Study of Quantum Decoherence in Neutral B Meson pairs produced at the $\Upsilon(5S)$ Resonance

Abstract

This thesis presents the development and validation of an analysis framework to study quantum decoherence in neutral B-meson pairs produced at the $\Upsilon(5S)$ resonance, using Monte Carlo simulated data from the Belle experiment. Unlike the $\Upsilon(4S)$, the $\Upsilon(5S)$ provides access to both antisymmetric ($C=-1$) and symmetric ($C=+1$) quantum states through the channels $B^0\bar{B}^0$, $B^{0*}\bar{B}^0+c.c.$, and $B^{0*}\bar{B}^{0*}$. Decoherence effects are studied with the Partial Spontaneous Disentanglement model, which introduces a fraction $\zeta$ of $B^{0*}\bar{B}^{*0}$ pairs disentangled at production. The analysis exploits the decay times of both neutral B mesons, for which time-dependent probability density functions are derived and extended to include detector-resolution effects. The framework is first validated on Signal Monte Carlo samples, where one-dimensional fits to $\Sigma t$ successfully recover the expected decoherence values for both entangled and disentangled configurations. Fits to Generic Monte Carlo samples, which include realistic backgrounds and event compositions, reveal strong correlations between the decoherence parameter and flavour-tagging wrong-tag fractions, particularly in the $C=-1$ channels. The results from these fits show significant deviations from expected $\zeta$ values and large uncertainties. Pure Toy Monte Carlo studies confirm that the fitter behaves linearly with respect to the injected decoherence parameter and show only small biases consistent with those observed in the nominal fits, but too small to fully explain them. To address the limitations of the single fits, a two-dimensional analysis in $(\Delta t,\Sigma t)$ is developed. By exploiting the full time-dependent information, the two-dimensional fit significantly reduces the statistical uncertainty on the decoherence parameter from $\sim\mathcal{O}(10\%)$ to $\sim\mathcal{O}(4\%)$ and mitigates correlations with the flavour-tagging parameters. This work constitutes a proof of concept for future analyses on Belle data and demonstrates the potential of the $\Upsilon(5S)$ system for studying quantum entanglement and decoherence in neutral B mesons.

Popular Abstract

Quantum physics describes a world that behaves very differently from our everyday experience. One of its strangest phenomena is quantum entanglement: two particles exist so deeply connected that measuring one instantly gives information about the other, even when they are separated. Albert Einstein famously described this as “spooky action at a distance”. Today, entanglement is at the heart of modern quantum technologies, but it also remains a fundamental question in physics itself: how stable is this quantum connection in real physical systems? This thesis explores quantum entanglement in pairs of particles called neutral B mesons. These particles are produced at the Belle experiment in Japan through high-energy electron–positron collisions and exist only for an extremely short time before decaying. Despite their brief lifetime, they provide a unique laboratory for studying the foundations of quantum mechanics. The work focuses on B meson pairs produced by the decay of the $\Upsilon(5S)$ resonance. Unlike the more commonly studied $\Upsilon(4S)$ state, the $\Upsilon(5S)$ can produce B mesons in several different quantum configurations. This makes the $\Upsilon(5S)$ a particularly powerful environment to investigate whether the quantum correlations between the two mesons remain perfectly coherent or whether some pairs behave as if they were produced without quantum correlations, an effect described here by a phenomenological decoherence model. To study this effect, I developed a statistical analysis framework based on Monte Carlo simulations of the Belle detector. The analysis reconstructs the decay times of the two B mesons and compares them with theoretical models that describe different degrees of entanglement. In particular, I studied two complementary observables: the difference between the two particles' decay times and their sum. Combining both pieces of information in a two-dimensional analysis significantly improves the precision of the measurement. The results show that the developed framework can distinguish between fully entangled and disentangled samples. While some biases and statistical limitations remain when realistic detector effects and backgrounds are included, the study demonstrates that the method is sensitive to decoherence effects and provides a proof of concept for future analyses on real Belle data. Beyond particle physics, this research contributes to a broader effort to understand how quantum correlations behave in complex and imperfect systems. Studying entanglement in unstable particles such as B mesons helps bridge the gap between abstract quantum theory and realistic environments where noise, measurement uncertainties, and decoherence all play a role.

• author: Beriet, Louise
• supervisor: Hans-Günther Moser; Johan Bijnens
• organization: Department of Physics; Particle and nuclear physics
• course: FYSM64 20261
• year: 2026
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Particle physics, Entanglement, Belle, Flavour physics
• language: English
• id: 9231736
• date added to LUP: 2026-06-08 14:59:20
• date last changed: 2026-06-08 14:59:20