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Introduction – “Historical Overview of Particle Physics, Accelerators and Detectors”

Introduction – “Historical Overview of Particle Physics, Accelerators and Detectors”. M. Velasco -- Lecture 1 & 2 Problems – 1.1, 1.2, 1.7, 1.12, 1.13. Particle Physics. What are the fundamental building “blocks” of the universe ? Visible matter, dark matter, dark energy…

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Introduction – “Historical Overview of Particle Physics, Accelerators and Detectors”

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  1. Introduction – “Historical Overview of Particle Physics, Accelerators and Detectors” M. Velasco -- Lecture 1 & 2 Problems – 1.1, 1.2, 1.7, 1.12, 1.13

  2. Particle Physics • What are the fundamental building “blocks” of the universe ? Visible matter, dark matter, dark energy… • What are the interactions between them? Gravitational, electro-weak, strong… • How can we explain the universe? • its history • its present form • its future • Is there a theory of everything? …bring us back at the beginning of the universe

  3. Clear relationship between energy of “particle” and “time”

  4. What is a particle and how we learn from them? • a small piece of matter... • characterized by • charge • mass • lifetime • spin • particles can scatter off each other like billiard balls • unlike billiard balls, most particles are unstable and decay • particles can be produced by colliding other particles and form bound states

  5. Models used to described general principles Small Fast Quantum Gravity What is missing? …

  6. Quantum FieldTheoriesincluded in Standard Model QED=Quantum Electro Dynamics QCD=Quantum Chromo Dynamics Electro-Weak

  7. n  K _ e  Production of Particles Primary cosmic rays: 90% protons, 9% He nuclei Air nuclei (Nitrogen & Oxygen) + + e+  Nuclear Reactors  alpha, neutrons, etc.

  8. Chronology of Early DiscoveriesInterplay with introduction of new detectors/particle sources • Electron (1897) J.J. Thompson • Cloud Chamber(1912) C.T.R.Wilson • Cosmic Rays(1913) V.F.Hess &C.Anderson • Discovery of Proton(1919) E. Rutherford • Compton Scattering gege (1923) C.T.R.Wilson • Waves nature of e’s(1927) C. Davisson 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

  9. Cloud Chamber • Supersaturated Gas • Cloud formation • Used until 1950’s • Condensation started around the ions generated by passing charged particles (ionization), and the resulting droplets were photographed. 9

  10. Scattering Geiger&Marsden , b source Zinc Sulphide Screen E. Rutherford 1927, Rutherford, as President of the Royal Society, expressed a wish for a supply of "atoms and electrons which have an individual energy far transcending that of the alpha and beta particles from radioactive bodies..."

  11. Penetrating Power    Neutron Paper sheet Lead Paraffin Aluminum

  12. Cross-Section 1 barn =10-24 cm2 Approximately the area of a proton Radii of nuclei~ fm • Distribution of scattering angles tell us about the force/particles • Precision required

  13. Accelerator technology The first successful cyclotron, built by Lawrence and his graduate student M. Stanley Livingston, accelerated a few hydrogen-molecule ions to an energy of 80,000 electron volts. (80KeV) 1932- 1MeV

  14. Particle guidance • In circular machines use magnetic field to guide particles along orbit (Lorentz force) • in early machines e.g. cyclotrons B field occupied entire accelerating plane • What about machines with larger energies like the one we need today? • Can you guess based on your basic knowledge of E&M ?

  15. Kinematics of circular accelerators • Use relativistic equations of motion (v = c) • Centripetal force = Lorentz force (magnetic) • mv2/r = mv = qvB/c v  B  = 1/(1 - v²/c²)  rev. freq.  = /2 = qB/2mc • at relativistic speeds v = c and momentum P = mc   = c/2 = qB/2P  = radius of orbit  P = qB/c or, P (GeV/c) = 0.3 B (Tesla)  (m) ( q = e ) In another words,  = (q/2mc) (1 - v²/c²) B • as particle accelerates, v increases,  B and/or  must increase to compensate • in electron synchrotrons (LEP)  fixed , B increases

  16. Summary of what the world was made of by 1932 • electrons (1897) • orbit atomic nucleus • photon (1905) • quantum of the electromagnetic field • proton (1911) • nucleus of lightest atom • neutron (1932) • neutral constituent of the nucleus  Required new experimental techniques...not stable … more questions

