arXiv subjects

Thomas McKelvey

Publications and source records attributed to Thomas McKelvey.

5 recordsLinked to original sources

arXiv2026

Critical Unstable Qubits in Particle Physics

We study in detail the dynamics of unstable two-level quantum systems by adopting the Bloch-vector representation. We identify a novel class of critical scenarios in which the so-called energy-level and decay-width vectors, ${\bf E}$ and ${\bfΓ}$, are orthogonal to one another, and the parameter $r = |{\bf Γ}|/(2|{\bf E}|)$ is less than~1. Most remarkably, we find that critical unstable qubit systems exhibit atypical behaviours like coherence--decoherence oscillations when analysed in an appropriately defined co-decaying frame of the system. By making use of a Fourier series decomposition, we define anharmonicity observables that quantify the degree of non-sinusoidal oscillation of a CUQ. We apply the results of our formalism to the neutral-meson systems and derive generic upper limits on these new observables. In particular, we provide a compilation table of all well-explored meson--antimeson two-level systems in terms of Bloch-sphere parameters.

quant-ph↗

arXiv2025

Critical Unstable Qubits: an Application to $B^0\bar{B}^0$-Meson System

We extend our previous work on a novel class of unstable qubits which we have identified recently and called them Critical Unstable Qubits (CUQs). The characteristic property of CUQs is that the energy-level and decay-width vectors, ${\bf E}$ and ${\bf Γ}$, are orthogonal to one another, and the key parameter $r = |{\bf Γ}|/|2{\bf E}|$ is less than 1. Most remarkably, CUQs exhibit two atypical behaviours: (i) they display coherence-decoherence oscillations in a co-decaying frame of the system described by a unit Bloch vector ${\bf b}$, and (ii) the unit Bloch vector ${\bf b}$ describing a pure CUQ sweeps out unequal areas during equal intervals of time, while rotating about the vector ${\bf E}$. The latter anharmonic phenomenon emerges beyond the usual oscillatory pattern due to the energy-level difference of the two-level quantum system, which governs an ordinary qubit. By making use of a Fourier series decomposition, we define anharmonicity observables that quantify the degree of non-sinusoidal oscillation of a CUQ. We apply the results of our formalism to the $B^0\bar{B}^0$-meson system and derive, for the first time, generic upper limits on these new observables.

hep-ph↗

arXiv2025

Leptogenesis and the relativistic degrees of freedom of the plasma

We investigate the impact of the temperature dependence of the relativistic degrees of freedom (dofs) of the plasma on lepton and baryon asymmetry. Motivated by the significant effect of the varying dofs on the tri-resonant leptogenesis particle model in low-scale leptogenesis, we show how this effect impacts the evolution of the lepton asymmetry in a simplified setup. We provide analytical approximations as well as numerical results showing that the simplified setup exhibits similar behavior as the concrete model. As the dofs enter the transport equations via the expansion rate of the Universe and the temperature of the plasma, we argue that any analysis must take these effects into account in order to be consistent.

hep-ph↗

arXiv2024

Quantum Coherence of Critical Unstable Two-Level Systems

We study in detail the dynamics of unstable two-level quantum systems by adopting the Bloch-sphere formalism of qubits. By employing the Bloch-vector representation for such unstable qubit systems, we identify a novel class of critical scenarios in which the so-called energy-level and decay-width vectors, ${\bf E}$ and ${\bfΓ}$, are orthogonal to one another, and the parameter $r = |{\bf Γ}|/(2|{\bf E}|)$ is less than 1. Most remarkably, we find that critical unstable qubit systems exhibit atypical behaviours like coherence--decoherence oscillations when analysed in an appropriately defined co-decaying frame of the system. In the same frame, a unit Bloch vector ${\bf b}$ describing a pure critical qubit will sweep out unequal areas during equal intervals of time, while rotating about the vector ${\bf E}$. These phenomena emerge beyond the usual oscillatory pattern due to the energy-level difference of the two-level quantum system. Interestingly enough, we observe that these new features will persist even for quasi-critical scenarios, in which the vectors ${\bf E}$ and ${\bfΓ}$ are not perfectly orthogonal to each other. Applications of our results to quantum information and to unstable meson--antimeson and other systems are discussed.

hep-ph↗

arXiv2024

Varying Entropy Degrees of Freedom Effects in Low-Scale Leptogenesis

We analyse in detail the effect of varying entropy degrees of freedom on low-scale leptogenesis models. As an archetypal model, we consider the Tri-Resonant Leptogensis${}$ (TRL) scenario introduced recently by the authors, where the neutrino-Yukawa coupling matrix is dictated by an approximate $\mathbb{Z}_n$ discrete symmetry (with $n=3,6$). TRL models exhibit no preferred direction in the leptonic flavour space and have the remarkable feature that leptogenesis can successfully take place even if all light neutrinos are strictly massless up to one-loop order. Most interestingly, for TRL scenarios with heavy Majorana neutrinos lighter than 100 GeV, temperature varying degrees of freedom associated with the entropy of the plasma have a dramatic impact on the predictions of the Baryon Asymmetry in the Universe (BAU), and may sensitively depend on the freeze-out sphaleron temperature $T_{\rm sph}$. We find that this is a generic feature of most freeze-out low-scale leptogenesis models discussed in the literature. In the same context, we consider heavy-neutrino scenarios realising dynamics related to critical unstable qudits in the thermal plasma and assess their significance in generating the BAU. The phenomenological implications of TRL scenarios at the intensity and high-energy frontiers are analysed.

hep-ph↗