The multiplicities of positive and negative pions, kaons and unidentified hadrons produced in deep-inelastic scattering are measured in bins of the Bjorken scaling variable x, the relative virtual-photon energy y and the fraction of the virtual-photon energy transferred to the final-state hadron z. Data were obtained by the COMPASS Collaboration using a 160 GeV muon beam of both electric charges and a liquid hydrogen target. These measurements cover the kinematic domain with photon virtuality Q^2 > 1 (GeV/c)^2, 0.004 < x < 0.4, 0.1 < y < 0.7 and 0.2 < z < 0.85, in accordance with the kinematic domain used in earlier published COMPASS multiplicity measurements with an isoscalar target. The calculation of radiative corrections was improved by using the Monte Carlo generator DJANGOH, which results in up to 12% larger corrections in the low-x region.

We report the measurement of the helicity asymmetry E for the pπ^{0} and nπ^{+} final states using, for the first time, an elliptically polarized photon beam in combination with a longitudinally polarized target at the Crystal Ball experiment at MAMI. The results agree very well with data that were taken with a circularly polarized photon beam, showing that it is possible to simultaneously measure polarization observables that require linearly (e.g., G) and circularly polarized photons (e.g., E) and a longitudinally polarized target. The new data cover a photon energy range 270-1400 MeV for the pπ^{0} final state (230-842 MeV for the nπ^{+} final state) and the full range of pion polar angles, θ, providing the most precise measurement of the observable E. A moment analysis gives a clear observation of the pη cusp in the pπ^{0} final state.

New results are presented on a high-statistics measurement of Collins and Sivers asymmetries of charged hadrons produced in deep inelastic scattering of muons on a transversely polarised $^6$LiD target. The data were taken in 2022 with the COMPASS spectrometer using the 160 \gevv\ muon beam at CERN, balancing the existing data on transversely polarised proton targets. The first results from about two-thirds of the new data have total uncertainties smaller by up to a factor of three compared to the previous deuteron measurements. Using all the COMPASS proton and deuteron results, both the transversity and the Sivers distribution functions of the $u$ and $d$ quark, as well as the tensor charge in the measured $x$-range are extracted. In particular, the accuracy of the $d$ quark results is significantly improved.

A set of measurements of azimuthal asymmetries in the production of pairs of identified hadrons in deep-inelastic scattering of muons on transversely polarised 6LiD (deuteron) and NH3 (proton) targets is presented. All available data collected in the years 2003–2004 and 2007/2010 with the COMPASS spectrometer using a muon beam of 160GeV/c at the CERN SPS were analysed. The asymmetries provide access to the transversity distribution functions via a fragmentation function that in principle may be independently obtained from e+e− annihilation data. Results are presented, discussed and compared to existing measurements as well as to model predictions. Asymmetries of π+π− pairs measured with the proton target as a function of the Bjorken scaling variable are sizeable in the range x>0.032, indicating non-vanishing transversity distribution and di-hadron interference fragmentation functions. As already pointed out by several authors, the small asymmetries of π+π− measured on the 6LiD target can be interpreted as indication for a cancellation of u and d-quark transversity distributions.

AbstractThe target asymmetry T, recoil asymmetry P, and beam-target double polarization observable H were determined in exclusive $$\pi ^0$$ π 0 and $$\eta $$ η photoproduction off quasi-free protons and, for the first time, off quasi-free neutrons. The experiment was performed at the electron stretcher accelerator ELSA in Bonn, Germany, with the Crystal Barrel/TAPS detector setup, using a linearly polarized photon beam and a transversely polarized deuterated butanol target. Effects from the Fermi motion of the nucleons within deuterium were removed by a full kinematic reconstruction of the final state invariant mass. A comparison of the data obtained on the proton and on the neutron provides new insight into the isospin structure of the electromagnetic excitation of the nucleon. Earlier measurements of polarization observables in the $$\gamma p \rightarrow \pi ^0 p$$ γ p → π 0 p and $$\gamma p \rightarrow \eta p$$ γ p → η p reactions are confirmed. The data obtained on the neutron are of particular relevance for clarifying the origin of the narrow structure in the $$\eta n$$ η n system at $$W = 1.68\ \textrm{GeV}$$ W = 1.68 GeV . A comparison with recent partial wave analyses favors the interpretation of this structure as arising from interference of the $$S_{11}(1535)$$ S 11 ( 1535 ) and $$S_{11}(1650)$$ S 11 ( 1650 ) resonances within the $$S_{11}$$ S 11 -partial wave.

