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PROFESSOR: Hello.

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In this lecture, we talk about
parity and parity violation.

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So we talk about a
discrete symmetry.

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A little bit of history
in particle physics--

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Lee and Yang in
the 1950s wondered

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if there's any experiment
testing parity invariance.

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There are many tests
which had to do

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with a strong interaction
and the electromagnetic

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interaction.

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But as it turned out,
parity invariance

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had never been tested
in the weak interaction.

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And so Lee and Yang
were motivated in this

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because it was a
puzzle at the time,

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so-called tau-theta puzzle,
which turned out to be kaon

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decays into various articles.

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And then that puzzle,
they solved the case

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with the same lifetime,
but different spin states.

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And so they couldn't
quite understand this,

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and were trying to
understand whether or not

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one can have an
experiment to test this.

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The experiments
they proposed is one

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where you look at the nuclei
and observe beta decay.

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So there's an electron
coming out of the beta decay.

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If you are able to align
the spin of the nuclei

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and put it in front
of a mirror, you

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see that the physics
going to the mirror state

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is not [INAUDIBLE] parity.

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You see that in the mirror, the
spin is the opposite direction,

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but the momentum
of the electrons,

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the electrons still
come out on the bottom.

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So there's clearly some
change in the physics going on

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in the mirrored state.

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So Madame Wu actually
took up this idea,

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and immediately tested
this in the same year.

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She was a Chinese-American
physicist, born in China,

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and then studied in Berkeley
together with famous people

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there.

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Lawrence is one of them.

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After the war or in the war,
she joined the Manhattan Project

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and made very, very
important contribution

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to the Manhattan Project
based on her thesis work.

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As a fun fact, she was married
to another Chinese-American,

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the grandson of
the first President

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of the Republic of China.

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As I said, in 1956, she
conducted the Wu experiment,

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and in the following year
received the Nobel Prize

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in physics for the finding.

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So really briefly, we're going
to have another discussion

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of the Wu experiment later on.

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What she did here is she
studied cobalt 60 decay

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to nickel, beta decays.

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And what she was able
to do experimentally

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with the magnetic field,
align the cobalt 60,

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and then just count the number
of electrons coming out.

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And it turns out that the
number of electrons coming out

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of the bottom and the
top are not the same.

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She did this by reverting
a magnetic field.

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And the experimental
data is shown here.

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You see as a function
of time, you just

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look at counting, which
is a function of time

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after injecting the
probe, and you revert.

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You do the measurement by
reverting the magnetic field.

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And you see very clearly
here that there's asymmetry

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in the beta decays.

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And that asymmetry
immediately tells you

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that there has to be parity
violation in beta decays

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or in weak interaction.

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So she found that this specific
picture here violates parity.

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And this is a dramatic signature
of the weak interaction.

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And we will later see how we
can actually understand it.

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Note that parity is conserved
in electromagnetic interaction

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and strong interaction.

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One more word on
parity inversion--

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you can define a
parity operator.

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And if you apply this parity
operator, for example,

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on the vector, these
x, y, and z components

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you simply turn the direction
over here with a minus sign.