New physics in $B$ meson mixing: future sensitivity and limitations

TL;DR

This paper assesses the future sensitivity of flavor physics experiments to new physics in B meson mixing, highlighting key measurement challenges and potential discovery reach up to 2 TeV.

Contribution

It provides a detailed analysis of future experimental constraints on new physics in B meson mixing, emphasizing the importance of measuring |V_{cb}| and addressing current tensions in semileptonic B decays.

Findings

Future experiments can probe new physics up to 2 TeV.

Measuring |V_{cb}| is crucial for maximizing sensitivity.

Resolving semileptonic B decay tensions impacts new physics searches.

Abstract

The mixing of neutral mesons is sensitive to some of the highest scales probed in laboratory experiments. In light of the planned LHCb Upgrade II, a possible upgrade of Belle II, and the broad interest in flavor physics in the tera-$Z$ phase of the proposed FCC-ee program, we study constraints on new physics contributions to $B_d$ and $B_s$ mixings which can be obtained in these benchmark scenarios. We explore the limitations of this program, and identify the measurement of $|V_{cb}|$ as one of the key ingredients in which progress beyond current expectations is necessary to maximize future sensitivity. We speculate on possible solutions to this bottleneck. Given the current tension with the standard model (SM) in semileptonic $B$ decays, we explore how its resolution may impact the search for new physics in mixing. Even if new physics has the same CKM and loop suppressions of flavor changing processes as the SM, the sensitivity will reach 2 TeV, and it can be much higher if any SM suppressions are lifted. We illustrate the discovery potential of this program.