No abstract

The antihydrogen formation by charge exchange between cold antiprotons and Rydberg positronium P * s is studied by using the Classical Trajectory Monte Carlo (CTMC) method. In absence of external magnetic field the cross section scaled by the fourth power of the P * s principal quantum number nP s shows an universal behaviour as a function of the ratio kv between the velocity of the Ps centre of mass and that of the positron in the classical circular orbit. At low velocity, below about kv 0.2 − 0.3, we show for the first time for Rydberg positronium, that the cross section increases as 1/k 2 v or, in equivalent way as 1/E cm P s with E cm P s being the P * s centre of mass energy. In this regime the distribution of the principal quantum number of the antihydrogen state is narrow at peaked around √ 2nP s while at higher kv values a broad distribution of antihydrogen states is produced. The study of the collision process in presence of moderate magnetic field (0.5-2 T) shows that there is an experimentally interesting region of kv with the cross section slightly higher than that in absence of field. However the presence of a magnetic field changes significantly the cross section behaviour as a function of kv, especially at low velocities, where reductions of the cross sections and deviations from the 1/k 2 v (1/E cm P s ) are observed. Our calculation shows for the first time a dependance of the cross section upon the angle between the magnetic field and the flight direction of the incoming P * s .

The precise measurement of forces is one way to obtain deep insight into the fundamental interactions present in nature. In the context of neutral antimatter, the gravitational interaction is of high interest, potentially revealing new forces that violate the weak equivalence principle. Here we report on a successful extension of a tool from atom optics—the moiré deflectometer—for a measurement of the acceleration of slow antiprotons. The setup consists of two identical transmission gratings and a spatially resolving emulsion detector for antiproton annihilations. Absolute referencing of the observed antimatter pattern with a photon pattern experiencing no deflection allows the direct inference of forces present. The concept is also straightforwardly applicable to antihydrogen measurements as pursued by the AEgIS collaboration. The combination of these very different techniques from high energy and atomic physics opens a very promising route to the direct detection of the gravitational acceleration of neutral antimatter.