abstract

Axial U(1) dynamics are characterised by large OZI violations. Here we review the phenomenology of \(\eta \) and \(\eta '\) production and decay processes, and its connection to the anomalous glue that generates a large part of the masses of these pseudoscalar mesons.

abstract

The production of \(\eta \) and \(\eta ^\prime \) mesons in photon- and hadron-induced reactions are studied in a combined analysis based on an effective Lagrangian approach in order to learn about the relevant production mechanisms and the possible role of nucleon resonances in these reactions.

abstract

The measurement of the quasi-free production of the \(\eta ^{\prime }\) meson has been carried out at the COSY-11 detection setup using a proton beam and a deuteron cluster target. The energy dependence of the cross-section is extracted using a fixed proton beam momentum of \(p_{\rm beam}=3.35\) GeV/\(c\) and exploiting the Fermi momenta of nucleons inside deuterons. The data cover a range of centre-of-mass excess energies from 0 to 24 MeV. Due to the low statistics at the present stage only upper limits for the cross-sections could be extracted. In this article we focus on the functioning and efficiency determination of the neutron detector.

abstract

Based on recent COSY-11 results of measurements of total cross- sections for the quasi-free \(pn\to pn\eta \) reaction we determine the isospin \(I=0\) component of the total cross-section for the \(NN\to NN\eta \) reaction down to the threshold. We show that the energy dependence of the total cross-section ratios \({\sigma _{I\,=\,0}(pn\to pn\eta )}/{\sigma (pp\to pp\eta )}\) and \({\sigma _{I\,=\,0}(pn\to pn\eta )}/\) \({\sigma (pn \!\to \!d\eta )}\) can be described using the Fäldt and Wilkin analytical parametrization of the nucleon–nucleon final state interaction.

abstract

We present a comparison of the two-body invariant mass distributions for the \(pp \to pp\eta \) and \(pp \to pp\eta '\) reactions, both measured at a nominal excess energy value of \(Q = 15.5\) MeV. For the \(pp \to pp\eta \) reaction, in addition, the differential cross-sections were extracted for an excess energy of \(Q = 10\) MeV. The comparison of the results for the \(\eta \) and \(\eta ^{\prime }\) meson production rather excludes the hypothesis that the enhancement observed in the invariant mass distributions is due to the interaction of the meson and the proton.

abstract

Photoproduction of mesons off the deuteron has been measured at a tagged photon beam of the Bonn ELSA electron accelerator with the combined Crystal Barrel–TAPS electromagnetic calorimeter for incident photon energies up to 2.5 GeV. The mesons have been detected in coincidence with recoil protons, neutrons and deuterons. This allows the investigation of meson production reactions off the quasifree nucleons bound in the deuteron, as well as coherent production off the deuteron. The comparison of the quasifree proton data to free proton data can pin down possible nuclear effects on the extracted cross-section data. The comparison of quasifree proton and neutron data is then used for the extraction of the isospin structure of the electromagnetic excitation. The quasifree data have been analyzed for the production of \(\eta \), \(\eta '\), \(\pi ^0\pi ^0\), and \(\pi ^0\eta \) mesons. Contributions from coherent production have been observed for \(\pi ^0\eta \) and \(\pi ^0\omega \) pairs.

abstract

In this paper the mixing angle and the \(\eta '\) gluonium content extraction from the \(R_{\phi } = \)Br\((\phi {\rm (1020)} \to \eta ' \gamma )/\)Br\((\phi {\rm (1020)} \to \eta \gamma )\) is updated. The \(\eta '\) gluonium content is estimated by fitting \(R_{\phi }\), together with radiative light vector to pseudoscalar gamma and pseudoscalar to vector transitions plus \(\eta ' \to \gamma \gamma \) and \(\pi ^0 \to \gamma \gamma \) decays. The extracted parameters are the gluonium fraction \(Z^2_G = 0.11 \pm 0.04\) and the pseudoscalar mixing angle \(\varphi _P = (40.7 \pm 0.7)^\circ \).

abstract

The \(\eta \)–\(\eta ^\prime \) pseudoscalar mixing angle and the gluonium content of the \(\eta ^\prime \) meson are deduced from an updated phenomenological analysis of \(V\to P\gamma \) and \(J/\psi \to VP\) decays. In absence of gluonium, the value of the mixing angle in the quark-flavour basis is found to be \(\phi _P=(41.5\pm 1.2)^\circ \) from \(V\to P\gamma \) and \(\phi _P=(40.5\pm 2.4)^\circ \) from \(J/\psi \to VP\). In presence of gluonium, the values for the mixing angle and the gluonic content of the \(\eta ^\prime \) wave function are \(\phi _P=(41.4\pm 1.3)^\circ \) and \(Z^2_{\eta ^\prime }=0.04\pm 0.09\) from \(V\to P\gamma \) and \(\phi _P=(44.5\pm 4.3)^\circ \) and \(Z^2_{\eta ^\prime }=0.28\pm 0.21\) from \(J/\psi \to VP\), respectively.

abstract

We describe the experimental method and consecutive steps of the analysis of the \(pp\to pp\eta '\) reaction measured by means of the COSY-11 detection setup. The conducted investigation aim at the determination of the total width of the \(\eta '\) meson directly from its mass distribution. The preliminary results show that the statistical error is in the order of \(\approx 10\) keV.

