1. Leptoni

2. Fermions: the elementary players
3rd generation
2/3
-1/3 -1
Why 3 families? Are there more?
The elementary particle families: fermions
1st generation
2nd generation
2/3
-1/3 -1
Quarks
Leptons and quarks form doublets under weak interactions
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 me < mm < mt
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 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. Flavour
Mass e
0.511 MeV m
MeV t
1777 MeV

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

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

13. 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)

14. Neutrino’s detected… (1956)
Cowan & Reines
Cowan nobel prize 1988 with 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+ 

15. 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

16. Scheme of the Reines and Cowan experiment
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

17. Helicity states For a massless fermion of positive energy, E = |p|
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

18. (Davis, Koshiba and Giacconi)
Anti-neutrino’s
Nobel prize 2002
(Davis, Koshiba and Giacconi)
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

19. 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”

20. 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,… N
2.984±0.0017
MZ0
 GeV
Z0
 GeV
Existence of only 3 neutrinos
Unless the undiscovered neutrinos have mass m>MZ/2

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