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Elementarteilchenphysik
Antonio Ereditato
University of Bern
historical development: discoveries of new particles
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The atom (around 1905)
Before ~1905, nobody really knew:
“ What does the inside of an atom look like ? ”
Thomson atom
Early
“plum-pudding”
model
Positive
charge
(uniformly
distributed)
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corpuscles
(electrons)
The positive charge is spread
out like a “plum-pudding”
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1906-1911
Electromagnetism predicted that the
heavy α particles should be only slightly
deflected by the “plum-pudding”
atom…but, contrary to expectations,
large scattering angles were detected:
Probability < 10-10 !
α
The atom must have a solid core capable of applying large
electric forces onto an incoming (charged) particle. The atom
is not an ‘elementary particle’
Ernest Rutherford
1871-1937
α
α
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Interpretation of the Rutherford
experiment
(discovery of the atomic nucleus)
soon followed by the Bohr atom
α
α
α
α
α
Electrons
~10-13 cm
Nucleus
Volume Nucleus = 4/3π (10-13)3 ~ 4 x 10-39 cm3
Volume Atom = 4/3π (10-8)3 ~ 4 x 10-24 cm3
~10-8 cm
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Fraction ~ 10-15
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The atomic nucleus
In addition to the “discovery” of the nucleus and of the proton, Rutherford also noted the need of
a “neutral” particle in the atomic nucleus, due to the disagreement between the atomic number of
an atom (number of positive charges) and its mass computed in atomic mass units.
In 1920 Rutherford proposed that the difference could be explained by the existence of a
neutrally charged particle (the neutron) within the atomic nucleus. This was conceived as a
“couple” of an electron orbiting around a proton.
As an example, the nitrogen nucleus (14N) would be composed of 14 protons and 7 (nuclear)
electrons. 7 more “external” electrons would have built up the nitrogen atom.
This hypothesis worked for a while, but was eventually questioned with the advent of quantum
mechanics (~1930). The Heisenberg principle: Δx⋅Δp ≥ ħ led to an energy of ~100 MeV for
nuclear electrons, larger than the nuclear binding energy and larger than the β electron energy.
More quantitative objections came from detailed QM calculations and the Pauli principle.
1932: Chadwick discovered the “neutron” and, soon after, Heisenberg worked out a nuclear
model with protons and neutrons.
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History: discovery of particles coming somewhere from the
Universe and hitting the Earth
~1900: charged electroscopes discharge with time, even if
accurately shielded from radioactive sources (ambient
radioactivity)
One would expect a strong reduction of this radioactivity with
increasing altitude (gamma absorption: 50% of the radiation to
survive after 80 m, 0% at the 330 m of the Eiffel Tower)
1910: Jesuit T. Wulf measured the radiation on top of the Eiffel
Tower still measuring a 60% factor
1912: Victor Hess discovered that
there are “particles” coming from the
outer space. The ionization even
increases with height!
Hess balloon flights: up to 5300 m
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Cosmic-ray spectrum
Direct measurements
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Indirect (ground based detectors)
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1912-1925: development of the first particle detectors
discovery of new particles
with cosmic-rays
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High energy cosmic-rays:
discovery of new particles
1932: Discovery of the antiparticle of the electron,
the positron (Anderson). Confirmed the existence
and prediction that anti-matter does exist (Dirac).
1936: Discovery of the muon
(Anderson and Neddermeyer, confirmed by Street and
Stevenson).
It’s very much like a “heavy electron”.
1947: Discovery of the pion (Powell).
Hadron lighter than the proton.
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Evidence for antimatter
Anderson’s experiment: cloud chamber (discovery of the positron)
Carl Anderson
1936 Nobel Prize
Direction of deflection inside a
magnetic field: it is a positively
charged particle
Momentum measured before and after the
6 mm thick lead plate:
pb = 63 MeV, pa = 23 MeV
Assuming a proton, the kinetic energy after
the plate would be ~200 keV, implying a range
in the cloud chamber gas of about 5 mm, not
compatible with the observed 50 mm:
positron track
The particle should have the electron mass
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Evidence for antimatter in this early bubble
chamber photo. The magnetic field in this
chamber makes negative particles curl left and
positive particles curl right. Many electronpositron pairs appear as if from nowhere, but are
in fact from photons, which don't leave a track.
Positrons (anti-electrons) behave just like the
electrons but curl in the opposite way because
they have the opposite charge. (One such
electron-positron pair is highlighted.)
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The muon and its role in the early times
The discovery of the muon was published in
Seth H. Neddermeyer and Carl. D. Anderson, Note on the Nature of Cosmic-Ray
Particles, Phys. Rev., Vol. 51, 884 1937.
