11 research outputs found
Helicity fractions of W bosons from top quark decays at NNLO in QCD
Decay rates of unpolarized top quarks into longitudinally and transversally
polarized W bosons are calculated to second order in the strong coupling
constant alpha_s. Including the finite bottom quark mass and electroweak
effects, the Standard Model predictions for the W boson helicity fractions are
F_L=0.687(5), F_+=0.0017(1), and F_-=0.311(5).Comment: 5 pages, 1 figur
Semileptonic decays in the limit of a heavy daughter quark
The rate of the semileptonic decay b to c l v is calculated with order
alphas^2 accuracy, as an expansion around the limit of equal masses of the b
and c quarks. Recent results obtained around the limit of the c-quark much
lighter than b are confirmed. Details of the new expansion method are
described.Comment: 7 pages, 5 figure
Radiative-nonrecoil corrections of order alpha^2 (Z alpha) E_F to the hyperfine splitting of muonium
We present results for the corrections of order alpha^2 (Z alpha) E_F to the
hyperfine splitting of muonium. We compute all the contributing Feynman
diagrams in dimensional regularization and a general covariant gauge using a
mixture of analytical and numerical methods. We improve the precision of
previous results.Comment: 5 pages, 3 figures; v2: corrected several typos and replaced figure
3, final results and conclusions unchange
Radiative-nonrecoil corrections of order alpha^2 (Z alpha)^5 to the Lamb shift
We present results for the corrections of order alpha^2 (Z alpha)^5 to the
Lamb shift. We compute all the contributing Feynman diagrams in dimensional
regularization and a general covariant gauge using a mixture of analytical and
numerical methods. We confirm results obtained by other groups and improve
their precision. Values of the 32 master integrals for this and similar
problems are provided.Comment: 8 pages, 4 figures; v2: added two references, changed comparison with
previous result, conclusions unchanged, matches published version; v3:
corrected typo in Table I, final results and conclusions unchange
Relation between bottom-quark MS-bar Yukawa coupling and pole mass
We calculate the O(alpha alpha_s) corrections to the relationships between
the MS-bar Yukawa couplings and the pole masses of the first five quark
flavours in the standard model. We also present the corresponding relationships
between the MS-bar and pole masses, which emerge as by-products of our main
analysis. The occurring self-energies are evaluated using the method of
asymptotic expansion.Comment: 20 pages, 5 figures, to appear in Nucl. Phys.
Threshold production of unstable top
We develop a systematic approach to describe the finite lifetime effects in
the threshold production of top quark-antiquark pairs. It is based on the
nonrelativistic effective field theory with an additional scale rho^(1/2) m_t
characterizing the dynamics of the top-quark decay, which involves a new
expansion parameter rho=1-m_W/m_t. Our method naturally resolves the problem of
spurious divergences in the analysis of the unstable top production. Within
this framework we compute the next-to-leading nonresonant contribution to the
total cross section of the top quark-antiquark threshold production in
electron-positron annihilation through high-order expansion in rho and confirm
the recently obtained result. We extend the analysis to the
next-to-next-to-leading O(alpha_s) nonresonant contribution which is derived in
the leading order in rho. The dominant nonresonant contribution to the
top-antitop threshold production in hadronic collisions is also obtained.Comment: 20 pages, 7 figures; v2: added a section on invariant mass cuts and
one reference, minor changes in Introduction, results unchanged, matches
published versio
Magnetic moment of a bound electron
Theoretical predictions underlying determinations of the fine structure constant alpha and the electron-to-proton mass ratio m_e/m_p are reviewed, with the emphasis on the bound electron magnetic anomaly g-2. The theory of the interaction of hydrogen-like ions with a magnetic field is discussed. The status of efforts aimed at the determination of O(alpha (Z alpha)^5) and O(alpha^2 (Z alpha)^5) corrections to the g factor is presented. The reevaluation of analogous corrections to the Lamb shift and the hyperfine splitting is summarized