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So I'm going to tell you about the discovery itself,
but there's quite a bit I have to tell you before that
and I have to do it in a very short time.
So we're going to cover all of quantum field theory,
all of particle physics in 18 minutes.
(Laughter) It's no small – You'll do fine.
We'll have an exam at the end.
So this is our experiment, the CMS experiment,
I've titled the talk
"Searching for the genetic code of our universe."
What we do in particle physics, in some sense, is very analogous to that
and I hope I can show you why that's the case.
Now like I said, it's quantum field theory,
it's Higgs field, Higgs mechanisms,
so we can get very, very obscure very quickly,
but what I'm gonna try to do is, with a lot images and fairly simple analogies
and some hopefully nice conceptualization of some of these things,
give you a sense of what we're doing and why we're doing it and it how it works
and why it's interesting and hopefully you could take some of this away with you.
Now the title is also kind of pretentious, I'd say,
but that depends a bit on your perspective.
I asked a friend of mine who's a physicist studying string theory
if she was interested in what we might learn at the LHC.
And she said, "No, not really."
I said, "Why not?" and she said, "Because it only pertains to our universe
and you know –"
So in some sense this is a very modest talk,
I'm only talking about our universe here, OK.
So I'm going to give you some background.
Now we have something we call the Standard Model of particle physics
and this took about 100 years to put together,
lots of theoretical physics had to be developed,
many subatomic particles had to be discovered.
And, it is really something like a new periodic table
of the most elementary particles that we've built.
This is what it looks like.
There was a famous Nobel prize winner
who flashed this slide at one point and said,
after decades and decades of research and billions of dollars,
"This is all we know."
But in a sense this is good, this is what we want,
we'd like to find a very simple, underlying explanation of the universe.
And what we discovered is that there are three sort of generations
of these kind of particles, quarks and leptons,
they're fermions, that means they have half-integer spin,
you don't have to worry about that.
And those things, these particles, are actually what build structure,
build the atoms, and things like this,
and then are particles that carry the force,
sort of glue the other particles together, very simple,
and there's a key piece, other piece that we hadn't found.
So let me show you this again –
this is one of the greatest achievements of 20th century science.
This is a look at the same particles,
but you can see their mass is on a log scale,
so there's quite a big difference between the lightest and the heaviest,
maybe a factor of a million.
Actually we only see these particles,
these make up the protons, for example, and the atoms.
But all the other ones turn out to be, even though we can't see them,
very crucial to how the universe is structured
and how everything behaves and that's why we do what we do, and I'll repeat that.
There's one missing piece and that's the Higgs particle,
at least there was missing.
Here you can see the masses of the quarks, they go from really tiny to really high,
and we don't know why.
That's one of the things we'd like to understand.
OK, so what is the Higgs particle?
So while we were developing
this modern theory of fundamental forces
in these directions, we kind of hit a snag.
The particles that carry the forces
have to be massless, we knew that from our equations,
but the data seemed to indicate otherwise.
And in fact we didn't understand why any particles should have mass
or what was mass for that matter.
Now massless particles, they move at the speed of light, OK.
And so theorists came up with an ingenious idea:
suppose there's a force field that fills the universe
that somehow slows particles down to below the speed of light.
That would effectively give them mass.
So in fact, as my predecessor was saying, you have something like this.
You have this field that fills the universe,
and as particles pass through it, they get kind of caught up in it.
Some more than others.
And that's how they become massive, they basically become slowed down.
And that's what the Higgs field is.
So what's the difference between a field and a particle?
Now this is where it gets a little bit counter-intuitive
and it's very hard to understand this without studying quantum field theory.
But fields have particles associated with them;
we call them field quanta, from quantum mechanics,
and they carry the force of that field.
Particles interact in fact by exchanging these force carriers.
And here, for example, I give you a very simple case,
where you have electrons
which are basically repelling each other by exchanging a photon.
So this is how forces work.
