Thursday, April 02, 2009

MAMI and beyond, Day Two

Hello again from Mainz, where I am at the conference "MAMI and beyond".

Today's first talk was by Barry Holstein (UMass Amherst), who spoke on "Hadronic physics and MAMI: past an future". The hadronic physics was cast mainly in the language of Chiral Perturbation Theory and its extensions. An interesting detail was the magnetic polarisability of the nucleon, which suggest that the nucleon in 10,000 times "stiffer" electromagnetically than a typical atom; this is in spite of the fact that the ability of the nucleon to transition to a Δ resonance ought to give it strongly paramagnetic properties from the quark spins; heutistically this is countered by the diamagnetism of the nucleon's pion cloud. Another feature that I found interesting was that the experimental determination of hadronic scattering lengths seems to be rather involved (possibilities mentioned involved the decay of pionium, or an analysis of the cusp structure in the energy dependence of K->3π or η->3π decays), and that the best way to determine them from theory is apparently from the lattice via Lüscher's formula for the volume-dependence of two-particle state energies.

The next speaker was Rory Miskimen (also UMass Amherst) talking about the measurement of nucleon polarisabilities in real and virtual Compton scattering. Real Compton scattering is, well, Compton scattering, virtual Compton scattering is the production of a photon in the scattering of an charged particle by a proton: γ*p -> pγ. Apparently the results from MAMI lie on a different curve from those from other experiments at other energies, which might suggest that there is something interesting happening around energies of Q2=0.3 GeV2.

The next two talks were by Bernard Pire (CPHT/Polytechnique) and Diego Bettoni (INFN Ferrara), who both talked about timelike processes. Due to my limited understanding of the relevant physics, I feel unable to give a summary of those talks, except that apparently it is quite difficult to disentangle the different form factors experimentally.

After that Fred Jegerlehner (Katowice and DESY Zeuthen) spoke about the running of the fine structure constant α. The running of α, which at zero energy is known to astounding precision, is of particular interest around the muon mass (where it enters the determination of the muon anomalous magnetic moment) and around the Z boson mass. The difficult part is to determine the contributions to the running of α coming from hadronic loops, the uncertainty about which causes a loss of five significant figures when evolving α from 0 to MZ. Using a method based on the Adler function (essentially a derivative of the self-energy with respect to the momentum squared), it should be possible to get a much more precise running of α by improving the understanding of low-energy hadronic contributions. Since most of the information needed in this approach would come from the Euclidean momentum region, the lattice might be able to help here.

After the lunch break, I skipped a couple of experimental talks to go over to the IWHSS workshop held next door and listen to a talk by Chris Michael about hadronic physics on the lattice. Chris presented approaches that can enable the determination of the nature of resonances and even the description of ρ -> ππ decays on the lattice.

After the coffee break, the lattice session of the MAMI conference took place: Meinulf Göckeler gave a summary of recent work towards the determination of generalised parton distributions on the lattice; Dru Renner at DESY Zeuthen works on this kind of thing, so I have heard about it a few times; it seems very hard each time I hear it, but I suppose saying "let's wait a few more years before starting on something like this" is not really an option.

Mike Peardon spoke about hadron spectroscopy on the lattice, giving a great introduction to lattice spectroscopy for the non-latticists in the audience. The highlight for lattice theorists was his mention of a new method that might replace noisy estimators for all-to-all propagators: a redefinition of quark smearing as a projection on the subspace spanned by the low modes of the Laplacian on a timeslice, enabling one to then exactly calculate all elements of the quark propagator out of this (relevant) subspace. The results shown looked rather promising, and the cost for diagonalising the Laplacian on a timeslice is of course much lower than that for diagonalising the Dirac operator as needed for the Dublin method of all-to-all correlators with low-modes.

Andreas Jüttner gave a talk about ongoing work to study mesonic form factors and (g-2). Using twisted boundary conditions to induce a momentum, he obtained very nice pion and K->π form factors. The (g-2) work is still in progress, but looks promising.

Silvia Necco gave an introduction to the links between Lattice QCD and Chiral Perturbation Theory, covering the extraction of SU(2) and SU(3) low-energy constants from Nf=2 and Nf=2+1 lattice simulations, and of the leading-order couplings Σ and F from simulations in the ε-regime.

Finally, Johann Kühn (Karlsruhe) spoke about precision physics in e+e- interactions, where the perturbative determination of the hadron-to-muon ratio R(s) has made it possible to precisely determine αs, mc and mb from experimental data (and the former two also from lattice simulations via the moments of current-current correlators).

In the evening, there was a social event: A string quartet played for us at the university's faculty of music in Mainz. The program was Mozart (Divertimento No. 1, KV 135), Schubert (String quartet No. 13 "Rosamunde) and Shostakovich (String quartet No 8 op. 110), the first two pieces quite pleasant, the last rather harrowing.

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