Thursday, December 14, 2006

Off-Lattice Links

JoAnne at Cosmic Variance has a nice introductory post about particle detectors. More about the subject, with a description of the CLEO-c detector, can be found at Superweak.

Peter Steinberg writes about nuclear toys, specifically an A.C. Gilbert U-238 Atomic Energy Lab. Nuclear toys for nuclear boys may sound like a bizarre relic from another age, but in fact, as recently as 2005, the Boy Scouts of America instituted a Nuclear Science merit badge with the advice of the APS Nuclear Physics Division.

Monday, December 11, 2006

Christmas presents

I'm sure everyone working in theoretical physics knows the problem: Many, probably most, of the people who are most important in your life -- family, friends, loved ones -- don't have the background to understand what your research is all about. So what will you do? A strict secret-agent-style "don't talk about work when at home" policy is deeply unsatisfying; after all, you went into academic research rather than into a more lucrative field such as, say, finance, because you are driven by a consuming interest in finding out how the universe works. But any attempt at a conversation about our work tends to end in a muddle of confusion ("What do you mean by a symmetry? What on earth is a group? Why do you have to prove that, isn't it obvious (or clearly false)? What's a simulation? How can a computer solve the equations if you can't, I mean, don't you have to tell it how to solve them? ...?") or worse yet, misunderstandings ("--- studies particles that form groups and have links between them, or something like that!") when talking to people lacking the basic concepts necessary to understand contemporary research, i.e. to virtually all non-scientists. Of course there always is the pop-sci approach (leave out mathematics and every other bit of "technical" detail and just focus on the beauty and wonder of it all), but the people closest to you usually want more than that: they want to be able to ask "How was your day/week?" and get a meaningful answer that does not exclude your research, which is after all what you spend most of your time doing. But if their background is in the arts or humanities, they don't just lack the technical knowledge of, e.g. group theory -- that could be easily remedied; no, most of the time they are actually deeply unfamiliar with mathematical reasoning and the general modes and methods of scientific research.

At Christmastime, there's always the question "what can I give to ---", and what better than to give the gift of greater understanding? For introducing people with a humanities background to the kind of ideas and ways of thinking used in some of the more abstract fields of theoretical physics, I have tried titles from Oxford University Press's Very Short Introduction series of books, such as the one on Particle Physics by Frank Close, the one on Quantum Theory by John Polkinghorne, the one on Cosmology by Peter Coles, or the one on Mathematics by Fields medalist Tim Gowers. I think they do a very good job at giving a non-technical introduction to their respective subjects that goes a good way beyond the usual pop-sci stuff without trying to make experts out of their readers.

For the more down-to-earth kind of physics, the most recent issue of PhysicsWorld (the one with a lattice on the front page), contains a very positive review of Louis Bloomfield's "How Everything Works", which is said to be a general-market version of the author's textbook "How Things Work" for physics courses for non-scientists.

Thursday, December 07, 2006

Lattice QCD makes title page

The latest issue of PhysicsWorld has a feature article on Lattice QCD by Christine Davies describing the recent progress made in confronting theory with experiment through unquenched lattice simulations. Among the highlights she mentions are the correct prediction of the mass of the Bc and the fact that the determinations of the quark masses and the strong coupling constant αs from unquenched lattice QCD are now more accurate than all other sources combined.

The article is very well written and should be easily understandable for anyone with a background in physics, and I would think that an informed layperson should also be able to learn something from it.

Axions discovered?

There is a forthcoming paper which claims the discovery of two particles decaying into electron pairs, which are tentatively identified as axions, with masses of 7 and 19 MeV respectively.

I am a little sceptical of this claim, based as it is on two narrow peaks being 3 standard deviations above the best fit to the background. I'm no experimentalist, but the situation appears to me to be too similar to the "discovery" of the pentaquarks, which later dissolved into statistical fluctuations as better data became available.

If this discovery turns out to be real, though, this would be huge news: the discovery of the axion would solve the strong CP problem (why is the CP-violating θ-angle in QCD so close to, or even identically, zero, when it generically should be of order one?) and might also contribute to solving the riddle of dark matter. There are a number of experiments looking for the axion, the best known being PVLAS, who claimed to have found a candidate with a mass in the meV (milli-eV) range, and the new experiments at CERN aiming to test their results.

So we should wait and see if axions have indeed been observed. If so, it would be great news for theoretical particle physics: it would be, as far as I am aware, the first discovery of a particle whose existence was conjectured based on naturalness considerations alone. If not, it would show once again that results based on low statistics should always be taken with great care.

Update: More criticism (by experimentalists) of the claimed discovery can be found on Chad Orzel's blog and on Superweak.

Physics Result of the Year, Anyone?

