Quark flavours

Physicist explores models of the tiniest building blocks


By Constanteyn Roelofs

The Standard Model for describing quarks, the tiny particles of matter, is incomplete and a Leiden quantum physicist is studying other orders. “The situation could change at any given time.”

There is no accounting for taste, they say, so why has a dissertation with the intriguing title A flavour of family symmetries in a family of flavour models by Reinier de Adelhart Toorop just been published? Well, that is because flavours are not only gastronomic features, they are also found in quantum physics, the study of the very tiniest of building blocks of the matter in, and around, us.
Physicists have had the recipe for stable matter for some time: a cloud of electrons around an atomic nucleus of neutrons and protons. The electrons are regarded as elementary, that is to say: they cannot be divided into smaller bits.
However, protons and neutrons can be divided up again, and when they are, physicists distinguish between two types of quarks: up and down, and there are always three per proton or neutron – a proton is up, up, down and a neutron is down, down, up. In addition, we have the mysterious neutrino, a particle without an electric charge and with almost no mass, that roams the universe. Neutrinos are the building blocks of all matter. Up and down are also known as “flavours” of quark. Electrons and neutrinos are not quarks, but they are related to each other and together they are known as leptons (from the Greek for “small”).
The whole is held together by three forces: the strong nuclear force that keeps atomic nuclei together, the weak nuclear force that causes things like radioactive decay and the electromagnetic force.
These things had scarcely been determined when yet more new flavours of quarks and leptons were discovered. It soon emerged that each quark had two brothers: up was escorted by “charm” and “top” while down was accompanied by “strange” and “bottom”. The electron proved to be related to the muon and the tau-lepton. And lastly, it was discovered that three neutrinos exist – which suddenly meant that there were three times two sets of flavours. These “brothers” from the second and third families have practically the same properties as the members of the first family. The most important properties, the electric charge and the spin are the same – it’s the mass that is different, and that can differ by a factor of a hundred, depending on the family member.
Interaction between the particles by means of the weak nuclear force, however, causes the heavy brothers in electrons and in up and down quarks to collapse within a fraction of a second, which is why we can’t find them in ordinary matter. We can only find the particles again when they occur in exceptional circumstances and to be able to study them thoroughly, these particles have to be generated in particle accelerators.
This categorisation is known as the Standard Model and can be used quite adequately to describe the results of the particle accelerator, but it is incomplete. The question as to why the families have such neat structures still hasn’t been answered. Why aren’t there four families, for instance? De Adelhart Toorop replies: “It is still a mystery. The existence of the three families is regarded as an experimental fact, but isn’t explained. But there has to be a reason for it.”
Accordingly, the Standard Model is expanded to included “fields”: theories that add new particles and interaction mechanisms.
In his dissertation, Reinier de Adelhart Toorop explores possible explanations for the similarities, the symmetry, in the families to explain the order in the particles and examines whether there is any structure in the way in which neutrinos start interacting by means of the weak nuclear force. Neutrinos occur in all three flavours “in the wild”, because the particles wobble, as it were, between the three flavours as they move through space. Depending on when you observe them, the neutrino can be small, medium or large, and this produces yet more models that can explain the cohesion between the particles. The strange thing is that the neutrinos cannot start interacting with each other in all states, but only in specific combinations. But, as yet, no one knows why.
Another possibility is that there is a relation between the family symmetry and the “Grand Unified Theory”, which states that, rather than having three different forces, there is a single force working at different energy levels. This would make the model much more straightforward but it is difficult to establish as the photons used to study the particles in the particle accelerators are also influenced by the energy level at which the experiments are conducted, compromising the research results.
His dissertation’s working title was The Beauty and the Beast, says the PhD student.
“Beauty” because the field of the family symmetries could produce suitable models in various ways, which, in turn, could be used to explain other mysteries of particle physics, such as the enigma of the dark matter and the possible existence of the Higgs particle. And the beastly? “The bad news is that the structure that that has inspired the family models is perhaps less convincing than people initially supposed.” The data from the interaction patterns in particular reveals many irregularities.
De Adelhart Toorop continues: “We can at least say that elementary particles physics seems to have found an answer to the question ‘What’s it made of?’. The question ‘Does it have a structure?’ is answered in the affirmative anyway, but how much structure is still a matter of debate.
And there are so many more answers to the question ‘Is there a reason for it?’ And when new answers are found to the first question – such as if particle accelerators were to find new, unexpected particles – the situation could change at any given time.”

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