In the fragmentation of a transversely polarized quark several left-right asymmetries are possible for the hadrons in the jet. When only one unpolarized hadron is selected, it exhibits an azimuthal modulation known as Collins effect. When a pair of o
ppositely charged hadrons is observed, three asymmetries can be considered, a di-hadron asymmetry and two single hadron asymmetries. In lepton deep inelastic scattering on transversely polarized nucleons all these asymmetries are coupled with the transversity distribution. From the high statistics COMPASS data on oppositely charged hadron-pair production we have investigated for the first time the dependence of these three asymmetries on the difference of the azimuthal angles of the two hadrons. The similarity of transversity induced single and di-hadron asymmetries is discussed. A new analysis of the data allows to establish quantitative relationships among them, providing for the first time strong experimental indication that the underlying fragmentation mechanisms are all driven by a common physical process.
New results for the double spin asymmetry $A_1^{rm p}$ and the proton longitudinal spin structure function $g_1^{rm p}$ are presented. They were obtained by the COMPASS collaboration using polarised 200 GeV muons scattered off a longitudinally polari
sed NH$_3$ target. The data were collected in 2011 and complement those recorded in 2007 at 160,GeV, in particular at lower values of $x$. They improve the statistical precision of $g_1^{rm p}(x)$ by about a factor of two in the region $xlesssim 0.02$. A next-to-leading order QCD fit to the $g_1$ world data is performed. It leads to a new determination of the quark spin contribution to the nucleon spin, $Delta Sigma$ ranging from 0.26 to 0.36, and to a re-evaluation of the first moment of $g_1^{rm p}$. The uncertainty of $Delta Sigma$ is mostly due to the large uncertainty in the present determinations of the gluon helicity distribution. A new evaluation of the Bjorken sum rule based on the COMPASS results for the non-singlet structure function $g_1^{rm NS}(x,Q^2)$ yields as ratio of the axial and vector coupling constants $|g_{rm A}/g_{rm V}| = 1.22 pm 0.05~({rm stat.}) pm 0.10~({rm syst.})$, which validates the sum rule to an accuracy of about 9%.
The COMPASS collaboration at CERN has measured diffractive dissociation of 190 GeV/$c$ pions into the $pi^-pi^-pi^+$ final state using a stationary hydrogen target. A partial-wave analysis (PWA) was performed in bins of $3pi$ mass and four-momentum t
ransfer using the isobar model and the so far largest PWA model consisting of 88 waves. A narrow $J^{PC} = 1^{++}$ signal is observed in the $f_0(980),pi$ channel. We present a resonance-model study of a subset of the spin-density matrix selecting $3pi$ states with $J^{PC} = 2^{++}$ and $4^{++}$ decaying into $rho(770),pi$ and with $J^{PC} = 1^{++}$ decaying into $f_0(980),pi$. We identify a new $a_1$ meson with mass $(1414^{+15}_{-13})$ MeV/$c^2$ and width $(153^{+8}_{-23})$ MeV/$c^2$. Within the final states investigated in our analysis, we observe the new $a_1(1420)$ decaying only into $f_0(980),pi$, suggesting its exotic nature.
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 u
sing 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.
Exclusive production of $etapi^-$ and $etapi^-$ has been studied with a $191,textrm{GeV}/c$ $pi^-$ beam impinging on a hydrogen target at COMPASS (CERN). Partial-wave analyses reveal different odd/even angular momentum ($L$) characteristics in the in
spected invariant mass range up to $3,textrm{GeV}/c^2$. 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 $a_2(1320)$ and $a_4(2040)$ are in line with this similarity. In contrast, a strong enhancement of $etapi^-$ over $etapi^-$ is found for the $L=1,3,5$ waves, which carry non-$qbar q$ quantum numbers. The $L=1$ intensity peaks at $1.7,textrm{GeV}/c^2$ in $etapi^-$ and at $1.4,textrm{GeV}/c^2$ in $etapi^-$, the corresponding phase motions with respect to $L=2$ are different.
A search for the exclusive production of the $Z_c^{pm}(3900)$ hadron by virtual photons has been performed in the channel $Z_c^{pm}(3900)rightarrow J/psi pi^{pm}$. The data cover the range from 7 GeV to 19 GeV in the centre-of-mass energy of the phot
on-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 $BR(Z_c^{pm}(3900)rightarrow J/psi pi^{pm} )times sigma_{ gamma~N rightarrow Z_c^{pm}(3900)~ N} /sigma_{gamma~N rightarrow J/psi~ N}$ of $3.7times10^{-3}$ has been established at the confidence level of 90%.
The COMPASS collaboration at CERN has investigated pion Compton scattering, $pi^-gammarightarrow pi^-gamma$, at centre-of-mass energy below 3.5 pion masses. The process is embedded in the reaction $pi^-mathrm{Ni}rightarrowpi^-gamma;mathrm{Ni}$, which
is initiated by 190,GeV pions impinging on a nickel target. The exchange of quasi-real photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, $Q^2<0.0015$,(GeV/$c$)$^2$. From a sample of 63,000 events the pion electric polarisability is determined to be $alpha_pi = (,2.0 pm 0.6_{mbox{scriptsize stat}} pm 0.7_{mbox{scriptsize syst}},) times 10^{-4},mbox{fm}^3$ under the assumption $alpha_pi=-beta_pi$, which relates the electric and magnetic dipole polarisabilities. It is the most precise measurement of this fundamental low-energy parameter of strong interaction, that has been addressed since long by various methods with conflicting outcomes. While this result is in tension with previous dedicated measurements, it is found in agreement with the expectation from chiral perturbation theory. An additional measurement replacing pions by muons, for which the cross-section behavior is unambigiously known, was performed for an independent estimate of the systematic uncertainty.
Exclusive production of the isoscalar vector mesons $omega$ and $phi$ is measured with a 190 GeV$/c$ proton beam impinging on a liquid hydrogen target. Cross section ratios are determined in three intervals of the Feynman variable $x_{F}$ of the fast
proton. A significant violation of the OZI rule is found, confirming earlier findings. Its kinematic dependence on $x_{F}$ and on the invariant mass $M_{pmathrm{V}}$ of the system formed by fast proton $p_mathrm{fast}$ and vector meson $V$ is discussed in terms of diffractive production of $p_mathrm{fast}V$ resonances in competition with central production. The measurement of the spin density matrix element $rho_{00}$ of the vector mesons in different selected reference frames provides another handle to distinguish the contributions of these two major reaction types. Again, dependences of the alignment on $x_{F}$ and on $M_{pmathrm{V}}$ are found. Most of the observations can be traced back to the existence of several excited baryon states contributing to $omega$ production which are absent in the case of the $phi$ meson. Removing the low-mass $M_{pmathrm{V}}$ resonant region, the OZI rule is found to be violated by a factor of eight, independently of $x_mathrm{F}$.
