Light new physics and the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>τ</mml:mi> </mml:math> lepton dipole moments

Testing new-physics (NP) scenarios that couple predominantly to the third generation is notoriously difficult experimentally, as exemplified by comparing limits for the τ lepton dipole moments to those of electrons and muons. In this case, extracting limits from processes such as e + e − → τ + τ − often relies on effective-field-theory (EFT) arguments, which allow for model-independent statements, but only apply if the NP scale is sufficiently large compared to the center-of-mass energy. In this work we offer a comprehensive analysis of light NP contributions to the τ dipole moments, providing a detailed account of the interpretation of asymmetry measurements in e + e − → τ + τ − that are tailored toward the extraction of dipole moments, for the test cases of new light spin-0 and spin-1 bosons. Moreover, we study the decoupling to the EFT limit in these scenarios and discuss the complementarity to constraints from other related processes, such as production in e + e − reactions. While covering a wide range of light NP scenarios, as specific case study we present a detailed discussion of a tauphilic gauge vector boson at Belle II.