The Antiproton Flux in Space Experiment

The understanding of the interactions and the transport mechanisms of cosmic rays through the interstellar medium and through the heliosphere of our sun is a fundamental question of astroparticle physics. For accurate investigations on the creation of primary cosmic rays it is necessary to know the processes that occur during the transport of these particles from their origin to Earth. A promising approach for testing our understanding of these processes is the research on cosmic ray antimatter. Since there is no primary source of antiparticles in the universe known to us until today, the only production mechanisms are via inelastic processes of other high-energy cosmic ray particles. A comparison of experimentally measured fluxes with predictions derived from current transport models thus can provide indications on the validity of these models. 

Due to technological advances in the last decades measurements became more and more effective. In 2011, PAMELA was the first experiment to discover geomagnetically trapped cosmic ray antiprotons, thus providing good experimental data to probe our understanding of the mechanisms of the creation of secondary antimatter particles and their trapping in Earth’s magnetic field. Although there exist some models to describe these processes, discrepancies between the measured data and the theoretical predictions remain. This could indicate to an inaccuracy of the models. Using our MAPT detector it is possible to measure antiprotons in an energy range of about 25 MeV – 100 MeV and thus, extend the measurement of PAMELA to lower energies.

 

The Antiproton Flux in Space experiment (AFIS) was established in 2012 and was the starting point of the development of the MAPT system.

 

The detection principle of antiprotons is based upon the signal from the stopping and annihilation process of the antiprotons inside the active detector region. This leads to a unique signal in the detector that can be distinguished from proton events or events from other particle species (see Figures). Nevertheless, as the ratio of antiprotons to protons is only one in 10 million, the correct selection of antiproton events is a very challenging task.  

Detector response of a 50 MeV proton (background event).
Detector response of a 50 MeV proton (background event).
Detector response of a 50 MeV antiproton (signal event).
Detector response of a 50 MeV antiproton (signal event).

Where do the antiprotons near Earth come from?

It is currently believed that there are two categories of antiproton sources in space. Hypothetically, antiprotons are predicted to be created in annihilation processes of dark matter particles. However, no antiprotons originating from these so-called primary sources have been observed yet, although current measurements have not excluded their existence. If proven to exist, they could radically alter our understanding of the formation of the universe.

 

The sources most interesting for the AFIS mission are of secondary origin: high-energy particles from the galactic cosmic radiation constantly batter Earth. If it were not for our planet's protective atmosphere, the radiation levels on the surface would very quickly kill all life on Earth. The mechanisms shielding us from the harmful radiation have been known for years: high-energy CR particles such as protons and light nuclei arriving at Earth hit the atmosphere and interact with air molecules, thereby loosing energy until they have lost enough to not significantly harm Earth's flora and fauna any more. These interactions include the direct creation of antiprotons, as well as the creation of antineutrons that subsequently decay to antiprotons.

 

 

Further interacting with air molecules they loose energy and therefore change the overall energy spectrum of the secondary sources. Because of that they are sometimes described as originating from a tertiary sources, although they originally are of secondary origin. 

Are there heavier antiparticles in the universe?

Besides antiproton measurements, the search for heavier antiparticles like antideuterons and antihelium attracted attention in the past years. The production of such particles in standard collision reactions is suppressed and their existence could be a hint for dark matter annihilation or decay.

 

Because the masses of these hypothetical dark matter particles are not known yet, the energy range of the search for their secondary decay particles is quite large. Although there are models that demand searches at low particle energies (hundreds of MeV/nucleon), most of the current particle physics experiments in space barely manage to measure in this energy region due to their design for higher particle energies.