Marian Smoluchowski
Institute of Physics

Jagiellonian University in Kraków

Doctoral thesis

Study of the eta -> g e+ e- decay using WASA-at-COSY detector system

Supervisor: prof. Paweł Moskal

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abstract

Defence year: 2012

HEP: arXiv:1203.5756v1

Since the eta meson is a short-lived neutral particle, it is not possible to investigate its structure via classical methods of particle scattering. To learn about its quark wave function, one studies those decay processes of this meson in which a pair of photons is produced, where at least one of them is virtual. The virtual photons have a non-zero mass and convert into lepton-antilepton pairs. Therefore, information about the quarks' spatial distribution inside the meson can be extracted from the lepton-antilepton invariant mass distributions by comparing the empirical results with QED predictions, which are based on the assumption that the meson is a point-like particle. The deviation from the expected behavior in the leptonic mass spectrum exposes the inner structure of the meson. This deviation is characterized by a form factor. The corresponding data were collected for the pd --> 3He eta reaction at a proton beam momentum of 1.69 GeV/c. The experiment was performed using the WASA-at-COSY detector in November 2008. 525 +/- 26 events of the eta --> g e+ e- decay channel were reconstructed. The analysis chain led to the determination of the value of the slope parameter, bp, equal to (2.27 +/- 0.73stat. +/- 0.46sys.) GeV-2, where the systematical uncertainty should be treated as an upper limit only. The estimation of the charge radius of the eta meson has been made resulting with the value of < r(eta)^2 >^(1/2) = (0.68 +/- 0.02) fm. @Version including minor corrections according to the comments of the referees

Publications

ARTICLES CONFERENCE PROCEEDINGS POSTERS

1. Measurement of the vec{n} p->d pi0 pi0 Reaction with Polarized Beam in the Region of the d*(2380) Resonance
   P. Adlarson, ..., E. Czerwiński, M. Hodana, J. Klaja, W. Krzemień, P. Moskal, I. Ozerianska, O. Rundel, M. Skurzok, M. Zieliński et al.
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   Eur. Phys. J. A 52 (2016) 147

   HEP: arXiv:1601.05253
   We report on a high-statistics measurement of the most basic double pionic fusion reaction vec{n}p -> dpi0pi0 over the energy region of the d*(2380) resonance by use of a polarized deuteron beam and observing the double fusion reaction in the quasifree scattering mode. The measurements were performed with the WASA detector setup at COSY. The data reveal substantial analyzing powers and confirm conclusions about the d? resonance obtained from unpolarized measurements. We also confirm the previous unpolarized data obtained under complementary kinematic conditions.
2. ABC Effect and Resonance Structure in the Double-Pionic Fusion to 3He
   P. Adlarson, ..., E. Czerwiński, M. Hodana, W. Krzemień, P. Moskal, I. Ozerianska, M. Skurzok, M. Zielinski, et al.
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   abstract

   Phys. Rev. C 91 (2015) 015201

   HEP: arXiv:1408.5744
   Exclusive and kinematically complete measurements of the double pionic fusion to 3He have been performed in the energy region of the so-called ABC effect, which denotes a pronounced low-mass enhancement in the pi pi-invariant mass spectrum. The experiments were carried out with the WASA detector setup at COSY. Similar to the observations in the basic pn->dpi0pi0 reaction and in the dd->4Hepi0pi0 reaction, the data reveal a correlation between the ABC effect and a resonance-like energy dependence in the total cross section. Differential cross sections are well described by the hypothesis of d* resonance formation during the reaction process in addition to the conventional t-channel Delta Delta mechanism. The deduced d* resonance width can be understood from collision broadening due to Fermi motion of the nucleons in initial and final nuclei.
3. Measurement of the eta -> pi(+)pi(-)pi(0) Dalitz plot distribution
   P. Adlarson, W. Augustyniak, W. Bardan, M. Bashkanov, FS. Bergmann, M. Berlowski, H. Bhatt, A. Bondar, M. Buscher, H. Calen, I. Ciepal, H. Clement, D. Coderre, E. Czerwinski, K. Demmich, E. Doroshkevich, R. Engels, A. Erven, W. Erven, W. Eyrich, P. Fedorets, K. Fohl, K. Fransson, F. Goldenbaum, P. Goslawski, A. Goswami, K. Grigoryev, CO. Gullstrom, F. Hauenstein, L. Heijkenskjold, V. Hejny, M. Hodana, B. Hoistad, N. Husken, A. Jany, BR. Jany, L. Jarczyk, T. Johansson, B. Kamys, G. Kemmerling, FA. Khan, A. Khoukaz, DA. Kirillov, S. Kistryn, H. Kleines, B. Klos, W. Krzemien, P. Kulessa, A. Kupsc, A. Kuzmin, K. Lalwani, D. Lersch, B. Lorentz, A. Magiera, R. Maier, P. Marciniewski, B. Marianski, BV. Martemyanov, UG. Meissner, M. Mikirtychiants, HP. Morsch, P. Moskal, H. Ohm, I. Ozerianska, EP. del Rio, NM. Piskunov, et al.
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   PHYSICAL REVIEW C 90:4 045207 (2014)
   The Dalitz plot distribution of the eta -> pi(+)pi(-)pi(0) decay is determined by using a data sample of 1.2 x 10(7) eta mesons from the pd -> He-3 eta reaction at 1 GeV collected by the WASA detector at COSY.
4. Study of the polarization degree for the -> p p ->pp eta measurement with WASA
   I. Ozerianska, P. Moskal, M. Hodana
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   Hyperfine Interact. 229 (2014) 1-3, 85-87
   Understanding of the production processes of the eta meson will strongly rely on the precise determination of spin observables. So far these observables have been determined only for few excess energies and with low statistics (Winter et al. Eur. Phys. J. A18, 355 2003; Czyzykiewicz et al. Phys. Rev. Lett. 98, 122003 2007; Balestra et al. Phys. Rev. C69, 064003 2004). In the year 2010 WASA detector was used for the measurement of the p?p?pp? reaction with the polarized proton beam of COSY (Moskal and Hodana J. Phys. Conf. Ser 295, 012080 2011). The measurement was done for the excess energy of Q = 15 MeV and Q = 72 MeV. In total about 106 events corresponding to the p?p?pp? reaction have been collected.
5. Measurement of the np->npi0i0 Reaction in Search for the Recently Observed d*(2380) Resonance
   P. Adlarson,..., E. Czerwiński, M. Hodana, W. Krzemień, P. Moskal, I. Ozerianska, M. Skurzok, M. Zielinski, et al.
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   Phys. Lett. B 743 (2015) 325-332

