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## What is the matter?

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**What is the matter?**Where is the antimatter? Professor Michael G Green Royal Holloway University of London**What is the matter? . . . . Where is the antimatter?**Where the hell …?**What is the matter? . . . . Where is the antimatter?**What is matter?**What is the matter? . . . . Where is the antimatter?**Where is the antimatter?**What is the matter? . . . . Where is the antimatter?**The concept of elements In Aristotle’s philosophy there were four elements Dalton (1808) listed, with weights, many elements we recognize today**What is the matter? . . . . Where is the antimatter?**The periodic table Mendeleev (1869) introduced the periodic table**What is the matter? . . . . Where is the antimatter?**The plum pudding model J J Thomson believed the electrons were embedded in a positively charged matrix - plum pudding**What is the matter? . . . . Where is the antimatter?**The structure of atoms Rutherford (1912) showed that atoms contain a central nucleus Electrons orbit nucleus with well-defined energy and ill-defined positions 10-10m**What is the matter? . . . . Where is the antimatter?**The structure of nuclei Nucleus contains protons with charge +e and uncharged neutrons 10-14 m**What is the matter? . . . . Where is the antimatter?**The structure of nucleons Neutrons and protons contain quarks 10-15m**What is the matter? . . . . Where is the antimatter?**The structure of quarks? ? There is no evidence for further structure <10-18m**What is the matter? . . . . Where is the antimatter?**Evidence for substructure Atom absorbs energy Electron energy increases Only certain energy levels (orbits) allowed Later ‘de-excites’**What is the matter? . . . . Where is the antimatter?**Evidence for substructure Measure size of struck objects (Rutherford 1912) 1970 - substructure of protons and neutrons discovered using electrons as projectiles**charge**quarks electron u e 2 3 e + 1 3 e - d -e What is the matter? . . . . Where is the antimatter? The constituents of matter Protons contain uud - charge = +e Neutrons contain udd - charge = 0**What is the matter? . . . . Where is the antimatter?**Prediction of antimatter Paul Dirac predicted existence of the positron in 1928 Dirac’s equation implies: positron mass = electron mass positron charge = +e The only equation in Westminster Abbey?**What is the matter? . . . . Where is the antimatter?**Discovery of antimatter Anderson (1932) discovered the positron predicted by Dirac**e**e + - What is the matter? . . . . Where is the antimatter? What is antimatter? Electrons and positrons annihilate to produce g-rays (energy) E = mc2**e**e - + What is the matter? . . . . Where is the antimatter? Production of e+e- pairs Inverse process also occurs, with g-rays becoming an electron-positron pair**+**e g - - + e e e - e - e E g > few MeV since thin metal 2 m c = 0.5 MeV e region of magnetic field What is the matter? . . . . Where is the antimatter? How to produce antimatter**u**e 2 3 e + quarks leptons n 1 3 e - d 0 -e What is the matter? . . . . Where is the antimatter? The neutrino ‘Invented’ by Pauli (1928), named by Fermi (1933) Discovered by Reines & Cowan (1956)**What is the matter? . . . . Where is the antimatter?**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 ‘Who ordered that?’ - I I Rabi**What is the matter? . . . . Where is the antimatter?**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)**u**e n s m d quarks leptons What is the matter? . . . . Where is the antimatter? The fundamental particles (1963)**ne**nm nt c t u 1956 1963 1976 1995 e m t s b d 1895 1936 1973 1978 1947 six leptons six quarks What is the matter? . . . . Where is the antimatter? The zoo grows larger**What is the matter? . . . . Where is the antimatter?**A particle accelerator Energy of electrons is about 20kV**What is the matter? . . . . Where is the antimatter?**The LEP accelerator Energy of electrons and positrons is about 100GeV**What is the matter? . . . . Where is the antimatter?**CERN Europe’s research laboratory for particle physics in Geneva.**What is the matter? . . . . Where is the antimatter?