The production of vector mesons in deep inelastic scattering is an interesting yet scarcely explored channel to study the transverse spin structure of the nucleon and the spin-dependence of fragmentation. The COMPASS collaboration has performed the first measurement of the Collins and Sivers asymmetries for inclusively produced rho(0) mesons. The analysis is based on the data set collected in deep inelastic scattering in 2010 using a 160 GeV/c mu(+) beam impinging on a transversely polarized NH3 target. The rho(0) mesons are selected from oppositely charged hadron pairs, and the asymmetries are extracted as a function of the Bjorken-xvariable, the transverse momentum of the pair and the fraction of the energy z carried by the pair. Indications for positive Collins and Sivers asymmetries are observed. (c) 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP(3).

The target asymmetry T, recoil asymmetry P, and beam-target double polarization observable H were determined in exclusive π0 and η photoproduction off quasi-free protons and, for the first time, off quasi-free neutrons. The experiment was performed at the electron stretcher accelerator ELSA in Bonn, Germany, with the Crystal Barrel/TAPS detector setup, using a linearly polarized photon beam and a transversely polarized deuterated butanol target. Effects from the Fermi motion of the nucleons within deuterium were removed by a full kinematic reconstruction of the final state invariant mass. A comparison of the data obtained on the proton and on the neutron provides new insight into the isospin structure of the electromagnetic excitation of the nucleon. Earlier measurements of polarization observables in the γp→π0p and γp→ηp reactions are confirmed. The data obtained on the neutron are of particular relevance for clarifying the origin of the narrow structure in the ηn system at W=1.68GeV. A comparison with recent partial wave analyses favors the interpretation of this structure as arising from interference of the S11(1535) and S11(1650) resonances within the S11-partial wave.

The COMPASS experiment at CERN used a upgraded transversely polarized deuteron target with a muon beam of 160 GeV/c for SIDIS measurements in 2022.The deuteron target material of 6 LiD installed in 3 cells was polarized by the DNP method with a 2.5 T magnet, a dilution refrigerator and three 70 GHz microwave oscillators.During the data-taking the target operates in a transversely oriented magnetic dipole field at 0.6 T. This frozen spin operation mode without DNP process leads to a slow depolarization.Nevertheless, it was important to reach 40 % polarization that is sufficient for spin asymmetry measurement.The newly installed 3 microwave oscillators played a significant role to increase the polarization.We obtained deuteron polarizations of more than 40 % in each cell for two days.The relaxation time of the deuteron polarizations during the data taking period was measured to be about 5000 h at 0.6 T and below 100 mK.

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.

We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive ρ ^0 meson muoproduction at COMPASS using 160 GeV/c polarised μ ^+ and μ ^- beams impinging on a liquid hydrogen target. The measurement covers the kinematic range 5.0 GeV/c^2 < W < 17.0 GeV/c^2, 1.0 (GeV/c)^2 < Q^2 < 10.0 (GeV/c)^2 and 0.01 (GeV/c)^2 < p_T^2 < 0.5 (GeV/c)^2. Here, W denotes the mass of the final hadronic system, Q^2 the virtuality of the exchanged photon, and p_T the transverse momentum of the ρ ^0 meson with respect to the virtual-photon direction. The measured non-zero SDMEs for the transitions of transversely polarised virtual photons to longitudinally polarised vector mesons (γ^*_T → V^_L) indicate a violation of s-channel helicity conservation. Additionally, we observe a dominant contribution of natural-parity-exchange transitions and a very small contribution of unnatural-parity-exchange transitions, which is compatible with zero within experimental uncertainties. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow one to evaluate in a model-dependent way the role of parton helicity-flip GPDs in exclusive ρ ^0 production.

The transversely polarized target (PT) of the COMPASS (NA58) collaboration at CERN has been used for Drell-Yan measurements in 2015 and 2018. The transverse spin structure of the proton has been studied using a negative pion beam and a solid ammonia target. Employing the dynamic nuclear polarization (DNP) method, proton polarization values of more than 80% have been routinely achieved after one day, at a homogeneous magnetic field of 2.5 T and using a He-3/He-4 dilution refrigerator. During the data-taking the target operates in a transversely oriented magnetic dipole field at 0.6 T. This so-called frozen spin operation mode without the DNP pumping process leads to a slow depolarization of the target material, which is further accelerated by the heat input of the pion beam, produced secondary particles and radiation damage effects to the target material. Ammonia has the highest resistance against radiation-induced depolarization among known solid target materials. The proton polarization has been measured by the nuclear magnetic resonance (NMR). Relaxation times of about 1100 h have been observed for the proton polarization resulting in an average polarization between 68% and 76% during about two weeks long data-taking periods. To achieve a systematic uncertainty of the polarization Delta P/P as low as 3.2% and a statistical one of less than 1.8% two large target cells with appropriate positioning of the NMR-coils have been built.