   HEP: arXiv:1409.2659
   Exclusive measurements of the quasi-free np->nppi0pi0 reaction have been performed by means of dp collisions at Td = 2.27 GeV using the WASA detector setup at COSY. Total and differential cross sections have been obtained covering the energy region sqrt_s = (2.35 - 2.46) GeV, which includes the region of the ABC effect and its associated d*(2380) resonance. Adding the d* resonance amplitude to that for the conventional processes leads to a reasonable description of the data. The observed resonance effect in the total cross section is in agreement with the predictions of Faeldt and Wilkin as well Albadajedo and Oset. The ABC effect, {it i.e.} the low-mass enhancement in the pi0pi0-invariant mass spectrum, is found to be very modest - if present at all, which might pose a problem to some of its interpretations.
6. Neutron-Proton Scattering in the Context of the d*(2380) Resonance
   P. Adlarson, ..., E. Czerwiński, M. Hodana, W. Krzemień, P. Moskal, I. Ozerianska, M. Skurzok, M. Zielinski, et al.
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   Phys. Rev. C 90 (2014) 035204

   HEP: arXiv:1408.4928
   New data on quasifree polarized neutron-proton scattering, in the region of the recently observed d* resonance structure, have been obtained by exclusive and kinematically complete high-statistics measurements with WASA at COSY. This paper details the determination of the beam polarization, checks of the quasifree character of the scattering process, on all obtained Ay angular distributions and on the new partial-wave analysis, which includes the new data producing a resonance pole in the 3D3-3G3 coupled partial waves at (2380+-10-i40+-5) MeV -- in accordance with the d* dibaryon resonance hypothesis. The effect of the new partial-wave solution on the description of total and differential cross section data as well as specific combinations of spin-correlation and spin-transfer observables available from COSY-ANKE measurements at Td = 2.27 GeV is discussed.
7. Charge Symmetry Breaking in dd->4Hepi0 with WASA-at-COSY
   P. Adlarson, ..., E. Czerwiński, M. Hodana, W. Krzemień, P. Moskal, I. Ozerianska, M. Skurzok, M. Zielinski, et al.
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   Phys. Lett. B 739 (2014) 44

   HEP: arXiv:1407.2756
   Charge symmetry breaking (CSB) observables are a suitable experimental tool to examine effects induced by quark masses on the nuclear level. Previous high precision data from TRIUMF and IUCF are currently used to develop a consistent description of CSB within the framework of chiral perturbation theory. In this work the experimental studies on the reaction dd->4He{pi}0 have been extended towards higher excess energies in order to provide information on the contribution of p-waves in the final state. For this, an exclusive measurement has been carried out at a beam momentum of p=1.2 GeV/c using the WASA-at-COSY facility. The total cross section amounts to sigma(tot) = (118 +- 18(stat) +- 13(sys) +- 8(ext)) pb and first data on the differential cross section are consistent with s-wave pion production.
8. Cross section ratio and angular distributions of the reaction p + d -> (3)He + eta at 48.8 MeV and 59.8 MeV excess energy
   P. Adlarson, E. Czerwiński, M. Hodana, W. Krzemień, P. Moskal, I. Ozerianska, M. Skurzok, M. Zielinski, et al.
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   Eur. Phys. J. A 50 (2014) 100

   HEP: arXiv:1402.3469
   We present new data for angular distributions and on the cross section ratio of the p+d -> (3)He + eta reaction at excess energies of Q = 48.8 MeV and Q = 59.8 MeV. The data have been obtained at the WASA-at-COSY experiment (Forschungszentrum Jülich) using a proton beam and a deuterium pellet target. While the shape of obtained angular distributions show only a slow variation with the energy, the new results indicate a distinct and unexpected total cross section fluctuation between Q = 20 MeV and Q = 60 MeV, which might indicate the variation of the production mechanism within this energy interval.
9. Measurement of the eta -> pi + pi - pi0 Dalitz plot distribution
   P. Adlarson,..., E. Czerwiński, M. Hodana, W. Krzemień, P. Moskal, I. Ozerianska, M. Skurzok, M. Zieliński, et al.
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   Phys. Rev. C 90 (2014) 045207

   HEP: arXiv:1406.2505
   Dalitz plot distribution of the eta-> pi+pi-pi0 decay is determined using a data sample of 1.2*10^7 eta mesons from pd->3He eta reaction at 1 GeV collected by the WASA detector at COSY.
10. Evidence for a New Resonance from Polarized Neutron-Proton Scattering
    P. Adlarson, ..., E. Czerwiński, M. Hodana, J. Klaja, W. Krzemień, P. Moskal, I. Ozerianska, M. Skurzok, M. Zieliński et al.
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    abstract

    Phys. Rev. Lett. 112 (2014) 202301

    HEP: arXiv:1402.6844
    Exclusive and kinematically complete high-statistics measurements of quasifree polarized vec_n p scattering have been performed in the energy region of the narrow resonance structure d* with I(JP)=0(3+), M ~2380 MeV/c2 and Gamma 70 MeV observed recently in the double-pionic fusion channels pn->dpi0pi0 and pn->dpi+pi-. The experiment was carried out with the WASA detector setup at COSY having a polarized deuteron beam impinged on the hydrogen pellet target and utilizing the quasifree process d p->np+p_spectator. That way the np analyzing power Ay was measured over a large angular range. The obtained Ay angular distributions deviate systematically from the current SAID SP07 NN partial-wave solution. Incorporating the new Ay data into the SAID analysis produces a pole in the 3D3->3G3 waves as expected from the d* resonance hypothesis.
11. Study of the eta meson production with polarized proton beam
    M. Hodana, P. Moskal, I. Ozerianska, M. Zieliński
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    abstract

