~~N ."$ II itOLit At .A Qo THE PHYSICS OF SUBATOMIC PARTICLES . S t e p h e n W o l f r a mm I 1 t • ~WNAI f' CONTENT S. Chapter One The Early History of Particle Physic s Chapter Two Some Basic Principle s Chapter Three The m clusion Principle, Antimatter, and Yukawas Hypothesi s Chapter Four he Proliferation of Particles Chapter Five eactions Chapter Six Symmetry and Structure Chapter Seven Interaction s Chapter Eight The Detection of Particle s Chapter Nine The Acceleration of Particle s Bibliography Appendix A Properties of Particles and Fields Appendix B Abbreviation s Appendix C Unit s Appendix 0 The Greek Alphabe t Appendix E Particle Accelerators Appendix F Physical constants CHAPTER ONE : THE EARLY HISTORY OF PARTICLE PHYSICS . In the fifth century B .C . a Greek philosopher named Democritus predicted th e existence of atoms (indivisible things), of which, in different patterns and motions , he believed everything to be made . At about this time, with the teaching of Democritu s and Leucippus, an idea of mass conservation and the discontinuity of matter began t o take shape . During its existence, the Pythagorean School of philosophers put forward fo r the first time the theory that light was composed of discrete corpuscles emitted from luminous bodies . In the first century B .C ., the Roman poet Lucretius colourfull y expressed these and other ideas on the nature of the universe, in his great didacti c poem De Rerum Natur0. ' (On the Nature of Things) . But however much these ancien t theories of discontinuity may seem to be ahead of their time, it should be remembere d that at the same time, almost equal support was gained for a continuous theory of th e universe, which was upheld. with much zeal by Heraclitus and the Eleatic philosophers . But neither of the two theories had any experimental proof to back them up, and s o were both of purely philosophical interest . Between the time of these Classical philosophers and the discovery of the firs t experimental basis for either theory, the controversy continued, and in the intervenin g twenty—three centuries many great scientists and philosophers, notably Newton and Descartes, considered the problem . On the whole, the continuous theory of matter an d light, as a wave, was favoured and it was not until the work of Brown in 1827 that the balance began to tip . He noticed some seemingly unprovoked movements of ligh t particles in aqueous solutions, and proposed a discontinuous molecular theory to accoun t for this . Then, after the work of Delsaux, Wiener, and Carbonelle in the period 186 3 to 1895, the Molecular Theory of solids, l iq uids, and gases, became firmly established . This had, among other things, the regular structures of crystals and the similaritie s between these and the proposed structures of molecules of particular substances (notably ' left— and right—handed Tartaric acid, as investigated by Pasteur), to back it up . It was after the discovery of electricity that the history of•Particle Physics a s a true science began . This discovery made Crookes investigate the nature of an electri c charge when passed thraagh a near—vacuum inside a sealed glass tube . In 1878 he made a spark from an induction coil traverse a sealed glass tube in which there was rarifie d inert gas, and observed a number of interesting phenomena . The first of these was th e rotation of the vanes of a radiometer, a tiny mica windmill, when it was place d inside one of these tubes, and this he attributed to molecular pressure . He als o performed the now classic experiment of putting flat aluminium discs of varying shap e between the two electrodes, and then observing the sharp shadows produced on a scree n at the end of the tube (by the positive electrode) . He found that light was evolved when the rays were stopped by the glass walls of the tube, and that this light wa s caused by an actual luminescence of the surface of the glass rather than by th e excitation of gas molecules . If this was happening then the emitted light would bea r some of the spectral characteristics of the gas filling the tube . Crookes also foun d that some substances, for example mica and quartz, did not emit light however close the y were brought to the negative electrode, and that, generally speaking, the mor e fluorescent the substance was, the greater the luminosity produced . Crookes also found that cathode rays discharged electroscopes (see chapter 8), and, which was even mor e important, they were deflected by a magnetic field . Crookes believed that he had found something even smaller than the atoms which had been believed to be the ultimat e stage in the division of matter and to behave like billiard-balls . In 1895 Jean Perrin, amid the controversy between scientists concerning the nature of cathode rays (some scientists, especially German ones, believed them to be due to some hitherto unknown process in the ether, while others believed them to consist of material particles with negative electric charges) repeated Crookes experiment o f deflecting cathode rays by a magnetic field . He proved that they were negativel y charged, and even collected the negative charges, but failed to perform any worthwhil e experiment on single particles . Thus he had practically proved that cathode rays wer e streams of electrified particles in rapid motion, and in 1897, J .J .Thomson, in hi s much celebrated experiment, discovered the constant ratio of the electric charges o f these particles to their masses . Although Hertz had previously tried to deflec t cathode rays by making them pass between two parallel metal plates across which a n electrostatic field had been produced by connection to a battery of electric cells , but had been unsuccessful, Thomson succeeded by using a more complete vacuum . Th e details of the experiment he performed were as follows : cathode rays were produced b y a hot wire at the cathode, and passed through two metal plugs which served as the anodes, and then between two aluminium plates 1 .5 ens . apart, both rectangular with dimension 5 ems . by 2 ems ., and finally hit a fluorescent screen on which they produce d a dot . When there was a high vacuum in the tube, the rays were seen to be repelled b y the negative plate and attracted by the positive one . The angle of deflection wa s shown to be directly proportional to the potential difference between the two plates . Thus the ratio of the electric charge of these particles to their mass was found t o be of the order of ten to the power of seven (ten million), a value much higher tha n any previously observed for other particles . The highest value then observed was tha t for the hydrogen ion, which was ten to the power of four . The charge to mass ratio for an electron is now accepted as being 1 .758796(6) x 10° C kg . But although Thomson s measurement of this fundamental constant was a great step forward, it gave no idea o f the independant values of either the electric charge or the mass of the cathode ra y particle . The cathode ray particle was christened the electron by Thomson . However, in 1924, R.A .Hillikan did manage to measure the charge of the electron o n its own . In his experiment oil droplets which had passed through a commercial atomiser using specially purified air, were allowed to fall in a large chamber, at the bottom of which there was a circular brass plate 22 ems, in diameter, with a pin-hole at it s centre . This plate formed one pole of an air condenser, whose other pole was a bras s plate held 16 mms . beneath the first by three ebonite rods . A three-way switch made it possible to control the charges of the plates, so that they had a potential differenc e of ten thousand volts when the switch was in two of its positions (in one, a given plate was positively charged, in the other it was negatively charged), and a potential difference of zero when it was in its third position . Any oil droplets which passed through the pin-hole were strongly illuminated, so that they could be observed, an d were then allowed to drop until they were very close to the negative plate (the lowe r one), when the switch was closed, and a potential difference was created between th e plates . This forced the oil droplet, which had been electrified by friction in th e atomiser, to rise, but when it was near the upper plate, the switch was opened agai n and the droplet fell under the influence of gravity . This cycle was repeated many times, and the time of fall, and therefore speed, of the droplet during each cycle was carefully measured . Millikan assumed, as had his predecessors, that the velocity of a droplet is proportional purely to the force acting upon it, and has nothing to do with the charg e on the droplet itself . As an electrified droplet passed through the air between th e two plates of the condenser, it sometimes picked up one or two ions, which increase d its electric charge, and thus the velocity with which it was attracted to the plate of opposite charge . It was found that the addition of an ion to the droplet caused a constant decrease in the time taken between the two plates on the upward journey , except in a few cases, when two ions had attached themselves onto the droplet, wher e the time was decreased by precisely twice the usual ampunt .