U-boson production in e+e- annihilations, psi and Upsilon decays, and Light Dark Matter

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Abstract
We recall how a new light gauge boson emerged in Supersymmetric extensions of the Standard Model with an extra singlet superfield, and how it could behave very much as a light pseudoscalar, with the corresponding symmetry broken at a scale higher than electroweak. Such a new gauge boson U, light and very weakly coupled, allows for Light Dark Matter particles, which could be at the origin of the 511 keV line from the galactic bulge. Could it be found directly in e+e- annihilations ? Not so easily, due to various constraints on its couplings, especially axial ones, leading to an axionlike behavior or extra parity-violation effects. Searches for the decay Upsilon -> gamma + invisible U may be used to constrain severely its axial coupling to the electron, f_eA = f_bA, to be less than about 10^-6 m_U(MeV). The vector coupling to the electron may in principle be larger. We find from g_mu-2, under reasonable assumptions, that this coupling can be at most as large as ~ 1.3 10^-3, for m_U < m_mu. Such a coupling of the order of 10^-3 could also be responsible for the somewhat large value of g_mu-2, compared to Standard Model expectations. The U couplings to electrons are otherwise likely to be smaller, e.g. \lsim 3 10^-6 m_U(MeV), if the couplings to neutrinos and electrons are similar. This restricts significantly the possibility of detecting a light U boson in e+e- -> gamma U. Despite that, U exchanges can provide annihilation cross sections of LDM particles of the appropriate size, even if this may require that light dark matter be relatively strongly self-interacting.

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