abstract

Two coupled channel effects connected with kaon pair production in proton-proton collisions are discussed. (1) Although there is ample evidence that the antikaon is strongly attracted to the recoil protons in \(pp\to K^+p\,\{K^-p\}\), residual effects of the \(K^+K^-\) interaction are seen, including a possible cusp at the \(K^0\bar {K}^0\) threshold. This is investigated within a simple \(K\)-matrix approach. (2) The production rates and invariant mass distributions for \(pp\to K^+p\,\{K^-p\}\) and \(pp\to K^+p\,\{\pi ^0{\mit \Sigma }^0\}\) are related using a separable potential description of the coupled \(K^-p/\pi ^0{\mit \Sigma }^0\) channels. It can be plausibly argued that this pair of reactions is driven through the production of the \({\mit \Lambda }(1405)\).

abstract

The near threshold production of \(K^{+}K^{-}\) pairs in proton–proton collisions has been investigated at the cooler synchrotron COSY below and above the threshold for the \(\phi \) meson. The experimental excitation function determined for the \(pp\rightarrow ppK^{+}K^{-}\) reaction differs from theoretical expectations including proton–proton final state interaction. The discrepancy may be assigned to the influence of \(K^{+}K^{-}\) or \(pK^{-}\) interaction. In this article we present distributions of the cross-section for the \(pp\rightarrow ppK^{+}K^{-}\) reaction as a function of the invariant masses of two and three particle subsystems at excess energies of \(Q=10\) MeV and 28 MeV.

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Lambda hyperon production via the reaction channel \(\vec {p} \,p \rightarrow p\,K^+ {\mit \Lambda }\) has been measured with a transverse polarised proton beam at an excitation energy of 40 MeV using the COSY-11 detection setup at the cooler synchrotron COSY in Jülich. By selecting events with kaon emission angles around 90\(^{\circ }_{\rm cm}\) the combination of differential cross-section and analysing power may allow to extract the pure spin triplet \(p {\mit \Lambda }\) scattering length. The preliminary analysis indicates an analysing power close to zero but the data are still under analysis. In addition to the hyperon production also \(\eta \) production data at 164 MeV excess energy will be extracted.

abstract

Investigations of the couplings of the scalar mesons \(f_0(980)\) and \(a_0(980)\) to pions and kaons are presented and discussed, from different analyses of an integrated luminosity of \(\sim 400\) pb\(^{-1}\) of KLOE data.

abstract

Formation of \(\eta \)-mesic nucleus, a bound state of an \(\eta \) meson in a nucleus, is reviewed in this paper. Three different theoretical approaches are used to calculate the binding energies and widths of such nuclei. The effect of \(\eta \)-mesic nucleus in pion double-charge-exchange reaction is discussed. Experimental efforts by different groups to detect the nucleus are also discussed. The ramifications of the theoretical and experimental studies of the bound state of \(\eta \) in a nucleus are pointed out.

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Excitation function as well as differential distributions for \(dp \rightarrow ppp \pi ^-\) reaction have been measured near the \(\eta \) production threshold in a search for a signal from decays of \(^3\)He–\(\eta \) bound state. We observe an enhancement in the excitation function above the \(\eta \) threshold. Calculation of the counting rate corresponding to the production of \(\eta \) mesons in the reaction \(dp \rightarrow ^3\)He \(\eta \) and their subsequent absorption on neutron in the \(^3\)He nucleus underestimates the observed enhancement by one order of magnitude. Also the differential distributions determined for the outgoing particles contradict the \(\eta \) absorption scenario.

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We conduct a search for the \(^4\)He–\(\eta \) bound state with the WASA-at-COSY facility via the measurement of the excitation function for the reaction \(dd \rightarrow ^3\)He \(p\,\pi ^-\). In first experiment performed in June 2008, we used COSY deuteron beam with a slowly ramped beam momentum corresponding to a variation of the excess energy for the \(^4\)He \(\eta \) system from \(-51.4\) MeV to 22 MeV. Here we report on the status of the measurement and the data evaluation.

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The production of \(\eta \) and \(\eta '\) mesons in nucleon–nucleon collisions near thresholds is considered within an one-boson exchange model. We show the feasibility of an experimental access to transition form factors.

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Eta meson production in the proton–proton scattering is dominated by the low-mass meson exchange. We present a brief study on how the type of the exchanged mesons within coupled-channel and multi-resonance model influences the scattering observables. We show under which circumstances the eta exchange may explain the shape of the observed cross-sections, and present a few selected results: total cross-section in the full energy range, and the proton–proton energy distribution at 15.5 MeV.

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Results of the near threshold measurements of the \(\eta \)-meson production in the \(dp\rightarrow dp\eta \) reaction are presented. The experiment was performed at the COSY-Jülich accelerator with the use of the COSY-11 apparatus. Data were taken for three values of deuteron beam momenta corresponding to excess energies of 3.1, 6.0 and 9.1 MeV for \(dp\eta \) reaction channel. Preliminary results were reported at the MESON-06 conference. Here we presented cross-section values re-examined taking into account the newest value of the \(\eta \)-meson mass. The energy dependence of the total cross-section confirms a strong effect of the final state interaction.

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A realistic estimate of the cusp effect in the \(\eta \rightarrow 3\pi ^{0}\) decay is required for the forthcoming high precision experiments. An estimate for the size of this effect was given recently within the framework of non-relativistic effective field theory. Here we present the next step towards the systematic way of fixing the parameters of the effective non-relativistic Lagrangian and consistency tests of the existing data on \(\eta \to \pi ^{0}\pi ^{0}\pi ^{0}\) and \(\eta \to \pi ^{+}\pi ^{-}\pi ^{0}\) decays.

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We describe experimental studies of the decays \(\eta \rightarrow \pi ^+\pi ^-\gamma ^{(*)}\), with the goal to test QCD anomalies and to search for flavor conserving CP violation.