A search for pairs of light neutral pseudoscalar bosons (A) resulting from the decay of a Higgs boson is performed. The search is conducted using LHC proton-proton collision data at $\sqrt{s}$ = 13 TeV, collected with the CMS detector in 2016$-$2018 and corresponding to an integrated luminosity of 138 fb$^{-1}$. The A boson decays into a highly collimated electron-positron pair. A novel multivariate algorithm using tracks and calorimeter information is developed to identify these distinctive signatures, and events are selected with two such merged electron-positron pairs. No significant excess above the standard model background predictions is observed. Upper limits on the branching fraction for H $\to$ AA $\to$ 4e are set at 95% confidence level, for masses between 10 and 100 MeV and proper decay lengths below 100 $μ$m, reaching branching fraction sensitivities as low as 10$^{-5}$. This is the first search for Higgs boson decays to four electrons via light pseudoscalars at the LHC. It significantly improves the experimental sensitivity to axion-like particles with masses below 100 MeV.
Invariant mass distribution of the four-electron system ($m_{4 e}$) for selected events (points), compared to the background-only fit (red) with its $68\%$ and $95\%$~CL uncertainty bands (green and yellow). A non-stacked benchmark signal (blue) for a Higgs boson decaying to a pair of ALPs with $m_a=20MeV$ and $c \tau = 10\,\mu\mathrm{m}$ is overlaid and normalized to a branching ratio of $4.6 \times 10^{-5}$, which corresponds to the $95\%$~CL upper limit value set by this analysis. The lower panel shows the same data after subtracting the background fit.
Observed (solid points) and expected (dashed lines) $95\%$ CL upper limits on the Higgs boson branching fraction to a pair of ALPs decaying into electron-positron pairs ($ H \to A A \to e e$), shown as a function of the ALP mass for benchmark proper decay lengths of 1 $\,\mu\mathrm{m}$ (upper left), 10 $\,\mu\mathrm{m}$ (upper right), and 100 $\,\mu\mathrm{m}$ (lower left). The green and yellow bands represent the one and two standard deviation confidence intervals around the expected limits. The lower right panel shows a map of the observed $95\%$ CL upper limit, shown as a color scale, as a function of the ALP mass $m_ A$ and proper decay length $c \tau$.
A map of the observed $95\%$ CL upper limit on the Higgs boson branching fraction for $ H \to A A \to4 e$, as a function of the ALP mass and the ratio of the ALP coupling to electrons to the energy scale of the ALP effective interaction.
The target asymmetry T, recoil asymmetry P, and beam-target double polarization observable H were determined in exclusive $\pi ^0$ and $\eta $ photoproduction off quasi-free protons and, for the first time, off quasi-free neutrons. The experiment was performed at the electron stretcher accelerator ELSA in Bonn, Germany, with the Crystal Barrel/TAPS detector setup, using a linearly polarized photon beam and a transversely polarized deuterated butanol target. Effects from the Fermi motion of the nucleons within deuterium were removed by a full kinematic reconstruction of the final state invariant mass. A comparison of the data obtained on the proton and on the neutron provides new insight into the isospin structure of the electromagnetic excitation of the nucleon. Earlier measurements of polarization observables in the $\gamma p \rightarrow \pi ^0 p$ and $\gamma p \rightarrow \eta p$ reactions are confirmed. The data obtained on the neutron are of particular relevance for clarifying the origin of the narrow structure in the $\eta n$ system at $W = 1.68\ \textrm{GeV}$. A comparison with recent partial wave analyses favors the interpretation of this structure as arising from interference of the $S_{11}(1535)$ and $S_{11}(1650)$ resonances within the $S_{11}$-partial wave.
Target asymmetry T, recoil asymmetry P, and polarization observable H for $\gamma p \to \pi^0 p$ as a function of the polar center-of-mass angle for bins at the given centroid c.m. energies.
Target asymmetry T, recoil asymmetry P, and polarization observable H for $\gamma n \to \pi^0 n$ as a function of the polar center-of-mass angle for bins at the given centroid c.m. energies.
Target asymmetry T, recoil asymmetry P, and polarization observable H for $\gamma p \to \eta p$ as a function of the polar center-of-mass angle for bins at the given centroid c.m. energies.
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