3.1. Proliferation of particle families
In the present standard theory of physics, the elementary particles are grouped in three main families called electronic, muonic and tauonic. Each family contains four members: two leptons and two quarks. For each member of a given family, there are corresponding members of the other two families which, as far as we know, differ only in masses. For instance, to the electron (0.5 MeV) correspond the muon (107 MeV), and the tauon (1.5 GeV).
The families are shown in table 1 with the best estimates of the masses. We have, so far, no evidence of any neutrino masses; only upper limits are quoted. The existence of the t-quark has not yet been established.
| electronic | muonic | tauonic | ?onic | ?onic |
| electron-neutrino | muon-neutrino | tauon-neutrino | ? | ? |
| < 10 eV | < 25 eV | < 100 MeV | ||
| electron | muon | tauon | ||
| 0.5 MeV | 0.1 GeV | 1.5 GeV | ? | ? |
| quark-u | quark-c | quark-t | ||
| 0.3 GeV (3 MeV) | 1.5 GeV | > 40 GeV | ? | ? |
| quark-d | quark-s | quark-b | ||
| 0.3 GeV (5 MeV) | 0.5 GeV | 5.0 GeV | ? | ? |
The question in every one's mind is how many more families are there in our world. A few years ago, high energy physics had hardly anything to say about this question.
Big-bang nucleosynthesis, on the other hand was making very definite predictions. The number of extra families could not be very large. At best, one or two. More probably, in fact, we already have come to the end of the list with our three families.
The latest CERN experiments (1990) have confirmed this prediction of primordial nucleosynthesis: the number of neutrino family is three. In the following paragraphs I will present the physical arguments behind these statements. Here I want to take the occasion to put some emphasis on the importance of the event I am talking about, because it seems to have passed largely unnoticed in the scientific community.
We should keep in mind that Big-Bang is, in the usual scientific context, quite an extravagant theory. Contrary to the scientific paradigm held unequivocally from the time of the ancient Greeks, throughout the Renaissance until quite recently, it places the universe in a historical framework. Instead of being the observer of an eternal unchanging realities, the astrophysicist become an historian exploring the past, in search of events which have given the world the properties it has today. A similar transition had already taken place in the life sciences, one century ago, when Darwin denied the fixity of the animal and plant species to introduce the notion of biological evolution. With the Big-Bang this notion is enlarged to the whole physical universe.
The more extravagant, (or out-of-the-beaten-path), a theory is, the better should be the proofs in his favour, before it is accepted. Confirmed predictions are always of prime values here, as it is always easier to find explanations to known facts than to predict correctly the result of a future observation or experimentation. After correctly predicting the existence of the fossil radiation, the theory has also passed successfully the test of the family proliferation. This is worth a double mention.