Probing MeV dark matter at low-energy e+e- colliders

It has been suggested that the pair annihilation of dark matter particles chi with mass between 0.5 and 20 MeV into e+e- pairs could be responsible for the excess flux (detected by the INTEGRAL satellite) of 511 keV photons coming from the central region of our Galaxy. The simplest way to achieve the required cross section while respecting existing constraints is to introduce a new vector boson U with mass M(U) below a few hundred MeV. We point out that over most of the allowed parameter space, the process e+e--->U(gamma), followed by the decay of U into either an e+e- pair or an invisible (nu(-)nu or chi(-)chi) channel, should lead to signals detectable by current B-factory experiments. A smaller, but still substantial, region of parameter space can also be probed at the Phi factory DAPhiNE.     

Comment on magnetic e+e- resonances in a central field

Within a simple semiclassical model it is shown that the presence of an external Coulomb field induces only slight changes in the energy of a resonant electron-positron state unless for extremely small distances, while the width increases strongly with the field strength.     

Sensitive search for neutral resonances in Bhabha scattering around 1.8 MeV/c2

The excitation function for Bhabha scattering was measured in the energy range between 760 and 840 keV (CM) in 1.4 keV steps with beams of monochromatic positrons scattered on a Be foil at angles between 79° and 101° (CM). The statistical accuracy was 0.5%. Up to an upper limit of 1.0 b eV/sr (CM) for the energy-integrated differential cross section (97% CL) no resonant scattering was observed. This limit corresponds to a maximal resonance contribution of 0.7% relatively to the Bhabha cross section, consistent with the upper bound, derived from precision measurements of the electron (g-2) factor.     

Heavy quarks in e+e- annihilation

There are many speculations that there exist quarks Q considerably heavier than the charmed quark. Their Q$\text{Q}$ states will display a far richer spectrum of monochromatic photon and hadron transitions than charmonium. The most important features of this spectrum — in particular, its dependence on the mass of Q — are outlined.     

Cltzster Effect in e+ e- Annihilation

Based on the picture of hadron cluster, some parameters about cluster are obtained in e⁺ e^- annihilation and it is found that the intermittent behavior in e⁺ e^- annihilation is closely related with short-range correlation from hadron cluster.