Tuesday, July 26, 2005

Lattice 2005, day one

Hello from Dublin. As promised, I'm going to try to deliver daily
reports from the plenary sessions. Unfortunately, getting wireless
internet access in the conference room has proved problematic, so
it'll have to be after the fact reports, rather than live blog
updates. These comments are subjective, and I can cover every talk,
so that's that.

After the usual introductory speechs the conference got off to a bang
with a talk by Julius Kuti, from the University of California San
Deigo. The topic was Lattice QCD and String Theory, which is a
growing field. There is a lot of interesting problems in the field,
from more abstract things to practical things. Julius spent most of
his talk on a practical goal, namely using lattice QCD simulations to
understand effective string models of QCD.

In some sense this is a return to the orgins of string theory. The
original idea was to model the gluon field connecting two quarks as a
peice of relativistic string. The naive application of this idea
didn't work, and so string theory went off in a totally different
direction. However, with all the things that have been learned about
it, effective (four dimensional) string models can now be
constructed. And lattice QCD is the ideal tool to test these models
against. There are some issues, as there always are, but the results
here were promising, and offer a lot of new territory to explore.

Next up was one of the best field theorists in the world, Martin
Luscher. He talked about effeciently simulating a certain type of
dynamical fermions (Wilson quarks, for the experts) much more
effeciently than they've been done before.

His idea was to split the lattice up into smaller hypercubic blocks,
about 0.5 fm on a side. Then you split your update algorithim into
three parts,

gluon part + inside block quark part + block boundry part

Now, in the standard way of doing things, all of these parts are
computed the same number of times (say 2000 times per lattice
point). What Luscher (and his collaborators) do is take advantage of
the physics of the system to drastically reduce the number of times
you have to compute the block boundry part, which is the most
expensive bit. The essential bit of physics is that the correlation
between points on the boundry, and points deep inside the cube is very
weak. This means you don't have to compute it's effects nearly as
often as when you compute the gluon effects.

As Luscher mentioned, comparing computer algorithms is a tricky
business, however his simulations with this new method seem to be a
factor of ten or more faster than comperable simulations with the
standard methods.

In the second plenary session we had a talk by Jim Napolitano, who is
an experimentalist working on the CLEO-C experiment. CLEO-C is
currently studying D meson physics in great detail at the CESR
accelertor at Cornell. One of the main motivations for CLEO-C is to
test lattice QCD predictions in the charm system, so that results in
the B meson system can be confidently predicted. Jim ran over a
number of new results from CLEO including the leptonic decay constant
fD, and the masses of two new mesons, the h_c and the \Upsilon(1D).

These measurements are very tough, they involve looking for rare
raditive transitions in decays of highly excited mesons. The reason
that the can be done at all is because CLEO has very good control over
the initial state. Basically, they're colliding electrons and
positrons right on top of a charm anti-charm quark resonance. This
resonance decays to a pair of D mesons, almost at rest. In most
cases, both D's decay in a shower of crap (pions, kaons, etc). But
sometimes one decays into a shower of crap, and one does something
rare. When this happens you're happy, because, from the shower of
crap you can learn everything about one of the D's that decayed. And
sinc the total momentum is nearly zero, conservation of momentum tells
you that it's the same for the D that decayed in a rare way. With
that information, and the final state of the rare decay, you can very
accurately reconstruct what happened. As usual, listening to an
experimental talk made me glad I'm in theory. What they do is really
hard :)

So there's lots going on here. I'll update tomorrow with the next
round of talks.