E. A. Kuraev

On the Pomeranchuk Singularity in Asymptotically Free Theories

Abstract We calculate the high-energy amplitude in the spontaneously broken Yang-Mills model within the leading logarithmic approximation, and find out that the vector meson is reggeised whereas the Pomeranchon is a fixed branch point located at the right hand from j = 1. The asymptotic freedom alters the situation in this model significally.

Quest for precision in hadronic cross sections at low energy: Monte Carlo tools vs. experimental data

We present the achievements of the last years of the experimental and theoretical groups working on hadronic cross section measurements at the low-energy e+e− colliders in Beijing, Frascati, Ithaca, Novosibirsk, Stanford and Tsukuba and on τ decays. We sketch the prospects in these fields for the years to come. We emphasise the status and the precision of the Monte Carlo generators used to analyse the hadronic cross section measurements obtained as well with energy scans as with radiative return, to determine luminosities and τ decays. The radiative corrections fully or approximately implemented in the various codes and the contribution of the vacuum polarisation are discussed.

Feynman rules for effective Regge action

Abstract Starting from the gauge invariant effective action in the quasi-multi-Regge kinematics (QMRK), we obtain the effective reggeized gluon (R)–particle (P) vertices of the following types: R P P , R R P , R R P P , R P P P , R R P P P , and R P P P P , where the on -mass-shell particles are gluons, or sets of gluons with small invariant masses. The explicit expressions satisfying the Bose-symmetry and gauge invariance conditions are obtained. As a comment to the Feynman rules for derivation of the amplitudes in terms of effective vertices we present a “vocabulary” for practitioners.

The dominant two-loop electroweak contributions to the anomalous magnetic moment of the muon

Abstract The leading two-loop electroweak contributions to the muon anomalous magnetic moment are calculated in the standard model. The diagrams with logarithmically enhanced contributions are selected. We separately discuss the two-loop diagrams containing heavy vector bosons and photons without virtual fermion loops and with lepton loops. The correction is −42 × 10 −11 and reaches about 22% of the one-loop result. Estimating the uncertainty from non-logarithmic two-loop and higher-order pieces allows to improve the electroweak contribution in the moment to (153 ± 5) × 10 −11 .

Hadronic cross sections in electron-positron annihilation with tagged photon

Events with tagged photons in the process of electron-positron annihilation into hadrons are considered. The initial state radiation is suggested to scan the hadronic cross section with the energy. QED radiative corrections are taken into account. The results for the total and exclusive cross sections are given in an analytic form. Some numerical estimates are presented.

Large angle QED processes at e+e− colliders at energies below 3 GeV

QED processes at electron-positron colliders are considerd. We present differential cross-sections for large--angle Bhabha scattering, annihilation into muons and photons. Radiative corrections in the first order are taken into account exactly. Leading logarithmic contributions are calculated in all orders by means of the structure-function method. An accuracy of the calculation can be estimated about 0.2%.

Small-angle electron-positron scattering with a per mille accuracy☆

Abstract The elastic and inelastic high-energy small-angle electron-positron scattering is considered. All radiative corrections to the cross section with the relative accuracy δσ / σ = 0.1% are explicitly taken into account. According to the generalized eikonal representation for the elastic amplitude, in higher orders only diagrams with one exchanged photon may be considered. Single-photon emission with radiative corrections and next-to-leading two-photon and pair production diagrams are evaluated, together with leading three-loop corrections. All contributions have been calculated analytically. We integrate the calculated distributions over typical for LEP 1 experiments intervals of angles and energies. To the leading approximation, the results are shown to be described in terms of kernels of electron structure functions. Some numerical results are presented.

Structure function method applied to polarized and unpolarized electron-proton scattering: A solution of the GE(p)/GM(p) discrepancy

The cross section for polarized and unpolarized electron-proton scattering is calculated by taking into account radiative corrections in leading and next-to-leading logarithmic approximation. The expression of the cross section is formally similar to the cross section of the Drell-Yan process, where the structure functions of the electron play the role of Drell-Yan probability distributions. The interference of the Born amplitude with the two-photon exchange amplitude (box-type diagrams) is expressed as a contribution to the K factor. It is calculated under the assumption that proton form factors decrease rapidly with the momentum transfer squared and that the momentum is equally shared between the two photons. The calculation of the box amplitude is done when the intermediate state is the proton or the {delta} resonance. The results of numerical estimations show that the present calculation of radiative corrections can bring into agreement the conflicting experimental results on proton electromagnetic form factors and that the two-photon contribution is very small.

Radiative corrections for pion and kaon production at e+e? colliders of energies below 2 GeV

Processes of electron--positron annihilation into charged pions and kaons are considered. Radiative corrections are taken into account exactly in the first order and within the leading logarithmic approximation in higher orders. A combined approach for accounting exact calculations and electron structure functions is used. An accuracy of the calculation can be estimated about 0.2%.

Excess low-energy photon pairs from pion annihilation at the chiral phase transition

Abstract The photon pair production by pion annihilation in a hot and dense medium at the chiral phase transition is investigated within a chiral quark model. As a direct consequence of this transition the σ meson appears as a bound state in the domain of temperatures and chemical potentials where the condition M σ (T,μ)≈2 M π (T,μ) is fulfilled. This effect results in a strong enhancement of the cross section for the pion annihilation process 2π→2γ compared with the vacuum case. The calculation of the photon pair production rate as function of the invariant mass shows a strong enhancement and narrowing of the σ meson resonance at threshold due to chiral symmetry restoration.