Neutral Current Couplings Research Articles

A determination of quark weak couplings at petra energies

Data on hadron production by e + e − annihilation at c.m. energies between 12 and 36.6 GeV have been collected using the JADE detector. They have been analysed in terms of single-photon and weak neutral-current exchange assuming production of quark-antiquark pairs with only d, u, s, c and b quarks to produce values for the quark weak neutral-current couplings. A further analysis in terms of the Glashow-Salam-Weinberg theory produced the result, sin 2 θ W = 0.22 ± 0.08. The theory has therefore been tested in a new energy domain and within the context of the neutral weak couplings of the first, second and third generation quarks.

Analysis of parity-violating asymmetries in elastic electron-nucleus scattering

Parity violating asymmetries in elastic electron scattering on nuclei with arbitrary spin and isospin are considered in detail, extending our previous work on this matter [1]. Examples are given for the target nuclei12C,40Ca,208Pb,3H,3He,41Ca,209Bi. Several ways of extracting information on the structure of weak neutral current couplings are presented. Besides, as a further possibility to check the validity of the standard gauge model [2] at intermediate energies, the chances to distinguish among different classes of gauge models are discussed.

Unique solution for the weak neutral current coupling constants in purely leptonic interactions

By combining results from the MARK-J at PETRA on Bhabha scattering, μ +μ - and τ +τ - production with recent world data from neutrino-electron scattering experiments, we determine unique values for the leptonic weak neutral current coupling constants g V and g A in the framework of electroweak models containing a single Z 0. In contrast to previous analyses, we only use data from purely leptonic interactions, and therefore avoid the inherent uncertainties resulting from the use of hadronic targets. From the MARK-J data alone in the context of the standard SU(2) ⊗ U (1) model of Glashow, Weinberg and Salam, we find sin 2 θ W=0.24±0.11.

Flavor-changing effective neutral-current couplings in the Weinberg-Salam model

We compute exactly the effective $\mathrm{Zd}\overline{s}$ coupling in the Weinberg-Salam model in the limit of vanishing external momenta. The result is of importance if the effect of the $t$ quark, which has presumably a rather large mass, is to be taken into account.

Why are the isoscalar neutral current axial-vector couplings and isoscalar nucleon anomalous moments small?

By using an argument based on the realization of chiral current algebras, we explain why the isoscalar neutral current axial-vector coupling and the isoscalar nucleon anomalous moments are small. We give an argument which predicts, ( G A N(0))p/( G A N(0)) n = ( H A N(0)) p/( H A N(0)) n = k p/ k n ≈ 1.79/−1.91, where k p and k n are the anomalous moments of the proton and neutron, respectively. ( G A N(0)) p and ( H A N(0)) p are the weak neutral current axial-vector coupling and the induced pseudoscalar coupling for the proton, respectively.

(Anti)-neutrino–deuteron scattering and the structure of the weak hadronic neutral currents

Neutrino(antineutrino)–deuteron elastic and neutral current breakup processes are studied to provide information about the structure of the weak hadronic neutral currents. The scattering cross sections for these processes have been calculated with the help of the impulse approximation. Thus, we have additional input for the determination of the weak hadronic neutral current couplings. Some of the popular models can be put to test as they give distinguishable predictions for these processes. Also since the deuterium filled bubble chambers will be exposed to neutrino beams, our results may serve as a guide as to what to expect theoretically.

A new relation among neutrino neutral current couplings and a constraint on the electron weak neutral current couplings

A new relation among the neutrino neutral current couplings to quarks and leptons is discussed. The relation permits a more direct test of the fermionic assignments in the Weinberg-Salam SU(2) L × U(1) model and SU(2) L × SU(2) R × U(1) models. This relation together with a relation derived by Bernabeu and Jarlskog allows a unique determination of neutrino electron weak neutral current couplings.

Neutral current couplings of higher generations

We show that longitudinal polarization of the outgoing lepton in e +e −→μ +(τ +)μ −(τ −) is expected at the energies of a vector meson resonance (Ψ, ϒ, T, …). On the basis of the standard SU(2) × U(1) theory, this parity violating observable is already appreciable (∼3%) for the ϒ, and it becomes ∼9% for a T at 30 GeV.

