J. Greiff

Thepp→ppπ+π−reaction studied in the low-energy tail of the Roper resonance

Exclusive measurements of the p p ->p p pi+ pi- reaction have been carried out at Tp = 775 MeV at CELSIUS using the PROMICE/WASA setup. Together with data obtained at lower energy they point to a dominance of the Roper excitation in this process. From the observed interference of its decay routes N* ->N sigma and N* ->Delta pi ->N sigma their energy-dependent relative branching ratio is determined.

Excitation functions of the analyzing power in elastic proton-proton scattering from 0.45 to 2.5 GeV

Abstract.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.

Exclusive measurements of pd→He3ππ: The ABC effect revisited

Abstract Exclusive measurements of the reactions p d → He 3 π + π − and p d → He 3 π 0 π 0 have been carried out at T p = 0.893 GeV 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 π 0 π 0 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.

Measurement of spin correlation parameters A(NN), A(SS), and A(SL) at 2.1-GeV in proton proton elactic scattering

At the Cooler Synchrotron COSY/Jülich spin-correlation parameters in elastic proton-proton (pp) scattering have been measured with a 2.11 GeV polarized proton beam and a polarized hydrogen atomic beam target. We report results for A(NN), A(SS), and A(SL) for c.m. scattering angles between 30 degrees and 90 degrees. Our data on A(SS)--the first measurement of this observable above 800 MeV--clearly disagrees with predictions of available pp scattering phase-shift solutions while A(NN) and A(SL) are reproduced reasonably well. We show that in the direct reconstruction of the scattering amplitudes from the body of available pp elastic scattering data at 2.1 GeV the number of possible solutions is considerably reduced.

Excitation functions of spin correlation parameters ANN, ASS, and ASL in elastic pol.p pol.p scattering between 0.45 and 2.5 GeV

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 COSY 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 2000 MeV/c covering angles theta_{c.m.} between 30 deg and 90 deg. 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.

Study of baryon and search for dibaryon resonances by the pp → ppπ+π− reaction

Exclusive measurements of the two-pion production channel pp → ppπ+π− have been carried out near threshold at CELSIUS with the PROMICE/WASA detector. They reveal \(pp \to pp*\left( {1440} \right) \to pp\sigma \to pp\left( {{\pi ^ + }{\pi ^ - }} \right)I = \ell = 0\) as the dominant process at these energies, however, the data exhibit also significant contributions from \({p^*}\left( {1440} \right) \to \Delta \pi \to p\left( {{\pi ^ + }{\pi ^ - }} \right)I = \ell = 0\) \({p^*}\left( {1440} \right) \to \Delta \pi \to p\left( {{\pi ^ + }{\pi ^ - }} \right)I = \ell = 0\) From the observed interference of these Roper decay routes their relative branching ratio is derived. The status on the search for N N-decoupled N N π-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.

Excitation functions of spin correlation parameters ANN, ASS, and ASL in elastic pp scattering between 0.45 and 2.5 GeV

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 COSY 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 2000 MeV/c covering angles theta_{c.m.} between 30 deg and 90 deg. 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.

ππ PRODUCTION IN PROTON-PROTON COLLISIONS

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 π0π0channels. 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 π0π0 channel moreover exhibit a low-mass enhancement in the π0π0 invariant mass spectrum, which is reminiscent of the ABC-effect found in double-pionic fusion to light nuclei.

Spectator tagging in quasi-free pn-reactions on deuterium at PROMICE/WASA, CELSIUS

A set of silicon detectors has been added to the PROMICE/WASA (P/W) experiment at CELSIUS. These detectors have been used for spectator-proton tagging in proton deuteron collisions to investigate proton-neutron reactions at intermediate energies. The performance of the setup has been tested by measuring the pd --> d pi(o)p(spectator) reaction.

Exclusive measurements of p d → He 3 π π : The ABC effect revisited

Abstract Exclusive measurements of the reactions p d → He 3 π + π − and p d → He 3 π 0 π 0 have been carried out at T p = 0.893 GeV 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 π 0 π 0 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.