Where is the antimatter? What is the matter? 13 March 1998 What is the matter? Where is the antimatter? Professor Michael G Green Royal Holloway University of London M G Green

The structure of matter... What is the matter? . . . . Where is the antimatter?

Where the hell …? What is the matter? . . . . Where is the antimatter? 13 March 1998 Where the hell …? Since time immemorial people have pondered on the world around them, as exemplified in this splendid cartoon by Sidney Harris. What is it made of? How does it work? Little by little we have developed a scientific view of matter and I will first briefly describe the current view of the smallest particles of matter and how we achieved that view. However if you are looking for complete answers you will be disappointed. Unfortunately at the end of the lecture we will still be pondering the question where the hell did it all come from, although I hope to have given you some insight into a physicist’s view of the Universe. What is the matter? . . . . Where is the antimatter? M G Green

Layman’s terms? What is the matter? . . . . Where is the antimatter? 13 March 1998 Layman’s terms? I am mindful of the warning supposedly given to Stephen Hawking by his publisher when he was writing ‘A brief history of time’. Namely that for each equation in the book he would halve the number of buyers. I haven’t quite reached that ideal. However I will tell you now that you will see just two equations this evening - the first will be completely incomprehensible to many in the audience but that won’t matter since it is where it appears that is important. The second one is the most famous equation of physics and will be known to almost all of you. What is the matter? . . . . Where is the antimatter? M G Green

What is matter? What is the matter? . . . . Where is the antimatter? 13 March 1998 What is matter? As we look around us we find that matter comes in many forms natural man-made small scale large scale simple complex inanimate animate So many forms in fact that it is natural to look for some simplifying way of classifying matter. What is the matter? . . . . Where is the antimatter? M G Green

Where is the antimatter? What is the matter? . . . . Where is the antimatter?

The concept of elements What is the matter? 13 March 1998 The concept of elements In Aristotle’s philosophy there were four elements The first attempt seems to have come from ancient Greece where the four elements of earth, water, air and fire were recognized. This scheme is quite attractive at a trivial level and if you get bored during the rest of the talk you could amuse yourselves constructing some of the objects around us from these four elements. However during the last few hundred years a more useful idea of elements developed. Substances such as gold, sulphur, lead, nitrogen were recognized as elemental in the sense that they were contained in different substances in different mixtures but could not themselves be broken down further. By the beginning of the last century Dalton had identified twenty elements and listed them together with their relative weights. Dalton (1808) listed, with weights, many elements we recognize today What is the matter? . . . . Where is the antimatter? M G Green

The periodic table Mendeleev (1869) introduced the periodic table What is the matter? 13 March 1998 The periodic table However the definitive word was given by Mendeleev in 1869 when he arranged the known elements in a beautiful pattern. Moreover his pattern was predictive in that some elements were missing with properties that could be predicted. All were later discovered. Thus by the end of the last century the concept of elements was well developed. The smallest piece of an element was known as an atom with atoms imagined as small spheres. However 100 years ago the electron was discovered and it became clear that it not only played an important role in electricity but was also contained inside atoms, i.e. atoms have sub-structure. Mendeleev (1869) introduced the periodic table What is the matter? . . . . Where is the antimatter? M G Green

The plum pudding model J J Thomson believed the electrons What is the matter? 13 March 1998 The plum pudding model J J Thomson believed the electrons were embedded in a positively charged matrix - plum pudding One of the first models to incorporate this feature, due to J J Thomson, was known as the ‘plum pudding model’ since he imagined that the electrons, with their negative charge, were scattered throughout some blob of positive matter. What is the matter? . . . . Where is the antimatter? M G Green

The structure of atoms 10-10 m Rutherford (1912) showed that atoms What is the matter? 13 March 1998 The structure of atoms Rutherford (1912) showed that atoms contain a central nucleus Electrons orbit nucleus with well-defined energy and ill-defined positions However it was in 1912 that the New Zealander Ernest Rutherford gave us our modern view of the atom when he showed that atoms have a positive nucleus surrounded at a relatively large distance by the electrons, a picture that everyone today recognizes as the symbol for the atom. The photograph shows Rutherford in his lab. The sign says ‘Talk softly please’ supposedly put there because the detectors were sensitive to noise. However the more likely explanation is that it was aimed at Rutherford by his colleagues, since he was renowned for his booming voice. Now it became clear what differentiates the elements - the number of electrons and the charge on the nucleus - for example hydrogen has one electron, helium has two, carbon six, lead eighty-two etc. However the story doesn’t stop there. 10-10 m What is the matter? . . . . Where is the antimatter? M G Green

