Why antideuterons?

Antideuteron limits from BESS, predicted antideuteron fluxes from different models, sensitivities for the running AMS-02 for 5 years and the planned GAPS experiments after three 35-day flights.

The existence of dark matter is proven and the investigation of its nature plays a key role in understanding structure formation after the big bang. Dark matter cannot be explained with known types of matter, therefore we are at the dawn of something significantly new. The importance of this problem becomes obvious by recalling that dark matter is more than five times more abundant than regular matter. Little is known about the nature of dark matter particles, except that they are relatively heavy, gravitationally interacting, and only very weakly interacting – if at all – with regular matter. Most searches for dark matter have concentrated on the neutralino, the lightest supersymmetric (SUSY) partner in many models. More recently dark matter from universal extra dimension (UED) theories has attracted intense interest. Together with the axion (a light dark matter candidate not discussed further), these theories are the most physically well-motivated. They provide a stable, heavy particle – the weakly interacting massive particle (WIMP). All WIMPs invoke physics beyond the standard model of particle physics.

If dark matter was in thermal equilibrium with the rest of the matter in the early universe and froze out when the temperature dropped due to expansion, it is a natural assumption that dark matter particles are able to interact with each other and produce known standard model particles. These particles would contribute to the known cosmic ray fluxes and, as the kinematic characteristics of these processes are different from the production mechanisms of the conventional cosmic rays, it could be possible to observe the imprint of dark matter in the diffuse cosmic ray spectra in the form of an excess (indirect searches). This self-interaction process is complementary to the scattering approach of the direct searches, and thus probes different parameter regions of dark matter theories. Cosmic ray antiparticles without primary astrophysical sources are ideal candidates for an indirect dark matter search, but recent results show that accomplishing this task with positrons and antiprotons appears to be challenging due to high levels of secondary/tertiary astrophysical background.

Antideuterons (bound antimatter: antiproton and antineutron) could also be produced in dark matter annihilations and are a potential breakthrough approach for its identification. Secondary antideuterons, like antiprotons, are produced when cosmic ray (anti)protons interact with the interstellar medium, but the production threshold for this reaction is higher for antideuterons than for antiprotons. Collision kinematics disfavor the formation of low-energy antideuterons in these interactions. Moreover, the steep energy spectrum of cosmic rays results in fewer particles with sufficient energy to produce secondary antideuterons, and those that are produced will have relatively large kinetic energy. As a consequence, a low-energy search for primary antideuterons has very low background. The signal dominated region can be defined to be below 1 GeV/n and the astrophysical background dominated region starts at a few GeV/n. The attractiveness of antideuteron searches becomes apparent by realizing that the signal-over-background ratio is about a factor of 100 in the low-energy region without introducing any boosting mechanisms, e.g., due to dark matter clumpiness, Sommerfeld enhancement, large galactic halo size. Introducing such effects the signal-to-background ratio would further increase. This is in contrast to dark matter signal predictions for positrons and antiprotons, which are typically only a relatively small additional contribution on top of the astrophysical background.

The Snowmass 2013 summer study for the cosmic frontier states that “[…] antideuteron searches are quite sensitive to models with low-energy neutralinos, and they maintain sensitivity up high neutralino masses” and that “[…] for the antideuteron searches the nominal or optimistic propagation and production parameters are very promising for dark matter searches” (arXiv:1310.7040). The final report of the strategic planning for U.S. particle physics comes to a similar conclusion: “Future experiments sensitive to antideuteron fluxes at low energies may provide incisive tests of some WIMP dark matter candidates” (P5 report). Many theoretical papers discuss aspects of antideuteron dark matter searches, with SUSY and UED theories being the most popular ones (please see publications).