Quantum effects in the search for new physics

Abstract

Quantum field theory is the backbone of modern particle physics. As the name implies, it is based on quantum mechanics, of which it is the consistent expansion to include special relativity. However, its quantum nature is not as evident when performing calculations for observables at collider experiments. Indeed, by construction, perturbation theory at leading order coincides with a classical theory, and quantum effect act as a parametrically small correction. Trying to bridge this gap, this thesis explores purely quantum phenomena as a discovery tool for new physics.

We start with an account of entanglement at colliders, focusing on the concrete example of 4-fermion scattering in the electroweak theory and including new physics dipole moments. We show that angular correlations of the fermions' decay products can restore (“resurrect”) interference contributions that are otherwise suppressed by small masses in cross sections, thus unlocking a quantum phenomenon. We also critically evaluate the new physics sensitivity of entanglement markers, by interpreting the angular correlations as spin correlation of the parent fermions; we show that there is no advantage in using entanglement markers, and the angular correlations perform equally or better.

We then move to a scenario where quantum interference plays an even more dramatic role. We study a new gauge boson that is nearly degenerate with the Standard Model $Z$ boson and show how interference between the two fundamentally alters predictions. We correct inconsistent treatments in older works, which worked in an uncontrolled approximation that suppressed interference. Surprisingly, we find that the quantum Zeno effect is critical to understanding the phenomenology of this system. This effect, which is not widely known in the particle physics community, is due to the mismatch of unitary evolution and the non-unitary nature of particle decay.

Finally, we shift our attention to nonrelativistic quantum mechanics, specifically exploring the connection between the quantum vacuum and the classical limit. Here, we uncover a surprising relationship between large-multiplicity scattering amplitudes and tunneling, offering new insights into how quantum effects persist even as systems approach classical behavior. This finding provides a fresh perspective on the interaction between quantum and classical mechanics in high-energy physics.