Brian Cox. Rockstar physicist who made space cool for many of us. He is a professor of
particle physics at the University of Manchester and has been the host of many science shows and
documentaries including Wonders of the Universe. Before he specialized in making science easy to
understand he wrote one very long and hard to understand PhD thesis and I've taken one for
the team and tried my best to read through this so that you can understand some of the
work he did as a student without having to slog through the whole thing yourself.
Here’s the first page. The thesis is titled: Double Diffraction Dissociation
at Large Momentum Transfer. Don't worry if that means nothing to you,
because it meant nothing to me last week but I think I've got a grip on it now. This was
submitted in 1998 at the University of Manchester to earn Brian Cox the
title of Doctor and permit his career to move forwards onto more research and professorship.
I learned a lot of interesting particle physics whilst just trying to understand
the thesis introduction. Turns out that the internal structure of a proton is
way more complicated than I knew and really this thesis is all about trying
to gain just a tiny bit more knowledge about what's going on inside a proton.
In physics we first learn that in an atom there is a nucleus containing positively
charged protons and neutral neutrons those are surrounded by negatively charged electrons.
Then we might learn that protons and neutrons are actually made up
of smaller particles called quarks but that is far from where we should stop.
Like charges repel and if electromagnetism was the only force in charge of the nucleus then
it would fall apart so what keeps the nucleus of an atom intact? There must be some really
strong force overcoming the repulsion that each proton feels from one another.
Protons and neutrons both experience the strong force but electrons don't.
What protons and neutrons have in common is that they both contain quarks so it
seems like quarks have something to do with the strong force and it turns out that is the case
because quarks can interact with the particle that carries the strong force, the gluon.
Gluons are massless particles that jump from quark to quark to mediate the strong force.
The study of the strong force and how it works is called quantum chromodynamics because it relates
to the color charge carried by the quarks. Quarks also come in the flavors up, down,
strange, charm, top, bottom and the anti-quarks of each of those such as anti-up and anti-down.
A quark/anti-quark pair will annihilate if they collide, the energy could turn into a gluon which
can then reappear as another quark/antiquark pair. We could have an up and an anti-up
combine to form a gluon which then becomes a down and an anti-down and that can continue.
Each gluon can split temporarily into a quark/anti-quark pair or even two gluons
so at any given moment the internal structure of a proton is actually very complicated. So much
so that throughout this thesis instead of saying quark or gluon when referring to particles inside
a proton, the word Parton is often used instead which means any particle that could be inside.
To find out about this complicated proton structure, Brian Cox studied
what happens when electrons are fired at protons in a particle accelerator.
Chapter 2 of the thesis is all about the H1 detector at the HERA particle accelerator in
Hamburg, Germany, which is where Brian performed these experiments. HERA ran
from 1992 to 2007 and could collide protons and electrons at high energy.
To see the unseeable there are three important parts of particle physics experiments;
accelerating particles, smashing them together, and recording that collision with detectors.
The particles in Brian’s experiments were accelerated in the hope of knocking a quark in
a proton hard enough that nothing else matters. If a kid tried to tackle a big rugby player,
even with impact, the rugby player would still have a grip on the ball. But if another rugby
player tackled them, there's a much greater chance the ball will fly out of their hands.
The rugby player might be a proton and the ball is a quark. In a violent high-energy
collision we could knock the quark clear of the proton and gain new information.
For charged particles like electrons and protons, we can accelerate them using electric fields. A
proton in an electric field will feel a force in the same direction the field is pointing.
The field might be made by two plates attached to a battery but instead of one long line of plates,
many accelerators are circular and work like a playground roundabout. If an adult pushes
the ride every time it comes past them, it will spin faster and faster with every push. HERA’s
synchrotron worked in a similar way using magnets to keep the particles in a circular
path with many points of acceleration. Getting to that top speed took a few different stages.
When it comes to detection the most common technique is to measure ionisation from
electrons getting knocked off of atoms. When a charged particle passes through matter,
the electric field of the particle knocks the negatively charged electrons from atoms
in the matter leaving positively charged ions. An external electric field pulls
the electrons and ions even further apart so they can’t recombine. The detector collects
those electrons allowing physicists to measure the ionisation at each point away from the collision.
If a runaway car lost its breaks and entered a cornfield, each plant would slow it down a
little. That's what a particle travelling through the detector is like. A fast and
heavy car would take a lot longer to come to a stop and so would a more energetic particle.
