On the running electromagnetic coupling constant at $M_Z$Körner, J.G.;Pivovarov, A.A.;Schilcher, K.
doi: 10.1007/s100530050498pmid: N/A
Abstract. We present a discussion on how to define the running electromagnetic coupling constant at \(M_Z\) or some other intermediate scale, e.g., \(m_\Upsilon\). We argue that a natural definition consistent with general requirements of the renormalization group should use Euclidean values of the momentum of the photon propagator as the appropriate scale. We demonstrate explicitly, through the evaluation of the running coupling constant at the scale of the \(\Upsilon\)-resonance mass, that the usual definition of the hadronic contribution with a principal-value prescription is inconsistent. In the determination of the value of \(\alpha\) at \(M_Z\), we use a Euclidean definition rather than the principal-value one, and as a result, the numerical difference is comparable in size to the errors caused by existing experimental and QCD inputs to the evaluation of \(\alpha(M_Z)\).
CP violation in two-body hadronic decays of $B_c$ mesonsDai, Youshan;Du, Dongsheng
doi: 10.1007/s100530050499pmid: N/A
Abstract. Using the next-to-leading-order low-energy effective Hamiltonian, the CP asymmetries for \(B_ c\)-meson decays into meson pairs are calculated in the spectator approximation. We do not compute the hadronic matrix elements directly; instead, we use the amplitude ratios to estimate the CP asymmetries. This is quite different from previous treatments in the literature. The values of the momentum squared carried by the virtual particles in timelike penguin diagrams are also carefully discussed. From our calculated results, the best decay modes to observe CP violation in \(B_ c\) decays would be \(B_ c^-\rightarrow{\bar D}^{*0}K^{*-}\), \({\bar D}^{0}K^{*-}\), \({\bar D}^{*0} K^{-}\), \({\bar D}^{0}K^{-}\), and \(B_ c^-\rightarrow{\eta}_ cD^{-}\), each of them needs about \(10^8\) of \(B_ c^{\pm}\) events in experiment.
CP violation in decays Z $\rightarrow$ 4 jetsNachtmann, O.;Schwanenberger, C.
doi: 10.1007/s100530050501pmid: N/A
Abstract. We analyse CP-violating effects in Z \(\rightarrow\) 4 jet decays, assuming the presence of CP-violating effective \(Z b \bar{b} G\)$ and \(Z b \bar{b} G G\) couplings. We discuss the influence of these couplings on the decay width. Furthermore, we propose various strategies of a direct search for such CP-violating couplings by using different CP-odd observables. The present data of LEP 1 should give significant information on the couplings.
Investigations of quark fragmentation universalityEdén, P.
doi: 10.1007/s100529900026pmid: N/A
Abstract. We propose event cuts in deep inelastic scattering, suitable for an examination of quark fragmentation universality. We compare the current hemisphere of the Breit frame with the hemispheres in \(\mathrm{e^+e^-}\) annihilation events, using Monte Carlo simulations. The agreement between the two processes is improved after the suggested event cuts are introduced. A method to study the scale evolution in the quark hemispheres using data from fixed-energy \(\mathrm{e^+e^-}\) experiments is presented. This makes it possible to use the high statistics from LEP1 also at scales below the \({\mrm Z}^0\) mass. We also discuss observables which are sensitive to the dynamics of regions closer to the remnant. The observables probe the relatively clean region on the current side of the hardest emission in the event, and can be used to distinguish between different assumptions about remnant effects and mechanisms for the parton evolution.
On the instanton-induced portion of the nucleon strangenessKlabučar, D.;Kumerički, K.;Melić, B.;Picek, I.
doi: 10.1007/s100529900056pmid: N/A
Abstract. We calculate the instanton contribution to the proton strangeness in an MIT bag enriched by the presence of a dilute instanton liquid. The evaluation is based on the expression of the nucleon matrix elements of bilinear strange-quark operators in terms of a model valence nucleon state and the interactions producing quark–antiquark fluctuations on top of that valence state. Our method combines use of the the evolution operator containing a strangeness source and the Feynman–Hellmann theorem. It enables one to evaluate the strangeness in different Lorentz channels in essentially the same way. Only the scalar channel is found to be affected by the interaction induced by the random instanton liquid.
Soft and hard Pomeron in the structure function of the proton at low x and low $Q^2$D'Alesio, U.;Metz, A.;Pirner, H.J.
doi: 10.1007/s100530050508pmid: N/A
Abstract. We study inclusive electroproduction on the proton at low x and low \(Q^2\) using a soft and a hard Pomeron. The contribution of the soft Pomeron is based on the Stochastic Vacuum Model, in which a nonperturbative dipole-dipole cross section can be calculated by means of a gauge invariant gluon field strength correlator. To model the hard Pomeron exchange we phenomenologically extend the leading order evolution of a power-behaved structure function, \(F_2 \propto x^{- \lambda}\), proposed by López and Ynduráin. This extension allows to consider both the case \(Q^2 = 0\) and the region of higher \(Q^2\) on the basis of the same parametrization. A good simultaneous fit to the data on \(F_2\) and on the cross section \(\sigma_{\gamma p}\) of real photoproduction is obtained for \(\lambda=0.37\). With four parameters we achieve a \(\chi^2/\textrm{d.o.f.} = 0.98\) for 222 data points. In addition, we use our model of the inclusive \(\gamma^{\ast} p\) interaction to compute the longitudinal structure function \(F_L\).
Baryon chiral perturbation theory in manifestly Lorentz invariant formBecher, T.;Leutwyler, H.
doi: 10.1007/PL00021673pmid: N/A
Abstract. We show that in the presence of massive particles, such as nucleons, the standard low-energy expansion, in powers of meson momenta and light-quark masses, in general converges in only part of the low-energy region. The expansion of the scalar form factor \(\sigma(t)\), for instance, breaks down in the vicinity of \(t=4M_\pi^2\). In the language of heavy baryon chiral perturbation theory, the proper behaviour in the threshold region results only if the multiple internal-line insertions generated by relativistic kinematics are summed up to all orders. We propose a method that yields a coherent representation throughout the low-energy region, while keeping Lorentz and chiral invariance explicit at all stages. The method is illustrated with a calculation of the nucleon mass and of the scalar form factor to order \(p^4\).