ar X iv : h ep - p h / 05 12 02 4 v 1 2 D ec 2 00 5 Quark - Lepton Universality and Large Leptonic Mixing

A unified description of fermionic mixing is proposed which assumes that in certain basis (i) a single complex unitary matrix V diagonalizes mass matrices of all fermions to the leading order, (ii) the SU(5) relation Md = M T l exists between the mass matrices of the down quarks and the charged leptons, and (iii) M † d = Md. These assumptions automatically lead to different mixing patterns for quarks and leptons: quarks remain unmixed to leading order (i. e. VCKM = 1) while leptons have non-trivial mixing given by a symmetric unitary matrix V 0 PMNS = V TV . V depends on two physical mixing angles and for values of these angles ∼ 20 − 25 it reproduces the observed mixing patterns rather well. We identify conditions under which the universal mixing V follows from the universal mass matrices of fermions. Relatively small perturbations to the leading order structure lead to the CKMmixing and corrections to V 0 PMNS. We find that if the correction matrix equals the CKM matrix, the resulting lepton mixing agrees well with data and predicts sin θ13 > 0.08.