NEUTRINO PHYSICS
The theoretical motivations, experimental searches/hints, and implications of neutrino mass are surveyed.
hep-ph↗
arXiv subjects
Paul Langacker
Publications and source records attributed to Paul Langacker.
The strong coupling, unification, and recent data
The prediction of the strong coupling assuming (supersymmetric) coupling constant unification is reexamined. We find, using the new electroweak data, $α_{s}(M_{Z}) \approx 0.129 \pm 0.010$. The implications of the large $α_{s}$ value are discussed. The role played by the $Z$ beauty width is stressed. It is also emphasized that high-energy (but not low-energy) corrections could significantly diminish the prediction. However, unless higher-dimension operators are assumed to be suppressed, at present one cannot place strong constraints on the super-heavy spectrum. Non-leading electroweak threshold corrections are also discussed.
hep-ph↗
Tests of the Standard Model and Searches for New Physics
The implications of precision $Z$ and $W$-pole and neutral current data for testing the standard electroweak model, determining its parameters, and searching for new physics, are described.
hep-ph↗
Solar Neutrinos
The status of the solar neutrino problem is reviewed. Attempts to explain the observed deficit and spectral distortion, both by astrophysical and particle physics methods, are described. It is argued that the comparison of all experiments strongly prefers the particle physics solutions.
hep-ph↗
Grand Unification and the Standard Model
The current status of gauge unification is surveyed both in the standard model and its minimal supersymmetric extension. Implications for proton decay, Yukawa unification, and the Higgs mass are described.
hep-ph↗
Implications of High Precision Experiments and the CDF Top Quark Candidates
We discuss the consequences of recent experimental results from CDF, SLC, LEP and elsewhere for the Standard Model and for new physics. A global fit to all indirect precision data yields $m_t = 175 \pm 11^{+17}_{-19}$~GeV, $\sin^2θ_{MS} = 0.2317(3)(2)$, and $α_s = 0.127(5)(2)$, where the central values are for $M_H = 300$ GeV and the second uncertainties are for $M_H \rightarrow 1000$ GeV (+) and 60 GeV (-). The $m_t$ value is in remarkable agreement with the value $m_t=174 \pm 16$ GeV suggested by the CDF candidate events. There is a slight preference for a light Higgs with $M_H < 730$ (880) GeV at 95\% c.l. if the CDF $m_t$ value is (not) included. The sensitivity is, however, due almost entirely to the anomalously large observed values for the $Z \rightarrow b\bar{b}$ width and left-right asymmetry. The value of alpha_s (from the lineshape) is clean theoretically assuming the Standard Model, but is sensitive to the presence of new physics contributions to the $Z \rightarrow b\bar{b}$ vertex. Allowing a vertex correction $δ_{b\bar{b}}^{\rm new}$ one obtains the significantly lower value alpha_s = 0.111 \pm 0.009, in better agreement with low energy determinations, and $δ_{b\bar{b}}^{\rm new} = 0.023 \pm 0.011$. There is now enough data to perform more general fits to parameters describing new physics effects and to separate these from $m_t$ and $M_H$. Allowing the parameter $ρ_0$, which describes sources of $SU(2)$ breaking beyond the Standard Model, to be free one finds $ρ_0=1.0012 \pm 0.0017 \pm 0.0017$, remarkably close to unity. One can also separate the new physics contributions to the oblique parameters $S_{\rm new}$, $T_{\rm new}$ and $U_{\rm new}$, which all take values consistent with zero. The effects of supersymmetry on the determination of the SM parameters are discussed.
hep-ph↗
Model Independent Determination of the Solar Neutrino Spectrum with and without MSW
Besides the opportunity for discovering new neutrino physics, solar neutrino measurements provide a sensitive probe of the solar interior, and thus a rigorous test of solar model predictions. We present model independent determinations of the neutrino spectrum by using relevant flux components as free parameters subject only to the luminosity constraint. (1) Without the Mikheyev-Smirnov-Wolfenstein (MSW) effect, the best fit for the combined data is poor. Furthermore, the data indicate a severe suppression of the $^7$Be flux relative to the $^8$B, contradicting both standard and nonstandard solar models in general; the $pp$ flux takes its maximum value allowed by the luminosity constraint. This pathology consistently appears even if we ignore any one of the three data. (2) In the presence of the two-flavor MSW effect, the current constraint on the initial $^8$B flux is weak, but consistent with the SSM and sufficient to exclude nonstandard models with small $^8$B fluxes. No meaningful constraint is obtained for the other fluxes. In the future, even allowing MSW, the $^8$B and $^7$Be fluxes can be determined at the $\pm$(15 -- 20)\% level, making competing solar models distinguishable. We emphasize that the neutral current sensitivity for $^7$Be neutrinos in BOREXINO, HELLAZ, and HERON is essential for determining the initial fluxes. The constraints on the MSW parameters in the model independent analysis are also discussed.
