We propose a new search for light scalar singlets in rare meson decays. For tiny interactions, the scalar is long-lived at detector scales and decays into displaced pairs of leptons or light mesons. We show that Belle II has a remarkable potential to probe scalars in the GeV range with couplings as small as 10 −5 . The predicted sensitivity is higher than at the long-baseline experiments FASER and NA62. We also investigate signatures of invisibly decaying scalars in rare meson decays with missing energy.

New physics not far above the TeV scale should leave a pattern of virtual effects in observables at lower energies. What do these effects tell us about the flavor structure of a UV theory? Within the framework of the Standard Model Effective Field Theory (SMEFT), we resolve the flavor structure of the Wilson coefficients in a combined analysis of top-quark and B-physics observables. We assume that the Yukawa couplings are the only sources of flavor symmetry breaking, a framework known as Minimal Flavor Violation. Our fits to LHC and b-factory measurements show that combining top and bottom observables is crucial to pin down possible sources of flavor breaking in a UV theory. This analysis includes the full analytic expansion of SMEFT coefficients in Minimal Flavor Violation and a detailed study of SMEFT effects in b → s flavor transitions.

In this paper, we describe the potential of the LHCb experiment to detect Stealth physics. This refers to dynamics beyond the Standard Model that would elude searches that focus on energetic objects or precision measurements of known processes. Stealth signatures include long-lived particles and light resonances that are produced very rarely or together with overwhelming backgrounds. We will discuss why LHCb is equipped to discover this kind of physics at the Large Hadron Collider and provide examples of well-motivated theoretical models that can be probed with great detail at the experiment.