The first observation of the η 1pt '(958)π^ + π^ - system in the charge-exchange reaction of pions at the VES setup of the U-70 accelerator is reported. The preliminary results of the study are presented. The invariant mass spectrum has a wide maximum at about 2.2 GeV. The η 1pt 'ρ^0 intermediate state dominates. The cross section is measured for the π^ - Be→ A 1ptη 1pt 'π^ + π^ - reaction, where A consists of nine nucleons (three protons and six neutrons) or eight nucleons and Δ (1232) isobar.

The reaction π^ - Be→A 1ptω (782)ϕ (1020) is studied at the VES setup. In the ωϕ mass spectrum, the peak near the threshold with quantum numbers J^PC = 0^ + + dominates. It is identified as f_0(1710) . The reaction cross section is measured and the product Br(f_0→ππ )Br(f_0→ωϕ ) is obtained in the one-pion-exchange approximation. The value obtained leads to a large partial width of the radiative decay J/ψ→γf_0 , which is the evidence of a significant glueball component for this resonance.

The status of the study of 2403 ± 98 events of the π^ - + Be→ ϕπ^0 + A 1pt^* reaction collected in the exposition of a Be target of the VES setup to 2.5 ×10^11 π^ - mesons with a momentum of 29 GeV/c is presented. The ϕπ^0 production in the vector state predominantly associated with the Δ (1232) isobar in the process with π -exchange dominance is observed. The ϕπ^0 mass spectrum has a complicated shape.

The charge-exchange reaction π ^- Be→A ω (782)ϕ (1020) with ω→π ^+π ^-π ^0 and ϕ→ K^+K^- is studied with the upgraded VES facility (U-70, Protvino) using a 29 GeV pion beam. The distribution of the ωϕ invariant mass shows a near-threshold enhancement. Partial-wave analysis reveals that an isoscalar scalar state with J^PC=0^++ dominates in this mass region. Considering the observed signal as an f_0 resonance, and using the one-pion-exchange model, the product of the branching fractions into two channels Br(f_0→ππ ) Br(f_0→ωϕ ) is measured. Based on this value, a large partial width is found for the radiative decay of J/ψ into this state, which suggests a significant glueball component. The result is discussed under alternative assumptions concerning an identification of the observed signal as f_0(1710) or f_0(1770) .

The charge-exchange reaction π^-p → n ω(783)ϕ(1020), ω→π^+π^-π^0, ϕ→ K^+K^- is studied with the upgraded VES facility (U-70, Protvino) in the interaction of a 29 GeV pion beam with a beryllium target. The distribution over the invariant mass of the system M_ωϕ shows a near-threshold signal. A partial wave analysis reveals that the scalar state (J^PC=0^++) dominates in this mass region. The observed signal can be described with a contribution of the known resonance f_0(1710). Using OPE approximation for the reaction π^-p → n f_0(1710) the product of branching fractions is found to be: Br(f_0(1710)→ππ)· Br(f_0(1710)→ωϕ) = (4.8 ± 1.2) · 10^-3.

We present the study of the production of double J/ψ mesons using COMPASS data collected with a 190 GeV/c π^- beam scattering off NH_3, Al and W targets. Kinematic distributions of the collected double J/ψ events are analysed, and the double J/ψ production cross section is estimated for each of the COMPASS targets. The results are compared to predictions from single- and double-parton scattering models as well as the pion intrinsic charm and the tetraquark exotic resonance hypotheses. It is demonstrated that the single parton scattering production mechanism gives the dominant contribution that is sufficient to describe the data. An upper limit on the double intrinsic charm content of pion is evaluated. No significant signatures that could be associated with exotic tetraquarks are found in the double J/ψ mass spectrum.

