Why do atoms exist?
Why is radioactivity such a weak force?
If it wasn't weak life would be impossible.
So we know that atoms are made up out of electrons
which are in a cloud around a central nucleus.
But why do they stick around and form a cloud?
Why don't they just fly away?
If they didn't have any mass they would always
travel at the speed of light and there would
be no way to glue them together with a nuclei
to make atoms.
So we need to understand where the electron
gets its mass.
Why are the weak forces responsible for radioactivity
so weak?
They're weak because the particle that generates
those forces is very heavy.
Why is it heavy?
Where does its mass come from?
This is the big puzzle that faced physicists
in the 1960s: where do particle masses come
from?
There was a theory proposed by Peter Higgs
and colleagues in 1964 which predicted that
there would be a universal field throughout
all of space and there would be a quantum
of that field that came to be called the Higgs
boson.
That Higgs boson was indeed discovered at
CERN at the Large Hadron Collider in 2012.
Let's go back to 1964.
At that time people were just beginning to
understand what we call the Standard model
of particle physics (which you can see on
my pullover).
We knew about the electron, we were just beginning
to understand that inside the nuclei there
were things called quarks, and people were
just beginning to write down theories of the
fundamental interactions, like the top line
of my pullover.
But there was a puzzle.
Where did those particle masses come from?
Several groups of theoretical physicists were
working on this problem and they were borrowing
ideas from other areas of physics, in particular,
the theory of superconductivity.
They took over those ideas and brought them
into the area of particle physics.
The most famous of those is Peter Higgs but
he wasn't the first.
Shortly before him two Belgian theorists,
François Englert and Robert Brout, proposed
a very similar idea and then many other people
came along afterwards.
What's the idea?
We're familiar with the idea of a field.
We talk about a gravitational field: the gravitational
field of the Sun keeps the Earth in its orbit.
We're familiar with electromagnetic fields:
we know about electric charges, they generate
fields, two charges attract if they're opposite
and repel when they're the same.
So they used this idea that there is this
thing that we call a field that extends through
space.
What Englert, Brout and Higgs proposed was
a new type of field, represented by φ on
my t-shirt, which was different from the gravitational
and electromagnetic fields.
it's constant throughout all the space, it's
the same in all directions, it's completely
homogeneous, isotropic and universal.
According to this theory, particles get their
masses from interacting with this universal
field.
I'm not going to go to the mathematics of
it but I will give you an analogy for thinking
about it.
Imagine that you are in Siberia in the middle
of winter.
So you've got this universal layer of snow,
absolutely everywhere.
And imagine that you're trying to cross Siberia.
If you're lucky you can have skis and you
go skimming very fast across the top, and
that's like a particle: think, for example,
about the photon, the particle of light that
does not interact with the Higgs field and
it always travels at the same very high speed.
In the same way the skier travels very fast
because he does not sink into the snow: the
analogy is that it doesn't interact with that
Higgs snowfield.
But maybe you don't have skis; maybe you have
snowshoes.
Then you're gonna go more slowly, you're going
to sink into the snow: that's like a particle
that interacts with the Higgs field, and because
it interacts with the Higgs field it goes
more slowly, it has a mass, it travels slower
than the speed of light.
And of course it's possible that maybe you
try to walk across Siberia in your boots.
If you do that you're gonna go very very slowly
because you interact very strongly with the
snow field: the analogy would be with a particle
that interacts very strongly with this universal
Higgs field and always travels much slower
than the speed of light.
So that's the theory: your mass depends on
how strongly you interact with this universal
Higgs field.
Mathematically it's represented by the third
line on my pullover.
How do you test this idea?
Well, what is snow made of?
What's the quantum of snow?
Snowflake.
So analogously there is a quantum, a smallest
unit of this Higgs field.
That's what we call the Higgs boson because
amongst all the people who proposed the idea
back in 1964 he was the only one who specifically
pointed out the necessary existence of this
particle.
Of course, there's many different shapes of
snowflakes and that's because snowflakes are
made up out of water molecules which you can
arrange in different ways.
In the same way we're still arguing about
whether there is just one Higgs boson or whether
there's many many different types of Higgs
boson or related particles which are made
up by rearranging some sort of more elementary
constituent in different ways.
For the moment we've only seen one Higgs boson
and it looks as if it's an elementary particle.
So this idea was proposed in 1964 but people
didn't pay a lot of attention.
Together with two colleagues, Mary Gaillard
and Dimitri Nanopoulos, in 1975 we were perhaps
the first to look at the Higgs boson systematically
to try to figure out what it would look like
in an experiment.
At that time, in 1970s, these ideas were still
regarded as being quite speculative and so
we actually noted in our paper we didn't want
to encourage big experimental searches for
this particle.
Fortunately, our advice was not taken.
In the 1990s and 2000s people built the Large
Hadron Collider at CERN, so it collides protons
at very high energies, it can make all sorts
of the new particles.
It had many different scientific objectives
but one of the principal objectives of the
Large Hadron Collider was to look for the
Higgs boson and try to establish once and
for all, does this mythical particle exists
or not.
The Large Hadron Collider started making high
energy collisions in 2010 and gradually in
2011 and the beginning of 2012 rumors started
going around that maybe two of the experiments
called ATLAS and CMS were perhaps seeing evidence
for something that might be the Higgs boson.
Finally on July 4th 2012 there was a big seminar
at CERN where the ATLAS and CMS experiments
both announced that they had discovered a
new particle which looked like it had the
right properties to be the Higgs boson.
As you can imagine, this was a really big
event in the world of particle physics.
Peter Higgs came to this presentation of these
results and he had a little bit of tears on
his eyes.
He said afterwards that he'd never expected
to see this particle discovered in his lifetime.
On July 4th 2012 we knew we discovered a new
particle but we could not be sure that is
was the Higgs boson.
We had to check that it had the right properties:
for example, it should couple to other particles
proportional to their mass, so we had to check
that.
Together with my student Tevong You we analysed
the data: everything looked good.
There were other properties of the Higgs boson
that needed to be checked and finally in 2013
everybody was convinced and Peter Higgs and
François Englert got the Nobel prize in 2013.
Now we have in our hands this particle called
the Higgs boson.
We're checking its properties to see maybe
there's some sort of deviation from the predictions
of the Standard model; we're looking to see
whether there's maybe other Higgs bosons,
we want to understand why the Higgs boson
has the mass that it does.
All these are open questions that remain to
be studied.
