Particle mixing and quantum reference frames

TL;DR

This paper addresses how to assign a rest frame to mixed-particle states, where classical boosts fail due to superposed momentum components. It extends the Wigner-Poincaré framework by promoting boost parameters to operator-valued quantities and applying quantum frame transformations (U(\hat{\Lambda})) to joint systems, thereby defining rest frames for mixed states such as neutrino flavor eigenstates. A key result is the relativity of entanglement: a QRF transformation can convert a product state into an entangled one (and vice versa) depending on the frame, with concrete demonstrations in neutrino and kaon contexts and a proposed entanglement witness that yields measurable signatures. The findings suggest observable consequences in accelerator-based experiments and offer a pathway toward a relational, possibly gravity-aware, formulation of quantum field theory where spacetime symmetries are themselves operator-valued.

Abstract

We discuss the role of quantum reference frames in providing a viable definition of rest frame for mixed particles. We then analyze the related concept of frame-dependent entanglement and its impact on the phenomenology of neutral mesons and neutrinos.