GBAR

GBAR. The Gravitational Behaviour of Antihydrogen at Rest (GBAR) experiment is a fundamental physics project located at the European Organization for Nuclear Research (CERN). It is designed to directly measure the gravitational interaction between matter and antimatter by observing the free fall of neutral antihydrogen atoms. The experiment aims to test the Weak Equivalence Principle—a cornerstone of Albert Einstein's general relativity—with antimatter, potentially revealing new physics beyond the Standard Model. By employing innovative techniques to produce, trap, and cool antihydrogen to ultra-low energies, GBAR represents a significant frontier in antiparticle research.

Overview

The GBAR collaboration was formally proposed in the early 2010s and approved for operation at CERN's Antiproton Decelerator (AD) facility, joining other pioneering experiments like ALPHA, ATRAP, and BASE. Its primary objective is to perform the first direct measurement of Earth's gravitational acceleration on antimatter, a test never before conducted with neutral antiatoms. The experiment leverages the unique infrastructure of the AD, which provides low-energy antiprotons, and the ELENA deceleration ring to further reduce antiproton energies. The conceptual and technical challenges of GBAR place it at the intersection of particle physics, atomic physics, and precision measurement, requiring advances in positron accumulation, ion trapping, and laser cooling.

Scientific goals and principles

The core scientific goal is to test the Weak Equivalence Principle, which states that all objects fall with the same acceleration in a gravitational field, independent of their composition. While extensively verified for ordinary matter—from Galileo Galilei's legendary experiments to modern tests with satellites like MICROSCOPE—it remains untested for antimatter. A violation could have profound implications, suggesting CPT symmetry breaking or hinting at theories like supersymmetry or quantum gravity. GBAR specifically aims to measure the gravitational acceleration of antihydrogen, the antimatter counterpart of hydrogen, with a precision of 1%. This requires producing antihydrogen ions (H̅⁺) to facilitate sympathetic cooling via laser techniques, ultimately creating ultra-cold, neutral antiatoms suitable for a free-fall measurement in a gravity field.

Experimental setup and components

The experimental apparatus is a complex integration of several key systems installed on the AD beamline. It begins with a positron accumulator, which uses a surko trap to collect positrons from a sodium-22 source. Simultaneously, antiprotons from ELENA are captured and combined with the positrons in a nested Penning trap to form positronium. A critical step involves directing a pulse of laser-excited positronium into a cloud of trapped antiprotons to produce antihydrogen ions via a two-step charge exchange process. These H̅⁺ ions are then sympathetically cooled by co-trapped beryllium ions and subsequently photo-detached by a laser to create neutral, ultra-cold antihydrogen atoms. These atoms are released into a free-fall tube, and their annihilation upon striking a detector grid is precisely timed to measure their acceleration due to gravity.

Results and findings

As an ongoing experiment, GBAR has achieved several critical milestones toward its ultimate measurement. The collaboration has successfully demonstrated the efficient production of positronium in the required dense clouds and has performed initial tests of the ion formation and cooling processes. Commissioning of the full apparatus with antiproton beams is underway, with the first synthesis of antihydrogen ions being a key recent objective. While the definitive gravity measurement is still forthcoming, early results on positron accumulation rates and trap efficiencies have been published, contributing valuable data to the broader field of antimatter research conducted by groups like ALPHA and BASE. The experiment continues to refine its techniques to achieve the necessary control over antiatom temperatures and densities.

Collaboration and institutions

GBAR is an international collaboration involving research institutes and universities from across Europe and beyond. The project is spearheaded by French laboratories including the French National Centre for Scientific Research (CNRS) and the French Alternative Energies and Atomic Energy Commission (CEA), with significant contributions from the University of Tokyo in Japan, the University of Liverpool in the United Kingdom, and the University of São Paulo in Brazil, among others. The collaboration benefits immensely from its location at CERN, which provides not only the antiproton beam but also a synergistic environment alongside other AD experiments. Funding and support are provided by national agencies such as the Japan Society for the Promotion of Science and the European Research Council, highlighting the global interest in answering one of physics' most fundamental questions.

Category:Antimatter experiments Category:CERN experiments Category:Physics experiments