Where are the missing solar neutrinos? One exciting possibility is that, on their way out through all the material why in the Sun, the neutrinos are changing type.The neutrinos emitted in the initial reactions will be electron- The bubble chamber Gargamelle arriving neutrinos, and so far the experiments are at CERN. Four "port holes" are visible have all been designed to detect along its length, where cameras were electron-neutrinos. So if the electron- positioned to take photographs of the neutrinos changed into muon- neutrinos bubble-tracks inside. (lower right) neutrinos or tau-neutrinos on their Tracks from a spray of particles way out of the Sun, they would not be produced when an invisible neutrino detected.This possibility is exciting hit a nucleus in the lqiuid. because theory shows that neutrinos Inside the proton can change in this way only if they In the 1970s physicists began using important? have some mass – although this mass neutrinos to learn about what goes must be very small as no other kinds on inside protons and neutrons – the of experiment have succeeded in particles that make up the nuclei of measuring it. atoms. In these experiments, billions of neutrinos were produced in high- To test the idea of changing neutrino energy particle accelerators at the types, UK physicists have joined forces European laboratory, CERN, near with others from Canada and the US Geneva, and at the Fermi National to build a new solar neutrino Accelerator Laboratory in Illinois. detector.The Sudbury Neutrino Observatory, 2070m deep in a One of the most famous experiments working nickel mine in Ontario, was with a 4.8m long detector called contains 1000 tonnes of ‘heavy’ water Gargamelle, which was filled with to detect the neutrinos. In heavy 18 tonnes of liquid Freon, a water, the hydrogen nuclei contain a compound of carbon, fluorine and neutron as well as a proton.This bromine.This is a particularly dense allows the neutrinos to react in liquid with the best chance of different ways, so that muon- and tau- stopping a neutrino. Occasionally, neutrinos can be detected, as well as a neutrino on its way through electron-neutrinos. Gargamelle would interact with a proton or a neutron in the liquid, Neutrinos in the Universe and the particles produced would Does it matter whether neutrinos leave visible trails of bubbles.This have any mass? Neutrinos appear to experiment, and others like it, showed be one of the truly fundamental types that protons and neutrons are built A diver inspects the of particle in the Universe, together from more fundamental particles, phototubes of the IMB with electrons, muons, taus and which are known as "quarks". detector – a vast tank various types of quark.These are the containing 8000 basic building blocks of the Universe, Inside the Sun tonnes of water in and it is important that any theory of The Sun is mainly hydrogen, and it a salt mine near the structure of matter should be releases energy when nuclei of Cleveland, Ohio. able to describe them properly.The hydrogen (the lightest element) fuse (below) Computer measurement of small masses for the to make helium (the next lightest) reconstruction of the neutrinos would have a tremendous at its core.These reactions emit a interaction of a impact on these theories. continuous stream of neutrinos as neutrino from protons change into neutrons in the supernova 1987a. Neutrinos with mass would also first stage of the fusion process.The influence theories of the evolution of energy we see as light from the Sun the Universe. Neutrinos are the takes an amazing 1 million years to commonest known particle, and if travel from the centre of the Sun, they have a small mass they could but the weakly interacting neutrinos comprise some, if not all, of the so- take only 3 seconds! So neutrinos called "dark matter" in the Universe. give us a direct view into the very This is matter that astronomers heart of the Sun. know must exist from studying the motion of stars and galaxies, but Physicists have been detecting which nevertheless does not shine, neutrinos from the Sun since the even invisibly through the emission of 1960s.The first famous experiment radio waves or X-rays, for example. If to do so used 380,000 litres of neutrinos turn out to make up a perchloroethylene (dry cleaning fluid) significant fraction of the dark contained in a tank 1500m below matter, they would be true "masters ground in a gold mine in South of the Universe"! Dakota.This detector has been observing neutrinos at the rate of about 1 every 2 days for more than The main tank of the solar neutrino detector in the Background picture: The main 25 years. However, this is only one- Homestake gold mine, spherical vessel of the Sudbury third of the rate expected. During South Dakota – 14.5m long Neutrino Observatory, in Canada, the 1980s, several other experiments and 6.1m across. is constructed from transparent began to look for solar neutrinos, all plastic.The geodesic structure using different techniques.They also surrounding it supports the 6400 detected too few neutrinos. phototubes that will detect light

Particle Physics and Astronomy from neutrino interactions. Research Council What is a neutrino? Neutrinos are probably the nearest thing to nothing Where do neutrinos Wolfgang Pauli predicted that something can be! They are tiny particles with no the existence of the electric charge and very little mass, if any.They neutrino in 1930, to solve a interact with other particles of matter so weakly that The Universe is full of neutrinos! Every (background) puzzle in radioactivity. they can pass right through the Earth as if it were not cubic centimetre of space contains a few The first there! But if neutrinos are almost nothing, how do we The Sun is a major come from? hundred neutrinos left over from the first evidence for know they exist at all? seconds of the Universe.These neutrinos radioactivity – source of neutrinos – were made in the heat of the Big Bang In the 1920s, scientists were puzzled because energy by day and by night! with which the Universe began. However, blurred images seemed to go missing in beta-decay.This is the form of formed on a neutrinos continue to be created in radioactivity in which the radioactive nucleus emits an processes much closer to home. photographic electron.To solve the problem,Wolfgang Pauli, an plate that Henri Austrian physicist, proposed that tiny neutral particles The Sun bathes the Earth in a continuous Becquerel left must be emitted