4,224 research outputs found
Revisiting Bremsstrahlung emission associated with Light Dark Matter annihilations
We compute the single bremsstrahlung emission associated with the pair
annihilation of spin-0 particles into electrons and positrons, via the
t-channel exchange of a heavy fermion. We compare our result with the work of
Beacom et al. . Unlike what is stated in the literature, we show that the
Bremsstrahlung cross section is not necessarily given by the tree-level
annihilation cross section (for a generalized kinematics) times a factor
related to the emission of a soft photon. Such a factorization appears only in
the soft photon limit or in the approximation where the masses of the particles
in the initial and final states are negligible with respect to the mass of the
internal particle. However, in the latter case, we do not recover the same
factor as for e^+ e- --> mu^+ mu^- gamma. Numerically the difference, in the
hard photon limit, is as large as a factor 3.6. However the effect on the upper
limit of the dark matter mass is not significant. Using gamma ray observations,
we obtain an upper limit on the dark matter mass of 30 or 100 MeV (depending on
the region that is considered for the 511 keV analysis) while Beacom et al.
found 20 MeV
Polarization and spin correlations of top quarks at a future e+ e- linear collider
We discuss the polarization and spin correlations of top quarks produced
above threshold at a future linear collider, including QCD radiative
corrections.Comment: 11 pages (LaTeX), 16 Postscript figure. Contribution to the
International Workshop on "Symmetry and Spin", PRAHA-SPIN99, Prague, Czech
Republic, September 5 -12, 199
The spin density matrix of top quark pairs produced in electron-positron annihilation including QCD radiative corrections
We calculate the spin density matrix of top quark pairs for the reaction
to order . As an application we show
next-to-leading order results for a variety of spin observables for the
system. These include the top quark and antiquark polarizations and
spin-spin correlations as a function of the center-of-mass energy
and of the top quark scattering angle for arbitrary longitudinal polarization
of the electron/positron beam.Comment: 26 pages, 13 Postscript figures, uses REVTE
The top-quark's running mass
We discuss the direct determination of the running top-quark mass from
measurements of the total cross section of hadronic top-quark pair-production.
The theory predictions in the MSbar scheme are very stable under scale
variations and show rapid apparent convergence of the perturbative expansion.
These features are explained by studying the underlying parton dynamics.Comment: 6 pages, 2 figures; to appear in Proceedings of the 9th International
Symposium on Radiative Corrections, RADCOR 2009, Ascona, Switzerland, October
200
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