    Acta Phys. Polon. B 45 (2014) 697-704

    HEP: arXiv:1401.7516
    The pp-> 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.
12. Measurement of the pn -> pp pi(0)pi(-) reaction in search for the recently observed resonance structure in d pi(0)pi(0) and d pi(+)pi(-) systems
    P. Adlarson, W. Augustyniak, W. Bardan, M. Bashkanov, FS. Bergmann, M. Berlowski, H. Bhatt, M. Buscher, H. Calen, I. Ciepal, H. Clement, D. Coderre, E. Czerwinski, K. Demmich, E. Doroshkevich, R. Engels, W. Erven, W. Eyrich, P. Fedorets, K. Fohl, K. Fransson, F. Goldenbaum, P. Goslawski, A. Goswami, K. Grigoryev, CO. Gullstrom, F. Hauenstein, L. Heijkenskjold, V. Hejny, F. Hinterberger, M. Hodana, B. Hoistad, A. Jany, BR. Jany, L. Jarczyk, T. Johansson, B. Kamys, G. Kemmerling, FA. Khan, A. Khoukaz, DA. Kirillov, S. Kistryn, J. Klaja, H. Kleines, B. Klos, M. Krapp, W. Krzemien, P. Kulessa, A. Kupsc, K. Lalwani, D. Lersch, L. Li, B. Lorentz, A. Magiera, R. Maier, P. Marciniewski, B. Marianski, M. Mikirtychiants, HP. Morsch, P. Moskal, BK. Nandi, H. Ohm, I. Ozerianska, EP. del Rio, NM. Piskunov, P. Plucinski, P. Podkopal, et al.
    abstract

    PHYSICAL REVIEW C 88:5 055208 (2013)
    Exclusive measurements of the quasifree pn -> pp pi(0)pi(-) reaction have been performed by means of pd collisions at T-p = 1.2 GeV using the wide angle shower apparatus (WASA) detector setup at the cooler synchrotron COSY (Institut fur Kernphysik, Julich). Total and differential cross sections have been obtained covering the energy region root s = (2.35-2.46) GeV, which includes the region of the ABC effect and its associated resonance structure. NoABCeffect, i.e., low-mass enhancement is found in the pi(0)pi(-) -invariant mass spectrum, in agreement with the constraint from Bose statistics that the isovector pion pair can not be in relative s wave. At the upper end of the covered energy region t-channel processes for Roper, Delta(1600) and Delta Delta excitations provide a reasonable description of the data, but at low energies the measured cross sections are much larger than predicted by such processes. Adding a resonance amplitude for the resonance at m = 2.37 GeV with Gamma = 70 MeV and I (J(P)) = 0(3(+)) observed recently in pn -> d pi(0)pi(0) and pn -> d pi(+)pi(-) reactions leads to an agreement with the data also at low energies.
13. Search for a dark photon in the pi(0) -> e(+)e(-)gamma decay
    P. Adlarson, W. Augustyniak, W. Bardan, M. Bashkanov, FS. Bergmann, M. Berlowski, H. Bhatt, A. Bondar, M. Buscher, H. Calen, I. Ciepal, H. Clement, D. Coderre, E. Czerwinski, K. Demmich, E. Doroshkevich, R. Engels, W. Erven, W. Eyrich, P. Fedorets, K. Fohl, K. Fransson, E. Goldenbaum, P. Goslawski, A. Goswami, K. Grigoryev, CO. Gullstrom, F. Hauensteinn, L. Heijkenskjold, V. Hejny, F. Hinterberger, M. Hodana, B. Hoistad, A. Jany, BR. Jany, L. Jarczyk, T. Johansson, B. Kamys, G. Kemmerling, FA. Khan, A. Khoukaz, S. Kistryn, J. Klaja, H. Kleines, DA. Kirillov, B. Klos, M. Krapp, W. Krzemien, P. Kulessa, A. Kupsc, A. Kuzmin, K. Lalwani, D. Lersch, L. Li, B. Lorentz, A. Magiera, R. Maier, P. Marciniewski, B. Marianski, UG. Meissner, M. Mikirtychiants, HP. Morsch, P. Moskal, BK. Nandi, H. Ohm, I. Ozerianska, EP. del Rio, et al.
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    PHYSICS LETTERS B 726:1-3 p. 187-193 (2013)
    The presently world largest data sample for pi(0) -> gamma e(+)e(-) decays studies containing nearly 5 x 10(5) events was collected using the WASA detector at COSY. A search for a dark photon U produced in the pi(0) -> gamma U -> gamma e(+)e(-) decay from the pp -> pp pi(0) reaction was carried out. An upper limit on the square of the U -> gamma mixing strength parameter epsilon(2) of 5 x 10(-6) at 90% CL was obtained for the mass range 20 MeV < M-U < 100 MeV. This result together with other recent experimental limits significantly reduces the M-U vs. epsilon(2) parameter space which could explain the presently seen deviation between the Standard Model prediction and the direct measurement of the anomalous magnetic moment of the muon. (C) 2013 Elsevier B.V. All rights reserved.
14. Investigation of the dd -> (3)Hen pi(0) reaction with the FZ Julich WASA-at-COSY facility
    P. Adlarson, W. Augustyniak, W. Bardan, M. Bashkanov, FS. Bergmann, M. Berlowski, H. Bhatt, M. Buscher, H. Calen, I. Ciepal, H. Clement, D. Coderre, E. Czerwinski, K. Demmich, E. Doroshkevich, R. Engels, W. Erven, W. Eyrich, P. Fedorets, K. Fohl, K. Fransson, F. Goldenbaum, P. Goslawski, A. Goswami, K. Grigoryev, CO. Gullstrom, C. Hanhart, F. Hauenstein, L. Heijkenskjold, V. Hejny, F. Hinterberger, M. Hodana, B. Hoistad, A. Jany, BR. Jany, L. Jarczyk, T. Johansson, B. Kamys, G. Kemmerling, FA. Khan, A. Khoukaz, DA. Kirillov, S. Kistryn, J. Klaja, H. Kleines, B. Klos, M. Krapp, W. Krzemien, P. Kulessa, A. Kupsc, K. Lalwani, D. Lersch, L. Li, B. Lorentz, A. Magiera, R. Maier, P. Marciniewski, B. Marianski, M. Mikirtychiants, HP. Morsch, P. Moskal, BK. Nandi, H. Ohm, I. Ozerianska, EP. del Rio, NM. Piskunov, P. Plucinski, et al.
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    PHYSICAL REVIEW C 88:1 014004 (2013)
    An exclusive measurement of the dd -> (3)Hen pi(0) reaction was carried out at a beam momentum of p(d) = 1.2 GeV/c using the WASA-at-COSY facility. Information on the total cross section as well as differential distributions was obtained. The data are described by a phenomenological approach based on a combination of a quasifree model and a partial wave expansion for the three-body reaction. The total cross section is found to be sigma(tot) = ( 2.89 +/- 0.01(stat) +/- 0.06(sys) +/- 0.29(norm)) mu b. The contribution of the quasifree processes ( with the beam or target neutron being a spectator) accounts for 38% of the total cross section and dominates the differential distributions in specific regions of phase space. The remaining part of the cross section can be described by a partial wave decomposition indicating the significance of p-wave contributions in the final state.
15. Measurement of the pn->pp pi0pi- Reaction in Search for the Recently Observed Resonance Structure in dpi0pi0 and dpi+pi-systems
    P. Adlarson, ..., E. Czerwiński, M. Hodana, J. Klaja, W. Krzemień, P. Moskal, I. Ozerianska, M. Skurzok, M. Zieliński et al.
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    abstract

