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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 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.

The COMPASS Collaboration at CERN has measured diffractive dissociation of 190  GeV/c pions into the π(-)π(-)π(+) final state using a stationary hydrogen target. A partial-wave analysis (PWA) was performed in bins of 3π mass and four-momentum transfer using the isobar model and the so far largest PWA model consisting of 88 waves. A narrow peak is observed in the f0(980)π channel with spin, parity and C-parity quantum numbers J(PC)=1(++). We present a resonance-model study of a subset of the spin-density matrix selecting 3π states with J(PC)=2(++) and 4(++) decaying into ρ(770)π and with J(PC)=1(++) decaying into f0(980)π. We identify a new a1 meson with mass (1414(-13)(+15))  MeV/c2 and width (153(-23)(+8))  MeV/c2. Within the final states investigated in our analysis, we observe the new a1(1420) decaying only into f0(980)π, suggesting its exotic nature.

Spin-averaged asymmetries in the azimuthal distributions of positive and negative hadrons produced in deep inelastic scattering were measured using the CERN SPS longitudinally polarised muon beam at 160 GeV/c160 GeV/c and a 6LiD target. The amplitudes of the three azimuthal modulations cos⁡ϕhcos⁡ϕh, cos⁡2ϕhcos⁡2ϕh and sin⁡ϕhsin⁡ϕh were obtained binning the data separately in each of the relevant kinematic variables x , z or View the MathML sourcepTh and binning in a three-dimensional grid of these three variables. The amplitudes of the cos⁡ϕhcos⁡ϕh and cos⁡2ϕhcos⁡2ϕh modulations show strong kinematic dependencies both for positive and negative hadrons