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Register a new userImproved exclusion limit for light dark matter from $e^+e^-$ annihilation in NA64
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The current most stringent constraints for the existence of sub-GeV dark matter coupling to Standard Model via a massive vector boson $A^prime$ were set by the NA64 experiment for the mass region $m_{A^prime}lesssim 250$ MeV, by analyzing data from the interaction of $2.84cdot10^{11}$ 100-GeV electrons with an active thick target and searching for missing-energy events. In this work, by including $A^prime$ production via secondary positron annihilation with atomic electrons, we extend these limits in the $200$-$300$ MeV region by almost an order of magnitude, touching for the first time the dark matter relic density constrained parameter combinations. Our new results demonstrate the power of the resonant annihilation process in missing energy dark-matter searches, paving the road to future dedicated $e^+$ beam efforts.
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A search for sub-GeV dark matter production mediated by a new vector boson $A$, called dark photon, is performed by the NA64 experiment in missing energy events from 100 GeV electron interactions in an active beam dump at the CERN SPS. From the analysis of the data collected in the years 2016, 2017, and 2018 with $2.84times10^{11}$ electrons on target no evidence of such a process has been found. The most stringent constraints on the $A$ mixing strength with photons and the parameter space for the scalar and fermionic dark matter in the mass range $lesssim 0.2$ GeV are derived, thus demonstrating the power of the active beam dump approach for the dark matter search.
A new evaluation of the hadronic vacuum polarization contribution to the muon magnetic moment is presented. We take into account the reanalysis of the low-energy ee annihilation cross section into hadrons by the CMD-2 Collaboration. The agreement between ee and $tau$ spectral functions in the $pipi$ channel is found to be much improved. Nevertheless, significant discrepancies remain in the center-of-mass energy range between 0.85 and $1.0 {rm GeV}/c^2$.The deviations from the measurement at BNL are found to be $(22.1 pm 7.2 pm 3.5 pm 8.0)~10^{-10}$ (1.9 $sigma$) and $(7.4 pm 5.8 pm 3.5 pm 8.0)~10^{-10}$ (0.7 $sigma$) for the ee- and $tau$-based estimates, respectively, where the second error is from the nonhadronic contributions and the third one from the BNL measurement. Taking into account the $rho^- - rho^0$ mass splitting determined from the measured spectral functions increases the $tau$-based estimate and leads to a worse discrepancy between the two estimates.
The usual interpretation of Bose-Einstein correlations (BEC) of identical boson pairs relates the width of the peak in the correlation function at small relative four-momentum to the spatial extent of the source of the bosons. However, in the tau-model, which successfully describes BEC in hadronic Z decay, the width of the peak is related to the temporal extent of boson emission. Some new checks on the validity of both the tau-model and the usual descriptions are presented.
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.
Results on Bose-Einstein correlations in e+e- ==> hadrons are reviewed.
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