Antimatter at CERN
AEgIS experimental hall (Nov/12) Marco Giammarchi and Rafael at the experimental hall of AEgIS (Dec/11).
AEgIS (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) is a physics experiment that takes place at the european laboratory CERN, using the antiprotons delivered by the ADaccelerator. AEgIS is a collaboration of physicists from all around the world.
The primary scientific goal of the AEgIS experiment is the direct measurement of the Earth’s gravitational acceleration g on antihydrogen. In the first phase of the experiment, a gravity measurement with 1% precision will be carried out by sending an antihydrogen beam through a classical Moire deflectometer coupled to a position sensitive detector. This will represent the first direct measurement of a gravitational effect on an antimatter system.
Figure 1. Sketch of the experimental setup region where antiprotons and positrons are manipulated to form and to accelerate antihydrogen. This region is at low temperature (~100 mK) in an axial magnetic field of 1 T (after Ref. 14).
Figure 1 shows a schematic drawing of the basic experimental setup that should reach an accuracy of 1% in the measurement of the matter-antimatter gravitational acceleration. The experiment is designed to allow higher precision measurements through radial cooling of the beam [15]. The essential steps leading to the production of antihydrogen (Hbar) and the measurement of its gravitational interaction in AEgIS with the use of CERN cold antiproton (pbar) are the following: i) accumulation of positrons (e+) in a Surko-type source and trap [16]; ii) capture and accumulation of pbar from the AD in a cylindrical Penning-Malberg trap [17]; iii) cooling of the pbar cloud to sub-K temperatures; iv) production of cold positronium (Ps) by bombardment of a cryogenic nanoporous material with an intense e+ pulse; v) two-steps laser excitation of Ps to a Rydberg state (Ps*) with principal quantum number n > 20; vi) pulsed formation of cold Rydberg antihydrogen by means of the resonant charge exchange interaction between Rydberg Ps* and cold pbar with a residual electron; vii) pulsed formation of an Hbar beam by Stark acceleration with inhomogeneous electric fields; viii) determination of g in a two-grating Moiré deflectometer coupled with a position-sensitive detector.
The formation and excitation of Ps to a Rydberg state represent a very interesting challenge. Our proposed technique of formation is conceptually similar to a charge exchange technique based on Rydberg Cs [18] which has been successfully demonstrated by ATRAP [19], but offers greater control of the final state distribution of and allows pulsed production of .
Construction has started on the AEgIS experiment, whose design is based upon the broad experience gained with the ATHENA and ATRAP experiments at the AD, a series of ongoing related experiments, tests and developments, as well as extensive simulations of critical processes (charge exchange production of , Stark acceleration and propagation through the Moiré deflectometer, resolution of the position-sensitive detector located at the end of the deflectometer). The proposed gravity measurement merges in a single experimental apparatus technologies already demonstrated or based on reasonable additional development.
For the initial phase of the experiment, obtaining samples of anti-atoms at 100 mK is an essential requirement. Gravity measurements with even higher precision, as well as competitive CPT tests through spectroscopy, are desirable, but will necessitate the development of novel techniques to attain even colder ensembles. The experiment has been designed with flexibility of the apparatus in mind, in order to allow a number of techniques, which may lead to such physics topics, to be implemented. One natural extension of the modular design is to incorporate, in a future stage, a magnetic decelerator and trap for , which will be spatially separated from the region where the anti-atoms are produced, similar to the devices currently being used to trap and study H atoms [20, 21]. The experience gained in the first phase of AEgIS with the formation of a beam will be used to optimize the design of such a trapping system.
References
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[6] ATRAP: http://hussle.harvard.edu/~atrap/
[7] ALPHA: http://alpha.web.cern.ch/alpha/
[8] ASACUSA: http://asacusa.web.cern.ch/asacusa/
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[14] G. Testera et al., AEgIS collaboration, Proc. of Cold Antimatter Plasmas and Application to Fundamental Physics Conference (Okinawa) vol. 1037 (AIP Conference Proceedings, 2008) p 5.
[15] AEgIS proposal: http://cdsweb.cern.ch/record/1037532/files/spsc-2007-017.pdf
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Some fundamental questions of modern physics relevant to unification of gravity with the other fundamental interactions, models involving vector and scalar gravitons, matter–antimatter symmetry can be enlightened via experiments with antimatter [10]. A quantum theory of gravitation necessarily constitutes a departure from the Einstein view of gravity as a geometric phenomenon and could potentially constitute a violation of the weak equivalence principle. This principle is a foundation of General Relativity and a large experimental effort is placed in testing its consequences in all possible fields: this research activity includes tests of the equality of the inertial and gravitational mass, the universality of the free fall, the search for non Newtonian corrections to the gravitational law, the measurement of the gravitational red shift, and the search for time variation of the fundamental constants. Measurements studying the equality of the inertial and gravitational mass of different macroscopic bodies or cold atoms have only been performed on ordinary matter. There are no direct measurements about the validity of the principle of equivalence for antimatter. At present, the validity of the equivalence principle for antimatter is extrapolated from the matter results or it is inferred using indirect arguments. Particularly interesting is that some quantum gravity models leave room for possible violations of the equivalence principle for antimatter [11]. Modern theories of gravity that attempt to unify gravity with the other forces of nature allow that, at least in principle, antimatter may fall differently from normal matter in the Earth‘s gravitational field. Specifically, as pointed out by Sherk [12], theories of supergravity are open to the possibility of a gravitational interaction which have different couplings for matter and antimatter.
The recent production of copious amounts of cold antihydrogen at CERN’s Antiproton Decelerator (AD) [3, 13] has paved the way for high-precision gravity experiments with neutral antimatter. In the first phase of the experiment, acceleration in a controlled way by an electric field gradient (Stark effect) and subsequent measurement of free fall in a Moiré deflectometer will allow a test of the weak equivalence principle. In a second phase, the antihydrogen will be slowed, confined and laser-cooled to perform CPT studies and detailed spectroscopy.
A new generation of antimatter experiments has been opened after the first experiments of antihydrogen production in laboratory conditions at the CERN laboratory [1] and at Fermilab [2]. These experiments produced hot , i.e. relativistic, in small quantities not suited to precision studies. Thereafter a program is underway at CERN with a facility dedicated to low energy and experiments. After the first production of cold by the ATHENA [3, 4] and ATRAP [5, 6] collaborations, second generation experiments, as ALPHA [7] and ASACUSA [8], are being performed for measuring the fundamental properties of this antiatom. AEgIS is an experiment approved by CERN with the goal of studying physics [9].