Speaker
Description
The GBAR (Gravitational Behaviour of Antihydrogen at Rest) experiment 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 $\mathrm{Be^+}$ ions. Afterward, 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 = $\mathrm{e^+ e^-}$ ) are mixed in the so-called reaction chamber 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 the 2024, we have improved the production rate of $\mathrm{\bar{H}}$ by an order of magnitude. The increased rate and better mastery of systematic effects enabled the measurement of the charge exchange cross section for the first reaction for ground state positronium and at two different antiproton kinetic energies: 4 and 6 keV. This cross section was measured only once in the past with protons for energies between 11 and 15 keV. Currently, GBAR is also in the process of measuring separately 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 both cross section measurements.
