How are antimatter-matter pairs kept separate?

Matter in the vampire test

It is no more than a tiny asymmetry between matter and its mirror image, the antimatter, that has led to an excess of matter in the universe. We owe our existence to it. Research groups at Max Planck Institutes in Heidelberg, Munich and Garching want to find out in different ways why matter - like vampires - has lost its mirror image.

Text: Thomas Bührke

The problem of why there is matter in the universe has been a problem for philosophers for centuries. Gottfried Wilhelm Leibniz put it in a nutshell: “Why is there something at all and not rather nothing?” The problem is not just a philosophical one, but also a physical one. Physicists have been looking for a solution to puzzles for decades. As in many areas of physics, symmetries play a decisive role here.

The mathematician Emmy Noether came across fundamental relationships between geometric symmetries in space and time and the laws of conservation of physics 100 years ago. The law of conservation of energy can be derived from such symmetries: In a closed system, energy can neither be lost nor generated. It follows that a perpetual motion machine is impossible. The maintenance of the total momentum, for example when two balls collide, can also be justified on the basis of symmetry specifications.

Over the past few decades, however, physicists have learned that it is not just about symmetries: "We already know these, the big puzzle is the asymmetries," says Michael Schmelling from the Max Planck Institute for Nuclear Physics, who is involved in one of the large experiments participates in the particle accelerator LHC des Cern in Geneva. Indeed, if the construction kit of the elementary particles were completely symmetrical, there would be no matter in the universe, hence neither the earth nor us humans.

An antiparticle to each elementary particle

The reason why perfect symmetry would have made the existence of matter impossible lies in the processes involved in the Big Bang. Because according to today's idea, the universe was filled with radiation and matter with unimaginably high temperature and density in the first billionths of a second. There was a seething mixture of particles that turned into radiation and back into matter.

But now physicists know that in such a chaos of particles and radiation, antiparticles also arise. This knowledge is also decades old: for every type of elementary particle there is a corresponding antiparticle that only differs in the sign of its electrical charge, but otherwise has exactly identical properties. The antiproton, for example, looks like a positively charged proton, but is negatively charged.

Although there is no doubt about the origin of antiparticles when the world is born, they are virtually non-existent in the universe. Because the two unequal partners have the fatal property of destroying each other in a flash of radiation when they meet. Conclusion for the Big Bang: If complete symmetry had prevailed at the time, as many particles as antiparticles would have been created in the sea of ​​radiation - and they would all have annihilated each other. The universe would then only contain radiation. So where does matter come from?

In order for matter to remain after the Big Bang, there must have been a tiny imbalance: When around a billion matter-antimatter pairs were annihilated, only a few particles were left. This difference seems very small, but we owe our existence to it. The physicists only have a vague idea of ​​how this asymmetry came about: “You can perhaps imagine it as a phase transition, similar to the freezing of water to ice,” explains Schmelling. "In the process, the asymmetry was frozen, so to speak, and the superiority of matter in the universe cemented."

This theory goes back to the Russian physicist and Nobel Peace Prize winner Andrei Sakharov. When he published it in 1967, he relied on an experiment that three years earlier had profoundly shaken physicists in their belief in natural symmetries. James Cronin and Val Fitch had investigated the decay of so-called K mesons in an accelerator at the Brookhaven National Laboratory. These particles consist of two quarks that belong to the elementary particles and are unstable. In a fraction of a second after they are created, they break down into other particles.

The standard model is stretchable in some ways

Cronin and Fitch studied the decays of K mesons and compared them with those of anti-K mesons. When they found a tiny difference in the two types of decay in the alcohol range, it was downright a shock for the specialist community. The complete symmetry between matter and antimatter was broken in this case, as the physicists say.

But they could not explain the excess of matter in the Big Bang in this way, the measured asymmetry is much too small for that, it should be a billion times larger. The theorists Toshihide Masukawa and Makoto Kobayashi built this asymmetry into the standard model of elementary particles, for which they received the 2008 Nobel Prize in Physics. Cronin and Fitch had already been honored with this award in 1980.

The standard model is like a construction kit that contains all known elementary particles and the forces acting between them. This model works great, but is in some ways stretchy. Although it determines the number and type of particles, it cannot predict certain physical quantities; they have to be taken from nature. This includes, for example, the masses that are then built into the model.

An asymmetry like that of the mesons can also be accommodated there without the building collapsing. However, only within a certain framework, and this has to be explored experimentally and theoretically. It only gets really exciting when researchers discover asymmetries that go beyond the limits of the standard model. Because only such discrepancies can explain the existence of matter; they would, however, force the physicist community to build a completely new theoretical structure instead of the old model and thus to create a kind of new physics.

