Search for exclusive photoproduction of Zc± (3900) at COMPASS

A search for the exclusive production of the Zc±(3900) hadron by virtual photons has been performed in the channel Zc±(3900)→J/ψπ± . The data cover the range from 7 GeV to 19 GeV in the centre-of-mass energy of the photon–nucleon system. The full set of the COMPASS data set collected with a muon beam between 2002 and 2011 has been used. An upper limit for the ratio BR(Zc±(3900)→J/ψπ±)×σγN→Zc±(3900)N/σγN→J/ψN of 3.7×10−3 has been established at the confidence level of 90%

Odd and even partial waves of ηπ− and η′π− in π−p→η(′)π−p at 191 GeV/c

Exclusive production of ηπ− and η′π− has been studied with a 191 GeV/c π− beam impinging on a hydrogen target at COMPASS (CERN). Partial-wave analyses reveal different odd/even angular momentum ( L ) characteristics in the inspected invariant mass range up to 3 GeV/c2 . A striking similarity between the two systems is observed for the L=2,4,6 intensities (scaled by kinematical factors) and the relative phases. The known resonances a2(1320) and a4(2040) are in line with this similarity. In contrast, a strong enhancement of η′π− over ηπ− is found for the L=1,3,5 waves, which carry non- qq¯ quantum numbers. The L=1 intensity peaks at 1.7 GeV/c2 in η′π− and at 1.4 GeV/c2 in ηπ− , the corresponding phase motions with respect to L=2 are different

Collins and Sivers asymmetries in muonproduction of pions and kaons off transversely polarised protons

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