abstract

Data on the production of \(\eta \) mesons on light nuclei near threshold are surveyed. The photoproduction of \(\eta {}^3\)He and elastic scattering from this nucleus are modelled in a multiple scattering scheme and the positions of the poles both here and in the scattering from neighbouring light nuclei are studied in an optical potential approach. The production of \(\eta {}^7\)Li is compared to that of \(\eta {}^3\)He within a cluster model. The decay of light \(\eta \)-mesic nuclei through pion or nucleon emission is also considered.

abstract

The properties of \(\eta '\)(958) meson at finite nuclear density is considered to have close connection with the effects of U\(_{\mathrm {A}}(1)\) anomaly at finite density. In this context, we are very much interested in the study of the \(\eta '\)(958)-mesic nuclei. As for the \(\eta \)-mesic nuclei, which have been studied for a long time, we think it is interesting and important to consider both effects of the coupling of \(\eta N\) system to \(N^*(1535)\) and of the mixing of \(\eta \) with \(\eta '\)(958). In this article, we briefly summarize recent results of our studies concerning with \(\eta '\)(958)- and \(\eta \)-mesic nuclei.

abstract

We discuss the role of QCD symmetries and confinement in understanding \(\eta \)- and \(\eta '\)-mesic nuclei. \(\eta \) and \(\eta '\) bound states in nuclei are sensitive to the flavour-singlet component in the meson. The bigger the singlet component, the more attraction and the greater the binding. Recent results on the \(\eta '\) mass in nuclei from the CBELSA/TAPS Collaboration are very similar to the prediction of the Quark Meson Coupling model. In the model, \(\eta \)–\(\eta '\) mixing induces a factor of two enhancement of the \(\eta \)-nucleon scattering length \(a_{\eta N}\) relative to the prediction with a pure octet \(\eta \) with real part about 0.8 fm.

abstract

Photoproduction of \(\eta \)-mesons has been measured for several light nuclei (\(^2\)H, \(^3\)He, \(^7\)Li) and photoproduction of \(\eta \pi \)-pairs was investigated for a deuteron target. The experiments were done at the tagged photon beam of the Mainz MAMI accelerator with the Crystal Ball/TAPS electromagnetic calorimeter. They aimed at two topics: the extraction of cross section data for the elementary reactions off neutrons and the investigation of the \(\eta \)-nucleus interactions, in view of \(\eta \)-mesic states. The main results are: the excitation function for the \(\gamma n\rightarrow n\eta \) reaction has a very narrow structure (width around 30 MeV) at final state invariant masses of 1670 MeV. Its nature is not yet understood. For the quasi-free production of \(\eta \pi \)-pairs, the analysis of all isospin channels (final states \(p\eta \pi ^0\), \(n\eta \pi ^0\), \(p\eta \pi ^-\), and \(n\eta \pi ^+\)) resulted in cross section ratios almost perfectly agreeing in the threshold region with a dominant contribution from the \(D_{33}\rightarrow \eta P_{33}(1232)\rightarrow \eta \pi N\) decay chain. The measurement of coherent \(\eta \)-production off \(^3\)He and \(^7\)Li nuclei confirmed a strong threshold enhancement accompanied by almost isotropic angular distributions for the \(^3\)He nucleus and much smaller deviations from PWIA approximations for the \(^7\)Li nucleus, supporting the special role of \(^3\)He as a candidate for \(\eta \)-mesic nucleus formation.

abstract

Differential and total cross sections for the \(dp \to \, ^{3}{\rm He}\,\eta \) reaction have been measured with both unpolarised and tensor/vector polarised deuteron beams at the COSY–ANKE experiment near threshold. The data have been taken by using a continuous beam energy ramp up to an excess energy \(Q\) of 10 MeV with essentially 100% acceptance of the detection system. The data allowed the determination of the energy dependence of the tensor analysing power \(t_{20}\) and hence of the two \(s\)-wave production amplitudes at threshold. The observed very weak energy dependence of these quantities supports the hypothesis that the rapid energy variation of the scattering amplitude for this reaction is dominantly given by a strong \(s\)-wave \(\eta ^3\)He final state interaction, which is discussed to be a signal for the existence of a possible quasi-bound eta-mesic state.

