269 research outputs found
Two-Loop QCD Corrections to the Heavy Quark Form Factors: Axial Vector Contributions
We consider the Z Q Qbar vertex to second order in the QCD coupling for an
on-shell massive quark-antiquark pair and for arbitrary momentum transfer of
the Z boson. We present closed analytic expressions for the two
parity-violating form factors of that vertex at the two-loop level in QCD,
excluding the contributions from triangle diagrams. These form factors are
expressed in terms of 1-dimensional harmonic polylogarithms of maximum weight
4.Comment: 57 pages, 5 figures. All the results in Section 6 of the paper are
available in an electronic form in the file formulas.in
Two-Loop QCD Corrections to the Heavy Quark Form Factors: Anomaly Contributions
We present closed analytic expressions for the order triangle
diagram contributions to the matrix elements of the singlet and non-singlet
axial vector currents between the vacuum and a quark-antiquark state. We have
calculated these vertex functions for arbitrary momentum transfer and for four
different sets of internal and external quark masses. We show that both the
singlet and non-singlet vertex functions satisfy the correct chiral Ward
identities. Using the exact expressions for the finite axial vector form
factors, we check the quality and the convergence of expansions at production
threshold and for asymptotic energies.Comment: 24 pages, 6 figure
Two-Loop N_F =1 QED Bhabha Scattering: Soft Emission and Numerical Evaluation of the Differential Cross-section
Recently, we evaluated the virtual cross-section for Bhabha scattering in
pure QED, up to corrections of order alpha^4 (N_F =1). This calculation is
valid for arbitrary values of the squared center of mass energy s and momentum
transfer t; the electron and positron mass m was considered a finite, non
vanishing quantity. In the present work, we supplement the previous calculation
by considering the contribution of the soft photon emission diagrams to the
differential cross-section, up to and including terms of order alpha^4 (N_F=1).
Adding the contribution of the real corrections to the renormalized virtual
ones, we obtain an UV and IR finite differential cross-section; we evaluate
this quantity numerically for a significant set of values of the squared center
of mass energy s.Comment: 24 pages, 15 figures. Formulas in Appendix B corrected, changes in
Section 3, references adde
Master Integrals for the 2-loop QCD virtual corrections to the Forward-Backward Asymmetry
We present the Master Integrals needed for the calculation of the two-loop
QCD corrections to the forward-backward asymmetry of a quark-antiquark pair
produced in electron-positron annihilation events. The abelian diagrams
entering in the evaluation of the vector form factors were calculated in a
previous paper. We consider here the non-abelian diagrams and the diagrams
entering in the computation of the axial form factors, for arbitrary space-like
momentum transfer Q^2 and finite heavy quark mass m. Both the UV and IR
divergences are regularized in the continuous D-dimensional scheme. The Master
Integrals are Laurent-expanded around D=4 and evaluated by the differential
equation method; the coefficients of the expansions are expressed as
1-dimensional harmonic polylogarithms of maximum weight 4.Comment: 38 pages, 6 figures, typos corrected, version accepted by Nucl. Phys.
Two-Loop Heavy-Flavor Contribution to Bhabha Scattering
We evaluate the two-loop QED corrections to the Bhabha scattering cross
section which involve the vacuum polarization by heavy fermions of arbitrary
mass m_f >> m_e. The results are valid for generic values of the Mandelstam
invariants s,t,u >> m_e^2.Comment: 13 pages, 6 figures. Equations in the appendix generalized to the
heavy-quark cas
Heavy-Quark Form Factors and Threshold Cross Section at O(\alpha_S^2)
During the last year, analytic expressions for the two-loop QCD corrections
to the form factors for the vector, axial-vector, scalar and pseudo-scalar
vertices involving a pair of heavy quarks, , were calculated. The
results are valid for arbitrary momentum transfer and mass of the heavy quarks.
These form factors have a number of applications, including anomalous
couplings, the cross section, and the
forward-backward asymmetry of heavy quarks. Here the threshold
cross section is presented with some new second order axial vector
contributions.Comment: 5 pages. Presented at the International Europhysics Conference on
High Energy Physics (HEP2005), July 21-27 2005, Lisboa, Portuga
Two-Loop N_F=1 QED Bhabha Scattering Differential Cross Section
We calculate the two-loop virtual, UV renormalized corrections at order
\alpha^4 (N_F=1) in QED to the Bhabha scattering differential cross section,
for arbitrary values of the squared c.m. energy s and momentum transfer t, and
on-shell electrons and positrons of finite mass m. The calculation is carried
out within the dimensional regularization scheme; the remaining IR divergences
appear as polar singularities in (D-4). The result is presented in terms of 1-
and 2-dimensional harmonic polylogarithms, of maximum weight 3.Comment: 61 pages, 4 figures. Overall sign mistakes in some formulas in
appendix corrected, references adde
QED vertex form factors at two loops
We present the closed analytic expression of the form factors of the two-loop
QED vertex amplitude for on-shell electrons of finite mass and arbitrary
momentum transfer . The calculation is carried out within the
continuous -dimensional regularization scheme, with a single continuous
parameter , the dimension of the space-time, which regularizes at the same
time UltraViolet (UV) and InfraRed (IR) divergences. The results are expressed
in terms of 1-dimensional harmonic polylogarithms of maximum weight 4.Comment: 53 pages, 3 figure
Decays of Scalar and Pseudoscalar Higgs Bosons into Fermions: Two-loop QCD Corrections to the Higgs-Quark-Antiquark Amplitude
As a first step in the aim of arriving at a differential description of
neutral Higgs boson decays into heavy quarks, , to second
order in the QCD coupling , we have computed the
amplitude at the two-loop level in QCD for a general neutral Higgs boson which
has both scalar and pseudoscalar couplings to quarks. This amplitude is given
in terms of a scalar and a pseudoscalar vertex form factor, for which we
present closed analytic expressions in terms of one-dimensional harmonic
polylogarithms of maximum weight 4. The results hold for arbitrary
four-momentum squared, , of the Higgs boson and of the heavy quark mass,
. Moreover we derive the approximate expressions of these form factors near
threshold and in the asymptotic regime .Comment: 56 pages, 2 figure
Next-to-next-to-leading soft-gluon corrections for the top quark cross section and transverse momentum distribution
I present results for top quark production in hadronic collisions at LHC and
Tevatron energies. The soft-gluon corrections to the differential cross section
are resummed at next-to-next-to-leading-logarithm (NNLL) accuracy via the
two-loop soft anomalous dimension matrices. Approximate
next-to-next-to-leading-order (NNLO) differential and total cross sections are
calculated. Detailed theoretical predictions are shown for the t tbar cross
section and the top quark p_T distribution at the Tevatron and the LHC.Comment: 23 pages, 14 figures; additional results and figure
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