A bstract With the advent of Belle II and the LHCb upgrade, the precision measurements of various B-Physics observables are on cards. This holds significant potential for delving into physics beyond the standard model of electroweak interactions. These measurements can also serve as means to establish limits on phenomena occurring at much finer length scales, such as quantum decoherence, which may arise due to potential discreteness in space-time or non-trivial topological effects. In this work, we set up the formalism to investigate the impact of quantum decoherence on several potential observables in B meson systems. The approach employs the trace-preserving Kraus operator formalism, extending unitary evolution to non-unitary dynamics while maintaining complete positivity. In this formalism, the decoherence effects are parametrized in terms of a single parameter. Through the analysis of purely leptonic, semileptonic, and non-leptonic decays of B mesons, we identify observables that could, in principle, be influenced by decoherence. The theoretical expressions are provided without neglecting the impact of decay width difference (∆Γ) and CP violation in mixing. Considering that many of these observables can be measured with high precision using the abundant data collected by LHCb and Belle II, our formalism can be applied to establish constraints on the decoherence parameter through multiple decay channels. This offers an alternative set-up for such studies, which, at present, are predominantly conducted in the neutrino sector.
The decays of the B meson into vector mesons, observed during the LHCb experiment, provide an ideal laboratory to investigate particle physics phenomena with quantum information theory methods. In this article, we focus on the decays yielding a pair of ϕ mesons to investigate the presence of entanglement in the spin correlations of the system and quantify the amount of Bell inequality violation it entails. Our results show that the present LHCb data allow access to entanglement and to the Bell inequality violation with a significance exceeding the 5σ threshold in both the cases. This demonstrates that the strong and electroweak interactions responsible for the B meson decay act as a source of entanglement and the quantum mechanics nature of high-energy phenomena. Particular attention is paid to the assessment of loopholes: deficiencies in the experimental setup which could invalidate the results of the Bell test.
In this thesis, we study the quantum entanglement of neutral B-mesons from the decay of the Υ(5S). The experiment which provided real data for this decay is the Belle experiment which ran from 1999 to 2010 and gathered about 121fb−1 of data for the Υ(5S). The decay of the Υ(5S) particle is expected to produce entangled pairs of neutral B-mesons, therefore any deviations of the theoretical predictions for such entangled pairs to the measured data can be attributed to an effect called Quantum Decoherence (QD). QD is a more general term for the loss of the quantum properties of a system. In this thesis QD will refer specifically to the loss of entanglement (=ˆ disentanglement). To start with, we will briefly go over the basics of the flavor physics of the neutral meson system and study effects of mixing and the time dependence of the flavor states for B0 mesons. Then we will use this framework to calculate the theoretical probability density of measuring possible states for these entangled systems and compare them to the disentangled system. We will further analyse how QD can manifest by considering the entanglement of formation, which refers to a system that consists of multiple entangled states. It will be shown how disentanglement can arise in such systems and then apply this theory directly to the B0-meson systems we expect from the Υ(5S) decay. In the end, there is a brief introduction to the experimental uncertainties of the Belle detector, which is captured by the resolution function, and its impact on the theoretical prediction.
In this thesis we study the robustness of entanglement in the B0 meson pairs at the Belle II and if andhow different quantum states can be distinguished. The first objective is to quantitatively analyse the influence of CP violation and ∆Γ ̸= 0 on concurrence, which is used to measure entanglement, of the quantum states |ψ−⟩ =1/√2 (|B0B ̄0 − |B ̄0B0⟩⟩) and |ψ⟩ = 1/2 [(|B0B ̄0⟩ − |B ̄0B0⟩) + i(|B0B ̄0⟩ + |B ̄0B0⟩)]. For the first state it remains constanteven for CP violation but for the second it is possible to see the effects of the parameters in discussion. Furthermore we study scattering among the B mesons in vacuum, scattering with matter and finally radiative emission at Υ(5S), the first scenario is rather trivial due to the simplified assumptions, such as only strong/electromagnetic interactions. As result one observes no change in the concurrence, which is therefore concluded to be affected only by weak interaction (mixing). The second and third scenarios are investigated with the help of open quantum systems and for both the concurrence decreases after the application of the quantum channels built with Kraus operators. At the last part of the thesis the relations between entanglement and the measurement basis are studied introducing an arbitrary phase φ in the Bloch sphere representation. One finds that |ψ(φ)⟩ can violate CHSH or not depending on φ and the choice of the measurement basis. The distinction of states on the equator of the Bloch sphere is possible using the correlation function Ef1,f2(t1,t2) constructed for flavor and CP tags. Further it is also possible to distinguish entangled states of the family |ψ(φ)⟩ from some separable states also using the correlation function and choosing the flavor-CP tag. A distinction of any kind using only flavor tags on both arms of the correlation function is shown not to be possible. Finally we show that the CP-violating parameter Cf and Sf , used to measure CP violation, are biased if a wrong assumption on the nature of the quantum state is made.
Neutrinos, due to their weakly interacting nature, provide us with a unique opportunity to test foundations of quantum mechanics over macroscopic distances. There has been considerable theoretical and experimental interest in examining the extent of violation of Leggett-Garg inequalities (LGI) in the context of neutrino flavour oscillations. While it is established that neutrino oscillations occur due to mass and mixing among the three generations of neutrinos, sub-dominant effects due to physics beyond the Standard Model (SM) are not yet ruled out. As we are entering the precision era in neutrino physics, it is possible that some of these effects may leave an imprint on data. Thus, it is worthwhile to investigate different physics scenarios beyond the SM and study their impact on oscillation probabilities. In the present work, we invoke damping effects (including decoherence and decay) on oscillation probabilities and study their implications on the LGI.
This thesis presents the development and validation of an analysis framework to study quantum decoherence in neutral B-meson pairs produced at the
Υ(5S) resonance, using Monte Carlo simulated data from the Belle experiment. Unlike the
Υ(4S), the
Υ(5S) provides access to both antisymmetric (
C=−1) and symmetric (
C=+1) quantum states through the channels
B0Bˉ0,
B0∗Bˉ0+c.c., and
B0∗Bˉ0∗. Decoherence effects are studied with the Partial Spontaneous Disentanglement model, which introduces a fraction
ζ of
B0∗Bˉ∗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
Σt 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
ζ 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
(Δt,Σt) is developed. By exploiting the full time-dependent information, the two-dimensional fit significantly reduces the statistical uncertainty on the decoherence parameter from
∼O(10%) to
∼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
Υ(5S) system for studying quantum entanglement and decoherence in neutral B mesons.
The weak measurement, proposed by Aharonov {\it et al.} in 1988, has been applied for various fields of physics for the purpose of precision measurement, which is made possible with the help of the 'postselection' specifying actively the final state in the physical process. Here we report for the first time the feasibility of applying the weak measurement in high energy particle physics, especially in measuring the
CP-violating parameters in
B meson decays, where the effective lifetime of the decay mode is expected to be prolonged statistically due to the postselection. Our analysis shows that, when adopted in the Belle II experiment at the SuperKEKB collider, the effective lifetime may be prolonged up to
2.6 times, and that the measurement precision of the
CP-violating parameters will be improved by about 20\%.