Studying rare processes is part and parcel of particle physics, providing stringent tests of the Standard Model. At the recent International Conference on High Energy Physics (ICHEP), the CMS and LHCb collaborations presented their latest research into one such process: the transformation of a beauty quark into a strange quark, a muon and the oppositely charged antimuon (b→sμ⁺μ⁻).

Flavor transitions—when one type of quark turns into another—can only take place via the weak interaction, one of the four fundamental forces. Quantum physics describes how other particles, not directly involved in a particular transition, can influence how that transition takes place. If a process happens differently than is predicted by the influence of the particles we know about, which make up the Standard Model, this difference could point to the existence of as-yet-undiscovered particles and interactions.

Some of these transitions are extremely rare, which makes them very sensitive to any potential contributions from particles that may exist beyond the Standard Model. The b→sμ⁺μ⁻ transition is one such process, only occurring approximately once in every 10 million decays of particles containing beauty quarks, known as beauty particles. Over the past decade, signs have begun to appear that this transition may not occur as expected.

Researchers at Friedrich Schiller University Jena and the University of Regensburg have developed a method for determining, using light, a quantum property of electrons that has hitherto been difficult to access. The focus is on what is known as Berry curvature—a quantity that describes certain geometric properties of the quantum mechanical states of electrons. The researchers measured it in a layer of the semiconductor tungsten diselenide, just one atomic layer thick, using ultrashort laser pulses. The results have been published in the latest issue of the journal Nature Photonics.

Electrons in a crystal do not simply move like tiny particles along fixed paths. Their behavior is determined by quantum mechanical states. Researchers summarize the geometric properties of these states under the term “quantum geometry.” An important parameter here is the Berry curvature. Among other things, it influences how electrons react to light or electric fields.

Until now, Berry curvature has mainly been calculated theoretically or investigated using comparatively complex experimental methods. The team from Jena and Regensburg is taking a different approach: it combines optical measurements with an analytical model and is thus able to quantitatively estimate the local Berry curvature of the material.