Speech of Professor Martinus Veltman Nobel prize of Physics 1999

(English)

MARTINUS J. G. VELTMAN* MacArthur Emeritus Professor of Physics University of Michigan Your excellencies, ladies and gentlemen. This talk will be about facts and mysteries in particle physics. Let me start by showing you some facts. On the picture every circle represents a particle. Some of them are familiar, in the sense that they are all around us. The top two rows contain the up and down quarks. These are the particles that you find when dissecting protons and neutrons. The nuclei of all atoms are made of protons and neutrons. Around these nuclei circle electrons, the particle on the fourth row. All matter that we see around us is therefore made of the particles that are contained in the top ensemble in the figure The rest of the particles is not part of the matter around us. When the first particle of the rest was found, the muon, some physicist exclaimed: "who ordered this ?" And now, after many years of research all the others have been found, and we still ask: who ordered them ? What you see in the figure are facts, hard facts. We speak of families. There are thus three families of particles. What makes it more amazing is this: for all we know the properties of the particles in the second and third family are precisely the same as those of the first family, except that they are heavier. They are subject to the same forces, with the same strength (except for some very weak forces discussed later). All these new particles have been given certain names, shown in the figure. To put it dramatically, assume that the first family had not existed. Forgetting for the moment the effect of the larger masses, we would have the same world! Instead of the up and down quark the nuclei would be made of charmed and strange quarks! And muons would be circling around these nuclei. Thus there would be no difference except that for example the President of the Swiss Confederation, Mr Ogi, would be even more charming but also much stranger! This is the great puzzle of particle physics today. It is so striking, these three identical families, and yet we have no clue why. It is extremely frustrating. The difference between these particles is their masses, and that provides us with at least a faint hope that we may find out. To understand this I must speak of the forces that we have come to understand. There are forces acting on the particles. The way forces act on the particles is described by perhaps the greatest discovery of the twentieth century, quantum mechanics, subtly different from Newtonian mechanics that describes for example in precise detail the movements of the planets around the sun. Quantum mechanics similarly applies to the movements of electrons around the nucleus in an atom. According to the laws of quantum mechanics to every force there corresponds a particle. To electromagnetic forces, for example, correspond photons. So there are a few more particles than those shown in the figure. We will not go into detail, except to say that some of them have been discovered here at CERN. We have at this time a very good understanding of the forces acting on the particles. We can calculate in minute detail how the particles interact with each other. It has been CERN, and in particular LEP, the machine that is now the focus of this celebration, that has made the most important contributions to measuring and verifying these effects. For example, on the basis of the LEP experiments we have been able, theoretically, to predict the mass of the top quark to within 5%. It was subsequently found, in 1995, at Fermi lab near Chicago, neatly with the predicted mass. The theory is really very good. The uncertainty would have been much less than 5% except for one missing link, a very strange animal. It is another force, so far not established experimentally, but yet an important and needed part of the theory. This is called the Higgs force. Associated with that force is a particle that we call the Higgs particle. While we think that we can guess its properties, we do feel unsure. There are crazy things about this Higgs particle. I will tell you about the craziest of them all. With forces are also associated fields. For example, there are electromagnetic forces and fields, well known and exploited everywhere today. Also to this Higgs force there must correspond a field, the Higgs field. And here comes the strangest fact of them all: according to the really well tested theory (by LEP) there must be a Higgs field all around us, and in fact all throughout the Universe. This field that we cannot feel directly, contains a gigantic amount of energy. According to Einstein mass and energy are essentially the same. From Newton we know that gravitation causes attraction, and the force is stronger if there is more mass. This Higgs field then has disastrous consequences with respect to gravitation. One would think that due to gravitation all that energy contained in the cosmic Higgs field would attract itself and thus the Universe would tend to collaps to something very small (the size of a football). That is clearly not the case. Something is very wrong. In the theory there is a close relationship between the mass of a particle and the strength of the Higgs force sensed by that particle. It has that in common with gravitation, whose strength is proportional to the mass of a particle. Unfortunately, on the particle level, we do not understand gravitation. So here is something else in which the particles of the families differ from each other: the Higgs forces are different. We may therefore have the faint hope that if we discover more about the Higgs force then we might also discover why there are three families! But it is really a faint hope. It happens that the theory and the data found by LEP allow us to guess properties of the Higgs particle. On the basis of that, we have a reasonable hope that it may be seen with the LHC, the machine that will take the place of LEP. This is by no means sure, but there is a reasonable hope. It may in fact just be within reach of LEP, that by means of an an unbelievable tour de force of the CERN engineering staff has been upgraded substantially. However, seeing it is one thing, studying it is another matter. Thus, to us, this extremely strange Higgs force may be the door to understanding other mysteries of particle physics. No one can even guess what there is. It may be utterly strange. It may have enormous consequences for our understanding of this world, including the structure of the whole Universe. We must know. The LHC may well be the key to this knowledge.