hep-ph↗
Theoretical Study of the Electroweak Interaction -- Present and Future
There have been several important recent developments in precision electroweak tests. These include: the new LEP energy scan during the 1993 run; the first high-precision results on the left-right asymmetry from the SLD Collaboration at SLAC; the probable discovery of the top quark by the CDF Collaboration at Fermilab and the determination of its mass. I will discuss the implications of these and earlier results for testing the standard model; for the standard model parameters, including the top quark mass, the Higgs mass, the weak mixing angle, and the strong coupling constant, $α_s$; and the search for new physics. In particular, given the CDF direct determination of $m_t$ it is now possible to severely constrain certain types of new physics by separating the contribution from new physics from the dependence on $m_t$.
hep-ph↗
Implications of Yukawa unification for the Higgs sector in supersymmetric grand-unified models
The SU(5) unification-scale relation h_b=h_tau between the b and tau Yukawa couplings severely constrains tan beta and m_t (even more so if h_t=h_b= h_tau holds) in supersymmetric models. We examine the implications of these constraints for the Higgs sector assuming universal soft breaking terms, and emphasize that both of these relations impose unique characteristics in terms of symmetries and of the spectrum. We further study the tan beta near 1 scenario, which is suggested by h_b=h_tau, and, in particular, the loop- induced mass of the light Higgs boson. We compare the effective potential and renormalization group methods and stress the two-loop ambiguities in the calculation of the mass. These and a large enhancement to the loop correction due to t-scalar left-right mixing considerably weaken the upper bound. Nevertheless, we find that for this scenario the Higgs boson is probably lighter than 110 GeV, and typically lighter than 100 GeV. Thus, it is in the mass range that may be relevant for LEPII. Our numerical results are presented in a self-contained manner in section V. In separate appendices we discuss the global symmetries of the Higgs potential, the issue of false (color-breaking) vacua, two-loop calculations, and the effect of an additional Higgs singlet. We show that the approximate constraints that are often used to eliminate color-breaking vacua are not always relevant.
hep-ph↗
Solar Model Uncertainties, MSW Analysis, and Future Solar Neutrino Experiments
Various theoretical uncertainties in the standard solar model and in the Mikheyev-Smirnov-Wolfenstein (MSW) analysis are discussed. It is shown that two methods of estimating the solar neutrino flux uncertainties are equivalent: (a) a simple parametrization of the uncertainties using the core temperature and the nuclear production cross sections; (b) the Monte Carlo method of Bahcall and Ulrich. In the MSW analysis, we emphasize proper treatments of correlation of theoretical uncertainties between flux components and between different detectors, the Earth effect, and multiple solutions in a combined $χ^2$ procedure. The MSW solutions for various standard and nonstandard solar models are also shown. The MSW predictions of the global solutions for the future solar neutrino experiments are given, emphasizing the measurement of the energy spectrum and the day-night effect in Sudbury Neutrino Observatory and Super-Kamiokande to distinguish the two solutions.
hep-ph↗
The Updated MSW Analysis and the Standard Solar Model Uncertainties
We update the analysis of the MSW and general astrophysical solutions to the combined solar neutrino observations by including the GALLEX II result. We also show that our parametrized flux uncertainties are equivalent to the Monte-Carlo results of Bahcall and Ulrich.
hep-ph↗
Testing the handedness of a heavy $W^{\prime}$ at future hadron colliders
We show that the associated production $pp\to W^{\prime}W$, and the rare decay $pp\to W'\to \bar\ell\ell W$ are useful tests of $W'$ couplings to fermions at future hadron colliders. For $M_{W'}\sim (1 - 3)$ TeV they would allow a clean determination on whether the $W'$ couples to $V-A$ or $V+A$ currents. As an illustration a model in which the $W^{\prime\pm}$ couples only to $V-A$ currents is contrasted to the left-right symmetric models which involve $V+A$ currents.
