What does this research mean for the field?

The Belle II experiment provides new measurements of lepton flavor universality in semileptonic B meson decays, including the first inclusive probe of the $b \to c \tau \nu$ transition. Novelty: ClaimNovelty.SYNTHESIS. Consensus alignment: ConsensusAlignment.NEUTRAL.

What question did this study set out to answer?

This research aims to examine lepton flavour universality through experimental tests in semileptonic B-meson decays.

March 1, 2026Open Access

Recent Belle II results on tests of lepton flavour universality in semileptonic B-meson decays

Key Points

This research aims to examine lepton flavour universality through experimental tests in semileptonic B-meson decays.
Analyzed Run 1 data from the Belle II experiment.
Measured branching fraction ratios using hadronic and semileptonic B-tagging.
Conducted an inclusive probe of the b to c tau nu transition.
Reported measurements of R(D*) using hadronic B-tagging.
Presented R(D) and R(D*) using semileptonic B-tagging.
Summarized the global experimental status and provided outlook for future sensitivity in Run 2.

Abstract

In this article, we review three recently published experimental results of lepton flavor universality tests in semileptonic B meson decays based on the Run 1 data sample of the Belle II experiment. We present measurements of the branching fraction ratios R (D^*) using hadronic B-tagging, alongside both R (D) and R (D^*) using semileptonic B-tagging. Additionally, we include the first inclusive probe of the b c transition via R (X /) using hadronic B-tagging. Finally, we summarize the current global experimental status and provide an outlook on the sensitivity prospects for the Run 2 target 10 ab^-1 integrated luminosity.

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