The structure of nuclei What is the matter? 13 March 1998 The structure of nuclei Nucleus contains protons with charge +e and uncharged neutrons The nucleus itself has structure and contains positively charged protons and neutral neutrons, the latter discovered by Chadwick in 1932. Can we go further down the levels of sub-structure? The answer is yes. 10-14 m What is the matter? . . . . Where is the antimatter? M G Green

The structure of nucleons What is the matter? 13 March 1998 The structure of nucleons Neutrons and protons contain quarks In the 1960s it became apparent that protons and neutrons have structure and contain objects called quarks, tiny in comparison with the neutrons and protons, but moving at very high speed and bound together by a force known as the strong nuclear force. The quarks will be the major players in my story this evening, but before we turn to look at them in detail there are two other issues I would like to consider. The first is the obvious one. Are there more layers? 10-15 m What is the matter? . . . . Where is the antimatter? M G Green

The structure of quarks? What is the matter? 13 March 1998 The structure of quarks? ? There is no evidence for further structure Currently the answer is that we don’t know of any deeper levels. Attempts have been made to find further sub-structure, some of which have been made within the particle physics group here at Royal Holloway, led recently by my colleague Terry Medcalf, but if such objects do exist they are smaller than 10-18 m. <10-18 m What is the matter? . . . . Where is the antimatter? M G Green

Evidence for substructure What is the matter? 13 March 1998 Evidence for substructure Atom absorbs energy Electron energy increases Secondly let me very briefly explain how we know about sub-structure. The first type of evidence is the existence of what are called ‘excited states’ of systems. If we shine light on atoms we can excite them - meaning that one of the electrons goes into a higher energy orbit. Later it will de-excite. We find the same effects in nuclei and thus know that they have substructure, and also in protons and neutrons. Only certain energy levels (orbits) allowed Later ‘de-excites’ What is the matter? . . . . Where is the antimatter? M G Green

Evidence for substructure What is the matter? 13 March 1998 Evidence for substructure Measure size of struck objects (Rutherford 1912) The second piece of evidence arises if we fire projectiles at material. Of course the projectiles must be tiny themselves. Rutherford and his co-workers used the nuclei of helium atoms, known as alpha particles. Because the nuclei are so tiny, almost all projectiles go nowhere near a nucleus and are therefore undeflected. A few, heading straight for the nucleus, are turned around by the electrostatic force between the alpha particle and the nucleus. A few others, passing close to the nucleus are deflected. Rutherford’s big step forward was to realize that detailed measurements on a large number of such collisions allowed him to determine the size of the nucleus. In a similar way around 1970, by firing electrons accelerated to close to the speed of light at protons and neutrons, it was clearly demonstrated that they have substructure. Moreover properties of that substructure could be determined. 1970 - substructure of protons and neutrons discovered using electrons as projectiles What is the matter? . . . . Where is the antimatter? M G Green

The constituents of matter What is the matter? 13 March 1998 The constituents of matter quarks electron 2 3 e + 1 - -e charge u d Thus we now have a very simple picture of the matter around us. It comprises a large number of different types of atoms, but they contain just protons, neutrons and electrons. Moreover the protons and neutrons contain just two types of quark. Note particularly the charges on each quark. It’s particularly interesting to point out that in this picture the difference between a proton and a neutron - one contains two up and one down quark, the other contains two down and one up quark. Moreover the charge on a proton appears to be exactly opposite that of the electron. If this were not so then theUniverse could not exist, but we still have no idea why this is the case. However the story of matter is not quite so simple as this. In reaching this picture in the 1960s several complications were discovered along the way. Protons contain uud - charge = +e Neutrons contain udd - charge = 0 What is the matter? . . . . Where is the antimatter? M G Green

