Neutral Current Phenomenology Research Articles

Contact interactions in low scale string models with intersectingD6-branes

We evaluate the tree level four fermion string amplitudes in the TeV string mass scale models with intersecting $D6$-branes. The coefficient functions of contact interactions subsuming the contributions of string Regge resonance and winding mode excitations are obtained by subtracting out the contributions from the string massless and massive momentum modes. Numerical applications are developed for the standard-model-like solution of Cremades, Ibanez, and Marchesano for a toroidal orientifold with four intersecting $D6$-brane stacks. The chirality conserving contact interactions of the quarks and leptons are considered in applications to high-energy collider and flavor changing neutral current phenomenology. The two main free parameters consist of the string and compactification mass scales, ${m}_{s}$ and ${M}_{c}$. Useful constraints on these parameters are derived from predictions for the Bhabha scattering differential cross section and for the observables associated to the mass shifts of the neutral meson systems $K\ensuremath{-}\overline{K}$, $B\ensuremath{-}\overline{B}$, $D\ensuremath{-}\overline{D}$ and the lepton number violating three-body leptonic decays of the charged leptons $\ensuremath{\mu}$ and $\ensuremath{\tau}$.

Lorentz spinors in elastic WW scattering and neutral currents

We propose a novel electroweak unification scheme that can be interpreted in terms of compositeness and an effective Lagrangian and show that it leads to a good high-energy behaviour of the reactions WW→WW, νν→WW and ee→WW. The weak vector bosons are no more gauge fields. The description of the reactions WLWL→WLWL and WTWL→WLWL becomes simpler than in the non-Abelian case: no seagull term is necessary and no scalar boson must be exchanged to get a good high-energy behaviour. We obtain a low-energy neutral current phenomenology that is identical with the usual one. The LEP II process ee→WW is analysed in some detail. The relative sign of the individual amplitudes can be determined. We find clear Lorentz spinor signals. They would be indicative of the existence of a Z′-boson as well as a heavy neutrino (N). The upper unitarity mass boundmN<890 GeV is derived.

Electroweak interactions with gauged baryon and lepton numbers.

The possibility that gauge symmetries associated with baryon and lepton numbers are spontaneously broken symmetries of nature is entertained. In this endeavor, the gauge group of electroweak interactions is SU(2)\ifmmode\times\else\texttimes\fi{}U${(1)}^{3}$. Consistent with neutral-current phenomenology, the spectrum of massive neutral gauge bosons consist of the ${Z}^{0}$ of the standard electroweak model and two additional neutral bosons with mass lower bounds of 120 and 210 GeV which makes these particles prospective candidates for production in the energy regimes of the CERN LEP, Fermilab Tevatron, and the Superconducting Super Collider.

Could composite interactions be detected at √ s = MZ?

In models in which both light fermions and technifermions are composite objects with common constituents, the couplings of the massive electroweak gauge bosons to light quarks and leptons can differ from their standard model counterparts. We consider the effects of the modified gauge boson couplings on low energy neutral current phenomenology and on the properties of the Z boson. We show that precise measurements at LEP and SLC of the total width of the Z, ratio of leptonic to hadronic partial widths, and left-right and forward-backward asymmetries should probe compositeness scales of 1–2.5 TeV.

The standard model and beyond

In these lectures, my aim is to present a status report on the standard model and some key tests of electroweak unification. Within that context, I also discuss how and where hints of new physics may emerge. To accomplish those goals, I have organized my presentation as follows. I survey the standard model parameters with particular emphasis on the gauge coupling constants and vector boson masses. Examples of new physics appendages are also commented on. In addition, I have included an appendix on dimensional regularization and a simple example which employs that technique. I focus on weak charged current phenomenology. Precision tests of the standard model are described and up-to-date values for the Cabibbo-Kobayashi-Maskawa (CKM) mixing matrix parameters are presented. Constraints implied by those tests for a 4th generation, extra Z' bosons, and compositeness are discussed. An overview of the physics of tau decays is also included. I discuss weak neutral current phenomenology and the extraction of sin/sup 2//theta/W from experiment. The results presented there are based on a global analysis of all existing data. I have chosen to concentrate that discussion on radiative corrections, the effect of a heavy top quark mass, implications for grand unified theories (GUTS), extramore » Z' gauge bosons, and atomic parity violation. The potential for further experimental progress is also commented on. Finally, I depart from the narrowest version of the standard model and discuss effects of neutrino masses, mixings, and electromagnetic moments. 32 refs., 3 figs., 5 tabs« less

Searching for a ( B− L) gauge boson in p [formula omitted] collisions

We explore the possibility that an extra neutral U(1) gauge boson of mass ∼ 250 GeV is responsible for two double jet-lepton-neutrino events which have been seen at CERN. A minimal extension of the standard model which gauges ( B− L) can lead to roughly one-third the observed number of events without violating low-energy neutral-current phenomenology.

