Single-pion production and the structure of the weak neutral current

We investigate the restrictions on the structure of the weak neutral current imposed by single pion production cross sections on single nucleons. A general vector (V), axial-vector (A) neutral current with |ΔI|?1 is assumed, where the isovector V,A neutral currents are the neutral members of the isotriplets containing the charged weak currents. From neutrino cross sections alone we derive bounds for the neutral current coupling constants. These bounds supplement the known constraints from inclusive scattering in a very useful way. More specific assumptions about the isoscalar neutral current are also considered. We discuss the resulting bounds using the existing neutrino data. Finally, it is shown that with the advent of antineutrino data for single pion production the neutral current coupling constants will be determined uniquely.     

Elastic neutrino proton scattering and the structure of the neutral current

Optimal constraints on the structure of a general V, A hadronic neutral current are derived from neutrino proton scattering and compared with corresponding results from inclusive neutrino scattering and single pion production by neutrinos. For an arbitrary axial component of the neutral current, restrictions for the vector coupling constants are obtained. It is shown that the most general neutral current which can be related to charged weak and electromagnetic currents accounts for all existing data on neutrino hadron scattering. The neutral current coupling constants are determined for the pure isovector model, the Salam-Weinberg model and the bottom-quark model. All three models lead to practically the same isovector couplings but they differ in the strength of the isoscalar current.     

On the structure of the hadronic neutral current

The coupling constants of the hadronic neutral current are determined independently of any gauge models. Essentially two solutions emerge from this analysis: either the isovector neutral current is predominantly vector and the isoscalar component predominantly axial-vector or vice versa.     

Gauge theories and neutral currents

We consider neutral current effects in some gauge theoretic models of weak and electromagnetic interactions. Asymptotic invariance under the gauge group is proposed and used for estimating deep-inelastic cross sections. Our analysis underlines the need for a systematic search for hadronic threshold effects in the coupling of neutral currents.     

Measurements of cross sections and forward-backward asymmetries at the Z resonance and determination of electroweak parameters

Abstract. We report on measurements of hadronic and leptonic cross sections and leptonic forward-backward asymmetries performed with the L3 detector in the years 1993–95. A total luminosity of was collected at centre-of-mass energies and which corresponds to 2.5 million hadronic and 245 thousand leptonic events selected. These data lead to a significantly improved determination of Z parameters. From the total cross sections, combined with our measurements in 1990–92, we obtain the final results: An invisible width of is derived which in the Standard Model yields for the number of light neutrino species . Adding our results on the leptonic forward-backward asymmetries and the tau polarisation, the effective vector and axial-vector coupling constants of the neutral weak current to charged leptons are determined to be and . Including our measurements of the forward-backward and quark charge asymmetries a value for the effective electroweak mixing angle of is derived. All these measurements are in good agreement with the Standard Model of electroweak interactions. Using all our measurements of electroweak observables an upper limit on the mass of the Standard Model Higgs boson of GeV is set at 95% confidence level. Received: 4 February 2000 / Published online: 18 May 2000     

Model Independent QED Corrections to the Process ep → eX

We give an exhaustive presentation of the semi-analytical approach to the model independent leptonic QED corrections to deep inelastic neutral current lepton-nucleon scattering. These corrections include photonic bremsstrahlung from and vertex corrections to the lepton current of the order φ(α) with soft photon exponentiation. A common treatment of these radiative corrections in several variables — leptonic, hadronic, mixed, Jaquet-Blondel variables — has been developed and double differential cross sections are calculated. In all sets of variables we use some structure functions, which depend on the hadronic variables and which do not have to be defined in the quark parton model. The remaining numerical integrations are twofold (for leptonic variables) or onefold (for all other variables). For the case of hadronic variables, all phase space integrals have been performed analytically. Numerical results are presented for a large kinematical range, covering fixed target as well as collider experiments at HERA or LEP⊗LHC, with a special emphasis on HERA physics.     

Neutral current anomalies: Difference between dynamical scaling violations and slow rescaling

We discuss the neutral current phenomena in order to discriminate between dynamical scaling violations and heavy quark productions. both of which could explain the charged current anomalies. In the Weinberg-Salam-GIM (W-S) model, “neutral current anomalies” should appear in addition to the charged current anomalies which result from dynamical scaling violations. The “neutral current anomalies” will be observed through the final hadronic invariant mass distributions, especially in $\text{v}$ neutral current processes. The predictions of the W-S model are compared with those of heavy quark models by investigating the changes of the ratios of neutral-to charged-current cross sections for $\text{v}\text{and}\text{v}$ as functions of the $\text{v}\text{/v}$ charged current cross section ratio.     