Prediction of antimatter What is the matter? 13 March 1998 Prediction of antimatter Paul Dirac predicted existence of the positron in 1928 Dirac’s equation implies: positron mass = electron mass positron charge = +e The first of these is antimatter, predicted by the British theoretical physicist Dirac in 1928 when he was developing an equation to describe the behaviour of the electron. Dirac was a very shy man; you see it in his demeanour as he gives a lecture. However the respect in which he is held is shown by his plaque in Westminster Abbey, placed there in 1995, which is reputedly the only equation in the Abbey. For Dirac the equation was obvious; however he could only find a solution to it describing the behaviour of the electron if there was also another solution which seemed to describe something with negative energy. This solution he eventually ascribed to the positron, the antiparticle of the electron. The only equation in Westminster Abbey? What is the matter? . . . . Where is the antimatter? M G Green

Discovery of antimatter What is the matter? 13 March 1998 Discovery of antimatter Four years later the positron was discovered by the American Carl Anderson in one of his pictures of the tracks of particles in a detector known as a cloud chamber. This device was in a magnetic field produced by the coils of cable you see in the photograph. The magnetic field has the effect of bending charged particles. From the direction of bend it is deduced that the particle has positive charge. From the amount of energy it loses as it passes through the plate in the middle of the chamber it is possible to deduce its mass. Note the size of his detector. Later we will see others, somewhat larger, but with similar features. We now know that all particles have antiparticles and I will now take a minute or two to discuss antimatter since it is not just the stuff of Star Trek science fiction, it is also science fact. Anderson (1932) discovered the positron predicted by Dirac What is the matter? . . . . Where is the antimatter? M G Green

E = mc2 What is antimatter? e + - What is the matter? 13 March 1998 What is antimatter? e + - Electrons and positrons annihilate to produce g-rays (energy) E = mc2 The simplest way to describe antimatter is in terms of its behaviour - when particle and antiparticle meet they annihilate and their mass is turned into energy in the form of g-rays, also called photons. This process is governed by the second and last equation you will see this evening, and surely known to the whole audience - Einstein’s famous equation E=mc2. Not only is this true for electrons and positrons, it also occurs for quarks and anti-quarks and more complicated objects like protons and anti-protons. You would also be in trouble if you met your anti-you. However this possibility is still someway off; it was not until 1996 that anti-hydrogen, the simplest anti-atom was produced. What is the matter? . . . . Where is the antimatter? M G Green

Production of e+e- pairs What is the matter? 13 March 1998 Production of e+e- pairs Inverse process also occurs, with g-rays becoming an electron-positron pair e + - The inverse to the annihilation process also occurs and g-rays with sufficient energy can convert to matter and antimatter. We can sketch it using a simple line drawing again. Evidence that it occurs is found on another of Anderson’s beautiful cloud chamber pictures where we see three g -rays have converted to electron-positron pairs just outside the cloud chamber. What is the matter? . . . . Where is the antimatter? M G Green

How to produce antimatter What is the matter? 13 March 1998 How to produce antimatter e - + e e g thin metal region of magnetic field E > few MeV since m c = 0.5 MeV 2 The inverse to the annihilation process also occurs and g-rays with sufficient energy can convert to matter and antimatter. We can sketch it using a simple line drawing again. Evidence that it occurs is found on another of Anderson’s beautiful cloud chamber pictures where we see three g -rays have converted to electron-positron pairs just outside the cloud chamber. What is the matter? . . . . Where is the antimatter? M G Green

u d n The neutrino quarks leptons What is the matter? 13 March 1998 The neutrino ‘Invented’ by Pauli (1928), named by Fermi (1933) Discovered by Reines & Cowan (1956) quarks leptons 2 3 e + 1 - -e u d n The next object I will tell you about is the neutrino. In 1928 Wolfgang Pauli realised that the phenomenon of radioactivity could only be explained if a new, neutral particle existed and was involved in radioactive decays. The name neutrino (little neutral one) was given by the Italian physicist Enrico Fermi after all attempts to find it in such decays had proved negative. The word seems to capture beautifully the elusiveness of the object. Eventually a few were discovered in 1956, close to a nuclear reactor, where they are emitted in huge numbers. In fact it is found in many nuclear processes, including those which drive the sun. The Universe contains huge numbers of them, indeed similar numbers to the number of electrons in the Universe. However their elusiveness is such that although a million million are passing through each one of us every second only one of that number will interact as they pass from one side of the earth to the other. The neutrino appears to be a close relative of the electron, its only significant difference being its charge. They are therefore grouped together as two objects known as leptons. What is the matter? . . . . Where is the antimatter? M G Green

