ALPHA is an international collaboration based at CERN, and whose aim is stable trapping of antihydrogen atoms, the antimatter counterpart of the simplest atom, hydrogen. By precise comparisons of hydrogen and antihydrogen, the experiment hopes to study fundamental symmetries between matter and antimatter.
Physicists have long wondered if the gravitational interaction between antimatter and matter might be different than that between matter and itself. Do atoms made of antimatter, like antihydrogen, fall at a different rate to those made of matter, or might they even fall up -- antigravity? There are many arguments that make the case that the interaction must be the same, but no-one has ever observed what an anti-atom does in a gravitational field - until now.
Today, the ALPHA Collaboration has published results in Nature Communications placing the first experimental limits on the ratio of the graviational and inertial masses of antihydrogen (the ratio is very close to one for hydrogen). We observed the times and positions at which 434 trapped antihydrogen atoms escaped our magnetic trap, and searched for the influence of a gravitational force. Based on our data, we can exclude the possibility that the gravitiational mass of antihydrogen is more than 110 times its inertial mass, or that it falls upwards with a gravitational mass more than 65 times its inertial mass.
Our results far from settle the question of antimatter gravity. But they open the way towards higher-precision measurments in the future, using the same technique, but more, and colder trapped antihydrogen atoms, and a better understanding of the systematic effects in our apparatus.
Read the paper on Nature Communications at http://dx.doi.org/10.1038/ncomms2787
Right now everyone at ALPHA is busy assembly the ALPHA-2 apparatust, the sucessor to ALPHA. The most recent parts to arrive have been the atom-trap cryostat built in TRIUMF in Vancouver, and the new superconducting solenoid, built by Oxford Instruments in the UK and financed by the Danish Carlsberg Foundation. They join the catching trap, designed by the Cockcroft Institute and the existing positron accumulator from ALPHA to make up the complete chain of apparatus being used in ALPHA-2.
The first antiprotons were caught last night in the new ALPHA2 catching trap, the first component of the next generation of the ALPHA experiment to be installed. This is the representation of the first 'hot dump' -- where we release the captured antiprotons, allowing them to annihilate on the surrounding apparatus. The annihilation converts the antiprotons into high-energy charged particles, which are counted by detectors surrounding the apparatus. Because we detect the annihilations at the same time as we release the trap, we can be sure that the antiprotons have been captured in the trap. Read more about the Penning trap in How ALPHA works.
The catching trap, designed in collaboration with staff at the Daresbury Laboratory and the Cockcroft Institute in the UK, will be responsible for cooling 5MeV antiprotons from the AD, and supplying them on demand to the ALPHA2 atom trap, which will be installed later this year. Construction has been taking place at the AD for the last month, and even though there's a long way to go before the apparatus achieves its full potential, this is a big milestone for us at ALPHA.
The construction team at CERN
Once you've trapped antihydrogen what do you do? You measure it! That's just what we've done. Published in Nature, we report the first resonant quantum transitions in antihydrogen atoms. We've used microwave radiation to change the internal state of the atom, from one which can be kept in our trap, to one that is kicked out. This process depends on the frequency of the microwave radiation and the magnetic field in the trap, so by changing both of these, we demonstrated that we had enough control and sensitivity to sucessfully carry out the experiment. This is by no means easy, as antihydrogen is not found in nature, but must be prepared in our apparatus from antiprotons made in the Antiproton Decelerator, and positrons from a radioactive source, Even more, it must have low enough energy to remain trapped in the magnetic fields making up our trap. Here's an animation describing how we do our measurement.
Eventually, we will use this technique to compare the structure of antihydrogen and hydrogen atoms, to search for difference between matter and antimatter, but In this first experiment, we do not yet have enough precision to test these fundamental symmetries. This is important, as the Universe has shown a preference for matter over antimatter as it has evolved, but so far, no measurements can explain why this came about. If matter and antimatter were truely identical, the Universe as we know it could not have come about. The next step at ALPHA is to construct an apparatus that will allow us to make these more precise measurements, using both microwave radiation, and laser light.
We've been waiting a long time for this result, so we're really happy -- the CERN People documentary has been following us through the process -- check out the first video here.