Radiative corrections to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>τ</mml:mi><mml:mo stretchy="false">→</mml:mo><mml:mi>π</mml:mi><mml:mo stretchy="false">(</mml:mo><mml:mi>K</mml:mi><mml:mo stretchy="false">)</mml:mo><mml:msub><mml:mi>ν</mml:mi><mml:mi>τ</mml:mi></mml:msub><mml:mo stretchy="false">[</mml:mo><mml:mi>γ</mml:mi><mml:mo stretchy="false">]</mml:mo></mml:math> : A reliable new physics test

The ratios ${R}_{\ensuremath{\tau}/P}\ensuremath{\equiv}\mathrm{\ensuremath{\Gamma}}(\ensuremath{\tau}\ensuremath{\rightarrow}P{\ensuremath{\nu}}_{\ensuremath{\tau}}[\ensuremath{\gamma}])/\mathrm{\ensuremath{\Gamma}}(P\ensuremath{\rightarrow}\ensuremath{\mu}{\ensuremath{\nu}}_{\ensuremath{\mu}}[\ensuremath{\gamma}])$ ($P=\ensuremath{\pi}$, $K$) provide sensitive tests of lepton universality $|{g}_{\ensuremath{\tau}}/{g}_{\ensuremath{\mu}}|=1$ and are a useful tool for new physics searches. radiative corrections to ${R}_{\ensuremath{\tau}/P}$ computed following large-${N}_{C}$ expansion deal with hadronic effects: Chiral Perturbation Theory is enlarged by including the lightest multiplets spin-one heavy states such that relevant Green functions well behaved at high energies. We find $\ensuremath{\delta}{R}_{\ensuremath{\tau}/\ensuremath{\pi}}=(0.18\ifmmode\pm\else\textpm\fi{}0.57)%$ $\ensuremath{\delta}{R}_{\ensuremath{\tau}/K}=(0.97\ifmmode\pm\else\textpm\fi{}0.58)%$, which imply ${|{g}_{\ensuremath{\tau}}/{g}_{\ensuremath{\mu}}|}_{\ensuremath{\pi}}=0.9964\ifmmode\pm\else\textpm\fi{}0.0038$ ${|{g}_{\ensuremath{\tau}}/{g}_{\ensuremath{\mu}}|}_{K}=0.9857\ifmmode\pm\else\textpm\fi{}0.0078$, compatible $1.8\ensuremath{\sigma}$ universality, respectively. test unitarity bind nonstandard effective interactions $\ensuremath{\tau}\ensuremath{\rightarrow}P{\ensuremath{\nu}}_{\ensuremath{\tau}}[\ensuremath{\gamma}]$ decays.