The charge-exchange reaction $\pi^-p \rightarrow n\,\omega(783)\phi(1020)$, $\omega \rightarrow \pi^+\pi^-\pi^0$, $\phi \rightarrow K^+K^-$ is studied with the upgraded VES facility (U-70, Protvino) in the interaction of a 29 GeV pion beam with a beryllium target. The distribution over the invariant mass of the system $M_{\omega\phi}$ shows a near-threshold signal. A partial wave analysis reveals that the scalar state ($J^{PC}=0^{++}$) dominates in this mass region. The observed signal can be described with a contribution of the known resonance $f_0(1710)$. Using OPE approximation for the reaction $\pi^-p \rightarrow n\, f_0(1710)$ the product of branching fractions is found to be: $Br(f_0(1710)\rightarrow \pi\pi)\cdot Br(f_0(1710)\rightarrow \omega\phi) = (4.8 \pm 1.2) \cdot 10^{-3}$.

We study the spin-exotic $J^{PC} = 1^{-+}$ amplitude in single-diffractive dissociation of 190 GeV$/c$ pions into $\pi^-\pi^-\pi^+$ using a hydrogen target and confirm the $\pi_1(1600) \to \rho(770) \pi$ amplitude, which interferes with a nonresonant $1^{-+}$ amplitude. We demonstrate that conflicting conclusions from previous studies on these amplitudes can be attributed to different analysis models and different treatment of the dependence of the amplitudes on the squared four-momentum transfer and we thus reconcile their experimental findings. We study the nonresonant contributions to the $\pi^-\pi^-\pi^+$ final state using pseudo-data generated on the basis of a Deck model. Subjecting pseudo-data and real data to the same partial-wave analysis, we find good agreement concerning the spectral shape and its dependence on the squared four-momentum transfer for the $J^{PC} = 1^{-+}$ amplitude and also for amplitudes with other $J^{PC}$ quantum numbers. We investigate for the first time the amplitude of the $\pi^-\pi^+$ subsystem with $J^{PC} = 1^{--}$ in the $3\pi$ amplitude with $J^{PC} = 1^{-+}$ employing the novel freed-isobar analysis scheme. We reveal this $\pi^-\pi^+$ amplitude to be dominated by the $\rho(770)$ for both the $\pi_1(1600)$ and the nonresonant contribution. We determine the $\rho(770)$ resonance parameters within the three-pion final state. These findings largely confirm the underlying assumptions for the isobar model used in all previous partial-wave analyses addressing the $J^{PC} = 1^{-+}$ amplitude.

Based on the observation of sizeable target-transverse-spin asymmetries in single-hadron and hadron-pair production in Semi-Inclusive measurements of Deep Inelastic Scattering (SIDIS), the chiral-odd transversity quark distribution functions h1q are nowadays well established. Several possible channels to access these functions were originally proposed. One candidate is the measurement of the polarisation of Λ hyperons produced in SIDIS off transversely polarised nucleons, where the transverse polarisation of the struck quark might be transferred to the final-state hyperon. In this article, we present the COMPASS results on the transversity-induced polarisation of Λ and Λ¯ hyperons produced in SIDIS off transversely polarised protons. Within the experimental uncertainties, no significant deviation from zero was observed. The results are discussed in the context of different models taking into account previous experimental results on h1u and h1d.

We study the spin-exotic $J^{PC} = 1^{-+}$ amplitude in single-diffractive dissociation of 190 GeV$/c$ pions into $\pi^-\pi^-\pi^+$ using a hydrogen target and confirm the $\pi_1(1600) \to \rho(770) \pi$ amplitude, which interferes with a nonresonant $1^{-+}$ amplitude. We demonstrate that conflicting conclusions from previous studies on these amplitudes can be attributed to different analysis models and different treatment of the dependence of the amplitudes on the squared four-momentum transfer and we thus reconcile their experimental findings. We study the nonresonant contributions to the $\pi^-\pi^-\pi^+$ final state using pseudo-data generated on the basis of a Deck model. Subjecting pseudo-data and real data to the same partial-wave analysis, we find good agreement concerning the spectral shape and its dependence on the squared four-momentum transfer for the $J^{PC} = 1^{-+}$ amplitude and also for amplitudes with other $J^{PC}$ quantum numbers. We investigate for the first time the amplitude of the $\pi^-\pi^+$ subsystem with $J^{PC} = 1^{--}$ in the $3\pi$ amplitude with $J^{PC} = 1^{-+}$ employing the novel freed-isobar analysis scheme. We reveal this $\pi^-\pi^+$ amplitude to be dominated by the $\rho(770)$ for both the $\pi_1(1600)$ and the nonresonant contribution. We determine the $\rho(770)$ resonance parameters within the three-pion final state. These findings largely confirm the underlying assumptions for the isobar model used in all previous partial-wave analyses addressing the $J^{PC} = 1^{-+}$ amplitude.