The muon Discovered in cosmic rays by Neddermeyer and Anderson (1936) What is the matter? 13 March 1998 The muon Discovered in cosmic rays by Neddermeyer and Anderson (1936) Appears identical to electron but 200 times as heavy Decays within 2.2 msec By 1936 Anderson had made yet another discovery in his cloud chamber - a completely new particle, seemingly identical to the electron but 200 times as heavy. This particle is produced in the cosmic radiation that continually bombards the earth but lives for only about a millionth of a second, decaying by a radioactive decay process. It seems to have no use at all in the Universe - an idea beautifully expressed by Isidore Rabi with the question ‘Who ordered that?’ This is still a challenging question as I hope will become apparent later. ‘Who ordered that?’ - I I Rabi What is the matter? . . . . Where is the antimatter? M G Green

What is the matter? 13 March 1998 Strange particles In 1947 Rochester and Butler discovered yet more new objects, now known to contain a third quark By the early 1960s beautiful patterns of particles containing three quarks or a quark and an antiquark were seen which were predictive (recall Mendeleev) Now at last I begin to make personal touch with the story, which next developed in 1947 - not directly I emphasize since I could hardly walk at the time. In that year Rochester and Butler in Manchester discovered some particles in their cloud chamber with unexpected behaviour which I will not attempt to describe. I’ll simply say that this behaviour led to them being called ‘strange particles’. We now know that the strange behaviour was the result of them containing a new quark, now known as the strange quark, rather like the down quark but very much heavier and decaying in about a millionth of a second. My own contact with this story is small but two-fold; the first being that I joined Butler’s group at Imperial College for my PhD in 1966. The second is that, perhaps surprisingly, there was still enough unknown about these strange particles, twenty years after their discovery, for me to be able to write a PhD about them. At around the same time it was noticed that many objects had been discovered during the proceeding 20 or so years fell into beautiful patterns with threefold symmetry. Moreover this could be explained assuming all such objects were made up of any three of the three quarks, or any quark and an antiquark. What is the matter? . . . . Where is the antimatter? M G Green

The fundamental particles (1963) What is the matter? 13 March 1998 The fundamental particles (1963) u d e n s m quarks leptons Thus in 1963 matter appeared to consist of three useful objects, the u and d quarks and the electron, three superfluous ones and the apparently equally useless antiparticles of them all. The virtue we see in this picture is a nice symmetric pattern between the quarks and the leptons (although to the best of my knowledge nobody drew this picture then). What is the matter? . . . . Where is the antimatter? M G Green

e m nm t nt ne u d t c s b The zoo grows larger six leptons 1956 1895 What is the matter? 13 March 1998 The zoo grows larger e m nm t nt ne six leptons 1956 1895 1963 1936 1973 u d t c s b six quarks 1947 1976 1995 1978 That was still not the end of the story and to cut it short at this point I will simply say that yet more fundamental objects have been discovered, and that now we have a beautiful pattern of three pairs of quarks and three pairs of leptons. They are shown here with their year of discovery. The t lepton was discovered in 1973 by a team led by Marty Perl, who had been a student of Isidor Rabi, who asked why the muon should exist. All three known as neutrinos appears to be very light (possibly with zero mass) and very stable, while four of the quarks and the other two leptons decay within less than a millionth of a second after they are produced. That leads to two important questions. Are there any more? - to which I will show in a bit that the answer is probably no. Why are the three pairs then? Before addressing these questions I will change tack a little since so far I have told you nothing about the experiments which have produced the description I have given. Let me now turn to that subject, illustrating it with a little of the work I and my colleagues here at Royal Holloway have been involved in during the last decade or so. What is the matter? . . . . Where is the antimatter? M G Green

We like elegant solutions What is the matter? 13 March 1998 We like elegant solutions Oh! Before doing that I should remark, again with the help of Sidney Harris, that the elegance of the model shown on the previous slide really is important. What is the matter? . . . . Where is the antimatter? M G Green

