Chung Ngoc Leung

Renormalization of the neutrino mass operator

Abstract A small neutrino Majorana mass can arise in the Standard Model as an effective dimension 5 operator. We calculate the renormalization of this operator in the minimal Standard Model and in its two-Higgs-doublet and supersymmetric extensions. Renormalization from the scale of lepton number violation (e.g., the Planck scale or a GUT scale) to the weak scale decreases the strength of this operator by an order of magnitude or more if the top quark and Higgs boson masses are large. Neutrino mixing angles also run with momentum. We show instances where small mixing at a high scale becomes large at the weak scale, and vice versa.

Classification of effective neutrino mass operators

Abstract We present a classification of SU(3)C×SU(2)L×U(1)Y gauge invariant ΔL=2 (L being lepton number) effective operators relevant for generating small Majorana neutrino masses. Operators of dimension up to 11 have been included in our analysis. This approach enables us to systematically identify interesting neutrino mass models. It is shown that many of the well-known models fall into this classification. In addition, a number of new models are proposed and their neutrino phenomenology is outlined. Of particular interest is a large class of models in which neutrinoless double beta decays arise at a lower order compared to the neutrino mass, making these decays accessible to the current round of experiments.

Chiral symmetry breaking in a uniform external magnetic field

Using the nonperturbative Schwinger-Dyson equation, we show that chiral symmetry is dynamically broken in QED at weak gauge couplings when an external magnetic field is present and that chiral symmetry is restored above a critical temperature which is much smaller than {radical}({vert_bar}eH{vert_bar}), where H is the magnetic-field strength. {copyright} {ital 1997} {ital The American Physical Society}

Possible violation of the equivalence principle by neutrinos.

We consider the effect of a long-range, flavor-changing tensor interaction of possible gravitational origin. Neutrino-mixing experiments provide the most sensitive probe to date for such forces{emdash}testing the equivalence principle at levels below 10{sup {minus}20}. Here we justify and generalize a formalism for describing such effects. The constraints from neutrino-mixing experiments on gravitationally induced mixing are calculated. Our detailed analysis of the atmospheric neutrino data confirms a remarkable result: the atmospheric neutrino data imply the same size violation of the equivalence principle as do the solar neutrino data. Additional tests of this suggestive result are discussed. {copyright} {ital 1996 The American Physical Society.}

Gauge independent approach to chiral symmetry breaking in a strong magnetic field

Abstract The gauge independence of the dynamical fermion mass generated through chiral symmetry breaking in QED in a strong, constant external magnetic field is critically examined. We show that the bare vertex approximation, in which the vertex corrections are ignored, is a consistent truncation of the Schwinger–Dyson equations in the lowest Landau level approximation. The dynamical fermion mass, obtained as the solution of the truncated Schwinger–Dyson equations evaluated on the fermion mass shell, is shown to be manifestly gauge independent. By establishing a direct correspondence between the truncated Schwinger–Dyson equations and the 2PI (two-particle-irreducible) effective action truncated at the lowest nontrivial order in the loop expansion as well as in the 1 / N f expansion ( N f is the number of fermion flavors), we argue that in a strong magnetic field the dynamical fermion mass can be reliably calculated in the bare vertex approximation.

Solar neutrinos and the principle of equivalence

We study the proposed solution of the solar neutrino problem which requires a flavor nondiagonal coupling of neutrinos to gravity. We adopt a phenomenological point of view and investigate the consequences of the hypothesis that the neutrino weak interaction eigenstates are linear combinations of the gravitational eigenstates which have slightly different couplings to gravity,

Atmospheric neutrino tests of neutrino oscillation mechanisms

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Neutrino mixing due to a violation of the equivalence principle

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Neutrino oscillations from string theory

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Probing the gravitational scale via running gauge couplings

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