Breaking CPT by mixed noncommutativity

The mixed component of the noncommutative parameter θ0 _μM, where μ=0,1,2,3 and M is an extra dimensional index, may violate four-dimensional CPT invariance. We calculate one- and two-loop induced couplings of θ_μ5 with the four-dimensional axial vector current and with the CPT odd dim=6 operators starting from five-dimensional Yukawa and U(1) theories. The resulting bounds from clock comparison experiments place a stringent constraint on θ_μ5, |θ_μ5|^-1/25×10¹¹ GeV. Orbifold projection and/or localization of fermions on a 3-brane lead to CPT-conserving physics, in which case the constraints on θ_μ5 are softened.