    Phys. Rev. C 88 (2013) 055208

    HEP: arXiv:1306.5130
    Exclusive measurements of the quasi-free $pn o pppi^0pi^-$ reaction have been performed by means of $pd$ collisions at $T_p$ = 1.2 GeV using the WASA detector setup at COSY. Total and differential cross sections have been obtained covering the energy region $sqrt s$ = (2.35 - 2.46) GeV, which includes the region of the ABC effect and its associated resonance structure. No ABC effect, {it i.e.} low-mass enhancement is found in the $pi^0pi^-$-invariant mass spectrum -- in agreement with the constraint from Bose statistics that the isovector pion pair can not be in relative s-wave. At the upper end of the covered energy region $t$-channel processes for Roper, $Delta(1600)$ and $DeltaDelta$ excitations provide a reasonable description of the data, but at low energies the measured cross sections are much larger than predicted by such processes. Adding a resonance amplitude for the resonance at $m$=~2.37 GeV with $Gamma$ =~70 MeV and $I(J^P)=~0(3^+)$ observed recently in $pn o dpi^0pi^0$ and $pn o dpi^+pi^-$ reactions leads to an agreement with the data also at low energies.
16. Isospin decomposition of the basic double-pionic fusion in the region of the ABC effect
    P. Adlarson, W. Augustyniak, W. Bardan, M. Bashkanov, T. Bednarski, FS. Bergmann, M. Berlowski, H. Bhatt, M. Buscher, H. Calen, H. Clement, D. Coderre, E. Czerwinski, K. Demmich, E. Doroshkevich, R. Engels, W. Erven, W. Eyrich, P. Fedorets, K. Fohl, K. Fransson, F. Goldenbaum, P. Goslawski, A. Goswami, K. Grigoryev, CO. Gullstrom, F. Hauenstein, L. Heijkenskjold, V. Hejny, F. Hinterberger, M. Hodana, B. Hoistad, A. Jany, BR. Jany, L. Jarczyk, T. Johansson, B. Kamys, G. Kemmerling, FA. Khan, A. Khoukaz, S. Kistryn, J. Klaja, H. Kleines, B. Klos, M. Krapp, W. Krzemien, P. Kulessa, A. Kupsc, K. Lalwani, D. Lersch, L. Li, B. Lorentz, A. Magiera, R. Maier, P. Marciniewski, B. Marianski, M. Mikirtychiants, HP. Morsch, P. Moskal, BK. Nandi, S. Niedzwiecki, H. Ohm, I. Ozerianska, EP. del Rio, P. Plucinski, P. Podkopal, et al.
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    abstract

    Phys. Lett. B 721 (2013) 229-236

    HEP: arXiv:1212.2881
    Exclusive and kinematically complete high-statistics measurements of the basic double-pionic fusion reactions pn -> d pi(0)pi(0), pn -> d pi(+)pi(-) and pp -> d pi(+)pi(0) have been carried out simultaneously over the energy region of the ABC effect using the WASA detector setup at COSY. Whereas the isoscalar reaction part given by the d pi(0)pi(0) channel exhibits the ABC effect, i.e. a low-mass enhancement in the pi pi-invariant mass distribution, as well as the associated resonance structure in the total cross section, the isovector part given by the d pi(+)pi(0) channel shows a smooth behavior consistent with the conventional t-channel Delta Delta process. The d pi(+)pi(-) data are very well reproduced by combining the data for isovector and isoscalar contributions, if the kinematical consequences of the isospin violation due to different masses for charged and neutral pions are taken into account. (C) 2013 Elsevier B.V. All rights reserved.
17. Investigation of the dd -> 3henpi0 reaction with the FZ Jülich WASA-at-COSY facility
    P. Adlarson, ..., W. Bardan, I. Ciepał, E. Czerwiński, M. Hodana, J. Klaja, W. Krzemień, I. Ozerianska, P. Moskal, M. Skurzok,M. Zieliński, et al.
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    abstract

    Phys. Rev. C 88 (2013) 014004

    HEP: arXiv:1304.3561
    An exclusive measurement of the dd?3Hen?0 reaction was carried out at a beam momentum of pd= 1.2 GeV/c using the WASA-at-COSY facility. Information on the total cross section as well as differential distributions was obtained. The data are described by a phenomenological approach based on a combination of a quasifree model and a partial wave expansion for the three-body reaction. The total cross section is found to be ?tot=(2.89?0.01stat?0.06sys?0.29norm) ?b. The contribution of the quasifree processes (with the beam or target neutron being a spectator) accounts for 38% of the total cross section and dominates the differential distributions in specific regions of phase space. The remaining part of the cross section can be described by a partial wave decomposition indicating the significance of p-wave contributions in the final state.
18. Search for a dark photon in the pi0->e+e-gamma decay
    P. Adlarson, ..., M. Hodana, W. Krzemień, P. Moskal, I. Ozerianska, M. Skurzok, M. Zieliński et al.
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    abstract

