historical introduction to elementary particles: leptons...

PHY771, 1/21/2016 Tomasz Skwarnicki 1

Historical introduction to Elementary Particles:

Leptons and Weak interactions

Tomasz SkwarnickiSyracuse University

• Griffiths, 2nd ed., 1.3-1.5,1.10


Yukawa’s prediction of a meson. Lepton, meson, baryon terminology.

• Protons and neutrons are held together in nuclei by a force much stronger than electrostatic repulsions of protons

• Yukawa in 1934:

– Strong interactions must be mediated by a particle

– Since strong forces drop quickly to zero outside the nucleus,

the strong force carrier must have a large mass

– From the size of a nucleus he estimated the mass of the

particle exchanged by nucleons to be between the electron

and proton masses (~100 MeV/c2)

– As middle-weight it got labeled “meson” , in between light-

weight electron (“lepton”) and heavy-weight proton (“baryon”)

Hideki Yukawa 1907-1981

Japan

Nobel Prize 1947

• Lepton-meson-baryon terminology has survived, but its meaning has changed

since then:

– Leptons are spin ½ particles which don’t participate in strong interactions,

which have no substructure (like electron)

– Baryons are half-integer spin (1/2 , 3/2, … ) particles which do interact strongly,

made out of 3 quarks (like proton, neutron); [also pentaquarks?]

– Mesons are integer spin particles (0, 1, 2, … ) which do interact strongly,

made out of quark anti-quark pair (like Yukawa meson) [also tetraquarks?]


Muon discovery 1937

• Anderson (positron discovery in 1932) and Nedermeyer discovered a mid-weight particle in cosmic rays in 1937 by having curvatures in magnetic field “in between” electron and proton

• Initially it was identified with Yukawa meson, thus called “mu meson” (later “muon”)

Anderson and Nedermeyer at Caltech with magnetic cloud chamber

• However, it was later shown (1946) that majority of cosmic mu mesons did not interact with nuclei – they were very penetrating

– Therefore, they were not Yukawa mesons!

– In fact, muons are leptons; a heavier version of electrons.

• To this day we don’t understand why nature needs muons.

• Muon discovery was the first time “second generation” particles were observed. However, people did not realize that right away because of the case of mistaken identity.

• To this day some old-timers call “muon” a “meson”. This is terribly wrong in today’s terminology since muon is a lepton.

Famous quote from

Isidor Rabi


Pion discovery 1946

• C.F.Powell developed a method of detecting cosmic rays in photographic emulsion and found one middle-weight particle decaying to another (1946):– There were two middle-weight

particles in cosmic rays! They were named pion (π) and muon (µ)

– muons are by far more common in cosmic rays at the ground level (it is the other way round in upper parts of the atmosphere)

– Pion is the particle Yukawa had predicted

– Yukawa received Nobel prize in 1947, Powell in 1950

Cecil Frank Powell 1903-1969England

π−

µ−

e−

Particle Mass

MeV/c2

e 0.5

µ 105.7

π− 139.6

p 938.3

n 939.6

Why kinks?

See next


Neutrinos and weak interactions • In 1930 electron energy spectrum in radioactive β decay was measured and

indicated that something else was also produced which was not visible as rays

Expected for

2-body decayZP → Z+1D e-

Expected for

3-body decayZP → Z+1D e- x

Since the endpoint of the observed

electron spectrum comes close to the 2-

body expectations, the missing particle

must be very light mx~0

The suggestion that an undetected

neutral particle was present came from

Pauli.

In 1933 Enrico Fermi published theory of βdecays and called the particle invented by

Pauli – neutrino (ν) i.e. “little neutral one”.

Z Z+1x

In Fermi theory (“4-fermion interactions”), β decays are mediated by a new type of force

called “weak”. Neutrinos are almost mass-less leptons (spin ½), which have only weak

charge. Participating only in weak interactions, neutrinos are super penetrating and

escape detection. Underlying process in β decay is n → p e- ν.


Back to Powell’s observation (1946)

π−

µ−

e−

ν

π− → µ− ν

The observed µ− energy

spectrum indicated 2-body

decay

The observed e− energy

spectrum indicated 3-body

decay

µ− → e− ν νSince neutrinos are present,

these are both weak decays.

In fact, these are dominant

decays of charged pions and

muons.

Since these are weak decays,

charged pion, muon and (free)

neutron are relatively long lived

– travel many meters before

decaying.

Proton and electron are

absolutely stable and don’t

decay.

Neutrons are often absolutely

stable in nuclei if their decay

would violate energy

conservation in nuclear βdecay.

ν

ν

Lifetime

τ cτ

e >5x1026 y

µ 2.2x10-6 s 658 m

π− 2.6x10-8 s 7.8 m

p >2x1029 y

n 880 s 2.6x1011 m

Actual average

decay paths are

longer than cτ and

depend on exact

velocity because

clock is ticking

slower for

relativistic

particles.

Average

decay time

for a particle

at rest.

π,µget stopped

by emulation

and decay at

rest


Direct observation of neutrino 1956

• Until 1950 neutrinos were observed only indirectly by their effects in decays of other particles – some people remained skeptical about their existence

• Neutrinos are so penetrating that they can easily pass a shield as thick as thousand light years. The only chance to detect them is to have a very intense source of them.

• Neutrinos are copiously produced in nuclear reactions.

• Cowan and Reines set up apparatus near Savannah River reaction in South Carolina with neutron flux 5x1013 ν/cm2/s and observed νννν pppp→ n eeee++++

(inverse of neutron β decay) in 1956. This firmly established existence of neutrions.

Fred Reines (1918-1998)

and

Clyde Cowan (1919-1974)

Reines received Nobel Prize in 1995

Sun is a distant, but a huge nuclear reactor.

