We carried out a model-independent search for light scalar ($s$) and pseudoscalar axionlike ($a$) particles that couple to two photons by using the high-energy CERN SPS H4 electron beam. The new particles, if they exist, could be produced through the Primakoff effect in interactions of hard bremsstrahlung photons generated by 100 GeV electrons in the NA64 active dump with virtual photons provided by the nuclei of the dump. The $a(s)$ would penetrate the downstream HCAL module, serving as a shield, and would be observed either through their $a(s)\to \gamma \gamma$ decay in the rest of the HCAL detector, or as events with a large missing energy if the $a(s)$ decays downstream of the HCAL. This method allows for the probing of the $a(s)$ parameter space, including those from generic axion models, inaccessible to previous experiments. No evidence of such processes has been found from the analysis of the data corresponding to $2.84\times {10}^{11}$ electrons on target, allowing us to set new limits on the $a(s)\gamma \gamma$-coupling strength for $a(s)$ masses below 55 MeV.

The improved results on a direct search for a new $X(16.7\mathrm{MeV})$ boson that could explain the anomalous excess of $e^{+}{e}^{-}$ pairs observed in the decays of the excited ${}^8{\mathrm{Be}}^{*}$ nuclei (“Berillium or X17 anomaly”) are reported. Interestingly, new recent results in the nuclear transitions of another nucleus, ${}^4\mathrm{He}$, seems to support this anomaly spurring the need for an independent measurement. If the $X$ boson exists, it could be produced in the bremsstrahlung reaction $e^{-}Z\to e^{-}ZX$ by a high energy beam of electrons incident on the active target in the NA64 experiment at the CERN Super Proton Synchrotron and observed through its subsequent decay into $e^{+}{e}^{-}$ pairs. No evidence for such decays was found from the combined analysis of the data samples with total statistics corresponding to $8.4\times {10}^{10}$ electrons on target collected in 2017 and 2018. This allows one to set new limits on the $X-e^{-}$ coupling in the range $1.2\times {10}^{-4}\lesssim {\epsilon }_e\lesssim 6.8\times {10}^{-4}$, excluding part of the parameter space favored by the X17 anomaly, and setting new bounds on the mixing strength of photons with dark photons ($A^{\prime }$) with a mass $\lesssim 24\mathrm{MeV}$. For the 2018 run, the setup was optimized to probe the region of parameter space characterized by a large coupling $\epsilon$. This allowed a significant improvement in sensitivity despite a relatively modest increase in statistics.

The inclusive double-spin asymmetry, $A_{1}^{p}$, has been measured at COMPASS in deepinelastic polarised muon scattering off a large polarised NH3 target. The data, collected in the year 2007, cover the range Q2 > 1 (GeV/c)^2, 0.004 < x < 0.7 and improve the statistical precision of g_{1}^{p}(x) by a factor of two in the region x < 0.02. The new proton asymmetries are combined with those previously published for the deuteron to extract the non-singlet spin-dependent structure function g_1^NS(x,Q2). The isovector quark density, Delta_q_3(x,Q2), is evaluated from a NLO QCD fit of g_1^NS. The first moment of Delta_q3 is in good agreement with the value predicted by the Bjorken sum rule and corresponds to a ratio of the axial and vector coupling constants g_A/g_V = 1.28+-0.07(stat)+-0.10(syst).The inclusive double-spin asymmetry, A_1^p, has been measured at COMPASS in deepinelastic polarised muon scattering off a large polarised NH3 target. The data, collected in the year 2007, cover the range Q2 > 1 (GeV/c)^2, 0.004 < x < 0.7 and improve the statistical precision of g_1^p(x) by a factor of two in the region x < 0.02. The new proton asymmetries are combined with those previously published for the deuteron to extract the non-singlet spin-dependent structure function g_1^NS(x,Q2). The isovector quark density, Delta_q_3(x,Q2), is evaluated from a NLO QCD fit of g_1^NS. The first moment of Delta_q3 is in good agreement with the value predicted by the Bjorken sum rule and corresponds to a ratio of the axial and vector coupling constants g_A/g_V = 1.28+-0.07(stat)+-0.10(syst).The inclusive double-spin asymmetry, A1p , has been measured at COMPASS in deep-inelastic polarised muon scattering off a large polarised NH 3 target. The data, collected in the year 2007, cover the range Q2>1(GeV/c)2 , 0.004