    Phys. Lett. B 726 (2013) 187

    HEP: arXiv:1304.0671
    The presently world largest data sample of pi0 --> gamma e+e- decays containing nearly 5E5 events was collected using the WASA detector at COSY. A search for a dark photon U produced in the pi0 --> gamma U --> gamma e+e- decay from the pp-->pppi^0 reaction was carried out. An upper limit on the square of the U-gamma mixing strength parameter epsilon^2 of 5e-6 at 90% CL was obtained for the mass range 30 MeV
19. Search for the eta-mesic 4He with WASA-at-COSY detector
    P. Adlarson, W. Augustyniak, W. Bardan, M. Bashkanov, T. Bednarski, FS. Bergmann, M. Berlowski, H. Bhatt, M. Buscher, H. Calen, H. Clement, D. Coderre, E. Czerwinski, K. Demmich, E. Doroshkevich, R. Engels, W. Erven, W. Eyrich, P. Fedorets, K. Fohl, K. Fransson, F. Goldenbaum, P. Goslawski, A. Goswami, K. Grigoryev, CO. Gullstrom, F. Hauenstein, L. Heijkenskjold, V. Hejny, F. Hinterberger, M. Hodana, B. Hoistad, A. Jany, BR. Jany, L. Jarczyk, T. Johansson, B. Kamys, G. Kemmerling, FA. Khan, A. Khoukaz, S. Kistryn, J. Klaja, H. Kleines, B. Klos, M. Krapp, W. Krzemien, P. Kulessa, A. Kupsc, K. Lalwani, D. Lersch, L. Li, B. Lorentz, A. Magiera, R. Maier, P. Marciniewski, B. Marianski, M. Mikirtychiants, HP. Morsch, P. Moskal, BK. Nandi, S. Niedzwiecki, H. Ohm, I. Ozerianska, EP. del Rio, P. Plucinski, P. Podkopal, et al.
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    abstract

    Phys. Rev. C 87 (2013) 035204

    HEP: arXiv:1301.0843
    An exclusive measurement of the excitation function for the dd->3Heppi- reaction was performed at the Cooler Synchrotron COSY-Juelich with the WASA-at-COSY detection system. The data were taken during a slow acceleration of the beam from 2.185 GeV/c to 2.400 GeV/c crossing the kinematic threshold for the eta meson production in the dd->4He-eta reaction at 2.336 GeV/c. The corresponding excess energy with respect to the 4He-eta system varied from -51.4MeV to 22MeV. The integrated luminosity in the experiment was determined using the dd->3Hen reaction. The shape of the excitation function for the dd->3Heppi- was examined. No signal of the 4He-eta bound state was observed. An upper limit for the cross-section for the bound state formation and decay in the process dd->(4He-eta)bound->3Heppi- was determined on the 90% confidence level and it varies from 20nb to 27nb for the bound state width ranging from 5MeV to 35MeV, respectively.
20. Abashian-Booth-Crowe resonance structure in the double pionic fusion to He-4
    P. Adlarson, W. Augustyniak, W. Bardan, M. Bashkanov, T. Bednarski, FS. Bergmann, M. Berlowski, H. Bhatt, KT. Brinkmann, M. Buscher, H. Calen, H. Clement, D. Coderre, E. Czerwinski, K. Demmich, E. Doroshkevich, R. Engels, W. Erven, W. Eyrich, P. Fedorets, K. Fohl, K. Fransson, F. Goldenbaum, P. Goslawski, K. Grigoryev, CO. Gullstrom, F. Hauenstein, L. Heijkenskjold, V. Hejny, F. Hinterberger, M. Hodana, B. Hoistad, C. Husmann, A. Jany, BR. Jany, L. Jarczyk, T. Johansson, B. Kamys, G. Kemmerling, FA. Khan, A. Khoukaz, S. Kistryn, J. Klaja, H. Kleines, B. Klos, W. Krzemien, P. Kulessa, A. Kupsc, K. Lalwani, D. Lersch, L. Li, B. Lorentz, A. Magiera, R. Maier, P. Marciniewski, B. Marianski, M. Mikirtychiants, HP. Morsch, P. Moskal, BK. Nandi, S. Niedzwiecki, H. Ohm, I. Ozerianska, C. Pauly, EP. del Rio, Y. Petukhov, et al.
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    abstract

    Phys. Rev. C 86 (2012) 032201

    HEP: arXiv:1206.6337
    Exclusive and kinematically complete high-statistics measurements of the double pionic fusion reaction dd?4He?0?0 have been performed in the energy range 0.8?1.4 GeV covering thus the region of the Abashian-Booth-Crowe effect, which denotes a pronounced low-mass enhancement in the ?? invariant mass spectrum. The experiments were carried out with the WASA detector setup at the cooler synchrotron at Forshungszentrum Julich GmbH. Similar to the observation in the basic pn?d?0?0 reaction, the data reveal a correlation between the ABC effect and a resonancelike energy dependence in the total cross section. The maximum occurs at m=2.37 GeV + 2mN, i.e., at the same position as in the basic reaction. The observed resonance width ??160 MeV can be understood from broadening due to Fermi motion of the nucleons in initial and final nuclei together with collision damping. Differential cross sections are described equally well by the hypothesis of a pn resonance formation during the reaction process.
21. Exclusive measurement of the eta->pi+ pi- gamma decay
    P. Adlarson, C. Adolph, W. Augustyniak, W. Bardan, M. Bashkanov, T. Bednarski, FS. Bergmann, M. Berlowski, H. Bhatt, KT. Brinkmann, M. Buscher, H. Calen, H. Clement, D. Coderre, E. Czerwinski, E. Doroshkevich, R. Engels, W. Erven, W. Eyrich, P. Fedorets, K. Fohl, K. Fransson, F. Goldenbaum, P. Goslawski, K. Grigoryev, CO. Gullstrom, C. Hanhart, L. Heijkenskjold, J. Heimlich, V. Hejny, F. Hinterberger, M. Hodana, B. Hoistad, M. Jacewicz, A. Jany, BR. Jany, L. Jarczyk, T. Johansson, B. Kamys, G. Kemmerling, O. Khakimova, A. Khoukaz, S. Kistryn, J. Klaja, H. Kleines, B. Klos, F. Kren, W. Krzemien, P. Kulessa, A. Kupsc, K. Lalwani, S. Leupold, B. Lorentz, A. Magiera, R. Maier, B. Marianski, P. Marciniewski, UG. Meissner, M. Mikirtychiants, HP. Morsch, P. Moskal, BK. Nandi, S. Niedzwiecki, H. Ohm, A. Passfeld, C. Pauly, et al.
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    abstract

