay channels and for the couplings discussed in the text.

Figure 2

Heavy neutrino flux at the sea level as a function of the neutrino energy and for the intensity coupling indicated in the text.

Figure 3

Schematic representation of the obstacle and the detector.

Figure 4

Photon flux integrated in energy as coming from a generic obstacle of 1 km of thickness as a function of the $N$ mass and for different distances to the obstacle.

Figure 5

Photon flux as coming from the obstacle described in the text as a function of the photon energy for different distances to the obstacle and different heavy neutrino masses.

Figure 6

Contours for the obtained number of events in the ($m_N$, $U^2$) plane are shown in dasheddotted lines. We include the bounds coming from $\pi$ decay, cosmological bounds, and also the bounds imposed by the Super-Kamiokande and Belle experiments. The arrows indicate the allowed region, and the labels close to the dotted-dashed curves indicate the photon number of events.