Public Thesis Defense of ZeQiang WANG - IRMP
sst |
Probing the τ dipole moments at the future lepton collider by ZeQiang WANG -
Lundi 12 octobre 2026 à 15h00 - Auditoire CYCL09a - Bâtiment de Hemptinne - Chemin du Cyclotron, 2 - 1348 Louvain-la-Neuve -
This thesis investigates the electromagnetic and weak dipole moments of the
tau lepton within the Standard Model Effective Field Theory (SMEFT). The
anomalous magnetic moments Δaτ and Δaτᶻ, together with the electric
dipole moments dτ and dτᶻ, provide sensitive probes of new physics related
to flavour, chirality, and CP violation. The relevant effects are described by
dimension-six electroweak dipole operators, rotated from the gauge basis to
the physical photon and Z directions.
At FCC-ee, the analysis combines e⁺e⁻ → τ⁺τ⁻, photon-induced γγ → τ⁺τ⁻,
radiative e⁺e⁻ → τ⁺τ⁻γ, and H → τ⁺τ⁻γ channels. Photon-induced production
provides the leading sensitivity to Δaτ, while the Tera-Z stage and Z-lineshape
scan strongly constrain the weak dipoles. The combined 95% CL sensitivities reach
|Δaτ| ≈ 2.8 × 10⁻⁵, |Δaτᶻ| ≈ 1.0 × 10⁻⁵, |dτ| ≈ 2.5 × 10⁻¹⁸ e cm, and
|dτᶻ| ≈ 4.4 × 10⁻¹⁹ e cm.
At a multi-TeV muon collider, the study includes direct annihilation, vector-boson
scattering, boson-associated production, and radiative Higgs decay. Suppressed
Standard Model–dipole interference makes quadratic SMEFT contributions increasingly
important, while energy-growing contact interactions strongly enhance associated
production. In particular, μ⁺μ⁻ → τ⁺τ⁻H becomes the most sensitive channel. At 14 TeV,
sensitivities reach |Δaτ| ≈ 6.7 × 10⁻⁶ and |dτ| ≈ 3.6 × 10⁻²⁰ e cm.
FCC-ee and a multi-TeV muon collider are highly complementary: the former exploits
luminosity and resonant enhancement, while the latter benefits from energy growth.
Together, they can improve the sensitivity to tau dipole moments by several orders of
magnitude.