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PROFESSOR: Hello, and
welcome back to 8.20,

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special relativity.

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In this little video,
I'm going to continue

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with my introduction, and
talk about the research I'm

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interested in.

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So this is not strictly on the
topic of special relativity,

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but you will see some
of the influences

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of my research in the class
as well as we move along.

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So what am I interested in,
and what am I working on?

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I work on the Large
Hadron Collider.

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You see behind me here a
picture of the CMS detector.

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CMS detector is one of
two omni-purpose detectors

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at the Large Hadron Collider.

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There's also LHCb and ALICE,
two more dedicated experiments.

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The Large Hadron
Collider collides

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protons at the highest
possible energies.

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In some units, 13 TeV--

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tera-electronvolts
collision energy.

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Collisions happen
around 40 million times

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per second in this machine
when it's operational.

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And we have made great progress
in understanding nature

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using this machine in
the last about decade.

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The Large Hadron Collider
started operating in 2009.

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They are currently
in a shutdown phase,

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but we hope to restart next
year with even higher center

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of mass energies
available for our studies.

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So why do we need a
machine like this?

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Colliding particles
at high energies

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allows us to probe the
structure of matter

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like with a big microscope.

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And so we can look very
deeply into the structure

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of the proton.

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At the very same time, we can--

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this high center of mass
energies and collisions

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produce perhaps new particles--
unexpected particles.

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We will later see
E equals mc squared

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as a result of
special relativity.

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And when you have
enough energy, you

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might be able to produce a
new particle of high mass.

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And so that's kind
of the Holy Grail,

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and what we're trying to do.

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And the other
thing we do here is

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by colliding protons and
sometimes even lead ions,

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we are able to create a very
hot and dense form of matter

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similar to the environment
after the Big Bang,

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and be able to study
this new form of matter.

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Let's see how mass and
matter are being built.

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If you take the table
in front of you,

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and you start looking
in detail, you

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start seeing
molecules and atoms.

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The atoms are built of
electrons and the nuclei.

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The nuclei itself is built
of protons and neutrons.

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And if you look more
precisely-- drill deeply

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into the structure-- you see
that a proton on the surface is

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built out of quarks--

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up quarks-- two up
quarks and a down quark.

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If you further investigate
the structure of the proton,

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you see that there's
much more going on.

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There's gluons-- particles
holding the quarks together.

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And there's also bunches
of quarks and anti-quarks.

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This is by now well understood.

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If you ask what is the
mass of the proton,

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it's about one
giga-electronvolts,

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or 938 mega-electronvolts.

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But where does the
mass come from?

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The mass of the
proton comes, in part,

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of the mass-- from the
mass of the quarks.

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But in most parts from
the gluons, or the field

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which holds the quarks together.

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That's kind of surprising,
but if you had 8.02 already,

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you know that there's
energy stored in a field,

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and that energy, again,
is equivalent to the mass.

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So the energy stored in the
gluon field holding the quarks

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together gives
mass to the proton.

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And this was quite well.

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There's a theory which
describes all of this.

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It's called QCD--
quantum chromodynamics.

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And if you-- with
some assumption,

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you can calculate the mass
of a bunch of particles.

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So this plot here
shows the light hadron

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spectrum which can be calculated
using just [INAUDIBLE]..

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What I'm actually
interested in is the mass

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of elementary particles.

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So this discussion so
far was a brief overview

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in how composite particles like
your table becomes massive.

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But how does the quark
itself acquire mass?

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How does an electron acquire
mass, or a muon and a tau.

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This picture here shows you
all known elementary particles.

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We can put them in three boxes--

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quarks-- those are
the particles--

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the up quarks and the down
quarks we found in the proton.

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The electron makes--
together with the proton

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makes the hydrogen atom.

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And there's neutrinos.

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Those are core electrons.

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And then there's force carrier.

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And we just met the gluons, but
there's also the photon, the W

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and the Z boson And the WZ
boson, they are themselves

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also massive particles.

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How do they acquire mass?

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The answer was found
by us about 8 years ago

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with the discovery of the
Higgs boson, a new particle.

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And the underlying
theory explains

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how particles acquire mass.

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And so basically solved, right?

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Not quite.

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So this is really
mysterious to see

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how different the masses of
those elementary particles

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actually are.

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You see on this
logarithmic table here.

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Again, here are our friends
the down quark, the up quark,

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and the electron.

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And if you compare
this, for example,

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with the heaviest known
elementary particle,

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the top quark, you
see many, many orders

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of magnitude difference.

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So how does this actually work?

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And then you see
some of the bosons--

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the force carriers are massive.

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Others, like the photons
and gluons, are massless.

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The answer to this was
the Higgs mechanism.

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And a very simple explanation
how the Higgs mechanism

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actually works for fermions for
those quarks-- so the electron,

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for example--

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is given in this cartoon.

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So the idea is that a
field fills all of space.

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It's basically a
property of the vacuum.

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And when you travel
an elementary particle

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through this vacuum, you
interact with this field.

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And the stronger you interact,
the more drag you kind of get.

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There's some sort of--
you feel an inertia.

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And this inertia is
what we know as the mass

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of the elementary particle.

