Studies of Quantum Entanglement in the 𝚼(𝟒𝑺) → $B^0\bar{B}^0$ system at Belle II using $𝑩^𝟎 → 𝑫^{(∗)−}𝝅^+$ decays

Sumitted to PubDB: 2026-01-28

Category: Master Thesis

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Principal Authors Maximilian Hattenbach, Allen Caldwell, Hans-Günther Moser
Date 2025-04-11
Belle II Number BELLE2-MTHESIS-2026-005
Abstract This thesis presents a study of quantum entanglement in the Υ(4S) → $B^0\bar{B}^0$ system using the hadronic $B^0 → D^{(∗)−}π^+$. As it is the first study of such kind at Belle II, this work is focused on establishing an analysis framework, which enables testing disentanglement in hadronic B0 modes. A novel event generator model has been implemented within basf2 to create a Signal MC samples with 1 Mio. BB events, corresponding to an integrated luminosity of 1ab−1 which are reconstructed in $B^0 → D^{(∗)−}π^+$ decays. The model of partial spontaneous disentanglement is implemented, where a fraction ζ of $B^0\bar{B}^0$ pairs disentangle right at production. The implementation is proven to be successful, as the model is sensitive to such entanglement effects. Fits to ∆t distributions for different values of ζgen in Signal MC samples return consistently the correct values for free parameter ζ within uncertainties. A subsequent Toy Monte Carlo study once more shows the model’s and fitting processes’ suitability for disentanglement studies, although some biased behavior for parameter ζ could not be fully resolved. However, this thesis provides a proof-of-concept for future probes of entanglement at Belle II. The question, whether unaccounted disentanglement effects introduce a bias in time-dependent CP violation measurements at Belle II is answered. Such effects in the region of ζ ∼ O(1 − 10%) is absorbed in form of a bias of 1 − 2% in wrong tag parameters. However, slight biases in these parameters do in general not translate to a shift in parameters ACP and SCP , thus the worry of an unaccounted systematic effect is for now mitigated.
Institute MPP

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