Measurement of Exclusive rho^0 rho^0 Production in Two-Photon Collisions at High Q^2 at LEP

Exclusive rho rho production in two-photon collisions involving a single highly virtual photon is studied with data collected at LEP at centre-of-mass energies 89GeV < \sqrt{s} < 209GeV with a total integrated luminosity of 854.7pb^-1 The cross section of the process gamma gamma^* -> rho rho is determined as a function of the photon virtuality, Q^2 and the two-photon centre-of-mass energy, Wgg, in the kinematic region: 1.2GeV^2 < Q^2 < 30GeV^2 and 1.1GeV < Wgg < 3GeV

Dark matter annihilation and its effect on CMB and Hydrogen 21 cm observations

If dark matter is made up of Weakly Interacting Massive Particles, the annihilation of these particles in halos results in energy being released, some of which is absorbed by gas, causing partial ionization and heating. Dark matter annihilation may result in partial ionization and gas heating at high redshifts, even before the formation of the first stars. It is shown that early ionization results in a transfer of power to higher multipoles in the large angle CMB polarization power spectra. Future CMB experiments may be able to place constraints on certain light dark matter models. We also investigate the effect of gas heating on the expected H21 cm power spectrum. Heating by particle annihilation results in a decrease in the amplitude of the H21 cm power spectrum as the gas temperature $T$ becomes comparable to the CMB temperature $T_\gamma$, and then an increase as $T > T_\gamma$. The result is a minimum in the power spectrum at the redshift for which $T \approx T_\gamma$. Only certain models (low particle masses $\sim$ 10 GeV, or favorable halo parameters) show this effect. Within these models, observations of the H21 cm power spectrum at multiple redshifts can help us obtain constraints on dark matter particle and halo properties.Comment: We correct an error in the calculation of $\kappa$ in Eq. 6. This results in a ~ factor 2 increase in the magnitude of the results. New plots have been computed. Replaced to reflect the published versio

Search for excited leptons at LEP

A search for charged and neutral excited leptons is performed in 217 pb(-1) of e(+)e(-) collision data collected with the L3 detector at LEP at centre-of-mass energies from 202 up to 209 GeV The pair- and single-production mechanisms of excited electrons, muons and taus, as well as of excited electron-, muon- and tau-neutrinos, are investigated and no signals are detected. Combining with L3 results from searches at lower centre-of-mass energies, gives improved limits on the masses and couplings of excited leptons. (C) 2003 Published by Elsevier B.V