$Λ_b\toΛ^{(*)}ν{\barν}$ and $b\to s$ $B$ decays

TL;DR

The paper analyzes baryonic FCNC decays $\Lambda_b\to\Lambda^{(*)}\nu\bar{\nu}$ in a framework that links them to mesonic $b\to s$ transitions. It uses a model-independent parametrization with $C_j^{\rm NP}=\mathcal{N}A_j\left(\frac{v}{M_{\rm NP}}\right)^{\alpha}$ to fit multiple observables including $R(K^{(*)})$, $\mathrm{Br}(B_s\to\mu^+\mu^-)$, $\mathrm{Br}(B^+\to K^+\mu^+\mu^-)$, and $P_5'$, along with $\mathrm{Br}(B^+\to K^+\nu\bar{\nu})$, under the constraint of $\mathrm{Br}(B^0\to K^{*0}\nu\bar{\nu})$. The best-fit results favor $\alpha\approx1.21$ and place a NP scale $M_{\rm NP}$ around $13\,{\rm TeV}$, with specific NP couplings $A_9$, $A_{10}$, and leptonic-neutrino coefficients. The analysis predicts a sizable enhancement for $\mathrm{Br}(\Lambda_b\to\Lambda\nu\bar{\nu})$ relative to the SM, while $\mathrm{Br}(\Lambda_b\to\Lambda^*\nu\bar{\nu})$ remains close to SM expectations; for $\alpha=2$ the NP window tightens to $[2.04,11.76]$ TeV. Implications include potential tests at future colliders like FCC-ee, while HL-LHC may struggle to probe the preferred $M_{\rm NP}$ range.

Abstract

The baryonic $b\to s$ transition $Λ_b\toΛ^{(*)}ν{\barν}$ is analyzed. We combine the mesonic counterpart $B^+\to K^+ν{\barν}$ and $B^0\to K^{*0}ν{\barν}$ as well as other observables involving $B$ mesons like $R(K^{(*)})$, ${\rm Br}(B_s\toμ^+μ^-)$, ${\rm Br}(B^+\to K^+μ^+μ^-)$, and $P_5'(B^+\to K^{*+}μ^+μ^-)$. We find that the new physics scale $M_{\rm NP}$ to be $2.04~{\rm TeV}\le M_{\rm NP} \le 11.76~{\rm TeV}$ (at $1σ$) for ordinary heavy new mediators. Our predictions for the branching ratios ${\rm Br}(Λ_b\toΛ^{(*)}ν{\barν})$ are $2.07 (1.07)$ times the standard model estimations, which could be verified at future colliders.

Figures (4)

• Figure 1: Allowed regions at the $2\sigma$ level for (a) $\alpha$ vs. $M_{\rm NP}$, (b) $C_{10}^{\rm NP}$ vs. $C_9^{\rm NP}$, (c) $C_{L,{\rm NP}}^{\nu_\tau}$ vs. $C_{L,{\rm NP}}^{\nu_e}$, (d) $C_{R,{\rm NP}}^{\nu_\tau}$ vs. $C_{L,{\rm NP}}^{\nu_e}$, and (e) $C_{R,{\rm NP}}^{\nu_\tau}$ vs. $C_{L,{\rm NP}}^{\nu_\tau}$.
• Figure 2: Allowed regions at the $2\sigma$ level for (a) ${\rm Br}(B^0\to K^{*0}\nu{\bar{\nu}})$ vs. ${\rm Br}(B^+\to K^+\nu{\bar{\nu}})$, (b) ${\rm Br}(\Lambda_b\to\Lambda^*\nu{\bar{\nu}})$ vs. ${\rm Br}(\Lambda_b\to\Lambda\nu{\bar{\nu}})$, (c) ${\rm Br}(\Lambda_b\to\Lambda\nu{\bar{\nu}})$ vs. ${\rm Br}(B^+\to K^+\nu{\bar{\nu}})$, (d) ${\rm Br}(\Lambda_b\to\Lambda^*\nu{\bar{\nu}})$ vs. ${\rm Br}(B^0\to K^{*0}\nu{\bar{\nu}})$, and (e) $R(\Lambda_b\to\Lambda^*\nu{\bar{\nu}})$ vs. $R(\Lambda_b\to\Lambda\nu{\bar{\nu}})$. Magenta lines are the SM predictions and the red stars are our best-fit points.
• Figure 3: Allowed regions at the $2\sigma$ level with fixed $\alpha=2$ (red dots) for (a) $C_9^{\rm NP}$ , (b) $C_{10}^{\rm NP}$, (c) $C_{L,{\rm NP}}^{\nu_e}$, (d) $C_{L,{\rm NP}}^{\nu_\tau}$, and (e) $C_{R,{\rm NP}}^{\nu_\tau}$ vs. $M_{\rm NP}$, respectively. Blue dots are for free $\alpha$.
• Figure 4: Allowed regions at the $2\sigma$ level with fixed $\alpha=2$ for (a) ${\rm Br}(B^0\to K^{*0}\nu{\bar{\nu}})$ vs. ${\rm Br}(B^+\to K^+\nu{\bar{\nu}})$, (b) ${\rm Br}(\Lambda_b\to\Lambda^*\nu{\bar{\nu}})$ vs. ${\rm Br}(\Lambda_b\to\Lambda\nu{\bar{\nu}})$, (c) ${\rm Br}(\Lambda_b\to\Lambda\nu{\bar{\nu}})$ vs. ${\rm Br}(B^+\to K^+\nu{\bar{\nu}})$, (d) ${\rm Br}(\Lambda_b\to\Lambda^*\nu{\bar{\nu}})$ vs. ${\rm Br}(B^0\to K^{*0}\nu{\bar{\nu}})$, and (e) $R(\Lambda_b\to\Lambda^*\nu{\bar{\nu}})$ vs. $R(\Lambda_b\to\Lambda\nu{\bar{\nu}})$ with respect to $M_{\rm NP}$, respectively.