What is matter What is antimatter

A comparison of matter and antimatter

Shortly after the Big Bang, around 13.8 billion years ago, matter and antimatter were initially created in equal proportions, only to almost completely annihilate each other shortly afterwards. All that remained was the matter of which our world is made today. The Standard Model of Particle Physics predicts perfect symmetry between matter and antimatter, but must therefore be incomplete. Welt der Physik spoke to Klaus Blaum from the Max Planck Institute for Nuclear Physics in Heidelberg about the difference between matter and antimatter, which researchers want to track down in the BASE experiment at the CERN research center, among other things.

The world around us - including us - is made of matter. The atoms are made up of protons, neutrons and electrons. Each of these particles has a counterpart, called an antiparticle. The antiparticles resemble matter in every respect - only their electrical charge is exactly the opposite. The positively charged positron corresponds to the negatively charged electron, and the negatively charged antiproton to the positively charged proton. But there are also corresponding antiparticles for electrically neutral particles such as the neutron. Here the difference in the charges of the quarks becomes noticeable, i.e. the elementary particles that make up the neutrons.

Klaus Blaum from the Max Planck Institute for Nuclear Physics

Klaus Blaum: “The neutron consists of an up quark with a charge of two thirds and two down quarks with a charge of minus one third each. This then gives the charge zero, so it is electrically neutral. With the antineutron, on the other hand, you have an up-transverse quark with a minus two thirds charge and two down-transverse quarks with plus a third charge, so that it is ultimately charge-neutral. So there are also the corresponding antiparticles for the neutral particles. "

The British physicist and Nobel Prize winner Paul Dirac was the first to track down antimatter in the 1920s - albeit by chance. Paul Dirac was working on a mathematical equation that describes the behavior of the electron. He found that this equation has not one but two solutions.

“Dirac has found a positive and a negative solution. The positive solution was interpreted as a solution for the description of particles and particle properties. At that time, he predicted the other solution - revolutionary in my view - as the solution of another type of particle, namely that of the antiparticles. "

Just a few years later, the positron predicted by Dirac could be detected in experiments. It occurs not only in cosmic rays that strike the earth's atmosphere from deep space, but also in certain radioactive decays. In principle, nothing speaks against the fact that chemical elements consist entirely of antimatter: perhaps anti-sodium, or anti-gold. In the laboratory, however, even the production of the second lightest chemical element is extremely time-consuming.

"The heaviest system that has ever been created - among other things in experiments that took place at the LHC -, is antihelium. Antihelium consists of two antiprotons and two antineutrons in the nucleus. Heavier systems require such high production rates or such high energies that they are not realistic. "

Among other things, physicists use what is currently the largest particle accelerator in the world, the Large Hadron Collider, to generate tiny amounts of antihelium. In nature, antimatter is only created in a few other processes.

"The little antimatter that we can still see is created, for example, by radioactive decay processes or when high-energy particles are created in galaxies and then hit the atmosphere, producing exotic antiparticles."

The BASE experiment at the CERN research center

A universe made almost entirely of matter - what bothers researchers is that in the course of the Big Bang around 13.8 billion years ago, matter and antimatter must have been created in exactly the same proportions. However, if particles and antiparticles meet, they destroy each other - all that remains is their energy in the form of radiation. Thus matter and antimatter should have completely annihilated each other shortly after the Big Bang. For reasons unknown so far, obviously not all matter has completely disappeared. This so-called process of baryogenesis occupies researchers, because theoretically matter and antimatter should be exactly the same.

“Our standard model says that the particle properties are the same as the antiparticle properties. This is accompanied by the so-called CPT symmetry. C stands for the English charge - the charge -, p for parity - the place - and t for time - time. That is, if I run the wave function of a particle under the transformation CPT, I get the antiparticle with identical properties. Such is our current understanding of the standard model. The theory says that the properties of particles and antiparticles are identical. "

So the theory cannot be entirely correct, or at least it is incomplete. There must be a break in the CPT symmetry, a tiny difference between matter and antimatter that caused some matter to be left after the Big Bang. Physicists are looking for an explanation. In precision experiments, they compare the properties of matter and antimatter. Klaus Blaum and his colleagues use the so-called BASE experiment at the CERN research center for this.

"We are currently only concerned with checking the ground state properties of matter and antimatter, in this case even of charged matter and antimatter - so we are examining protons and antiprotons."

First, the scientists checked whether the proton and antiproton differ in terms of their charge and mass.

“The ratio of charge to mass can be determined by the number of revolutions that a proton or an antiproton makes in a strong magnetic field. You can measure this very precisely with special technologies and then compare the number of revolutions. That means, we let these particles go around hundreds of millions of times, count the number of revolutions for a certain time window and compare the number of revolutions with that of the antimatter. And so you can find out whether one of the particles is a little heavier or has a different charge compared to its partner. "

Penning trap - the heart of the BASE experiment

The researchers found that this ratio is exactly the same - down to the eleventh decimal place. Similarly, a single fly on board an airplane weighing several hundred tons would be measurable. Currently, the aim of the BASE experiment is to precisely record the magnetic moment of the proton and antiproton. The magnetic moment of a particle can be thought of as a kind of bar magnet: the proton and antiproton each have a north and a south pole.

“The question is how strong this bar magnet is. We try to compare this strength and have also measured it most precisely worldwide in a sub-experiment by BASE - to 2 x 10-9 exactly. We are also working on increasing the proton by an order of magnitude, and we have already succeeded in making this measurement. And we are now trying to do the same thing on the antiproton in order to ultimately be able to compare the magnetic properties - that is, how strong is the bar magnet in the case of the proton and the antiproton. "

The theory suggests that the difference between matter and antimatter could be noticeable in the magnetic properties such as the magnetic moment - but how big or small this difference should be cannot be theoretically estimated. And so the researchers have no choice but to continuously improve the accuracy of their measurements - the experiments to determine the magnetic moment of the antiproton are still ongoing.