The COMPASS Collaboration experiment recently discovered a new isovector resonancelike signal with axial-vector quantum numbers, the a_{1}(1420), decaying to f_{0}(980)π. With a mass too close to and a width smaller than the axial-vector ground state a_{1}(1260), it was immediately interpreted as a new light exotic meson, similar to the X, Y, Z states in the hidden-charm sector. We show that a resonancelike signal fully matching the experimental data is produced by the decay of the a_{1}(1260) resonance into K^{*}(→Kπ)K[over ¯] and subsequent rescattering through a triangle singularity into the coupled f_{0}(980)π channel. The amplitude for this process is calculated using a new approach based on dispersion relations. The triangle-singularity model is fitted to the partial-wave data of the COMPASS experiment. Despite having fewer parameters, this fit shows a slightly better quality than the one using a resonance hypothesis and thus eliminates the need for an additional resonance in order to describe the data. We thereby demonstrate for the first time in the light-meson sector that a resonancelike structure in the experimental data can be described by rescattering through a triangle singularity, providing evidence for a genuine three-body effect.

A 32-channel 12-bit ADC-32ATC analog-to-digital conversion module with signal shape digitization at 40 MHz, which was been developed for the VES experiment at the U-70 accelerator, is presented. The analog shapers for this module, the measurement methods, the data-processing techniques, and the experience gained in using this module in the VES experiment are described. The module is intended to register single pulses with a stable shape or a linear combination of a few such pulses with positive coefficients. It can be used for calorimeters, as well as for Cherenkov and scintillation counters, in fixed-target experiments in high-energy physics.

Hadron leptoproduction in Semi-Inclusive measurements of Deep-Inelastic Scattering (SIDIS) on unpolarised nucleons allows one to get information on the intrinsic transverse momentum of quarks in a nucleon and on the Boer-Mulders function through the measurement of azimuthal modulations in the cross section. These modulations were recently measured by the HERMES experiment at DESY on proton and deuteron targets, and by the COMPASS experiment using the CERN SPS muon beam and a 6LiD target. In both cases, the amplitudes of the cos⁡ϕh and cos⁡2ϕh modulations show strong kinematic dependences for both positive and negative hadrons. It has been known since some time that the measured final-state hadrons in those SIDIS experiments receive a contribution from exclusive diffractive production of vector mesons, particularly important at large values of z, the fraction of the virtual photon energy carried by the hadron. In previous measurements of azimuthal asymmetries this contribution was not taken into account, because it was not known that it could distort the azimuthal modulations. Presently, a method to evaluate the contribution of the exclusive reactions to the azimuthal asymmetries measured by COMPASS has been developed. The subtraction of this contribution results in a better understanding of the kinematic effects, and the remaining non-zero cos⁡2ϕh modulation gives indication for a non-zero Boer-Mulders effect.

The antiparticle-over-particle multiplicity ratio is measured in deep-inelastic scattering for negatively and positively charged kaons and, for the first time, for antiprotons and protons. The data were obtained by the COMPASS Collaboration using a 160 GeV muon beam impinging on an isoscalar 6LiD target. The regime of deep-inelastic scattering is ensured by requiring Q2 > 1 (GeV/c)2 for the photon virtuality and W>5 GeV/c2 for the invariant mass of the produced hadronic system. Bjorken-x is restricted to the range 0.01 to 0.40. Protons and antiprotons are identified in the momentum range from 20 GeV/c to 60 GeV/c and required to carry a large fraction of the virtual-photon energy, z>0.5. In the whole studied z-region, the p¯ over p multiplicity ratio is found to be below the lower limit expected from calculations based on leading-order perturbative Quantum Chromodynamics (pQCD). Kaons were previously analysed in the momentum range 12 GeV/c to 40 GeV/c. In the present analysis this range is extended up to 55 GeV/c, whereby events with larger virtual-photon energies are included in the analysis and the observed K− over K+ ratio becomes closer to the expectation of next-to-leading order pQCD. The results of both analyses strengthen our earlier conclusion that at COMPASS energies the phase space available for single-hadron production in deep-inelastic scattering should be taken into account in the standard pQCD formalism.

