INT Workshop on Hadronic Light-by-Light Contribution to the Muon Anomaly (Seattle: Feb. 28 – Mar 4, 2011) David Hertzog, University of Washington

Our Challenges (see web) 1.Can agreement be reached on the individual and combined theoretical contributions to the hadronic light- by-light (HLbL) contribution to the muon anomalous magnetic moment, a μ, based on QCD-inspired models? 2.Can the lattice approach lead to a result having sufficient precision to check the models or to independently establish the HLbL value? 3.Which data that can be obtained at Frascati, and at other facilities, are essential to constrain the theoretical calculations and what theoretical developments are required to connect data to model predictions?

Our Motivation New muon g-2 experiment aims for uncertainty  a  = 16 x SM contribution uncertainty must be reduced to realize maximum impact of comparison of Expt. to Theory HLbL – well, it seems complicated – is an easy target to blame, especially by those people who haven’t spent much time thinking about it – EXAMPLES

Arguments made against a new g-2 “You’ll never be able to get the HLbL uncertainty low enough to interpret the result” Many in the community “Although the ultimate limit to the comparison with the SM expectation may be uncertainties in the theory (particularly the light-by-light contribution), the pastwork at BNL, … was limited by statistics” FNAL PAC Mar 2009 “There have been significant advances in the theoretical understanding of g-2, although the uncertainty on the light-by-light contribution remains a worry.” FNAL PAC Nov 2009 Questions to Collaboration from DOE Intensity Frontier Review Aug – 1. Provide a basis for the claimed future evolution of the theory syst. errors (e.g., how much improvement over current status is expected from current/future e+e‐  hadrons experiments? From improved theory calc of light‐by‐light corrections? ) – 2. Describe how you plan to use LQCD results or additional experimental data (which specific measurements) to improve or cross‐check the precision of your result.

What do people say to us in the hallways? We get “reminded” of – the famous sign error – the intrinsic naivety of theory uncertainties in hadronic models – The difficulty of ever getting a precision calculation from the lattice – That there is no data to support the calculations And mostly that, – This is HARD stuff !!

Yes. This is Hard Stuff We need to know a small contribution to 10% and then we’re done Do we need this tomorrow? – NO Do we need to start today? – YES

How to get there 1.Gather the best people in the world to talk about it – This is that meeting 2.Write down our intermediate conclusions and our working plan – That should be the product of this meeting – A White Paper, suitable for publication 3.Repeat in a few years time to reassess