Particle Physics Division

Aims:

One of the fundamental questions of today’s physics concerns the action of gravity upon antimatter. No experimental direct measurement had been successfully performed with antimatter particles in the 2010's. 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 is then 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)

Location

-The GBAR experiment started its installation in the CERN AD hall in December 2016. It is connected to the new deceleration ring ELENA recently build in the AD hall, and takes data since 2022.

Contribution of IRFU

IRFU initiated this project, has studied and built an intense beam of positrons based on a small electron linear accelerator.

It developed the trapping and storage of the positrons, the production of positronium with positrons, and studied the interactions of protons/antiprotons with the positronium.

 

Size of the project

GBAR is a collaboration of 17 institutes in 9 countries

The IRFU team received initial 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.

Contacts

Patrice PEREZ

Laszlo LISZKAY