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MARKUS KLUTE: Welcome
back to 8.701.

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With this lecture,
I'd like to introduce

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the major players of this
class, the particles,

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fundamental particles, but also
some of the compound particles,

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which play a role in
the discussions we'll

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have over the next weeks.

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For centuries, people
believed that atoms

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are the most fundamental
constituents of matter.

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The name atom comes
from the Greek atomic,

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which means not divisible.

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But as you know today, electrons
and nuclei build an atom.

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But even those nuclei are
not fundamental particles.

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As you see nicely
here in this picture,

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the nucleus can be built out
of many neutrons and protons,

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and even those
protons and neutrons

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are not fundamental particles.

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A proton, for example,
as depicted here,

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has three components
and three constituents,

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two up quarks and
one down quark.

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A neutron, then, is built out
of one down quark and two up

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quarks.

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It's kind of important to
understand and appreciate

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the size of those particles,
specifically the difference

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in size.

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Comparing here an atom, a
typical atom, of the size of 10

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to the minus 10 meters,
and that compares

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to a nucleon, which can be a
few 10 to the minus 15 meters.

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When we talk about a
proton, we typically

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like to use units
of femtometers,

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which is 10 times 10
to the minus 15 meters.

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The tremendous size different
or the expansive finding

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of the famous gold
foil experiment, which

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found that an atom
basically is made out

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of a nothingness, empty
space, and a very dense

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charged core, the nucleus.

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So you see this very
much in this picture

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and by comparing those
order of magnitudes.

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Not shown in this picture
here is a particle which

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doesn't really like to interact
with anybody else or which

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the forces it's interacting with
are so weak that it cannot be

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found, and that is a neutrino.

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A neutrino is not that
different from an electron

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or from a quark.

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It's just that the
interactions it participates in

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are only the big force
as we understand today.

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Just to be clear, when I talk
about a fundamental particle,

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I talk about a particle
which has no size,

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it's infinitely small.

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It has no substructure,
meaning it cannot be broken up

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into constituents and
can also not be excited.

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Having said that, this is our
current understanding of nature

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and of those particles.

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Experimentally, we can
only probe those particles

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to a certain scale
or size, and we'll

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talk later about how
precisely we actually

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do know that a
quark is fundamental

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or an electron is fundamental.

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In this discussion here
and most of the lecture,

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I talk about the standard
model in particle physics.

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It is a fact that
our measurements

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and our experimental
findings are

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in fantastic agreement with
this very predictive theory.

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The only experimental
deviation from this

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is the fact that we measure
the mass of neutrinos

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to be non-zero.

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As a consequence, you could say
the standard model is broken

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or we found physics
beyond the standard model,

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but it is actually
rather straightforward

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to extend the standard model
to accommodate neutrino masses.

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So we can just forget
about this small fact

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and assume that the standard
model describes nature

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as we know it.

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In the last week of this class,
we'll talk about motivation,

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why we thinks that the
standard model, in fact,

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is not complete, and one of the
big drivers here is the fact

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that we cannot describe
all observations in nature,

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specifically the
observation of dark matter,

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with the standard model
in particle physics.

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But that's for a later date.

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Looking into some more
detail, the standard model

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has sets of particles,
some particles which

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carry forces and some
are metaparticles.

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The ones who carry forces
are all spin-one particles,

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and their bosons.

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We have, in the standard model,
describe three interactions.

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The electromagnetic
interactions,

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which are known from light,
electromagnetic phenomena,

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chemistry.

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The atom's behavior
molecule is determined

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by electromagnetic interactions.

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And then there's a
strong interaction.

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The name already tells
you that it's strong.

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It's very strong.

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The first carrier
here is the gluon.

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The gluons, there's
eight, and then

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differentiated by
the so-called color,

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which is an interesting effect.

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And then, there is the
weak interaction carried

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by the W boson and the Z boson.

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They are different in their own
right because they carry mass.

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They are massive particles.

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And they're actually quite
heavy, about 80 to 100 times as

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heavy as a proton.

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The weak interaction is
responsible for neutron decays,

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also responsible for
the burning of the sun.

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And in our nuclear physics part,
we talk in detail about what

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that all means.

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Gravitational effects
are not considered

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in the standard model.

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They are very,
very weak compared

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to the strength of the other
forces we'll discuss here.

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But it's technically
very difficult

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to actually accommodate
gravity as part

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of a quantum [INAUDIBLE] theory.

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And therefore, we will
simply ignore this fact.

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And this is yet
another reason why

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you can consider
the standard model

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to be incomplete as a model
or theory describing nature.

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The matter particles
themselves are all fermions.

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They have spin half.

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And they come in three
different generations.

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The only difference between
one generation to the next

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is the fact that those
particles have different mass.

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In other words, their
coupling to the Higgs field

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is different.

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And that's the only difference
between those particles.

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There's consequences
out of this, for example

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heavier particles indicating
to lighter particles.

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We differentiate between
the quarks and leptons.