There are other ways, other processes, that are much more complex
and they become very counter-intuitive but this is a good way to look at things.
Now quantum field theory I've mentioned –
The basis here is that energy and mass are equivalent.
So, strange things can happen, actually, too, in quantum field theory.
You can have a particle and an antiparticle
pop into existence out of empty space.
Something like this.
Here you have 2 top quarks, and then they can vanish back into it.
This is called a quantum fluctuation and these are virtual particles.
It sounds a bit magical but it's actually really critical
to everything we understand.
And it has very far reaching consequences.
So, in fact the structure of the universe turns out,
because of these virtual particles being everywhere,
to depend on particles that don't exist in the usual sense.
Some of which existed earlier,
when the universe was much hotter and much younger.
And this is why we do what we do, we're trying to find these particles
to understand how they affect our universe.
Here for instance is an event, an event display,
of some of the first top quarks ever seen,
in the 1990's at Fermilab.
So, what makes us so sure that this Higgs particle should exist?
Well, the theory has very predictable consequences.
For instance, it predicts these very heavy force carriers
of the weak nuclear force,
the W and the Z particles.
The W should have a mass of about 80 GeV.
Now this unit I'll come back to in a second.
The Z should have a mass of about 91 GeV
and the proton has a mass of 0.9 GeV.
So, these particles are much heavier than the proton,
even though they're much smaller.
Now, when they're made, they're very unstable
and they decay almost instantly.
And we can see the tracks of the decay products
and we can see energy deposits from the decay products in our detectors,
and we can use these to reconstruct the mass of the original particle
[or] many of its other properties.
So here's, for example, what we predict we would see
if we look for Z particles decaying to muons.
You count the number of events at different mass values,
you'd expect to find a peak.
This is what a particle resonance looks like,
a peak at the mass of 91.1.
And then there's some background from other things.
Now let me show you what we actually see – this is it.
The black dots show our measurements.
So the Z and the W were exactly as we predicted they would be,
and this really made us take this idea of the Higgs very seriously.
Now there are fundamental connections between particles
and this is where it gets kind of interesting.
Fundamental particles actually all interact with each other all the time,
through these virtual reality kind of interactions that I mentioned.
So the mass of the W particle, for instance, depends a lot on the mass
of the top quark and a little bit on the mass of the Higgs.
And it is through this kind of a process
a W can decay into a top and a bottom quark,
and then those can fuse back together and become a W again.
A W can radiate a Higgs and re-absorb it and become a W again.
These things are happening all the time.
Basically the identity of any of these elementary particles
is really not separable from what it can become or decay into.
And this is how the universe works at a very, very basic level.
How is this possible? Well, here's a good way to visualize it.
The vacuum of space-time is really a very interesting place.
Imagine that you have kind of an invisible fabric,
that cloaks all these particles that could exist
and encodes how they could interact.
That's really what space-time is.
Not anything can happen in space-time, only these kind of things.
These virtual particles are always waiting
for an opportunity to interact with real particles.
So if you provide enough energy in a very small region,
you can pull particles from this fabric into our reality.
And to some extent that's exactly what we do.
In fact if the energy is large enough, we can pull up particles
that are very heavy, that we've never seen before.
And these are the keys to understanding the underlying code of our universe.
So, how do you get a lot of energy in one small spot?
Well, we do it with what we call: "Large Hadron Collider".
Let me show you that, [I] have a nice picture of it.
Basically we have rings of magnets that focus beams and circulate them.
Each time they go round we give them a little acceleration with an electric field.
And then when they get energetic enough we switch them to another ring that's bigger,
we can make the particles accelerate to even higher energies
and then finally to this yellow one which is the Large Hadron Collider.
And it shows you the scale of things, 100 meters underground.
It took a long time to paint those stripes by the way.
And here's another view of it,
and you can now get kind of a sense of how big it is, because you can see
Geneva airport, there, it's really quite a huge machine.
