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HK: Physik der Hadronen und Kerne

HK 10: Postersitzung: Theorie

HK 10.13: Poster

Tuesday, March 20, 2001, 10:30–12:45, R

Isospin Breaking in Neutron Beta Decay — •Norbert Kaiser — Physik-Department, Technische Universität München

Present precision measurements of the neutron life time lead to a CKM matrix element |V_ud|=0.9715±0.0014 which is almost three standard devitations off the value √1−|V_us|²−|V_ub|²=0.9754±0.0004 inferred from heavy quark decays etc. We investigate the possibility whether isospin-breaking effects in the neutron-to-proton vector transition matrix element ⟨ p|V₀⁺|n ⟩=g_v ≠ 1 could eventually close this gap. For that we calculate in chiral perturbation theory the effect of pion-loops on the vector-current transition matrix element ⟨ p|V₀⁺|n ⟩ taking into account the mass difference of the charged and neutral pion mass m_π⁺−m_π⁰= 4.6 MeV. We find a negligibly small isospin breaking effect of g_v−1 = −(g_A²+1/3) [(m_π⁺−m_π⁰)/4π f_π]² ≃ −3.2 · 10⁻⁵. The critical quantity in the analysis of neutron beta-decay precision measurements is thus the radiative correction term Δ_R = 0.0246± 0.0009 as calculated by Sirlin in 1978. We intend to take a fresh look at this quantity in baryon chiral perturbation theory. Supported in part by BMBF