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

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.

Wednesday, July 27, 2005

Lattice 2005, day two

Hello again from Dublin. Day two of Lattice 2005 was the "busy" day,
with three plenary sessions, a parallel session, and the poster
session all in one day. Twelve solid hours of physics, which is
rather tiring, particularly since I was presenting a poster. As such,
today's update of the Plenary sessions will be brief.

The morning started off with a talk by Herbert Neuberger on simulations
of large N field theory. As discovered by 't Hooft, SU(N) gauge
theory simplifies as you take the limit N -> infinity. However, this
is hard to do in Lattice QCD, as you would need infinite sized
matrices. However, there are techniques for attacking the problem.
Neuberger reviewed the interesting phase structure you see in this
system. The lattice version of large N QCD has 6 different phases.

Next up was Simon Catterall, who reviewed his work on Lattice
supersymmetry. In the early days of both supersymmetry and lattice
QCD, it wasn't thought possible to put a supersymetric theory on the
lattice without badly breaking the supersymmetry. However in recent
years, a few different methods have been discovered. The basic idea
is you construct a continuum theory with lots of supersymmetry and
arrange things such that when you put the theory on the lattice, a
remmenant of the supersymmetry remains. Simon reviewed his method for
doing this, and briefly touched on some of the possible applications
of these methods.

After a coffee break, the sessions shifted in focus a little bit.
Chris Dawson reviewed the state of Kaon Phenomenology on the lattice.
The focus here is on kaon decays, which are hard to do in lattice
QCD. For example, a kaon can decay into two pions. This is extremely
hard to compute, since pions are very large, it's hard to fit two of
them inside your finite lattice box.

We swapped last names for the next talk, Chris Dawson became Chris
Michael, who gave a lively review of the state of hadronic decays on
the lattice. The people I work with are interested in doing very high
precision calculations. This is good, however, it limits you to a
small number of thing you can calculate. However lattice QCD can, in
principle, calculate many many more interesting strong interaction
processes. Chris gave an update of the state of some of these
calculations, which are very very hard to do. You have an unstable
particle in the intitial state, two or more hadrons in the final
state, and a transition at some point between. I

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.

Tuesday, July 19, 2005


Been a while since I posted an update, so I thought I'd check in. The physics blog world is abuzz with the creation of a new physics group blog cosmic variance which features Sean Carroll, Mark Trodden, JoAnne Hewett, Clifford Johnson, and Risa Wechsler. A nice mix of cosmology, particle phenomenology and string theory.

I've been busy calculating, and preparing for Lattice 2005, which is the big annual conference for lattice field theory. This year it's being held at Trinity college in Dublin. They have wireless around, so I should be able to liveblog at least some of the sessions. I'm also going to try to post nightly updates.

An amusing note, the built in spellchecker for blogger flags "blog" and "liveblog" as spelling errors.