 
Hello.
My name is Roman Zwicky.
I'm from the
University of Edinburgh
and I work in particle physics.
And in this MOOC I'll
be presenting to you
the theory of strong
interactions, also known
as quantum chromodynamics.
I hope you'll enjoy.
 
Strong and weak force, the two
missing microscopic forces.
As a matter of fact, there
are four fundamental forces
of nature, of which we
have seen two so far.
Gravitation corresponding
to attraction of masses,
as seen in part two, and
electromagnetism corresponding
to attraction of charges
as seen in part three.
They are well known in
our microscopic world.
For example,
through the orbiting
of planets and the propagation
of light respectively.
 
In the advent of the
age of particle physics,
it became clear that they're
ought to be more forces.
First, the nucleus made
up of protons and neutrons
is held together very strongly.
Gravitation is too weak
and electromagnetism
is repelling as protons
carry positive charge.
Hence, there has to be
a strong force binding
a nucleus together,
which operates
on the microscopic
or nuclear scale.
Second, beta decay.
The neutron decays into a
proton plus other particles
with very small probability.
It, therefore, seemed
reasonable to assume
that this decay is
governed by a weak force.
In the following
lectures we're going
to discuss the nature of the
strong and the weak force,
what the force carriers
are analogous to photon
and electromagnetism,
and why they're not
experienced in the
macroscopic world.
Concerning the latter,
I shall remind you
that the potential
in electromagnetism
and gravitation is proportional
to 1 over the distance.
This is due to the
force carriers -
the photon and the graviton
- having no rest mass.
By virtue of e equals
MC squared equals 0,
no virtual energy
needs to be created
and the force carriers
can propagate freely.
We will see that the
potential of the strong force
is proportional to the distance
and, therefore, radically
different.
The potential of the weak
force is exponentially
suppressed by a mass associated
with the weak force carriers.
The Higgs mechanism
has got everything
to do with this mass.
Strong interactions.
The way the theory of strong
interactions were uncovered
is rather subtle.
Information came
from a never ending
serious of discoveries
of new particles.
It eventually led to the idea
that the discovered particles
are bound states made of more
elementary constituents called
quarks.
We will see that their
proliferation can be understood
as coming from different
arrangements of these quarks.
For the sake of clarity,
we shall from now on mostly
explain the facts, since the
full story of the discovery
of the strong interactions
is rather intricate.
Quantum chromodynamics.
 
