Transverse–Spin Quark Distributions from Asymmetry Data and Symmetry Arguments

The transversity and the Sivers distribution functions of quarks incorporate important information about the transverse-spin and transverse-momentum structure of nucleons. We show how these distributions can be directly determined point by point from leptoproduction asymmetry data collected for various targets and produced hadrons by the COMPASS Collaboration. Only simple symmetry relations are used in the extraction.

COMPASS: meson spectroscopy and low-energy meson dynamics

Within the broad physics programme of the COMPASS collaboration at CERN SPS, soft reactions of high-energy hadron beams on different nuclear targets are investigated. Aiming at a better understanding of the strong interaction, novel results range from tests of chiral symmetry breaking to properties of diffractively produced meson resonances studied in their multi-particle decays, such as the a1(1420) with unusual properties. The talk will highlight the challenges of the employed experimental techniques, and as well of the analysis methods, that are developed to a large extent along with the analysis of the world’s largest data sets in the field.

Dipole Polarizabilities of π±-Mesons

The main experimental works, where dipole polarizabilities of charged pions have been determined, are considered. Possible reasons for the differences between the experimental data are discussed. In particular, it is shown that the account of the [Formula: see text]-meson gives a significant correction to the value of the polarizability obtained in the latest experiment of the COMPASS collaboration. We present also new fit results for the [Formula: see text] reaction.

Λ/Λ̄ Longitudinal Spin Transfer from Asymmetric Nucleon s/s̄ Distribution

We investigate quark to [Formula: see text] and [Formula: see text] longitudinal spin transfers in the light-cone SU(6) quark sepctator-diquark model and try to analyze the possible origins for the spin transfer difference between them measured by the COMPASS collaboration. The intermediate heavier hyperon decay processes are considered, while the final hadron transverse momentum is also included. We find that after taking into account the asymmetric nucleon [Formula: see text] distribution effect, the results we get are qualitatively comparable with the difference of the COMPASS experimental data.

Program of COMPASS-II at CERN

COMPASS collaboration has started in 2012 a five-year program of new measurements, which are outlined in the 'COMPASS-II' proposal. The two new major projects of the proposal are measurements of polarized Drell-Yan process in π- scattering off transversely polarized protons and studies of GPDs via measurements of Deeply Virtual Compton Scattering and Hard Exclusive Meson Production in muon scattering off a liquid hydrogen target. In addition, high statistics SIDSI measurements with muon beams and a study of charged pion and kaon polarizabilities via Primakoff reactions with hadron beams are also foreseen as a part of this program.

Pion polarizabilities: Theory vs Experiment

The values of charged pion polarizabilities obtained in the framework of chiral perturbation theory at the level of two-loop accuracy are compared with the experimental result recently reported by COMPASS Collaboration. It is found that calculated value for the dipole polarizabilities [Formula: see text] fits quite well the experimental result [Formula: see text].

RADIATIVE π-γ TRANSITIONS OF EXCITED LIGHT-QUARK MESONS IN THE COVARIANT OSCILLATOR QUARK MODEL

A study of the light-meson couplings to the single photon in the framework of the covariant oscillator quark model (COQM) is reported. The transition rates of {ρ(770), b1(1235), a1(1260), a2(1320), π2(1670), ρ3(1690), ρ(1700)}± into π±γ, being expected to be measured by the COMPASS collaboration in their Primakoff-reaction experiments, are calculated. It could be useful not only for understanding the internal structures of observed light-quark mesons and their quark-model classification but also for the ongoing experimental studies by COMPASS.