[Paper Review] Lepton-Flavor-Dependent Angular Analysis of $B o K^\ast \ell^+\ell^-$

Belle Collaboration, S. Wehle+201 more|arXiv (Cornell University)|Jan 1, 2016

Particle physics theoretical and experimental studiesPhysics and Astronomy109 citations

TL;DR

This paper presents the first lepton-flavor-dependent angular analysis of the B⁰ → K*⁰ℓ⁺ℓ⁻ decay using 711 fb⁻¹ of data from the Belle experiment at KEKB. It measures angular observables P′₄ and P′₅ and tests lepton flavor universality, finding a 2.6σ deviation in the muon mode at 4 < q² < 8 GeV²/c⁴—consistent with the LHCb P′₅ anomaly—while all modes remain compatible with the Standard Model overall.

ABSTRACT

We present a measurement of angular observables and a test of lepton flavor universality in the $B o K^\ast \ell^+\ell^-$ decay, where $\ell$ is either $e$ or $\mu$. The analysis is performed on a data sample corresponding to an integrated luminosity of $711~\mathrm{fb}^{-1}$ containing $772 imes 10^{6}$ $B\bar B$ pairs, collected at the $\Upsilon(4S)$ resonance with the Belle detector at the asymmetric-energy $e^+e^-$ collider KEKB. The result is consistent with Standard Model (SM) expectations, where the largest discrepancy from a SM prediction is observed in the muon modes with a local significance of $2.6\sigma$.

Motivation & Objective

• To perform the first lepton-flavor-dependent angular analysis of B⁰ → K*⁰ℓ⁺ℓ⁻ decays using a large data sample.
• To measure angular observables P′₄ and P′₅ separately for electron and muon final states.
• To test lepton flavor universality by comparing observables between e⁺e⁻ and μ⁺μ⁻ modes.
• To assess the compatibility of the results with Standard Model predictions and to search for new physics signals.
• To investigate the origin of the P′₅ anomaly previously reported by LHCb in the same q² region.

Proposed method

• The analysis uses 711 fb⁻¹ of e⁺e⁻ collisions at √s = 10.52 GeV, collected at the Υ(4S) resonance with the Belle detector.
• The decay B⁰ → K*⁰ℓ⁺ℓ⁻ is reconstructed using kinematic reconstruction of the K*⁰ → K⁺π⁻ decay and the dilepton system.
• Angular observables are extracted via unbinned maximum-likelihood fits to the full 4D angular distribution in cosθₗ, cosθₖ, φ, and q².
• The differential decay rate is parametrized using the formalism of Altmannshofer et al. (2009), with observables P′₄ and P′₅ derived from the angular coefficients.
• Systematic uncertainties are evaluated using control samples, simulation, and data-driven methods, including signal and background modeling.
• Lepton flavor universality is tested via the observables Q₄ and Q₅, defined as the difference in P′₄ and P′₅ between electron and muon modes.

Experimental results

Research questions

• RQ1 Is there a measurable lepton-flavor dependence in the angular observables P′₄ and P′₅ of B⁰ → K*⁰ℓ⁺ℓ⁻ decays?
• RQ2 Does the measured angular distribution in the muon mode show a significant deviation from the Standard Model prediction in the q² region 4–8 GeV²/c⁴?
• RQ3 Are the results for electron and muon modes consistent with lepton flavor universality, as tested via Q₄ and Q₅?
• RQ4 Does the observed tension in P′₅ for muons align with the LHCb P′₅ anomaly in the same q² region?
• RQ5 What is the significance of the largest discrepancy between data and SM predictions, including systematic uncertainties?

Key findings

• The largest deviation from the Standard Model prediction is observed in the muon mode, with a local significance of 2.6σ in the P′₅ observable for 4 < q² < 8 GeV²/c⁴.
• In the same q² region, the electron mode shows a 1.3σ deviation, and the combined analysis of all channels yields a 2.5σ tension, both including systematic uncertainties.
• The Q₄ and Q₅ observables—measuring lepton flavor universality violation—show no significant deviation from zero, with Q₄ = 0.448 ± 0.392 ± 0.076 and Q₅ = 0.498 ± 0.410 ± 0.095 in the 4–8 GeV²/c⁴ bin.
• All measured observables, including P′₄ and P′₅ for both electron and muon modes, are consistent with Standard Model expectations within uncertainties.
• The results are compatible between electron and muon final states, with no evidence for lepton flavor universality violation in the Q₄ and Q₅ observables.
• The observed 2.6σ tension in P′₅ for muons in the 4–8 GeV²/c⁴ region is in agreement with the LHCb P′₅ anomaly, suggesting a possible common origin in new physics.