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Register a new userSearch for light vector boson production in $e^+e^- rightarrow mu^+ mu^- gamma$ interactions with the KLOE experiment
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We have searched for a light vector boson $U$, the possible carrier of a dark force, with the KLOE detector at the DA$Phi$NE epm collider, motivated by astrophysical evidence for the presence of dark matter in the universe. Using epm collisions collected with an integrated luminosity of $239.3$~pb$^{-1}$, we look for a dimuon mass peak in the reaction epmto$mu^+ mu^-$gam, corresponding to the decay $Utomu^+mu^-$. We find no evidence for a $U$ vector boson signal. We set a 90% CL upper limit for the mixing parameter squared between the photon and the $U$ boson of 1.6$times$10$^{-5}$ to 8.6$times$10$^{-7}$ for the mass region $520<m_{rm U}<980$ MeV.
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We searched for evidence of a Higgsstrahlung process in a secluded sector, leading to a final state with a dark photon U and a dark Higgs boson h, with the KLOE detector at DAFNE. We investigated the case of h lighter than U, with U decaying into a muon pair and h producing a missing energy signature. We found no evidence of the process and set upper limits to its parameters in the range 2m_mu<m_U<1000 MeV, m_h<m_U.
The recent interest in a light gauge boson in the framework of an extra U(1) symmetry motivates searches in the mass range below 1 GeV. We present a search for such a particle, the dark photon, in ${rm e^+ e^-}rightarrow {rm U}gamma$, U$rightarrow pi^+pi^-$ based on 28 million $mathrm{e^+ e^-} rightarrow pi^+ pi^-gamma$ events collected at DA$Phi$NE by the KLOE experiment. The $pi^+ pi^-$ production by initial-state radiation compensates for a loss of sensitivity of previous KLOE ${rm U} rightarrow mathrm{e^+ e^-}$, $mu^+mu^-$ searches due to the small branching ratios in the $rho-omega$ resonance region. We found no evidence for a signal and set a limit at 90% CL on the mixing strength between the photon and the dark photon, $varepsilon^2$, in the U mass range between $527$ and $987$~MeV. Above 700 MeV this new limit is more stringent than previous ones.
We have measured the ratio $sigma(e^+e^-rightarrowpi^+pi^-gamma)/sigma(e^+e^-rightarrow mu^+mu^-gamma)$, with the KLOE detector at DA$Phi$NE for a total integrated luminosity of $sim$ 240 pb$^{-1}$. From this ratio we obtain the cross section $sigma(e^+e^-rightarrowpi^+pi^-)$. From the cross section we determine the pion form factor $|F_pi|^2$ and the two-pion contribution to the muon anomaly $a_mu$ for $0.592<M_{pipi}<0.975$ GeV, $Delta^{pipi} a_mu$= $({rm 385.1pm1.1_{stat}pm2.7_{sys+theo}})times10^{-10}$. This result confirms the current discrepancy between the Standard Model calculation and the experimental measurement of the muon anomaly.
We present the first measurements of branching fractions of rare tau-lepton decays, $tau^- rightarrow pi^- u_{tau} ell^+ ell^-$ ($ell = e$ or $mu$), using a data sample corresponding to 562 fb$^{-1}$ collected at a center-of-mass energy of 10.58 GeV with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. The $tau^- rightarrow pi^- u_tau e^+ e^-$ decay is observed for the first time with 7.0$sigma$ significance. The partial branching fraction determined by the structure-dependent mechanisms mediated by either a vector or an axial-vector current for the mass region $M_{pi e e}>1.05$ GeV/$c^2$ is measured to be $mathcal{B}(tau^-rightarrow pi^- u_tau e^+ e^-)[M_{pi^- e^+ e^-}>1.05~{rm GeV}/c^2] = (5.90 pm 0.53 pm 0.85 pm 0.11) times 10^{-6}$, where the first uncertainty is statistical, the second is systematic, and the third is due to the model dependence. In the full phase space, due to the different detection efficiencies for the structure-dependent mechanisms mediated by axial-vector and vector currents, the branching fraction varies from $mathcal{B}_{A}(tau^-rightarrow pi^- u_tau e^+ e^-) = (1.46 pm 0.13 pm 0.21) times 10^{-5}$ to $mathcal{B}_{V}(tau^-rightarrow pi^- u_tau e^+ e^-) = (3.01 pm 0.27 pm 0.43) times 10^{-5}$, respectively. An upper limit is set on the branching fraction of the $tau^- rightarrow pi^- u_tau mu^+ mu^-$ decay, $mathcal{B}(tau^-rightarrow pi^- u_tau mu^+ mu^-) < 1.14 times 10^{-5}$, at the 90% confidence level.
A search for the lepton-flavour violating decays $B^0_s to e^{pm} mu^{mp}$ and $B^0 to e^{pm} mu^{mp}$ is performed with a data sample, corresponding to an integrated luminosity of 1.0 fb$^{-1}$ of $pp$ collisions at $sqrt{s} = 7$ TeV, collected by the LHCb experiment. The observed number of $B^0_s to e^{pm} mu^{mp}$ and $B^0 to e^{pm} mu^{mp}$ candidates is consistent with background expectations. Upper limits on the branching fractions of both decays are determined to be $BR(B^0_s to e^{pm} mu^{mp}) < 1.1 ,(1.4) times 10^{-8}$ and $BR (B^0 to e^{pm} mu^{mp}) < 2.8 ,(3.7) times 10^{-9}$ at 90% (95%) confidence level (C.L.). These limits are a factor of twenty lower than those set by previous experiments. Lower bounds on the Pati-Salam leptoquark masses are also calculated, $M_{rm LQ} (B^0_s to e^{pm} mu^{mp}) > 107$ TeV/c$^2$ and $M_{rm LQ} (B^0 to e^{pm} mu^{mp}) > 126$ TeV/c$^2$ at 95% C.L., and are a factor of two higher than the previous bounds.
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