The Division of Particles and Fields of the American Physical Society has been having its annual meeting at UC Riverside during th past few days, and some of the plenary talks have been put on-line.
Particularly interesting is the talk by Jamie Rosenzweig about Advanced Accelerators: Near and Far Future Options. It reviews ongoing development of existing technologies for use in the next (post-LHC) generation of accelerators, including the superconducting RF cavity technology recently chosen for use in a possible electron-positron linear collider. But it also covers some of the more exotic acceleration technologies that people are thinking about, including optical lasers and plasma wake-fields. Some of these technologies, if they could be made to work, hold the promise of creating much higher accelerating gradients and might allow the construction of much higher energy linear accelerators. The future of particle physics may end up depending on the success of these efforts.
The review of New Models of Electroweak Symmetry Breaking is interesting, although mainly in that it shows that the ideas going around about this aren’t very compelling, and perhaps some dramatically new ones are needed. The reviews of heavy flavor and neutrino physics give a good idea of the current experimental situation. Still to be posted are talks by Clifford Johnson on “Current Trends in String Theory” and by Sean Carroll on cosmology. Carroll also has an interesting discussion of the current state of tests of general relativity on his weblog.
Conceptually the charmonium states can be thought of like positronium (that’s the reason for the name), but you cannot use a pure 1/r potential in a model. Model calculations typically use a 1/r + r or 1/r+log(r) potiential, where the latter term “mock’s up” confinement. Even then you have to account for relativitic corrections, which can be quite large for charm. For the bottom system, the potiential model approach works better.
This only works because the quarks are heavy, so essentially non-relativistic. You cannot use this approach for bound states with light quarks, or to compute decays. But, to the extent that a 1/r + r potential is “funky QED” it can be done for heavy quarkonium states.
If you’re interested in advanced accelerators, you might want to consider coming to our ‘Advanced Acceleration Concepts’ Workshop. It’s held in the US every other year – this year’s was hosted by Brookhaven Nat’l Lab and held at SUNY Stony Brook. There’s a good website if you want to see the range of topics and interests covered – http://www.bnl.gov/atf/AAC04.htm .
Also – you might want to visit the physics dept. at Columbia. Tom Marshall has been a long-time participant and contributor to this field.
My point was just that the spectrum of c/c-bar resonances look a lot like the energy levels of positronium, so there must be something going on that is similar in terms of the binding force. Someone gave a talk about the similarities when I was at Oxford 20 years ago, but I can’t remember all the details.
Well on refreshing my memory there are other resonances for c-cbar states, see here:
http://www.e835.to.infn.it/people/gollwitz/pdg_charmonium.html
Chris,
Charmonium is just another name for the J/psi meson (c-cbar). It’s made in electron-positron scattering and decays into pions, so
something like
e+e- –> c-cbar = J –> pions
-drl
It’s remarkable that any experimental information at all could be pulled from the theory, and to be honest I don’t know exactly how this was done. Someone?
By hook and crook. Before QCD there was the SU(3) models of Gell-Mann. And the whole current algebra programme. Both of these produced information that follows from symmetry considerations in QCD. Then there was Bjorken scaling, which pointed the way to the full theory of QCD.
Nowadays, there are plenty of ways to extract information from QCD
1) Perturbation theory for high energy processes
2) Lattice calculations
3) Effective field theory analysis (chiral perturbation theory, heavy quark effective theory, …)
You really can’t even naively imagine QCD like “a strong funky QED”
Charmonium?
Chris,
You really can’t even naively imagine QCD like “a strong funky QED” because the basic interactions are completely different, i.e. in QCD one has gluon loops as well as quark-antiquark pairs in the vacuum.
Plus, the coupling is on the order of 1 rather than 1/100, so even the simplest calculations are intractable.
It’s remarkable that any experimental information at all could be pulled from the theory, and to be honest I don’t know exactly how this was done. Someone?
-drl
I am a bit rusty, so can someone remind me why one could not have quarks being extremely heavy with extremely high binding energy, making it always energetically preferable to produce composites in scattering events rather than lone quarks?
With strongly interacting relativistic particles like quarks or gluons, you really can’t usefully think about the mass of a state as being a sum of masses of its composites and a potential energy contribution. In the limit of massless quarks the spectrum includes:
1. massless pions that are in some sense composites of two quarks.
2. massive baryon states that are in some sense composites of three quarks.
3. massive “glueball” states that in some sense have no quarks.
There’s no way of understanding these things in terms of free particles bound by a potential. For one thing, as you note, you can’t even define the mass of an unbound quark since such a state carries infinite energy.
Although I’ve heard something like this before, and even read articles by Frank Wilczak on the same subject, I still find Matthew Nobes’ comment hard to understand. A helium atom and a neutron weigh less than a deuterium atom and a tritium atom. The difference, the binding energy, is available to be released by a fusion interaction. Similarly, a neutron weighs more than a proton plus an electron plus a neutrino, and hence decays with a positive release of energy. The rule is mass of bound particle equals mass of constituents minus mass equivalent of binding energy. Therefore if two or three massless quarks are stably bound together with a positive binding energy, it would appear that the bound meson or nucleon should weigh less than zero. What gives?
I guess it somehow relates to “asymptotic freedom / infrared slavery” and hence the quarks are only massless when confined, but are actually infinitely massive when deconfined. Could one then say that most of the mass comes from the fact that the quarks are only imperfectly confined? At least this way the binding energy has the right sign. In this way of speaking, the “massless pion” is massless only because the two infinitely massive quarks are bound together by an equally infinite amount of binding energy.
Jim Graber
Well, the standard model does not predict masses for the elemtary particles, that much is true. It is interesting to note though, that if you take QCD with two massless quark flavours, you still get a theory that roughly approximates the real world. That is, very light pions (in this case massless) and nucleons with approximately the right mass.
This is because most of the mass that we see around us is actulally the binding energy of the gluons in protons and neutrons. The actual masses of the up and down quarks contribute very little to this.
Now of course it’s very hard to predict the proton mass using QCD, even with just two flavours of massless quarks. But it is possible in principle, and it’s something that can be done in the framework of QCD alone.
In the standard model, the masses of elementary particles are the product of the electroweak vacuum symmetry breaking mass scale and the strength of the coupling of the particle to the field causing the symmetry breaking. No one knows why either of these two factors take the values they do.
I haven’t studied particle physics (or any advanced physics) and have a simple question: do any theories predict masses of particles? From what I understand, this is cited as a failure of the Standard Model, so I am curious to know if other theories predict masses for any of the elementary particles.
Hi William,
I may take you up on that offer next time I’m in Berkeley. I’ve never understood why this sort of research doesn’t get more attention given its potential importance.
Sean Carroll’s DPF talk on cosmology has now been posted.
(You say “still to be posted” so this is an update)
It is great to see such a nice plug for advanced acclerators in your blog. These devices have still a long way to go to be useful for particle physics. However I expect that they will be the basis of a user facilty for other fields of physics in the next decade. If you visit LBNL, we will be happy to give you a tour of our laer wakefield laboratory.