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Welcome back to 8.701. Our second part of the chapter on QCD is on elastic electron-proton scattering. So elastic electron-proton scattering in general has a long history, going back into the 1960s. Very famous examples are the MIT-SLAC or SLAC-MIT experiment which led to the Nobel Prize in physics for Jerry Friedman, Henry Kendall, and Richard Taylor in 1990. And the most recent and highest energetic electron-positron experiments were conducted at DESY in Hamburg, at the so-called HERA ring. And we'll go back into those experiments. They are famous for not just elastic electron-positron scattering, but inelastic and deep inelastic electron-positron scattering, which is going to be subject of future lectures. What we are trying to do here-- you might still wonder why this is topic of a QCD lecture. What we're trying to do is study the structure of protons. And the way to look at this is in these Feynman diagrams, we're using this photon here to probe into the ...

Welcome back to 8.701. In this lecture, we'll talk about CP symmetry or CP violation. In previous lectures, we discussed that the weak interaction is not invariant under parity and charge conjugation transformation. But now we can ask the question, how about CP-- so transformation which does change conjugation and parity transformation. The classical example to show parity violation is the decay of a pion. So we have here this charge pion with spin 0. And it decays into a muon and a neutrino, an anti-muon and a neutrino. And so since the neutrino is left-handed, the-- [INAUDIBLE] decay coming onto muon needs to be left-handed as well. So if you do parity transformation of this decay, you see that the outcoming muon would be right-handed. On the other hand, there is no right-handed neutrino. And therefore, this decay is not possible. So this is not-- so this mirror symmetry is not realized in nature, as a consequence of the weak interaction. So similarly, we could do a...