Roles of scalar mesons in charmless $\Lambda_b$ decays

We first study the charmless two-body $\Lambda_b$ decays with the scalar mesons as the final states and predict that ${\cal B}(\Lambda_b\to \Lambda f_0(980,1500))=(2.9\pm 0.7,12.4\pm 3.8)\times 10^{-6}$ and ${\cal B}(\Lambda_b\to p K^{*-}_0(800,1430))=(1.9\pm 0.5,14.1\pm 4.5)\times 10^{-6}$. With the resonant $f_0(980,1500)\to (\pi^+\pi^-,K^+K^-)$ and $K^{*-}_0\to \bar K^0 \pi^-$ decays, we then obtain ${\cal B}(\Lambda_b\to \Lambda (\pi^+\pi^-,K^+K^-))=(4.2\pm 1.0,3.5\pm0.7)\times 10^{-6}$ and ${\cal B}(\Lambda_b\to p\bar K^0 \pi^-)=(10.4\pm2.9)\times 10^{-6}$, in comparison with the data of $(4.6\pm 1.9,15.9\pm2.6)\times 10^{-6}$ and $(12.6\pm 4.0)\times 10^{-6}$ from LHCb, respectively. Our results for $\Lambda_b\to \Lambda\pi^+\pi^-$ and $\Lambda_b\to p\bar K^0 \pi^-$ would be regarded as the first evidences of the scalar meson productions in the anti-triplet $b$-baryon decays. The smaller predicted value of ${\cal B}(\Lambda_b\to \Lambda K^+K^-)$ indicates the existences of other resonant contributions to the decay, such as $\Lambda_b\to K^-(N^{*+}\to)\Lambda K^+$.