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Andrzej Czarnecki

Publications and source records attributed to Andrzej Czarnecki.

At least 127 records · Page 7Linked to original sources
arXiv1997

Two-loop QCD corrections to the heavy quark pair production cross section in e+e- annihilation near the threshold

We present the O(alpha_s) corrections to the cross section for the reaction e+e- --> gamma^* --> Q \bar Q in the energy region close to the threshold. We assume that the energy of the reaction is such that both the perturbative expansion in the strong coupling constant and expansion in the relative velocity of the heavy quarks can be used. We explicitly obtain terms O(alpha_s^2/beta^2, alpha_s^2/beta, alpha_s^2) in the relative correction to the threshold cross section. Using the ideas of asymptotic expansions, we demonstrate how an expansion of Feynman diagrams in the threshold region is constructed. From this analysis we obtain a matching relation between the vector current in full QCD and the quark-antiquark current in NRQCD at leading order in 1/m and the second order in the strong coupling constant.

hep-ph↗
arXiv1997

Nonfactorizable QCD and Electroweak Corrections to the Hadronic Z Boson Decay Rate

We present an analysis of two-loop mixed QCD and electroweak corrections to the decay of the Z boson into light quarks. We find that the naive factorization of QCD and electroweak corrections does not describe correctly the two-loop effects. The nonfactorizable corrections shift the width of the Z boson by approximately -0.55(3) MeV and increase the central value of the strong coupling constant determined at LEP by 0.001.

hep-ph↗
arXiv1997

Using muons to probe for new physics

Searches for new physics using muons are reviewed. Particular attention is paid to muon number non-conserving processes, like the decay muon-->e+gamma and muon-electron conversion in muonic atoms. Also, experimental determinations and theoretical predictions for the muon anomalous magnetic moment are reviewed.

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

Neutron Electric Dipole Moment In The Standard Model: Valence Quark Contributions

We present a complete three-loop calculation of the electric dipole moment of the $u$ and $d$ quarks in the Standard Model. For the $d$ quark, more relevant for the experimentally important neutron electric dipole moment, we find cancellations which lead to an order of magnitude suppression compared with previous estimates.

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↗
arXiv1997

Threshold behavior of Feynman diagrams: the master two-loop propagator

An asymptotic expansion of the two-loop two-point ``master'' diagram with two masses $m$ and $M$, on the mass shell $Q^2=M^2$, is presented. The treatment of the non-analytical terms arising in the expansion around the branching point is discussed. Some details of the calculation of a new class of two-loop integrals are given.

hep-ph↗
arXiv1996

Computing Techniques for Two-Loop Corrections to Anomalous Magnetic Moments of Leptons

Two-loop electroweak corrections to the electron, muon, and tau lepton anomalous magnetic moments have been computed recently. Effects of hadronic contributions to the photon propagator in two-loop QED corrections have also been reanalysed. The common technique used in both calculations was asymptotic expansion of Feynman diagrams in the ratio of lepton mass to some heavy mass scale. In this talk we present some details of this method paying particular attention to the expansion of multi-scale diagrams.

hep-ph↗
arXiv1996

On the Dipole Moments of Fermions at Two Loops

Complete two-loop electroweak corrections to leptonic anomalous magnetic moments have been calculated recently using asymptotic expansions of Feynman diagrams. Techniques developed in that context can also be applied to the electric dipole moments (EDM) of fermions. In the Standard Model the EDM was proved to vanish at two loops. In this talk we discuss a simplification of that proof.

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↗
arXiv1996

Electroweak corrections to the muon anomalous magnetic moment

The two-loop electroweak radiative corrections to the muon's anomalous magnetic moment, $a_μ\equiv (g_μ-2)/2$, are presented. We obtain an overall 22.6\% reduction in the electroweak contribution $a_μ^{\rm EW}$ from $195\times 10^{-11}$ to $151(4)\times 10^{-11}$. Implications for the full standard model prediction and an upcoming high precision measurement of $a_μ$ are briefly discussed. Some aspects of the calculations are discussed in detail.

hep-ph↗
arXiv1996

Electroweak corrections to the muon anomalous magnetic moment

The bosonic two-loop electroweak radiative corrections to the muon's anomalous magnetic moment, $a_μ\equiv (g_μ-2)/2$, are presented. We find $Δa_μ^{\rm EW}({\rm 2\,loop\, bosonic})/ a_μ^{\rm EW}({\rm 1\,loop})\approx {α\over π}\left(-3.6 \ln\left({M_W^2\over m_μ^2}\right) +0.10 \right)\approx -0.11$ for $M_{\rm Higgs}\approx 250$ GeV. Combining that result with our previous two-loop fermionic calculation, we obtain an overall 22.6\% reduction in $a_μ^{\rm EW}$ from $195\times 10^{-11}$ to $151(4)\times 10^{-11}$. Implications for the full standard model prediction and an upcoming high precision measurement of $a_μ$ are briefly discussed. We also give the two-loop electroweak corrections to the anomalous magnetic moments of electron and tau lepton; they result in a reduction of the one-loop estimates by 35\% and 15\%, respectively.

hep-ph↗
arXiv1996

Two-loop QCD corrections to $b\to c$ transitions at zero recoil

Complete two-loop QCD corrections to $b\to c$ transitions are presented in the limit of zero recoil. Vector and axial-vector coefficients $η_{A,V}$ are calculated analytically in the limit of equal beauty and charm masses, and a series appoximation is obtained for the general mass case. $η_A$ is crucial for the determination of the absolute value of the Cabibbo-Kobayashi-Maskawa matrix element $V_{cb}$. The two-loop effects enhance the one-loop corrections by 21\%, removing a major theoretical uncertainty in the value of $|V_{cb}|$. Including two-loop QCD effects and previously neglected electroweak and Coulomb corrections we find $|V_{cb}| = 0.0379 \pm 0.0020({\rm stat}) \pm 0.0025({\rm syst}) \pm 0.0011({\rm theory})$.

hep-ph↗
arXiv1995

Electroweak Radiative Corrections To Polarized Møller Scattering Asymmetries

One loop electroweak radiative corrections to left-right parity violating Møller scattering ($e^-e^-\to e^-e^-$) asymmetries are presented. They reduce the standard model (tree level) prediction by 40$\pm 3$ \% where the main shift and uncertainty stem from hadronic vacuum polarization loops. A similar reduction also occurs for the electron-electron atomic parity violating interaction. That effect can be attributed to an increase of $\sin^2θ_W(q^2)$ by $3\%$ in running from $q^2=m_Z^2$ to 0. The sensitivity of the asymmetry to ``new physics'' is also discussed.

hep-ph↗