A variant of the Stückelberg formalism for massive gauge fields and applications

A variant of the Stückelberg formalism for massive neutral and non- Abelian gauge fields studied in detail. Applications to hadron electrodynamcs. are discussed. The major new results is that if the isovector, hadronic, electroma magnetic current is proportional to the neutral ? meson field operator, the finite c-number Schwinger term in the current commutators can be eliminated altogether by introducing an unphysical, negative metric, scalar particle without violating unitarity.     

SU(3) symmetry for the vector meson-nucleon interaction

When comparing data on VBB coupling constants with SU(3) one usually assumes Sakurai's idea that the ?-meson couples universally to the isovector current. In order to use this assumption one has to continue the coupling from the vector meson pole to t=0. The implications for the coupling constants depend upon how this continuation is carried out. In this paper we present arguments to expect simple continuation properties for VNN form factors defined in analogy with the Sachs electromagnetic form factors, rather than for the vector and tensor form factors. We show that in this case experimental data are consistent with SU(3), Sakurai's universality idea and with Zweig's rule. Furthermore we find $\text{F}\text{D}$ ratios close to the SU(6) prediction.     

Simplest photonuclear reactions accompanied by the excitation of isovector giant dipole and quadrupole resonances: Semimicroscopic description

A semimicroscopic approach based on the continuum version of the random-phase approximation (CRPA) and on a semiphenomenological inclusion of the fragmentation effect is applied to describing cross sections for photoabsorption and direct plus semidirect and inverse reactions accompanied by the excitation of isovector giant dipole and quadrupole resonances. In addition to the spinless part of the Landau-Migdal interaction and a partly self-consistent phenomenological mean field of the nucleus, that version of the approach which is used here takes into account isovector separable velocity-dependent forces, as well as the effect of the fragmentation shift of the giant-resonance energy. The results obtained by calculating various features of the aforementioned cross sections for a number of magic and semimagic medium-mass nuclei are compared with respective experimental data.     

Issues in large-angle scattering

We analyze phenomenologically several models for the high-energy scattering of hadrons through fixed, large angles. The emphasis is on trying to isolate and understand those aspects of hadronic forces which are important at large angles. We review the fixed-angle-lower bounds derived from analyticity and discuss how simple geometrical concepts can be used to guide our extrapolation of cross sections away from the forward and backward peaks into the large-angle region. This extrapolation is important in understanding whether or not we need a new, hard component of the hadronic force to interpret the data. We try to isolate the important features of dual models, statistical models, and constituent models and to clarify the possibility of experimental distinction between these approaches.     

Nuclear spin dependent atomic parity violation,nuclear anapole moments and the hadronic axial neutral current

Left-right asymmetries in atomic transitions, depending upon the nuclear spin, could be a source of information on the neutral hadronic axial current. We show that the relevant electroweak parameter can be extracted from experiment by measuring hyperfine component ratios which do not involve the knowledge of the atomic wave function. In the standard electroweak model, the parity violating electron-nucleus interaction associated with the hadronic axial neutral current is accidently suppressed and, as a consequence, dominated by the electron interaction with the nuclear anapole moment, which describes the effect of the parity violating nuclear forces on the nucleus electromagnetic current. One of our objectives was to identify the various physical mechanisms which determine the size of the nuclear anapole moments. As an important step, we have established a simple relation between the anapole moment and the nuclear spin magnetization. From this relation it follows that the computation of the anapole moment can be reduced to that of one-body operators. The basic tool is a unitary transformationW which eliminates the one-body parity violating potential from the nuclear hamiltonian. It generalizes, to more realistic situations, a procedure used by F.C. Michel in the case of constant nuclear density. The fact that the transformationW does not commute with the residual spin-dependent nucleon-nucleon interaction can be accounted for — within some approximation — by a renormalization of the effective coupling constants which appear in the one-body reduction of the two-body parity violating nucleon-nucleon interaction induced by meson exchange. A particular attention was paid to nuclear correlation effects. They are treated semi-empirically in the independent pair approximation. The nuclear anapole moments of⁸⁵Rb,¹³³Cs, and²⁰⁹Bi have been evaluated for three sets of parity violating meson-nucleon coupling constants, taking into account configuration mixing effects in a semi-empirical way. We suggest a possible strategy to disentangle the axial neutral current from the anapole moment contribution. It requires experiments, accurate to few tenths of a percent, performed on several heavy nuclei. The results should be collected in a two-dimensional plot involving a suitably chosen set of variables (X, Y). In an ideal situation — small theoretical uncertainties —the points corresponding to various nuclei should fall on a straight line which crosses the lineX=0 at a point the ordinate of which is the sought for axial coupling constant.     