Polarised electron-hadron scattering and weak interaction theory

It is shown that the naive valence quark parton model assumptions used to calculate the parity violation in polarisede-d scattering are not needed in order to extract informations from the experimental data about the neutral current couplings. Much more general assumptions which can be justified within the QCD approach are sufficient in order to arrive at essentially the same conclusions for the neutral current coupling parameters

Natural relations among neutrino neutral current couplings in gauge theories

We show that two relations among neutrino neutral current couplings derived by Bernabeu and Jarlskog for SU(2) × U(1) gauge theories remain valid for arbitrary gauge groups of the type G 1L × G 2R, where G 1L is a group of rank m ⩾ 2 and is either a simple group or a direct product of simple groups provided at least m − 1 diagonal generators satisfy the relations 2 T u jL + T d jL + T e jL = 0 and T u jL + 2 T d jL + T v jL = 0. These conditions are sufficient to make the BJ relation natural, independent of the mechanism of symmetry breaking. Implications of these conditions for theories of flavor dynamics are discussed.

Weak neutral-current couplings of muons

Inversion of both the charge and the helicity of muon beams is considered as a possibility to determine the weak neutral-current couplings of muons, in particular the right-handed weak charge I R 3( μ) and sin 2θ without using the parton model.

Determination of the Electronic Neutral-Current Couplings from e-d and e-p Scattering in Connection with the Effects of Sea Components

We make analyses of the parity-violating asymmetries of electron-deuteron and electron­ proton deep inelastic scatterings including sea quark contribution am! adopting three different kinds of parton distribution functions. Both results from electron-deuteron and from electron­ proton scatterings coincide with the prediction of the Weinberg-Salam model. Our results are found to be insensitive to the difference among the three kinds of parton distribution functions. \Ve also study the x-, y- and sin21lw-dependences of AIQ' in the Weinbcrg-Salam model. Experiments at higher y values will be useful for more precise determination of the Weinberg angle llw. § I. Introduction For these years enormous progress has been made in the determination of the weak neutral-current couplings of quarks and of leptons. In addition to the ex­ periments on neutrino-induced neutral-current reactions, measurements of the parity violating asymmetries A./Q' in the inelastic scattering of longitudinally polarized electrons from deuterium targetll allow a more discriminating study of neutral­ current couplings. So far many analyses of neutral-current data have been pre­ sented, and are divided into two classes. One is to test the validity of the models of unified gauge theories of weak and electromagnetic interactions. The other is to determine, in a phenomenological and model-independent way, the space-time and isospin structures of the hadronic neutral-currents and the space-time structure of the leptonic ones. Model-independent analyses of neutrino-nucleon reactions have been made by Abbott-Barnett,') Hung-Sakurai, 3l Langacker-Sidhu,'J Claudson­ Paschos-Sulak5) and others. They made a unique or almost unique determination of the weak neutral-current couplings of u and d quarks. Furthermore, Konuma and the present author 6J and Abbott-Barnettn have determined the couplings of the electronic weak neutral-current, using the above mentioned solutions of u and d quarks, experimental value of 1/ Q' in electron-deuteron (c-d) scattering and elas­ tic neutrino-electron data. Our result has indicated that there exists a umque solution of the electronic weak coupling constants which is consistent with the

Neutral-current quark coupling determination using neutrino-neutron and neutrino-proton deep-inelastic cross sections

There is some question about the reliability of inclusive-pion-production analyses as used in previous determinations of the weak neutral-current couplings of u and d quarks. We are able to eliminate this input altogether by using new neutrino and antineutrino data for the ratio of neutral-current neutron-to-proton deep-inelastic cross sections, sigma (..nu..n ..-->.. ..nu..X)/sigma (..nu..p ..-->.. ..nu..X). Another new input to our model-independent analysis is the Q/sup 2/ dependence of elastic neutrino-proton scattering. The final values determined for the neutral-current couplings are consistent with those we obtained previously. For purposes of comparison, we also present a new analysis of high-energy inclusive-pion data.

Model-independent analysis of the weak interaction in e − -N elastic scattering experiments

We discuss what model-independent information about the parity non-conserving electron-quark neutral current interaction can be obtained in the experiments on elastic scattering of longitudinally polarized electrons on unpolarized nucleons. Two possible forms of the neutron electric form factor G E γn ( q 2) are used in our analysis. The interpretation of the P-odd asymmetry, measured in these experiments, in terms of electron-quark neutral current coupling constants depends on the form chosen for G E γn ( q 2). We note also that the polarized e −-p elastic scattering experiments might be used in principle to study the q 2 dependence of G E γn ( q 2).

