The influence of invisible light particles on $Λ_b \to ΛE_{\mathrm{miss}}$

TL;DR

The paper addresses the Belle II $B \to K E_{\mathrm{miss}}$ excess by exploring whether invisible light particles can modify $b \to s E_{\mathrm{miss}}$ transitions, focusing on the baryonic decay $\Lambda_b \to \Lambda E_{\mathrm{miss}}$ and its three-body channels $\Lambda_b \to \Lambda \phi \bar{\phi}$ and $\Lambda_b \to \Lambda \psi \bar{\psi}$. It formulates detailed NP models in both parity and chiral bases, derives differential decay rates and observables, and evaluates correlations with $\mathcal{B}(B \to K E_{\mathrm{miss}})$ under eight NP scenarios with a high NP scale $\Lambda = 10$ TeV. Numerically, invisible scalars or spin-1/2 particles can enhance $d\mathcal{B}/dq^2$ above thresholds and suppress the $q^2$-dependent $P^\Lambda_L$, while the $\mathcal{B}(B \to K E_{\mathrm{miss}})$–$P^\Lambda_L$ correlation in the chiral basis provides clear discriminants of hadronic current chirality. The work highlights that future precision measurements of $P^\Lambda_L$ (e.g., at FCC-ee) could decisively test these new-physics scenarios and deepen understanding of $b \to s E_{\mathrm{miss}}$ transitions.

Abstract

In this work, we study the contribution of invisible light particles to $Λ_b \to ΛE_{\mathrm{miss}}$, particularly the three-body decays $Λ_b \to Λφ\barφ$ and $Λ_b \to Λψ\barψ$. The differential branching ratio of $Λ_b \to ΛE_{\mathrm{miss}}$ and the $q^2$-dependent longitudinal polarization asymmetry of $Λ$ in scenarios explaining the Belle II excess are presented. In the chiral basis, we investigate the correlations between $\mathcal{B}(B \to K E_{\mathrm{miss}})$ and $\mathcal{B}(Λ_b \to ΛE_{\mathrm{miss}})$, as well as between $\mathcal{B}(B \to K E_{\mathrm{miss}})$ and $P^Λ_{L}$, in eight distinct new physics scenarios. We find that the $P^Λ_{L}$ can be used to distinguish the chirality of the hadronic current part in the effective operators, which is similar to the cases in the two-body decays $Λ_b \to Λφ$ and $Λ_b \to ΛV$.

Figures (3)

• Figure 1: Predictions for the differential branching ratio of $\Lambda_b \to \Lambda E_\mathrm{miss}$ in the SM and SM+$\phi$ (left panel) as well as the SM and SM+$\psi$ (right panel), using the best-fit mass and couplings from Ref. Bolton:2024egx.
• Figure 2: Predictions for the $q^2$-dependent longitudinal polarization asymmetry of $\Lambda$ in the SM and SM+$\phi$ (left panel) as well as the SM and SM+$\psi$ (right panel), using the best-fit mass and couplings from Ref. Bolton:2024egx.
• Figure 3: The figure displays the $\mathcal{B}(B \to K E_{\mathrm{miss}})-\mathcal{B}(\Lambda_b \to \Lambda E_{\mathrm{miss}})$ correlation (left panel) and $\mathcal{B}(B \to K E_{\mathrm{miss}})-P^{\Lambda}_{L}$ correlation (right panel) for different NP scenarios. $X,Y \in \{L,R\}$.The SM predictions are represented by black rectangles. The light orange regions indicate the present experimental range \ref{['eq:BKexp']} quoted by Belle-II.