The principle of the equivalence of gravitational and inertial mass is one of the cornerstones of general relativity. Considerable efforts have been made and are still being made to verify its validity. A quantum-mechanical formulation of gravity allows for non-Newtonian contributions to the force which might lead to a difference in the gravitational force on matter and antimatter. While it is widely expected that the gravitational interaction of matter and of antimatter should be identical, this assertion has never been tested experimentally. With the production of large amounts of cold antihydrogen at the CERN Antiproton Decelerator, such a test with neutral antimatter atoms has now become feasible. For this purpose, we have proposed to set up the AEGIS experiment at 0168-583X/$ -see front matter Ó 2007 Published by Elsevier B.V.

Exploring the WEP with a pulsed cold beam of antihydrogen View the table of contents for this issue, or go to the journal homepage for more 2012 Class. Quantum Grav. 29 184009

We have developed a new method, based on the ballistic transfer of preaccumulated plasmas, to obtain large and dense positron plasmas in a cryogenic environment. The method involves transferring plasmas emanating from a region with a low magnetic field (0.14 T) and relatively high pressure (10 ÿ9 mbar) into a 15 K Penning-Malmberg trap immersed in a 3 T magnetic field with a base pressure better than 10 ÿ13 mbar. The achieved positron accumulation rate in the high field cryogenic trap is more than one and a half orders of magnitude higher than the previous most efficient UHV compatible scheme. Subsequent stacking resulted in a plasma containing more than 1:2 10 9 positrons, which is a factor 4 higher than previously reported. Using a rotating wall electric field, plasmas containing about 20 10 6 positrons were compressed to a density of 2:6 10 10 cm ÿ3 . This is a factor of 6 improvement over earlier measurements.

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.