For the first time, a cloud of positronium, the lightest atom found in nature, has been cooled using a laser. The long-awaited experimental proof of this process was achieved by the scientific collaboration of the AEgIS experiment, significantly supported by the INFN (National Institute for Nuclear Physics) and the University of Brescia. A research group from the University of Brescia is also part of the AEgIS scientific collaboration.
The result, published in Physical Rview Letters as an Editor’s Highlight, was obtained using a particular laser system based on an alexandrite crystal, specifically developed to meet the requirements of this experiment: high intensity, wide bandwidth, and long pulse duration. The temperature of the positronium atoms produced from a porous silica target at room temperature, hit by a beam of positrons, decreased from 380 kelvin to 170 kelvin, corresponding to a decrease in the transverse component of velocity from 54 km/s to 37 km/s.
Positronium is a lighter sibling of hydrogen, with a positron replacing the proton. Consequently, it is about 2,000 times lighter than hydrogen, making it the lightest atom in nature. It is an unstable atom: in a vacuum and in the ground state, with parallel spins of the two particles, it annihilates with a lifetime of only 142 nanoseconds (billionths of a second). Cooling positronium must therefore occur during its short lifetime, making it challenging to reproduce this process compared to ordinary atoms. The use of a pulsed laser with a wide bandwidth has the advantage of cooling a large fraction of the positronium cloud, doubling its effective lifetime and making a greater number of atoms available for further experiments after cooling.
The scientific goal of the AEgIS experiment, one of the experiments operating in the Antimatter Factory at CERN, is to measure the gravitational acceleration of antihydrogen as a test for the weak equivalence principle of Einstein’s antimatter, one of the cornerstones of the theory of General Relativity, which states that a body in free fall in a vacuum under the influence of a gravitational field follows a trajectory in space independent of the body’s composition. In the case of AEgIS, antihydrogen is obtained through a reaction between excited positronium and trapped antiprotons. The slower the positronium velocity, the higher the probability of antihydrogen formation, hence the importance of cooling positronium atoms to reduce their kinetic energy as much as possible.
The University of Brescia has been part of the AEgIS collaboration since its foundation in 2011. In particular, the Brescia group has always been involved in the scintillators and photomultipliers that allow researchers to “see” antimatter annihilations.
“Antimatter annihilations, which occur when it collides with ordinary matter, cannot be seen with the naked eye, obviously, but sophisticated instrumentation is required to observe them,” explains Prof. Nicola Zurlo of the Department of Civil Engineering Environment Territory Architecture and Mathematics of the University of Brescia and coordinator of group 3 (Nuclear Physics) at the INFN Section of Pavia. “This is why the main AEgIS apparatus is surrounded by plastic detectors that, properly calibrated with a procedure developed over the years by our research group in Brescia, allow distinguishing the signal produced by positron annihilations from that of antiproton annihilations, as well as from those produced by cosmic rays and natural radioactivity present in the room where the experiment is located, at CERN.”