And it is so huge because the particles are accelerated
to such high energies that the magnets
are limited in how well they can keep them on track.
So we have to build a very big machine, and there we have four experiments.
Two of them I'll talk about a little bit today: my experiments at CMS,
or an experiment I'm part of, I don't own it, and ATLAS.
But there are couple of others, LHCb and ALICE
that are very dedicated to very specific things and I won't go into those.
It's a bit like Swiss chocolate; I'll let you think about that for a second.
The LHC magnets that keep the particles on their track,
they store a huge amount of energy.
In fact, it's enough to melt 12 tons of copper,
that's how much energy there is in these magnets.
It is the kinetic energy of an A380 at 700 km/h.
How much energy is stored in the actual beams?
Well, it's equivalent to 90kg of TNT or 15kg of chocolate.
I bet you didn't know that chocolate has more calories than TNT.
OK, now let me tell you about the experiments.
The experiments are very big
because we ram these protons together at really high energy.
Things can come out at really high energy and we want to measure those things.
We have to build very, very large experiments
to be able to bend the particle tracks in magnetic fields
and actually measure their momenta.
So here's ATLAS and I'll show you how it looked as it was being built.
This is 30 storeys underground
and there you see a person standing amids it.
Now lots get filled in here in fact
and I'll show you in fact with CMS a little bit more.
ATLAS is just like CMS in the sense that there's about 40 countries involved,
hundreds of institutions and thousands of physicists.
So CMS, this is the experiment I'm heading right now,
we had to build it on the surface and then lower it.
And this piece right here, the central piece of the experiment,
is 4.4 million pounds
and had to be lowered 30 stories with only, if you look there,
we had 3 inches of clearance, so it was quite tricky to do that.
And here, this sets the scale, if you go back to this picture,
you see the magnets, comes hard to tell how big this giant solenoid magnet is,
if come here you can see, it's quite big, it's the largest magnet ever built.
We also recycled some things to build,
actually old casings from the Russian military,
were made into parts of our experiment.
And here I show what's happening as we're inserting the central tracking system.
OK, and then this is the picture I showed at the beginning,
this is when the detector was ready to close
this is actually the beam line here – OK?
And that's where the protons go and then they collide
in the center of the detector which is a bit over here to the left.
OK, there are a lot of people involved,
this is 1/8th of the people that were involved in the CMS experiment.
And as the presenter mentioned before me
there were about 4000 people involved altogether.
OK, so how do we reconstruct what happens in a collision?
This is the detector looked at end on
and you notice it's kind of got a lot of cylinders involved.
And if I replace it with a cartoon, you can see this.
All the different cylinders are different kinds of detectors
that detect different properties of the particles as they pass through them.
And when we sum up all the information from all the different layers,
we can tell if they're pions, muons, kaons, etc.
And that's how we reconstruct these things.
Now we collide these two beams of protons, as I mentioned,
each beam has 1380 bunches, each bunch has 160 billion protons.
Lots of numbers, they collide 4 different places,
and whenever they cross,
even though there's a 160 billion protons in each bunch,
you only get about 20 or 30 pairs of protons that collide.
And usually what happens is, they just break up.
The proton breaks up, the quarks go flying out,
and you make new particles, but it's not very interesting.
Sometimes though, it gets very interesting.
Let me just show you a simple event, this is our first event of the 2012 era,
And this is a real event, everything you're seeing now is simulated,
but you'll see what the event actually looks like when the two bunches cross –
Here's 30 pairs of protons fly,
and those are the tracks of the particles coming out,
and the blue represent energy deposits
in the energy measuring part of the experiment.
OK, we're done, come on.
Aha good, I can't see we get past this one.
Ah, very good. Now, if two quarks inside hit very hard,
you can have so much energy that you can produce something really interesting.
Here I show for instance a diagram of two quarks interacting,
forming a very energetic gluon and then decaying to top quarks.
