[Paper Review] Future prospects for exploring present day anomalies in flavour physics measurements with Belle II and LHCb

J. Albrecht, F. U. Bernlochner+4 more|arXiv (Cornell University)|Sep 29, 2017

Particle physics theoretical and experimental studiesPhysics and Astronomy40 references21 citations

TL;DR

This paper evaluates the future sensitivity of Belle II and LHCb to current anomalies in flavour physics, particularly deviations in lepton flavour universality and CKM matrix parameters. By simulating future datasets and scanning new physics contributions to Wilson coefficients, it projects that both experiments will independently confirm or rule out the 4σ tensions in semi-leptonic decays and $B\to K^{(*)}\mu^+\mu^-$ observables with high significance by 2030.

ABSTRACT

A range of flavour physics observables show tensions with their corresponding Standard Model expectations: measurements of leptonic flavour-changing neutral current processes and ratios of semi-leptonic branching fractions involving different generations of leptons show deviations of the order of four standard deviations. If confirmed, either would be an intriguing sign of new physics. In this manuscript, we analyse the current experimental situation of such processes and for the first time estimate the combined impact of the future datasets of the Belle II and LHCb experiments on the present tensions with the Standard Model expectations by performing scans of the new physics contribution to the Wilson coefficients. In addition, the present day and future sensitivity of tree-level CKM parameters, which offer orthogonal tests of the Standard Model, are explored. Three benchmark points in time are chosen for a direct comparison of the estimated sensitivity between the experiments. A high complementarity between the future sensitivity achieved by the Belle II and LHCb experiments is observed due to their relative strengths and weaknesses. We estimate that all of the anomalies considered here will be either confirmed or ruled out by both experiments independently with very high significance by the end of data-taking at Belle II and the LHCb upgrade.

Motivation & Objective

• To assess the future experimental sensitivity of Belle II and LHCb to persistent anomalies in flavour physics, including deviations in lepton flavour universality and CKM matrix parameters.
• To evaluate the combined impact of Belle II and LHCb datasets on resolving tensions between experimental measurements and Standard Model predictions.
• To explore the complementarity between Belle II and LHCb in probing new physics through tree-level and loop-level flavour-changing processes.
• To project the resolution of current anomalies—particularly in $B\to K^{(*)}\mu^+\mu^-$ and semi-tauonic decays—by 2030 using realistic luminosity and efficiency assumptions.
• To investigate the future precision of tree-level CKM parameters as orthogonal tests of the Standard Model, independent of loop-induced processes.

Proposed method

• Performs scans of new physics contributions to Wilson coefficients to model potential NP effects in $b\to s\ell^+\ell^-$ and $b\to c\tau\bar{\nu}$ transitions.
• Uses extrapolated event yields based on anticipated integrated luminosities, production rates, and reconstruction efficiencies for Belle II and LHCb.
• Incorporates expected systematic uncertainties from experimental and theoretical sources, including trigger efficiency improvements in LHCb's software trigger upgrade.
• Projects sensitivities at three milestone years—2020, 2024, and 2030—using data-taking schedules from Belle II and LHCb.
• Compares projected sensitivities across multiple observables: $|V_{ub}|$, $|V_{cb}|$, $\gamma$, $R(D^{(*)})$, and angular observables in $B^0\to K^{*0}\mu^+\mu^-$.
• Employs a combination of experimental projections and theoretical modeling to estimate the significance of resolving current anomalies.

Experimental results

Research questions

• RQ1 Can Belle II and LHCb independently resolve the 4σ tension in the ratio of semi-tauonic to semi-leptonic $B$ decays?
• RQ2 To what extent will the combined datasets from Belle II and LHCb improve sensitivity to deviations in $B\to K^{(*)}\mu^+\mu^-$ angular observables compared to current measurements?
• RQ3 How will future measurements of $|V_{ub}|$, $|V_{cb}|$, and $\gamma$ from tree-level decays test the consistency of the Standard Model CKM matrix?
• RQ4 What is the projected significance of confirming or excluding new physics contributions in the Wilson coefficient space by 2030?
• RQ5 How do the complementary strengths of Belle II (precision $B$-physics at $\Upsilon(4S)$) and LHCb (high-energy $b$-hadron production) enhance overall sensitivity to anomalies?

Key findings

• By 2030, both Belle II and LHCb are projected to achieve sufficient sensitivity to either confirm or rule out the current 4σ deviation in the $R(D^{(*)})$ ratio with high significance.
• The angular observables $S_4$, $S_5$, and $F_L$ in $B^0\to K^{*0}\mu^+\mu^-$ are expected to reach precisions that will allow a clear test of the Standard Model in the $q^2$-dependent regions.
• The projected uncertainty on $|V_{ub}|$ and $|V_{cb}|$ from tree-level decays will be reduced to the level of a few percent, enabling a stringent test of CKM unitarity.
• The combination of Belle II and LHCb data will allow for a global fit to Wilson coefficients with significantly improved constraints on new physics models such as leptoquarks or Z' bosons.
• The sensitivity of LHCb to $B\to K^{(*)}\mu^+\mu^-$ observables is expected to improve by a factor of 2–3 by 2030 due to increased luminosity and trigger efficiency.
• The high complementarity between Belle II and LHCb arises from their differing kinematic regimes and systematics, with Belle II excelling in semi-leptonic decays and LHCb in high-$q^2$ and forward $b$-hadron studies.