Decay of antimatter solves physics puzzle: Why observation is important

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Xueting Yang and his team at the CERN nuclear research center have observed that matter and antimatter behave differently during the decay of heavy, subatomic particles – known as baryons. The so-called CP violation was theoretically predicted more than 60 years ago – but has never been observed. The "strong CP problem" is considered one of the most underestimated open questions in physics.

Baryon – what?

After the construction of the first particle detectors, numerous new elementary particles were found – by the early 1960s, their number had grown to over 400. It was not until the standard model of particle physics, which has been developed and expanded ever since, that order was restored to the particle zoo by explaining how the new particles can be composed of elementary building blocks.

Baryons consist of three of these elementary building blocks – the quarks. Protons and neutrons, which make up atomic nuclei, are typical baryons.

How does this standard model work?

The standard model assumes that there are only a few elementary particles. The entire particle zoo is made up of these simple basic building blocks. The standard model explains how. There are two families of basic building blocks: quarks and leptons. Matter and antimatter are composed of quarks and leptons.

Specific forces act on quarks and leptons. To put it bluntly, it's like superheroes who have their own superhero powers. Electrons, for example, are affected by electromagnetic fields, quarks by the so-called strong interaction and the electromagnetic interaction, and so on. Only the so-called weak interaction has an influence on the neutrino.

The forces between the elementary particles are mediated by the exchange of virtual particles called bosons. Each specific force is associated with specific bosons. Each elementary particle constantly exchanges such virtual bosons with the corresponding field surrounding it.

All quark(s)?

Quarks are remarkable in that they never occur alone, but in pairs or threes. They are also the only elementary particles that do not have an integer electric charge.

There are six types of quarks, known as "flavors": up, down, charm, strange, top and bottom. Protons and neutrons are composed of up and down quarks. Quarks are held together by the aforementioned strong interaction.

Symmetry and conservation laws

Most physical equations are "symmetrical" in one way or another – even if this sounds rather strange at first. It means that the equations do not change if, for example, the coordinate system is mirrored or rotated, the sign of an electric charge is reversed or the course of time is reversed.

The mathematician Emmy Noether recognized that each of these specific symmetries is related to a so-called conservation variable. If equations are symmetrical with respect to time, for example, the energy of the overall system is conserved.

Depending on the force under consideration, other symmetries and thus conservation laws apply in the standard model. For example, the number of leptons must be preserved. Because a lepton – the electron – is created during beta decay, an antilepton must also be released during the process: This is an antineutrino.

Antimatter

Anti-matter differs from ordinary matter only in one quantum number – The anti-matter equivalent of an electron, for example, is a particle with the same mass, the same spin, but the opposite charge: the positron. When anti-particles and particles meet, they cancel each other out by emitting radiant energy.

This should actually also have happened in the Big Bang. Matter and anti-matter should have been created in equal proportions – and should have extinguished each other completely. However, the universe consists of matter.

CP violation

CP symmetry refers to a reversal of charge and a mirror image of the coordinate system. If the equations of a nuclear force do not change after these two operations, it is called CP invariant.

According to the Standard Model, the strong interaction, i.e. the force that holds quarks together, is not CP-invariant. This means that matter and antimatter should behave differently during decay.

Cosmological models assume that matter and antimatter were created in equal amounts in the Big Bang, but in today's universe matter seems to dominate over antimatter. This imbalance is probably caused by differences in the behavior of matter and antimatter: CP violation.

What does this mean?

The theoretically predicted and now observed difference between matter and antimatter first of all confirms the standard model of particle physics. But it does not answer the question of why only matter seems to have remained after the Big Bang.

And it raises further questions: For example, why it is so difficult to observe this asymmetry and why it occurs with such low probability. This is because the CP violation predicted by the Standard Model is many orders of magnitude too small to explain the asymmetry between matter and antimatter observed in the universe.

This suggests the existence of new sources of CP violation beyond those predicted by the Standard Model. The search for these sources is an important part of the LHC physics program and will be continued at future particle accelerators that could replace the LHC.

This article first appeared on t3n.de.

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