→ is all matter made up of water? Thale’s followers: matter made up of 4 fundamental elements → earth, fire, air and water Democritus (~350 BC): matter made up from small, indivisible particles atoms Example: What happens if a piece of metal is cut into smaller and smaller pieces? Ans: if matter is continuous, piece is infinitely divisible Democritus: at some stage reach immutable atoms (indivisible) Epicurus of Samos (342-270BC): expanded idea of atomism Snag: atomism disputed by Plato and Aristotle matter continuous, made up of four elementary principles hotness, coldness, dryness and wetness 2. The chemistry of the elements Lavoisier (1734-1794): observations on combustion suggested that matter comprised discrete elements, and that matter was conserved in chemical reactions the chemical elements hydrogen, oxygen, carbon, sodium etc
since q/me = 1.76 x 1011 C/kg (Thomson) and qe = 1.6 x 10-19 C (Millikan) deduce me = 9.1 x 10-31 C/kg 2.Canal rays and the proton Thomson (~1890): does anode produce +ve rays? +ve rays detected when slit put in cathode measure q/m ratio of +ve rays using Thomson method Results vx much smaller than electron case q/m much smaller than electron case q/m depends on gas in tube largest q/m value for hydrogen other gases have q/m simple fractions of H value Deduce +ve particles (ions) due to e collision with gas atoms charge equal and opposite to electron mass much larger than electron (from q/m) H ion is lightest (named proton)
Experiment -particle bombardment of B gas products bombard paraffin wax behind target Results protons knocked out of paraffin Inference 1. En transferred to protons by massive particle 2. mn mp (from proton tracks) a 2nd nuclear particle with proton mass but no charge P.S. 1932: complete atomic model nucleus contains protons and neutrons electrons orbit atomic nucleus explains atomic structure, isotopes, radioactivity e.g. 3. Radioactivity Becquerel (1896): element uranium producing mysterious radiation that could penetrate black paper and fog photographic film - independent of temp, press, E-field Rutherford (1900): Becquerel radiation contains 3 components α,β and γ rays α rays: massive particles of double +ve charge β rays: same q/m as electrons γ rays: similar to X-rays, but higher energy and penetration Rutherford and Soddy (1903): some atoms can spontaneously disintegrate
Wolfgang Pauli (1936): postulates neutrino massless, chargeless particle (detected in 1956) no p+ + e- + Splitting the ‘atom’ 1932: Cockcroft and Walton (Rutherford group, Cambridge) Experiment Linear accelerator: protons accelerated in electric field to 1 MeV Fired at Lithium atoms: alpha particles detected on zinc screens Explanation Li atoms split into 2 helium nuclei First transmutation of elements by artificial means Verification of E = mc2 (Einstein) Verification of quantum tunnelling (Gamow) Nobel Prize (1951)
Higher fraction of K.E. converted into new particles VII The quark model of particle physics (i) The quark model Hundreds of new particles Similar to hundreds of new elements Periodic Table: repeated patterns → elements are composed of atoms (protons etc) Does a similar pattern exist for elementary particles? Gellmann (1961): arranged particles into patterns using group theory The eightfold way Predicts new particle (Ω-); mass and charge Detected 1963 Gellmann and Zweig (1964): particles contain more fundamental units? quarks: up, down strange fractional charges Quark model: meson = quark-antiquark pair hadron = 3 quarks (p = uud, n = udd, Ω- = sss ) Explains all properties of hadrons, resonances and stable particles (ii) the search for quarks
1st generation: all of ordinary matter 2nd, 2rd generation; cosmic rays and accelerators Note: total charge in each generation = 0 2. The weak interaction Recall : no → p + e- + ( β-decay ) Problem: Coupling calculations divergent Lee, Rosenbluth and Yang (1940s); Weak boson ‘Messenger’ role similar to Yukawa pion, but for weak decay no → W- → p + e- + e Sheldon, Glashow and Weinberg (1968): gauge theory Three ‘exchange’ particles necessary to describe weak interaction W+, W-, Z0 Extremely heavy particles; integer spin (bosons) Conservation laws in weak interactions: quark flavour not conserved parity not conserved Weak neutral currents (CERN 1973) Quark or lepton absorbs neutral Z boson Detection of W and Z bosons (CERN, 1983) Z0 → e+ + e- (UA1) W+ → e+ + e W- → e- + e (UA2)