  17. Postulates  to explain what was observed & known at that time • 1927 Dirac’s relativistic quantum mechanics • antiparticles: for every particle there exists an antiparticle with same mass, lifetime, spin, but opposite charge • 1931 the positive electron (positron) found • 1930 Pauli’s neutrino • energy conservation in beta (b) decay requires the existence of a light, neutral particle • n  p+ + e- +  (e- = b) • observed in 1956  Why it took so long? • … To come …1937 Yukawa’s pion to explain inter-nuclear forces

  18. Just as the equation x2=4 can have two possible solutions (x=2 OR x=-2), so Dirac's equation could have two solutions, one for an electron with positive energy, and one for an electron with negative energy. Dirac interpreted this to mean that for every particle that exists there is a corresponding antiparticle, exactly matching the particle but with opposite charge. For the electron, for instance, there should be an "anti-electron" called the positron identical in every way but with a positive electric charge. E2 – p2c2 = (m0c2) 2“relativistic invariant” (same value in all reference frames)

  19. 1931 the positive electron (positron)

  20. Neutrinos must be present to account for conservation energy & momentum __ Wolfgang Pauli • Large variations in the emission velocities of the  particle seemed to indicate that both energy and momentum were not conserved. • This led to the proposal by Wolfgang Pauli of another particle, the neutrino, being emitted in  decay to carry away the missing mass and momentum.

  21. 1937: Theory of nuclear forces mc2200 MeV for Rint10 -13 cm Hideki Yukawa Existence of a new light particle (“meson”) as the carrier of nuclear forces (140GeV) Relation between interaction radius & meson mass m:

  22. 1932-1947 • Neutron(1932) J. Chadwick • Triggered Cloud Chamber(1932) P.Blackett • Muon(1937) S.H. Neddermeyer • Muon Decay(1939) B.Rossi, Williams • Kaon(1944) L. Leprince-Ringuet • Pion(1947) .H.Perkins,G.P.S.Occialini 19001910 1920 1930 1940 1950 1960 1970 1980 1990 2000

  23. Emulsion heavily used in the early days of Cosmic Ray experiments • Dates back to Becquerel (1896) • Three components • silver halide (600mm thick) • plate • target • Grain diameter 0.2mm • Still the highest resolution device

  24. Emulsion  used in discovery of m, p, k, etc. m Scale 100mm

  25. The particle “Zoo”  Cosmic rays 1st , followed by accelerator • 1947: strange particles • K0+-, K+++- • p+- • ,  • long lifetime  ~ 10-10 s • more particles... • p, •  • short lifetime  ~10-24 s

  26. 1947-1953 • Efficient production of particles with higher masses is going to required high energy • Before 50’s E=mc2 was still just a theory… • Next period will required the development of both accelerators in addition to detectors • Cockcroft and Walton…

  27. Energy and momentum for relativistic particles m: relativistic mass m0: rest mass energy associated with rest mass “classical” kinetic energy (velocity v comparable to c) Speed of light in vacuum c = 2.99792x108 m / s Total energy: Expansion in powers of (v/c): Momentum:

  28. Cockcroft & Walton Accelerator • First artificial splitting of nucleus • First transmutation using artificially accelerated particles • First experimental verification of • E = mc2

  29. Relativity • “Mass” not conserved  Energy & Momentum are conserved Experimental verification ofE = mc2 17.3 MeV 1 MeV Proton + Lithium 2 a particles + Energy

  30. Other discoveries between 1947-1953 • Scintillation Counters(1947) F. Marshall • pion decay(1947) C. Lattes • Unstable V’s(1947) G.D.Rochester • Semi-Conductor Detectors(1949) K.G.McKay • SparkChambers(1949) J.W.Keuffel • K Meson decays(1951) R.Armenteros 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

  31. Basic principles of particle detection Ionization + excitation of atomic energy levels energy loss Passage of charged particles through matter Interaction with atomic electrons K p ionization (neutral atom  ion+ + free electron) p e excitation of atomic energy levels (de-excitation  photon emission) m Momentum Mean energy loss rate – dE /dx • proportional to (electric charge)2 of incident particle • for a given material, function only of incident particle velocity • typical value at minimum: -dE /dx = 1 – 2 MeV /(g cm-2) What causes this shape?

