arXiv subjects

Kirill Melnikov

Publications and source records attributed to Kirill Melnikov.

188 records · Page 11Linked to original sources

arXiv1997

Lepton pair production by a high energy photon in a strong electromagnetic field

Using impact-factor representation, we consider the lepton pair production by an incident high-energy photon in a strong electromagnetic field of a nucleus. By summing leading terms of perturbation series, we obtain a simple formula for the amplitude, valid to all orders in ${\cal O}(αZ)$ and arbitrary field of the nucleus. Using these results, we derive, in a simple manner, the results for the lepton pair production by a virtual incident photon in a Coulomb field. For real incident photon our results coincide with the known ones. Also, a particular example of a non-Coulomb potential is discussed in some detail.

hep-ph↗

arXiv1997

Non-Abelian Dipole Radiation and the Heavy Quark Expansion

Dipole radiation in QCD is derived to the second order in $α_s$. A power-like evolution of the spin-singlet heavy quark operators is obtained to the same accuracy. In particular, ${\cal O}(α_s^2)$ relation between a short-distance low-scale running heavy quark mass and the $\barMS$ mass is given. We discuss the properties of the effective QCD coupling $\aw(E)$ which governs the dipole radiation. This coupling is advantageous for heavy quark physics.

hep-ph↗

arXiv1997

Complete O(alpha_s^2) Corrections to Zero-Recoil Sum Rules for B-->D* Transitions

We present the complete O(alpha_s^2) corrections to the Wilson coefficient of the unit operator in the zero-recoil sum rule for the B-->D* transition. We include both perturbative and power-suppressed nonperturbative effects in a manner consistent with the operator product expansion. The impact of these corrections on |V_cb| extracted from semileptonic B-->D* decays near zero recoil is discussed. The mixing of the heavy quark kinetic operator with the unit operator at the two loop level is obtained. O(alpha_s) corrections to a number of power-suppressed operators are calculated.

hep-ph↗

arXiv1997

Techniques for computing two-loop QCD corrections to b-->c transitions

We have recently presented the complete O(alpha_s^2) corrections to the semileptonic decay width of the b quark at maximal recoil. Here we discuss various technical aspects of that calculation and further applications of similar methods. In particular, we describe an expansion which facilitates the phase space integrations and the treatment of the mixed real-virtual corrections, for which Taylor expansion does not work and the so-called eikonal expansion must be employed. Several terms of the expansion are given for the O(alpha_s^2) QCD corrections to the differential semileptonic decay width of the b --quark at maximal recoil. We also demonstrate how the light quark loop corrections to the top quark decay rate can be obtained using the same methods. We briefly discuss the application of these techniques to the calculation of the O(alpha_s^2) correction to zero recoil sum rules for heavy flavor transitions.

hep-ph↗

arXiv1997

Two-loop QCD Corrections to $b \to c$ Transitions at Zero Recoil: Analytical Results

We present analytical results for the $O(α_s ^2)$ contributions to the functions $η_A$ and $η_V$ which parameterize QCD corrections to semileptonic $b \to c$ transitions at zero recoil. Previously obtained approximate results are confirmed. The methods of computing the relevant two-loop diagrams with two mass scales are discussed in some detail.

hep-ph↗

arXiv1997

Two-loop QCD corrections to semileptonic b decays at maximal recoil

We present a complete $O(α_s^2)$ correction to the differential width of the inclusive semileptonic decay $b\to clν_l$ at the kinematical point of vanishing invariant mass of the leptons, $q^2=0$. Together with the recently computed $O(α_s^2)$ correction at the upper boundary of the lepton invariant mass spectrum, this new information permits an estimate of the $O(α_s^2)$ effect in the total inclusive semileptonic decay width $b\to clν_l$. We argue that the non-BLM part of the $O(α_s^2)$ correction gives at most 1% correction to the inclusive semileptonic decay width $b\to clν_l$. This significantly improves the credibility of extracting $|V_{cb}|$ from the inclusive semileptonic decays of the b-hadrons.

hep-ph↗

arXiv1996

An estimate of the QCD corrections to the decay Z --> W u d

We present an estimate of perturbative QCD corrections to the decay $Z \to Wu\bar d$. A simple approximate approach is described in detail. The difference of masses of $M_Z$ and $M_W$ is used as an expansion parameter. A complete analytical formula for a part of the corrections is also presented.

hep-ph↗

arXiv1994

Threshold Effects in Two--Photon Decays of Higgs Particles

The couplings of Higgs particles to two photons are analyzed in the threshold region, where the Higgs mass is about twice the loop--particle mass leading to possible bound state formation. The amplitude of the pseudoscalar decay $A\to γγ$ for pseudoscalar masses in the range of $(t\bar t)$--bound state masses is calculated in leading and next--to--leading order in $α_s$. It is shown that the real and imaginary parts of the form factor are significantly different from the lowest order perturbative one.

hep-ph↗