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MARKUS KLUTE: Welcome
back to 8.701.

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In this short
section, you're going

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to look at the weak
interaction a little bit more,

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and specifically discuss
neutral currents.

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We looked in some detail
at charged currents--

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specifically, the
interaction with quarks.

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So here, I'm going to look
at the Z boson specifically,

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and the weak interaction
via the neutral current.

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So studying those
two processes here,

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where there is an
electron and a positron

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through some process including
a Z boson and a photon

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and resulting in a
muon and an anti-muon--

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those processes have been
studied in great detail

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at SLAC and at CERN, at the SLC,
and the Large Electron-Positron

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Collider.

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So if we want to calculate
the cross-section

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and study the cross-section
of the center of mass energy,

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we see a number of
interesting effects.

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At low energies, and
at very large energies,

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the cross-section runs with
1 over the energy squared.

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But at the mass of the Z boson,
we see this enormous resonance

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here.

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The cross-section at the
resonance from the Z boson

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is about 200 times that
of just a photon exchange.

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So this allows you to study the
Z boson with great precision

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at those colliders.

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You have sizable
cross-section when you are

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in electron-positron colliders.

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And then you can,
with precision,

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look at, what is the rate
into a muon/anti-muon?

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What is the rate into
a quark/anti-quark?

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And so on.

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And you can study the mass,
the width of the Z boson

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with an enormous
level of precision.

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Again, so I will not go
into too much detail here.

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And please have a look at
chapter 9.6 in Griffiths,

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for example.

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But there's many other resources
where you can learn more

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about [? neutral ?] currents.

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Neutral currents,
electroweak neutral currents

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are specifically important
in the study of neutrinos,

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as we will discuss more
in the lectures as well.