arXiv subjects
Publications and source records attributed to Paul Langacker.
Aspects of theoretical electroweak physics are summarized, including the status of electroweak radiative corrections, the hadronic contribution to the running of alpha, global fits to precision data and their implication for testing the standard model and constraining new physics, and electroweak baryogenesis.
In the gauge-mediation framework the soft supersymmetry breaking mass parameters of the supersymmetric standard model are induced by the gauge interactions of some messenger fields. The parameters exhibit flavor universality which is dictated by the gauge interactions and which efficiently eliminates new dangerous contributions to flavor changing neutral currents. However, the Higgs potential in this framework typically contains an unacceptable hierarchy between its dimensionful parameters (the $μ$-problem of gauge mediation). We show that the problem can be resolved if the Higgs potential arises dynamically once an intermediate U(1)' sector is integrated out rather than arising radiatively from some Yukawa interactions at the messenger scale. As an added benefit, such models may naturally avoid new contribution to CP violating amplitudes. The proposed framework is described, explicit examples are given and its phenomenology is explored. The $μ$ problem is resolved in this case by the low-energy U(1)' dynamics which could be tested in future collider experiments.
Indirect precision data are used to constrain the masses of possible extra Z^prime bosons and their mixings with the ordinary Z. We study a variety of Z^prime bosons as they appear in E_6 and left-right unification models, the sequential Z boson, and the example of an additional U(1) in a concrete model from heterotic string theory. In all cases the mixings are severely constrained (sin theta < 0.01). The lower mass limits are generally of the order of several hundred GeV and competitive with collider bounds. The exception is the Z_psi boson, whose vector couplings vanish and whose limits are weaker. The results change little when the rho parameter is allowed, which corresponds to a completely arbitrary Higgs sector. On the other hand, in specific models with minimal Higgs structures the limits are generally pushed into the TeV region.
The importance of non-zero neutrino mass as a probe of particle physics, astrophysics, and cosmology is emphasized. The present status and future prospects for the solar and atmospheric neutrinos are reviewed, and the implications for neutrino mass and mixing in 2, 3, and 4-neutrino schemes are discussed. The possibilities for significant mixing between ordinary and light sterile neutrinos are described.
We review the experimental and theoretical status of the standard electroweak theory, and its fundamental parameters. We obtain the global best fit result for the Standard Model Higgs boson of M_H = 107^{+67}_{-45} GeV, and find the 95% upper limit of 255 GeV. Parameters describing physics beyond the Standard Model are discussed as well. Particular emphasis is given to implications for supersymmetric extensions of the Standard Model.
Efficient oscillations between ordinary (active) and sterile neutrinos can occur only if Dirac and Majorana mass terms exist which are both small and comparable. It is shown that this can occur naturally in a class of string models, in which higher-dimensional operators in the superpotential lead to an intermediate scale expectation value for a scalar field and to suppressed Dirac and Majorana fermion masses.
We study the electroweak and $U(1)^{'}$ symmetry breaking patterns in models with the particle content of supersymmetric $E_{6}$, including standard model singlets $S$ and exotic quarks $D,~\bar{D}$. Motivated by free fermionic string models, we do not require $E_{6}$-type relations between Yukawa couplings. In particular, we assume that baryon and lepton numbers are conserved, so that the exotic quarks can be light. Gauge invariance allows Yukawa interactions between $S$ and Higgs doublets, and between $S$ and the exotic quarks, allowing radiative $U(1)^{'}$ symmetry breaking and the generation of an effective $μ$ parameter at the electroweak scale. For both the $E_{6}$ $ψ$ and $η$ models, universal soft supersymmetry breaking parameters and Yukawa universality at the high (string) scale do not yield acceptable low energy phenomenology. Relaxing universality, we find solutions with phenomenologically acceptable values of $M_{Z^{'}}$ and the $Z-Z^{'}$ mixing angle. In addition, by varying the $U(1)^{'}$ charge assignments due to the mixing of $U(1)_χ$ and $U(1)_ψ$ of $E_{6}$, it is possible to have acceptable low energy phenomenology with universal boundary conditions.
Many extensions of the standard model, especially grand unified theories and superstring models, predict the existence of additional Z' bosons and associated exotic chiral supermultiplets. It has recently been argued that for classes of string motivated models with supergravity mediated supersymmetry breaking there are two scenarios for the additional Z's: either the mass is in the accessible range < O(1 TeV), providing a natural solution to the mu problem and implications for the Higgs and sparticle masses and for the LSP; or, when the breaking is associated with a D-flat direction, at an intermediate scale, providing a possible explanation for the hierarchies of quark and charged lepton masses and new possibilities for neutrino masses. Related work, examining the detailed structure of specific perturbative string vacua for D and F-flat directions, surviving U(1)'s and exotics, and effective couplings, is briefly described.
In quasi-realistic string models that contain an anomalous U(1) the non-zero Fayet-Iliopoulos term triggers the shifting of the original vacuum to a new one along some flat direction, so that SUSY is preserved but the gauge group is partially broken. The phenomenological study of these models thus requires as a first step the mapping of the space of flat directions. We investigate F- and D-flat directions in several three-generation SU(3)_C x SU(2)_L x U(1)_Y free-fermionic string models and discuss the typical scenarios that generically arise. When they exist, we systematically construct the flat directions that preserve hypercharge, only break Abelian group factors, and can be proven to remain F-flat to all orders in the non-renormalizable superpotential.
We review the status of precision electroweak physics with particular emphasis on the extraction of the Higgs boson mass. Global fit results depend strongly on the used value for the hadronic contribution to alpha(M_Z). We emphasize, however, that the general tendency for a light Higgs persists when using any of the recently obtained values for alpha(M_Z), and is also less dependent on deviating observables such as A_LR than in the past.