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Quarks partake in the
strong interactions,

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while electrons are neutral
in the strong interaction.

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And then we have seen
neutrinos already

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and, electron-type particles.

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Electron-type particles,
charged leptons,

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they have electric charge.

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So they couple to photons
by neutrinos bond.

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I'll show you here again one
of those striking differences.

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An electron is 9 times 10 to
the minus 31 kilograms heavy.

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We like to talk about
units in a different class.

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But that's 511 keV.

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While the neuron is about 200
times heavier, and even heavier

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the tau lepton.

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And as I said
before, that allows

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that tau lepton [INAUDIBLE]
neurons to decay

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into lighter particles.

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We'll talk about this and how
we can use those decays in order

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to learn about the
standard model.

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And then there is
the Higgs boson.

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And again, the Higgs
boson, discovered in 2012,

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plays a very special role
in the standard model.

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We talk about the
Lagrange is a theory which

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describes the standard model,
or which is the basic--

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is a theory itself.

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And later on, in this theory
we introduce a potential which

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is shown here-- it's
this Mexican hat--

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if you want-- potential.

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And what you see here
is that in this picture,

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that the lowest energy
state of this potential

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breaks a symmetry.

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So it's away from the 0 point.

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And that symmetry
breaking then gives mass

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to the W and the Z boson,
which I was describing before.

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And the coupling of meta
particles to the Higgs field

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gives mass at this point.

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All this on a later date.

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I thought here, I'd also
give you how CERN actually

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depicts the Higgs boson.

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July 4th is a special
day in the US,

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but it's also a
special day at CERN,

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because the Higgs discovery
has been announced on this day.

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And at CERN, typically, the
menu is enriched on that day

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by the Higgs boson itself
in the form of pizza, which

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takes the shape of
event displays--

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displays of
proton-proton collisions

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into the Higgs boson
and then further decays.

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We'll look at some of those
event displays also later.

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Well, that completes the
elementary-- the fundamental

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particles of the standard
model, particles which describe

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almost everything around us.

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So we have seen the charged
and neutral leptons.

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We have seen the quarks.

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We have seen the force carriers,
with the W/Z bosons, the photon

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and the gluon, and
the Higgs boson, which

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is kind of the very special
particle holding this

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all together.

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An interesting point
here is, again,

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looking a little bit at history
and when those particles have

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been discovered, and also
when those particles have

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been explained--

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and I don't want to
read this all to you.

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You see that the earliest
discovery was a discovery

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by J.J. Thomson of the
electron, and the latest,

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the completion of the particles
in the standard model--

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the Higgs boson-- in 2012.

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Interesting for the
Higgs boson, the time

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between the
theoretical discovery

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by Peter Higgs and
friends was about 50 years

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before the
experimental discovery.

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But then there is also
composite particles.

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The things around you are
all composite particles.

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Here, we can differentiate
between mesons and baryons.

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Mesons are particles
which are made out

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of quarks and antiquark pairs
[? for the ?] bound states.

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And they are bosonic, because
you add 2 1/2 particles

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together.

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One example is a pion,
which is made out

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of up quarks and
down quarks that can

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be charged in neutral pions.

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And the zoo of particle
increases quite quickly

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if you then consider that it's
not just up and down quarks

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making those particles.

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But you could add strangeness to
that, meaning a strange quark.

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And so you see here in this
picture, the mesons, the pions,

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etas, but also then neutrons
and charged particles

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with different charges,
and then kaons,

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which are particles
which have one

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s quark in addition to an
up quark and down quark.

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And then there's baryons.

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They are made out
of three quarks.

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We have already seen the
protons and neutrons.

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But also here, you can see
a different configuration.

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We'll introduce the
concept of isospin.

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You can see here the
proton and the neutron,

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and then strangeness being
added with one or two

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components of strangeness.

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And then there is this
isospin component as well.

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This situation becomes
complex very, very quickly.

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But we'll look at
this in more detail.

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And then again,
putting those bound

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states together-- bound
states of protons and neutrons

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through the strong
force gives us

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a rich table of nuclei
on isotope tape.

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Here, you can describe nuclei
by the number of protons, which

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is typically called Z, and
the number of neutrons,

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which is typically
called N. The sum,

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N plus Z is the atomic mass.

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Now, each proton and neutron
are about 1 geV heavy.

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And then the atomic mass
is simply the sum of them.

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So you already know
explicitly how heavy

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your isotope might be.

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We'll talk about the fact
that those masses are not

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quite the sum of
the masses later,

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because there is some
binding energy involved.

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And then isotopes can
be stable or unstable.

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They can decay in
various processes.

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You can combine them.

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It's very interesting to
understand how they're actually

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being created in our solar
system or the universe

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in general.

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So with this, I would
like to conclude

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this part of the introduction.

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So we have seen
the major players

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of this course with the
fundamental particles,

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but also the compound particles,
meson, baryons, and nuclei.

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And there's a few more
points in the introduction

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before we then dive
into a little bit more

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of the theoretical discussion.