Tuesday, July 31, 2007

Lattice 2007 -- Day Two

The second day started with the annual experimental talk, which was given by Diego Bettoni, who spoke about FAIR (Facility for Antiproton and Ion Research). After an overview of the accelerator facilities involved, he spoke about charmonium spectroscopy. The advantage of studying charmonium systems in pbar-p annihilation reactions is that states of all quantum numbers can be produced directly, as opposed to e+-e- annihilation which gives only 1-- states directly and all others via radiative decays only. Studies of the χc and ηc states were presented. Planned studies are searches for exotic charmonium hybrids and for glueballs, measurements of the in-nuclear-medium mass shifts of the D meson mass, studies of double hypernuclei (nuclei with two nucleons replaced by hyperons), measurements of the proton form factor in the timelike region, and reversed deeply virtual Compton scattering, all at PANDA, and studies of nucleon structure with polarised antiprotons at PAX. As always, the experimental talk was somewhat sobering, as it pointed out the huge gaps in one's (or at least my) knowledge of experimental physics.

Next was Craig McNeile speaking about hadron spectroscopy. Topics were the η and η' mesons, the 0++ spectrum, the controversial κ meson, distinguishing qqbar mesons from tetraquarks and molecules, the glueball spectrum and the search for glueballs within the meson spectrum, the changing and mixing in the 0++ spectrum from unquenching, the f0(600)/σ meson, and comparisons between different unquenched studies, including the different values obtained for r0.

After the coffee break, we got to the "staggered wars" plenary. Mike Creutz opened with a talk on "why rooting fails". The crux of his argument as I understood it was that rooting averages over the four tastes, which have pairwise opposite chiralities, leading to a theory that is not a theory of a single chiral fermion. The postulated manifestation of this was an incorrect singular behaviour of the 't Hooft vertex in the rooted theory, which could lead to the wrong physics in singlet channels, particularly the mass of the η'.

The opposite point of view was presented by Andreas Kronfeld. He argued that the group structure of staggered symmetries is much more complex than usually considered, and that the "phantom" Goldstone bosons coming from the tastes removed by rooting cancel in physical correlation function. He then proceeded to counter the points raised in Creutz's criticism of rooted staggered quarks, arguing that rooting turns the quark mass m into its absolute value |m|, that the staggered taste-singlet chirality is not the same as naive chirality, and does in fact track the topological index correctly if the chiral and continuum limits are taken in the right order.

The final plenary talk was an ILDG status report delivered by Carleton DeTar. The ILDG (International Lattice Data Grid) is the union of national grid applications from Europe, the UK, Japan, Australia and the US, which is intended to allow sharing of lattice configurations, and eventually propagators, between collaborations. They have developped portable data formats (a markup language called QCDml and a binary format for lattice configurations), as well as the grid software. While the permissions policies of the various collaborations are still an issue in some cases, the general tendency seems to be that it is now easier to download unquenched configurations than to generate quenched configurations, which will put the last nail into the coffin of the (already quite dead) quenched approximation over the next couple of years.

After the lunch break, there were parallel sessions. Some remarkable talks were about non-QCD physics on the lattice: Julius Kuti talked about getting Higgs physics from the lattice by using a lattice theory as the UV completion of the Standard Model, Simon Catterall talked about exploring gauge-gravity duality through simulations of N=4 super-Yang-Mills quantum mechanics as the dual of a type IIa string theory with D0 branes, and Jun Nishimura talked about non-lattice Monte Carlo simulations of SYM quantum mechanics as the dimensional reduction of a theory that might be M-theory.

The poster session was interesting, if a tad chaotic, for which I blame the Bavarian beer. I didn't get to see all the posters, since I spent too much time talking to people I knew who had posters.


amused said...

If the MILC folks were genuinely serious about checking that the rooting is ok they would redirect their efforts away from heuristic hand-waving "plausibility arguments" and into actual calculations of quantities where a problem, if it exists, is most likely to show up. But such calculations (e.g. of the eta' mass) really don't seem to be a priority for MILC.

Georg said...

I have to say I am a little tired with anonymous accusations of intellectual dishonesty against the "staggered community". To turn the argument around: the "staggered skeptics" could just as easily destroy the fourth-root trick once and for all by calculating the η' mass on the MILC lattices and showing that it comes out unphysical, instead of presenting theoretical arguments. -- Accusations, however veiled, don't help; results do.

still amused said...

Your response leaves me quite incredulous... Please distinguish between accusations of intellectual dishonesty (which I did not make)and saying that people are not doing what they could and should do (imo) to verify that their approach is valid. These are completely different things!

"To turn the argument around: the "staggered skeptics" could just as easily destroy the fourth-root trick once and for all by calculating the η' mass on the MILC lattices and showing that it comes out unphysical..."

No, I am sorry, the burden of doing this lies with the people who use this approach. In case you don't know, calculating the eta' mass in LQCD is very tough, requiring huge investment of time and computer resources. You can't expect other groups to put their own research programs on hold to do this. And I suspect many "skeptics" don't have the necessary computing resources at their disposal to do this calculation, even if they wanted to.

In response to the rest of what you wrote and your general tone, let me just mention that unlike you I have published papers on the 4th root business, and in fact they were supportive of it (well, one of them was). I'm an agnostic, not a skeptic. Hopefully, after having worked on this, it is ok for me to have some views on the situation and express them here. The view I expressed in the last comment was also influenced by having talked with a very relevant person but I'm obviously not going to start relating the contents of private conversations here.

Since you seem to have a problem with my anonymity I'll send you an email to reveal who I am once I get home from Regensburg. The view expressed in my last comment is my honest opinion, but I would never have written it here if I had to sign my name to it - it's not exactly going to endear me to influential people..

"Accusations, however veiled, don't help; results do"

In the unlikely event that my little rant here provokes MILC to care more about calculating things like the eta' mass then I would consider it a big help!

Georg said...

First, please note that I did not intend to insult you in any way; the tone of your comment suggested to me that you did not believe MILC were serious about ensuring the correctness of the fourth-root trick, which to me sounds as if MILC were essentially deceiving themselves. If that is not what you intended to say, I apologise for misreading you.

The problem with anonymity is that it is very difficult to tell how much the other person knows, and how serious they are. You don't need to identify yourself if you don't want to. On the other hand, you should not be upset if an anonymous comment is not seen in the context of its author's publication record.

I am aware that the η' is very hard to do, but would have thought that the "clean kill" of rooted staggered should be worth it.

Finally, let me say that I am often a little bewildered by how personally most people seem to take the staggered business. In the end, it will all go away when the progress in computing power makes staggering redundant. Until then, it is my (obviously rather irrelevant) opinion that the field benefits from having results from as wide a range of fermion discretisations as possible. If any of them is wildly incorrect, it should show up eventually.