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Register a new user$e^{+}e^{-}$ pairs from a nuclear transition signaling an elusive light neutral boson
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Electron-positron pairs have been observed in the 10.95-MeV $0^-to0^+$ decay in $^{16}$O. The branching ratio of the e$^+$e$^-$ pairs compared to the 3.84-MeV $0^-to2^+$ $gamma$ decay of the level is deduced to be $20(5)times10^{-5}$. This magnetic monopole (M0) transition cannot proceed by $gamma$-ray decay and is, to first order, forbidden for internal pair creation. However, the transition may also proceed by the emission of a light neutral $0^{-}$ or $1^{+}$ boson. Indeed, we do observe a sharp peak in the $e^{+}e^{-}$ angular correlation with all the characteristics belonging to the intermediate emission of such a boson with an invariant mass of 8.5(5) MeV/c$^2$. It may play a role in the current quest for light dark matter in the universe.
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The improved results on a direct search for a new X(16.7 MeV) boson which could explain the anomalous excess of $e^+e^-$ pairs observed in the excited 8Be nucleus decays (Berillium anomaly) are reported. Due to its coupling to electrons, the X boson could be produced in the bremsstrahlung reaction e-Z -> e-ZX by a high-energy beam of electrons incident on active target in the NA64 experiment at the CERN SPS and observed through its subsequent decay into $e^+e^-$ pair. No evidence for such decays was found from the combined analysis of the data samples with total statistics corresponding to 8.4times 10^{10} electrons on target collected in 2017 and 2018. This allows to set the new limits on the $X$--$e^-$ coupling in the range 1.2 times 10^{-4} < epsilon_e < 6.8 times 10^{-4}, excluding part of the parameter space favored by the Berillium anomaly. We also set new bounds on the mixing strength of photons with dark photons (A) from non-observation of the decay $A to e^+e^-$ of the bremsstrahlung A with a mass below 24 MeV.
We report the results of a search for a light pseudoscalar particle $a$ that couples to electrons and decays to $e^+e^-$ performed using the high-energy CERN SPS H4 electron beam. If such pseudoscalar with a mass $simeq 17$ MeV exists, it could explain the ATOMKI anomaly. We used the NA64 data samples collected in the visible mode configuration with total statistics corresponding to $8.4times 10^{10}$ electrons on target (EOT) in 2017 and 2018. In order to increase sensitivity to small coupling parameter $epsilon$ we used also the data collected in 2016-2018 in the invisible mode configuration of NA64 with a total statistics corresponding to $2.84times 10^{11}$ EOT. A thorough analysis of both these data samples in the sense of background and efficiency estimations was already performed and reported in our previous papers devoted to the search for light vector particles and axion-like particles (ALP). In this work we recalculate the signal yields, which are different due to different cross section and life time of a pseudoscalar particle $a$, and perform a new statistical analysis. As a result, the region of the two dimensional parameter space $m_a - epsilon$ in the mass range from 1 to 17.1 MeV is excluded. At the mass of the ATOMKI anomaly the values of $epsilon$ in the range $2.1 times 10^{-4} < epsilon < 3.2 times 10^{-4}$ are excluded.
The transverse momentum cross section of $e^+e^-$ pairs in the $Z$-boson mass region of 66-116 GeV/$c^2$ is precisely measured using Run II data corresponding to 2.1 fb$^{-1}$ of integrated luminosity recorded by the Collider Detector at Fermilab. The cross section is compared with quantum chromodynamic calculations. One is a fixed-order perturbative calculation at ${cal O}(alpha_s^2)$, and the other combines perturbative predictions at high transverse momentum with the gluon resummation formalism at low transverse momentum. Comparisons of the measurement with calculations show reasonable agreement. The measurement is of sufficient precision to allow refinements in the understanding of the transverse momentum distribution.
The study of light hadrons is central to the understanding of confinement--a unique property of QCD. The quark model describs mesons as bound states of quarks and antiquarks. LQCD and QCD-motivated models for hadrons, however, predict a richer spectrum of mesons that takes into account not only the quark degrees of freedom but also the gluonic degrees of freedom. A selection of recent progress in the light-quark sector with unprecedented high-statistics data sets from $e^+e^-$ experiments are reviewed.
A vector-dominance two-photon exchange model is proposed to explain the recently observed production of $rho^0rho^0$ and $rho^0phi$ pairs in $e^+e^-$ annihilation at 10.58 GeV with the BaBar detector. All the observed features of the data --angular and decay distributions, rates-- are in agreement with the model. Predictions are made for yet-unobserved final states.
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