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.

Precise numerical evaluation of Feynman integrals plays a central role in particle physics. While some methods focus on individual phase-space points, others exploit differential equations to efficiently propagate results across kinematic regions. LINE is a novel open-source program that integrates both strategies within a single tool, computing boundary values and propagating them accordingly. Written primarily in C, it is designed for performance and scalability, enabling large-scale computations on clusters without relying on proprietary software. In this talk, I will introduce the core ideas behind LINE and present a few representative applications.

Gravitational wave (GW) observations have opened new avenues for probing Beyond Standard Model (BSM) physics. While most detections involve typical black hole (BH) or neutron star (NS) mergers, events like GW190814, GW190425, and GW230529 observed by the LIGO-Virgo-KAGRA (LVK) collaboration include at least one compact object whose nature—either a binary neutron star (BNS) or a low-mass black hole (LMBH)—remains uncertain. One possible formation channel for such LMBHs involves dark matter (DM) capture and accumulation in NSs, leading to a collapse-induced transmutation of the NS into a BH of comparable mass. I will discuss how DM capture in NSs can lead to the formation of LMBHs, highlighting the relevant parameter space that governs this process. I will also show how current GW observations can constrain portions of this DM parameter space. In the second part, I will focus on the gravitational waveforms of BNS mergers versus LMBH mergers, examining their distinguishing features and assessing the capability of current and future detectors to differentiate between these two scenarios.

The cross section for an infrared safe observable in hadron-hadron collisions can be written as the convolution of parton distribution functions (PDFs) for the two hadrons and a perturbatively calculable hard scattering function, with power suppressed corrections. The arguments that this PDF factorization works at all orders of perturbation theory involves the cancellation of what are called Glauber singularities from soft gluons that interact with both hadrons. I illustrate the arguments used in a 1988 paper with Collins and Sterman by applying these arguments to graphs at the lowest order in which the Glauber problems appear. We can see in this simple example how the contributions from Glauber singularities disappear. This sort of analysis is perhaps useful for the analysis of "non-global" logarithms in certain observables.

In this talk, we derive the coupled dynamics between the bubble wall and the plasma from first principles using nonequilibrium quantum field theory. The commonly used equation of motion of the bubble wall in the kinetic approach is shown to be incomplete. In the language of the two-particle-irreducible effective action, the conventional equation misses higher-loop terms generated by the condensate-particle type vertices (e.g.,~ φϕχ2, where φ is the background field describing the bubble wall, ϕ the corresponding particle excitation and χ another particle species in the plasma). From the missing terms, we identify an additional dissipative friction which is contributed by particle production processes from the condensate-particle type vertices. We also show how other transmission processes beyond the 1-to-1 elementary transmission studied in the literature for ultrarelativistic bubble walls, e.g., 1-to-1 mixing and 1-to-2 transition radiation, can be understood from the kinetic approach.

Major improvements in the theoretical aspects of Cosmology have been possible in recent years due to QFT-inspired methods, such as the effective field theory of large-scale structure (EFTofLSS). In this talk, I will explore further connections between high-energy physics and cosmology. I will present a systematic approach to renormalizing the galaxy bias parameters using path integrals and a finite cutoff scale Λ. I will derive the differential equations of the Wilson-Polchinski renormalization group that describe the evolution of the finite-scale bias parameters with Λ, analogous to the β-function running in QFT. I will then discuss how the RG-flow of EFTofLSS can lead to improvements in the extraction of cosmological parameters and also serve as a tool for sanity checks.

Optomechanical sensors have seen recent innovations which make them more useful as leading detectors of ultralight dark matter. To date they remain a less-studied class of detectors among the rapidly developing field of quantum sensors, and a full study of the potential of these sensors to detect scalar, pseudoscalar and vector DM couplings is needed. After reviewing the necessary theoretical background to calculate their sensitivity, I will give some new insights into optimisation of these systems for DM detection and propose a new detector which can lead to new bounds on DM couplings, focusing on B-L vector DM as an illustration. This new setup, proposed in collaboration with the optomechanics group at UCL, is feasible with current technologies at their disposal, and can operate at the standard quantum limit (SQL). Finally I will briefly discuss the prospects of improving on these bounds by reaching SQL on resonance and other exciting near-future prospects.