abstract

The connection of the binding energy and width of possible bound \(\eta \)-mesic states is given to the complex scattering length for \(s\)-states in the hope that, with knowledge of the final state interaction, this could be useful in searches of these states. In spite of the strong direct potential dependence of both observables, this connection is seen to be very model independent even for various nuclei once the influence of also the effective range is considered. The importance of this term is pointed also for data analyses. Although the nucleus considered here is \(^{12}\)C, extension to other nuclei is implied in the background work.

abstract

The role played by subthreshold meson–baryon dynamics is demonstrated in \(K^-\)-atom, \(\bar K\)-nuclear and \(\eta \)-nuclear bound-state calculations within in-medium models of \(\bar K N\) and \(\eta N\) interactions. New analyses of kaonic atom data reveal appreciable multi-nucleon contributions. Calculations of \(\eta \)-nuclear bound states show, in particular, that the \(\eta N\) scattering length is not a useful indicator of whether or not \(\eta \) mesons bind in nuclei nor of the widths anticipated for such states.

abstract

We search for an evidence of \(\eta \)-mesic \({^4\textrm {He}}\) with the WASA-at-COSY detector. Two dedicated experiments were performed at the Cooler Synchrotron COSY-Jülich. The experimental method is based on the measurement of the excitation functions for the two reaction channels: \(dd \rightarrow {^3\textrm {He}} p \pi ^{-}\) and \(dd \rightarrow {^3\textrm {He}} n \pi ^{0}\), where the outgoing \(N\)–\(\pi \) pairs originate from the conversion of the \(\eta \) meson on a nucleon inside the \({\textrm {He}}\) nucleus. In this contribution, the experimental method is shortly described and preliminary excitation functions from the 2010 data are presented.

abstract

The \(pp \to pp \eta \) reaction was investigated at excess energies of 15 MeV and 72 MeV using the azimuthally symmetric WASA detector and a polarized proton beam of the Cooler Synchrotron COSY. The aim of the studies is the determination of partial wave contributions to the production process of the \(\eta \) meson in nucleon–nucleon collisions. Here, we present preliminary results of the extraction of the position of the interaction region with respect to the WASA detector and preliminary results on the degree of polarization of the COSY proton beam used in the experiment.

abstract

The \(\eta \) meson can be bound to atomic nuclei. Experimental search is discussed in the form of final state interaction for the reactions \(dp\to {^3\text {He}}\eta \) and \(dd\to {^4\text {He}}\eta \). For the latter case, tensor polarized deuterons were used in order to extract the \(s\)-wave strength. For both reactions, complex scattering lengths are deduced: \(a_{{}\rm {^3He}\eta } = \left [ { \pm \left ( {10.7 \pm 0.8_{ - 0.5}^{ + 0.1} } \right ) + i \, \left ( {1.5 \pm 2.6_{ - 0.9}^{ + 1.0} } \right )} \right ]\) fm and \(a_{^4\text {He}\eta } = \left [ { \pm \left ( {3.1 \pm 0.5} \right ) + i \, \left ( {0 \pm 0.5} \right )} \right ]\) fm. For the reaction \(p^6\)Li\(\to ^7\)Be\(\eta \), more data in the threshold region are necessary to draw any conclusions. In a two-nucleon transfer reaction under quasi-free conditions, \(p^{27}\text {Al}\to {^3\text {He}}X\) was investigated. The system \(X\) can be the bound \(^{25}\text {Mg}\otimes \eta \) at rest. When a possible decay of an intermediate \(N^*(1535)\) is required, a highly significant bump shows up in the missing-mass spectrum. The data give for a bound state a binding energy of \(13.3\pm 1.6\) MeV and a width of \(\sigma =4.4\pm 1.3\) MeV.