Therefore, the researchers continue to search for such deviations from perfect symmetry. Lately they have been concentrating on a different type of mesons: the B mesons, which come in different variants. The currently ideal instrument for this is the LHC, in which protons rotate in opposite directions and collide with the highest energy. In the resulting fireballs, B mesons and their antipartners form among many other particles, whose decay particles are analyzed with the LHCb detector.

Schmelling's group played a key role in the development and construction of a silicon detector for this device, which is the size of a three-story house. The silicon detector alone occupies an area of ​​around eleven square meters and can determine the passage of a charged particle with an accuracy of 0.05 millimeters, i.e. about the thickness of a human hair.

After being a physicist in the USA and Japan at B0-Mesons had already discovered an asymmetry of eight percent, the LHCb collaboration focused on brother meson B.0sthat can be produced in large numbers in the LHC. Then three years ago the surprise: When comparing the decays of B0s- Mesons and their antimesons found an asymmetry of a size never seen before - namely 27 percent. Was that finally the hot lead that led to the mysterious preference for matter in the Big Bang?

Unfortunately no - this very strong asymmetry can probably still be explained in the context of the Standard Model, as the theorists quickly reported. Only a value that does not fit there could be an indication of the physics beyond the Standard Model, which could make the excess of matter understandable. Scientists with the LHC are currently searching meticulously for her - so far unsuccessfully.

However, the data stream of the LHC is far from being fully evaluated, and the search for a symmetry violation continues in the decays of other meson types. But Michael Schmelling wants to look for another effect that would shake the foundations of today's physics: that the characteristics of a meson decay, such as the lifetime of the particles, depend on the direction in space - i.e. on the arrangement of the experiment is oriented with respect to the fixed stars.

A wealth of experiments have confirmed that space is isotropic, i.e. that it does not have any particular preferred direction. It is physically irrelevant in which direction you send a ray of light in free space, it will always move in the same way and at the same speed. The most precise experiments confirm this to within 15 places behind the decimal point. But what about the decay properties of particles and antiparticles?

In order to approach this question, one has to think about the forces acting between particles, which the standard model kit contains. With light, only the electromagnetic force plays a role. When particles decay, the so-called weak force comes into play, which only works in the atomic nucleus. It is theoretically conceivable that this weak force interacts with an unknown, hypothetical energy field that pervades the room. The idea is no coincidence. Cosmologists discovered in 1998 that there is such an energy field in the universe: dark energy. It acts like a pressure in a steam boiler, drives the universe apart and allows it to expand at an accelerated rate.

So one could imagine a direction-dependent background field that feels the weak force but not the electromagnetic one. Then it would be possible that the characteristics of a particle decay depend on the direction in which one is moving relative to this background field - just as the speed of a ship also depends on whether it is moving with or against the current. That is all hypothetical, says Schmelling: "But we want to check it out."

We are looking for variations over the course of a day

The task now is to compare the decays and other properties of particles and antiparticles relative to the hypothetical energy field, i.e. depending on the orientation of the experimental setup to the fixed stars. "If there is a directional dependency, we have to see variations with the period of a day because the orientation to the fixed stars is different at night than during the day," says Schmelling. The data is already in place and the LHC will provide more in the future.

The experiment at the LHC, which traces the asymmetry between matter and antimatter, is also to be supplemented by another accelerator experiment. If all goes well, it will start in two years. After an eight-year renovation phase, the SuperKekB accelerator at the research center in Tsukuba, Japan, should run at full speed. In two separate rings, each three kilometers in circumference, electrons and anti-electrons (positrons) run in opposite directions and collide at one point.

SuperKekB is smaller than the LHC and does not accelerate the particles to such a high energy by any means, but the latter is set in such a way that many more pairs of B mesons and their antiparticles are created during the collisions than at the LHC - and then decay again immediately. Physicists therefore like to speak of the B-factory. In this system there is a much weaker background of other particles, so that the data analysis is easier than at the LHC. In addition, this facility can be used to study types of decay of the B mesons, which are basically hidden from the LHC.

From the end of 2018, the superfactory is expected to produce up to 40 times more B-mesons per unit of time than its predecessor - and it held the world record until it was shut down in 2010. In order to be able to precisely analyze the decay products of the particles, the old detector called Belle, which detects the particles produced during meson decay, had to be technically improved considerably.

The central element of Belle II is a vertex detector, with which the flight direction and the place of origin, known as the vertex, of a particle can be determined down to a hundredth of a millimeter. The core of this instrument is a pixel vertex detector, which in turn consists of 40 image sensors. One of these sensors has 200,000 individual pixels.

