Production of heavy vector particles in interactions of leptons with the massless gauge bosons

The cross section for vector leptoquark production in electron–gluon collisions is calculated analytically using the Lagrangian with the minimal couplings between the leptoquarks and the gauge fields of the standard model. It is found that the cross section significantly exceeds the corresponding quantity previously presented in the literature. The cross section of exclusive W boson production in neutrino–photon scattering emerges as a by-product of this Letter. The obtained results can be used for studies at ep colliders.     

Mass and width of unstable gauge bosons

Zeitschrift für Physik C Particles and Fields - It has been proposed that the mass of theZ 0 boson should be defined as the real part, and the width of theZ 0 as the imaginary part of the...     

Electroproduction of alpha particles from various nuclei

Alpha particles from the electrodistintegration of six nuclei with Z between 29 and 73 have been observed. Energy spectra at 50° in the lab for all six nuclei and angular distributions for copper and thulium have been measured. The cross sections exhibit a broad peak whose magnitude decreases with increasing Z; the energy of the peak increases as Z increases. The copper angular distributions are isotropic, but the thulium distributions show strong forward peaking above 16 MeV. This peaking is taken as possible evidence of the existence of α-clusters in this nucleus.     

Massive supersymmetric quantum gauge theory

We continue the study of the supersymmetric vector multiplet in a purely quantum framework. We obtain some new results which make the connection with the standard literature. First we construct the one‐particle physical Hilbert space taking into account the (quantum) gauge structure of the model. Then we impose the condition of positivity for the scalar product only on the physical Hilbert space. Finally we obtain a full supersymmetric coupling which is gauge invariant in the supersymmetric sense in the first order of perturbation theory. By integrating out the Grassmann variables we get an interacting Lagrangian for a massive Yang‐Mills theory related to ordinary gauge theory; however the number of ghost fields is doubled so we do not obtain the same ghost couplings as in the standard model Lagrangian.     

Electroweak precision data and right-handed gauge bosons

The implication of recent electroweak precision data for left-right symmetric models is examined. We establish a lower bound on the charged and neutral right-handed gauge bosons independent of the right-handed neutrino mass and of any restrictions or implied symmetries on the right KM matrix.     

Dynamical gauge bosons from fundamental fermions

In a class of four-dimensional, non-linear fermionic lagrangians, a classical gauge symmetry induces dynamical gauge fields at the quantum level. The construction requires flavour (or families) as well as the “unification” of all gauge couplings at the common Landau pole mass.     

Higgs bosons in supersymmetric models (I)

We describe the properties of Higgs bosons in a class of supersymmetric theories. We consider models in which the low-energy sector contains two weak complex doublets and perhaps one complex gauge-singlet Higgs field. Supersymmetry is assumed to be either softly or spontaneously broken, thereby imposing a number of restrictions on the Higgs boson parameters. We elucidate the Higgs boson masses and present Feynman rules for their couplings to the gauge bosons, fermions and scalars of the theory. We also present Feynman rules for vertices which are related by supersymmetry to the above couplings. Exact analytic expressions are given in two useful limits — one corresponding to the absence of the gauge-singlet Higgs field and the other corresponding to the absence of a supersymmetric Higgs mass term.     

Decoupling of supersymmetric particles

The possibility of a heavy supersymmetric spectrum at the Minimal Supersymmetric Standard Model is considered and the decoupling from the low energy electroweak scale is analyzed in detail. The formal proof of decoupling of supersymmetric particles from low energy physics is stated in terms of the effective action for the particles of the Standard Model that results by integrating out all the sparticles in the limit where their masses are larger than the electroweak scale. The computation of the effective action for the standard electroweak gauge bosons , Z and is performed by integrating out all the squarks, sleptons, charginos and neutralinos to one-loop. The Higgs sector is not considered in this paper. The large sparticle masses limit is also analyzed in detail. Explicit analytical formulae for the two-point functions of the electroweak gauge bosons to be valid in that limit are presented. Finally, the decoupling of sparticles in the S, T and U parameters is studied analitically. A discussion on how the decoupling takes place in terms of both the physical sparticle masses and the non-physical mass parameters as the -parameter and the soft-breaking parameters is included. Received: 27 March 1998 / Published online: 5 October 1998     

