New physics in B meson mixing: Future sensitivity and limitations

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 jV cb j 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 Cabibbo-Kobayashi-Maskawa 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.

Theoretical challenges for flavor physics

“…The scale of dimension-6 operators contributing to neutral meson mixing is typically constrained to be higher by the data than the scales of operators affecting decays. A recent analysis of future sensitivities observed that beyond the Belle II and LHCb data taking in this decade, future improvements are hindered by the expected uncertainty of |V cb | [12]. To go beyond current expectations and reach the per mille level, not yet known theoretical progress would be needed.…”

“…(The theory uncertainties are subject to some recent discussions [10].) At the FCC-ee, the achievable experimental uncertainty has been estimated to be about 2.5 × 10 −5 for both quantities [11,12], which would allow a measurement of A d SL even at the SM level. These measurements would help probe BSM physics, as well as help discriminate between models, should BSM physics be discovered in other processes.…”

Probing squared four-fermion operators of SMEFT with meson-mixing

“…( 2) based on their contributions to different meson-mixing observables. We exploit the bounds provided in [13,14] on generic NP contributions. Despite being an effect coming from a dim.-8 operator and being generated at one-loop, given the previous enhancements and the precision with which many of these observables are known, one reaches a sensitivity to multi-TeV NP effects, as shown in Tab.…”

Heavy-quark opportunities and challenges at FCC-ee

“…In making this argument, it is useful to recall the results of a model-independent study of possible BSM contributions to the B 0 − B 0 and B 0 s − B 0 s mixing amplitudes, reported in Ref. [10]. The box diagrams that drive the oscillations and carry CP-violating phases are natural entry points for any heavy degrees of freedoms from BSM physics.…”