If a particle hits such a pixel, it generates a very small signal in it, which is amplified in the pixel itself. “With their 50 by 60 micrometers, the pixels are small marvels in themselves,” says the spokesman for the international detector collaboration Christian Kiesling, who conducts research at the Max Planck Institute for Physics in Munich. The pixel vertex detector was designed and built there and at the MaxPlanck Society's semiconductor laboratory in Munich. “The development of this globally unique detector cost us a lot of sweat,” says the scientist.

With Belle II, the researchers at SuperKekB also want to study those types of decay of the B mesons that are extremely rare. Theoreticians make very precise predictions for these, in other words: Here the standard model is not so flexible and can best be checked experimentally. The program also includes the investigation of other unstable particles - always in the hope of finding an asymmetry somewhere between the particle and the corresponding antiparticle that can explain the excess matter in the world.

Whether at the LHC or the SuperKekB - the decay experiments take place at extremely high energies. The search for the asymmetry between matter and antimatter can, however, also pursue other paths. The alternative is simply to compare the properties of elementary particles and their antiparticles with the greatest possible accuracy. Apart from the sign, these should be identical. Any further difference, no matter how small, would contradict today's physics. Even the elastic standard model leaves no room for maneuver here.

A possible asymmetry in the magnetic moment

A group led by Klaus Blaum, Director at the Max Planck Institute for Nuclear Physics, is studying the properties of protons, the nuclei of hydrogen atoms, and antiprotons. So far, the researchers have most precisely compared the charge-to-mass ratio of the two particles. This combination is easier to measure experimentally than the individual quantities. To do this, a proton or an antiproton is first transferred into a vacuum container, where it is captured and stored by an electric and a magnetic field. The particle then performs a circular movement around the axis of the magnetic field, which can be precisely measured and from which the measured variable is obtained (MAXPLANCK RESEARCH 3/2010, page 46 ff.). “This experiment is very delicate and requires a lot of experience because we only work with a single proton or antiproton,” says Klaus Blaum. In mid-2015, the base collaboration, headed by Blaum's former colleague Stefan Ulmer, published the world's most accurate result to date in the specialist magazine NATURE. According to this, the charge-to-mass ratio for both particles is the same to within less than a billionth.

The researchers are now using this experimental experience to compare another characteristic quantity of protons and antiprotons: the magnetic moment. This can be remotely thought of as the strength of the magnetic field that a single proton creates. It is extremely small and more difficult to measure than the charge-to-mass ratio. However, according to theoretical predictions, it could be a hot candidate for an asymmetry between matter and antimatter. In the year before last, an international collaboration, in which the Heidelberg group, among others, the University of Mainz, the GSI in Darmstadt and the Riken Research Institute in Japan are involved, succeeded in determining the magnetic moment of the proton to within three billionths of a second. World record!

Next, the researchers want to make the corresponding measurement on the antiproton. To do this, however, the physicists have to bring their equipment to Cern, where a small accelerator, the antiproton decelerator, supplies the cold antiprotons. "We want to measure the magnetic moment of a single antiproton there and increase the accuracy ten or a hundred times by the end of 2018," explains Blaum. This is a race against time, because in September 2018 the LHC will be switched off for a longer maintenance break, and then antiproton production will also come to a standstill.

Experimenting with anti-hydrogen, i.e. atoms that consist of an antiproton and an anti-electron (positron), is even more challenging. These experiments are currently only possible worldwide at Cern. The first tricky problem is to bring antiprotons and positrons together and cool them down enough that they combine to form anti-atoms. The second problem occurs precisely at this moment: unlike their two components, the antiatoms are electrically neutral and cannot be easily captured and stored.

But why all this effort with atoms when investigations on elementary particles such as protons and their counterparts from the anti-world are easier? One reason is again the precision that is possible with measurements on atoms.Because hardly any quantum physical value is measured as precisely as a certain transition of the electron in the hydrogen atom. Physicists understand a transition to be the raising of an electron to a higher or bringing it down to a lower energy state.

Exaggeratedly asked: Does the antiapple fall up?

The energy exchanged during the transition can be measured so precisely because Theodor W. Hänsch, Director at the Max Planck Institute for Quantum Optics in Garching, developed the so-called frequency comb, for which he was awarded the Nobel Prize in Physics in 2005. This technology makes it possible to measure the frequency of the hydrogen transfer with a spectrometer with an accuracy of 14 places after the decimal point. So if you want to find minimal differences between matter and antimatter, this technique is the most accurate. Masaki Hori's group at the Max Planck Institute in Garching has been working on this feat since 2008 as part of the international Atrap collaboration.

A second property can be measured in the anti-hydrogen that could reveal a difference between matter and antimatter: the free fall solely under the influence of gravity, which can only be demonstrated in electrically neutral particles. Alban Kellerbauer and his colleagues at the Max Planck Institute for Nuclear Physics are working on such experiments.