Speaker
Description
The origin of matter-antimatter asymmetry is one of the big unanswered questions in modern physics. Various experiments look for an explanation of this asymmetry by searching for CPT symmetry violations and by testing the Weak Equivalence Principle with antimatter. The GBAR (Gravitational Behaviour of Antihydrogen at Rest) experiment at CERN seeks to measure the gravitational acceleration of antimatter with precision better than 1% using ultracold antihydrogen atoms. To obtain ultracold antihydrogen atoms, a multi-step process is used: first, antihydrogen ions are formed and sympathetically cooled using Be$^+$ ions. Afterwards, the extra positron is photo-detached from cooled antihydrogen ions to obtain ultracold antihydrogen atoms with temperatures on the order of 10 $\mathrm{\mu K}$.
Currently, the GBAR experiment is working towards the first-ever synthesis of the antihydrogen ions using the following scheme: first, antiprotons ($\mathrm{\bar{p}}$) and positronium (Ps = e$^+$ e$^-$) are mixed in the so-called reaction cavity to obtain (hot) antihydrogen atoms ($\mathrm{\bar{H}}$). These $\mathrm{\bar{H}}$ atoms then undergo a second charge exchange with Ps, producing $\mathrm{\bar{H}^+}$ ions.
The first step, the production of (hot) $\mathrm{\bar{H}}$ atoms, has been achieved for the first time in 2022, and at the end of 2024, we have improved the production rate of $\mathrm{\bar{H}}$ by an order of magnitude. The increased rate and better control of systematic effects enabled the measurement of the charge exchange cross section of the first reaction at 4 and 6 keV antiproton energies. This cross section was measured only once previously with protons at energies between 11 and 15 keV. Last year, GBAR also performed measurements of the cross section for the second charge-exchange reaction, but with an energy-tunable pulsed hydrogen beam obtained from photo-neutralisation of $\mathrm{H^-}$ provided by ELENA. In this talk, we will report on the results of these cross section measurements, and the current status and future plans of the experiment.