Gravitational Behaviour of Antihydrogen at Rest


One of the fundamental questions of todays 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 has been approved by CERN in May 2012.


The R&D programme of IRFU consists in demontrating the feasibility of the production of H+ ions with the use of a target of positronium atoms (the bound state of an electron and a positron). This target, when bombarded with antiprotons, should allow to combine its positrons with the incoming antiprotons. 

Several challenges must be overcome , with the following ones being studied in IRFU:
-create positronium atoms with density of order 1012 cm-2.
-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        (Antihydrogen Laser PHysics Apparatus)
ATRAP        (Antihydrogen Trap Collaboration)
ASACUSA    (Atomic Spectroscopy And Collisions Using Slow Antiprotons)
AEGIS        (Antimatter Experiment: Gravity, Interferometry, Spectroscopy)


-The appatus to measure the performances of the converter of positons into positronium is located at ETH Zurich 

-The SOPHI and SELMA projects are located in the CEA-Saclay site (former Saturne accelerator Hall)

-The confinement and extraction tests of the stored positrons were performed at RIKEN (Wako, Japan), and now at Saclay

-The GBAR experiment will take place at CERN in the AD/ELENA Hall.


Contribution of IRFU

IRFU (formerly DAPNIA) initiated this project.

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

It studies the conversion of positrons into positronium in collaboration with CEA/IRAMIS/LSI who has an expertise in the use of positrons to measure point like defects in materials ands CNRS/CEMHTI. This research is performed together with a group from ETH Zurich at CERN.

  Size of the project

GBAR is a collaboration of 14 institutes and 49 physicists.

The IRFU team received funding from IRFU, the French National research Agency (ANR - SOPHI and POSITRAP projects) and the district of Essonne. This project has a human size allowing to have an overall vision of the way an experiment works. The goal, ambitious and complex, is fundamental for physics, because the discovery of antigravity would lead to revisit the classical models at the most fundamental level.


  • CEA/IRFU -  Saclay (France)
  • CEA/IRAMIS - Saclay (France)
  • CNRS/CSNSM - Orsay (France)
  • ETH - Zurich (Switzerland)
  • ILL - Grenoble (France)
  • IPCMS - Strasbourg (France)
  • JGU - Mainz (Germany)
  • Lebedev - Moscow (Russia)
  • LKB - Paris (France)
  • NCBJ - Otvock (Poland)
  • RIKEN -Wako (Japan)
  • U. of Swansea (UK)
  • U. of Tokyo - Komaba (Japan)
  • Tokyo U. of Science (Japan)
  • U. of Uppsala (Sweden)





Last update : 01/17 2017 (2095)

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