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We derive a unitarized model for the peripheral production of the three-pion system in the isobar approximation. The production process takes into account long-range t-channel pion exchange. The K-matrix approach is chosen for the parameterization of the scattering amplitude. Five coupled channels are used to fit the COMPASS spin-density matrices for the JPCMϵ = 2−+0+ sector. Preliminary results of the fit are presented.

Spin Density Matrix Elements (SDMEs) are determined for exclusive ω meson production on unpolarized protons, in the COMPASS kinematic region of 1.0 (GeV/c)2 < Q2 < 10.0 (GeV/c)2, 5.0 GeV/c2 < W < 17.0 GeV/c2 and 0.01 ( GeV/c ) 2 < p T 2 <0.5 ( GeV/c ) 2 . Using extracted the preliminary SDMEs values the hypothesis of S-Channel Helicity Conservation (SCHC) is studied. Certain matrix elements that correspond to the transition γ T * → V L ( e.g. r 00 5 ) indicate violation of SCHC in exclusive ω production. A sizable contribution of unnatural parity exchange amplitudes is found for exclusive ω meson muoproduction, and there is a clear indication of its decrease with increasing W. The extracted longitudinal-to-transverse cross section ratio is 0.553 ± 0.044 ± 0.020.

Diffractive production of π - π - π + and π - π 0 π 0 final states is the subject of comprehensive studies performed recently by the VES and the COM-PASS experiments. COMPASS pioneered the application of novel methods of partial-wave analysis: mass-independent PWA inmultiple (m 3π , t’)-cells, mass-dependent analysis of spin-density matrices performed simultaneously in all measured t’ bins, the analysis with freed shapes of π + π - isobars. In addition, COMPASS observed a new narrow state: a 1 (1420) . VES has world-leading data samples on π - π - π + and π - π 0 π 0 , that yield compatible results and show the potential for a detailed comparison of isospin relations between different decay channels, using the PWA methods with fixed and freed shapes of ππ isobars.

The COMPASS Collaboration has recently published an article \"Multiplicities of positive and negative pions, kaons, and unidentified hadrons from deep-inelastic scattering of muons off a liquid hydrogen target\", Phys. Rev. D 112 (2025) 012002. In contrast to earlier COMPASS publications on similar topics, the aforementioned article features an enhanced treatment of QED radiative corrections, employing the DJANGOH Monte Carlo generator. This methodological improvement led to corrections that are up to 12% larger in the low-x, high-z region compared to the previously applied ones. To ensure consistent treatment of COMPASS data sets obtained using both isoscalar and proton targets, this paper presents an updated set of isoscalar multiplicities based on DJANGOH-derived radiative corrections. The present results supersede those published in Phys. Lett. B 764 (2017) 1 and Phys. Lett. B 767 (2017) 133.

The Compass spectrometer at CERN has collected a large data set for diffractive three-pion production of 46 × 10 6 exclusive events. Based on previous conventional Partial-Wave Analyses (PWA), we performed a “freed-isobar PWA” on the same data, removing model assumptions on the dynamic isobar amplitudes for dominating waves. In this analysis, we encountered continuous mathematical ambiguities, which we were able to identify and resolve. This analysis gives an unprecedented insight in the interplay of 2 π and 3 π dynamics in the process. As an example we show results for a spin-exotic wave$J_{{X^ - }}^{PC} = {1^{ - + }}$wave.

Its cornerstone were studies of hadron production in deep inelastic scattering (DIS), which can be interpreted in the transverse-momentum-dependent (TMD) factorisation framework, allowing to access the distributions of polarisation and transverse momentum of quarks within the nucleon in the language of TMD PDFs, and the hadronisation in terms of TMD fragmentation functions. The data collected with a liquid hydrogen target in 2016-2017 will soon bring new information on the transverse momentum and may allow for the first time to extract the transverse polarisation of quarks within an unpolarised nucleon described by the Boer-Mulders function. The unique data collected with a transversely polarised deuteron target have already improved the knowledge of the d-quark transversity (transverse counterpart of the helicity PDF), reducing the uncertainties by a factor of 2.5 at large Bjorken x, and are yet to yield a number of interesting results.

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 a 1 (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.

While the spectrum of non-strange light mesons is well known, many predicted strange mesons have not yet been observed, and many potentially observed states require further confirmation. Using the \\(K^-\\) component of the hadron beam at the M2 beamline at CERN, we study the strange-meson spectrum with the COMPASS experiment. The flagship channel is the \\(K^-^-^+\\) final state, for which COMPASS has obtained the world's largest sample. Based on this sample, we have performed the most detailed and comprehensive partial-wave analysis of this final state to date. For example, we observe a clear signal from the well-known \\(K_2^*(1430)\\), and for the first time we study the \\(K_2(1770)\\), \\(K_2(1820)\\), and \\(K_2(2250)\\) in a single analysis. We also find evidence for a supernumerary signal called \\(K(1630)\\), suggesting that this signal is a pseudoscalar exotic strange meson.

The goal of the COMPASS experiment at CERN is to study the structure and dynamics of hadrons. The two-stage spectrometer used by the experiment has large acceptance and covers a wide kinematic range for charged as well as neutral particles and can therefore measure a wide range of reactions. The spectroscopy of light mesons is performed with negative (mostly π−) and positive (p, π+) hadron beams with a momentum of 190 GeV/c. The light-meson spectrum is measured in different final states produced in diffractive dissociation reactions with squared four-momentum transfer t to the target between 0.1 and 1.0 (GeV=c)2. The flagship channel is the π−π−π+ final state, for which COMPASS has recorded the currently world’s largest data sample. These data not only allow to measure the properties of known resonances with high precision, but also to observe new states. Among these is a new axial-vector signal, the a1(1420), with unusual properties. Novel analysis techniques have been developed to extract also the amplitude of the π−π+ subsystem as a function of 3π mass from the data. The findings are confirmed by the analysis of the π−π0π0 final state.

The COMPASS Collaboration has recently published an article \"Multiplicities of positive and negative pions, kaons, and unidentified hadrons from deep-inelastic scattering of muons off a liquid hydrogen target\", Phys. Rev. D 112 (2025) 012002. In contrast to earlier COMPASS publications on similar topics, the aforementioned article features an enhanced treatment of QED radiative corrections, employing the DJANGOH Monte Carlo generator. This methodological improvement led to corrections that are up to 12% larger in the low-x, high-z region compared to the previously applied ones. To ensure consistent treatment of COMPASS data sets obtained using both isoscalar and proton targets, this paper presents an updated set of isoscalar multiplicities based on DJANGOH-derived radiative corrections. The present results supersede those published in Phys. Lett. B 764 (2017) 1 and Phys. Lett. B 767 (2017) 133.