Differential branching fractions and isospin asymmetries of $B \to K^{(*)}μ^{+}μ^{-}$ decays (1403.8044v3)

Abstract: The isospin asymmetries of $B \to K\mu^+\mu^-$ and $B \to K^{*}\mu^+\mu^-$ decays and the partial branching fractions of the $B^0 \to K^0\mu^+\mu^-$, $B^+ \to K^+\mu^+\mu^-$ and $B^+ \to K^{*+}\mu^+\mu^-$ decays are measured as functions of the dimuon mass squared, $q^2$. The data used correspond to an integrated luminosity of 3$~$fb$^{-1}$ from proton-proton collisions collected with the LHCb detector at centre-of-mass energies of 7$\,$TeV and 8$\,$TeV in 2011 and 2012, respectively. The isospin asymmetries are both consistent with the Standard Model expectations. The three measured branching fractions, while individually consistent, all favour lower values than their respective Standard Model predictions.

• The paper presents high-precision measurements of differential branching fractions and isospin asymmetries that align with Standard Model predictions, especially in the low q² region.
• It employs a detailed q² bin analysis and normalization with well-known decay modes to minimize systematic uncertainties and probe for signals of new physics.
• The findings constrain possible beyond Standard Model contributions and establish a robust methodology for future high-luminosity LHCb studies.

Differential Branching Fractions and Isospin Asymmetries in $B \to K^{(*)}\mu^+\mu^-$ Decays

The paper under review presents a paper conducted by the LHCb collaboration focusing on the differential branching fractions and isospin asymmetries of the decays $B \to K\mu^+\mu^-$ and $B \to K^{*}\mu^+\mu^-$ as functions of the dimuon mass squared, denoted as $q^2$. The analysis utilizes data corresponding to an integrated luminosity of 3 fb$^{-1}$ collected from proton-proton collisions with the LHCb detector at center-of-mass energies of 7 TeV and 8 TeV during the years 2011 and 2012.

Key Measurements

1. Isospin Asymmetries (\AI):
   • The isospin asymmetry {$A_I$} for the decay processes under consideration is measured to be consistent with the predictions of the Standard Model (SM). Specifically, the values obtained are within the expected $\mathcal{O}(1\%)$ in the low $q^2$ region. These results align with SM predictions, which assert that any deviation should be very small, especially in high-energy physics contexts where SM processes dominate.
2. Branching Fractions:
   • The branching fractions for the decays $B^0 \to K^0\mu^+\mu^-$, $B^+ \to K^+\mu^+\mu^-$, and $B^+ \to K^{*+}\mu^+\mu^-$ were measured. While these measurements also appear to favor slightly lower values than theoretical predictions, they remain consistent with SM expectations within the experimental uncertainties.

Methodology and Analysis

The measurements were achieved by conducting a detailed analysis of the differential branching fractions across different $q^2$ bins. This approach allows for a more sensitive probe to new physics by minimizing the dominant theoretical uncertainties associated with form factor calculations through the use of ratios or asymmetries in observables.

Furthermore, to ensure a high sensitivity and minimal systematic uncertainties, normalizations were conducted using modes with well-known branching fractions, such as those involving $\jpsi$ resonances decaying to dimuons. The paper also involved improvements in detector alignment parameters, reconstruction algorithms, and event selection criteria over previous analyses, which increased the overall precision of the results.

Implications and Future Prospects

This research holds several key implications for both theoretical and experimental particle physics. These precise measurements provide essential tests for the SM by probing for potential contributions from physics beyond the SM (BSM). The consistency of the results with SM predictions, especially regarding isospin asymmetries, sets significant constraints on various BSM theories that predict deviations in these observables.

Looking towards the future, the methods employed set a precedent for upcoming high-luminosity runs of LHCb, where larger datasets will allow for even more precise determinations of these observables. Increased precision will further tighten the constraints on BSM theories and possibly reveal deviations indicative of new physics phenomena.

Overall, the paper represents a significant contribution to the domain of flavor physics and exemplifies the sophisticated methodologies employed at LHCb to probe the intricate behaviors of subatomic particles as dictated by the fundamental principles of the SM, while remaining vigilant for new physics signals.