We present an updated analysis of astrophysical solutions, two-flavor MSW solutions, and vacuum oscillation solutions to the solar neutrino anomaly. The recent results of each of the five solar neutrino experiments are incorporated, including both the zenith angle (day-night) and spectral information from the Kamiokande experiment, and the preliminary Super-Kamiokande results. New theoretical developments include the use of the most recent Bahcall-Pinsonneault flux predictions (and uncertainties) and density and production profiles, the radiative corrections to the neutrino-electron scattering cross section, and new constraints on the Ga absorption cross section inferred from the gallium source experiments. From a model independent analysis, arbitrary astrophysical solutions are excluded at more that 98% C.L. even if one ignores any one of the three classes of experiment, relaxes the luminosity constraint, or allows more suppression of the 7Be than 8B flux. The data is well described by large and small mixing angle two-flavor MSW conversions, MSW conversions into a sterile neutrino with small mixing, or vacuum oscillations. We also present MSW fits for nonstandard solar models parameterized by an arbitrary solar core temperature or arbitrary 8B flux.
The status of precision electroweak data, tests of the standard model, determination of its parameters, and constraints on new physics, are surveyed.
The experimental data, as well as theoretical considerations allow (and, in some cases, require) the Universe at present to rest in a false vacuum, whose approximate stability imposes constraints on the model parameters. Under very general and mild conditions, the Universe would have ended up in the standard vacuum even if the potential has deeper minima, provided there was a period in which the temperature was > 1 TeV. In many cases, the zero temperature tunneling rate is much smaller than the inverse age of the Universe. Future experiments may reveal that the physical vacuum is not entirely stable. Implications for the cosmological constant are discussed.
The scalar potential of the MSSM may have local and global minima characterized by non-zero expectation values of charged and colored bosons. Even if the true vacuum is not color and charge conserving, the early Universe is likely to occupy the minimum of the potential in which only the neutral Higgs fields have non-zero vev's. The stability of this false vacuum with respect to quantum tunneling imposes important constraints on the values of the MSSM parameters. We analyze these constraints using some novel methods for calculating the false vacuum decay rate. Some regions of the MSSM parameter space are ruled out because the lifetime of the corresponding physically acceptable false vacuum is small in comparison to the present age of the Universe. However, there is a significant fraction of the parameter space that is consistent with the hypothesis that the Universe rests in the false vacuum that is stable on a cosmological time scale.
The mixing of a new $Z'$ boson with the $Z$ significantly improves the fit to the LEP precision electroweak data, provided that the $Z'$ couples mainly to quarks. If $M_{Z_2}<200$~GeV, the $s$-channel $Z_2$ production and $(W,Z,γ)Z_2$ pair production cross sections at the Tevatron give an excess above QCD of $b\bar b$ and $(W,Z,γ)b\bar b$ events, respectively, with invariant mass $m(b\bar b)\approx M_{Z_2}$, which provide viable signals for detection of the $Z_2$. The interference of the $Z_2$ with $γ,Z_1$ in $e^+e^-\to \bar bb(\bar cc)$ at LEP\,1.5 energies is correlated with $R_b(R_c)$ and may be observable.
We address the mass ranges of new neutral gauge bosons and constraints on the accompanying exotic particles as predicted by a class of superstring models. Under certain assumptions about the supersymmetry breaking parameters we show that breaking of an additional U(1)' symmetry is radiative when the appropriate Yukawa couplings of exotic particles are of order one, analogous to the radiative breaking of the electro-weak symmetry in the supersymmetric standard model due to the large top-quark Yukawa coupling. Such large Yukawa couplings occur for a large class of string models. The Z' and exotic masses are either of O(M_Z), or of a scale intermediate between the string and electro-weak scales. In the former case, M_Z'=O(1 TeV) may be achievable without excessive fine tuning, and is within future experimental reach. (This paper is a summary, with a more phenomenological emphasis and additional discussion, of the results in our earlier article "Implications of Abelian Extended Gauge Structures from String Models", hep-ph/9511378.)
Within a class of superstring vacua which have an additional non-anomalous $U(1)'$ gauge factor, we address the scale of the $U(1)'$ symmetry breaking and constraints on the exotic particle content and their masses. We also show that an extra gauge $U(1)'$ provides a new mechanism for generating a naturally small effective $μ$ term. In general, existing models are not consistent with all phenomenological constraints; however, they do provide a testing ground to address the above issues, yielding a set of concrete scenarios. Under the assumptions that the spontaneous $U(1)'$ breaking takes place in the observable sector and that the supersymmetry breaking scalar mass square terms are positive at the string scale, the breaking of $U(1)'$ symmetry is radiative. It can take place when the appropriate Yukawa couplings of exotic particles are of order one, which occurs for $Z_2\times Z_2$ fermionic orbifold constructions at symmetric points of moduli space. The $Z'$ mass is either of ${\cal O}(M_Z)$, when the symmetry breaking is due to a single standard model singlet, or is of a scale intermediate between the string and electro-weak scales, determined by the radiative corrections (or by competing non-renormalizable operators), when the breaking is due to two or more mirror-like singlets. In the former case, the $M_{Z'}/M_Z$ hierarchy achievable without excessive fine tuning is within future experimental reach.
Precision electroweak data allow one to test the standard model, constrain its parameters, and search for the effects of some kinds of new physics. The results of the most recent data from LEP, SLC, and elsewhere are described, as are their predictions for $m_t$, $M_H$, and $α_s$. The implications for the two major classes of extensions of the standard model, supersymmetry/unification and compositeness, are described.