Constraints on New Physics from decays of polarized $Λ_b^0$ baryons at the FCC-ee

TL;DR

The paper investigates how FCC-ee Z^0 decays, producing polarized Λ_b^0 baryons, enable a comprehensive angular analysis of Λ_b^0 → Λ(pπ^−) μ^+ μ^− with 34 observables. By simulating the detector response (IDEA) and employing a maximum-likelihood 5D angular fit, it demonstrates that polarization substantially enriches sensitivity to the Wilson coefficients $C_9$ and $C_{10}$ beyond unpolarized analyses, albeit with realistic systematic considerations. The approach highlights the potential of a high-luminosity Z^0 factory to tighten constraints on new-physics scenarios in b→sℓℓ transitions, motivating further detailed studies and method refinements. Overall, the work shows that polarized baryon decays at FCC-ee can provide significant gains in flavor-physics precision and NP discrimination.

Abstract

The $Z^0$ bosons produced in electron-positron collisions at the potential Future Circular Collider (FCC-ee) provide unique opportunities for flavour physics. The non-zero polarization of \Lb baryons produced in $Z^0$ decays enables access to a much larger set of observables than at the LHC, where the \Lb baryons are produced unpolarized. This paper presents a toy angular analysis of $Λ_b^0\to Λ(\to pπ^-)μ^+μ^-$ decays using simulation samples of collisions at the FCC-ee reconstructed using the IDEA detector concept and assuming a dataset of $6\times 10^{12}$ $Z^0$ bosons. While the statistical sensitivity achieved for individual angular observables is not expected to significantly exceed that from the LHCb Upgrade II experiment, the addition of the polarized observables leads to a significant improvement of the knowledge on the Wilson coefficients $C_9$ and $C_{10}$.

Figures (8)

• Figure 1: Location and magnitude of the backgrounds (left) after reconstruction without further selections, (center) including the topological, and (right) including all selections (dimuon mass) for the dimuon mass (hadron masses).
• Figure 2: 1- and 2-dimensional projections of the efficiency model. For the 1-dimensional projections, the simulation samples (including calibration and after removal of phase-space structures) are displayed as black data points.
• Figure 3: AUC for classifying a toy of an efficiency model from the simulation samples. The nominal model is shown in green. The dashed line at 0.5 represents inseparable samples. Values below 0.5 are impossible and indicate a problem with the classifier or the input data. The horizontal axis indicates the complexity of the model given by the number of coefficients.
• Figure 4: Uncertainties in the measurement for different levels of $p-\pi$ separation obtained as the deviation of an observable from the truth.
• Figure 5: Sensitivity of a fit to the real and imaginary parts of the Wilson coefficients $C_9$ and $C_{10}$ for a measurement using either all observables, only unpolarized, or only polarized observables assuming perfect particle identification.