With a series of measurements of antiproton-production cross sections in 2023 and 2024, the Apparatus for Meson and Baryon Experimental Research (AMBER) experiment provides new experimental data to improve the accuracy of cosmic-ray models required to interpret current cosmic-ray measurements, such as from the AMS-02 experiment on the International Space Station.
Among today’s big questions in particle physics is the origin of the imbalance between matter and antimatter in the Universe as it seems that there is nothing made from antimatter therein. Even though space-based experiments such as AMS-02 detected antiprotons within our Galaxy, it is not precisely known how many of them are expected to be produced by collisions of cosmic rays with the interstellar medium, which consists mainly of hydrogen and helium nuclei. Knowing this flux precisely is, however, essential to identify potential additional and more exotic sources of antimatter, such as dark matter, that increase the flux of antiprotons in our Galaxy.
Thus, recreating collisions between cosmic rays and hydrogen and helium nuclei in a laboratory such as CERN and comparing the measured antiproton production to theoretical models that are used to predict the cosmic-ray antiproton flux are key elements to understand the antimatter content of the Universe. Linking these results to the number of antiprotons detected by space-based detectors can provide an indirect probe of dark matter if an excess of antiprotons from expectations is identified.
At AMBER, cosmic-ray collisions are reproduced in the North Area of the Super Proton Synchrotron (SPS) by colliding a proton beam with different energies with different targets, such as hydrogen, deuterium, and helium. The collision energies reached in the experiment are closer to the relevant energies of cosmic-ray collisions in the Galaxy compared to what is realised by current colliders, such as the LHC, and allows to imitate collisions between cosmic rays with helium and hydrogen nuclei more realistically.
In total, AMBER took data with 12 different configurations of collision energy and target material to allow systematic studies of antiproton production as a function of energy and target material. To determine the protons from the rest of the hadrons in the beam, the experimentalists used so-called CEDAR detectors, which create a specific signal when a proton flies through the detector. Antiprotons produced in the collisions are identified with AMBER’s Ring Imaging CHerenkov detector (RICH), which is one of the world’s best.
“The analysis is still ongoing, but with having around 6 million antiprotons identified in only one of our 12 different data sets, the data show a very good coverage of the phase space with a small statistical uncertainty,” says Giordano. “These data will help to shed light on how dark matter might contribute to the imbalance of matter and antimatter in the Universe.”
AMBER’s antimatter production-cross section is the first experiment in a row of three approved programmes of the AMBER Phase-1 Proposal, which also includes the proton radius measurement and the study of the emergence of the hadron mass.