hep-ph↗
The Bottom Mass Prediction in Supersymmetric Grand Unification; Uncertainties and Constraints
Grand unified theories often predict unification of Yukawa couplings (e.g., $h_{b} = h_τ$), and thus certain relations among fermion masses. The latter can distinguish these from models that predict only coupling constant unification. The implications of Yukawa couplings of the heavy-family in the supersymmetric extension of the standard model (when embedded in a GUT) are discussed. In particular, uncertainties associated with $m_{t}$ and $m_{b}$, threshold corrections at the low-scale, and threshold and nonrenormalizable-operator corrections associated with a grand-unified sector at the high-scale are parametrized and estimated. The implication of these and of the correlation between $m_{t}$ and the prediction for $α_{s}$ are discussed. Constraints on the $\tanβ$ range in such models and an upper bound on the $t$-quark pole mass are given and are shown to be affected by the $α_{s}-m_{t}$ correlation. Constraints on the low-scale thresholds are found to be weakened by uncertainties associated with the high-scale.
hep-ph↗
Five Phases of Weak Neutral Current Experiments from the Perspective of a Theorist
I give my personal perspective on the past, present, and future of weak neutral current experiments, emphasizing the experimental inputs; the theoretical difficulties and inputs; the role of model independent and global analyses; and the implications at each stage. hep-ph/yymmnnn
hep-ph↗
The Earth Effect in the MSW Analysis of the Solar Neutrino Experiments
We consider the Earth effect in the MSW analysis of the Homestake, Kamiokande, GALLEX, and SAGE solar neutrino experiments. Using the time-averaged data and assuming two-flavor oscillations, the large-angle region of the combined fit extends to much smaller angles (to $\sin^22θ\simeq 0.1$) than when the Earth effect is ignored. However, the additional constraint from the Kamiokande II day-night data excludes most of the parameter space sensitive to the Earth effect independent of astrophysical uncertainties, and leaves only a small large-angle region close to maximal mixing at 90\% C.L. The nonadiabatic solution remains unaffected by the Earth effect and is still preferred. Both theoretical and experimental uncertainties are included in the analysis.
hep-ph↗
Precision Tests of the Standard Model
The implications of recent precision $Z$-pole, $W$ mass, and weak neutral current data for testing the standard electroweak model, constraining the $t$ quark and Higgs masses, \alsz, and grand unification are discussed. A fit to all data yields $\siz = 0.2328 \pm 0.0007$ (\msb) or $\sinn \equiv 1 - \mw^2/\mz^2 = 0.2267 \pm 0.0024$ (on-shell), where the uncertainties are mainly from \mt. In the standard model one predicts $\mt = 150^{+19 + 15}_{-24 - 20}$ GeV, where the central value assumes \mh = 300 GeV and the second uncertainty is for \mh $\ra$ 60 GeV ($-$) or 1 TeV (+). In the minimal supersymmetric extension of the standard model (MSSM) one predicts $\mt = 134^{+23}_{-28} \pm 5$ GeV, where the difference is due the light Higgs scalar expected in the MSSM. There is no significant constraint on \mh \ until \mt \ is known independently.
hep-ph↗
Determination of $Z'$ Gauge Couplings to Quarks and Leptons at Future Hadron Colliders
We point out that at future hadron colliders the ratio of cross sections for $pp\to Z'\to \ell^+\ell^-$ in two rapidity bins is a useful probe of the relative couplings of the $Z'$ to $u$ and $d$ quarks. Combined with the forward-backward asymmetry, the rare decay modes $Z'\to W \ellν_\ell$, and three associated productions $pp\to Z'V$ ($V=Z,W,γ$), and assuming inter-family universality, small $Z-Z'$ mixing, and the $Z'$ charge commuting with the $SU_{2L}$ generators, three out of four normalized couplings could be extracted. An analysis of the statistical uncertainties expected for the above probes at the LHC for typical models with $M_{Z'}\simeq1$ TeV shows that one lepton coupling and two combinations of quark couplings could be determined to around 5\%, 20\%, and 30\%, respectively. This allows for a clear distinction between models.
hep-ph↗
Solar Neutrinos
The status of solar neutrino experiments and their implications for both nonstandard astrophysics ({\it e.g.,} cool sun models) and nonstandard neutrino properties ({\it e.g.,} MSW conversions) are discussed. Assuming that all of the experiments are correct, the relative rates observed by Kamiokande and Homestake are hard to account for by a purely astrophysical solution, while MSW conversions can describe all of the data. Assuming the standard solar model, there are two allowed regions for MSW conversions into $ν_μ$ or $ν_τ$, with the non-adiabatic solution giving a better fit than the large angle. For conversions into sterile neutrinos there is only a nonadiabatic solution. Allowing both MSW conversions and nonstandard astrophysics, the data simultaneously determine the temperature of the core of the sun to within five percent, consistent with the standard solar model prediction. The implications of the atmospheric $ν_μ/ν_e$ ratio and of a hot component of the dark matter are briefly discussed, and the expectations of theoretical models motivated by grand unification are summarized.
hep-ph↗