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The first measurement of transverse-spin-dependent azimuthal asymmetries in the pion-induced Drell-Yan (DY) process is reported. We use the CERN SPS 190  GeV/c π^{-} beam and a transversely polarized ammonia target. Three azimuthal asymmetries giving access to different transverse-momentum-dependent (TMD) parton distribution functions (PDFs) are extracted using dimuon events with invariant mass between 4.3  GeV/c^{2} and 8.5  GeV/c^{2}. Within the experimental uncertainties, the observed sign of the Sivers asymmetry is found to be consistent with the fundamental prediction of quantum chromodynamics (QCD) that the Sivers TMD PDFs extracted from DY have a sign opposite to the one extracted from semi-inclusive deep-inelastic scattering (SIDIS) data. We present two other asymmetries originating from the pion Boer-Mulders TMD PDFs convoluted with either the nucleon transversity or pretzelosity TMD PDFs. A recent COMPASS SIDIS measurement was obtained at a hard scale comparable to that of these DY results. This opens the way for possible tests of fundamental QCD universality predictions.

The COMPASS Collaboration at CERN has measured the transverse spin azimuthal asymmetry of\ud charged hadrons produced in semi-inclusive deep inelastic scattering using a 160 GeV μ+ beam and a\ud transversely polarised NH3 target. The Sivers asymmetry of the proton has been extracted in the Bjorken\ud x range 0.003 < x < 0.7. The new measurements have small statistical and systematic uncertainties of\ud a few percent and confirm with considerably better accuracy the previous COMPASS measurement. The\ud Sivers asymmetry is found to be compatible with zero for negative hadrons and positive for positive\ud hadrons, a clear indication of a spin–orbit coupling of quarks in a transversely polarised proton. As\ud compared to measurements at lower energy, a smaller Sivers asymmetry for positive hadrons is found\ud in the region x > 0.03. The asymmetry is different from zero and positive also in the low x region,\ud where sea-quarks dominate. The kinematic dependence of the asymmetry has also been investigated and\ud results are given for various intervals of hadron and virtual photon fractional energy. In contrast to the\ud case of the Collins asymmetry, the results on the Sivers asymmetry suggest a strong dependence on the\ud four-momentum transfer to the nucleon, in agreement with the most recent calculations

Measurements of the Collins and Sivers asymmetries for charged pions and charged and neutral kaons produced in semi-inclusive deep-inelastic scattering of high energy muons off transversely polarised protons are presented. The results were obtained using all the available COMPASS proton data, which were taken in the years 2007 and 2010. The Collins asymmetries exhibit in the valence region a non-zero signal for pions and there are hints of non-zero signal also for kaons. The Sivers asymmetries are found to be positive for positive pions and kaons and compatible with zero otherwise

The NA62 experiment reports the branching ratio measurement $$ \mathrm{BR}\left({K}^{+}\to {\pi}^{+}\nu \overline{\nu}\right)=\left({10.6}_{-3.4}^{+4.0}\left|{}_{\mathrm{stat}}\right.\pm {0.9}_{\mathrm{syst}}\right)\times {10}^{-11} $$ BR K + → π + ν ν ¯ = 10.6 − 3.4 + 4.0 stat ± 0.9 syst × 10 − 11 at 68% CL, based on the observation of 20 signal candidates with an expected background of 7.0 events from the total data sample collected at the CERN SPS during 2016–2018. This provides evidence for the very rare K+→$$ {\pi}^{+}\nu \overline{\nu} $$ π + ν ν ¯ decay, observed with a significance of 3.4σ. The experiment achieves a single event sensitivity of (0.839 ± 0.054) × 10−11, corresponding to 10.0 events assuming the Standard Model branching ratio of (8.4 ± 1.0) × 10−11. This measurement is also used to set limits on BR(K+→ π+X), where X is a scalar or pseudo-scalar particle. Details are given of the analysis of the 2018 data sample, which corresponds to about 80% of the total data sample.