CERN What is the matter? . . . . Where is the antimatter? 13 March 1998 CERN Through much of my career I have carried out experiments at the European Laboratory for Particle Physics, known as CERN. Straddling the French-Swiss border near Geneva it occupies two large sites and has a number of smaller sites around the circumference of its largest particle accelerator called LEP, which I will talk more about shortly. The main site is close to Geneva Airport; indeed even nearer than we are here to Heathrow, making visits there particularly straightforward. It has an annual budget of about #400M, paid by 19 member European countries whose flags you see flying here and normally put out when there are VIP visitors to CERN. The UK annual contribution is about #60M. What is the matter? . . . . Where is the antimatter? M G Green

A particle accelerator What is the matter? 13 March 1998 A particle accelerator Let me say just a little about particle accelerators. You are all familiar with one - the television set. In a television electrons are boiled off a piece of metal by heating it and then accelerating them through a potential of about 20,000 volts (which is why you are advised not to poke you fingers in the back of a television). They are focused and directed to the screen by electric and magnetic fields where they produce the picture by making materials of the screen fluoresce. Energy of electrons is about 20kV What is the matter? . . . . Where is the antimatter? M G Green

The LEP accelerator Energy of electrons and positrons is about 100GeV What is the matter? 13 March 1998 The LEP accelerator The largest accelerator at CERN is called is called ‘LEP’ - Large Electron Positron collider, shown very schematically here. In recent years, together with other members of our group, I have been using LEP for experiments. It contains very similar elements to a television set, including some 3,000 magnets to steer the particles in a circular path, and acceleration devices. The vacuum pipe containing the particles is a few cm in diameter and 27km in circumference. A marvellous feature of this machine is that electrons go in one direction and positrons travel in the opposite direction because of their opposite charge and identical mass. Energy of electrons and positrons is about 100GeV What is the matter? . . . . Where is the antimatter? M G Green

CERN Europe’s research laboratory for particle physics in Geneva. What is the matter? . . . . Where is the antimatter?

LEP What is the matter? . . . . Where is the antimatter? 13 March 1998 LEP In a tunnel roughly 50m underground the particles are accelerated to an energy of about 100 billion electron volts compared to the 20,000 electron volts in our televisions. Travelling at close to the speed of light take about 100ms to make one circuit. What is the matter? . . . . Where is the antimatter? M G Green

Inside the LEP tunnel LEP is 27km in circumference What is the matter? 13 March 1998 Inside the LEP tunnel LEP is 27km in circumference Four bunches of electrons and positrons circulate inside the vacuum pipe 100ms for a complete circuit About one electron-positron collision per second The electrons and positrons are in eight small bunches, each roughly the size of a pin. Generally the bunches pass through each other but about once a second an electron in one bunch collides with a positron in another bunch and annihilate. Here you see the magnets that bend the beams around the 27km circumference circle. What is the matter? . . . . Where is the antimatter? M G Green

Electron-positron collisions Annihilation produces energy - mini Big Bang Electron (matter) Particles and antiparticles are produced Positron (antimatter) g e- e+ E = mc2 What is the matter? . . . . Where is the antimatter?

The ALEPH detector End view What is the matter? 13 March 1998 The ALEPH detector End view The detectors required for this task are huge. During the 1980s about 200 physicists and engineers from 30 universities and research laboratories, including Royal Holloway, designed and built a detector called ALEPH. Of cylindrical shape, closed by two endcaps, it is some 10m long and 10m high. It weights about 3,000 tons. It is full of detectors, most of them somewhat similar to Geiger counters, which delineate the paths of the emerging particles. A large superconducting solenoid produces a magnetic field which bends the charged particles, allowing their momenta to be measured. Perhaps you recall Anderson’s detector earlier with its magnetic coils and a detector to show the paths of particles being bent in the magnetic field. What is the matter? . . . . Where is the antimatter? M G Green

What is the matter? 13 March 1998 Collisions in ALEPH The tracks of the particles emerging from each collision are recorded on computer and can be replayed later, rather as snowy footprints are recorded in a photograph. I want to show you just two results from the very many important advances made in particle physics by the LEP collider. What is the matter? . . . . Where is the antimatter? M G Green

ALEPH - a LEP particle detector What is the matter? . . . . Where is the antimatter?