    Phys. Lett. B 707 (2012) 243-249

    HEP: arXiv:1107.5277
    An exclusive measurement of the decay eta --> pi+ pi- gamma has been performed at the WASA facility at COSY. The eta mesons were produced in the fusion reaction pd --> 3He X at a proton beam momentum of 1.7 GeV/c. Efficiency corrected differential distributions have been extracted based on 13340pm140 events after background subtraction. The measured pion angular distribution is consistent with a relative p-wave of the two-pion system, whereas the measured photon energy spectrum was found at variance with the simplest gauge invariant matrix element of eta --> pi+ pi- gamma. A parameterization of the data can be achieved by the additional inclusion of the empirical pion vector form factor multiplied by a first-order polynomial in the squared invariant mass of the pi+ pi- system.
22. pi0 pi0 Production in proton-proton collisions at Tp=1.4 GeV
    P. Adlarson, C. Adolph, W. Augustyniak, M. Bashkanov, T. Bednarski, FS. Bergmann, M. Berlowski, H. Bhatt, KT. Brinkmann, M. Buscher, H. Calen, H. Clement, D. Coderre, E. Czerwinski, E. Doroshkevich, R. Engels, W. Erven, W. Eyrich, P. Fedorets, K. Fohl, K. Fransson, F. Goldenbaum, P. Goslawski, K. Grigoryev, CO. Gullstrom, L. Heijkenskjold, J. Heimlich, V. Hejny, F. Hinterberger, M. Hodana, B. Hoistad, M. Jacewicz, M. Janusz, A. Jany, BR. Jany, L. Jarczyk, T. Johansson, B. Kamys, G. Kemmerling, O. Khakimova, A. Khoukaz, S. Kistryn, J. Klaja, H. Kleines, B. Klos, F. Kren, W. Krzemien, P. Kulessa, A. Kupsc, K. Lalwani, B. Lorentz, A. Magiera, R. Maier, B. Marianski, P. Marciniewski, M. Mikirtychiants, HP. Morsch, P. Moskal, BK. Nandi, S. Niedzwiecki, H. Ohm, A. Passfeld, C. Pauly, EP. del Rio, Y. Petukhov, N. Piskunov, et al.
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    abstract

    Phys. Lett. B 706 (2012) 256-262

    HEP: arXiv:1107.0879
    The reaction pp->pppi0pi0 has been investigated at a beam energy of 1.4 GeV using the WASA-at-COSY facility. The total cross section is found to be (324 +- 21_systematic +- 58_normalization) mub. In order to to study the production mechanism, differential kinematical distributions have been evaluated. The differential distributions indicate that both initial state protons are excited into intermediate Delta(1232) resonances, each decaying into a proton and a single pion, thereby producing the pion pair in the final state. No significant contribution of the Roper resonance N*(1440) via its decay into a proton and two pions is found
23. Abashian-Booth-Crowe effect in basic double fusion: A new resonance?
    P. Adlarson, C. Adolph, W. Augustyniak, V. Baru, M. Bashkanov, T. Bednarski, FS. Bergmann, M. Berlowski, H. Bhatt, KT. Brinkmann, M. Buscher, H. Calen, H. Clement, D. Coderre, E. Czerwinski, E. Doroshkevich, C. Ekstrom, R. Engels, W. Erven, W. Eyrich, P. Fedorets, K. Fohl, K. Fransson, F. Goldenbaum, P. Goslawski, K. Grigoryev, V. Grishina, CO. Gullstrom, J. Hampe, C. Hanhart, L. Heijkenskjold, V. Hejny, F. Hinterberger, M. Hodana, B. Hoistad, M. Jacewicz, M. Janusz, A. Jany, BR. Jany, L. Jarczyk, T. Johansson, B. Kamys, G. Kemmerling, O. Khakimova, A. Khoukaz, S. Kistryn, J. Klaja, H. Kleines, B. Klos, F. Kren, W. Krzemien, P. Kulessa, S. Kullander, A. Kupsc, K. Lalwani, B. Lorentz, A. Magiera, R. Maier, P. Marciniewski, B. Marianski, M. Mikirtychiants, P. Moskal, HP. Morsch, BK. Nandi, S. Niedzwiecki, H. Ohm, et al.
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    abstract

    Phys. Rev. Lett. 106 (2011) 242302

    HEP: arXiv:1104.0123
    We report on an exclusive and kinematically complete high-statistics measurement of the basic double-pionic fusion reaction pn->dpi0pi0 over the full energy region of the ABC effect, a pronounced low-mass enhancement in the pipi-invariant mass spectrum. The measurements, which cover also the transition region to the conventional t-channel DeltaDelta process, were performed with the upgraded WASA detector setup at COSY. The data reveal the Abashian-Booth-Crowe effect to be uniquely correlated with a Lorentzian energy dependence in the integral cross section. The observables are consistent with a narrow resonance with m=2.37??GeV, Gamma aprox 70 MeV and I(JP)=0(3+) in both pn and DeltaDelta systems. Necessary further tests of the resonance interpretation are discussed.
24. Study of the eta --> e+e-gamma decay with the WASA-at-COSY
    M. Hodana
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    abstract

    Int. J. Mod. Phys. A 26 (2011) 650-651
    In October 2008, the WASA-at-COSY collected more than 10⁷ events for the pd-->³He reaction. The aim is the determination of the invariant mass of the lepton pairs created in the Dalitz decay eta --> e⁺e^-gamma. The e⁺e^- invariant mass spectrum is directly related to the distribution of the four-momentum squared of the virtual photon from the eta-->gamma*gamma process and hence it allows for the study of the transition form factor which in turn reflects the structure of the decaying meson.
25. Measurement of the eta->3pi0 Dalitz plot distribution with the WASA detector at COSY
    WASA-at-COSY Collaboration: C. Adolph, ..., E. Czerwiński, R. Czyżykiewicz, M. Hodana, M. Janusz, J. Klaja, P. Klaja, W. Krzemień, P. Moskal, M. Zieliński, et al.
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    abstract

