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.

The main goal of the AEgIS experiment at CERN is to test the weak equivalence principle for antimatter. AEgIS will measure the free-fall of an antihydrogen beam traversing a moir'e deflectometer. The goal is to determine the gravitational acceleration ḡ with an initial relative accuracy of 1% by using an emulsion detector combined with a silicon μ-strip detector to measure the time of flight. Nuclear emulsions can measure the annihilation vertex of antihydrogen atoms with a precision of ∼ 1–2 μm r.m.s. We present here results for emulsion detectors operated in vacuum using low energy antiprotons from the CERN antiproton decelerator. We compare with Monte Carlo simulations, and discuss the impact on the AEgIS project.

The goal of the AEḡIS experiment at the Antiproton Decelerator (AD) at CERN, is to measure directly the Earth's gravitational acceleration on antimatter by measuring the free fall of a pulsed, cold antihydrogen beam. The final position of the falling antihydrogen will be detected by a position sensitive detector. This detector will consist of an active silicon part, where the annihilations take place, followed by an emulsion part. Together, they allow to achieve 1% precision on the measurement ofḡ with about 600 reconstructed and time tagged annihilations. We present here the prospects for the development of the AEḡIS silicon position sentive detector and the results from the first beam tests on a monolithic silicon pixel sensor, along with a comparison to Monte Carlo simulations.-2 -2 antimatter by measuring the Earth's gravitational acceleration g for antihydrogen. Several attempts 3 have been made in the past to measure the gravitational constant for antimatter, both for charged 4 [2, 3] and neutral antiparticles [4, 5, 6]. However, none of these experiments brought to conclusive 5 results. Recently, a study from the ALPHA collaboration [7] sets limits to the ratio of gravitational 6 mass to the inertial mass of antimatter but is yet far from testing the equivalence principle. Another 7 experiment, GBAR, [8] has been proposed but not yet built. 8 Cold antihydrogen (100 mK) in Rydberg states will be produced through the charge exchange 9 reaction between Rydberg positronium and cold antiprotons stored in a Penning trap [9]. Applying 10 an appropriate electric field will accelerate the formed antihydrogen in a horizontal beam, with a 11 typical axial velocity distribution spanning a few 100 m/s [10]. 12Some of the trajectories will be selected through a moiré deflectometer [11], which will consist 13 of two vertical gratings producing a fringe pattern on a downstream annihilation plane (see fig. 2). 14 This plane will be the first layer of the position sensitive detector where the antihydrogen will 15 impinge with energies of the order of meV and annihilate. The vertical deflection of the pattern 16 is proportional to the gravitational constant to be measured. Over a flight path of ∼ 1 m, the 17 deflection is expected in the order of ∼ 20 µm for a 1 g vertical acceleration [1]. A vertical 18 resolution better than 10 µm is required to meet the goal of 1% precision on theḡ measurement 19 with 600 reconstructed and time tagged annihilations [12]. 20According to the current design, the position sensitive detector will be a hybrid detector con-21 sisting of an active silicon part, where the annihilation takes place, followed by an emulsion part 22 65 the fragmentation of excited hadronic systems into individual hadrons, whereas the FTFP model 66 [23] relies on a string model to describe the interactions between quarks. 67The CHIPS and FTFP models differ in the production rate and in the composition of the 68 annihilation products. CHIPS produces heavy nuclear fragments in only 20 % of the events while 69 FTFP...

AEḡIS experiment's main goal is to measure the local gravitational acceleration of antihydrogenḡ and thus perform a direct test of the weak equivalence principle with antimatter. In the first phase of the experiment the aim is to measureḡ with 1% relative precision. This paper presents the antihydrogen production method and a description of some components of the experiment, which are necessary for the gravity measurement. Current status of the AEḡIS experimental apparatus is presented and recent commissioning results with antiprotons are outlined. In conclusion we discuss the short-term goals of the AEḡIS collaboration that will pave the way for the first gravity measurement in the near future.

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