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Leptoni

Leptoni. Why 3 families? Are there more?. 2nd generation. 3rd generation. 2/3. 2/3. - 1/3. - 1/3. 0. 0. -1. -1. Fermions: the elementary players. The elementary particle families: fermions. 1st generation. Quarks. Leptons and quarks form doublets under weak interactions. Leptons.

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Leptoni

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  1. Leptoni

  2. Why 3 families?Are there more? 2nd generation 3rd generation 2/3 2/3 -1/3 -1/3 0 0 -1 -1 Fermions: the elementary players The elementary particle families: fermions 1st generation Quarks Leptons and quarks form doubletsunder weakinteractions Leptons

  3. Muons Where first observed in 1936, in cosmic rays Cosmic rays have two components: • Primaries:high-energy particles coming from outer space mostly H2 nuclei 2) Secondaries:particles produced in collisions primaries-nuclei in the Earth atmosphere • m’s are 200 heavier than e and are very penetrating particles • Electromagnetic properties of m’s are identical to those of • electron (upon the proper account of the mass difference) • Tauons Is the heaviest of the leptons, discovered in e+e- annihilation experiments in 1975

  4. Leptons • Leptons are s = ½ fermions, not subject to strong interactions • me < mm < mt • Electron e-, muon m- and tauon t- have corresponding neutrinos: • ne, nm and nt • Electron, muon and tauon have electric charge of e-. • Neutrinos are neutral • Neutrinos have very small masses • For neutrinos only weak interactions have been observed so far

  5. Anti-leptons are positron e+, positive muons and tauons and • anti-neutrinos • Neutrinos and anti-neutrinos differ by the lepton number. • For leptons La = 1 (a = e,m or t) • For anti-leptons La = -1 • Lepton numbers are conserved in any reaction

  6. Consequence of the lepton nr conservation: some processes are not allowed..... Lederman, Schwarts, Steinberger • Neutrinos • Neutrinos cannot be registered by detectors, there are only indirect indications of them • First indication of neutrino existence came from b-decays of a nucleus N

  7. Electron is a stable particle, while muon and tauon have a finite • lifetime: • tm = 2.2 x 10-6 s and tt = 2.9 x 10-13 s • Muon decay in a purely leptonic mode: • Tauon has a mass sufficient to produce even • hadrons, but has leptonic decays as well: • Fraction of a particular decay mode with respect to all possible decays is called branching ratio (BR) • BR of (a) is 17.84% and of (b) is 17.36%

  8. Important assumptions: • Weak interactions of leptons are identical like electromagnetic ones (interaction universality) • 2) One can neglect final state lepton masses for many basic calculations • The decay rate for a muon is given by: • Where GF is the Fermi constant • Substituting mm with mt one obtains decay rates of tauon leptonic • decays, equal for (a) and (b). It explains why BR of (a) and (b) • have very close values

  9. Using the decay rate, the lifetime of a lepton • is: • Here l stands for m and t. Since muons have basically one decay mode, B= 1 in their case. Using experimental values of B and formula for G, one obtaines the ratio of m and t lifetimes: • Again in very good agreement with independent • experimental measurements • Universality of lepton interaction proved to big extent. Basically no difference between lepton generations,apart from the mass

  10. Unique electron energy? Experimental electron energy  events Energy conservationviolated?  electron energy Crisis around 1930 • Observations: • Nuclear -decay: • 3H →3He+e- • Matter is made of: • Particles: , e-, p • Atoms: Small nucleus of protons surrounded by a cloud of electrons before Pauli:

  11. Variable electron energy! Pauli’s hypothesis Pauli:

  12. What is a b-decay ? It is a neutron decay: • Necessity of neutrino existence comes from the apparent energy • and angular momentum non-conservation in observed reactions • For the sake of lepton number conservation, electron must be • accompanied by an anti-neutrino and not a neutrino! • Mass limit for can be estimated from the precise measurements • of the b-decay: • Best results are obtained from tritium decay • it gives (~ zero mass)

  13. Neutrino’s detected… (1956) • Cowan & Reines • Cowan nobel prize 1988with Perl (for discovery of -lepton) • Intense neutrino flux from nuclear reactor Scintillator counters and target tanks Power plant (Savannah river plant USA) Producing e -capture  n e+e annihilation e e+ 

  14. An inverse b-decay also takes place: • However the probability of these processes is very low. • To register it one needs a very intense flux of neutrinos • Reines and Cowan experiment (1956) • Using antineutrinos produced in a nuclear reactor, possible to • obtain around 2 evts/h • Acqueous solution of CdCl2 (200 l + 40 kg) used as target • (Cd used to capture n) • To separate the signal from background, “delayed coincidence” • used: signal from n appears later than from e

  15. Scheme of the Reines and Cowan experiment 2m 2m • Antineutrino interacts with p, producing n and e+ • (b) Positron annihilates with an atomic electron produces fast • photon which give rise to softer photon through Compton effect • (c) Neutron captured by a Cd nucleus, releasing more photons

  16. Helicity states For amassless fermion of positive energy, E = |p| helicity H measures the sign of the component of the particle spin, in the direction of motion: H=+1  right-handed (RH) H=-1  left handed (LH) c is a LH particle or a RH anti-particle • Helicity is a Lorentz invariant for massless particles • If extremely relativistic, also massive fermions can be described by Weyl equations

  17. Nobel prize 2002 (Davis, Koshiba and Giacconi) Anti-neutrino’s • Davis & Harmer • If the neutrino is same particle as anti-neutrino then close to power plant: • Reaction not observed: • Neutrino-anti neutrino not the same particle • Little bit of 37Ar observed: neutrino’s from cosmic origin (sun?) • Rumor spread in Dubna that reaction did occur: Pontecorvo hypothesis of neutrino oscillation -615 tons kitchen cleaning liquid -Typically one 37Cl  37Ar per day -Chemically isolate 37Ar -Count radio-active 37Ar decay e + 37Cl  e + 37Ar

  18. Flavour neutrino’s • Neutrino’s from π→+ identified as  • ‘Two neutrino’ hypothesis correct: e and  • Lederman, Schwartz, Steinberger (nobel prize 1987) “For the neutrino beam method and the demonstration of the doublet structure of the leptons through the discovery of the muon neutrino”

  19. LEP (1989-2000) • Determination of the Z0 line-shape: • Reveals the number of ‘light neutrinos’ • Fantastic precision on Z0 parameters • Corrections for phase of moon, water level in Lac du Geneve, passing trains,… Existence of only 3 neutrinos • Unless the undiscovered neutrinos have mass m>MZ/2

  20. Discovery of -neutrino (2000) DONUT collaboration Production and detection of -neutrino’s ct t t nT nt Ds nt

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