Particle Physics_A Very Short Introduction

by Frank Close

  Kaons are made of a quark and an antiquark and as such are an equal mixture of matter and antimatter. The neutral kaon (Ko) consists of a down quark and a strange antiquark, while its antiparticle consists of a down antiquark and a strange quark. The Ko and are thus different particles, but they are intimately related through the weak force which, rather surprisingly, allows a Ko to change to a , and vice versa, via interactions between their quarks and antiquarks. What this effect means is that once a neutral kaon or neutral anti-kaon is created some quantum mechanical ‘mixing’ begins to occur.

  These in-between mixtures are known as the KS (S for ‘short’) and the KL (L for ‘long’). The K-Long lives about 600 times longer than the K-Short. The important feature is that the states K-Long and K-Short behave differently in the combined ‘mirrors’ of CP. The two states decay in different ways, the K-Short to two pions, the K-Long to three pions. If CP symmetry were perfect this pattern of decay would always be true. The K-Long, for example, would never decay to two pions. However, as Cronin and Fitch and their colleagues first observed, in about 0.3% of cases the K-Long does decay to two pions.

  The question now in the minds of many physicists is whether the ‘accident’ of three generations is what has led to the dominance of matter in our universe. Theory implies that CP violation should be a large effect in the case of B mesons, which are similar to kaons but with the strange quark replaced by a bottom quark. The B-meson system is now the subject of intensive experimental investigation and the first signs of a large asymmetry have been reported.

  The LHC will produce large numbers of bottom particles and investigations of CP asymmetry for these particles will be a major part of the programme there. To this end there is a dedicated experiment named LHCb.

  Some questions for the future

  Now for the really bizarre. According to the latest theories the three dimensions of space and that of time are just a part of a more profound universe. There are dimensions that are beyond perception by our usual senses but which could be revealed in forthcoming high-energy experiments at CERN.

  To make some sense of this, imagine a universe perceived by flatlanders who are aware of only two dimensions. We, with our greater awareness, know of a third. So we can imagine two flat plates separated by, say, a millimetre. The effects of forces on one plate could leak across the gap but the flatlanders would not realize this. They would perceive the remnant effects, which would be feeble in comparison to the effects when restricted to the flat plane universe that they experience.

  Now imagine us as ‘flatlanders’ in a universe with higher dimensions. The idea is that gravity appears feeble to us because it is the effect of the other forces leaking out into the higher dimensions in our universe. So when we feel gravity, we are feeling the effect of the other unified forces that have leaked away into the higher dimensions leaving a trifling remnant to do its work. One could even imagine particles moving from our ‘flatlander’ dimensions into the higher dimensions and in effect ‘disappearing’ from the universe as we know it.

  Thus in the new experiments at the LHC at CERN, physicists will be on the lookout for signs of particles ‘spontaneously’ appearing or vanishing. If such a phenomenon is found to occur in some systematic way, this could provide evidence that we are indeed like flatlanders, and that there are dimensions in nature beyond the three space and one time that we currently experience.

  We have reached a point where it is beginning to be hard to distinguish science fact from science fiction. But a century ago, much of what we take for granted today would have been beyond the imagination of H. G. Wells. A hundred years from now there will be material in the science text books as yet undreamed of. Some fifty years ago I read a book that told of the wonders of the atom as they were then being revealed, and of the strange particles that were showing up in cosmic rays. Today I am writing about them for you. Perhaps in another half century you may be updating the story for yourself. Good luck.

  Further reading

  The following suggestions for further reading are not intended to form a comprehensive guide to the literature on particle physics.

  This section includes some ‘classics’ that are out of print but which should be available through good libraries or second-hand bookshops, on the ground or via the Internet (such as at www.abebooks.com).

  Frank Close, The Cosmic Onion: Quarks and the Nature of the Universe (Heinemann Educational, 1983). An account of particle physics in the 20th century for the general reader.

  Frank Close, Lucifer’s Legacy (Oxford University Press, 2000). An interesting introduction to the meaning of asymmetry in antimatter and other current and future areas of particle physics.

  Frank Close, Michael Marten, and Christine Sutton, The Particle Odyssey (Oxford University Press, 2003). A highly illustrated popular journey through nuclear and particle physics of the 20th century, with pictures of particle trails, experiments, and the scientists.