The Standard Model and Beyonda

In these lectures, my aim is to present a status report on the standard model and some key tests of electroweak unification. Within that context, I also discuss how and where hints of new physics may emerge. To accomplish those goals, I have organized my presentation as follows. I survey the standard model parameters with particular emphasis on the gauge coupling constants and vector boson masses. Examples of new physics appendages are also commented on. In addition, I have included an appendix on dimensional regularization and a simple example which employs that technique. I focus on weak charged current phenomenology. Precision tests of the standard model are described and up-to-date values for the Cabibbo-Kobayashi-Maskawa (CKM) mixing matrix parameters are presented. Constraints implied by those tests for a 4th generation, extra Z' bosons, and compositeness are discussed. An overview of the physics of tau decays is also included. I discuss weak neutral current phenomenology and the extraction of sin/sup 2//theta/W from experiment. The results presented there are based on a global analysis of all existing data. I have chosen to concentrate that discussion on radiative corrections, the effect of a heavy top quark mass, implications for grand unified theories (GUTS), extramore » Z' gauge bosons, and atomic parity violation. The potential for further experimental progress is also commented on. Finally, I depart from the narrowest version of the standard model and discuss effects of neutrino masses, mixings, and electromagnetic moments. 32 refs., 3 figs., 5 tabs« less

Phenomenology of weak isoscalar vector bosons

Motivated by the hypothesis of compositeness of leptons, quarks and vector bosons the effects of a weak isoscalar vector boson Y coupled to the weak hypercharge current, and, alternatively, the effects of an isoscalar vector boson Y L coupled to the left-handed part of the weak hypercharge current only, are investigated within the framework of an effective electroweak lagrangian theory incorporating W(Y) dominance. We discuss experimental implications for the neutral current phenomenology at presently available as well as at LEP and SLC energies. From the experimental data in neutrino scattering we deduce that the Y(Y L ) can hardly have a mass below 250 GeV. An enhanced forward-backward asymmetry A FBin e +e − → μ + μ − is expected at PETRA and TRISTAN energies. In fact, the one standard deviation effect seen at PETRA in A FB (e +e − → μ + μ −) can naturally be explained through the existence of such an isoscalar boson.

Addendum to "Prospects for a second neutral vector boson at low mass in SO(10)"

An SO(10) model proposed previously is modified so as to allow for a legitimate fermion mass spectrum consistent with a low-mass ${Z}_{2}$. This drastically changes the neutral-current phenomenology of the original theory. The modified model reproduces more of the low-energy phenomenology of the standard model. All axial-vector-type neutral-current couplings at ${q}^{2}=0$ are identical to those in the standard model. The coupling of ${Z}_{2}$ to $u\overline{u}$ is enhanced, making it easier to detect the ${Z}_{2}$ in $\mathrm{pp}$ and $\overline{p}p$ colliders.

Testing the standard model by precise determinations ofW±andZmasses

We analyze the standard model's predictions for ${m}_{W}$, ${m}_{Z}$, and their interdependence. Simple formulas are given. Criteria for testing the theory at the tree and loop levels via precise mass measurements are proposed. Effects due to large Higgs-boson and top-quark masses as well as additional generations are illustrated. Constraints from neutral-current phenomenology are discussed. Present experimental values of ${m}_{W}$ and ${m}_{Z}$ are compared with theoretical expectations.

Probing residual family-unification symmetry through neutral currents

Lower bounds on the masses of neutral intermediate vector bosons within models augmenting known strong electromagnetic-weak interaction symmetries with an SU(2) symmetry unifying electron and muon or electron and mirror-electron families are derived from present constraints on rare processes. One such model, fully consistent with Glashow-Weinberg-Salam neutral current phenomenology, permits the occurrence of a neutral intermediate vector boson with a mass between 90 and 100 GeV accompanied by a lighter vectorlike partner whose mass is only minimally constrained by e +e −→μ +μ − asymmetry measurements. Alternatively, if the lighter neutral boson within the model has a mass of 89 GeV, then a heavier neutral intermediate vector boson may have a mass as low as 95 GeV, depending on the value for the gauge coupling of the “interfamilial” gauge symmetry.

A grand unification of flavour with testable predictions

We present a grand unification of flavour based on the group O (14) = SO(14) ⊗ K'. The novel feature of this model is that the discrete symmetry K' is not the internal parity. This implies a fermion assignment different from the usual O(14) with important consequences on the low energy phenomenology. One new fermion family with V - A weak interactions and four new families with V + A character are predicted, with masses less than 250 GeV. The cosmological bound on light neutrino species forces us to keep an SU(2) family group acting on the V + A families unbroken until low energy. Depending on the hierarchy pattern, either we recover the standard model predictions or a new scenario emerges with substantial departures in the neutral current phenomenology.