together with the electrons.These stream of neutrinos. Each second around under some neutral particles would escape undetected with some 60 billion neutrinos from the Sun pass of the energy released in the process. through each square centimetre of the uranium salts Earth's surface facing the Sun, and most of in 1896. Pauli’s particles became known as "neutrinos", but them pass swiftly out of the other side! they were not proved to exist until the 1950s. By then, These neutrinos are produced in the physicists had discovered nuclear fission, and knew nuclear reactions that make the Sun – and that if neutrinos existed then billions would be other stars – shine. Stars also produce emitted by the reactions in a nuclear bomb or a neutrinos when they die, in the nuclear reactor.Two Americans, Clyde Cowan and spectacular stellar collapses known as Fred Reines, realised that a reactor would emit so supernovae. Indeed, most of the energy many neutrinos that a few might interact in a released in a supernova is carried away by detector, even though most of the neutrinos would go the invisible neutrinos, rather than by the straight through. In 1956, they were able to send Pauli light we see. a telegram to say that they had succeeded – they had at last detected a neutrino. Closer to Earth, neutrinos are created in Fred Reines (back) and Clyde the atmosphere when cosmic rays – Cowan at the controls of the Since then, physicists have discovered that there are energetic particles from outer space – experiment that first three types of neutrino.The neutrinos emitted in collide with atomic nuclei in the upper beta-decay are related to the familiar electron. atmosphere. Other neutrinos are emitted demonstrated the existence of Indeed, they behave just like electrons with no electric neutrinos. (inset) The prototype Supernova 1987a (ringed) by the ground beneath our feet, in the charge.The other two types of neutrino are like beta-decay of naturally occurring neutrino detector appeared suddenly on 23 uncharged versions of charged particles related to the radioactive nuclei in the Earth. – a 300-litre tank of liquid electron – the "muon" and the "tau".To emphasise February 1987 – flooding surrounded by phototubes. the differences between them, the three neutrinos are space with neutrinos. Neutrinos are a by-product of all nuclear called the "electron-neutrino", the "muon-neutrino" reactors.They are emitted by the and the "tau-neutrino". radioactive products of the reactions at the core of the reactor.They can also be made in controlled conditions at particle accelerator laboratories.Accelerators produce high-energy beams of particles such as protons.When the energetic protons hit something, neutrinos are among the products of the particle more collisions. How do you see a neutrino? Further Information Particle Physics and Astronomy Neutrinos The main way for neutrinos to interact with other matter is through what is If you would like to know more about Research Council known as the "weak force".This is much neutrinos, try the following resources: The Particle Physics and Astronomy Research Council weaker than the electromagnetic force World Wide Web sites (PPARC) is the UK Government strategic science A first stop for particle physics investment agency for research in: that electrically charged particles, such as in the UK Particle Physics electrons, experience. So, while electrons http://hepweb.rl.ac.uk/ppUK/ Cosmic rays – their Astronomy can be easily stopped in matter, neutrinos All about neutrinos Space Science trails seen here in a can pass right through the Earth; indeed, it http://wwwlapp.in2p3.fr/~verkindt This support is provided by funding UK researchers spark chamber – would take 1000 light years (10,000 billion /neutrinos/aneut.html and by ensuring that they have access to world-class produce neutrinos in km) of water to be sure of catching a The Sudbury Neutrino Observatory facilities in the UK and overseas.The Council pays neutrino from the Sun! Surprisingly home page the UK’s subscription to the science programmes of the atmosphere. enough, however, water is often used to http://www.sno.phy.queensu.ca/ the European Laboratory for Particle Physics (CERN) and the European Space Agency (ESA). detect neutrinos. An introduction to particle physics http://durpdg.dur.ac.uk/lbl/cpep/ PPARC recognises the importance of its science in So how can we hope to observe a adventure.html assisting wealth creation; in post-graduate training; and in motivating young people towards an interest and career neutrino? Fortunately neutrino reactions Books in science generally. are random, and although it takes an Christine Sutton unimaginable amount of material to be Spaceship Neutrino Cambridge University Press 1992 Front cover: Bubble-tracks created by particles produced sure of stopping a neutrino, there is a in a neutrino collision inside the Big European Bubble chance that a neutrino will interact Gordon Fraser, Egil Lillestol and Chamber at CERN, Switzerland. Inge Sellevag sooner or later. For a distance of only a The Search for Infinity few metres this chance is very small Mitchell Beazley 1994 indeed, but with a large enough number of neutrinos to begin with and a large detector – and a great deal of patience – physicists can detect neutrinos.The trick Neutrinos are also a is to look for what happens when the by-product of nuclear neutrino interacts with another particle – Particle Physics and like seeing an invisible man knock Astronomy Research Council power stations, such as this Polaris House something over. In water, neutrinos can hit North Star Avenue one at Torness, Scotland. electrons out of atoms with so much force Swindon that the electrons emit light. Physicists PPARC also publishes information on Wiltshire SN2 1SZ a variety of topics related to physics Tel:+44 (0) 1793 442000 Particle Physics and Astronomy can then detect the light. Neutrinos can and astronomy, to help increase public Fax: +44 (0) 1793 442002 Research Council also change protons in atomic nuclei into awareness of the UK’s achievements Email: [email protected] neutrons, which can then be detected in science and technology. Find out through their reactions. In fact, this is the more by visiting the PPARC World Acknowledgements: Wide Web site: way that Cowan and Reines detected Photos supplied by: AIP Niels Bohr Library, Los Alamos National Laboratory, http://www.pparc.ac.uk IMB, CERN, European Southern Observatory, SNO and Scottish Nuclear Ltd neutrinos in 1956.