Magnetic fields also cause charged particles to move in curved paths with low-energy particles
curving more, so detectors can gather information about the path, charge and energy of a particle.
Chapter 3 of The Thesis discusses more of the underlying theory motivating
the research it's very advanced and mathematical he goes into more depth
about quantum chromodynamics and how to reconcile it with other theories.
There is mention of the pomeron, a mysterious virtual particle. Where the gluon carries
the strong force between quarks, the pomeron is thought to carry the strong force between protons.
Chapter 4 shows the results of some experiments and makes
mention of observed jets. Jets are actually pretty cool and weird.
Turns out that quarks can't exist on their own once they’ve been knocked out of the proton.
This rubber band represents the strong force connecting two quarks. When the two ends are
close there is little force but when the ends are separated the force increases.
Think of the rubber band stretching as a quark and an antiquark moving away from
each other after a collision. As the rubber band becomes tighter, its energy increases
until it snaps. The same concept applies to quarks and antiquarks. As they move apart,
the strong force between them grows stronger until it becomes too intense for the available space to
contain. The energy at this breaking point transforms into a new quark/antiquark pair.
Now there are two quark/antiquark pairs, the original pair still moving apart and the newly
created one. When it comes to rubber bands, the process ends at this point, but with quarks,
it's just the beginning. The original pair and the newly created pair continue to move
away from each other and this process repeats, converting energy into the mass of new particles
until the adjacent pairs are no longer moving away. Then, the quark-antiquark
pairs join to form mesons that travel in a straight line along the original direction.
Mesons are small particles made up of one quark and one antiquark and examples include the pion
and kaon. In this wild process, quarks and gluons have become a jet of mesons.
Brian compares the energy distribution of observed jets to simulations. Brian says he can't draw any
statistically significant conclusions from these first experiments because
he encountered many difficulties including that some parts of jets
fell outside of the detector. He also had high uncertainties in identifying some jets. These
difficulties lead him to think up a new approach for the next chapters.
Chapter 5 is where the name of the thesis comes from. Double Diffraction Dissociation
at Large Momentum Transfer. Let’s break down the title. Dissociation refers to
a system of particles breaking apart into two or more separate final states after a
collision. Diffraction means the particles are scattered. Double means we’ve got two
separate systems in the final state. Large momentum transfer (|t|) just means that a
large amount of momentum is transferred from the incident particles to the scattered ones.
The example studied throughout this work is a high-energy photon (the basic unit of light)
interacting with a proton and producing two jets, call them X and Y, in the final state.
X is the backward jet from the photon side and Y is the forward Jet from the proton side.
The new focus here and in chapter 6 is in measuring a Double dissociation Differential
Cross Section. A cross-section is a measure of the size of the interaction area between
two particles in a collision. It’s like a probability that the interaction will
occur. Brian measured the probability of the double dissociation interaction as a function
of energy and the angle of particles relative to the beam axis, to see if the interaction
is more or less likely as the energy of the particles involved in the collision increase.
In the summary, we can see that Ryan Cox has compared the cross-section results to a
perturbative Quantum chromodynamics model and found that it was consistent with a
colorless object, the theorized pomeron, being exchanged in the interaction. The
results were not conclusive enough to say for sure but that finding was still one of
the main contributions of the thesis. Another contribution was having measured the double
dissociation differential cross-section in this specific energy range for the first time.
Since this thesis was published in 1998, particle physics has made a lot of progress but the ideas
Brian studied are still relevant. Efforts were made to combine the quark models described here
with the standard model of particle physics. The standard model does not include a pomeron,
so with our current understanding, whilst it was a useful mathematical object, it doesnt seem the
pomeron is a real particle, in the sense that it isnt detectable. Interestingly however,
since modern string theory arose from trying to explain the results
of scattering experiments involving particles with 2 or more quarks,
the mathematics describing pomerons has a new use in string theory and has lived to see another day.
You might associate Brian Cox more with the study of space than particles but
astrophysics and cosmology stand hand in hand with particle physics. Many of the particles studied
in colliders were only present in the universe during the earliest moments after the big bang.
At that time the universe was very hot and particles were moving extremely quickly
with a lot of energy. This thesis studied particles with high energy and experiments
like these are recreating the conditions of the early universe. Through this we
can understand how matter came together and why the universe contains anything at all.
Particle physics is a way to perform experiments to understand the wonders
of the universe rather than relying on observations of space alone.
Thank you for watching this video and thanks to my Patreon supporters
for making it possible. A special shout out to today's Patreon cat of the day Stanley.