    Phys. Lett. B 677 (2009) 24-29

    HEP: arXiv:0811.2763
    In the first production run of the WASA experiment at COSY, the eta decay into three neutral pions was measured in proton?proton interactions at a proton beam kinetic energy of 1.4 GeV. The Dalitz plot of the three pions was studied using fully reconstructed events, and the quadratic slope parameter ? was determined to be . The result is consistent with previous measurements and further corroborates the importance of pion?pion final state interactions.
26. Measurement of the eta -> 3 pi(0) Dalitz plot distribution with the WASA detector at COSY
    C. Adolph, M. Angelstein, M. Bashkanov, U. Bechstedt, S. Belostotski, M. Berlowski, H. Bhatt, J. Bisplinghoff, A. Bondar, B. Borasoy, M. Buscher, H. Calen, K. Chandwani, H. Clement, E. Czerwinski, R. Czyzykiewicz, G. D'Orsaneo, D. Duniec, C. Ekstrom, R. Engels, W. Erven, W. Eyrich, P. Fedorets, O. Felden, K. Fransson, D. Gil, F. Goldenbaum, K. Grigoryev, A. Heczko, C. Hanhart, V. Hejny, F. Hinterberger, M. Hodana, B. Hoistad, A. Izotov, M. Jacewicz, M. Janusz, BR. Jany, L. Jarczyk, T. Johansson, B. Kamys, G. Kemmerling, I. Keshelashvili, O. Khakimova, A. Khoukaz, K. Kilian, N. Kimura, S. Kistryn, J. Klaja, P. Klaja, H. Kleines, B. Klos, A. Kowalczyk, F. Kren, W. Krzemien, P. Kulessa, S. Kullander, A. Kupsc, A. Kuzmin, V. Kyryanchuk, J. Majewski, H. Machner, A. Magiera, R. Maier, R. Marciniewski, W. Migdal, UG. Meissner, et al.
    abstract

    PHYSICS LETTERS B 677:1-2 p. 24-29 (2009)
    In the first production run of the WASA experiment at COSY, the eta decay into three neutral pions was measured in proton-proton interactions at a proton beam kinetic energy of 1.4 GeV. The Dalitz plot of the three pious was Studied using 1.2 x 10(5) fully reconstructed events. and the quadratic slope parameter alpha was determined to be -0.027 +/- 0.008(stat) +/- 0.005(syst). The result is consistent with previous measurements and further corroborates the importance of pion-pion final state interactions. (C) 2009 Elsevier B.V. All rights reserved.
27. Dynamics of the near threshold eta meson production in proton-proton interaction
    R. Czyzykiewicz, P. MoSkal, HH. Adam, A. Budzanowski, E. Czerwinski, D. Gil, D. Grzonka, M. Hodana, M. Janusz, L. Jarczyk, B. Kamys, A. Khoukaz, K. Kilian, P. Klaja, B. Lorentz, W. Oelert, C. Piskor-Ignatowicz, J. Przerwa, B. Rejdych, J. Ritman, T. Sefzick, M. Siemaszko, M. Silarski, J. Smyrski, A. Taschner, K. Ulbrich, P. Winter, M. Wolke, P. Wustner, M. Zielinski, W. Zipper
    abstract

    PROC. PROCEEDINGS OF THE 11TH WORKSHOP ON THE PHYSICS OF EXCITED NUCLEONS p. 219-+ (2008)
    We present the results of measurements of the analysing power for the pp -> pp eta reaction at the excess energies of Q = 10 and 36 MeV, and interpret these results within the framework of the meson exchange models. The determined values of the analysing power at both excess energies are consistent with zero implying that the eta meson is produced predominantly in s-wave.
28. Mechanism of near-threshold production of the eta meson
    R. Czyzykiewicz, P. Moskal, HH. Adam, A. Budzanowski, E. Czerwinski, D. Gil, D. Grzonka, M. Hodana, M. Janusz, L. Jarczyk, B. Kamys, A. Khoukaz, K. Kilian, P. Klaja, B. Lorentz, W. Oelert, C. Piskor-Ignatowicz, J. Przerwa, B. Rejdych, J. Ritman, T. Sefzick, M. Siemaszko, J. Smyrski, A. Taschner, K. Ulbrich, P. Winter, M. Wolke, P. Wustner, W. Zipper
    abstract

    PHYSICAL REVIEW LETTERS 98:12 122003 (2007)
    Measurements of the analyzing power for the pp -> pp eta reaction have been performed at excess energies of Q=10 and 36 MeV. The determined analyzing power is essentially consistent with zero, implying dominance of the s wave at both excess energies. The angular dependence of the analyzing power, combined with the isospin dependence of the total cross section for the eta meson production in nucleon-nucleon collisions, reveal that the excitation of the nucleon to the S-11(1535) resonance is predominantly due to the exchange of the pi meson between the colliding nucleons.
29. Mechanism of the close-to-threshold production of the eta meson
    R. Czyzykiewicz, P. Moskal, H. -H. Adam, A. Budzanowski, E. Czerwinski, D. Gil, D. Grzonka, M. Hodana, M. Janusz, L. Jarczyk, B. Kamys, A. Khoukaz, K. Kilian, P. Klaja, B. Lorentz, W. Oelert, C. Piskor-Ignatowicz, J. Przerwa, B. Rejdych, J. Ritman, T. Sefzick, M. Siemaszko, J. Smyrski, A. Taeschner, K. Ulbrich, P. Winter, M. Wolke, P. Wuestner, W. Zipper
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    abstract

    Phys. Rev. Lett. 98 (2007) 122003

    HEP: arXiv:hep-ex/0611015
    Measurements of the analysing power for the p(pol)p --> ppeta reaction have been performed in the close-to-threshold energy region at beam momenta of p_{beam}=2.010 and 2.085 GeV/c, corresponding to excess energies of Q=10 and 36 MeV, respectively. The determined analysing power is essentially consistent with zero implying that the eta meson is produced predominantly in the s-wave at both excess energies. The angular dependence of the analysing power, combined with the hitherto determined isospin dependence of the total cross section for the eta meson production in nucleon-nucleon collisions, reveal a statistically significant indication that the excitation of the nucleon to the S_{11}(1535) resonance, the process which intermediates the production of the eta meson, is predominantly due to the exchange of the pi meson between the colliding nucleons.