Now, you've not seen these kind of diagrams before,
but if I showed you the masses involved,
it's kinda like throwing 2 ping-pong balls at each other,
and having 2 bowling balls come out,
as the top quarks are so much more massive.
Let me show you the lead-lead collision, this is just fun.
Now we're throwing 2 lead atoms at each other,
so you have 400 protons and neutrons colliding.
And that's what that looks like.
So, we often talk about these detectors as something like a camera.
They have about 80 million pixels, but they're not ordinary cameras.
They take up to 40 million pictures per second, which is pretty hard to do,
and the pictures are three-dimensional with extremely high precision,
one micron level precision.
And the detectors at 15 and 31 million pounds each
are not very portable.
Some of the challenges we have are that these collisions are very frequent.
Right now there's about 16 million per second
and the things we're looking for are really rare.
So the Higgs events we're looking for, some are 1 in a trillion.
So we have to run a long time, continuously round the clock,
many, many collisions have to be collected.
We keep about 1000 of these 16 million every second,
and that's still a lot of data, in fact.
In fact we end up with about 22 petabytes of data per year,
a petabyte, I think is a million gigabytes, correct?
So, there's a ton of data, we have to transfer it out all over the world,
basically to process it, because it's too much,
to hold in one place. So it goes out to 34 countries,
about a 100,000 computers are involved.
Alright, so I'm going to show you the Higgs searches finally.
Here is a Higgs event, we think,
or a possible candidate, and what you see,
are lots of low energy tracks. This is debris from the protons breaking up.
It's not very interesting, but you notice these 2 big red bars, alright?
Those are actually 2 photons coming out sideways;
they're very, very energetic.
This is a very rare event and this is what we look for.
The Higgs particle could decay to 2 photons and they would look something like this.
But there are lots of other ways of making two photons
and so you end up with a background of events
that's very smooth like this.
But, if you find an excess in anyone place at a particular mass value, OK –
that's indication of a possible new particle.
And in fact this little bump is only a few hundred events, OK –
that's an excess corresponding to a couple of hundred events,
at about 125 GeV – and it took how many collisions to find it?
Well, it took ten to the fifteen.
So it took a long time running
and a lot of sifting through the data to find these guys.
But this little bump, actually, really represents a major discovery.
Here in ATLAS we see a different kind of event that we're looking for,
the Higgs can also decay to two Z particles, I mentioned those before,
and they can decay to electrons as well as muons,
and this is an event with 4 electrons.
You reconstruct the Z's and then you reconstruct what you get from the two Z's
and you find, in fact, lots of things that you would expect.
This is a hard to read display and has lots of data that matches expectations,
but there's one place where the data is much above expectation,
around 150, 125, and if you look at CMS
on the blown-up scale, we also see an excess at 125,
So these little tell-tale signs, actually,
are what tell us that we have something new
and we've just begun to see it emerging.
It's very, very new. They point to a major discovery.
Both experiments see excesses at the mass of 125,
in several different channels. There are some I didn't show you.
And after very intricate studies and very careful checks
that took us months and 100s and 100s of people involved,
everything held up – we know this is not something we've seen before.
Everything is consistent with what's expected for the Higgs
and the significance statistically is adequate to claim a discovery.
But this is really just the beginning.
Here is the cover of the publication
with the two results that came out in July.
It's been 48 years – I've just 2 slides left –
since the Standard Model Higgs boson was predicted.
It's been 20 years to design
and build these very complex accelerator and experiments,
the most complex experiments ever built in the history of physics.
It took 3 years to acquire the data
and it really took a generation of intense effort by thousands
of physicists, engineers and technicians to make this all possible.
So what's next?
Well, we have to figure out what it is.
We're pretty sure it's the Higgs, we're sure it's a Higgs, I should say,
but we have to study its properties, because there's a chance
it's not the simple standard model Higgs.
In which case we have something of a revolution,
which could help us understand a lot of things.
And that could take us to new frontiers, actually.
So stay tuned.
(Applause)