New approach to axial coupling constants in the QCD sum rule

We derive new QCD sum rules for the axial coupling constants by considering the correlation functions of axial-vector currents in a one-nucleon state. The QCD sum rules tell us that the axial coupling constants are expressed by nucleon matrix elements of quark-gluon composite operators which are related to the sigma terms and the moments of parton distributions. The results for the isovector axial coupling constants and the eighth component of the SU(3)(f) octet are in good agreement with experiment.     

Hadron production from a boiling quark soup: A thermodynamical quark model predicting particle ratios in hadronic collisions

We present a thermodynamical quark model which can predict cross sections for particle production in hadronic interactions at high energies. In this model a hadronic collision gives rise to a soup of quarks and antiquarks at some temperature kT ? 170 MeV. Results for inclusive meson production cross sections look promising in comparison with experiments. We give a formula for the inclusive cross section.     

Massive μ pair production in a vector field theory model

We treat massive electrodynamics as a model for the production of massive μ pairs in high-energy hadronic collisions. The dominant diagrams in perturbation theory are identified and analyzed. These graphs have an eikonal structure which leads to enormous cancellations in the two-particle inclusive cross section but not in the n-particle production cross sections. Under the assumption that these cancellations are complete, a Drell-Yan structure appears in the inclusive cross section but the particles accompanying the μ pairs have a very different structure compared to the parton model. The pionization region is no longer empty of particles as in simple parton models.     

Analysis of the neutral-current interaction in the inclusive neutrino reactions

We attempt a general phenomenological analysis of the neutral weak current in the inclusive neutrino reactions using the parton model as a tool. From the recently reported data on these processes we determine the strengthH of the neutral-current interaction as well as the amount of theVA interference. We find (H/G)²=0·54±0·06 whereG is the Fermi coupling constant and theVA interference contribution turns out to be 33±23%. We also discuss the comparison of the data with various models for the neutral hadronic current. An erratum to this article is available at .     

Sum rules in perturbation theory

Sum rules derived from isospin current algebra are examined in perturbation theory for nucleons interacting with neutral pseudoscalar mesons and isovector pions. It is shown that the well-known sum rules for electromagnetic isovector form factors of nucleons and pions are correct in zero and second order of the meson-nucleon coupling constant.     

Analysis of electromagnetic nucleon form factors

Electromagnetic nucleon form factors have been determined from Rosenbluth plots and, independently, by fitting a dispersion ansatz to electron-nucleon scattering cross sections, allowing for a renormalization of the data in both cases. The ?-exchange contribution was taken from a Frazer-Fulco-type analysis based on new πN. Pole terms with adjustable parameters were used for the other parts of the spectral functions. Only the Dirac isoscalar and the Pauli isovector spectral function show a pronounced dipole- like bump-dip structure. The bumps belong to ω- and ?-exchange and the dips presumably to ø- and ω′(1250)-exchange in the first case and to ?′(1250)-exchange in the second case. The results for vector meson-nucleon coupling constants are compared with predictions from SU(3). Values for the nucleon radii are given which are only weakly model dependent.     

Total photoproduction cross section at very high energy

In this paper we apply to the photoproduction total cross section a model we have proposed for purely hadronic processes and which is based on QCD mini-jets and soft gluon re-summation. We compare the predictions of our model with the HERA data as well as with other models. For cosmic rays, our model predicts substantially higher cross sections at TeV energies than models based on factorization, but lower than models based on mini-jets alone, without soft gluons. We discuss the origin of this difference.     

Rigorous sum rules and bounds on pion-pion scattering amplitudes with positivity

We derive the complete set of crossing symmetric forward pion-pion amplitudes with non-negative absorptive parts obeying twice-subtracted dispersion relations. We give a method to eliminate subtraction constants for these amplitudes by making a simultaneous use of subtracted dispersion relations for the amplitude and its inverse, and unitarity. We thus deduce from axiomatic field theory (i) lower bounds on forward real parts in terms of physical forward amplitudes, (ii) lower bounds on S- and P-wave scattering lengths in terms of forward amplitudes in any part of the physical region, (iii) sum rules on scattering lengths in terms of physical forward amplitudes which provide a complete test of forward dispersion relations, (iv) upper bounds on scattering lengths in terms of total cross sections and (v) absolute sum rule inequalities on total and forward differential cross sections in terms of the pion mass alone which provide new tests of dispersion relations not involving scattering lengths.