The Violation of Fundamental Symmetries with B Mesons (VFSB) workshop was held at Fermilab from December 15–19, 2025, and has successfully concluded. The workshop brought together the flavor physics community to share recent experimental and theoretical progress and to discuss future directions in the field. Throughout the week, theorists and experimentalists engaged in in-depth discussions on current puzzles in flavor physics, emerging theoretical ideas, and the most promising measurements to pursue in the coming years.
The workshop covered the topics including flavor-changing neutral current (FCNC), lepton flavor violation (LFV), semileptonic decays, CP violation, and dark sector. From MIT, Christoph Paus, Eluned Smith, and Zhangqier Wang participated in the workshop. Eluned and Qier served as conveners of the session on FCNC&LFV, with Qier chairing one of the sessions. Christoph Paus presented a talk on flavor-physics opportunities at FCC-ee and participated as a panelist in a discussion session on ‘Dark Sector and Future Detectors’. The panel discussed advances in detector technologies, theoretical priorities, the growing role of AI-based techniques, and the main experimental and theoretical challenges in the field.
The major highlight of the workshop was the presentation of the most recent BaBar results on the measurements of and . These observables are defined as the ratios of branching fractions for decays between the and light-lepton () channels. The anomaly has been a long-standing tension in flavor physics for over a decade, which some people even jokingly claim will never go away (page 4).
For the first time, BaBar presented new measurements of and using a semileptonic tag, providing results independent of previous measurements. The new result (BaBar 25) is consistent with the SM prediction, though inconsistent with their own previous result (BaBar 12) which was produced with an completely uncorrelated tagger, a hadronic tagger. Simply averaging all results will drastically reduce the tension of the world average and thus, it is very likely that the R(D*)/R(D) anomaly will be disappearing sooner than many have anticipated. The averaging working group will have to have some discussions about how to handle some of the results which apparently disagree at over three standard deviations.
The workshop was highly successful, with more than 100 participants and a lively exchange between the experimental and theoretical communities. Several social events were held, including a reception at the Swiss Consulate with beautiful views of Chicago, and Fermilab experiments tours at the end of the workshop.