J. C. Street and E. C. Stevenson, New Evidence for the Existence of a Particle
Intermediate Between the Proton and Electron, Phys. Rev. 52, 1003 (1937).
Isidor Rabi: “who ordered this?”
Before that the fundamental particles were presumed to be electrons, protons and
the (then) newly discovered neutron. The discovery brought attention to the
prediction by Yukawa in 1935 that an intermediate mass "meson" might be
responsible for the nuclear strong force. Yukawa had predicted a mass of about 100
MeV and the muon had a mass very close to that.
Moreover, “the mesons” decayed emitting electrons, and Yukawa’s nuclear quanta
were expected to be responsible for β-radioactivity by disintegrating into electrons
and undetectable neutrinos.
Was the new intermediate-mass particle the responsible for the strong
interaction?
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Yukawa Theory (1935)
Relativistic Klein-Gordon equation:
For m = 0 reduces to the wave propagation equation, with Ψ either the wave
function of the photon or the potential U(r). For a static potential we obtain:
1 ∂ ⎛ 2 ∂U ⎞ m 2 c 2
∇ U(r) = 2 ⎜ r
⎟ = 2 U(r)
⎝
r ∂r
∂r ⎠ h
2
The solution is:
The analogous equation for EM (Laplace):
∇ 2U(r) = 0
with
gives
This implies that g can be called the “nuclear charge”
€
For a typical R =10-13 cm one obtains mc2 ~ 100 MeV
The Yukawa theory was compatible with the assumption
that the “meson” was the quantum of the strong interaction
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From 1941 and through the difficult years of World War II, three young Italian physicists,
Conversi, Pancini and Piccioni, carried on a series of observations of mesons stopped in
matter, which seemed at the beginning to support Yukawa’s predictions. At the end of 1946,
they reported that the rates of absorption of mesons in light materials were in strong
disagreement with the theory.
The experiment was based on the magnetic separation of positive and negative muons.
A positive particle would have been repelled by the nucleus and decay as in vacuum. A
negative one would have been captured by a nucleus and, as the quanta of the strong
interaction, quickly be absorbed.
The results of the experiment showed instead that also negative muons decayed when at rest
in Carbon, rather than being absorbed by the nucleus.
This disproved the possibility for the muon to be the quantum of strong interaction.
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Discovery of the pion
Cecil Powell and colleagues (Bristol University) used nuclear
emulsions to see charged tracks in the upper atmosphere. In 1947,
they announced the discovery of a particle called the π-meson or
pion (π) for short.
Pion (π)
comes to rest
here, and then
decays:
π!µ+ν
One neutrino is also
produced but escapes
undetected.
Muon (µ)
comes to rest
here, and then
decays:
µ ! e +ν +ν
µ
Cecil Powell
1950 Nobel Prize
Two more neutrinos
are also produced but
escape undetected.
e
π
The π (140 MeV) was then assumed to be the mediator of the strong interaction at short
distances (the Yukawa particle). Today we know that the actual mediator is the massless gluon.
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Interaction of an antiproton in a bubble
chamber: 8 pions are produced. One of
them (positive) decays into a muon and
a muon-neutrino
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The pion
•  Pions are produced copiously in strong interactions
•  Charged pions decay by weak interaction
•  The neutral pion decays by electromagnetic interaction
•  All pions have spin 0.
m = 135.0 MeV, τ = 8.4×10−17 s
m = 139.6 MeV, τ = 26×10−9 s
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“Strangeness”
In 1947 Rochester and Butler observed the joint production of two unstable particles.
Pais (1952) proposed the “associated production” for these particles. These particles
were always observed together: one with a mass of ~500 MeV (the K meson), the
other heavier than the proton (hyperons: Λ, Σ), producing the so-called V events.
These particles had other “strange” properties:
1)  Produced in strong interaction (fast)
2)  Decaying by weak interaction (slow)
The solution by Nishijima and Gell-Mann (1953):
New additive quantum number S (strangeness), conserved in strong and EM
interaction, violated in weak
K!
Λ, Σ !
p, n, π !
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S= +1 or -1
S= -1
S= 0
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V
“Strange” particles are produced in
pair (ΔS = 0) via strong interaction
π
but they decay weakly (ΔS = ±1)
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Examples of reactions involving “strange” particles
strong
EM
weak
weak
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Discovery of many more particles (1950-1960)
Because one has no control over cosmic rays (energy, types of particles, location,
etc), scientists focused their efforts on accelerating particles in the lab and
smashing them together. Generically people refer to them as “particle
accelerators”.
Circa 1950, these particle accelerators began to uncover many new particles. Most
of these particles are unstable and decay very quickly, and hence had not been
seen in cosmic rays. Notice the discovery of the proton’s antiparticle, the
antiproton, in 1955: more antimatter.
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CERN: European Laboratory for Particle Physics
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