We have performed the most comprehensive resonance-model fit of π − π − π þ states using the results of our previously published partial-wave analysis (PWA) of a large data set of diffractive-dissociation events from the reaction π − þ p → π − π − π þ þ p recoil with a 190 GeV=c pion beam. The PWA results, which were obtained in 100 bins of three-pion mass, 0.5 < m 3π < 2.5 GeV=c 2 , and simultaneously in 11 bins of the reduced four-momentum transfer squared, 0.1 < t 0 < 1.0 ðGeV=cÞ 2 , are subjected to a resonance-model fit using Breit-Wigner amplitudes to simultaneously describe a subset of 14 selected waves using 11 isovector light-meson states with J PC ¼ 0 −þ , 1 þþ , 2 þþ , 2 −þ , 4 þþ , and spin-exotic 1 −þ quantum numbers. The model contains the well-known resonances πð1800Þ, a 1 ð1260Þ, a 2 ð1320Þ, π 2 ð1670Þ, π 2 ð1880Þ, and a 4 ð2040Þ. In addition, it includes the disputed π 1 ð1600Þ, the excited states a 1 ð1640Þ, a 2 ð1700Þ, and π 2 ð2005Þ, as well as the resonancelike a 1 ð1420Þ. We measure the resonance parameters mass and width of these objects by combining the information from the PWA results obtained in the 11 t 0 bins. We extract the relative branching fractions of the ρð770Þπ and f 2 ð1270Þπ decays of a 2 ð1320Þ and a 4 ð2040Þ, where the former one is measured for the first time. In a novel approach, we extract the t 0 dependence of the intensity of the resonances and of their phases. The t 0 dependence of the intensities of most resonances differs distinctly from the t 0 dependence of the nonresonant components. For the first time, we determine the t 0 dependence of the phases of the production amplitudes and confirm that the production mechanism of the Pomeron exchange is common to all resonances. We have performed extensive systematic studies on the model dependence and correlations of the measured physical parameters.

The improved results on a direct search for a new X(16.7 MeV) boson that could explain the anomalous excess of e + e − pairs observed in the decays of the excited 8 Be * nucleus ("Berillium anomaly") are reported. The X boson could be produced in the bremsstrahlung reaction e − Z → e − ZX by a high energy beam of electrons incident on the 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 8.4 × 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 × 10 −4 e 6.8 × 10 −4 , excluding part of the parameter space favored by the Berillium anomaly. The non-observation of the decay A → e + e − allows also to set the new bounds on the mixing strength of photons with dark photons (A ) with a mass 24 MeV.Recently, the search for new light bosons weakly coupled to SM particles was additionally inspired by the observation in the ATOMKI experiment by Krasznahorkay et al. [1,2] of a ∼7σ excess of events in the invariant mass distribution of e + e − pairs produced in the nuclear transitions of the excited 8 Be * to its ground state via internal pair creation. It was shown that this anomaly can be interpreted as the emission of a protophobic gauge boson X with a mass of 16.7 MeV decaying into e + e − pair [3,4]. This explanation of the anomaly was found to be consistent with the existing constraints assuming that the X has non-universal coupling to quarks, coupling to electrons in the range 2 × 10 −4 e 1.4 × 10 −3 and lifetime 10 −14 τ X 10 −12 s. It is interesting that a new boson with such relatively large couplings to charged leptons could also resolve the so-called (g µ − 2 ) anomaly, a discrepancy between measured and predicted values of the muon anomalous magnetic moment. This has motivated worldwide efforts towards the experimental searches, see, e.g., Refs. [5,6], and studies of the phenomenological aspects of light vector bosons weakly coupled to quarks and leptons, see, e.g., and also earlier works of Refs. [13][14][15][16]. The latest experimental results from the ATOMKI group show a similar excess of events at approximately the same invariant mass in the nuclear transitions of another nucleus, 4 He [17]. This further increases the importance of independent searches for a new particle X.Another strong motivation to search for new light bosons decaying into e + e − pair comes from the dark matter puzzle. An interesting possibility is that in addition to gravity a new force between the dark sector and visible matter, carried by a new vector boson A , called dark photon, might exist [18,19]. Such A could have a mass

The COMPASS collaboration has collected the currently largest data set on diffractively produced π − π − π þ final states using a negative pion beam of 190 GeV=c momentum impinging on a stationary proton target. This data set allows for a systematic partial-wave analysis in 100 bins of three-pion mass, 0.5 < m 3π < 2.5 GeV=c 2 , and in 11 bins of the reduced four-momentum transfer squared, 0.1 < t 0 < 1.0 ðGeV=cÞ 2. This two-dimensional analysis offers sensitivity to genuine one-step resonance production, i.e. the production of a state followed by its decay, as well as to more complex dynamical effects in nonresonant 3π production. In this paper, we present detailed studies on selected 3π partial waves with J PC ¼ 0 −þ , 1 þþ , 2 −þ , 2 þþ , and 4 þþ. In these waves, we observe the well-known groundstate mesons as well as a new narrow axial-vector meson a 1 ð1420Þ decaying into f 0 ð980Þπ. In addition, we present the results of a novel method to extract the amplitude of the π − π þ subsystem with I G J PC ¼ 0 þ 0 þþ in various partial waves from the π − π − π þ data. Evidence is found for correlation of the f 0 ð980Þ and f 0 ð1500Þ appearing as intermediate π − π þ isobars in the decay of the known πð1800Þ and π 2 ð1880Þ.