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So there is an equivalence
between how strongly you're

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coupled to the vacuum-- to the
Higgs field, and your mass.

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And so a top quark couples
strongly to this Higgs field,

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while an electron only slightly.

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Great.

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So we have understood
everything.

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So the question
is why do we still

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collide protons and
bosons at the LHC?

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Is there anything
else to be discovered?

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So it turns out that we have
a very sophisticated theory

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describes those particles
and their interactions,

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but this theory fails to
explain all of the observations

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we have in nature.

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And so that is
kind of the driving

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force behind the experiment
I'm conducting right now.

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And so for example, we know
that there is dark matter.

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When we look at the rotation
of stars and galaxies,

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we find that they don't behave
as you would expect simply

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based on the distribution
of matter in those galaxies.

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There must be something else
out there, and that's what--

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since it's not visible--

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is called dark matter.

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And those dark matter--

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dark matter could be
a particle we might

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be able to produce at the LHC.

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So that's an
interesting question.

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Also when you look
out into the universe,

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we see a lot of matter.

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You don't see a
lot of antimatter.

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So there must be an
asymmetry between how

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matter and antimatter
is being produced.

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And so that is also not
fully understood yet.

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And then there's more question.

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For example, those neutrinos,
they are really, really light.

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On this logarithmic
scale, I had a cut off,

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and then the neutrino masses.

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Do neutrinos acquire
mass as an electron

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does, as a top quark does, or
is it a different mechanism?

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We don't know.

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Gravity is not even included
in the standard model.

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And the fact that
the Higgs boson

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was discovered at a specific
mass which is rather small it's

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also a little bit unnatural.

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And so there is this
entire list of questions

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and unresolved mysteries
which we're trying to answer.

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And the way we do this
is with big cameras.

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So this is CMS detector.

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There's a similar
picture that's behind me.

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You can think about
it as a big camera

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looking at the interaction of
the collision of two protons.

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And it starts off with around
this interaction region

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with pieces of silicon, which we
use to track charged particles

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going through.

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We put all of this
in a magnetic field,

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and if you listen
to 8.02 already,

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you know the charged
particle in magnetic field,

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they follow a curvature.

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And from the radius
of the curvature,

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we can calculate the
momentum of those particles.

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And then we stop the
particles in order

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to measure their energy.

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So we do this in kilometers.

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And what we use here is the
lead tungstate electromagnetic

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calorimeter, and a second
calorimeter for particles

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which are harder to stop.

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So those are called
atomic calorimeters.

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And then the silver
part in the middle

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here gives the CMS
detector its name.

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It's the solenoid.

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It's a 2-- a 3.8 Tesla
superconducting magnet.

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And then we have more detectors
there to see whether or not

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some particles
might escape, and we

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try to measure those as well.

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There's another
very nice picture.

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After opening the detector,
you see this silver thing

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in the middle here.

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It's the pipe in
which the protons

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zoom through the detector,
and broaden into collision

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in the very center part of it.

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And then we take those pictures.

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Here's one, and this
is a very famous one.

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It's a Higgs candidate event,
where the Higgs boson might

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have decayed into two Z
bosons, and then those Z

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bosons themselves decayed again
into a pair of electrons shown

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here, and a pair of muons here.

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And then we can use those
individual particles

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to reconstruct the property
of the Higgs boson.

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Here's another candidate where
there's two photons being

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reconstructed.

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And again, those
two photons can then

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be used in order to
reconstruct, for example,

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the mass of the particle,
which is the original first two

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protons.

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So we have done this
for the last years,

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and collected quite some data.

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And if we look at the
entirety of the data,

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we can make this plot here.

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And what this plot here shows
is the mass of the particle

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and the coupling of the
particle to the Higgs field.

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And what you see, there is a
linear relationship in this

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log-log plot between those
two, and that gives us some

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confidence that the elementary
particle-- like a muon here,

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like a tau lepton, and
a bottom quark here,

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like a top quark here--

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they acquire mass through the
coupling of the Higgs field.

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And so there's this
linear relationship--

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the correspondence
between mass and coupling

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to the Higgs field.

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Great.

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So we have this all
together, and it gives us

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a complete theory.

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Again, there are a large number
of open mysteries and questions

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we'd like to answer.

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And the way I look at
it is it's a little

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similar to the exploration
of Christopher Columbus.

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So what we're trying
to do is to go

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to higher and higher
energies, to higher and higher

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intensities to find
out whether or not

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we find first hints of
something new and unexplored.

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So we made this discovery.

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We made the discovery
of the Higgs boson,

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but whether or not this particle
is really the Higgs boson

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is still out there.

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We're trying to measure it
with more and more precision.

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Maybe we find deviations
from its expected properties

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to the ones we observe.

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Similarly, Christopher Columbus,
when he sailed off from Spain,

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he tried to reach the Indies
or Asia, and in his lifetime,

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he never figured out--
they didn't actually

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accomplish this.

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And similarly, maybe we have
discovered a new particle

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00:12:57,340 --> 00:13:00,430
which helps us to
understand more

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about the inner structure
of particle [INAUDIBLE]..