R(b), R(c) and New Physics: An Updated Model Independent Analysis

We analyze LEP and SLC data from the 1995 Summer Conferences as well as from low energy neutral current experiments for signals of new physics. The reasons for doing this are twofold, first to explain the deviations from the standard model observed in $R_b$ and $R_c$ and second to constrain non-standard contributions to couplings of the $Z^0$ boson to all fermions and to the oblique parameters. We do so by comparing the data with the Standard Model as well as with a number of test hypotheses concerning the nature of the new physics. These include non-standard $Zb\bar{b}$-, $Zc\bar{c}$- and $Zs\bar{s}$-couplings as well as the couplings of the $Z^0$ to fermions of the entire first, second and third generations and universal corrections to all up- and down-type quark couplings (as can arise \eg\ in $Z'$ mixing models). We find that non-standard $Zb\bar{b}$ couplings are both necessary and sufficient to explain the data and in particular the $R_b$ anomaly. It is not possible to explain $R_b$, $R_c$ and a value of the strong coupling constant consistent with low energy determinations invoking only non-standard $Zb\bar{b}$- and $Zc\bar{c}$-couplings. To do so one has to have also new physics contributions to the $Zs\bar{s}$ or universal corrections to all $Zq\bar{q}$ couplings.Comment: 25 pages, Plain TeX, 10 Tables, 3 Figures, Analysis extended to include universal corrections to all NC quark couplings (such as can arise in Z' mixing models), to be published in IJMP

Naturally Degenerate Neutrinos

The solar neutrino problem, atmospheric neutrino problem, and the existence of hot dark matter can all be economically accounted for using only the three known neutrinos if these neutrinos all have nearly degenerate masses of a few eV. We show how to generate this pattern of neutrino masses in a natural way within a `see-saw' framework.Comment: 10 pages, plain TeX, McGill-94/07, NEIP-94-003 (This is the version which is to appear in the journal. We have cleaned up typos, added references, and slightly expanded our treatment of radiative corrections.

Shifting RbR_b with AFBbA^b_{FB}

Precision measurements at the $Z$ resonance agree well with the standard model. However, there is still a hint of a discrepancy, not so much in $R_b$ by itself (which has received a great deal of attention in the past several years) but in the forward-backward asymmetry $A^b_{FB}$ together with $R_b$. The two are of course correlated. We explore the possibilty that these and other effects are due to the mixing of $b_L$ and $b_R$ with one or more heavy quarks.Comment: 11 pages, 1 Figure, LaTex fil

An Elusive Z' Coupled to Beauty

By extending the standard gauge group to SU(3)_c \times SU(2)_L \times U(1)_Y \times U(1)_X with X charges carried only by the third family we accommodate the LEP measurement of R_b and predict a potentially measurable discrepancy in A_{FB}^{b} in e^+e^- scattering and that D^0\bar{D}^0 mixing may be near its experimental limit. The Z', which explicitly violates the GIM mechanism, can nevertheless be naturally consistent with FCNC constraints. Direct detection of the Z' is possible but challenging.Comment: 12 pages, plus 1 Postscript figure, uses revtex, Discussion of FCNC extende

THE SENSITIVITY TO NEW PHYSICS OF A LEP SCAN IN 1995

We study the implications of possible off-peak measurements in the 1995 LEP run, in regard to probing physics beyond the Standard Model. To do so, we determine the accuracy with which various nonstandard couplings can be expected to be measured in the three different scan scenarios recently discussed by Clarke and Wyatt. We find that each scan scenario allows greater sensitivity to a different set of new physics couplings. Oblique parameters are best measured with the longest scan, while nonstandard fermion couplings to the Z tend to be better constrained (albeit only marginally) if all of the 1995 LEP measurements are taken on the Z peak.Comment: Plain TeX, 9 pages, no figures. We have streamlined our presentation by omitting observables of our Class B. All else is completely unchanged

R_b and New Physics: A Comprehensive Analysis

We survey the implications for new physics of the discrepancy between the LEP measurement of $R_b$ and its Standard Model prediction. Two broad classes of models are considered: ($i$) those in which new Z\bbar b couplings arise at tree level, through $Z$ or $b$-quark mixing with new particles, and ($ii$) those in which new scalars and fermions alter the Z \bbar b vertex at one loop. We keep our analysis as general as possible in order to systematically determine what kinds of features can produce corrections to $R_b$ of the right sign and magnitude. We are able to identify several successful mechanisms, which include most of those which have been recently been proposed in the literature, as well as some earlier proposals (\eg\ supersymmetric models). By seeing how such models appear as special cases of our general treatment we are able to shed light on the reason for, and the robustness of, their ability to explain $R_b$.Comment: 60 pages, 8 figures, plain tex, uses epsf. Final version to appear in Phys. Rev. D; propgating sign error corrected in eqs. 78, 87, 88, 89, 98, and 107; results unchange

Negative s and Light New Physics

Motivated by the difference between SLD's recent measurement of ALR and the corresponding LEP results, we explore which kinds of new particles can (1) contribute dominantly to new physics through oblique corrections, (2) produce negative values for S and T, and (3) not be in conflict with any other experiments, on or off the Z resonance. We are typically led to models which involve new particles which are not much heavier than MZ/2, and so which may also have implications for other experiments in the near future. For such light particles, we show how the oblique-parameter analysis of purely Z-pole data requires the interpretation of the data in terms of modified parameters, S' and T', whose difference from S and T improves the available parameter space of the models.Comment: plain TeX, 16 pages, 6 figures attached as a uuencoded file, McGill-94/27, NEIP-94-00