abstract

Based on an effective Lagrangian approach, we performed a combined analysis of the free and quasi-free \(\gamma N\! \to \! \eta ' N\), \(NN\! \to \! NN\eta '\), and \(\pi N\! \to \! \eta ' N\) reactions. Considering spin-1/2 and -3/2 resonances, we found that a set of above-threshold resonances \(\{ S_{11}, P_{11}, P_{13}\}\), with fitted mass values of about \(M_R = 1925, 2130\), and \(2050\) MeV, respectively, and the four-star sub-threshold \(P_{13}(1720)\) resonance reproduce best all existing data for the \(\eta '\) production processes from threshold to \(\sqrt {s} \sim 2.35\) GeV. All three above threshold resonances found in the present analysis are essential and indispensable for the good quality of the present fits. A very small \(\eta ' N\) scattering length of \(a_{\eta ' N} \sim (0.017 + i 0.005)\) fm is obtained.

abstract

Meson–nucleus bound systems have been providing precious information on the meson properties in nuclear medium, which is leading to understanding of non-trivial structure of the QCD vacuum. Two related experimental projects are discussed. One is a pionic atom factory project at RIBF and the other is a spectroscopy of \(\eta ^\prime \)-mesic nuclei in GSI/FAIR. The former is aiming at high precision systematic spectroscopy of pionic atoms followed by challenges for pionic atoms with unstable nuclei. The latter is aiming at spectroscopy of \(\eta ^\prime \)-mesic nuclei in \((p,d)\) reactions by inclusive and semi-exclusive measurement.

abstract

We describe a new high precision measurement of the production cross section for the \(\eta '\) meson in proton–proton collisions \(\sigma _{pp\to pp\eta '}\) for five beam momenta at low access energy region \(Q\) conducted at the COSY-11 detection system together with an updated results of all other previous measurements of \(\sigma _{pp\to pp\eta '}\) at COSY-11.

abstract

Collisions in a system of two particles at energies close to a bound state in different channels are discussed. Next, the bound state decays into a third coupled channel. A phenomenological approach to \(dd\rightarrow \pi ^- p~^3\)He reaction is presented.

abstract

The AMADEUS experiment deals with the investigation of the low-energy kaon-nuclei hadronic interaction at the DA\(\Phi \)NE collider at LNF-INFN, which is fundamental to solve longstanding open questions in the non-perturbative strangeness QCD sector. AMADEUS step 0 consisted in the analysis of 2004–2005 KLOE data, exploring the \(K^-\) absorptions in H, \({}^4\)He, \({}^{9}\)Be and \({}^{12}\)C in-flight and at-rest. With AMADEUS step 1, a dedicated pure carbon target was implemented in the central region of the KLOE detector, providing a high statistic sample of pure at-rest \(K^-\) nuclear interaction. For the future, a dedicated setup to perform detailed studies is under preparation. Our present analysis deals with the major open questions in hadron nuclear physics in the strangeness sector, namely how hadron masses and interactions change in nuclear environment, with a search ranging from kaonic nuclear clusters to strange baryon resonances and in-medium modification of the resonance parameters.

abstract

We report on preliminary results of the J-PARC E15 experiment aiming to search for the simplest kaonic-nuclear bound-state, \(K^-pp\), via in-flight \(^3\)He(\(K^-, n\)) reaction. The first physics data-taking was performed with \(5 \times 10^9\) incident kaons on the \(^3\)He target, and \(3 \times 10^5\) neutrons were collected by a forward neutron counter placed 15 m away from center of the target at 0 degrees. The semi-inclusive neutron spectrum shows clear peak structure composed of the quasi-elastic \(K^-n \to K^-n\) and the charge exchange \(K^-p \to \bar {K^0}n\) reactions as expected.