Chad Orzel has a poll about which physics result of 2006 deserves to be called The Physics Result Of The Year. Surprisingly, so far nobody seems interested in nominating their favourite result for this honour. Which is strange, because A) the APS has a list from which you could simply pick one, and B) nobody says you can't nominate your own results if you feel so inclined. This isn't the Nobel Prize, after all.

On the lattice QCD front, I'm going to be both bold and modest at the same time and nominate the recent progress in the debate about the validity of the fourth-root trick for staggered fermions as the result of the year. It isn't really a result, because the debate isn't completely resolved, but there are results now where there was mostly just conjecture before, so this is definitely progress.

Anyway, please go Chad's fine blog and nominate your favourite result of the year. You wouldn't want the public to think physicists have no enthusiasm for their own work, would you?

Tuesday, December 05, 2006

Hadronic Physics from Lattice QCD

As a matter of fact, I have no idea how my small circle of reader is composed with respect to physics expertise or professional position, but I like to pretend that some of my readers are physicists with a genuine interest in, but no real experience with, lattice QCD. It is to these (imagined, and perhaps imaginary) readers that I want to issue a book recommendation, just in time for inclusion on their holiday wishlist.

The book in question is "Hadronic Physics from Lattice QCD", edited by Anthony M. Green, published by World Scientific. The aim of this book is to provide an introduction to lattice QCD for non-specialist readers such as nuclear and particle physicists, and while it cannot replace one of the various introductory testbooks (such as Montvay and Münster or Rothe) as required reading for people interested in pursuing original research in the field, I think it succeeds very well at giving the non-specialist a much better idea of the how and what, the strengths and the limitations, of lattice QCD.

The book is a collection of independent chapters by different authors, each of which focusses on a specific issue of interest that can be studied using lattice QCD.

The first chapter, by Craig McNeile, starts with a basic introduction to lattice QCD and its methods, including a discussion of systematic errors including how they can be reduced via unquenching, improved actions and chiral perturbation theory. He then proceeds to give an overview of the masses of stable mesons and baryons that can be measured accurately, as well as an introduction to the use of maximal entropy methods to determine spectral functions from lattice data, and some of the methods used to incorporate electromagnetic effects and to study unstable particles on the lattice, both of which are rather hard problems.

The second chapter, by Chris Michael, is devoted to a discussion of exotics, or states that are neither conventional mesons nor baryons: glueballs, and their mixing with scalar mesons of the same quantum numbers, hybrid mesons (mesons that contain a gluonic excitation along with a quark-antiquark pair), and hadronic molecules (states consisting of several hadrons bound by their residual strong interactions).

The third chapter, by Gunnar Bali, discusses the quark-antiquark potential, starting from the static quark potential and its relation to Wilson loops, the strong coupling expansion on the lattice, the confining string picture and perturbative calculations of the potential, and going on to discuss some aspects of quark-antiquark and nucleon-nucleon potentials for nonstationary particles.

The fourth chapter, by Rudolf Fiebig and Harald Markum, is concerned with the difficult topic of hadronic interactions in lattice QCD. After describing some of the issues that arise in a 2+1 dimensional "toy" model, they discuss the highly sophisticated techniques that are used to extract information on pion-nucleon, nucleon-nucleon and pion-pion interactions from lattice QCD. This chapter has an appendix which describes aspects of improvement of lattice actions, an important ingredient in any lattice project aiming for precise predictions.

The fifth chapter, by Anthony Green, discusses "bridges" between lattice QCD and nuclear physics, such as nuclear effective field theories and potential models that are founded upon, or at least inspired by, QCD.

All chapters have extensive bibliographies that should function as excellent starting places for readers who wish to learn more about the subject.

Monday, December 04, 2006

The arXiv is changing

Via Urbano Franca: The arXiv preprint archive is changing the way it labels papers with effect from 1st January 2007. The familiar arch-ive/YYMMNNN identifiers like hep-lat/0411026 will be gone (although they will be retained for old papers), and new identifiers of the form YYMM.NNNN will take their place. The stated reason for this is that the math archive is getting dangerously close to 1000 submissions a month, which would break the existing indentifier system. The new identifiers will no longer be assigned on an archive-by-archive basis; including the archive will be done as in 0701.1234 [hep-lat]. The new system is expected to be good for a number of years, and after that five-digit identifiers YYMM.NNNNN will be needed.

This change appears to be orthogonal to the other announced big change in the physics arXiv, although it is possible that the latter is considered redundant now. A slightly more open information policy on the part of the arXiv might be nice from time to time, but I suspect they are afraid that more openness might offer more inroads to cranks and crackpots, so I kind of understand their policy of semi-secret decision-making. Still, I think it probably couldn't hurt too much if they sent out informative emails to registered authors from time to time.