Thursday, April 02, 2009

MAMI and beyond, Day One

Hello from Schloss Waldthausen near Mainz, where I am attending the conference "MAMI and beyond".

The meeting started this morning with welcome speeches by the VP for research of Mainz University, the VP of physics of the German Research Foundation, and the acting director of the Mainz nuclear physics institute. This was followed by the first talk, given by Ulf G. Meissner (Bonn University), who spoke about "Hadron physics at the 1 GeV scale and its impact". He paid particular attention to isospin violating effects, which can come from both QED and QCD sources, since the up and down quarks differ in both mass and charge. MAMI experiments could measure isospin violating effects in πN scattering, η -> 3π and η' -> ηππ decays, and in Kaon photoproduction on the proton, for all of which there are higher-order predictions from some versions of chiral perturbation theory. Beyond MAMI, interesting isospin violating effects are the mass splittings of heavy baryon mutliplets, where the mass of the cdd Σc0 is greater that that of the cud Σc+, even though md>mu, but the ordering of the Σs baryons is the normal one, an effect that may be explained by the presence of a new operator appearing in the O(p2) χPT effective Lagrangian for heavy quarks, which has a different sign for c and b quarks because of their different electrical charge.

After the coffee break, Jens Erler (UNAM, Mexico) talked about "Low-energy tests of the Standard Model and beyond". Low-energy probes, such as leptonic decays, flavour-changing neutral current contributions to Kaon decays, first row CKM matrix unitarity tests, tests of CP violation, search of nucleon and lepton electric dipole moments, atomic parity violation, (g-2) measurements and many more from particle, nuclear and atomic physics, can surprisingly probe very high energies by placing extremely stringent limits on various kinds of beyond-the-Standard-Model physics, excluding in many cases BSM contributions from scales below a few 100 TeV or so. This makes them a very useful complement to high-energy collider experiments that search for BSM particles and processes in a more direct manner.

The next talk was an overview of form factors given by Carl Carlson (College of William and Mary). The point that stuck to my mind most prominently was that measurements of hydrogen hyperfine splitting when combined with proton structure measurements and calculations are accurately predicted to more than 1 p.p.m. and show now evidence of new physics to such accuracy.

After the lunch break, Constantia Alexandrou (Cyprus University) gave an overview of nucleon structure on the lattice, concentrating on Nf=2 studies using dynamical twisted mass, Wilson clover or overlap fermions. Special attention was drawn to the fact that it is now becoming possible to simulate at the physical pion mass, and that the first such simulations have recently been done by the Wuppertal group.

This was followed by another experimental talk by Volker Burkert (Jefferson Lab). What I took home from this talk was that there is experimental support for the notion that the Roper resonance is a radial excitation of the nucleon, and that there is such a thing as femtotomography, where an image of the charge distribution inside a hadron is created from the Fourier transform of its structure functions.

After this, Mauro Anselmino (INFN Torino) spoke about the spin structure of the nucleon from a mostly theoretical point of view, followed after the coffee break by Klaus Rith (Erlangen-Nürnberg University) speaking about the same from a mostly experimental point of view. The "spin crisis" caused by the discovery that the quark spins only contribute about 33% of the nucleon's spin still appears somewhat unresolved. The gluons appear to contribute very little, and the contributions of the angular momenta of up and down quarks, which must make up the remainder, interestingly have opposite sign. A lot of research still seems to be ongoing in this very complex area, and I honestly don't understand enough of it to be able to give a decent summary of the enormous amount of information contained in these talks.

The same is true (and to an even larger extent) of the experimental talks that followed, and to which I didn't pay the necessary attention in any case, since I had to deal with several pressing matters by email.