abstract

In this article, studies of the near threshold \(pp\to ppK^+K^-\) reaction conducted with the COSY-11 and the ANKE detectors are reviewed. In particular, recent investigations on the \(K^+K^-\) final state interaction are revisited taking into account updated cross sections of the COSY-11 experiment. These studies resulted in the new value of \(K^+K^-\) effective range amounting to: \({\rm Re}(b_{K^{+}K^{-}}) = -0.2^{+0.8_{\rm stat}+0.4_{\rm sys}}_{-0.6_{\rm stat}-0.4_{\rm sys}}~{\rm fm}\) and \({\rm Im}(b_{K^{+}K^{-}}) = 1.2^{+0.5_{\rm stat}+0.3_{\rm sys}}_{-0.3_{\rm stat}-0.3_{\rm sys}}~{\rm fm}\). The determined real and imaginary parts of the \(K^+K^-\) scattering length were estimated to be: \(\left |{\rm Re}(a_{K^{+}K^{-}})\right | = 10^{+17_{\rm stat}}_{-10_{\rm stat}}~{\rm fm}\) and \({\rm Im}(a_{K^{+}K^{-}})\! =\! 0^{+37_{\rm stat}}_{-10_{\rm stat}}~\mathrm {fm}\).

abstract

Studies of the low-energy \(\bar {K}N\) interaction have been performed based on X-ray spectroscopy of light kaonic atoms by the SIDDHARTA experiment, where the \(K\)-series lines of kaonic hydrogen and deuterium, and the \(L\)-series lines of kaonic \(^3\)He and \(^4\)He were measured, as well as kaonic atom X-ray transitions produced in Kapton. This article deals with the short summary of the results of these measurements in the SIDDHARTA experiment.

abstract

Hadron modifications in nuclear matter are discussed in connection to chiral symmetry restoration and/or hadronic many body effects. Experiments with photon, proton and heavy ion beams are used to probe properties of hadrons embedded in nuclear matter at different temperatures and densities. Most of the information was gathered for the light vector mesons \(\rho \), \(\omega \) and \(\phi \). HADES is a second generation experiment operating at GSI (Darmstadt) with the main aim to study in-medium modifications by means of dielectron production at the SIS18/Bevelac energy range. Large acceptance and excellent particle identification capabilities allow also for measurements of strangeness production. These abilities combined with the variety of beams provided by the SIS18 lead to a characterization of properties of the dense baryonic matter properties created in heavy ion collisions at these energies. A review of recent experimental results obtained by HADES is presented, with the main emphasis on hadron properties in nuclear matter.

abstract

The mass shift of the vector mesons in nuclei is known to be a powerful tool for investigating the mechanism of generating hadron mass from the QCD vacuum. The mechanism is known to be the spontaneous breaking of chiral symmetry. One of the approach to investigate spontaneous breaking of chiral symmetry in nuclear media is formation of meson nucleus bound state and measurring energy level of the state which will be a direct connection to the \(\langle \overline {q}q\rangle \) expectation value in nucleus. We propose the experiment to study in-medium mass modification of the \(\phi \) meson using the formation of \(\phi \)-meson bound state at J-PARC/Japan. We demonstrate that a clear missing-mass spectrum can be obtained efficiently by \((\overline {p}, \phi )\) spectroscopy together with the \({\mit \Lambda }\) tagging, using the primary reaction channel \(\overline {p} p \rightarrow \phi \phi \). A systematic study over several nuclear targets will yield a unique, definitive and precise determination of the in-medium mass modification of the vector meson \(\phi (s\overline {s})\). This paper gives an overview of the physics motivation and the detector concept, and explains the direction of the initial research and development effort.

abstract

The use of scattering length of particle–target interaction due to real-valued potential to study the bound states of the particle–target system is well known in nuclear and atomic physics. In view of the current interest in using \(\eta \)-nucleus scattering length to infer the existence of \(\eta \)-mesic nucleus, we derive general analytic expressions that relate the binding energy and half-width of an unstable bound state to the complex-valued scattering length due to the same particle-target interaction.

abstract

Next-to-leading order corrections to using Watson final-state interaction theory to extract \(\eta \)-nucleus scattering length from measurements are discussed. For certain classes of \(\eta \)-nucleus reactions, the need to take into account interference effects due to the presence of two competing processes is also elucidated.