  Gordon Fraser (ed.), The Particle Century (Institute of Physics, 1998). The progress of particle physics through the 20th century.

  Brian Greene, The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory (Jonathan Cape, 1999). A prize-winning introduction to the ‘superstrings’ of modern theoretical particle physics.

  Tony Hey and Patrick Walters, The Quantum Universe (Cambridge University Press, 1987). An introduction to particle physics and quantum theory.

  George Johnson, Strange Beauty: Murray Gell-Mann and the Revolution in Twentieth-century Physics (Jonathan Cape, 2000). A biography of Murray Gell-Mann, the ‘father’ of quarks.

  Gordon Kane, The Particle Garden: Our Universe as Understood by Particle Physicists (Perseus Books, 1996). An introduction to particle physics and a look at where it is heading.

  Robert Weber, Pioneers of Science (Institute of Physics, 1980). Brief biographies of physics Nobel Prize winners from 1901 to 1979.

  Steven Weinberg, The First Three Minutes (Andre Deutsch, 1977; Basic Books, 1993). The first three minutes after the Big Bang, described in non-technical detail by a leading theorist.

  Steven Weinberg, Dreams of a Final Theory (Pantheon Books, 1992; Vintage, 1993). A ‘classic’ on modern ideas in theoretical particle physics.

  W. S. C. Williams, Nuclear and Particle Physics, revised edn. (Oxford University Press, 1994). A detailed first technical introduction suitable for undergraduates studying physics.

  Glossary

  alpha particle: two protons and two neutrons tightly bound together; emitted in some nuclear transmutations; nucleus of a helium atom

  angular momentum: a property of rotary motion analogous to the more familiar concept of momentum in linear motion

  antimatter: for every variety of particle there exists an antiparticle with opposite properties such as the sign of electrical charge. When particle and antiparticle meet they can mutually annihilate and produce energy

  anti(particle): antimatter version of a particle, for example antiquark, antiproton

  atom: system of electrons encircling a nucleus; smallest piece of an element that can still be identified as that element

  B: symbol for the ‘bottom meson’

  B-factory: accelerator designed to produce large numbers of particles containing bottom quarks or antiquarks

  baryon: class of hadron; made of three quarks

  beta decay (beta radioactivity): nuclear or particle transmutation caused by the weak force, resulting in the emission of a neutrino and an electron or positron

  boson: generic name for particles with integer amount of spin, measured in units of Planck’s constant; examples include carriers of forces, such as photon, gluon, W and Z bosons and the (predicted) spinless Higgs boson

  bottom(ness): property of hadrons containing bottom quarks or antiquarks

  bottom quark: most massive example of quark with electric charge –1/3

  bubble chamber: form of particle detector, now obsolete, revealing the flightpaths of electric
ally charged particles by trails of bubbles

  CERN: European Centre for Particle Physics, Geneva, Switzerland

  charm quark: quark with electric charge +2/3; heavy version of the up quark but lighter than the top quark

  collider: particle accelerator in which beams of particles moving in opposing directions meet head on

  colour: whimsical name given to property of quarks that is the source of the strong forces in the QCD theory

  conservation: if the value of some property is unchanged throughout a reaction, the quantity is said to be conserved

  cosmic rays: high-energy particles and atomic nuclei coming from outer space

  cyclotron: early form of particle accelerator

  down quark: lightest quark with electrical charge –1/3; constituent of protons and neutrons

  electromagnetic force: fundamental force that acts through forces between electrical charges and the magnetic force

  electron: lightweight electrically charged constituent of the atom

  electroweak force: theory uniting the electromagnetic and weak forces

  eV (electronvolt): unit of energy; the amount of energy that an electron gains when accelerated by one volt

  E = mc2 (energy and mass units): technically the unit of MeV or GeV is a measure of the rest energy, E = mc2, of a particle, but it is often traditional to refer to this simply as mass, and to express masses in MeV or GeV

  fermion: generic name for a particle with half-integer amount of spin, measured in units of Planck’s constant. Examples are the quarks and leptons.

  flavour: generic name for the qualities that distinguish the various quarks (up, down, charm, strange, bottom, top) and leptons (electron, muon, tau, neutrinos), thus flavour includes electric charge and mass

  gamma ray: photon; very high-energy electromagnetic radiation

  generation: quarks and leptons occur in three ‘generations’. The first generation consists of the up and down quarks, the electron and a neutrino. The second generation contains the charm and strange quark, the muon, and another neutrino, while the third, and most massive, generation contains the top and bottom quarks, the tau, and a third variety of neutrino. We believe that there are no further examples of such generations.

