Gravitational Behaviour of Antihydrogen at Rest

Buffer gas trap built at Saclay for the trapping of positrons


One of the fundamental questions of today’s physics concerns the action of gravity upon antimatter. No experimental direct measurement has ever been successfully performed with antimatter particles. CERN has thus launched a research program with the Antiproton Decelerator (AD) allowing to prepare a measurement of the effect of gravity on antihydrogen atoms.

The primary aim of this experiment is to determine how antimatter reacts to gravity. A first test will be to verify the sign of the gravitational acceleration for antimatter, as a theory opens the possibility for it to be negative, which would translate as an elevation rather than a fall of an antimatter atom only submitted to the gravity force of the Earth. Other theories predicting less spectacular deviations with respect to standard gravitation will also be tested.

The GBAR project, initiated by an IRFU team, has been approved by CERN in May 2012. It started as a R&D program of IRFU demonstrating the feasibility of an intense beam of slow positrons. This beam is needed to create a target of positronium atoms (the bound state of an electron and a positron), allowing the production of H+ ions when bombarded with antiprotons.

Several challenges must be overcome, with the following ones being studied in IRFU:

  • Create a slow positron source of high intensity (108/s) ; the usual radioactive sources uses to make antihydrogen are limited at about 106/s.
  • Create positronium atoms with density of order 1012 cm-2, using a mesoporous silica substrate;
  • Exploit these in a very short time, of order their decay time period of 142 ns, with the aim to form a target for the antiprotons.


It will then be possible to create neutral anti-hydrogen H as well as positive anti-hydrogen ions H+. Note that if protons are used as incident particles instead of antiprotons, one obtains hydrogen and H- ions.

Competing Experiments

ALPHA-g (Antihydrogen Laser PHysics Apparatus)
AEGIS (Antimatter Experiment: Gravity, Interferometry, Spectroscopy)


-The GBAR experiment started its installation in the CERN AD hall in December 2016. It will be connected to the new deceleration ring ELENA recently build in the AD hall

-The SOPHI and SELMA projects are located in the CEA-Saclay site (former Saturne accelerator Hall). The SOPHI facility received an ANR grant and is the prototype of the positron source built by GBAR at CERN. SELMA is the small linear electron accelerator feeding the source. Another ANR grant supported the ANTION project, devoted to study the interaction between protons and positronium. It will measure the production of hydrogen atoms at Saclay, then later at CERN the production of H- ions, then of antihydrogen and of H+ ions.


Contribution of IRFU

IRFU initiated this project.

It studies and builds an intense beam of positrons based on a small electron linear accelerator.

It studies the trapping ond storage of the positrons;

It studies the production of positronium with positrons, and the interactions of protons/antiprotons with the positronium.


Size of the project

GBAR is a collaboration of 19 institutes in 9 countries

The IRFU team received funding from IRFU, the French National research Agency (ANR) (for SOPHI, POSITRAP and ANTION projects) and from the district of Essonne (for the SELMA project).

This project has a human size compared to the big CERN experiments; it is highly multidisciplinary, dealing with particle physics, atomic physics, accelerator physics, laser…
The goal, ambitious and complex, is fundamental for physics, because any deviation of the antimatter gravity compared to the matter one would lead to revisit the classical models at the most fundamental level.


Patrice PEREZ




Last update : 06/12 2017 (2095)

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