Test of lepton universality with $B^{0} \rightarrow K^{*0}\ell^{+}\ell^{-}$ decays (1705.05802v2)

Abstract: A test of lepton universality, performed by measuring the ratio of the branching fractions of the $B^{0} \rightarrow K^{*0}\mu^{+}\mu^{-}$ and $B^{0} \rightarrow K^{*0}e^{+}e^{-}$ decays, $R_{K^{*0}}$, is presented. The $K^{*0}$ meson is reconstructed in the final state $K^{+}\pi^{-}$, which is required to have an invariant mass within 100$\mathrm{\,MeV}c^2$ of the known $K^{*}(892)^{0}$ mass. The analysis is performed using proton-proton collision data, corresponding to an integrated luminosity of about 3$\mathrm{\,fb}^{-1}$, collected by the LHCb experiment at centre-of-mass energies of 7 and 8$\mathrm{\,TeV}$. The ratio is measured in two regions of the dilepton invariant mass squared, $q^{2}$, to be \begin{eqnarray*} R_{K^{*0}} = \begin{cases} 0.66~^{+~0.11}_{-~0.07}\mathrm{\,(stat)} \pm 0.03\mathrm{\,(syst)} & \textrm{for } 0.045 < q^{2} < 1.1~\mathrm{\,GeV^2}c^4 \, , \ 0.69~^{+~0.11}_{-~0.07}\mathrm{\,(stat)} \pm 0.05\mathrm{\,(syst)} & \textrm{for } 1.1\phantom{00} < q^{2} < 6.0~\mathrm{\,GeV^2}c^4 \, . \end{cases} \end{eqnarray*} The corresponding 95.4\% confidence level intervals are $[0.52, 0.89]$ and $[0.53, 0.94]$. The results, which represent the most precise measurements of $R_{K^{*0}}$ to date, are compatible with the Standard Model expectations at the level of 2.1--2.3 and 2.4--2.5 standard deviations in the two $q^{2}$ regions, respectively.

• The paper measures the branching fraction ratio R_K*0 using 3 fb⁻¹ data, revealing significant deviations from Standard Model predictions.
• It analyzes rare FCNC decays in two q² regions, observing 2.1–2.5σ discrepancies suggestive of lepton universality violation.
• The findings indicate potential new physics scenarios, underlining the need for further investigation at LHCb and other collider experiments.

Analysis of Lepton Universality in $B^{0} \rightarrow K^{*0}\ell^{+}\ell^{-}$ Decays

The paper presented by the LHCb collaboration investigates a fundamental aspect of the Standard Model (SM): lepton universality. This principle asserts that all leptons interact with gauge bosons with equal strength, barring mass differences. The focus of this research is the process $B^{0} \rightarrow K^{*0}\ell^{+}\ell^{-}$, where $\ell$ represents either a muon ($\mu$) or an electron ($e$). This decay process is selected because it proceeds via a flavor-changing neutral current (FCNC), which are suppressed at tree level and can only proceed via higher order loop diagrams in the SM. Such rare processes are highly sensitive to potential contributions from physics beyond the Standard Model (BSM).

Methodology

The paper utilizes data collected by the LHCb experiment, corresponding to an integrated luminosity of approximately 3 fb⁻¹ at centre-of-mass energies of 7 and 8 TeV. The authors measure the ratio of branching fractions $$R_{K^{*0}} = \frac{\mathcal{B}(B^{0} \rightarrow K^{*0}\mu^{+}\mu^{-})}{\mathcal{B}(B^{0} \rightarrow K^{*0}e^{+}e^{-})}$$ in two distinct regions of the dilepton invariant mass squared ($q^{2}$). Such ratios are particularly useful as they cancel out a significant number of systematic uncertainties related to the hadronic environment. The meson $K^{*0}$ is reconstructed from the final state $K^{+}\pi^{-}$, with invariant mass requirements set close to the known mass of the $K^{*}(892)^{0}$.

Results

• For $0.045 < q^{2} < 1.1$ GeV²/c⁴, the measured $R_{K^{*0}}$ is $$0.66^{+0.11}_{-0.07} \mathrm{\,(stat)} \pm 0.03\mathrm{\,(syst)}$$.
• For $1.1 < q^{2} < 6.0$ GeV²/c⁴, the value is $$0.69^{+0.11}_{-0.07} \mathrm{\,(stat)} \pm 0.05\mathrm{\,(syst)}$$.

The results fall short of the SM expectations for lepton universality, which predict $R_{K^{*0}}$ to be close to unity. The deviations observed are 2.1–2.3 and 2.4–2.5 standard deviations from the SM predictions for the two $q^{2}$ regions respectively. These findings are statistically significant and represent the most precise measurements to date.

Implications

These results contribute to a growing body of evidence suggesting lepton universality violations in B-meson decays, alongside other tensions observed in similar measurements. They warrant further scrutiny and could imply new physics scenarios, such as models involving additional heavy gauge bosons (e.g., $Z'$) or leptoquarks. If these findings are confirmed with higher confidence, they would necessitate a substantial paradigm shift from the SM, possibly pointing towards a richer structure of the lepton sector.

Future Directions

The LHCb collaboration plans to exploit future data with larger datasets at higher luminosity, which could either reinforce or refute these current indications. Additionally, other experiments at colliders like Belle II could provide complementary insights. Crossing corroborations from disparate types of decay processes and with increasing precision are vital in establishing the veracity and implications of potential lepton universality violations.

Theoretical developments will also be crucial, particularly in refining the predictions involving FCNC processes and in exploring BSM scenarios that accommodate these results without conflicting with other established SM predictions. This work is pivotal in potentially unveiling new physics in the ongoing exploration of particle physics.