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Rare decays of $K$ and $B$ mesons provide a powerful probe of dark sectors with light new particles. We show that the pair production of $O(100,{rm MeV})$ dark states can be probed with the decays of $K_L$ mesons, owing to the enhanced two-body kinematics, $K_Lto X_1X_2$ or $X_2X_2$. If either or these two particles is unstable, e.g. $X_2to X_1pi^0$, $X_2to X_1gamma$ or $X_{1,2}to gammagamma$, such decays could easily mimic $K_Lto pi^0 uoverline{ u}$ signatures, while not being ruled out by the decays of charged kaons. We construct explicit models that have enhanced $K_L$ decay signatures, and are constrained by the results of the KOTO experiment. We note that recently reported excess events can also be accommodated while satisfying all other constraints ($B$ decays, colliders, beam dumps). These models are based on the extensions of the gauge and/or scalar sector of the theory. The lightest of $X_{1,2}$ particles, if stable, could constitute the entirety of dark matter.
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We review the two and three-body baryonic $B$ decays with the dibaryon (${bf Bbar B}$) as the final states. Accordingly, we summarize the experimental data of the branching fractions, angular asymmetries, and $CP$ asymmetries. In the approach of perturbative QCD counting rules, we study the three-body decay channels. Using the $W$-boson annihilation (exchange) mechanism, the branching fractions of $Bto {bf B bf bar B}$ are shown to be interpretable. In particular, we review the $CP$ asymmetries of $Bto {bf Bbar B}M$, which are promising to be measured by the LHCb and Belle II experiments.
This article summarizes recent developments in $Bto D^{(ast)}tau u$ decays. We explain how to extract the tau leptons production properties from the kinematics of its decay products. The focus is on hadronic tau decays, which are most sensitive to the tau polarizations. We present new results for effects of new physics in tau polarization observables and quantify the observation prospects at BELLE II.
We study the collider phenomenology of dark matter pair production at the LHC in simplified dark matter models and in the MSSM. Among the large space of dark matter models, we focus on two particular models where a fermionic dark matter candidate interacts with the Standard Model via the exchange of either a vector mediator in the s-channel or coloured scalar mediators in the t-channel. We find that the simplified models are capable of reproducing the predictions of the MSSM to some extent in simplified supersymmetric scenarios, but lack the complexity to descibe the complete theory over the full supersymmetric parameter space.
We consider the fidelity of the vector meson dominance (VMD) assumption as an instrument for relating the electromagnetic vector-meson production reaction $e + p to e^prime + V + p$ to the purely hadronic process $V + p to V+p$. Analyses of the photon vacuum polarisation and the photon-quark vertex reveal that such a VMD Ansatz might be reasonable for light vector-mesons. However, when the vector-mesons are described by momentum-dependent bound-state amplitudes, VMD fails for heavy vector-mesons: it cannot be used reliably to estimate either a photon-to-vector-meson transition strength or the momentum dependence of those integrands that would arise in calculations of the different reaction amplitudes. Consequently, for processes involving heavy mesons, the veracity of both cross-section estimates and conclusions based on the VMD assumption should be reviewed, e.g., those relating to hidden-charm pentaquark production and the origin of the proton mass.
We discuss the possibility of observing a loosely bound molecular state in a B three-body hadronic decay. In particular we use the QCD sum rule approach to study a $eta^prime-pi$ molecular current. We consider an isovector-scalar $I^G J^{PC}= 1^-~0^{++}$ molecular current and we use the two-point and three-point functions to study the mass and decay width of such state. We consider the contributions of condensates up to dimension six and we work at leading order in $alpha_s$. We obtain a mass around 1.1 GeV, consistent with a loosely bound state, and a $eta^prime-pirightarrow K^+ K^-$ decay width around 10 MeV.
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