F. Kren, M. Bashkanov, D. Bogoslawsky, H. Calén, H. Clement, L. Demiroers, C. Ekström, K. Fransson, J. Greiff, L. Gustafsson, B. Höistad, G. Ivanov, M. Jacewicz, E. Jiganov, T. Johansson, O. Khakimova, S. Keleta, I. Koch, S. Kullander, A. Kupść, P. Marciniewski, R. Meier, B. Morosov, C. Pauly , H. Petrén, Y. Petukhov, A. Povtorejko, R.J.M.Y. Ruber, K. Schönning, W. Scobel, T. Skorodko, B. Shwartz, J. Stepaniak, P. Thörngren-Engblom, V. Tikhomirov, G.J. Wagner, M. Wolke, A. Yamamoto, J. Zabierowski, and J. Zlomanczuk Physikalisches Institut der Universität Tübingen, D-72076 Tübingen, Germany Joint Institute for Nuclear Research, Dubna, Russia The Svedberg Laboratory, Uppsala, Sweden Hamburg University, Hamburg, Germany Uppsala University, Uppsala,Sweden Forschungszentrum Jülich, Germany Budker Institute of Nuclear Physics, Novosibirsk, Russia Soltan Institute of Nuclear Studies, Warsaw and Lodz, Poland High Energy Accelerator Research Organization, Tsukuba, Japan

Exclusive measurements of the reaction pp -> dpi+pi0 have been carried out at T_p = 1.1 GeV at the CELSIUS storage ring using the WASA detector. The isovector pi+pi0 channel exhibits no enhancement at low invariant pipi masses, i. e. no ABC effect. The differential distributions are in agreement with the conventional t-channel Delta-Delta excitation process, which also accounts for the observed energy dependence of the total cross section. This is an update of a previously published version -- see important note at the end of the article.

Exclusive measurements of the reaction pp→dπ+π0 have been carried out at Tp=1.1GeV at the CELSIUS storage ring using the WASA detector. The isovector π+π0 channel exhibits no enhancement at low invariant ππ masses, i.e. no ABC effect. Therefore this most basic isovector double-pionic fusion reaction qualifies as an ideal test case for the conventional t-channel ΔΔ excitation process. Indeed, the obtained differential distributions reveal the conventional t-channel ΔΔ mechanism as the appropriate reaction process, which also accounts for the observed energy dependence of the total cross section.

The WASA 4π multidetector system, aimed at investigating light meson production in light ion collisions and η meson rare decays at the CELSIUS storage ring in Uppsala is presented. A detailed description of the design, together with the anticipated and achieved performance parameters are given.

At CELSIUS-WASA the two-pion production in proton-proton collisisons has been measured exclusively from threshold up to the energy regime, where both of the collision partners are expected to be excited to the Δ state. The measurements constitute the first kinematically complete data samples of solid statistics in this energy range. Most of the data have been obtained for the π + π - and π 0 π 0 channels. Whereas at near-threshold energies the differential distributions can be succesfully explained by chiral dynamics and Roper excitation, respectively, the data for the π + π - channel in the ΔΔ region can be described only, if the special configuration (ΔΔ) 0 + is assumed. The data for the π 0 π 0 channel moreover exhibit a low-mass enhancement in the π 0 π 0 invariant mass spectrum, which is reminiscent of the ABC-effect found in double-pionic fusion to light nuclei.

At CELSIUS-WASA the two-pion production in proton-proton collisisons has been measured exclusively from threshold up to the energy regime, where both of the collision partners are expected to be excited to the. state. The measurements constitute the first kinematically complete data samples of solid statistics in this energy range. Most of the data have been obtained for the pi(+)pi(-) and pi(0)pi(0) channels. Whereas at near-threshold energies the differential distributions can be succesfully explained by chiral dynamics and Roper excitation, respectively, the data for the pi(+)pi(-) channel in the Delta Delta region can be described only, if the special configuration (Delta Delta)(0+) is assumed. The data for the pi(0)pi(0) channel moreover exhibit a low-mass enhancement in the pi(0)pi(0) invariant mass spectrum, which is reminiscent of the ABC-effect found in double-pionic fusion to light nuclei.

The EDDA-Detector at the Cooler-Synchrotron COSY/J\"ulich has been operated with an internal CH$_2$ fiber target to measure proton-proton elastic scattering differential cross sections. For the data analysis knowledge of beam parameters, like position, width and angle, are indispensable. We have developed a method to obtain these values with high precision from the azimuthal and polar angles of the ejectiles only, by exploiting the coplanarity of the two final state protons with the beam and the kinematic correlation. The formalism is described and results for beam parameters obtained during beam acceleration are given.

Thin polypropylene fibers have been used for internal experiments in storage rings as an option for hydrogen targets. The change of the hydrogen content due to the radiation dose applied by the circulating proton beam has been investigated in the range 1 × 106–2 × 108 Gy at beam momenta of 1.5–3 GeV/c by comparing the elastic pp-scattering yield to that from inelastic p-carbon reactions. It is found that the loss of hydrogen as a function of applied dose receives contributions from a fast and a slow component.

The pp -> NN pi pi and pd -> He-3 pi pi reactions have been measured exclusively at CELSIUS using the WASA 4 pi 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 He-3 data have been taken at T-p = 0.893 GeV, where the maximum of the so-called ABC effect is expected. For the He-3 case a huge low-mass enhancement is observed in the pi(0)pi(0) invariant mass M-pi 0 pi 0, 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-pi 0 pi 0 spectrum develops a strong low-mass enhancement similar to the one observed in the He-3 pi(0)pi(0) channel.

