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New measurements of the CP asymmetry in B->\psi K_S, a_{\psi K_S}, by the BABAR and BELLE collaborations are consistent with the Standard Model prediction for sin2\beta. These measurements, however, leave open the possibility that a_{\psi K_S} is well below the Standard Model prediction. We identify deviations from the `reasonable ranges' of hadronic parameters that can lead to low values of sin2\beta. New physics, mainly in B^0-\barB^0 mixing and/or K^0-\barK^0 mixing, can explain low values of a_{\psi K_S} in two ways: either by allowing for values of sin2\beta below the Standard Model prediction or by modifying the relation between a_{\psi K_S} and sin2\beta., Comment: 14 pages

Supersymmetric models with an approximate CP, $10^{-3} \lsim \phi_{CP} \ll 1$, are a viable framework for the description of nature. The full high energy theory has exact CP and horizontal symmetries that are spontaneously broken with a naturally induced hierarchy of scales, $\Lambda_{CP} \ll \Lambda_{H}$. Consequently, the effective low energy theory, that is the supersymmetric Standard Model, has CP broken explicitly but by a small parameter. The $\epsilon_{K}$ parameter is accounted for by supersymmetric contributions. The predictions for other CP violating observables are very different from the Standard Model. In particular, CP violating effects in neutral B decays into final CP eigenstates such as $B \to \psi K_{S}$ and in $K \to \pi\nu\bar{\nu}$ decays are very small. This framework, though, is strongly disfavored by the recent measurements of $\epsilon^\prime_{K}/\epsilon_{K}$., Comment: 8 pages, LaTeX; to appear in the proceedings of the Erice International School on Basics and Highlights in Fundamental Physics, August 1999

Models with extended quark sector affect the CP asymmetry in the $B\to \psi K_{S}$ decay, $a_{\psi K_{S}}$, in two ways: First, the top-mediated box diagram is not necessarily the only important contribution to $B-\bar{B}$ mixing. Second, the $3\times 3$ CKM matrix is no longer unitary. We analyze the constraints that follow from the CDF measurement, $a_{\psi K_{S}} =0.79^{+0.41}_{-0.44}$, on the mixing parameters of extended quark sectors. Most noticeably, we find significant constraints on the phase of the relevant flavor changing $Z$ coupling in models with extra down quarks in vector-like representations. Further implications for the CP asymmetry in semileptonic $B$ decays are discussed., Comment: 12 pages, LaTeX, 3 figures

Recently, the CDF collaboration has reported a measurement of the CP asymmetry in the $B\to\psi K_S$ decay: $a_{\psi K_S}=0.79^{+0.41}_{-0.44}$. We analyze the constraints that follow from this measurement on the size and the phase of contributions from new physics to $B-\barB$ mixing. Defining the relative phase between the full $M_{12}$ amplitude and the Standard Model contribution to be $2\theta_d$, we find a new bound: $\sin2\theta_d\gsim-0.6 (-0.87)$ at one sigma (95% CL). Further implications for the CP asymmetry in semileptonic B decays are discussed., Comment: 13 pages, harvmac, 3 figures; v2: a discussion of new physics effects on tree level decays added; references added; accepted for publication in Physical Review Letters

We investigate supersymmetric models where neither R-parity nor lepton number nor baryon number is imposed. The full high energy theory has an exact horizontal U(1) symmetry that is spontaneously broken. Quarks and Higgs fields carry integer horizontal charges but leptons carry half integer charges. Consequently, the effective low energy theory has two special features: a U(1) symmetry that is explicitly broken by a small parameter, leading to selection rules, and an exact residual Z_2 symmetry, that is lepton parity. As concerns neutrino parameters, the Z_2 symmetry forbids contributions from R_p-violating couplings and the U(1) symmetry induces the required hierarchy. As concerns baryon number violation, the Z_2 symmetry forbids proton decay and the U(1) symmetry provides sufficient suppression of double nucleon decay and of neutron - antineutron oscillations., Comment: 12 pages, harvmac

We investigate supersymmetric models in which an anomalous $U(1)_{X}$ symmetry explains the Yukawa hierarchy, and the related $D_{X}$-term plays a role in supersymmetry breaking. We use a bottom-up approach to model building. Phenomenological viability leads to a scenario with degenerate squark and slepton spectra and with heavy gauginos. Features of a K\"ahler potential that allows for such a scenario are described., Comment: 13 pages, LaTeX; analysis limited to phenomenological constraints, version accepted for publication in Phys. Lett. B

We use a $U(2)\times U(1)$ horizontal symmetry in order to construct supersymmetric models where the flavor structure of both quarks and leptons is induced naturally. The supersymmetric flavor changing neutral currents problem is solved by the degeneracy between sfermions induced by the U(2) symmetry. The additional U(1) enables the generation of mass ratios that cannot be generated by U(2) alone. The resulting phenomenology differs from that of models with either abelian or $U(2)\times GUT$ symmetries. Our models give rise to interesting neutrino spectra, which can incorporate the Super-Kamiokande results regarding atmospheric neutrinos., Comment: 12 pages, LaTeX, typos corrected in eqs. 28,40

We construct phenomenologically viable supersymmetric models where CP is an approximate symmetry. The full high energy theory has exact CP and horizontal symmetries that are spontaneously broken with a naturally induced hierarchy of scales, $\Lambda_{CP}\ll\Lambda_H$. Consequently, the effective low energy theory, that is the supersymmetric Standard Model, has CP broken explicitly but by a small parameter. The $\epsilon_K$ parameter is accounted for by supersymmetric contributions. The predictions for other CP violating observables are very different from the Standard Model. In particular, CP violating effects in neutral B decays into final CP eigenstates such as $B\ra\psi K_S$ and in $K\ra\pi\nu\bar\nu$ decays are very small., Comment: 19 pages, harvmac