Three neutrinos ... Number of different neutrinos = 2.994 ± 0.011 What is the matter? 13 March 1998 Three neutrinos ... Number of different neutrinos = 2.994 ± 0.011 nm nt ne u d t c s b e t m The first is derived from the rate at which interactions of electrons and positrons occur. This varies considerably around 90 GeV because of the presence of a resonance known as the Z boson as the physicists in the audience will know. The energy dependence is predicted but depends on the number of different types of neutrino for reasons that I will not discuss. The result of comparing theory and experiment is that the number of different types of neutrino is 2.993 ± 0.011, clearly consistent with 3, although the third has not yet been observed. This is an exceedingly important result. We have observed three charged leptons and for good theoretical reasons believe that the leptons come in pairs. This suggests that our picture of the leptons is complete. For somewhat weaker theoretical reasons, but strong aesthetic reasons, it also suggests that three pairs of quarks we have observed is also the complete story. s measures rate at which e+e- collisions occur What is the matter? . . . . Where is the antimatter? M G Green

… and no further substructure What is the matter? 13 March 1998 … and no further substructure mass Excited states produced if substructure exists e + - g * The other result I will show is one from Terry Medcalf and Julian von Wimmersperg in our group here, who have looked for and found no evidence for substructure of quarks and leptons at LEP energy. They have looked for excited states of e.g. electrons, which would then be expected to decay by emitting g -rays, just like excited atoms as we saw earlier. Evidence for this would be in the form of a peak in a plot of the electron-photon invariant mass in interactions producing electrons and g-rays. No such peak is seen and the data, shown as points with error bars, fits beautifully what is expected (the red distribution) assuming no such object exists with a mass less than about 80 GeV. This result is equivalent to the statement that quarks and leptons are less than 10-18 m in size. What is the matter? . . . . Where is the antimatter? M G Green

The story so far e nm t nt ne m u d t c s b What is the matter? 13 March 1998 The story so far e nm t nt ne m u d t c s b The everyday world contains two quarks and the electron. Additional quarks and leptons have been observed with six of each in total; most decay very rapidly. All particles have an antiparticle. So before moving on to the final step let me summarize the important aspects of our story so far. When energy turns to mass equal numbers of particles and antiparticles are produced. What is the matter? . . . . Where is the antimatter? M G Green

Evolution of the Universe -270 o ? heavy elements formed in stars stars and galaxies exist, atoms form neutrons quark "soup" 15 billion years 1 million years 1 second 10 - 15 deg 9 6000 -255 3 minutes helium nuclei microwave background radiation fills universe 300,000 years 4000 life on earth, molecules dominates matter and protons 1 billion years s What is the matter? 13 March 1998 Evolution of the Universe The Universe began with a “Big Bang” about 15 billion years ago Big Bang I now turn to the really big question - the connection between the discussion so far and our existence in the Universe. The first important point to make is that many aspect of the evolution of the Universe from the Big Bang to our existence on Earth some 15 billion years later are quite well understood in very general terms. We know quite a lot about the evolution of life, we know how heavy elements essential to life formed in massive stars and supernovae, we know how galaxies and stars form, how atoms formed as the Universe cooled, and even how nuclei and nucleons formed. All of this is relatively straightforward biology, chemistry and atomic, nuclear and particle physics and takes us back in our understanding to somewhere about one billionth of a second after the Big Bang. This is the point at which our experiments literally run out of energy at present and real speculation begins. What is the matter? . . . . Where is the antimatter? M G Green

The Big Bang What happened at times less than 10-9s is uncertain What is the matter? 13 March 1998 The Big Bang What happened at times less than 10-9s is uncertain What happened in that era is therefore debated amongst physicists with great fervour. Nevertheless there are some general principles that we understand. What is the matter? . . . . Where is the antimatter? M G Green

Evolution with matter-antimatter symmetry What is the matter? 13 March 1998 Evolution with matter-antimatter symmetry For example, let’s try to understand what would happen to the Universe if at that time, a billionth of a second after the beginning, equal numbers of particles and antiparticles exist. This is a reasonable assumption since it is what we observe in our experiments at accelerators where the energy density is the same. The density of particles at this time is enormous. They collide and annihilate producing photons, which then turn back into particle and antiparticle pairs. The number of photons and particle-antiparticle pairs is roughly equal. Eventually, after about one second, the energy of the photons is too small for any more particle-antiparticle pairs to be produced and only the annihilation process continues. Since we started with equal numbers of particles and antiparticles we cannot have an excess of either and, indeed only photons will be left in the Universe. This is manifestly what did not happen - our Universe is different. However it’s not so different. The Universe does contain huge numbers of photons, by now at very low energy and known as the ‘microwave background’. Measurements shown that there are about 109 photons in the Universe for every proton. How this number arose is an area of great interest to particle physicists and astrophysicists. Eventually such a universe contains only photons (almost true for our Universe - cosmic microwave background) What is the matter? . . . . Where is the antimatter? M G Green

