17 Very Elementary Particle Physics Martinus J.G. Veltman Abstract This address was presented by Martinus J.G. Veltman as the Nishina Memorial Lecture at the High Energy Accelerator Research Organization on April 4, 2003, and at the University of Tokyo on April 11, 2003 Introduction Today we believe that everything, matter, radiation, gravita- tional fields, is made up from elementary particles. The ob- ject of particle physics is to study the properties of these el- ementary particles. Knowing all about them in principle im- plies knowing all about everything. That is a long way off, we do not know if our knowledge is complete, and also the way from elementary particles to the very complicated Uni- verse around us is very very difficult. Yet one might think that at least everything can be explained once we know all about the elementary particles. Of course, when we look to Martinus J.G. Veltman such complicated things as living matter, an animal or a hu- c NMF man being, it is really hard to see how one could understand all that. And of course, there may be specific properties of particles that elude the finest detection instruments and are yet crucial to complex systems such as a human being. In the second half of the twentieth century the field of elementary particle physics came into existence and much progress has been made. Since 1948 about 25 Nobel prizes have been given to some 42 physicists working in this field, and this can be seen as a measure of the amount of ingenuity involved and the results achieved. Relatively little of this is known to the general public. Martinus J.G. Veltman (1931 – ). Nobel Laureate in Physics (1999) University of Michigan (USA) at the time of this address M. J. G. Veltman: Very Elementary Particle Physics, Lect. Notes Phys. 746, 371–392 (2008) DOI 10.1007/978-4-431-77056-5 17 c Nishina Memorial Foundation 2008 372 Martinus J.G. Veltman In this short account a most elementary introduction to the subject is presented. Itmaybeasmallsteptohelpbridgingthe gap between the scientific knowledge achieved and the general understanding of the subject. Photons and Electrons Photons are very familiar to us all: all radiation consists of photons. This includes radio waves, visible light (from red to dark blue), X-rays, gamma rays. The first physics Nobel prize, in 1901, was given to R¨ontgen, for his discovery of X-rays or r¨ontgen rays. The 1921 physics Nobel prize was awarded to Einstein for his discovery that light is made up from particles, photons. Einstein is most famous for his theory of relativity, but it is his discovery of photons that is mentioned by the Swedish Academy. Indeed, many think that this is perhaps his most daring and revolutionary discovery! The difference between all these types of photons is their energy. The photons of radio waves have lower energy than those of visible light (of which red light photons are less energetic than blue light photons), those of X-rays are of still higher energy, and the gamma rays contain photons that are even more energetic than those in X-rays. In particle physics experiments the photon energies are usually very high, and then one deals with individual photons. The energy of those photons starts at 100,000,000,000 times that of the photons emitted by portable phones. Fig. 17.1 Especially at lower energies the number of photons involved is staggering. To give an idea: a 1 watt portable phone, when sending, emits roughly 10,000,000,000,000,000,000,000 photons per second. Together these photons make the wave pattern of radio waves. Also in visible light the number of photons is generally very large. Interestingly, there exist devices, called photo-multipliers that are so sensitive that they can detect single photons of visible light. The photons of visible light are one million times as energetic as those emitted by a portable phone, and while the number of photons emitted is correspondingly lower the amount is still enormous. 17 Very Elementary Particle Physics 373 Electrons are all around us. They move in wires in your house, make light, and function in complicated ways in your computer. They also make the pictures on your TV or computer screen. In the tube displaying the picture electrons are accelerated and deflected, to hit the screen thereby emitting light. Fig. 17.2 In the tube, on the left in the picture, a piece of material called the cathode is heated. As a consequence electrons jump out of the material, and if nothing were done they would fall back. However, applying an electric field of several thousand volt they are pulled away and accelerated. Then they are deflected, usually by means of magnetic fields generated by coils, deflection coils. They make the beam of elec- trons move about the screen. There they paint the picture that you can watch if you have nothing better to do. Thus inside your TV tube there is an accelerator, although of rather low energy as accelerators go. The energy of the electrons is expressed in electron-Volts. An electron has an energy of one electron-Volt (abbreviated to eV) after it has been accelerated by an electric field of 1 Volt. In a television tube the field may be some- thing like 5000 V, thus the electrons in the beam, when they hit the screen, have an energy of 5000 eV. In particle physics the energies reached are much higher, and one uses the unit MeV. One MeV is one million eV. Thus the electrons inside the TV tube have an energy of 0.005 MeV. Try to remember that unit, because it is used everywhere in particle physics. Related units are GeV (1GigaeV = 1000MeV) and TeV (1 Tera eV = 1000GeV). 374 Martinus J.G. Veltman Making New Particles In order to make particles one must first make an energetic beam of electrons or protons. Protons can be found in the nucleus of an atom together with neutrons. The simplest nucleus is that of hydrogen, it contains only one proton. The picture shows a hydrogen atom, one proton with one electron circling around it. The electron is negatively charged, the proton has exactly the same charge but of the opposite sign. The total charge of a hydrogen atom is the sum of these two and Is thus zero, it is electrically neutral. Fig. 17.3 The first step then is to take a box filled with hydrogen. Subsequently a strong spark, an electric discharge, is produced in that box. This amounts to a beam of electrons moving through the hydrogen. When it meets a hydrogen atom on its way the electron is knocked out of the atom, and there results an electron and a proton drifting in that box. Ionization is the name given to the process of stripping electrons from an atom. Fig. 17.4 The next step is to apply an electric field. If the field is positive on the left then the electrons are pulled to the left and the protons, having the opposite charge, are pulled to the right. Making a little window (of some thin material) the protons emerge on the right hand side and one has a proton beam. Likewise one has en electron beam on the left hand side, but this is not an efficient way to make electron beams. One can do it better as shown by the TV tube. 17 Very Elementary Particle Physics 375 To produce new particles one must create a minuscule bubble of concentrated energy. From Einstein’s equation E = mc2 we know that matter is a form of energy, and in order to make a particle of some mass one must have an energy bubble of at least that much energy. The bubble will decay into whatever is possible, sometimes this, sometimes that. But always, the energy equivalent to the sum of the masses of the particles that it decays into is less than the initial energy of the bubble. That is why particle physicists want forever bigger accelerators. Those bigger machines create ever more energetic bubbles, and very heavy particles, not seen before, may appear among the decay products of those bubbles. To create a bubble of high energy the beam of protons coming out of the ioniza- tion chamber is accelerated to high energy, and then the protons are smashed into other particles, for example particles in an atomic nucleus. Thus one simply shoots the beam into a suitable material, a target. Whenever there is a collision of a particle in the beam with a nucleus a bubble will form and that bubble will decay into all kinds of particles. In this way new particles can be found. Those particles, themselves little bubbles of energy, are usually unstable and they decay after a little while and that is why they are not seen in the matter around us. There are only a few stable particles, mainly photons, electrons, protons and neutrinos. Fig. 17.5 The new particles found are given names, and their properties are studied. In the early days these particles were called mesons. For example one finds copiously π-mesons, or briefly pions, particles with a mass about 260 times bigger than the electron mass. Those pions decay mostly into a muon (a μ-meson, a particle whose mass is about 200 times that of the electron) and a neutrino (ν, mass 0). The muon, while relatively long-lived, decays in an electron, a neutrino and an anti-neutrino. Anti-particles have almost the same properties as particles (they have the same mass, but opposite charge), but that will be discussed later.