Nejc Košnik

A bstract: We study the weak magnetic and electric dipole moments of the τ lepton as precision tests of the Standard Model (SM) and probes of heavy New Physics (NP). We present an updated SM prediction for the τ weak magnetic dipole moment at one loop, including a careful assessment of theoretical uncertainties from electroweak scheme dependence. Working within the SM Effective Field Theory, we derive comprehensive current constraints on the τ dipole operators from a combination of observables: the τ weak and electromagnetic dipole moments, high-mass Drell-Yan tails at the LHC, Z partial decay widths, and the electron electric dipole moment. Finally, we assess the prospects of measuring the SM value of the τ weak magnetic moment at the FCC- ee Tera- Z run, and project the sensitivities of the leading observables to heavy NP at FCC- ee and HL-LHC, incorporating informed assumptions about improvements of systematic uncertainties. We find that the τ weak dipole moments are already among the leading probes of the τ dipole operators, and will become increasingly dominant at future colliders.

We argue that a detailed analysis of semileptonic decays can test the possibility of a massive neutrino. The key observable, related to the forward–backward asymmetry, is exactly zero for a massless neutrino but becomes nonzero if the neutral lepton is heavy and interacts with Standard Model fields via lefthanded operators. For righthanded interactions, this quantity differs significantly from zero even for a massless righthanded neutrino. We demonstrate this explicitly using the example of a pseudoscalar meson decaying into another pseudoscalar meson. A similar discussion applies to decays into a vector meson, with an additional subtlety addressed in this work.

Probing the C P nature of the Higgs Yukawa couplings is a promising avenue for unraveling new physics effects. In this work, we investigate the possible single- and two-field UV origins of C P -violating leptonic Yukawa couplings, using the Standard Model effective field theory as a stepping stone. We demonstrate a rich set of constraints on the UV model parameters, including direct Higgs measurements, electric and magnetic dipole moments of leptons, charged lepton flavor violating observables, and electroweak precision tests. Studying representative flavor assumptions we find that the precision constraints are often, but not always, more constraining than the dedicated LHC analyses of modified leptonic Yukawa couplings.

A bstract: We explore CP-violating (CPV) effects of heavy New Physics in flavour-changing quark dipole transitions, within the framework of Standard Model Effective Field Theory (SMEFT). First, we establish the relevant dimension six operators and consider the Renormalisation Group (RG) evolution of the appropriate Wilson coefficients. We investigate RG-induced correlations between different flavour-violating processes and electric dipole moments (EDMs) within the Minimal Flavour Violating and U(2) ³ quark flavour models. At low energies, we set bounds on the Wilson coefficients of the dipole operators using CPV induced contributions to observables in non-leptonic and radiative B , D and K decays as well as the neutron and electron EDMs. This enables us to connect observable CPV effects at low energies and general NP appearing at high scales. We present bounds on the Wilson coefficients of the relevant SMEFT operators at the high scale Λ = 5 TeV, and discuss most sensitive CPV observables for future experimental searches.

We present constraints on the left-handed dimension-6 interactions that contribute to semileptonic and leptonic decays of K, D, pions and to nuclear beta decay. We employ the flavour covariant description of the effective couplings, identify universal CP phases of New Physics and derive constraints from decay rates and CP-odd quantities. As a result, we can predict the maximal effects of such flavoured NP in D decays from stringent K decay constraints and vice-versa.