Speaker
Description
Summary
The principal of equivalence between gravitational mass and inertial mass is a foundation of general relativity. The universality of free-fall (UFF), the experimental evidence on which the weak equivalence principle is based, has been tested to be valid to a very high precision (1 part in 10 trillion) by many experiments using a variety of techniques [1]. General relativity is a classical theory which makes no distinction between matter and antimatter particles. However, there has never been a direct verification of the weak equivalence with antimatter. Furthermore, it also claimed that an asymmetry in the interaction of gravity with matter and antimatter could be a signature of quantum theories of gravity [2].
The scientific goal of the AEgIS experiment is to measure, in the first instance, the acceleration of antihydrogen in the Earth’s gravitational field with a 1% accuracy. In the original proposal for the experiment it was planned to use a silicon strip tracking detector to determine the position of the annihilation vertex with a 10 um position resolution. Monte Carlo simulations suggest that to measure gbar with a 1% accuracy with a detector that has a 10um position resolution, will require the detection of 10,000 antihydrogen atoms. However, only 500 antihydrogen atoms are required for the 1% gbar measurement if the antihydrogen annihilation is reconstructed with a 1 um position resolution. The order of magnitude reduction in the required antihydrogen flux is particularly significant in light of the considerable technical difficulties that must be overcome to produce an antihydrogen beam.
In light of the considerable benefit to AEgIS of a 1 um position sensitive detector, a detector based on nuclear emulsions is currently under development. Nuclear emulsions are photographic films which are optimized for use as particle tracking detectors and are still the most precise particle tracking detector technology currently available. A typical detector consists of a gel with a suspension of silver bromide crystals, in which a track is formed after the passage of an ionizing particle. After chemical development silver grains along the path of the track, with dimensions of 1 μm or less, are visible with an optical microscope. With just a single 50 μm layer it is possible for the full three-dimensional reconstruction of the particle track and a measurement of the energy loss. The intrinsic resolution for tracks in the nuclear emulsion is a mere 50 nm.
In order to evaluate the suitability of a nuclear emulsion based gbar detector for AEgIS, a series of measurements were performed during the 2012 antiproton decelerator (AD) run at CERN. The 5 MeV antiprotons from the AD were directed onto a nuclear emulsion the surface of which was partially covered with a 20 um stainless steel foil. The foil simulates the separation window which will be used in the actual experiment to isolate the emulsion detector from the ultra high vacuum region. The antiproton annihilation vertex was reconstructed with an impact parameter resolution of 1.0 um / 1.4 um in the region uncovered / covered by the stainless steel foil.
A proof of principle of the complete deflectometer was tested by passing antiprotons through a small moiré deflectometer coupled to an emulsion film. Tests were also performed to check that the emulsions could operate in the AD radiation environment. This talk will describe these measurements and detail further progress towards the realization of the final free-fall detector and the gbar measurement in general.
[1] B. Heckel et al., Advances in Space Research, 25(6):1225 – 1230, 2000.
[2] M. Nieto et al., Physics Reports, 205(5):221 – 281, 1991.