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.

PANDA (anti-Proton ANnihiliation at DArmstadt) is planned to be one of the four main experiments at the future international accelerator complex FAIR (Facility for Antiproton and Ion Research) in Darmstadt, Germany. It is going to address fundamental questions of hadron physics and quantum chromodynamics using cooled antiproton beams with a high intensity and and momenta between 1.5 and 15 GeV/c. PANDA is designed to reach a maximum luminosity of 2x10^32 cm^2 s. Most of the physics programs require an excellent particle identification (PID). The PID of hadronic states at the forward endcap of the target spectrometer will be done by a fast and compact Cherenkov detector that uses the detection of internally reflected Cherenkov light (DIRC) principle. It is designed to cover the polar angle range from 5{\deg} to 22{\deg} and to provide a separation power for the separation of charged pions and kaons up to 3 standard deviations (s.d.) for particle momenta up to 4 GeV/c in order to cover the important particle phase space. This document describes the technical design and the expected performance of the novel PANDA Disc DIRC detector that has not been used in any other high energy physics experiment (HEP) before. The performance has been studied with Monte-Carlo simulations and various beam tests at DESY and CERN. The final design meets all PANDA requirements and guarantees suffcient safety margins.

AbstractThe antiproton experiment PANDA at FAIR is designed to bring hadron physics to a new level in terms of scope, precision and accuracy. In this work, its unique capability for studies of hyperons is outlined. We discuss ground-state hyperons as diagnostic tools to study non-perturbative aspects of the strong interaction, and fundamental symmetries. New simulation studies have been carried out for two benchmark hyperon-antihyperon production channels: $${\bar{p}}p \rightarrow {\bar{\varLambda }}\varLambda $$ p ¯ p → Λ ¯ Λ and $${\bar{p}}p \rightarrow {\bar{\varXi }}^+\varXi ^-$$ p ¯ p → Ξ ¯ + Ξ - . The results, presented in detail in this paper, show that hyperon-antihyperon pairs from these reactions can be exclusively reconstructed with high efficiency and very low background contamination. In addition, the polarisation and spin correlations have been studied, exploiting the weak, self-analysing decay of hyperons and antihyperons. Two independent approaches to the finite efficiency have been applied and evaluated: one standard multidimensional efficiency correction approach, and one efficiency independent approach. The applicability of the latter was thoroughly evaluated for all channels, beam momenta and observables. The standard method yields good results in all cases, and shows that spin observables can be studied with high precision and accuracy already in the first phase of data taking with PANDA.

A measurement of the double-polarization observable $E$ for the reaction $\gamma p\to \pi^0 p$ is reported. The data were taken with the CBELSA/TAPS experiment at the ELSA facility in Bonn using the Bonn frozen-spin butanol (C$_4$H$_9$OH) target, which provided longitudinally-polarized protons. Circularly-polarized photons were produced via bremsstrahlung of longitudinally-polarized electrons. The data cover the photon energy range from $E_\gamma =600$~MeV to $E_\gamma =2310$~MeV and nearly the complete angular range. The results are compared to and have been included in recent partial wave analyses.

The reaction $\gamma p \rightarrow p \pi^0 \eta$ has been studied with the CBELSA/TAPS detector at the electron stretcher accelerator ELSA in Bonn for incident photon energies from threshold up to 3.1 GeV. This paper has been motivated by the recently claimed observation of a narrow structure in the M$_{N\eta}$ invariant mass distribution at a mass of 1678 MeV. The existence of this structure cannot be confirmed in the present work. Instead, for E$_{\gamma}$ = 1400 - 1500 MeV and the cut M$_{p\pi^0} \le 1190 $ MeV a statistically significant structure in the M$_{p\eta}$ invariant mass distribution near 1700 MeV is observed with a width of $\Gamma\approx 35$ MeV while the mass resolution is $\sigma_{res}$ = 5 MeV. Increasing the incident photon energy from 1420 to 1540 MeV this structure shifts in mass from $\approx 1700 $ to 1725 MeV; the width increases to about 50 MeV and decreases thereafter. The cross section associated with this structure reaches a maximum of $\approx$ 100 nb around E$_{\gamma} \approx$ 1490 MeV (W $\approx $ 1920 MeV), which coincides with the $p a_0$ threshold. Three scenarios are discussed which might be the origin of this structure in the M$_{p\eta}$ invariant mass distribution. The most likely interpretation is that it is due to a triangular singularity in the $\gamma p \rightarrow p a_0 \rightarrow p \pi^0 \eta$ reaction