Hierarchy of symmetry breakings and neutral currents in anSU(6) grand unification model

In anSU(6) grand unification model with eight quarks and eight leptons belonging to 15-plet and singlet representations, the symmetry is spontaneously broken by the sequenceSU(6)→SU(3) c ×SU(2)×U(1)×U(1)→SU(3) c ×U(1). Fror two cases of symmetry breakings the effective weak neutral current coupling constants are compared with experiment. For theSU(3) c ×SU(2)×U(1)×U(1)×→SU(3) c ×U(1) symmetry breaking, the coupling constants reproduce the Weinberg-Salam model with a small correction term. Agreement with the experimental mean values is improved with the correction term. Parity violation in atomic physics is also discussed.

Forward-backward asymmetry ine−+e+→q+q¯→π,K,D,…,+X

The forward-backward asymmetry ${A}_{M}$ due to weak-electromagnetic interference effects in the inclusive process ${e}^{\ensuremath{-}}+{e}^{+}\ensuremath{\rightarrow}M+X$, where $M$ is a pseudoscalar meson, is calculated in terms of the axial-vector weak neutral-current couplings ${b}_{q}$ of the quarks composing $M$, their electric charges ${Q}_{q}$, and the ratio $D(x)$ of their ($q\ensuremath{\rightarrow}M$) fragmentation probabilities. Thus, if ${b}_{q}$ and ${Q}_{q}$ are considered known, (i) a measurement of ${A}_{M}$ effectively determines $D(x)$, and (ii) for mesons composed of a light quark and a heavy quark a theoretical argument indicates that ${A}_{M}$ is independent of $D(x)$ so that the result of a measurement of ${A}_{M}$ is immediately predictable.

Parity violation in helium and cesium and the effect of electron-electron interaction

We investigate the parity violating effect in helium and cesium atoms due to the exchange of a Z 0-meson between atomic electrons. It is found that this effect is negligible for both atoms in comparison with the contribution from the electron-nucleus interactions, if we take the neutral-current coupling between electrons as predicted by the SU (2) × U (1) theory and sin 2 θ W ≈ 0.3.

Neutral currents in left-right symmetric gauge models

We analyse all the neutral-current phenomena following from the general class of gauge models based on the group SU(2) L ⊗ SU(2) R ⊗ U(1). It is found that the neutral-current couplings in these models bear a remarkable similarity to those in the standard Weinberg-Salam gauge model. The parameter which plays the role of sin2ϑw is found to lie between 0 and 1/2. Comparison with experimental data shows that even a model with the ratio of the masses of the twoZ bosons as small as 1.9 is not ruled out.

Higher-order induced axial-vector isoscalar neutral current in gauge theories

We calculate the strength of the axial-vector isosinglet neutral hadronic current coupling to neutrinos and electrons in the one-loop approximation in a class of gauge theories that forbid such couplings at the tree level. These theories are based on the gauge group $\mathrm{SU}{(2)}_{L}\ifmmode\times\else\texttimes\fi{}\mathrm{U}(1)$ and $\mathrm{SU}{(2)}_{L}\ifmmode\times\else\texttimes\fi{}\mathrm{SU}{(2)}_{R}\ifmmode\times\else\texttimes\fi{}\mathrm{U}(1)$, respectively. The coupling strength comes out the same in both classes of models and is small compared to the corresponding coupling strength for isovector hadronic neutral current which is present at the tree level. We compare our results with the gluon-induced heavy-quarks contribution and find that the latter dominate, except for the case in which hadronic axial-vector currents are coupled to the electron vector current so that strong-interaction effects are suppressed by $1\ensuremath{-}4{{sin}^{2}\ensuremath{\theta}}_{W}$ and our results are dominant. That case may provide a clear way of testing gauge theories of weak and electromagnetic interactions.

Neutral current couplings and SU(2) ⊗ U(1) gauge models

Combining neutral current (NC) data in a statistically correct way we determine: (i) the weak NC couplings in neutrino-nucleon and -electron scattering independently of gauge models; (ii) the weak isospin T 3 of the right-handed fermions u R, d R and e R , the Z mass, and sin 2 θ W in SU(2) ⊗ U(1) models with standard left-handed fermion multiplet assignment two Higgs doublets. Both determinations support the Weinberg-Salam model, although neither considerable right-handed fermion singlet-doublet mixing involving new heavy fermions can be excluded, nor the existence of a second Higgs doublet.