acta physica slovaca

Abstract: Recent insights and open questions connected with the production of eta-mesons from hadrons are presented. The discussion includes the etaN, etaNN, etad and eta3He systems with emphasis on final state interactions.

Abstract: Differential and total cross sections of the p d -> p d eta reaction are estimated in the region below the NN threshold. At the lowest energies, it is predicted that a two-step model involving an intermediate pion should be the most important but, at higher energies, a pick-up mechanism with a spectator proton should become dominant. The overall production rate is underestimated by about a factor of two compared with experimental data, which present angular distributions that are more featureless than those of the theoretical calculations. The analogous two-step model is extended to describe the low energy p d -> K+ d Lambda and inclusive K+ production data.

Abstract: We present preliminary results from dp -> 3He X (X=pi0, eta) measurements near the eta production threshold. The data were taken during a slow ramping of the COSY internal deuteron beam scattered on a proton target. The  3He ejectiles were registered with the COSY-11 detection setup. The ongoing data analysis should deliver high precision data for the dp -> 3He eta total and differential cross sections for the excess energies in the range from threshold up to 9 MeV. The preliminary excitation function for the reaction dp -> 3He pi0 does not show any structure which could originate from the decay of 3He-eta bound state. We present also a threshold excitation curve for the dp -> 3He ,X channel. Contrary to corresponding results from SATURNE we see no cusp in the vicinity of the eta threshold.

Abstract: Using a chiral unitary approach for meson-baryon scattering in the strangeness zero sector, where the N*(1535) and  Delta*(1700) resonances are dynamically generated, we study the reactions gamma p -> pi0 eta p and gamma p -> pi0 K0 Sigma+ at photon energies at which the final states are produced close to threshold. Among several reaction mechanisms, we find the most important is the excitation of the Delta*(1700) state which subsequently decays into a pseudoscalar meson and the N*(1535). Hence, the reaction provides useful information with which to test current theories of the dynamical generation of the low-lying 1/2- and 3/2- states. Predictions are made for cross sections and invariant mass distributions which can be compared with forthcoming experiments at ELSA.

Abstract: Some experimental studies of eta production and eta interactions performed or presently under way by the GEM collaboration at COSY Julich are reviewed.

Abstract: A combined analysis of the existing data on the reactions gamma p -> p eta' and pp -> pp eta', based on a relativistic meson exchange model of hadronic interactions, is presented.

Abstract: Gluonic degrees of freedom play an important role in the masses of the eta and eta' mesons. We discuss eta- and eta'-nucleon and nucleus interactions where this glue may be manifest. Interesting processes being studied in experiments are eta' production in proton-nucleon collisions close to threshold and possible eta-nucleus bound-states.

Abstract: The high statistics data from the pp -> pp eta reaction measurement, delivered by the COSY-11 collaboration, are now being evaluated using the correlation femtoscopy technique. This method is based on the relative momentum correlations of two emitted protons and may permit determination of the size of the reaction volume. For the very first time, we apply an intensity interferometry technique to study the mechanism of the meson production via the nucleon-nucleon interaction close to the kinematical threshold. We invented a method to determine correlation function for the pp eta system free from the physical multi-pion production background. We show the comparison of experimental results with theoretical predictions and appraise the accuracy achieved for the determination of the size of the emission source.

Abstract: The cross section for the process np -> eta d in the region near threshold is calculated within a three-body model. Non-relativistic and relativistic results are compared. The shape of the cross-section is seen to be determined by the eta d final-state interaction alone, in both relativistic and non-relativistic calculations.

Abstract: Based on the attractive eta-nucleon interaction, the possible existence of eta-mesic nuclear states has been discussed since some time. Nevertheless these states have not been seen clearly in experiments. The current status of experiments at COSY and at GSI searching for signals of eta-mesic states in light nuclear systems, by using different methods, is presented.