Parity violation Macroscopic systems obey the same physical laws in a mirror system, e.g. planetary motion “parity conservation”. θ q cos 1 ) ( c v I - = b-decay (weak interaction) does not conserve parity. Discovered in 1956 in polarized 60Co decay. What is the matter? . . . . Where is the antimatter?

P violation - CP conservation Parity violation leads to an asymmetry for neutrinos -only left-handed ones exist. Changing particle to antiparticle (C) then applying the parity operation (P) produces the right-handed antineutrino, which exists “CP conservation” What is the matter? . . . . Where is the antimatter?

CP violation in K0 decays What is the matter? 13 March 1998 CP violation in K0 decays s - d K0 u,c,t - - - W d s - K0 W u,c,t - - - Phases of the amplitudes for the two processes are not equal ‘CP violation’ (1964) Occurs only because there are three families of quarks However such an asymmetry does turn up in some objects made up of a quark and an antiquark with the property that they can turn into their own antiparticle. An example is the K0 meson, consisting of a d and an sbar quark, which can change into its own antiparticle in a two-step process involving W boson and u, c and t quarks. These three quarks are an important feature. The inverse can also occur. In 1964 an effect was discovered, known technically as CP violation, which arises because the two processes are slightly different. Again being technical for an instant, the phases of the two amplitudes are different. It can only happen if there is the possibility of there being three quarks in the intermediate steps of the processes. When discovered it seemed esoteric and an a rather irrelevant if somewhat interesting effect and was not understood. It requires there to be three families of quarks (their existence was not known at the time). However, because the effect is very small, and in spite of 33 years of experimental and theoretical effort since the discovery, there are still inconsistencies in experimental results and large uncertainties in details of the theory. Moreover this is the only process in which CP violation has been observed. The importance of the effect is that it allows particles to turn into anti-particles at a different rate to the inverse, the second of Sakharov’s conditions. What is the matter? . . . . Where is the antimatter? M G Green

CP violation Leads to beautiful interference effects and non-exponential decay distributions 5 10 15 20 25 30 10 6 5 4 3 2 1 COUNTS / 0.5 x 10 -10 s t (10 s) 5 10 15 k p + - DISTRIBUTION What is the matter? . . . . Where is the antimatter?

The Sakharov conditions What is the matter? 13 March 1998 The Sakharov conditions Antimatter can turn into matter if: (a) proton decay occurs (b) there is a matter-antimatter asymmetry (CP violation) (c) there is thermal non- equilibrium The first major contribution to our understanding came in 1964 when the Russian theoretical physicist, Andrei Sakharov, known to many people for quite different reasons, realized that three conditions would allow antimatter to turn into matter and thus in principle solve the problem. I will now make some comments on the first two of these while leaving the third unexplained. Sakharov (1964) What is the matter? . . . . Where is the antimatter? M G Green

What is the matter? 13 March 1998 Proton decay Life on earth implies protons exist, on average, for >1023 seconds Searches for proton decay have set limits >1032 seconds d u X e+ - proton p0 Proton decay converts quarks into leptons - important only in early stages of the Big Bang but a small effect will remain Proton decay is, in principle, a disaster for us since, as we have seen, they constitute the very material from which we are made. However many theoretical models include that possibility and heroic efforts have been made to discover their decay. Some recent ones, in which some of my UK colleagues here in the audience tonight have been involved, are capable of discovering the decay of a single proton in 1000 tons of matter. However the process involved, even though undiscovered now, would have happened at a very much larger rate in the conditions of much higher energy at the beginning of the Universe. Let’s then consider this as a possibility and continue the argument. The next problem to face is that if protons can decay in this way, then so too can antiprotons in equal numbers and we have not achieved a matter-antimatter asymmetry. However antiprotons will decay similarly What is the matter? . . . . Where is the antimatter? M G Green

