A recent analysis by the LHCb Collaboration suggests the existence of three narrow pentaquarklike states—the $P_c(4312)$, $P_c(4440)$, and $P_c(4457)$—instead of just one in the previous analysis [the $P_c(4450)$]. The closeness of the $P_c(4312)$ to the $\overline{D}{\mathrm{\Sigma }}_c$ threshold and the $P_c(4440)\text{and}{P}_c(4457)$ to the ${\overline{D}}^{*}{\mathrm{\Sigma }}_c$ threshold suggests a molecular interpretation of these resonances. We show that these three pentaquarklike resonances can be naturally accommodated in a contact-range effective field theory description that incorporates heavy-quark spin symmetry. This description leads to the prediction of all the seven possible $S$-wave heavy antimeson-baryon molecules [that is, there should be four additional molecular pentaquarks in addition to the $P_c(4312)$, $P_c(4440)$, and $P_c(4457)$], providing the first example of a heavy-quark spin symmetry molecular multiplet that is complete. If this is confirmed, it will not only give us an impressive example of the application of heavy-quark symmetries and effective field theories in hadron physics, it will also uncover a clear and powerful ordering principle for the molecular spectrum, reminiscent of the SU(3)-flavor multiplets to which the light hadron spectrum conforms.

In the present work, we assign the newly observed $P_c(4312)$ as a $I(J^P)=\frac{1}{2}(\frac{1}{2}{)}^{-}$ molecular state composed of ${\mathrm{\Sigma }}_c\overline{D}$, while $P_c(4440)$ and $P_c(4457)$ as ${\mathrm{\Sigma }}_c{\overline{D}}^{*}$ molecular states with $I(J^P)=\frac{1}{2}(\frac{1}{2}{)}^{-}$ and $\frac{1}{2}(\frac{3}{2}{)}^{-}$, respectively. In this molecular scenario, we investigate the $P_c\to J/\psi p$ process of these three states and further predict the ratios of the $\mathcal{B}(P_c\to J/\psi p)$ and those of $\mathcal{B}({\mathrm{\Lambda }}_b\to P_{c}K)$ between these three states, which could serve as a crucial test of the present molecular scenario.

Rare meson decays are among the most sensitive probes of both heavy and light new physics. Among them, new physics searches using kaons benefit from their small total decay widths and the availability of very large datasets. On the other hand, useful complementary information is provided by hyperon decay measurements. We summarize the relevant phenomenological models and the status of the searches in a comprehensive list of kaon and hyperon decay channels. We identify new search strategies for under-explored signatures, and demonstrate that the improved sensitivities from current and next-generation experiments could lead to a qualitative leap in the exploration of light dark sectors.

Heavy-quark symmetry as applied to heavy hadron systems implies that their interactions are independent of their heavy-quark spin (heavy-quark spin symmetry) and heavy flavor contents (heavy flavor symmetry). In the molecular hypothesis the $X(3872)$ resonance is a $1^{++}$ $D^{*}\overline{D}$ bound state. If this is the case, the application of heavy-quark symmetry to a molecular $X(3872)$ suggests the existence of a series of partner states, the most obvious of which is a possible $2^{++}$ $D^{*}{\overline{D}}^{*}$ bound state for which the two-body potential is identical to that of the $1^{++}$ $D^{*}\overline{D}$ system, the reason being that these two heavy hadron-antihadron states have identical light-spin content. As already discussed in the literature, this leads to the prediction of a partner state at 4012 MeV, at least in the absence of other dynamical effects which might affect the location of this molecule. However the prediction of further heavy-quark symmetry partners cannot be made solely on the basis of symmetry and requires additional information. We propose to use the one-boson-exchange model to fill this gap, in which case we will be able to predict or discard the existence of other partner states. Besides the isoscalar $2^{++}$ $D^{*}{\overline{D}}^{*}$ bound state, we correctly reproduce the location and quantum numbers of the isovector hidden-bottom $Z_b(10610)$ and $Z_b(10650)$ molecular candidates. We also predict the hidden-bottom $1^{++}$ $B^{*}{\overline{B}}^{*}$ and $2^{++}$ $B^{*}{\overline{B}}^{*}$ partners of the $X(3872)$, in agreement with previous theoretical speculations, plus a series of other states. The isoscalar, doubly charmed $1^{+}D{D}^{*}$ and $D^{*}{D}^{*}$ molecules, and their doubly bottomed counterparts are likely to bind, providing a few instances of explicitly exotic systems.

Motivated by renewed evidence for new physics in $b\to s\ell \ell$ transitions in the form of LHCb’s new measurements of theoretically clean lepton-universality ratios and the purely leptonic $B_s\to {\mu }^{+}{\mu }^{-}$ decay, we quantify the combined level of discrepancy with the Standard Model and fit values of short-distance Wilson coefficients. A combination of the clean observables $R_K$, $R_{K^{*}}$, and $B_s\to \mu \mu$ alone results in a discrepancy with the Standard Model at $4.0\sigma$, up from $3.5\sigma$ in 2017. One-parameter scenarios with purely left-handed or with purely axial coupling to muons fit the data well and result in a $\sim 5\sigma$ pull from the Standard Model. In a two-parameter fit of new-physics contributions with both vector and axial-vector couplings to muons the allowed region is much more restricted than in 2017, principally due to the much more precise result on $B_s\to {\mu }^{+}{\mu }^{-}$, which probes the axial coupling to muons. Including angular observables data restricts the allowed region further. A by-product of our analysis is an updated average of $\mathrm{BR}(B_s\to {\mu }^{+}{\mu }^{-})=(2.8\pm 0.3)\times {10}^{-9}$.