Data taken by the DELPHI experiment at centre-of-mass energies of 183 GeV and 189 GeV with a total integrated luminosity of 212 pb^{-1} have been used to search for the supersymmetric partners of the electrons, muons, and taus in the context of the Minimal Supersymmetric Standard Model (MSSM). The decay topologies searched for were the direct decay ({\tilde \ell} -> \ell {\tilde \chi_1^0}), producing acoplanar lepton pairs plus missing energy, and the cascade decay ({\tilde \ell} -> \ell {\tilde \chi_2^0} > \ell \gamma {\tilde \chi_1^0}), producing acoplanar lepton and photon pairs plus missing energy. The observed number of events is in agreement with Standard Model predictions. The 95% CL excluded mass limits for selectrons, smuons and staus are m_{\tilde {e}} \leq 87 GeV/c^2, m_{\tilde {\mu}} \leq 80 GeV/c^2 and m_{\tilde {\tau}} \leq 75 GeV/c^2, respectively, for values of \mu=-200 GeV/c^2 and tan(beta)=1.5.

We report on a measurement of hard exclusive pi(0) muoproduction on the proton by COMPASS using 160GeV/cpolarised mu+ and mu- beams of the CERN SPS impinging on a liquid hydrogen target. From the average of the measured mu+ and mu- cross sections, the virtual-photon proton cross section is determined as a function of the squared four-momentum transfer between initial and final proton in the range 0.08 (GeV/c)(2) <vertical bar t vertical bar < 0.64 (GeV/c)(2). The average kinematics of the measurement are Q(2) = 2.0 (GeV/c)(2), nu = 12.8 GeV, x(Bj) = 0.093and -t = 0.256 (GeV/c)(2). Fitting the azimuthal dependence reveals a combined contribution by transversely and longitudinally polarised photons of (8.2 +/- 0.9(stat-1.2)(+1.2)vertical bar(sys)) nb/(GeV/c)(2), as well as transverse-transverse and longitudinal-transverse interference contributions of (-6.1 +/- 1.3(stat- 0.7)(+0.7) vertical bar sys) nb/(GeV/c)(2) and (1.5 +/- 0.5(stat-0.2)(+ 0.3) vertical bar(sys)) nb/(GeV/c)(2), respectively. Our results provide important input for modelling Generalised Parton Distributions. In the context of the phenomenological Goloskokov-Kroll model, the statistically significant transversetransverse interference contribution constitutes clear experimental evidence for the chiral-odd GPD ET. (C) 2020 The Author(s). Published by Elsevier B.V.

The transverse spin asymmetries measured in semi-inclusive leptoproduction of hadrons, when weighted with the hadron transverse momentum $P_T$, allow for the extraction of important transverse-momentum-dependent distribution functions. In particular, the weighted Sivers asymmetries provide direct information on the Sivers function, which is a leading-twist distribution that arises from a correlation between the transverse momentum of an unpolarised quark in a transversely polarised nucleon and the spin of the nucleon. Using the high-statistics data collected by the COMPASS Collaboration in 2010 with a transversely polarised proton target, we have evaluated two types of $P_T$-weighted Sivers asymmetries, which are both proportional to the product of the firsttransverse moment of the Sivers function and of the fragmentation function. The results are compared to the standard unweighted Sivers asymmetries and used to extract the first transverse moments of the Sivers distributions for $u$ and $d$ quarks.