It produces ~1011 ν/cm2/s at Earth’s surface.

They constantly pass through you …

Nowadays there are experiments studying

solar neutrions.


Antineutrinos • In cosmic rays both negative and positive pions (and muons) were observed.

• Like electrons and positrons, these are particles and antiparticles to each other.

• Neutral particle can be its own antiparticle (e.g. photon), but not necessarily (antineutron is different from neutron)

• Is there an antineutrino?

π− → µ− νπ+ → µ+ ν

β− decay: n → p e- νβ+ decay: p → n e+ ν

(cannot happen for a free

proton, but it does happen for

protons in some nuclei!)

Is one of these antineutrino ?

Crossing symmetry

If

AB → CD

happens, then cross-reactions

A → BCD

AC → BD

CD → AB

…

can also happen

(if kinematically allowed)

Suppose n → p e- ν i.e. reactors produce antineutrinosCowan and Reines:

νννν pppp→ n eeee++++

This cross-reaction must exist: νννν nnnn→ p eeee−−−−

But if neutrino is the same as antineutrino then reactor experiments must also show:

νννν nnnn→ p eeee−−−−

Not observed! (A. Davies in 1956)

Antineutrinos exist!


Lepton number and its conservation

• Konopinski and Mahmoud 1953 defined a lepton number (L):– L(lepton) = +1

– L(antilepton)= -1

– L(not lepton)= 0

• and suggested that total lepton number is a conserved quantity (like electric charge conservation) :– Σi Li = const

• Reaction sought by Davies would violate this conservation law:


Σi Li 1+0=1 0+1=1


Σi Li -1+0=-1 0+1=1

Implications for other decays:

π− → µ− νπ+ → µ+ ν

Σi Li 0 1+(-1)=0

µ− → e− ν ν

Σi Li 1 1+1+(-1)=1


Conservation of electron and muon numbers

• Allowed by lepton number conservation, but not observed (we are still looking for it…)

• This leads to a postulate that there are distinct electron neutrino and muon neutrinos, and that lepton numbers defined separately for electron and muon families are conserved

µ− → e− γ

Σi Li 1 1+0=1

µ− → e− γ

Σi Lµ i 1 0+0=0

Σi Le i 0 1+0=1

µ− → e− νe νµ

Σi Lµ i 1 0+ 0 +1=1

Σi Le i 0 1+(-1)+0=0

OK


Confirmation of two-neutrino hypothesis 1962

• Hypothesis that there are two distinct neutrinos:

electron neutrino and muon neutrino (each with

its own antiparticle) was confirmed experimentally

by Lederman–Schwartz–Steinberger in 1962:

π− → µ− νµ

An accelerator based experiment at Brookhaven (NY), in which muon

anti-neutrino beam was produced via:

and directed into proton-reach target:

ννννµµµµ pppp→ n µµµµ++++

ννννµµµµ pppp→ n eeee++++

Observed

Not observed


Tau lepton 1974-77

• Discovered at SPEAR collider at SLAC with LBL detector

τ− → µ− νµ ντ

τ+ → e+ νe ντ

e+ e−→ τ+ τ−

Martin Perl

1927 –

US

Nobel Prize1995

• Yet much heavier version of electron

• Much shorter lifetime

• Comes with its own neutrino. Tau lepton number is a conserved quantity.

• Tau lepton discovery was the first time “third generation” particles were observed.

• The same puzzle as with muon – who ordered that?


Intermediate boson of weak interactions – W±

• In Fermi theory of weak interactions, four-fermions had contact interactions.

• Consistent formulation of quantum field theory for weak interactions proved theoretically more difficult than for electromagnetic (QED), but eventually was accomplished …

• It gave a good quantitative description of the data on weak interactions, but had a divergent behavior when extrapolated to reactions at very high energies

• This could be fixed by introduction of a heavy, spin-1 (i.e. “vector boson”), charged weak-force carrier (W-)


• Consistent quantum field theory of weak interactions by

Glashow-Weinberg-Salam (~1961-1968) made a number of

bold predictions:

– In addition to weak interactions mediated by W± (“charged currents”)there should also be a heavy, neutral intermediate vector boson of weak interactions – Z0 (“neutral currents”)

– Weak and electromagnetic interactions are different aspects of “electroweak” interactions (one theory describes both).

– What makes W±,Z0 have large masses (~ 90 x proton mass), while γhas no mass, are interactions with a scalar (spin 0) Higgs field, which should also manifest itself in a form of a real scalar particle H0

– Given various experimental constraints on the parameters of this theory, W and Z0 masses were precisely predicted. Predictions for H0

mass were very uncertain.

Electroweak theory - W±,Z0,H0


Experimental confirmation of neutral weak currents

• Neutrino scattering on protons observed at CERN

in Gargamelle bubble chamber in 1973

Glashow-Weinberg-Salam get Nobel Prize in 1979

e- e-

ννννµµµµ eeee---- → ννννµµµµ eeee----


Observation of W±,Z0 1983

• Observed at Super Proton Synchrotron (SPS) at CERN in 1983 with masses as predicted

Carlo Rubbia

1934 –

Italy

Nobel Prize

1984

WWWW---- → eeee---- ννννeeee

ZZZZ0000 → eeee+ + + + eeee----

Simon van der Meer

1925 –2011

Dutch


Higgs discovery - H0 2012

• Observed at Large Hadron Collider at CERN in 2012 with mass close to the lower end of the predicted range

Peter Higgs

1929 –

UK (Edinburgh)

Nobel Prize 2013

Francois Englert

1932 –

Belgium

HHHH0000 → γγγγγγγγ

HHHH0000 → µµµµ++++µµµµ−−−−µµµµ++++µµµµ−−−−

historical introduction to elementary particles: leptons...

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