[Paper Review] Measurement of lepton universality parameters in $B^+ o K^+\ell^+\ell^-$ and $B^0 o K^{*0}\ell^+\ell^-$ decays

LHCb Collaboration, Aaij, R.+1062 more|arXiv (Cornell University)|Jan 1, 2022

Particle physics theoretical and experimental studies14 citations

TL;DR

This paper presents the first simultaneous measurement of lepton universality (LU) in B⁺→K⁺ℓ⁺ℓ⁻ and B⁰→K*⁰ℓ⁺ℓ⁻ decays using 9 fb⁻¹ of LHCb data from 2011–2018. By applying stringent particle identification and multivariate selections, it achieves higher signal purity and improved statistical sensitivity, yielding RK and RK* values in two q² intervals that are consistent with the Standard Model and supersede previous LHCb measurements.

ABSTRACT

A simultaneous analysis of the $B^+ o K^+\ell^+\ell^-$ and $B^0 o K^{*0}\ell^+\ell^-$ decays is performed to test muon-electron universality in two ranges of the square of the dilepton invariant mass, $q^2$. The measurement uses a sample of beauty meson decays produced in proton-proton collisions collected with the LHCb detector between 2011 and 2018, corresponding to an integrated luminosity of $9$ $ ext{fb}^{-1}$. A sequence of multivariate selections and strict particle identification requirements produce a higher signal purity and a better statistical sensitivity per unit luminosity than previous LHCb lepton universality tests using the same decay modes. Residual backgrounds due to misidentified hadronic decays are studied using data and included in the fit model. Each of the four lepton universality measurements reported is either the first in the given $q^2$ interval or supersedes previous LHCb measurements. The results are compatible with the predictions of the Standard Model.

Motivation & Objective

• To test muon-electron lepton universality in rare b→sℓ⁺ℓ⁻ decays with enhanced precision using a simultaneous analysis of B⁺→K⁺ℓ⁺ℓ⁻ and B⁰→K*⁰ℓ⁺ℓ⁻ modes.
• To reduce systematic uncertainties by directly modeling residual misidentified hadronic backgrounds from data, improving signal purity beyond prior LHCb studies.
• To supersede previous LHCb LU measurements by achieving higher statistical sensitivity per unit luminosity through optimized trigger strategies and tighter particle identification.
• To validate the robustness of the analysis by performing cross-checks and using pseudoexperiments to quantify systematic shifts from improved background modeling.
• To provide the most precise measurements of RK and RK* in two q² intervals (low- and central-q²), serving as a null test of the Standard Model.

Proposed method

• Simultaneous unbinned extended maximum-likelihood fit to the invariant mass distributions of B⁺ and B⁰ decays, using data from 9 fb⁻¹ of pp collisions at √s = 7, 8, and 13 TeV.
• Application of multivariate selections and strict particle identification (PID) criteria to suppress misidentified and partially reconstructed backgrounds.
• Use of resonant B→K*J/ψ decays (J/ψ→ℓ⁺ℓ⁻) as normalization channels to determine detector correction factors, ensuring consistency between B⁺ and B⁰ modes.
• Direct estimation of residual misidentified backgrounds (e.g., K→μ, π→μ) from data and inclusion in the fit model to improve systematic uncertainty control.
• Calibration of simulation using control samples and efficiency corrections derived from data-driven methods to ensure accurate modeling of detector effects.
• Use of pseudoexperiments to quantify systematic shifts due to improved PID and background modeling, particularly for electron-mode backgrounds.

Experimental results

Research questions

• RQ1 What is the value of the lepton universality ratio RK in the low-q² (0.1–1.1 GeV²/c⁴) and central-q² (1.1–6.0 GeV²/c⁴) intervals for B⁺→K⁺ℓ⁺ℓ⁻ decays?
• RQ2 How does the measurement of RK* in the same q² intervals compare to the Standard Model prediction and previous LHCb results?
• RQ3 To what extent do improved particle identification and background modeling reduce systematic uncertainties compared to prior LHCb LU measurements?
• RQ4 Are the measured RK and RK* values consistent with the Standard Model within the combined statistical and systematic uncertainties?
• RQ5 Can the simultaneous analysis of B⁺ and B⁰ decays reduce correlations and improve sensitivity compared to independent measurements?

Key findings

• The measured RK in the low-q² region is 0.994⁺⁰.⁰⁹⁰₋₀.⁰⁸² (stat) ⁺⁰.⁰²⁹₋₀.⁰²⁷ (syst), consistent with the Standard Model prediction of 0.9936.
• The RK in the central-q² region is 0.949⁺⁰.⁰⁴²₋₀.⁰⁴¹ (stat) ⁺⁰.⁰²²₋₀.⁰²² (syst), showing a shift toward the SM value compared to previous LHCb results.
• The RK* in the low-q² region is 0.927⁺⁰.⁰⁹³₋₀.⁰⁸⁷ (stat) ⁺⁰.⁰³⁶₋₀.⁰³⁵ (syst), and in the central-q² region is 1.027⁺⁰.⁰⁷²₋₀.⁰⁶⁸ (stat) ⁺⁰.⁰²⁷₋₀.⁰²⁶ (syst), both consistent with the SM prediction of 0.9832 and 0.9964 respectively.
• The combined compatibility of all four measurements with the Standard Model is evaluated via a χ² test, yielding a p-value of 0.812 and a significance of 0.2 standard deviations, indicating no evidence for new physics.
• Systematic shifts in RK and RK* are primarily attributed to improved treatment of misidentified hadronic backgrounds and stricter PID criteria, with a total systematic shift of 0.064 for RK compared to prior results.
• The new measurements supersede previous LHCb results for RK (central-q²), RK* (low- and central-q²), and are the first such measurements in the low-q² region for RK.