The study of baryon excitation spectra provides insight into the inner structure of baryons. So far, most of the world-wide efforts have been directed towards N^* and spectroscopy. Nevertheless, the study of the double and triple strange baryon spectrum provides independent information to the N^* and spectra. The future antiproton experiment P ANDA will provide direct access to final states containing a pair, for which production cross sections up to μ b are expected in p̅ p reactions. With a luminosity of L=10^31 cm ^-2 s ^-1 in the first phase of the experiment, the expected cross sections correspond to a production rate of ∼ 10^6 events /day . With a nearly 4π detector acceptance, P ANDA will thus be a hyperon factory. In this study, reactions of the type p̅ p → ^+ ^*- as well as p̅ p → ^*+ ^- with various decay modes are investigated. For the exclusive reconstruction of the signal events a full decay tree fit is used, resulting in reconstruction efficiencies between 3 and 5

The antiproton experiment PANDA at FAIR is designed to bring hadron physics to a new level in terms of scope, precision and accuracy. In this work, its unique capability for studies of hyperons is outlined. We discuss ground-state hyperons as diagnostic tools to study non-perturbative aspects of the strong interaction, and fundamental symmetries. New simulation studies have been carried out for two benchmark hyperon-antihyperon production channels: $${\bar{p}}p \rightarrow {\bar{\varLambda }}\varLambda $$ p ¯ p → Λ ¯ Λ and $${\bar{p}}p \rightarrow {\bar{\varXi }}^+\varXi ^-$$ p ¯ p → Ξ ¯ + Ξ - . The results, presented in detail in this paper, show that hyperon-antihyperon pairs from these reactions can be exclusively reconstructed with high efficiency and very low background contamination. In addition, the polarisation and spin correlations have been studied, exploiting the weak, self-analysing decay of hyperons and antihyperons. Two independent approaches to the finite efficiency have been applied and evaluated: one standard multidimensional efficiency correction approach, and one efficiency independent approach. The applicability of the latter was thoroughly evaluated for all channels, beam momenta and observables. The standard method yields good results in all cases, and shows that spin observables can be studied with high precision and accuracy already in the first phase of data taking with PANDA.

The reaction $$\gamma p \rightarrow p \pi ^0 \eta $$ has been studied with the CBELSA/TAPS detector at the electron stretcher accelerator ELSA in Bonn for incident photon energies from threshold up to 3.1 GeV. This paper has been motivated by the recently claimed observation of a narrow structure in the $$M_{N\eta }$$ invariant mass distribution at a mass of $$1678\,\hbox {MeV}/\hbox {c}^2$$ . The existence of this structure cannot be confirmed in the present work. Instead, for $$E_{\gamma }$$ = 1400–1500 MeV and the cut $$M_{p\pi ^0} \le 1190 \,\hbox {MeV}/\hbox {c}^2$$ a statistically significant structure in the $$M_{p\eta }$$ invariant mass distribution near 1700 $$\hbox {MeV}/\hbox {c}^2$$ is observed with a width of $$\varGamma \approx 35$$ $$\hbox {MeV}/\hbox {c}^2$$ while the mass resolution is $$\sigma _{res}$$ = 5 $$\hbox {MeV}/\hbox {c}^2$$ . Increasing the incident photon energy from 1420 to 1540 MeV this structure shifts in mass from $$\approx $$ 1700 $$\hbox {MeV}/\hbox {c}^2$$ to $$\approx $$ 1725 $$\hbox {MeV}/\hbox {c}^2$$ ; the width increases to about 50 $$\hbox {MeV}/\hbox {c}^2$$ and decreases thereafter. The cross section associated with this structure reaches a maximum of $$\approx $$ 100 nb around $$E_{\gamma } \approx $$ 1490 MeV (W $$\approx $$ 1920 MeV), which coincides with the $$p a_0$$ threshold. Three scenarios are discussed which might be the origin of this structure in the $$M_{p\eta }$$ invariant mass distribution. The most likely interpretation is that it is due to a triangular singularity in the $$\gamma p \rightarrow p a_0 \rightarrow p \pi ^0 \eta $$ reaction.