A universe with asymmetry What is the matter? 13 March 1998 A universe with asymmetry Perhaps one in every 109 antiquarks turned into a quark very early in the life of the Universe After the matter-antimatter annihilation a small amount of matter will be left (about one proton for 109 photons) Ignoring the third condition, except to say that non-thermal equilibrium almost certainly occurred early in the Universe, let’s see the consequences of our discussion. Let’s imagine one antiparticle in 109 turning into a particle in the first billionth of a second after the Big Bang. The same annihilation process we saw earlier continues, but when it has finished we now have one particle left over for every 109 photons, just as we find in the Universe today. We thus conclude that, although matter consists of two different quarks and the electron, we could not be here today if the other quarks did not exist to play their crucial role in the first billionth of a second of the life of the Universe. Thus we may not have an answer to Rabi’s question about the muon - Who ordered that? - but we may begin to see why these other objects have to exist. What is the matter? . . . . Where is the antimatter? M G Green

Current problems 1. We have never observed proton decay What is the matter? 13 March 1998 Current problems 1. We have never observed proton decay 2. Precise measurements of CP violation in K0 decay are difficult and there are uncertain theoretical corrections 3. Cosmological models do not easily explain the ratio of 109 photons for each proton in the Universe Well it’s a beautiful idea with several problems. We haven’t observed proton decay CP violation is not well-understood The ratio of 1 in 109 is not understood Clearly more information is needed and during the next few years a collaboration of particle physicists, including some of us from Royal Holloway, will be trying to throw more light on the problem of CP violation. What is the matter? . . . . Where is the antimatter? M G Green

CP violation in B0 decays What is the matter? 13 March 1998 CP violation in B0 decays Similar effect expected in B0 d - b B0 u,c,t - - - W d b - B0 W u,c,t - - - CP violation is predicted to occur in decays of the B0 meson, a similar object to the K0 meson, but with a b quark rather than an s quark. Until very recently experimental limitations made it impossible to observe the effect for the B0 meson. Now a large collaboration, including some of the particle physics group at Royal Holloway, is building an experiment in Stanford, California, which will begin taking data in about one year from now. For the following five or more years we will be making detailed measurements of CP violation for the B0 meson. First measurements starting 1999, Stanford, California What is the matter? . . . . Where is the antimatter? M G Green

Matter-antimatter asymmetry In 1964 it was discovered that the radioactive decay of antimatter differs by a tiny amount from the decay of matter. Since then progress in understanding has been very slow: experiments are very difficult; astronomy is an observational science, not experimental (cannot repeat the Big Bang). BUT we have learned that the matter-antimatter asymmetry can only occur if there are three pairs of quarks. What is the matter? . . . . Where is the antimatter?

The Stanford Linear Accelerator What is the matter? 13 March 1998 The Stanford Linear Accelerator The experiment is known as BaBar, since it is studying Bs and Bbars. It makes use of the two-mile long Stanford Linear Accelerator to produce, accelerate and store electrons and positrons before colliding them, just like LEP. The energy is only about 10GeV, significantly less than that of LEP, but tuned to be at the maximum for the production of the B mesons. What is the matter? . . . . Where is the antimatter? M G Green

What is the matter? 13 March 1998 The BaBar detector What is the matter? . . . . Where is the antimatter? M G Green

BaBar experiment at SLAC What is the matter? . . . . Where is the antimatter?

What is the matter? 13 March 1998 Summary The everyday world is made from up and down quarks and the electron. Experiments tell us that six quarks and six leptons exist. The “extra” ones seem to be needed to explain why there is an asymmetry between matter and antimatter and hence why we exist. However it is likely to be a long time before we have a good understanding of what happened in the first fraction of a second of the Universe’s existence I will leave the last word this evening to Sidney Harris. We understand a lot about the Universe but there are still many unanswered questions. Perhaps the ultimate one is Why? I said at the beginning of the lecture that I would be giving you a physicist’s view of the Universe. Whether or not the ultimate question is susceptible to a scientific answer is still unknown. What is the matter? . . . . Where is the antimatter? M G Green

THE END What is the matter? Where is the antimatter?