Leptons

Known particles at the end of the 30’s •  Electron •  Proton •  Photon •  Neutron •  Positron •  Muon

•  Pion

Neutrino: a particle whose existence was hypotesized without a discovery!

Faraday, Goldstein, Crookes, J. J Thomson (1896)

Avogadro, Prout (1815)

Einstein (1905), Compton (1915)

Chadwick (1932)

Conventional birth date of Nuclear Physics

Anderson (1932)

Cosmic rays interaction studies. Pion/Muon separation

Discovery of first elementary particles

Muon decay (1930)

Decayeletrontrack

The mesotron puzzle..

Conversi, Pancini, Piccioni (1947) experiment

Conversi, Pancini, Piccioni (1947) experiment

Conversi, Pancini, Piccioni (1947) experiment

Conversi, Pancini, Piccioni (1947) experiment

Conversi, Pancini, Piccioni (1947) experiment

Conversi, Pancini, Piccioni (1947) experiment

Pion discovery

Pion discovery

2/7.105)( cMeVm =−µ

14

Pion and Muon decay sequence: a cascade of decays

Pion discovery (1947, Lattes, Powell Occhialini)

Muon decay

Nuclear Emulsion

)106.2( 8 s−−− ×=→ τνµπ µ

m(π − ) = 139.6 MeV / c2

)102.2( 6 se e−−− ×=→ τννµ µ

In all these decays, neutrinos are emitted !

Muon decay scheme

15

Pion – Muon

The pion in term of quarks

Experimental strategy: Exposure of Emusions to Cosmic Rays

e→→ µπ

Pion – Muon

Leptons •  Leptons are s = ½ fermions, not subject to strong interactions

me < mµ < mτ

•  Electron e-, muon µ- and tauon τ- have corresponding neutrinos:

νe, νµ and νt •  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

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

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛ +++

τµ ν

τν

µ

ν e

e

101101011011

µνµν e

enumbermuonnumberelectronnumberleptonLepton

−−−−

Nonep

YesnpNoeNoepnYesepn

e

e

+→+

+→+

+→

+→+

+→+

+

+

−−

−

−

µ

µ

ν

µν

γµ

ν

ν

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

•  First indication of neutrino existence came from β-decays of a nucleus N

eeAZNAZN ν+++→ −),1(),(

•  Electron is a stable particle, while muon and tauon have a finite lifetime:

τµ = 2.2 x 10-6 s and ττ = 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%

µννµ ++→ −−ee

τµ

τ

ννµτ

νντ

++→

++→−−

−−

)(

)(

bea e

assumptions

:

1)  1) 

Weak

interactions of leptons are interactions identical like

of

leptons electromagnetic are identical ones like( interaction electromagnetic universality ones)

(2) interactionOne can universalityneglect ) final

can neglect masses for final many state lepton basic masses calculations for The manydecay rate for a muon basic given

calculations

The decay rate for a muon

is given by: Where GF is the Fermi constant Substituting mµ with mτ one ,

for (a) and (b). It obtainsexplains why

BR of (a) and (b) havesame very closedecay values

3

52

195)(

πννµ µµ

mGe F

e =++→Γ −−

Using the decay rate, the

Using the

decay lifetime of a lepton

is Here l stands : for µ and τ. Since have basically one decayHere l mode, B= 1 in stands for µ and τ. case. Using experimental

Since

good agreement with valuesindependent experimental of B and formula for Γ, measurements one

•  Universality obtaines of lepton the ratio of µ and τ lifetimes: In no

very

)()(le

lel el

elBνννν

τ −−

−−

→Γ→

=

75

103.1178.0 −⋅≈⎟⎟⎠

⎞⎜⎜⎝

⎛⋅≈

τ

µ

µ

τ

ττ

mm

The tau search CERN PS

CERN PS

of the PAPLEP (Proton-AntiProton into LEpton Pairs)

It starts the search for the 3rd sequential lepton family, a replica of the first two.The “Heavy Lepton and its neutrino”

Searching for acoplanar lepton pairs of opposite chargesIt starts the search for the 3rd sequential lepton family, a replica of the

νHL

HL"

#$%

&'

The tau discovery The tau was then

was

reaction searched for by Zichichi in 1967 in the e+ e–→τ+ τ– reaction at the ADONE ring in Frascati which did not have enough of the new lepton

energy

The maximum ADONE energy was √s=3

to produce a

GeV, below the threshold for pair– production √s=3.554 of the new lepton20% less

.

!!) A lower limit for the heavy lepton (HL) mass was obtained

Simplfied from

Nuovo Cimento 17A (1973) 383

HL is here

The tau discovery 1971. M. Pearl e co. same idea at SPEAR (e+e- with E= 8 GeV) 1975. τ discovery with the Mark I experiment

Common processes

e+ + e− → e+ + e− 2 e (showers) opposite sign, collinearse+ + e− → µ+ + µ− 2 µ (penetrating) opposite sign, collinearse+ + e− → π + +π − 2 π (hadrons) opposite sign, collinearse+ + e− → π + +π − +π ˚ 2 π (hadrons) opposite sign, non collinears

Signal e+ + e− → τ + +τ − τ + → e+ +ν 's / τ − → µ− +ν 's e+ + e− → τ + +τ − τ + → µ+ +ν 's / τ − → e− +ν 's

Topology: eµ pair of opposite sign, non collinears Background: non-identified hadrons

e

µ

1977. PLUTO and DASP @ DESY confirm discovery 1976. HL is called τ from τριτον, the third (P. Rapidis)

Neutrinos: the crisis around 1930 •  Matter is made of:

–  Particles: γ, e-, p –  Atoms: Small nucleus of

protons surrounded by a cloud of electrons

before Pauli: Unique electron

energy?

Experimental electron energy

→ electron energy

→ e

vent

s

Observations: Nuclear β-decay:

3H →3He+e-

Energy conservation violated?

Pauli: Variable electron energy!

Pauli's letterofthe4 thofD ecember1930

DearRadioactiveLadiesandGentlemen,

Asthebeareroftheselines,towhomIgraciouslyaskyoutolisten,willexplaintoyouinmoredetail,howbecauseofthe"wrong"statisticsoftheNandLi6nucleiandthecontinuousbetaspectrum,Ihavehituponadeseperateremedytosavethe"exchangetheorem"ofstatisticsandthelawofconservationofenergy. Namely,thepossibility thattherecouldexist in thenucleielectr ically neutralparticles,thatI w ishtocallneutrons,w hichhavesp in1/2 andobey theexclusionprinciple andwhichfurtherdifferfromlightquantainthattheydonottravelwiththevelocityoflight.Themassoftheneutronsshouldbeofthesameorderofmagnitudeastheelectronmassandinanyeventnotlargerthan0.01protonmasses.T hecontinuousbetaspectrumw ould thenbecomeunderstandablebytheassumptionthat in betadecayaneutronisemitted in additiontotheelectron suchthat thesumoftheenergiesoftheneutronandtheelectron isconstant... … Unfortunately, I cannotappear in T ubingenpersonally sinceI am indispensablehereinZ urichbecauseofaballonthen ightof6/7 D ecember. Withmybestregardstoyou,andalsotoMrBack.Yourhumbleservant.W.Pauli

Pauli’s hypothesis

Fermi theory of β decay

•  Whatisaβ-decay?Itisaneutrondecay:

• Necessityofneutrinoexistencecomesfromtheapparentenergyandangularmomentumnon-conserva=oninobservedreac=ons• Forthesakeofleptonnumberconserva=on,electronmustbeaccompaniedbyanan=-neutrinoandnotaneutrino!• Masslimitforcanbees=matedfromtheprecisemeasurementsoftheβ-decay:

• Bestresultsareobtainedfromtri=umdecay

itgives(~zeromass)

eepn ν++→ −

eν

emMEm Nee ν

−Δ≤≤

eeHeH ν++→ −33

2/2 ceVme≤ν

•  Themostpowerfulavailablesourcesofneutrinos,beforetheconstruc=onofprotosinchrotrons(60)werethenuclearreactors.

•  BytheprocessesoffissionνeareproducedwithaspectrumofenergiesofafewMeV.Afewtensofmetersfromthecoreofareactorof1GW,theflowisenormous Φ ≈1017m-2s-1

•  Electronicneutrinosandan=neutrinoscanberevealedthroughtheelectronic"inversebetadecay",butthecrosssec=onismicroscopic

Electron Neutrino detection

σ νe + p→ e+ + n( ) ≈10–47 Eν / MeV( )2 m2

Electron Neutrino detection σ νe + p→ e+ + n( ) ≈10–47 Eν / MeV( )2 m2

•  Rate for p target Eν= 1MeV W1=Φσ ≈ 10–30 s–1 •  So, for a total rate of: W = 10–3 Hz ⇒ Np = 1027 •  If the target is made of H2O (10 p), in a mole (18 g) there are NA 10/18 = 3.3 1023 protons •  So, one needs about 3000 moles ⇒ 50 kg • Detection efficiency, fiducial volume/total volume. Lets’ put ≈ 1/4 ⇒ Total mass needed ≈ 200 kg

The main problem is not the needed mass (albeit this was remarkable in 1958), but the control of the ”backgrounds” : •  n from the reactor •  background induced by cosmic rays •  natural radioactivity

Electron Neutrino detection (1956) •  Cowan & Raines

–  Cowan nobel prize 1995 with Perl (for discovery of τ-lepton)

•  Intense neutrino flux from nuclear reactor

•  Inverse β decay

γγ

ν

+→+

+→+

−+

+

ee

enpeby followed

Power plant 0.7 GW (Savannah river plant USA) Producing νe

Scheme of the Reines and Cowan experiment 2m

2m

Target = 200 l of H2O e+ immediately annihilates in two γ’s at 180 ˚ between them, which go in two different containers of liquid scintillator adjacent. Compton electrons produce a flash of light. H2O is a good moderator and in a few tens of µs a neutron is thermalized. H2O doped with 40 kg of Cd which has a large cross section for capture of thermal n. The retarded γ’s are revealed in the scintillator. Detector at 10 m below a building (cosmic) + lot of care in shielding

•  Observed: 3±0.2 events/h •  Background ⇒ small •  Cross section ≈ expected value

“Neutrino” detected, finally

6-Nov-17

Science 124 (1956) 104

We now know it was electron antineutrino

Muon Neutrino detection 1959. B. Pontecorvo (in Russia) and M. Schwartz (in US) proposed independently the use of neutrino beams produced in accelerator (they show that intensities should be enough). Which neutrinos:

π + → µ+ +ν? π − → µ− +ν?

1960. Lee e Yang. Should be different from the electron neutrino otherwise:

µ± → e± + γ1962. Schwartz, Lederman, Steinberger experiment. The proton beam extracted from the AGS at BNL is sent against a target. Hadrons and µ are filtered by13.5 m of Fe and neutrons with paraffin

Muon Neutrino detection

172 6 Historical track detectors

Between two discharges the produced ions are removed from the detec-tor volume by means of a clearing field . If the time delay between thepassage of the particle and the high-voltage signal is less than the mem-ory time of about 100 µs, the efficiency of the spark chamber is close to100%. A clearing field, of course, removes also the primary ionisation fromthe detector volume. For this reason the time delay between the passageof the particle and the application of the high-voltage signal has to bechosen as short as possible to reach full efficiency. Also the rise time ofthe high-voltage pulse must be short because otherwise the leading edgeacts as a clearing field before the critical field strength for spark formationis reached.

Figure 6.12 shows the track of a cosmic-ray muon in a multiplate sparkchamber [5, 44].

If several particles penetrate the chamber simultaneously, the proba-bility that all particles will form a spark trail decreases drastically withincreasing number of particles. This is caused by the fact that the firstspark discharges the charging capacitor to a large extent so that less volt-age or energy, respectively, is available for the formation of further sparks.This problem can be solved by limiting the current drawn by a spark.In current-limited spark chambers partially conducting glass plates aremounted in front of the metallic electrodes which prevent a high-currentspark discharge. In such glass spark chambers a high multitrack efficiencycan be obtained [45, 46].

Fig. 6.12. Track of a cosmic-ray muon in a multiplate spark chamber [44].

6.5 Spark chambers 171

Fig. 6.10. Single muon track in a stack of polypropylene-extruded plastic tubes.Such extruded plastic tubes are very cheap since they are normally used aspacking material. Because they have not been made for particle tracking, theirstructure is somewhat irregular, which can clearly be seen [37].

sparkgap

RL

scintillatorphotomultiplier

photomultiplier scintillator

particle trajectory

coincidence

20 kV

C

R

discriminators

Fig. 6.11. Principle of operation of a multiplate spark chamber.

In a spark chamber a number of parallel plates are mounted in a gas-filled volume. Typically, a mixture of helium and neon is used as countinggas. Alternatingly, the plates are either grounded or connected to a high-voltage supply (Fig. 6.11). The high-voltage pulse is normally triggeredto every second electrode by a coincidence between two scintillation coun-ters placed above and below the spark chamber. The gas amplification ischosen in such a way that a spark discharge occurs at the point of thepassage of the particle. This is obtained for gas amplifications between108 and 109. For lower gas amplifications sparks will not develop, whilefor larger gas amplifications sparking at unwanted positions (e.g. at spac-ers which separate the plates) can occur. The discharge channel followsthe electric field. Up to an angle of 30◦ the conducting plasma chan-nel can, however, follow the particle trajectory [8] as in the track sparkchamber.

•  A number of parallel plates are mounted in a gas filled volume (typically, a mixture of He and Ne)

•  Plates are alternatively connected to ground and to a high voltage supply •  The high-voltage pulse is triggered by a coincidence between two

scintillation counters placed above and below the spark chamber •  Gas amplification between 108 and 109 results in a spark discharge along

the trajectory of the particle.

a muon track

6-Nov-17

•  34 “single muon” events observed

•  Additional 8 events compatible with background

•  No electron observed

•  Conclusion: the neutrino that is born together with a µ in the π decay when interacts produce a µ, not e.

•  Two different conserved quantities exist, lepton flavours: ne and nµ

Muon Neutrino detection

How do electrons look like

Exposure of the chambers at the 400 MeV electron beam at Cosmotron

Muon Neutrino detection

39

),,( duup=

),,( ddun=

+− ee ,+− µµ ,

),(,),( uddu == −+ ππ

γ

ν

Leptons (heavier copies of the electron)

The photon

The neutrino, postulated to explain beta decay and observed in inverse beta decay, is always associated to a charged lepton.

The hadrons, particles made up of quarks and obeying mainly to strong nuclear interaction

Classification of elementary particles

Anti-neutrino’s vs neutrino’s

•  Davis & Harmer –  If the neutrino is same

particle as anti-neutrino then close to power plant:

Ar Cl

3718

3717 +→+

+→+

+→+

−

−

++

e

pen

nep

e

e

e

ν

ν

ν

νe + 37Cl → e- + 37Ar

-615 tons kitchen cleaning liquid -Typically one 37Cl → 37Ar/day -Chemically isolate 37Ar -Count radio-active 37Ar decay

•  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

Nobel prize 2002

(Davis, Koshiba and Giacconi)

Flavour neutrino’s

•  Neutrino’s from π→µ+ν identified as νµ

–  ‘Two neutrino’ hypothesis correct: νe and νµ

–  Lederman, Schwartz, Steinberger (nobel prize 1988)

“For the neutrino beam method and the demonstration of the doublet structure of the leptons through the discovery of the muon neutrino”

Discovery of τ-neutrino (2000) DONUT collaboration Production and detection of τ-neutrino’s

τ

ντ

ντ

τ

cτ

νΤΔs

Neutrino flavours

•  Neutrinos cannot be directly detected

•  The charged lepton produced by the neutrino interaction in the detector identifies the neutrino flavour

Neutrino flavour CHANGES In the last 15 years we learnt that neutrino change flavour, provided time (flight distance) is given them to do so

•  Oscillations and flavour conversion in matter, prove that neutrinos, contrary to the Standard model have non-zero mass

•  Flavour states are superposition (mixing) of mass eigenstates

Flavour Mass Lifetime

e 0.5 MeV ∞

µ 106 MeV 2.2 µs

τ 1777 MeV 0.29 ps

We observed three couples of leptons (tre “families”, “generations”) One lepton is charged (e–, µ–, τ– ), the other is “its” neutrino (νe, νm, νt) e–, µ– e τ– have all the same characteristics, except for the mass Charged leptons makes gravitational, electromagnetic and weak interactions. Neutrinos makes gravitational and weak interactions.

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,…

LEP (1989-2000): the 3 neutrino families

Nν 2.984±0.0017

MZ0 91.1852±0.0030 GeV

ΓZ0 2.4948 ±0.0041 GeV

Existence of only 3 neutrinos Unless the undiscovered neutrinos have mass mν>MZ/2