Neutral current phenomenology of supersymmetric SU(2) × U(1) × ≈U(1) models

The neutral current sector of a class of supersymmetric SU(2) × U(1) × ≈ U(1) models is parametrized. Bounds on the neutral boson masses are obtained from the low energy data, and the implications of future experimental findings for these models are discussed.

Implications of lepton-flavor mixing for neutral-current phenomenology

Neutral-current data are reanalyzed incorporating arbitrary mixings among an arbitrary number of lepton generations. Within experimental uncertainties, the determination of electron neutral-current couplings is unaltered. For inelastic ..nu../sub ..mu../ (nu-bar/sub ..mu../) scattering to give a value of sin/sup 2/theta consistent with this, ..mu.. flavor must remain isolated from all others.

Low-energy phenomenology and neutral-weak-boson masses in theSU(2)L×SU(2)R×U(1)B−Lelectroweak model

Within the framework of the $\mathrm{SU}{(2)}_{L}\ifmmode\times\else\texttimes\fi{}\mathrm{SU}{(2)}_{R}\ifmmode\times\else\texttimes\fi{}\mathrm{U}{(1)}_{B\ensuremath{-}L}$ electroweak group, a model-independent analysis of the neutral-current phenomenology at low energy is given both for Dirac and Majorana neutrinos. It is found that if the Higgs structure is not fixed, solutions substantially different from the standard model are still allowed by the present neutral-current data. It is shown how the upper limit on the backward-forward asymmetry in $e\overline{e}\ensuremath{\rightarrow}\ensuremath{\mu}\overline{\ensuremath{\mu}}$ constrains not only the neutral-weak-boson masses but also the mass ratio of the two Majorana neutrinos predicted by the "minimal" left-right-symmetric model.

Low-energy neutral current phenomenology and grand unified theories

We derive necessary and sufficient conditions to be satisfied by any expanded electroweak gauge model in order to reproduce the standard model low-energy neutral current predictions. These conditions imply several constraints on the neutral gauge boson masses and quantum number assignments for the ordinary fermions. Using these conditions, we prove that the popular grand unified theories based on the gauge groups SO(10) and E 6 can only accomodate trivial extensions of the standard model. As a consequences, if any of these grand unified models works at some energy scale, present low-energy neutral current phenomenology implies that the Z-boson must be produced with the expected mass and couplings to the ordinary fermions. Any additional neutral gauge boson (with the possible exception of very heavy ones) could only be produced in hadronic collisions and it would not decay in e +e −.

Effects of the spin-1 partner of the goldstino (gravitino) on neutral current phenomenology

Spontaneously broken supersymmetrc theories involve, in addition to the standard neutral gauge boson, a second one which is, partly, the superpartner of the massless spin- 3 2 goldstino (or massive spin- 3 2 gravitino). The mass and the coupling strength of the new boson are not fixed but both might be very small. In the simplest example, the new boson is axially coupled to leptons and quarks, and neutrino scattering experiments restrict its mass to be less than 300 MeV/ c 2, approximately.

Problem of the isoscalar axial-vector current in neutral-current phenomenology

The matrix element of the isoscalar axial-vector current between nucleon states is not simply related to the corresponding coupling constant appropriate for analyzing inclusive reactions. The consequences for the theoretical analysis of elastic neutrino-proton scattering and of parity violation in atomic hydrogen and deuterium are discussed.

The price of natural parity conservation in the neutral current sector

We study the requirement that the neutral current sector in SU(2) L ⊗ SU(2) R ⊗ U(1) gauge models be naturally parity conserving in the tree approximation, and therefore calculable. A class of models which reproduces automatically the Weinberg-Salam neutrino neutral current phenomenology cannot be made natural in this sense and at the same time compatible with charged current phenomenology. At best one has small but uncalculable violations of both Weinberg-Salam neutrino phenomenology and parity conservation in neutral currents, as well as proliferation of fermion and scalar fields and possible difficulties with neutral strangeness changing transitions. A second class of models has calculable p parity violation in neutral currents and allows a more restricted fermion and scalar sector, but the Weinberg-Salam neutrino phenomenology does not emerge naturally. For the latter class we evaluate the parity violation induced by radiative corrections, relevant to atomic physics experiments. While technically calculable, it is not practically so, because of their dependence on unknown parameters of the Higgs sector. We also discuss radiative corrections to Cabibbo universality in these models.

Neutral current and diffractive production of vector mesons in neutrino scattering

We examine the consequences of the neutral-current phenomenology by supplementing the Glashow, Iliopoulos, and Maiani model with generalized vector-meson dominance and Regge-pole phenomenology. We feel that the diffractive production of the vector mesons in exclusive as well as inclusive reactions with high-energy neutrino beams would test the experimental validity of this model.