Ernst Rutherford decided to probe
the atom using fast moving alpha
He got his students Geiger and
Marsden to fire the
positively charged‑ α particles at‑
very thin gold foil and observe how
they were scattered.
The diagram summarises his
Most of the α particles passed‑
straight through the foil, but to
his surprise a few were scattered
back towards the source.
Rutherford said that this was
rather like firing a gun at tissue
paper and finding that some
bullets bounce back towards you!
Rutherford soon realised that the
positive charge in the atom must
be highly concentrated to repel
the positive a particles in this‑
The diagram shows a simple
The ball is rolled towards the hill
and represents the a particle.‑
The steeper the `hill' the more
highly concentrated the charge.
The closer the approach of the
steel ball to the hill, the greater
its angle of deflection.
• In 1911 Rutherford described his
nuclear model of the atom. He
• All of an atom's positive charge
and most of its mass is
concentrated in a tiny core.
• Rutherford called this the
• The electrons surround the
nucleus, but they are at relatively
large distances from it.
• The atom is mainly empty space!
Can we use this model to explain the
α particle scattering?‑
The concentrated positive charge
produces an electric field which is
very strong close to the nucleus.
The closer the path of the α particle to‑
the nucleus, the greater the
electrostatic repulsion and the greater
Most α particles are hardly‑
deflected because they are far
away from the nucleus and the
field is too weak to repel them
The electrons do not deflect the
α particles because the effect of‑
their negative charge is spread
thinly throughout the atom.
Using this model Rutherford
calculated that the diameter of the
gold nucleus could not be larger than
This diagram is not to scale. With a 1
mm diameter nucleus the diameter of
the atom would have to be 10 000 mm
or 10 m!
What makes a particle “elementary” ?
A particle is
elementary if it has
no inner structure
(i.e not “made” of
some even smaller
Which particles were considered
elementary throughout History?
1911 : Rutherford discovers the nucleus.
Transmutation reactions showed that the
hydrogen nucleus played a specific role (4
7N --> 18
9F --> 17
8O + 1
1p) . Rutherford named it
proton (protos = first)
1932 : Chadwick discovers the neutron, which
is not stable when isolated, and decays as
follows : n p + e-
(+ ¯νe). The proton,
electron and neutron account for all the
atoms of all the elements in the Universe.
This was the “simplest” elementary
particle set ever described. A small
number of particles, a small number of
LEPTON (leptos = light) : e-
BARYONS (baryos = heavy) : p , n
However, some problems were already
1. The photon : Photoelectric effect ;
2. Antiparticles : Discovery of the
positron by Anderson (1932), studying
cosmic rays. Many more particles
would be discovered in cosmic rays…
3. Mesons : These particles were first postulated
by Yukawa (1935) to explain the force that binds
the nucleus together. Being of intermediate
masses, they were called mesons (mesos =
4. Neutrinos : Necessary to preserve E
conservation in β decay
From the particle garden to the jungle :
In 1937, Anderson discovered the muon μ. The μ proved to
be some sort of heavier electron (lepton).
The muon decays into through β
μ νμ + e-
I.I Rabi, Nobel 1944
In 1947, pions (mesons) were detected in cosmic
rays. They were thought of as Yukawa’s mediator
particle for the strong interaction. The Universe
was in order again, except for the muon, which
played no visible role.
In December 1947, new mesons were found : the
kaons. The place got crowded again…
With the use of particle accelerators in the 50’s,
many new particles were discovered. Some of
them were « strange » because they were
produced by the strong force but decayed through
the weak force.
Moreover, some rules seemed to be missing to predict if a
decay could occur or not :
Why is π-
+ n is impossible ?
In 1953, Gell-Mann and Nishijima came with a simple and
elegant idea. Each particle was to be assigned a
«strangeness », and the overall strangeness had to be
conserved during a collision (not through decay).
There were then THREE laws of conservations for
Baryonic number (proton like particles)
Each vertex has an arrow going in and
one going out.
These represent a lepton – lepton or
Quarks or leptons are solid straight lines
Exchange particles are either wavy
(Photons, W, Z) or curly (gluons).
Time usually flows from left to right.
Arrows from left to right represent
particles moving forward in time.
Arrows from right to left represent
antiparticles moving forward in time.
(think of them as moving left to right).
Vertices are linked by a line representing
an exchange particle
Charge and colour are conserved at each
What is happening here?
Now you try to construct:
Quarks CANNOT be found alone.
This is known as the “quakr
confinement” – that is quarks
cannot be observed in isolation.
The Higgs boson is a sub-atomic particle
that acts as the intermediary between
the Higgs field and other particles. All
fields are mediated by bosons, some of
which pop into and out of existence
depending on the state of the field, sort
of like how rain drops emerge out of a
cloud when it reaches a certain point.
Finding the Higgs boson
confirmed that the Higgs field
exists, and that field enables
mass to be explained