Friday, September 04, 2015

Fundamental Parameters from Lattice QCD, Day Four

Today's first speaker was Andreas Jüttner, who reviewed the extraction of the light-quark CKM matrix elements Vud and Vus from lattice simulations. Since leptonic and semileptonic decay widths of Kaons and pions are very well measured, the matrix element |Vus| and the ratio |Vus|/|Vud| can be precisely determined if the form factor f+(0) and the ratio of decay constants fK/fπ are precisely predicted from the lattice. To reach the desired level of precision, the isospin breaking effects from the difference of the up and down quark masses and from electromagnetic interactions will need to be included (they are currently treated in chiral perturbation theory, which may not apply very well in the SU(3) case). Given the required level of precision, full control of all systematics is very important, and the problem of how to properly estimate the associated errors arises, to which different collaborations are offering very different answers. To make the lattice results optimally usable for CKMfitter &Co., one should ideally provide all of the lattice inputs to the CKMfitter fit separately (and not just some combination that presents a particularly small error), as well as their correlations (as far as possible).

Unfortunately, I had to miss the second talk of the morning, by Xavier García i Tormo on the extraction of αs from the static-quark potential, because our Sonderforschungsbereich (SFB/CRC) is currently up for review for a second funding period, and the local organizers had to be available for questioning by panel members.

Later in the afternoon, I returned to the workshop and joined a very interesting discussion on the topic of averaging in the presence of theoretical uncertainties. The large number of possible choices to be made in that context implies that the somewhat subjective nature of systematic error estimates survives into the averages, rather than being dissolved into a consensus of some sort.