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
Last week, we promised that you could have a look into our #antiproton decelerator, starting with the first half of the device that can be seen in the picture below:
But first, a few words on how it works.
The ELENA decelerator provides us with antiprotons that have been decelerated to a certain energy: 100 keV (kilo electron-Volt), to express it in a convenient unit. For the purpose of the our experiment, we want even lower energy: 1 keV.
Well, if you want to stop a beam of 100 keV, make it hit a wall of 100 kV (kilo Volts). This is what we already mentioned when we introduced the decelerator: