Muon decay (1930)

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)

Decay eletron track

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 )

( MeV c

m µ⁻ =

14

Pion and Muon decay sequence: a cascade of decays

Pion discovery (1947, Lattes, Powell Occhialini)

Muon decay

Nuclear Emulsion

) 10

6 . 2

( ⁻⁸ s

−

− →

µ ν τ

π

m(

π

) 10

2 . 2

( ⁶ s

e⁻ _e ⁻

− → ν ν τ = ×

µ _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

→ µ π

Pion – Muon

Leptons

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

< m

< m

•  Electron e

, muon µ

and tauon τ

have corresponding neutrinos:

ν

, ν

and ν

•  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

The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again.

•  Anti-leptons are positron e+, positive muons/tauons and anti-neutrinos

•  Neutrinos and anti-neutrinos differ by the lepton number.

  For leptons L

= 1 (a = e,µ or τ)   For anti-leptons L

= -1

•  Lepton numbers are conserved in any reaction

⎟⎟⎠

⎞

⎜⎜⎝

⎛

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⎟⎟⎠

⎞

⎜⎜⎝

⎛ ^{+ + +}

µ

ν

τ ν

µ ν

e

1 0

1

1 0

1

0 1

1

0 1

1

ν

µ ν

e

number muon

number electron

number lepton

Lepton

−

−

−

−

No n

e p

Yes n

p

No e

No e

p n

Yes e

p n

e e

+

→ +

+

→ +

+

→

+

→ +

+

→ +

+ +

−

−

−

−

µ µ

ν

µ ν

γ µ

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 e

A Z

N A

Z

N ( , ) → ( + 1 , ) +

+ ν

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

τ

= 2.2 x 10

s and τ

= 2.9 x 10

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%

ν

ν µ

→ e

+

+

τ µ

τ

ν ν

µ τ

ν ν

τ

+ +

→

+ +

→

−

−

−

−

) (

) (

b

e

a

assumptions : 1) 

1) 

Weak

interactions interactions of leptons are identical like of

leptons are identical electromagnetic ones like

( interaction universality ) electromagnetic ones (2) interaction One can can neglect neglect universality final final state ) lepton masses masses for for many basic calculations The decay rate for a muon given many basic

calculations

The decay rate for a muon is

given by:

Where G

is the Fermi constant

Substituting m

with m

one ,

for (a) and (b). It explains why obtains BR of (a) and (b) have same decay very close values

3 5 2

) 195

( µ → ^e + ν

+ ν

= ^G

^m π

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 decay

Here l mode, B= 1

in stands for µ and τ. case. Using experimental Since

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

•  Universality of lepton obtaines

the ratio of µ and τ lifetimes:

In no very

) (

) (

l e

l l e

e l

e l

B

ν ν

ν

τ

ν

−

−

→ Γ

= →

7 5

10 3 . 1 178

.

0 ⎟⎟ ≈ ⋅

⎠

⎞

⎜⎜ ⎝

⋅ ⎛

≈

τ µ µ

τ

τ τ

m

m

The tau search

CERN PS CERN PS

of the PAPLEP (Proton- AntiProton into LEpton Pairs)

It starts the search for the 3

sequential lepton family, a replica of the first two.

The “Heavy Lepton and its neutrino”

Searching for acoplanar lepton pairs of opposite charges

It starts the search for the 3

sequential lepton family, a replica of the

ν_HL 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 lepton 20% 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, collinears e⁺ + e⁻ → µ⁺ + µ⁻ 2 µ (penetrating) opposite sign, collinears e⁺ + e⁻ →π⁺ +π⁻ 2 π (hadrons) opposite sign, collinears e⁺ + e⁻ →π⁺ +π⁻ +π˚ 2 π (hadrons) opposite sign, non collinears

Signal

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

→ events

Observations:

Nuclear β-decay:

3

H →

He+e-

Energy

conservation

violated?

Pauli:

Pauli's letter of the 4th of D ecember 1930

Dear Radioactive Ladies and Gentlemen,

As the bearer of these lines, to whom I graciously ask you to listen, will explain to you in more detail, how because of the "wrong" statistics of the N and Li⁶ nuclei and the continuous beta spectrum, I have hit upon a deseperate remedy to save the "exchange theorem" of statistics and the law of conservation of energy.

N am ely, the possibility that there could exist in the nuclei electrically neutral particles, that I w ish to call neutrons, w hich hav e spin 1/2 and obey the exclusion principle and which further differ from light quanta in that they do not travel with the velocity of light. The mass of the neutrons should be of the same order of magnitude as the electron mass and in any event not larger than 0.01 proton masses. T he continuous beta spectrum w ould then becom e understandable by the assum ption that in beta decay a neutron is em itted in addition to the electron such that the sum of the energies of the neutron and the electron is constant...

…

U nfortunately, I cannot appear in T ubingen personally since I am indispensable here in Z urich because of a ball on the night of 6/7 D ecem ber. With my best regards to you, and also to Mr Back.

Your humble servant . W. Pauli

Pauli’s hypothesis

Fermi theory of β decay

•  What is a β-decay ? It is a neutron decay:

•  Necessity of neutrino existence comes from the apparent energy and angular momentum non-conserva=on in observed reac=ons •  For the sake of lepton number conserva=on, electron must be accompanied by an an=-neutrino and not a neutrino!

•  Mass limit for can be es=mated from the precise measurements of the β-decay:

•  Best results are obtained from tri=um decay

it gives (~ zero mass)

e

p

n → +

+ ν

ν

m

M E

m

−

Δ

≤

≤

e

He

H →

+

+ ν

3

/

2 eV c

m

≤

•  The most powerful available sources of neutrinos, before the construc=on of protosinchrotrons (60) were the nuclear reactors.

•  By the processes of fission ν

are produced with a spectrum of energies of a few MeV. A few tens of meters from the core of a reactor of 1 GW, the flow is enormous Φ ≈ 10

m

s

•  Electronic neutrinos and an=neutrinos can be revealed through the electronic "inverse beta decay", but the cross sec=on is microscopic

Electron Neutrino detection

σ ν

(

)

⁽

⁾

Electron Neutrino detection

σ ν (

+ p → e

+ n ) ^{≈ 10}

^{( E}

^{/ MeV )}

^m

•  Rate for p target E

= 1MeV W

= Φσ ≈ 10

s

•  So, for a total rate of: W = 10

Hz ⇒ N

= 10

•  If the target is made of H

O (10 p), in a mole (18 g) there are N

10/18

= 3.3 10

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

γ γ ν

+

→ +

+

→ +

− +

+

e e

e n

e p

by followed

Power plant 0.7 GW

(Savannah river plant USA)

Producing ν

Scheme of the Reines and Cowan experiment

2m

2m

Target = 200 l of H

O

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.

H

O is a good moderator and in a few tens of µs a neutron is

thermalized.

H

O 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

+ γ

1962. 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 the passage of the particle and the high-voltage signal is less than the mem- ory time of about 100µs, the efficiency of the spark chamber is close to 100%. A clearing field, of course, removes also the primary ionisation from the detector volume. For this reason the time delay between the passage of the particle and the application of the high-voltage signal has to be chosen as short as possible to reach full efficiency. Also the rise time of the high-voltage pulse must be short because otherwise the leading edge acts as a clearing field before the critical field strength for spark formation is reached.

Figure6.12shows the track of a cosmic-ray muon in a multiplate spark chamber [5,44].

If several particles penetrate the chamber simultaneously, the proba- bility that all particles will form a spark trail decreases drastically with increasing number of particles. This is caused by the fact that the first spark 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 are mounted in front of the metallic electrodes which prevent a high-current spark discharge. In such glass spark chambers a high multitrack efficiency can 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 as packing material. Because they have not been made for particle tracking, their structure is somewhat irregular, which can clearly be seen [37].

spark gap

R_L

scintillator photomultiplier

photomultiplier scintillator

particle trajectory coincidence

20kV

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 counting gas. Alternatingly, the plates are either grounded or connected to a high- voltage supply (Fig. 6.11). The high-voltage pulse is normally triggered to every second electrode by a coincidence between two scintillation coun- ters placed above and below the spark chamber. The gas amplification is chosen in such a way that a spark discharge occurs at the point of the passage of the particle. This is obtained for gas amplifications between 10⁸ and 10⁹. For lower gas amplifications sparks will not develop, while for larger gas amplifications sparking at unwanted positions (e.g. at spac- ers which separate the plates) can occur. The discharge channel follows the electric field. Up to an angle of 30^◦ the conducting plasma chan- nel can, however, follow the particle trajectory [8] as in the track spark chamber.

•  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 10

and 10

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: n

and n

Muon Neutrino detection

How do electrons look like

Exposure of the chambers at the 400 MeV electron beam at

Cosmotron

Muon Neutrino detection

39

) ,

,

( u u d p =

) , ,

( u d d n =

+

−

e e ,

+

−

µ µ ^,

) , ( ,

) ,

( u d = d u

=

+

π

π

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

37 18 37

17 → +

+

+

→ +

+

→ +

−

− + +

e

p e

n

n e

p

e e e

ν ν ν

ν _e + ³⁷ Cl → e ^- + ³⁷ Ar

-615 tons kitchen cleaning liquid -Typically one

Cl →

Ar/day -Chemically isolate

Ar

-Count radio-active

Ar decay

•  Reaction not observed:

–  Neutrino-anti neutrino not the same particle

–  Little bit of

Ar 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 ν

–  ‘Two neutrino’ hypothesis correct: ν

and ν

–  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

ν_τ ν_τ

τ

cτ

ν_Τ

Δ_s

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 ( ν

^, ν

^, ν

⁾ 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 Z

line- shape:

Reveals the number of ‘light neutrinos’

Fantastic precision on Z

parameters

Corrections for phase of moon, water level in Lac du Geneve, passing

trains,…

LEP (1989-2000): the 3 neutrino families

N_ν 2.984±0.0017

M_Z0 91.1852±0.0030 GeV Γ_Z0 2.4948 ±0.0041 GeV

Existence of only 3 neutrinos

Unless the undiscovered neutrinos

have mass m

>M

/2