Meson factories provide the answer

It may well be that the explanation of broken CP-symmetry also provides a raison d’être for the second and third particle families. These resemble the first family in many respects, but are so short-lived that they cannot form anything lasting in our world. One possibility is that these capricious particles fulfilled their most important function at the beginning of time when their presence guaranteed the broken symmetry that made matter win against antimatter. How nature solved this problem is, as mentioned before, something we do not yet know in detail. The broken symmetry needs to be reproduced many, many times to create all the matter that gives us our star-scattered sky.

Kobayashi and Maskawa’s theory also indicated that it should be possible to study a major violation of symmetry in B-meson particles, which are ten times heavier than their cousins, the kaons. However, broken symmetry occurs extremely rarely in B-mesons, so immense quantities of these particles are needed to find just a few that break the symmetry. Two gigantic constructions housing the BaBar particle detectors at the SLAC accelerator at Stanford, California and Belle at the KEK accelerator at Tsukuba in Japan produced more than one millionB-mesons a day in order to follow their decay in detail. As early as 2001, both independent experiments confirmed the symmetry violation of the B-mesons, exactly as Kobayashi and Maskawa’s model had predicted almost 30 years earlier.

This meant the completion of the Standard Model, which has worked well for many years. Almost all the missing pieces of the puzzle have fallen into place in accordance with the boldest of predictions. All the same, the physicists are still not content.

Symmetry lies hidden under spontaneous violations

As already explained, the Standard Model comprises all of the known elementary particles and three of the four fundamental forces. But why are these forces so different? And why do the particles have such different masses? The heaviest one, the top quark, is more than three hundred thousand times heavier than the electron. Why do they have any mass at all? The weak force stands out in this respect again: its messenger particles, W and Z, are much heavier, while its ally, the photon, which conveys the electromagnetic force, lacks mass at all.

Most physicists believe that another spontaneous broken symmetry, called the Higgs mechanism, destroyed the original symmetry between forces and gave the particles their masses in the very earliest stages of the universe.

The road to this discovery was mapped out by Yoichiro Nambu when, in 1960, he was the first to introduce spontaneous symmetry violation into elementary particle physics. It is for this discovery that he is now awarded the Nobel Prize in Physics. To begin with, Nambu worked on theoretical calculations of another remarkable phenomenon in physics, superconductivity, when electric currents suddenly flow without any resistance. Spontaneous symmetry violation that described superconductivity was later translated by Nambu into the world of elementary particles, and his mathematical tools now permeate all theories concerning the Standard Model.

We can witness more banal spontaneous symmetry violations in everyday life. A pencil standing on its point leads a completely symmetrical existence in which all directions are equal. But this symmetry is lost when it falls over — now only one direction counts. On the other hand, its condition has become more stable, the pencil cannot fall any further, it has reached its lowest level of energy.

A vacuum has the lowest possible energy level in the cosmos. In fact, a vacuum in physics is precisely a state with the lowest possible energy. But it is not empty by any means. Since the arrival of quantum physics, a vacuum is defined as full of a bubbling soup of particles that pop up, only to immediately disappear again in ubiquitously present but invisible quantum fields. We are surrounded by many different quantum fields across space; the four fundamental forces of nature are also described as fields. One of them, the gravitational field, is known to us all. It is the one that keeps us down on earth and determines what is up and what is down.

Nambu realised at an early date that the properties of a vacuum are of interest for studies of spontaneous broken symmetry. A vacuum, that is, the lowest state of energy, does not correspond to the most symmetrical state. As with the fallen pencil, the symmetry of the quantum field has been broken and only one of many possible field directions has been chosen. In recent decades, Nambu’s methods of treating spontaneous symmetry violation in the Standard Model have been refined; they are frequently used today to calculate the effects of the strong force.

Higgs provides mass

The question of the mass of elementary particles has also been answered by spontaneous broken symmetry of the hypothetical Higgs field. It is thought that at the Big Bang the field was perfectly symmetrical and all the particles had zero mass. But the Higgs field, like the pencil standing on its point, was not stable, so when the universe cooled down, the field dropped to its lowest energy level, its own vacuum according to the quantum definition. Its symmetry disappeared and the Higgs field became a sort of syrup for elementary particles; they absorbed different amounts of the field and got different masses. Some, like the photons, were not attracted and remained without mass; but why the electrons acquired mass at all is quite a different question that no one has answered yet.

Like other quantum fields, the Higgs field has its own representative, the Higgs particle. Physicists are eager to find this particle soon in the world’s most powerful particle accelerator, the brand new LHC at Cern in Geneva. It is possible that several different Higgs particles will be detected — or none at all. Physicists are prepared, a so-called supersymmetric theory is the favourite among many to extend the Standard Model. Other theories exist, some more exotic, some less so. In any case, they are likely to be symmetrical, even though the symmetry may not be evident at first. But it is there, keeping itself hidden in the seemingly messy appearance.

(http://www.nobelprize.org/nobel_prizes/physics/laureates/2008/public.html)

Exercises