1. The eta-meson production with the polarized proton beam with WASA-at-COSY
   I. Ozerianska, M. Hodana , P. Moskal
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   published in: Proceedings of XV Advanced Research Workshop on High Energy Spin Physics
   In November 2010, the azimuthal symmetric WASA detector and the polarized proton beam of COSY, have been used to collect a high statistics sample of (vec)p->ppeta reactions in order to determine the analyzing power as a function of the invariant mass spectra of the two particle subsystems. Here, we show studies of the infuence of the beam and target characteristics such as location and direction on the determination of the polarization.
2. Studies of Systematic Uncertainties of Polarization Estimation for Experiments with the WASA Detector at COSY
   M. Hodana, P. Moskal, I. Ozerianska
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   published in: Acta Phys. Polon. B Suppl. 6 (2013) 1041-1052
   In November 2010, the azimuthally symmetric WASA detector and the polarized proton beam of COSY have been used to collect a high statistics sample of ?p p ? pp? reactions in order to determine the analyzing power as a function of the invariant mass spectra of the two-particle subsystems. Here, we show studies of the influence of the beam and target characteristics such as location and direction on the determination of degree of the polarization.
3. Study of the eta -> e+ e-gamma Decay Using WASA-at-COSY Detector System
   M. Hodana, P. Moskal for the WASA - at - COSY Collaboration
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   published in: EPJ Web Conf. 37 (2012) 09017
   At the turn of October and November 2008, the WASA-at-COSY collaboration performed an experiment to collect data on the eta meson decays in pd-->3He eta reactions. This report presents results of a study of the eta-->e+e-g conversion decay. The desired feature of this decay is that the final state e+e- pairs come from the conversion of a virtual gamma quantum and, therefore, they constitute a rich source of knowledge about the electromagnetic structure of decaying meson. This is a very important characteristic when it comes to study of a short-lived neutral particle like the eta meson, because the classical methods of particle scattering are not applicable here. The analysis performed on one third of the p-d data sample collected using the WASA-at-COSY detector, lead to the extraction of the eta transition form factor as a function of the e+e- mass and to the calculation of the slope parameter, related to the charge radius of the eta meson.
4. Study of eta Meson Production with a Polarized Proton Beam
   I. Ozerianska, P. Moskal, M. Hodana for the WASA-at-COSY Collaboration
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   published in: Proceedings of the second International PrimeNet Workshop (2012) 87
   These are the proceedings of the second PrimeNet Workshop, held in September 26-28, 2011, at the campus of Forschungszentrum J'{u}lich, Germany. This workshop is part of the activities in the project 'Study of Strongly Interacting Matter' (acronym HadronPhysics2), which is an integrating activity of the Seventh Framework Program of EU. This HP2 project contains several activities, one of them being the network PrimeNet having the focus on Meson Physics in Low-Energy QCD. This network is created to exchange information on experimental and theoretical ongoing activities on mainly eta and eta-prime physics at different European accelerator facilities and institutes.
5. Study of the eta --> e+ e- gamma decay
   M. Hodana, P. Moskal for the WASA-at-COSY Collaboration
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   published in: Proceedings of the second International PrimeNet Workshop (2012) 45
   These are the proceedings of the second PrimeNet Workshop, held in September 26-28, 2011, at the campus of Forschungszentrum J'{u}lich, Germany. This workshop is part of the activities in the project 'Study of Strongly Interacting Matter' (acronym HadronPhysics2), which is an integrating activity of the Seventh Framework Program of EU. This HP2 project contains several activities, one of them being the network PrimeNet having the focus on Meson Physics in Low-Energy QCD. This network is created to exchange information on experimental and theoretical ongoing activities on mainly eta and eta-prime physics at different European accelerator facilities and institutes.
6. Study of the eta meson production with the polarised proton beam
   P. Moskal, M. Hodana
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   published in: J. Phys. Conf. Ser. 295 (2011) 012080
   The azimuthally symmetric WASA detector and the polarised proton beam of COSY, enable investigations of energy and angular dependence of the beam analysing power in the pp --> pp eta reaction. The aim of the studies is the determination of the partial wave contributions via interference terms which are inaccessible from spin averaged observables. The partial wave decomposition is mandatory for the understanding of the reaction dynamics and for the determination of the ?-proton interaction, independently of the theoretical paradigm used.
7. Studies of the eta meson decays with WASA
   A. Kupsc, P. Adlarson, M. Berłowski, H. Bhatt, K. Chandwani, D. Coderre, M. Hodana, M. Janusz, Ch. Pauly, Ch. F. Redmer, N. Shah, J. Stepaniak, P. Vlasov, A. Winnemöller, J. Złomanczuk and L. Yurev
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   published in: PoS CD09 (2009) 046
   Studies of ? meson decays with the WASA detector are a major goal of the experimental program at COSY-Juelich. In 2008 a sample of 107 eta mesons was collected in pd --> 3He eta reaction close to threshold without trigger bias on the decay channels. The status of the analysis of various decay channels is presented.
8. Dynamics of the eta meson production in proton-proton collisions
   M. Hodana, A. Kupść, P. Moskal
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   published in: AIP Conf. Proc. 950 (2007) 268-272
   We briefly describe the present knowledge about the mechanism of the eta meson production in the collisions of nucleons. We stress the role of the spin degrees of freedom and discuss a possibility of investigations of the spin observables for the pp --> pp eta reaction using the WASA?at?COSY apparatus and the vertically polarized proton beam of COSY.

1. Study of the eta meson production with the polarized proton beam
   I. Ozerianska, P.Moskal, M. Hodana
   MesonNet 2013

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2. Study of the eta --> e+e-gamma Decay using WASA-at-COSY Detector System
   M. Hodana, P. Moskal
   12th International Workshop on Meson Production, Properties and Interaction, Kraków, Poland

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3. Study of the eta meson production with the polarized proton beam
   I. Ozerianska, P.Moskal, M. Hodana
   arXiv:1204.5509 [nucl-ex] ,PrimeNet Workshop

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