Gravitational Behaviour of Antihydrogen at Rest

Buffer gas trap built at Saclay for the trapping of positrons


One of the fundamental questions of today 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 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 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, and of antihydrogen and  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

  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 - SOPHI, POSITRAP and ANTION 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.


  • CSNSM/IN2P3 Orsay (FR)
  • ETH Zurich (CH)
  • ILL Grenoble (FR)
  • IPCMS Strasbourg (FR)
  • Lebedev Moscow (RU)
  •  LKB Paris, (FR)
  •  JGU Mainz (GE),
  •  NCBJ Otwock-Swierk (PL)
  •  RIKEN (JP)
  • Swansea University (UK)
  • University of Tokyo, Komaba, (JP)
  • Kyoto University (JP)
  • Uppsala University (SE)
  • Stockholm University (SE)
  • Seoul National University (KR)
  • Institute for Basic Science Daejon (KR)
  • Korea University (KR)
  • Ulsan National Institute (KR)



Patrice PEREZ






Last update : 02/08 2017 (2095)

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