**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**Annihilation produces energy - mini Big Bang**g g e- e+ Positron (antimatter) Electron (matter) g g Particles and antiparticles are produced What is the matter? . . . . Where is the antimatter? Electron-positron collisions E = mc2**End view**What is the matter? . . . . Where is the antimatter? The ALEPH detector**What is the matter? . . . . Where is the antimatter?**Collisions in ALEPH**What is the matter? . . . . Where is the antimatter?**ALEPH - a LEP particle detector**ne**nm nt e t m u c t d s b What is the matter? . . . . Where is the antimatter? Three neutrinos ... Number of different neutrinos = 2.984 ± 0.008 s measures rate at which e+e- collisions occur**e**e e e e * + - - g mass What is the matter? . . . . Where is the antimatter? … and no further substructure Excited states produced if substructure exists**ne**nm nt u c t e t m d s b What is the matter? . . . . Where is the antimatter? The story so far 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. When energy turns to mass equal numbers of particles and antiparticles are produced.**What is the matter? . . . . Where is the antimatter?**Matter-antimatter asymmetry in the Universe When energy turns to mass equal numbers of particles and antiparticles are produced. This observation creates problems for our understanding of the present day Universe, which appears to contain only matter and essentially no antimatter**life on earth,**15 billion years molecules 1 billion years form heavy elements 1 million years stars and formed galaxies 300,000 years in stars exist, microwave background atoms 3 minutes radiation form helium fills universe nuclei 1 second formed neutrons and protons - quark "soup" 10 10 s formed matter dominates ? 10 15 deg 10 10 deg Big Bang Big Bang 10 9 deg o 6000 o 4000 o -255 What is the matter? . . . . Where is the antimatter? o -270 Evolution of the Universe The Universe began with a “Big Bang” about 15 billion years ago**What is the matter? . . . . Where is the antimatter?**The Big Bang What happened at times less than 10-9s is uncertain**What is the matter? . . . . Where is the antimatter?**Evolution with matter-antimatter symmetry Eventually such a universe contains only photons (almost true for our Universe - cosmic microwave background)**What is the matter? . . . . Where is the antimatter?**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 Sakharov (1964)**e+**d X u - u u p0 u proton What is the matter? . . . . Where is the antimatter? Proton decay Life on earth implies protons exist, on average, for >1023 seconds Searches for proton decay have set limits >1032 seconds Proton decay converts quarks into leptons - important only in early stages of the Big Bang but a small effect will remain However antiprotons will decay similarly**θ**θ v q = - q I ( ) 1 cos c What is the matter? . . . . Where is the antimatter? Parity violation Macroscopic systems obey the same physical laws in a mirror system, e.g. planetary motion “parity conservation”. b-decay (weak interaction) does not conserve parity. Discovered in 1956 in polarized 60Co decay.**P**n n L R C CP n n L R 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?**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.**K0**W W s d u,c,t - - - - - - - - - - u,c,t u,c,t s s d d K0 W W d s u,c,t - - K0 K0 What is the matter? . . . . Where is the antimatter? CP violation in K0 decays Phases of the amplitudes for the two processes are not equal ‘CP violation’ (1964) Occurs only because there are three families of quarks**6**10 s -10 5 10 4 10 COUNTS / 0.5 x 10 5 10 15 t -10 (10 s) 3 10 + - k 0 p p DISTRIBUTION 2 10 1 10 5 10 15 20 25 30 t -10 (10 s) What is the matter? . . . . Where is the antimatter? CP violation Leads to beautiful interference effects and non-exponential decay distributions**What is the matter? . . . . Where is the antimatter?**A universe with CP violation 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)**What is the matter? . . . . Where is the antimatter?**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**-**- b b B0 W W B0 W W - - - - - - - - u,c,t u,c,t d d b d d b u,c,t u,c,t - - B0 B0 What is the matter? . . . . Where is the antimatter? CP violation in B0 decays Similar effect expected in B0 First measurements starting 1999, Stanford, California