Friday, July 29, 2005

Lattice 2005, day four

Well, if day two was the long day, day three was the short one. There
were no plenary sessions, just a short moring parallel session, then
excursions in the afternoon. It was a very nice day to be out on the
Irish countryside, so that was very nice.

Day four had the plenary sessions I was most interested in, a close
collaborator, Quentin Mason, started off the day talking about lattice
perturbaiton theory, which is what I do. Quentin has completed the
two-loop calculation of the light quark mass, which allows for a much
more accurate determination. Quentin also reviewed the determination
of the strong coupling constand, which I've covered in a previous
post.

Next up was Zoltan Ligeti, who reveiwed progress in heavy quark
physics from a non-lattice perspective. There's been lots of activity
on this front over the past few years, with the development of a new
expansion, the Soft Colinear Expansion. This theory is complicated
enough that I won't even try to explain it.

Finally, another collaborator, Masataka Okamoto, gave a very nice
overview of the status of lattice calculations of the CKM matrix.
This the the matrix which tells you how the various types (or
flavours) of quarks interact in the standard model. It has nine
entries (not all of them are independent) most of which can be
computed from lattice QCD + an experimental result. Masataka has done
a large amount of work, both doing many of the calculations himself,
and collecting everything into a coherent picture.

In the standerd model, the CKM matrix is unitary. If you accept that
assumption, the Masataka has produced a complete determination of the
CKM matrix from lattice QCD, experimental measurements, and the
unitarity of the matrix. Of course, it would be nice to test the
unitarity, from just theory+experiment with no extra assumption. In
that case, you can check row by row in the matrix. Masataka showed
how one row is completly determined without assuming unitarity. And
in that row, the matrix is unitary, up to the errors. It will be a
big challenge to repeat this for the other two rows.

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