Theorie Palaver

The CP violation observed in the hadronic decays of charmed mesons remains a puzzling open question for theorists. Calculations relying on the assumption of inelastic final-state interactions occurring between the pairs of pions and kaons fall short of the experimental value. It has been pointed out that a third channel of four pions can leave imprints on the CP asymmetries of the two-body decays. At the same time, plenty of data are available for rare decays such as \(D^0\to\pi^+\pi^-\ell^+\ell^-\), which provide a promising environment for the search for new physics. With this motivation, we study the cascade topology \(D^0\to a_1(1260)^+(\to \rho(770)^0\pi^+)\,\pi^-\), which has been measured to contribute significantly to the \(4\pi\) decays of the same meson, and estimate its effect on the branching ratio of the rare decays. I will also comment on the possibility of this topology contributing to the decay amplitude of \(D^0\to\pi^+\pi^-\) and by extension to the related CP asymmetry.

Muon conversion — the process of a bound muon decaying into an energetic electron — is one of the best probes of charged lepton flavor violation. The experimental limit is soon expected to improve by four orders of magnitude, thus calling for precise predictions on the theory side. Equally important are precise predictions for muon decay-in-orbit, the main background for muon conversion. While the calculation of electromagnetic corrections to the two processes above the nuclear scale does not involve significant challenges, it becomes substantially more complex below that scale due to multiple scales, bound-state effects and experimental setup. In this talk, I present a systematic framework that addresses these challenges by resorting to a series of effective field theories. Combining Heavy Quark Effective Theory (HQET), Non-Relativistic QED (NRQED), potential NRQED, Soft-Collinear Effective Theory I and II, and boosted HQET, I derive a factorization theorem and present the renormalization group equations.

Top-quark pair production in association with a jet is a key process at the LHC. Its high sensitivity to the top mass and the increasing experimental precision call for the QCD corrections to be computed at the next-to-next-to-leading order (NNLO). In this seminar, I will present the computation of the two-loop Feynman integrals required to obtain NNLO QCD predictions in the leading colour approximation. These integrals are characterised by significant algebraic complexity—stemming from the multi-scale, five-particle kinematics—as well as analytic complexity, due to the appearance of nested square roots and elliptic functions. I will discuss modern methods for tackling multi-scale integrals in a way that is suitable for phenomenology, and outline first steps to extend these techniques to cases involving elliptic functions.

By employing the Bloch-sphere formalism, I will present a novel class of unstable qubits, which are called Critical Unstable Qubits (CUQs). The characteristic property of CUQs is that the energy-level and decay-width Pauli vectors, E and Γ, are orthogonal to one another, and the key parameter r = |Γ|/(2|E|) is less than 1. A remarkable feature of CUQs is that they exhibit atypical behaviours like coherence-decoherence oscillations when analysed in an appropriately defined co-decaying frame of the system. In the same frame, I will show how a unit Bloch vector b describing a pure CUQ sweeps out unequal areas during equal intervals of time, while rotating about the vector E. These phenomena emerge beyond the usual oscillatory pattern due to the energy-level difference of a standard two-level quantum system. I will illustrate how these new features are relatively robust and persist even for quasi-CUQs, in which the vectors E and Γ are not perfectly orthogonal to each other. I discuss potential applications of our results to quantum information and to unstable meson-antimeson and other systems.

Electroweakly interacting stable particles in the (1 − 10) TeV mass range can be a dark matter candidate with rich testability. In particular, gamma-ray line-like features are expected to be a smoking-gun signature for indirect detection. In this framework, one fundamental question follows: How can we distinguish DM spin among DM candidates with the same electroweak interaction by contrasting their predictions? A straightforward but crucial effort is to derive annihilation spectra with higher accuracy. However, one encounters complexities due to non-perturbative corrections following the mass hierarchy between heavy DM and electroweak mediators. In this talk, we present how to construct Soft-Collinear Effective field Theory (SCET) to systematically resum large Sudakov logarithmic corrections for spin-0, 1/2, and 1 DM, and achieve accurate prediction. We focus especially on spin-1 DM, which is the last piece to complete all the possible predictions. After specifying the leading operators to describe heavy DM annihilation in SCET, we discuss how to extract spin-dependence from the predicted gamma-ray spectrum.

I will introduce our IBP package NeatIBP, which automatically generates small-size integration-by-parts (IBP) identities for Feynman integrals. Based on the syzygy and module intersection techniques, the generated IBP identities’ propagator degree is controlled and thus the size of the system of IBP identities is shorter than that generated by the standard Laporta algorithm. Resently, we updated NeatIBP with some new featrues, such as, spanning cut and the reduction interface of NeatIBP and Kira. I will also give some powerful applications of NeatIBP.