Abstract: Investigation of the dd -> 4He eta reaction has been a part of the experimental programme at the ANKE facility of COSY-Julich. In a first stage measurements were made at two excess energies: 2.6 MeV, 7.7 MeV as well as one energy below threshold for physical background studies. The experimental technique and the results are presented. The total cross sections obtained link well with the existing data, though there is some ambiguity at the higher energies, where some earlier data were taken with a polarised beam. Measured angular distributions indicate the onset of higher partial waves at only 7.7 MeV above threshold.

Abstract: The $pp\rightarrow NN pi pi  and  pd -> 3He pi pi reactions have been measured exclusively at CELSIUS using the WASA 4pi detector with pellet target system. With regard to the first reaction measurements have been conducted from the near-threshold region up to T_p=1.45 GeV. For the double-pionic fusion to 3He data have been taken at T_p=0.893 GeV, where the maximum of the so-called ABC effect is expected. For the 3He case a huge low-mass enhancement is observed in the pi0 pi0 invariant mass M_{pi0 pi0}, whereas only a moderate low-mass enhancement is seen in M_{pi+ pi-}. The pp -> pp pi pi reaction in the near threshold region can be well described by Roper excitation and chiral dynamics, however, at higher energies, in the Delta Delta region, data are at variance with theoretical predictions. In particular the M_{pi+  pi-} spectrum behaves phase-space like - in accordance only with the special (Delta Delta)_{0+} configuration - and the M_{ pi0 pi0} spectrum develops a strong low-mass enhancement similar to the one observed in the 3He pi0 pi0 channel.

Abstract: Production of omega in the pd -> 3He omega reaction is currently being studied at the CELSIUS storage ring with the WASA 4pi detector. Older data on  omega production from various experiments show a suppression in the cross section near the kinematic threshold. New data have been collected with WASA during 35 eight-hour shifts. A preliminary analysis of these data shows omega production in the pd -> 3He omega reaction.

Abstract: A short overview of eta and eta' decays is given with an emphasis on what can be learned for the strong interaction from them. The talk consists of a short introduction to Chiral Perturbation Theory, a discussion of eta -> 3pi beyond p^4 and some of the physics involved in eta' -> eta pi pi, 3pi as well as an overview of anomalous processes.

Abstract: Various decays of eta and  eta' are investigated within the framework of U(3) chiral effective field theory in combination with a relativistic coupled-channels approach. Final state interactions are included by deriving s- and p-wave interaction kernels for meson-meson scattering from the chiral effective Lagrangian and iterating them in a Bethe-Salpeter equation. Very good agreement with experimental data is achieved.

Abstract: We improve the calculations of the  eta -> pi0 gamma gamma decay within the context of meson chiral lagrangians. We use a chiral unitary approach for the meson-meson interaction, thus generating the a_0(980) resonance and fixing the longstanding sign ambiguity on its contribution. This also allows us to calculate the loops with one vector meson exchange, thus removing a former source of uncertainty.  In addition we ensure the consistency of the approach with other processes, first, by using vector meson dominance couplings normalized to agree with radiative vector meson decays, and, second, by checking the consistency of the calculations with the related gamma gamma -> pi0 eta reaction. We find an eta -> pi gamma gamma decay width of 0.47 +- 0.10 eV,  in clear disagreement with published data but in remarkable agreement with the most recent measurement.

Abstract: The status of the KLOE data taking  and the topics related to the eta, eta' physics which will be addressed by the analysis of a sample of 2.5 fb^-1 of integrated luminosity at the Frascati phi-factory, DAPhiNE, are summarized.

Abstract: The KLOE experiment at the Frascati phi factory DAphiNE has collected  ~450 pb^-1 in the 2001 -- 2002 data taking. The study of the eta -> 3pi dynamics, through a fit to the Dalitz plot density distribution, performed with these data is discussed in this paper. A  precise measurement of the "slope parameters"  can improve the knowledge of the decay amplitude and allows to test theoretical predictions at the level of precision needed to extract the quark mass ratio (m_s^2-\hat{m}^2) / (m_d^2-m_u^2) from the decay rate (\hat{m} is the u,d average mass).