Instanton effects in supersymmetric gauge theories

We investigate how in supersymmetric gauge theories non-perturbative effects are able to generate non-trivial vacuum properties otherwise forbidden by perturbative non-renormalization theorems. This conclusion can be reliably drawn since the constancy of certain Green functions — due to supersymmetry (SUSY) — allows one to connect vacuum-dominated large distances with short-distance behaviour which is reliably computed by instanton methods. In all the cases we discuss (without matter, with massive or massless matter in real representations and, finally, with matter in complex representations) instanton calculations imply the occurrence of a variety of condensates. For the pure SUSY gauge theory, a gluino condensate induces the spontaneous breaking of Z_2N. For massive super-quantum chromodynamics (SQCD) we find a peculiar mass dependence of matter condensates whose origin is traced to mass singularities of non-zero mode instanton contributions. These contributions force the massless limit of SQCD to differ from the strictly massless case, in which the spontaneous breaking of chiral symmetries is induced. Inconsistency with an anomaly equation forces either infinite matter condensates or spontaneous SUSY breaking in the massless cases. For non-constant Green functions, instantons are shown to provide new calculable short-distance singularities of an obvious non-perturbative nature.     

Non-perturbative aspects in supersymmetric gauge theories

We review our present understanding of those non-perturbative features of supersymmetric gauge theories that are believed to determine the properties of their ground states (vacua). A wide variety of theories is discussed in detail: pure Yang-Mills, theories with massive or massless real matter fields, theories with chiral matter, both for SU(N) and for the case of a general compact gauge group (as for instance E₈). Depending on some general features of the theory under consideration, various (perhaps) unexpected phenomena are shown to occur. Among these the breakdown of the (perturbatively established) non-renormalization theorem, the occurrence of runaway vacua in certain limits, the spontaneous dynamical breaking of supersymmetry itself in some chiral theories. Throughout the report we restrict ourselves to the confining picture instanton method, occasionally complementing it with the information coming from chiral and supersymmetric Ward-Takahashi identities. We compare our results with the ones suggested earlier by effective Lagrangian methods and, only briefly, with those obtained by other groups in the Higgs picture.     

Vacuum alignment in supersymmetric gauge theories

The vacuum alignment problem is analyzed in the context of supersymmetric gauge theories with dynamical symmetry breaking. Three cases are distinguished, depending on whether the vacuum expectation value of the weak gauge current superfield in vacuum characterized by the orientation Ω is zero for all Ω, for some Ω, or for no Ω. In the first case, a non-renormalization theorem is proved to all orders in the weak coupling, showing that the usual criterion of minimizing the vacuum energy density is insufficient to determine the alignment, and possible resolutions of the problem are discussed. The second case is similar, except that the possible alignments are resricted to the range of Ω giving a vanishing VEV and the weak gauge group may then be broken non-minimally. In the third case, supersymmetry is itself broken at the tree level by the weak interactions. The supersymmetric generalization of the Schwinger mechanism for dynamical mass generation is described.     

Massless gauge bosons other than the photon

Gauge bosons associated with unbroken gauge symmetries, under which all standard model fields are singlets, may interact with ordinary matter via higher-dimensional operators. A complete set of dimension-six operators involving a massless U(1) field, gamma('), and standard model fields is presented. The mu-->egamma(') decay, primordial nucleosynthesis, star cooling, and other phenomena set lower limits on the scale of chirality-flip operators in the 1-15 TeV range if the operators have coefficients given by the corresponding Yukawa couplings. Simple renormalizable models induce gamma(') interactions with leptons or quarks at two loops, and may provide a cold dark matter candidate.