1,066 research outputs found
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.Comment: 22 pages, revte
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
Dipole Excitation of Dipositronium
The energy interval between the ground and the P-wave excited states of the
recently discovered positronium molecule Ps_2 is evaluated, including the
relativistic and the leading logarithmic radiative corrections, E_P-E_S = 0.181
586 7(8) a.u. The P-state, decaying usually via annihilation, is found to decay
into the ground state by an electric dipole transition 19 percent of the time.
Anticipated observation of this transition will provide insight into this
exotic system.Comment: 5 page
QCD corrections to the t-->H+b decay within the minimal supersymmetric standard model
I present the contribution of gluinos and scalar quarks to the decay rate of
the top quark into a charged Higgs boson and a bottom quark within the minimal
supersymmetric standard model, including the mixing of the scalar partners of
the left- and right-handed top quark. I show that for certain values of the
supersymmetric parameters the standard QCD loop corrections to this decay mode
are diminished or enhanced by several 10 per cent. I show that not only a small
value of 3 GeV for the gluino mass (small mass window) but also much larger
values of several hundreds of GeV's have a non-neglible effect on this decay
rate, against general belief. Last but not least, if the ratio of the vacuum
expectation values of the Higgs bosons are taken in the limit of I
obtain a drastic enhancement due to a \ dependence in the couplings.Comment: UQAM-PHE-94/01, 6 pages, plain tex, 4 figures not included, available
under request via mail or fa
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
alpha^2 corrections to parapositronium decay: a detailed description
We present details of our recent calculation of alpha^2 corrections to the
parapositronium decay into two photons. These corrections are rather small and
our final result for the parapositronium lifetime agrees well with the most
recent measurement. Implications for orthopositronium decays are briefly
discussed.Comment: 18 pages, late
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.Comment: 9 pages, revte
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