Quantum electrodynamics,
or QED for short,
can be understood as the
theory of interactions
between electrically charged
objects such as the electron
and the electromagnetic
force carrier, the photon.
In analogy, the strong
force can be understood
as the interaction between
particles called quarks
carrying colour charge and
the strong force carrier,
the gluon.
The quarks are matter
particles who spin one half
and the gluons are
force carriers spin one.
The most important different
to QED is that the gluons also
carry colour charge
and, therefore,
can interact with itself -
which I have drawn in a set
of so-called Feynman graphs.
This is due to the color
charge being more elaborate.
More precisely, the colour
charge can have three different
qualities which are called
red, green, and blue.
The analogue of plus or minus
for the electric charge may be
thought of as red and anti-red.
All these colourful words led
particle physicist to name
the theory of
strong interactions,
quantum chromodynamics,
or QCD for short.
The term strong
interaction and QCD
shall be used
interchangeably from now on.
This is the QCD kinetic
minds potential term,
in so-called Lagrangian form,
which I merely write down
because it fits on one line.
The first term corresponds
to the analogue of the Dirac
equation for QED as
seen in part three.
The second term gives
rise to the analogue
of Maxwell's equations in
vacuum, as seen in part two.
You're not supposed to be
familiar with these equations
for the remainder
of this course.
It is remarkable,
though, that one,
if not the, most
complicated theory in nature
can be written on
one line and yet
is so hard to solve as such.
For completeness we mentioned
that the quarks also
carry electric charges.
In QED we have seen that there
are electrons, muons, and taus,
where the muons and taus can be
thought of as heavy electrons.
Similarly, the quarks
come in different types,
called flavours.
Actually, there are six
flavours: up, down, strange,
charm, beauty, and top,
in increasing weight.
The last quark, the top, was
only directly confirmed in 1995
at Fermilab near Chicago.
It is time to return to the
beginning of this lecture
and fit the protons
and the neutrons
back into the new picture.
They correspond to a up,
up, down and up, down,
down bound states.
They up quark and
down quark carry
2/3 and minus 1/3 electric
charge, respectively.
Therefore, the proton made out
of up, up, down, carries 2/3
plus 2/3 minus 1/3 equals
1 unit of electric charge.
And the neutron,
made out of up, down,
down, carries 2/3
minus 1/3 minus 1/3
equals 0 units of
electric charge.
The quarks are bound
together into the proton
by continuously
exchanging gluons.
Let us add some terminology.
Bound states of so called
quarks are called hadrons.
This explains why
the CERN experiment
is called the large
hadron collider,
as two hadrons - namely, protons
- are collided at high energy.
The large range of particles
discovered by the physicists
in the 1950s can therefore
be understood as variations
thereof interchanging changing
the light flavours of up,
down and strange quarks.
For example, the bound
state made out of up, down
and strange is
known as the lambda
and weighs approximately
1.2 times the proton mass.
This scheme can thought of as
the analogue of the periodic
table of particle physics.
This is a nice picture,
but there are still
paradoxical aspects.
QED as well as QCD
are gauge theories
and with the caveats of
the yet to be discussed
Higgs mechanism, imply that
the force carriers have no mass
and, therefore, should act
at the macroscopic scale.
Here is the paradox.
How come the strong
force only operates
on the microscopic scale?
This is related
to our next topic
of confinement and
asymptotic freedom.
Confinement and
asymptotic freedom.
Possibly the most
puzzling aspect of QCD
is that, mathematically,
it is defined
in terms of quarks and gluons.
But neither have ever appeared
in a particle detector
in isolated form.
That this can never
happen is known
as the confinement hypotheses.
Whereas the hydrogen
atom can be split
into its two constituents,
the electron and the proton,
the same is not possible
for the hadrons.
This was a real puzzle.
It is, therefore, not
surprising that it was largely
thought that quarks
had no reality,
but were merely convenient
mathematical objects,
bringing some order into
the world of hadrons.
In 1969, experiments in
deep inelastic scattering
gave the community new results.
In those experiments energetic
photons probed the proton.
The best interpretation
was that the quarks
could be thought of as free
particles within the proton.
By free, we mean that
no force acts on them.
Let us summarise the situation.
Experiments suggests that the
quarks interact weakly at short
distances (known as asymptotic
freedom) and very strong
at large distances,
(confinement) binding coloured
objects such as quarks and
gluons into bound states.
The challenge for
particle physicists
was to show that these two
properties are contained
within the QCD equations.
This is easily said, but totally
counterintuitive, as in QED, it
works just the other way around.
More precisely, charges are
known to be screened in QED.
Imagine a charge
placed into the vacuum.
A virtual pair of
opposite charges
is created from the vacuum,
then the initial charge
will attract the one
of opposite charge.
This is analogous to
a polarised medium.
The net effect is
that from a distance,
it will look like
there is less charge.
To see this, we place a charge
into such a configuration
and we see that the
virtual charges counteract
the effect of the
initial charge.
Therefore the
electromagnetic force
gets stronger at short distance
due to quantum effects.
The idea of asymptotic
freedom in QCD
is to turn this
picture upside down.
In 1973, it was indeed
shown mathematically,
using the QCD equations, that
the force does become smaller
at short distances.
The discovery of
asymptotic freedom in QCD
led to the Nobel Prize for
Politzer, Gross and Wilczek
in 2004.
Possibly we should add
that the word asymptotic
refers to small distances.
The new key feature,
as with respect to QED,
is the self interacting
nature of the gluons
which turns the screening
picture-- described
for QED-- upside down.
The fact that the force is
small at short distances
complies with the force being
strong at large distances.
Using the QCD
equations, it has been
shown through elaborate
computer simulations
as performed at
Edinburgh University,
that the force is indeed
strong at large distances
and leads to the confinement
of quarks inside the hadrons.
An ab initio issue analytical
approach to confinement
is one of the great outstanding
problems of particle physics.
At the qualitative
level, the lines
of forces for two QED and
QCD charges look as follows.
The photon flux lines spread
out into the entire space
and, therefore,
weaken with distance.
The gluon flux lines are
squeezed into one dimension
and do not weaken.
The force is, therefore,
independent of the distance.
This implies that the
potential scales linearly
with the distance, as
announced at the beginning
of the lecture.
Let us summarise the
picture of confinement.
One cannot isolate
coloured particles.
Only colour neutral particles
can propagate over long
distances.
This is the explanation of why
the strong interactions are
of short range.
The force carrier of the
strong interactions, the gluon,
has colour charge
and can, therefore,
not act at a long range
despite having no rest mass.
Let us mention, as it is
of importance for the Higgs
discovery plots, that when
a quark pair is pulled apart
to a certain distance then it
is energetically favourable
to break up into two
colour neutral quark pairs.
This leads to the formation
of so-called jets of colour
neutral particles in
high energy experiments.
The puzzle of why the weak force
is not of long range nature
has a very different solution
and has got everything
to do with the Higgs
mechanism, to be
discussed in
forthcoming lectures.
That's it from me.
I hope you have enjoyed.
 
