Superheavy dark matter and ANITA’s anomalous events

The ANITA experiment, which is designed to detect ultrahigh-energy neutrinos, has reported the observation of two anomalous events, directed at angles of 27° and 35° with respect to the horizontal. At these angles, the Earth is expected to efficiently absorb ultrahigh-energy neutrinos, making the origin of these events unclear and motivating explanations involving physics beyond the Standard Model. In this study, we consider the possibility that ANITA’s anomalous events are the result of Askaryan emission produced by exotic weakly interacting particles scattering elastically with nuclei in the Antarctic ice sheet. Such particles could be produced by superheavy (approximately ${10}^{10}–{10}^{13}\mathrm{GeV}$) dark matter particles decaying in the halo of the Milky Way. Such scenarios can be constrained by existing measurements of the high-latitude gamma-ray background and the ultrahigh-energy cosmic-ray spectrum, along with searches for ultrahigh-energy neutrinos by IceCube and other neutrino telescopes.

Figure 1

The relative acceptance of ANITA-III, as a function of the polarization angle and the emergence angle of the primary, for Askaryan emission observed within 5° of the angles of the two ANITA anomalous events (27° and 37°). Both events are observed to be predominantly horizontally polarized ($|{\theta }_{\mathrm{pol}}|<10°$), suggesting that the emission is compatible with primary emergence angles of approximately 30°.

Figure 2

ANITA-III’s relative acceptance as a function of emergence angle (left) and observed elevation angle (right) for a range of shower energies, assuming no Earth absorption as is appropriate for a primary particle with small scattering cross section. Each energy is normalized separately. In the right frame, we mark the elevation angles of the anomalous ANITA-I and ANITA-III events. ANITA’s anomalous events can be reasonably interpreted as Askaryan emission from showers with energies in the range of approximately $E_{\text{shower}}\sim {10}^9–{10}^{11}\mathrm{GeV}$.

Figure 3

The effective exposure of ANITA and IceCube to ultrahigh-energy hadronic showers. The dashed line represents ANITA’s exposure neglecting any attenuation in the Earth, as is appropriate for showers initiated by particles with very small scattering cross sections, ${\sigma }_{\chi N}\ll {\sigma }_{\nu N}$. The dotted curve includes the level of attenuation predicted for Standard Model neutrinos. The effective exposure of IceCube to high-energy showers is approximately $1{\mathrm{km}}^3\times 4\pi \mathrm{sr}$ (neglecting any absorption in the Earth). Note that we do not consider IceCube’s exposure to muon tracks, as the particles under consideration interact only through elastic scattering.

Figure 4

The rate of ultrahigh-energy shower events at IceCube from the decays of superheavy dark matter into exotic weakly interacting particles, $X_d\to \chi \chi$, normalizing $f/{\tau }_{X_d}$ to produce two events over the total flight time of ANITA (115 days). The gray band is the 90% confidence band around this rate. Given that IceCube has not yet observed any such events, the scenario presented here can explain the two anomalous events observed by ANITA so long as $E_{\text{shower}}\gtrsim (1-2)\times {10}^9\mathrm{GeV}$, corresponding to $m_{X_d}\gtrsim (1-2)\times {10}^{10}\mathrm{GeV}$.

Figure 5

The distribution of $\chi$ energies after passing through the Earth with an emergence angle of 30°. Results are shown for elastic scattering cross sections with nucleons that are 10% (red), 1% (blue), or 0.1% as large as the total neutrino-nucleon cross section ($f={10}^{-1}$, ${10}^{-2}$ or ${10}^{-3}$). We consider $\chi$’s with an initial energy of $10^{10}\mathrm{GeV}$ (left), $10^{11}\mathrm{GeV}$ (center), and $10^{12}\mathrm{GeV}$ (right).