Abstract: A sample of 3 x 10^7 eta's collected with the Crystal Ball spectrometer was used to calculate a precise value of the pi0 slope parameter of the eta -> 3 pi0 Dalitz plot, and a new branching ratio for eta -> pi0 gamma gamma. For the first time the upper limit for the CP violating eta -> 4 pi0 decay was obtained as well as upper limits for the C violating decays eta -> gamma pi pi0, eta -> gamma pi0 pi0 pi0, and eta -> 3 gamma. A new Crystal Ball program has started at the Mainz Microtron. Among other experiments the program includes a study of neutral and charged decays of eta and eta'. The first data set of about 3 x 10^7 eta's was obtained in 2004.

Abstract: The Wide Angle Shower Apparatus (WASA) has been transferred from the storage ring CELSIUS (TSL, Uppsala, Sweden) to the cooler synchrotron COSY (FZ Julich, Germany). In this presentation, the status and planning of the WASAatCOSY project are reported. The main physics aspects are rare decays of eta and eta' mesons and isospin violation in deuteron-deuteron interactions

Abstract: Photoexcitation of the nucleon yields structures in the total absorption cross section that indicate the existence of resonances. In order to explore resonance contributions, the total photabsorption cross section does not deliver a satisfying set of information. Fixing degrees of freedom by selection of partial reaction channels achieved a more detailed set of information over the last years. Further defined conditions could be reached in the recent past by fixing the polarization orientation and type between photon beam and target. This allows to observe double-polarization observables of the differential photoabsoption cross section. This article gives an overview of the double-polarization observables G and E and presents what can be learned from the polarization dependent photoproduction of the eta-and pi pi-mesons.

Abstract: Analysis of eta -> pi+ pi- e+ e- decay channel is discussed and preliminary results are presented. The eta mesons were produced in pd -> 3He eta reaction at 892.5 MeV incident proton energy with 3He measured in zero-degree spectrometer.

Abstract: The fascinating physics underlying eta and eta' mesons can be studied theoretically and experimentally in various contexts. In this presentation we want to turn our attention to two important uncorrelated aspects of this vivid research field which provide yet unexpected challenges or surprises. First we discuss open questions in the theoretical treatment of Coulomb-interaction in the context of reaction processes like p p -> p p eta. Then we review eta-eta' and sigma(600)-f_0(980) mixing in the U(3) x U(3) Linear Sigma Model and extract information on eta-eta' mixing and the K*_0(800) resonance from meson-meson scattering.

Abstract: The CELSIUS-WASA setup has been used to measure the production of 3 pi0 in pp interactions at Tp = 1360 and 1450 MeV. A cross section for the prompt pp -> pp 3 pi0 and resonant pp -> pp (eta -> 3 pi0) reaction channel is determined. The efficiency corrected Dalitz plot and density distribution for the $eta -> 3pi0 decay are shown, together with a fit of the quadratic slope parameter alpha. Our preliminary result for alpha is -0.027 +- 0.009(stat) +- 0.01(syst).

Abstract: The analyzing power A_y for the p p -> p p eta reaction has been determined at the beam momentum p_{beam} = 2010 MeV/c, corresponding to the excess energy Q = 10 MeV. In the paper the method of the data analysis is briefly presented.

Abstract: This article is dedicated to the current status of the eta mass measurement with the Crystal Ball detector at the MAMI facility in Mainz. The reaction gamma + p -> p + eta is used for the determination of the eta production threshold. The two main eta decay modes are analysed. The energy calibration of the tagger microscope shows that the error for the eta mass determination has improved in comparison to the previous Mainz experiment.

Abstract: We present preliminary results from the KLOE experiment on the Br(eta -> pi0 gamma gamma), the ratio of the two branching fractions Br(phi -> eta' gamma)/Br(phi -> eta gamma) with the pi+ pi- 7 gamma final state and a preliminary measurement of the eta mass using the decay phi -> eta gamma, eta -> gamma gamma