L’antimatière tombe-t-elle vers le haut ?

Le principe d’équivalence est au centre de la théorie de la relativité générale ; selon ce principe, testé avec une précision toujours plus fine au cours des dernières décennies, toute la matière tombe à la même vitesse. Trois collaborations (ALPHA, AEgIS et GBAR) préparent actuellement des expériences auprès du Décélérateur d’antiprotons du CERN afin de vérifier si ce principe est valable également pour l’antimatière, en mesurant la manière dont les atomes d’antihydrogène tombent sous l’effet de la gravité. Toute différence par rapport à des atomes d’hydrogène normal suggérerait que des effets quantiques entrent en ligne de compte ; nous aurions alors besoin d’une théorie quantique de la gravité.

Measuring the effect of gravity on antimatter is a long-standing story. It started with a project at Stanford in 1968 that attempted to measure the free fall of positrons, but a trial experiment with electrons showed that environmental effects swamped the effect of gravity and the final experiment was not performed. In the 1990s, the PS200 experiment at CERN’s LEAR facility attempted the same feat with antiprotons, but the project ended with the termination of LEAR before any robust measurement could be made. To date, indirect measurements have set limits on the deviation from standard gravity at the level of 10–6.

Thanks to advances in cooling and trapping technology, and the construction of a new synchrotron at CERN called ELENA, three collaborations are now preparing experiments at CERN’s Antiproton Decelerator (AD) facility to measure the behaviour of antihydrogen (a positron orbiting an antiproton) under gravity. The ALPHA experiment has already analysed its data on the trapping of antihydrogen atoms to set upper limits on differences in the free-fall rate of matter and antimatter, and is now designing a new set-up. AEgIS is currently putting its apparatus through its paces, while GBAR will start installation in 2017.

Given that most of the mass of antinuclei comes from massless gluons, it is extremely unlikely that antimatter experiences an opposite gravitational force to matter and therefore “falls” up. Nevertheless, precise measurements of the free fall of antiatoms could reveal subtle differences that point to a crack in our current understanding.

Violating equivalence

To date, most efforts at the AD have focused on looking for CPT violation by comparing the spectroscopy of antihydrogen to its well-known matter counterpart, hydrogen. Now we are in a position to test Einstein’s equivalence principle with antimatter by directly measuring the free fall of antiatoms on Earth. The equivalence principle is the keystone of general relativity and states that all particles with the same initial position and velocity should follow the same trajectories in a given gravitational field. On the other hand, quantum theories such as supersymmetry or superstrings do not necessarily lead to an equivalent force on matter and antimatter (technically, the terms related to gravity in the Lagrangians are not bound to be the same for matter and antimatter). This is also the case when Lorentz-symmetry violating terms are included in the Standard Model of particle physics.

Any difference seen in the behaviour of antimatter and matter with respect to gravity would mean that the equivalence principle is not perfect and force us to understand quantum effects in the gravitational arena. Experiments performed with free-falling matter atoms have so far found no difference to that of macroscopic objects. Such tests have set limits at the level of one part in 1013, but have not yet been able to test the equivalence principle at the level where supersymmetric or other quantum effects would appear. Since the amplitude of these effects could be different for antimatter, the AD experiments might have a better opportunity to test such quantum effects. Any difference would probably not change anything in the observable universe, but it would point to the necessity of having a quantum theory of gravity.

AEgIS plans to measure the vertical deviation of a pulsed horizontal beam of cold antihydrogen atoms, generated by bringing laser-excited positronium moving at several km/s into contact with cold antiprotons, travelling with a velocity of a few hundred m/s. The resulting highly excited antihydrogen atoms are then accelerated horizontally and a moiré deflectometer used to measure the vertical deviation, which is expected to be a few microns given the approximately 1 m-long flight tube of AEgIS. Reaching the lowest possible antiproton temperature minimises the divergence of the beam and therefore maximises the flux of antihydrogen atoms that end up on the downstream detector.

In GBAR, which takes advantage of advances in ion-cooling techniques, antihydrogen ions (H+) are produced with velocities of the order of 0.5 m/s. In a second step, the anti-ions will be stripped of one positron to give an ultra-slow neutral antiatom that is allowed to enter free fall. The time of free fall over a height of 20 cm is as long as 200 ms, which is easily measurable. These numbers correspond to the gravitational acceleration known for matter atoms, and the expected sensitivity to small deviations is 1% in the first phase of operation.

The ALPHA-g experiment will release antihydrogen atoms from a vertical magnetic atom trap and record their positions when they annihilate on the walls of the experiment. In a proof-of-principle experiment using the original ALPHA atom trap, the acceleration of antihydrogen atoms by gravity was constrained to lie anywhere between –110 g and 65 g. ALPHA-g improves on this original demonstration by orienting the trap vertically, thereby enabling better control of the antiatom release and improving sensitivity to the vertical annihilation position. In the new arrangement, antihydrogen gravitation can be measured at the 10% level, which would already settle the question of whether antimatter falls up or down, but improvements in cooling techniques will allow measurements at the 1% level. A long-term aspiration of the ALPHA-g project is to use techniques that cause antihydrogen atoms to interact with a beam of photons, promising a sensitivity in the 10–6 range.

Cooling matter

In the case of AEgIS, the deflectometer principle that underpins the measurement has already been demonstrated with matter atoms and with antiprotons, while the time-of-flight measurement is straightforward in the case of GBAR. The difficulty for the experiments lies in preparing sufficient numbers of antiatoms at the required low velocities. ALPHA has already demonstrated trapping of several hundred antiatoms at a temperature below 0.5 K, corresponding to random velocities of the order 10 m/s. The antiatoms are formed by letting the antiprotons traverse a plasma of positrons located within the same Penning trap.

A different scheme is used in AEgIS and GBAR to form and possibly cool the antiatoms and anti-ions. In AEgIS, antiprotons are cooled within a Penning trap and receive a shower of positronium atoms (bound e+e pairs) to form the antiatoms. These are then slightly accelerated by electric fields (which act on the atoms’ induced electric-dipole moments) so that they exit the charged particle trap axially in the form of a neutral beam. For GBAR, the antiproton beam traverses a cloud of positronium to form the anti-ions, which are then cooled to a few μK by forcing them to interact with laser-cooled beryllium ions.

In this race towards low energies, ALPHA and AEgIS are located on the beam at the AD, which delivers 5 MeV antiprotons. While AEgIS is already commissioning its dedicated gravity experiment, ALPHA will move from spectroscopy to gravity in the coming months. GBAR, which will be the first experiment to make use of the beam delivered by ELENA, is now beginning installation and expects first attempts at anti-ion production in 2018. ELENA will decelerate antiprotons coming from the AD from 5 MeV to just 100 keV, making it more efficient to trap and store antimatter. Following commissioning first with protons and then with hydrogen ions, ELENA should receive its first antiprotons in the middle of 2017 (CERN Courier December 2016 p16). Along with precision tests of CPT invariance, this facility will help to ensure that any differences in the gravitational antics of antimatter are not missed.