  32. + - - + - + + + - - - + - + - + - + + - - + + - - - + Most detectors at that time based on Ionization • Charged particles • interaction with material “track of ionisation”

  33. Ionization Density of electrons • Important for all charged particles • Bethe-Bloch Equation velocity Mean ionization potential (10ZeV)

  34. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Ionization • In low fields the ions eventually recombine with the electrons • However under higher fields it is possible to separate the charges Note: e-’s and ions generally move at a different rate + + E + + + + + +

  35. 1953-1968 • Neutrino (1953) F. Reines • Bubble Chamber(1953) D.A. Glaser • K+ Lifetime(1955) L.W.Alvarez • Flash Tubes(1955) M. Conversi • Spark Chamber(1959) S. Fukui • Streamer Chambers(1964) B.A.Dolgoshein • MWPC(1968) G. Charpak 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

  36. The Stanford two-mile electron linear accelerator (SLAC) CERN LEP-1984-1999 SC 1957-1990 Synchrotron Radiation SLAC

  37. Before we move to accelerator based measurement let’s talk about neutrinos - nPuzzle in b – decay: the continuous e- energy spectrum First measurement by Chadwick (1914) Radium E: 210Bi83 (a radioactive isotope produced in the decay chain of 238U) If  – decay is (A, Z)  (A, Z+1) + e–, then the emitted electron is mono-energetic  e- total energy E = [M(A, Z) – M(A, Z+1)]c2 (neglecting the kinetic energy of the recoil nucleus ½p2/M(A,Z+1) << E)

  38. Theory of -decay - decay: n  p + e- +  + decay: p  n + e+ +  (e.g., 14O814N7 + e+ + ) : the particle proposed by Pauli (named “neutrino” by Fermi) : its antiparticle (antineutrino) Enrico Fermi Fermi’s theory:  particles emitted in  – decay need not exist before emission – they are “created” at the instant of decay Prediction of  – decay rates and electron energy spectra as a function of only one parameter: Fermi coupling constant GF (determined from experiments)

  39. First neutrino detection  + p  e+ + n 2 m H2O + CdCl2 I, II, III: Liquid scintillator (Reines, Cowan 1953) E = 0.5 MeV • detect 0.5 MeV -rays from e+e–  (t = 0) • neutron “thermalization” Followed by capture in Cd nuclei • Emission of delayed -rays (average delay ~30s) Event rate at the Savannah River nuclear power plant: 3.0  0.2 events / h in agreement with expectations

  40. Muon decay ± e± +  +  Cosmic ray muon stopping in a cloud chamber and decaying to an electron decay electron track p: muon momentum c0.66 km Decay electron momentum distribution Muon spin = ½ Muon lifetime at rest:  = 2.197x10 - 6 s 2.197s Muon decay mean free path in flight:  muons can reach the Earth surface after a path  10 km because the decay mean free path is stretched by the relativistic time expansion

  41. Lepton Number Conservation Electron, Muon and Tau Lepton Number We find that Le , Lm and Lt are each conserved quantities

  42. n  p + e-+ne m+ e+ + ne +nm Lm -1 0 0 Le 0 0 +1 -1 -1 Le B +1 0 +1 -1 0 +1 0 0 Lm -1 0 0 X X Le 0 -1 0 Lepton Number Conservation • . • . • . • . m+ e+ + g

  43. Other conserved quantities Baryon Number Conservation When we collide particles together, we find that the number ofbaryons is conserved. A + B C + D • For each baryon, we simply assign B = +1(protons, neutrons,for example) • For each anti-baryon ,we assign B = -1(antiprotons, antineutrons,for example) •  Compute the total baryon number on each side and they must be equal!

  44. Baryon number conservation B = +1 for baryon in a decay or reaction, and B = -1 for each anti-baryon, then the total baryon number must be the same before and after the process. Eg p+ + n  p+ + p+ + n + p- 1 1  1 1 1 -1 • . p+ + n  p+ + p+ + p- 1 1  1 1 -1 X

  45. Recall: We had many new types of matter! More and More Mystery particles Fermilab:Bubble Chamber Photo

  46. Strange particles observed:Long lifetimes & Heavy

  47. Invention of a new, additive quantum number “Strangeness” (S) (Gell-Mann, Nakano, Nishijima, 1953) • conserved in strong interaction processes: • not conserved in weak decays: S = +1: K+, K° ; S = –1: , ±, ° ; S = –2 : °, – ; S = 0 : all other particles (and opposite strangeness –S for the corresponding antiparticles)

  48. Summary of strangeness puzzles & their contribution to the SM 1944-47:Strangeness  quark model  Basis for QCD 1956:Parity violation Spin-dependence of weak interactions 1964:Suppression of Flavour Changing NC Suggested charm quark  Properties of the neutral currents 1964:CP violation  Absolute matter-antimatter asymmetry…

  49. s u Puzzle #1 -- Strange particles observed:Long lifetimes & Heavy Strangeness - produced by strong interaction • conserved by strong interactions •  these strange particles produced in pairs d g s u u d

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