A new project called GBAR (Gravitational Behaviour of Antihydrogen at Rest) aims to measure the free-fall acceleration of neutral antihydrogen atoms in the Earth’s gravitational field. Initiated by Patrice Perez (CEA / Irfu), GBAR is an international collaboration involving fifteen institutes in France, Germany, Japan, Poland, Russia, the United Kingdom and Switzerland. Several groups from the LKB are involved in this collaboration. The project is described in the proposal that was submitted to the CERN SPSC Committee in September 2011 and approved in May 2012. The project is currently being installed at CERN.
The steps necessary to carry out the GBAR experiment are as follows:
Production of an intense flow of low energy positrons (a few MeV) from the interaction on a thin tungsten target of a 10 MeV electron beam produced by a small accelerator.
Selection of positrons and suppression of electron and gamma ray backgrounds with a magnetic separator.
Moderation of the positrons down to a few eVs.
Accumulation of positrons in a Penning-Malmberg high-field trap, where they cool to a few meV and are then ejected in less than 100 ns onto a porous silicate target to form a dense cloud of ortho-positronium.
Excitation of positronium to gain an important factor on their effective cross-section of interaction with antiprotons.
Interaction with the very low energy antiproton beam extracted from the Antiproton Decelerator AD, followed by the ELENA ring at CERN, to produce antihydrogen atoms and ions.
Ion accumulation and sympathetic cooling to 10μK.
Photo-detachment at the threshold of excess positrons and measurement of the free fall of antihydrogen atoms.
In the longer term, greater accuracy of measurement can be achieved by spectroscopy of antihydrogen levels in the gravity field, based on the method used for ultra cold neutrons at the ILL (Institut Laue Langevin).
In this programme, the LKB teams are in charge of the following elements
Positronium excitation and photodetachment
(Team Simple systems metrology and fundamental tests)
Cooling of antihydrogen ions
(Team Trapped Ions)
Study of the quantum reflection of antihydrogen atoms and spectroscopy of quantum states in the gravitational field
(Team Quantum Fluctuations and Relativity with the ILL).
Motivations and principe
Motivations and principe
GBAR’s scientific motivation is to test Einstein’s Equivalence Principle with antimatter. The validity of the principle of equivalence for antimatter is a scientific question whose interest is strongly reinforced by the observation of the acceleration of the expansion of the Universe, which raises fundamental questions about gravitation theories. This discovery triggered very large projects in astrophysics. The introduction of dark energy to take account of observations leads to open questions related to particle physics. In addition, the matter content of the Universe seems to be dominated by so-called dark matter, whose nature and properties are still unknown. These observational facts suggest that our understanding of gravity remains incomplete.
The main features of the proposed experiment follow an idea of J. Walz and T.W. Hänsch (General Relativity and Gravitation 36, (2004) 561). The originality of the idea is to produce the positive antihydrogen ion before the antihydrogen atom.Ions can be cooled to temperatures of the order of μK (velocities of the order of m/s), then the excess positrons photodetached by laser in order to recover the neutral atom and observe its free fall. This process can be adapted to minimize pulse transfer in the vertical direction. The temperature reached in cooling the ions is the main source of systematic error. The ion is produced by a process of charge exchange between antiprotons and positroniums.
The measurement requires a large production of antihydrogen at very low speeds, which is an experimental challenge. Sympathetic cooling of ions to below 100μK is an essential part of the method. Accuracy in measuring the acceleration of antihydrogen atoms is statistically limited and depends on ion temperature. For example, at 10μK, an accuracy of 1% is obtained with 1500 atoms. This is the target accuracy for the first phase of the experiment, which could be achieved in a few weeks of operation. A longer duration would allow measurement at the 0.1% level.In a next generation, a new technique using spectroscopy of the quantum levels of antihydrogen atoms in the gravity field could give even better precision on the measurement of acceleration.