Tuesday, February 23, 2010

The forces of nature IV - The weak force

The last force of nature is the weak force. Actually, its name is a bit misleading, but this will be explained later. This force is something which is even less accessible to our daily experience, but it is as much of relevance to our very existence as the other forces. The most direct evidence of it is already a nuclear process, the so-called beta decay. In such a decay, a neutron transforms, or decays, into a proton, an electron, and an electron-anti-neutrino. Precisely such decays, or the inverse process of neutron formation, is of quite central importance for suns. In the interior of suns, this process is very much involved in the generation of heat and light, which makes live on earth possible. But it is also of relevance in the formation of heavy elements, like lead, during supernova explosions. So, much of the things we see around us exist only due to the weak force. The details of how this works in suns and novas is very intricate, and complicated to model. These are central questions in the field of nuclear astrophysics. However, this has little connection to the research illustrated in this blog, so I will skip it. Many excellent resources on these questions can be found elsewhere on the web.

For my purposes, let me delve deeper into what happens during beta decays. The proton and neutron are themselves build from quarks, and it is actually not the neutron which transforms, but one of the quarks. The neutron consists out of two down quarks and one up quark, while the proton has two up quarks and one down quark. Thus, to get from a neutron to a proton, one down quark has to be exchanged for an up quark. This is exactly what the weak force is doing. For this to happen one of the agents of the weak force needs to become involved. This agent is called the W boson. There are actually two of these, which differ by their electric charge, one having a positive charge like the proton, the other a negative charge like the electron. What happens is that the down quark emits a negatively charged W boson - the down quark itself has a third of the charge of the electron - and by this transforms into an up quark, which has two thirds of the charge of the proton. This transforms the neutron into a proton. The emitted W boson then decays into the electron, which carries the electric charge, and the electron-anti-neutrino, which are observed.

So that is how the weak force acts. But why are the W bosons not itself detected like the photons. Are they strongly bound like the gluons? The answer is no. The reason is rather different. The W bosons have a large mass, which is actually just about half the one of the top quark. They are therefore some of the heaviest particles found so far. Hence, it is favorable for them to decay into lighter particles after traveling a short distance. That is then also the reason why the force appears so weak: It can only act over a very short distance, before its agents decay, and the resulting particles start to act differently.

When investigating this phenomenon more in detail, it turns out that the there is another agent of it, the so-called Z boson. This is electromagnetically neutral, and about ten percent heavier than the W bosons. It is thus the second-heaviest elementary particle we know so far. Because of its properties, it turns out that it often acts very much like a very heavy copy of the photon. Indeed, upon closer inspection it is found that the photon and Z boson are not two particles apart, but mix quite heavily with each other: At long distances what looks like the photon is more of a Z boson at short distances. This is because of the mass of the Z boson, which is so much heavier than the photon and can therefore not travel far without decaying. But at short distances it looks more like a Z boson.

Therefore, and because of the charges of the W bosons, the description of both interactions - the weak and the electromagnetic ones - rest on a common theory, the so-called electroweak theory. QED, described earlier, is actually only its long-distance face. At short distances, reached in modern particle physics experiment, the unification of both forces into one is very evident.

There are two things odd about the weak part of this interaction. First, it acts actually not directly on the particles described earlier, say the up quark, but only on certain combinations of them. Therfore, saying before that the down quark decays is not quite right. It is more like that a combination of the down quark and a strange quark which appears as a quantum fluctuation inside the neutron act together to produce the W boson, and by this change into the up quark and a quantum fluctuation of a charm quark. These quantum fluctuations inside the neutron and proton are not strange - that is something which is natural in quantum physics that things just pop up and vanish here and there. That will be looked at in detail later. The strange thing is that such combinations are necessary. Why this is so is one of the big questions of the theory.

The other thing strange is that the weak interactions makes a difference between left and right. In fact, it prefers left over right as much as possible. This has directly observable consequences. For example, if a decaying neutron is put into a magnetic field, the emitted electron has a preferred direction with respect to the field. Neither the strong nor the electromagnetic force has such a preference. This may be seen as an oddity of nature, at first sight. However, it has very profound consequences for our understanding of nature, as will be discussed next.

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