arXiv subjects
Publications and source records attributed to O. Nachtmann.
Observables with optimised sensitivity to the $WWZ$ and $WWγ$-couplings in $e^+ e^- \to W^+ W^-$ are investigated. They allow for separate studies of CP violation and of absorptive parts in the amplitude. We have calculated expected statistical errors on extracted couplings at $\sqrt{s} = 500 GeV$, with unpolarised or longitudinally polarised beams.
We discuss the influence of anomalous tbar-t-G couplings on total and differential tbar-t production cross sections in hadron-hadron collisions. We study in detail the effects of a chromoelectric and a chromomagnetic dipole moment, d' and μ', of the top quark. In the d'-μ' plane, we find a whole region where the anomalous couplings give a zero net contribution to the total top production rate. In differential cross sections, the anomalous moments have to be quite sizable to give measurable effects. We estimate the values of d' and μ' which are allowed by the present Tevatron experimental results on top production. A chromoelectric dipole moment of the top violates CP invariance. We discuss a simple CP-odd observable which allows for a direct search for CP violation in top production.
We re-derive and discuss two low energy theorems which relate the potential of a static quark-antiquark pair with the total energy and action stored in the flux tube between the sources. In lattice QCD these relations are known as Michael's sum-rules, but we give an essential correction to one of them. Then we relate the low energy theorems to the virial theorem for a heavy quark-antiquark bound state. Finally we compare the results for the flux tube formation, which have been calculated in the model of the stochastic vacuum, with the low energy theorems and obtain consistency. From this we conclude that the model describes the non perturbative gluon dynamics of QCD at a renormalization scale, where the strong coupling constant is given by $\al_s = 0.57$.
We discuss the effect of CP-violating $Zb\bar{b}$, $Zb\bar{b}G$ and $Zb\bar{b}γ$ couplings on the width $Γ(Z\rightarrow b\bar bX)$. The presence of such couplings leads in a natural way to an increase of this width relative to the prediction of the standard model. Various strategies of a direct search for such CP-violating couplings by using CP-odd observables are outlined. The number of $Z$ bosons required to obtain significant information on the couplings in this way is well within the reach of present LEP experiments.
We discuss the production of soft photons in high energy hadron-hadron collisions. We present a model where quarks and antiquarks in the hadrons emit ``synchrotron light'' when being deflected by the chromomagnetic fields of the QCD vacuum, which we assume to have a nonperturbative structure. This gives a source of prompt soft photons with frequencies $ω<= 300 MeV$ in the c.m. system of the collision in addition to hadronic bremsstrahlung. In comparing the frequency spectrum and rate of ``synchrotron'' photons to experimental results we find some supporting evidence for their existence. We make an exclusive--inclusive connection argument to deduce from the ``synchrotron'' effect a behaviour of the neutron electric formfactor $G_E^n(Q^2)$ proportional to $(Q^2)^{1/6}$ for $Q^2 < 20 fm^{-2}$. We find this to be consistent with available data. In our view, soft photon production in high energy hadron-hadron and lepton-hadron collisions as well as the behaviour of electromagnetic hadron formfactors for low $Q^2$ are thus sensitive probes of the nonperturbative structure of the QCD vacuum.
We present a study of parity (P) violating polarization rotations of atoms in external electric fields. Five different types of rotations are identified and the consequences of time reversal invariance (T) are discussed. The role played by Zeldovich's electric dipole moment of unstable states is elucidated. To calculate the effects, we use the standard model of elementary particle physics where P violation in atoms is due to the exchange of the Z boson between the quarks in the nucleus and the atomic electrons. We consider in detail hydrogen-like systems in $n=2$ states, especially $^1_1$H, $^4_2$He$^+$, and $^{12}_6$C$^{5+}$. We discuss ways to obtain in some cases large enhancement factors for the P-violating polarization rotations and show that e.g. for $^1_1$H one could in principle observe rotations as large as a few percent, where for a statistically significant result one would need a total of $10^{15}$ polarized atoms. We point out that some of our P-violating polarization rotations are very sensitive to the nuclear spin-dependent part of the P-violating Hamiltonian which receives a contribution from the polarized strange quark density in polarized nuclei.