  GeV: unit of energy equivalent to a thousand million (109) eV

  gluon: massless particles that grip quarks together making hadrons; carrier of the QCD forces

  hadron: particle made of quarks and/or antiquarks, which feels the strong interaction

  Higgs boson: massive particle predicted to be the source of mass for particles such as the electron, quarks, W and Z bosons

  ion: atom carrying electric charge as a result of being stripped of one or more electrons (positive ion), or having an excess of electrons (negative ion)

  K (kaon): variety of strange meson

  keV: a thousand eV

  kinetic energy: the energy of a body in motion

  LEP: Large Electron Positron collider at CERN

  lepton: particles such as the electron and neutrino that do not feel the strong force and have spin 1/2

  LHC: Large Hadron Collider; accelerator at CERN

  linac: abbreviation for linear accelerator

  MACHO: acronym for Massive Compact Halo Object

  magnetic moment: quantity that describes the reaction of a particle to the presence of a magnetic field

  mass: the inertia of a particle or body, and a measure of resistance to acceleration; note that your ‘weight’ is the force that gravity exerts on your mass, so you have the same mass whether on Earth, on the Moon, or in space, even though you may be ‘weightless’ out there.

  meson: class of hadron; made of a single quark and an antiquark

  MeV: a million eV

  meV: a millionth of an eV

  molecule: a cluster of atoms

  microsecond: one millionth of a second

  muon: heavier version of the electron

  nanosecond: one billionth of a second

  neutrino: electrically neutral particle, member of the lepton family; feels only the weak and gravitational forces

  neutron: electrically neutral partner of a proton in the atomic nucleus, which helps stabilize the nucleus

  parity: the operation of studying a system or sequence of events reflected in a mirror

  picosecond: one millionth of a millionth of a second

  photon: massless particle that carries the electromagnetic force

  pion: the lightest example of a meson; made of an up and/or down flavour of quark and antiquark

  Planck’s constant (h): a very small quantity that controls the workings of the universe at distances comparable to, or smaller than, the size of atoms. The fact that it is not zero is ultimately the reason why the size of an atom is not zero, why we cannot simultaneously know the position and speed of an atomic particle with perfect precision, and why the quantum world is so bizarre compared to our experiences in the world at large. The rate of spin of a particle is also proportional to h (technically, to units or half-integer units of h divided by 2π)

  positron: antiparticle of an electron

  proton: electrically charged constituent of the atomic nucleus

  QCD (quantum chromodynamics): theory of the strong force that acts on quarks

  QED (quantum electrodynamics): theory of the electromagnetic force

  quarks: seeds of protons, neutrons, and hadrons

  radioactivity: see beta decay

  SLAC: Stanford Linear Accelerator Center, California, USA

  SNO: Sudbury Neutrino Observatory, underground laboratory in Sudbury, Ontario

  spark chamber: device for revealing the passage of electrically charged particles

  spin: measure of rotary motion, or intrinsic angular momentum, of a particle; measured in units of Planck’s constant

  strange particles: particles containing one or more strange quarks or antiquarks

  strange quark: quark with electrical charge –1/3, more massive than the down quarks but lighter than the bottom quark

  strangeness: property possessed by all matter containing a strange quark or antiquark

  strong force: fundamental force, responsible for binding quarks and antiquarks to make hadrons, and gripping protons and neutrons in atomic nuclei; described by QCD theory

  Superkamiokande: underground detector of neutrinos and other particles from cosmic rays, located in Japan

  SUSY (supersymmetry): theory uniting fermions and bosons, where every known particle is partnered by a particle yet to be discovered whose spin differs from it by one half

  symmetry: if a theory or process does not change when certain operations are performed on it, then we say that it possesses a symmetry with respect to those operations. For example, a circle remains unchanged after rotation or reflection; it therefore has rotational and reflection symmetry.

  synchrotron: modern circular accelerator

  tau: heavier version of the muon and electron

  top quark: the most massive quark; has charge +2/3