Thread by @GBAR_Experiment

Before continuing our visit of the GBAR installation, a ⬇️thread⬇️ for a more detailed introduction.
GBAR is an experiment at @CERN that will measure the gravitational acceleration of #antimatter on Earth, that we note "g" with a bar above ("g-bar" afterwards).
#gravity #physics

For matter, g is 9.81 m/s². It has never been directly measured for #antimatter and, in its first stage, GBAR will be able to give a value for g-bar with a 1 % relative precision. #CERN #gravity #physics

Any deviation of g-bar from g would be a major discovery, helping to explain why our observable universe is made of matter. #CERN #antimatter #gravity #physics

Obviously, a negative g-bar (gravitational repulsion) would be even more exciting! There are arguments against this; however, as long as it has not been directly measured, it cannot be ruled out. #CERN #antimatter #gravity #physics

To measure g-bar, the idea of GBAR is simple: let’s drop #antimatter and measure its free fall time. To make this free fall experiment, we need very slow and cold neutral antimatter. #CERN #gravity #physics

1) Slow, because we want to see the deflection due to #gravity over a reasonable distance (otherwise, one needs huge & pricy vacuum vessel and detectors). In GBAR, slow is about 1 m/s, so #antimatter can typically fall by 20 cm over a 20 cm horizontal distance. #CERN #physics

2) Cold means that the falling #antimatter particles all have very similar initial velocities, to avoid a large dispersion in the measured free fall times. In particular, initial vertical velocity distribution is what will limit our precision on g-bar. #CERN #gravity #physics

3) Neutral, because #gravity is so weak that a tiny electric or magnetic field would ruin an experiment with a charged antimatter particle. This is why we need to use antihydrogen (H-bar). #CERN #antimatter #physics

Other experiments successfully make and study antihydrogen atoms at #CERN (home.cern/science/physic…), but the way their antiatoms are produced does not provide cold enough particles for our purpose. And cooling antihydrogen is really hard! #antimatter #gravity #physics

However, manipulating and cooling a charged particle is easier… In particular, you can use a laser-cooled ion species as a freezer for another ion species (sympathetic cooling). The most convenient ion “freezer” for us is Beryllium, charge +1. #CERN #antimatter #gravity #physics

Of course, there is a trick, because you don’t want to see your #antimatter annihilating with the beryllium ions! It means that the antimatter species must also have a + charge: then Coulomb repulsion prevents the annihilation. #CERN #gravity #physics

Therefore, we need to produce a positive ion of antihydrogen: H-bar+, made of 1 antiproton and 2 positrons, the antimatter equivalent of H-. After cooling, a laser kicks out one positron and we obtain a cold antihydrogen atom ready to fall. #CERN #antimatter #gravity #physics

A large part of the GBAR experimental set-up is dedicated to the production of H-bar+. The antiprotons we get from @CERN...(instagram.com/p/Bp4GBQUF4bQ/)
#antimatter #gravity #physics

… while we produce our own positrons, using an electron accelerator (home.cern/news/news/phys…). #CERN #antimatter #gravity #physics

Last year, we introduced you to the positron trapping: instagram.com/p/BrSv0qdl5Uz/
Next, you will learn more about our recipe to make antihydrogen ions.
End of the thread.
#CERN #antimatter #gravity #physics