Exclusive measurements of the reactions pd→He3π+π− and pd→He3π0π0 have been carried out at Tp=0.893GeV at the CELSIUS storage ring using the WASA detector. The π+π− channel evidences a pronounced enhancement at low invariant ππ masses—as anticipated from previous inclusive measurements of the ABC effect. This enhancement is seen to be even much larger in the isoscalar π0π0 channel. The differential distributions prove this enhancement to be of scalar–isoscalar nature. ΔΔ calculations give a good description of the data, if a boundstate condition is imposed for the intermediate ΔΔ system.

The pp -> ppx(x = gamma gamma, pi(0)pi(0), pi(+)pi(-)) reactions have been measured at CELSIUS in the energy range T-p = 775 - 1360 MeV using the WASA 4 pi detector with hydrogen pellet target. Close to threshold the pp pi(+)pi(-) data are in accordance with the Roper resonance concept and/or a dynamic formation of the sigma meson. At higher energies the formation of the Delta Delta system becomes increasingly dominant. The pp pi(0)pi(0) channel basically shows similar features, however, develops a peculiar enhancement at small invariant masses M-pi 0 pi 0 - in resemblance of Bose-Einstein correlations. Alternatively, the enhancement could be also indicative of a (dynamic) isospin breaking due to pi pi rescattering in the or channel. The latter is favored by the results on pd -> He-3 x (x = pi(0)pi(0), pi(+)pi(-)), where the observed pi(0)pi(0) enhancements are still much larger. In the gamma gamma channel a line at M gamma gamma = 2m(pi) is observed. The possible nature of this line is discussed.

Excitation functions of the spin correlation coefficients A(NN)(p(lab),theta(c.m.)),A(SS)(p(lab),theta(c.m.)), and A(SL)(p(lab),theta(c.m.)) have been measured with the polarized proton beam of the Cooler Synchrotron and an internal polarized atomic beam target. Data were taken continuously during the acceleration for proton momenta p(lab) ranging from 1000 to 3300 MeV/c (kinetic energies T-lab 450-2500 MeV) as well as for discrete momenta of 1430 MeV/c and above 1950 MeV/c covering angles theta(c.m.) between 30(degrees) and 90(degrees). The data are of high internal consistency. Whereas A(SL)(p(lab,)theta(c.m.)) is small and without structures in the whole range, A(NN) and, even more, A(SS) show a pronounced energy dependence. The angular distributions for A(SS) are at variance with predictions of existing phase-shift analyses at energies beyond 800 MeV. The impact of our results on phase-shift solutions is discussed. The direct reconstruction of the scattering amplitudes from all available pp elastic scattering data considerably reduces the ambiguities of solutions.

Excitation functions AN(plab,Θc.m.) of the analyzing power in elastic proton-proton scattering have been measured in an internal target experiment at the Cooler Synchrotron COSY with an unpolarized proton beam and a polarized atomic hydrogen target. Data were taken continuously during the acceleration and deceleration for proton kinetic energies Tlab (momenta plab) between 0.45 and 2.5 GeV (1.0 and 3.3 GeV/c) and scattering angles 30 ° ⩽ Θc.m. ⩽ 90°. The results provide excitation functions and angular distributions of high precision and internal consistency. The data can be used as calibration standard between 0.45 and 2.5 GeV. They have significant impact on phase shift solutions, in particular on the spin triplet phase shifts between 1.0 and 1.8 GeV.

The pp→ppx(x=γγ, π0π0, π+π-) reactions have been measured at CELSIUS in the energy range Tp=775-1360 MeV using the WASA 4π detector with hydrogen pellet target. Close to threshold the ppπ+π- data are in accordance with the Roper resonance concept and/or a dynamic formation of the σ meson. At higher energies the formation of the ΔΔ system becomes increasingly dominant. The ppπ0π0 channel basically shows similar features, however, develops a peculiar enhancement at small invariant masses Mπ0π0 - in resemblance of Bose-Einstein correlations. Alternatively, the enhancement could be also indicative of a (dynamic) isospin breaking due to ππ rescattering in the σ channel. The latter is favored by the results on pd→3 He x (x=π0π0, π+π-), where the observed π0π0 enhancements are still much larger. In the γγ channel a line at Mγγ=2mπ is observed. The possible nature of this line is discussed.

Exclusive measurements of the two-pion production channel \(pp \rightarrow pp\pi^ + \pi^-\) have been carried out near threshold at CELSIUS with the PROMICE/WASA detector. They reveal \(pp \rightarrow pp^*(1440)\rightarrow pp\sigma \rightarrow pp(\pi^ + \pi^-)_{I = \ell = 0}\) as the dominant process at these energies, however, the data exhibit also significant contributions from \(p^*(1440) \rightarrow \Delta \pi \rightarrow p(\pi^ + \pi^-)_{I = \ell = 0}\). From the observed interference of these Roper decay routes their relative branching ratio is derived. The status on the search for NN-decoupled \(NN\pi\)-resonances is reviewed with regard to recent experimental searches in the pionic double charge exchange in nuclei, the two-pion production in nucleon-nucleon collisions, the photo pion-production on the deuteron and the electro pion-production in nuclei.

A brief description is given of the CELSIUS ring, of the unique pellet target system and of the CELSIUS/WASA facility. Preliminary results on the production of etas and neutral pions in proton-proton collisions are presented. The cross section for non-resonant three-pion production is only 2 mub which is an advantage for the future rare eta decay studies. Two-pion production is two orders of magnitude more frequent. As expected no etas are seen in the two-pion mass spectrum since an eta is forbidden by CP invariance.