Search

Abstract Coherent double neutral-pion photoproduction on the deuteron, γ d → π 0 π 0 d , has been experimentally studied at incident photon energies ranging from 0.75 to 1.15 GeV. The total cross section as a function of the γd center-of-mass energy shows resonance-like behavior, which peaks at approximately 2.47 and 2.63 GeV. The measured angular distribution of deuteron emission is rather flat, which cannot be reproduced by the kinematics of quasi-free π 0 π 0 production with deuteron coalescence. In π 0 d invariant-mass distributions, a clear peak is observed at 2.14 ± 0.01 GeV / c 2 with a width of 0.09 ± 0.01 GeV / c 2. The spin-parity of this state is restricted to 1 + , 2 + or 3 − from the angular distributions of the two π 0 s. The present work shows strong evidence for the existence of an isovector dibaryon resonance with a mass of 2.14 GeV / c 2. The 2 + assignment is consistent with the theoretically predicted D 12 state, and also with the energy dependence of the πd partial-wave amplitude P 2 3 for the π ± d → π ± d and π + d → p p reactions. [ABSTRACT FROM AUTHOR]

Recently a two-step process has been proposed for the double-pionic fusion to deuterium p n (I = 0) → d π + π −. Its calculation is solely based on total cross section data for the two sequential single-pion production steps p n (I = 0) → p p π − followed by p p → d π +. Though this sequential process was aimed to explain the dibaryon resonance d ⁎ (2380) peak in double-pionic fusion, we demonstrate that this is not the case. It rather fits to a possible broad bump at 2.31 GeV in the energy dependence of the p n → d π 0 π 0 reaction, which was recently interpreted as a consequence of dibaryonic excitations in isoscalar single-pion production. [ABSTRACT FROM AUTHOR]

Exclusive measurements of the quasi-free p n → p p π − and p p → p p π 0 reactions 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 T p = 0.95 – 1.3 GeV ( s = 2.3 – 2.46 GeV ), which includes the regions of Δ ( 1232 ) , N ⁎ ( 1440 ) and d ⁎ ( 2380 ) resonance excitations. From these measurements the isoscalar single-pion production has been extracted, for which data existed so far only below T p = 1 GeV . We observe a substantial increase of this cross section around 1 GeV, which can be related to the Roper resonance N ⁎ ( 1440 ) , the strength of which shows up isolated from the Δ resonance in the isoscalar ( N π ) I = 0 invariant-mass spectrum. No evidence for a decay of the dibaryon resonance d ⁎ ( 2380 ) into the isoscalar ( N N π ) I = 0 channel is found. An upper limit of 180 μb (90% C.L.) corresponding to a branching ratio of 9% has been deduced. [ABSTRACT FROM AUTHOR]

The total cross sections were measured for coherent double neutral-pion photoproduction on the deuteron at incident energies below 0.9 GeV for the first time. No clear resonance-like behavior is observed in the excitation function for W γ d = 2.38 – 2.61 GeV , where the d ⁎ ( 2380 ) dibaryon resonance observed at COSY is expected to appear. The measured excitation function is consistent with the existing theoretical calculation for this reaction. The upper limit of the total cross section is found to be 0.034 μb for the dibaryon resonance at W γ d = 2.37 GeV (90% confidence level) in the γ d → π 0 π 0 d reaction. [ABSTRACT FROM AUTHOR]

Recently it has been shown by exclusive and kinematically complete experiments that the appearance of a narrow resonance structure in double-pionic fusion reactions is strictly correlated with the appearance of the so-called ABC effect, which denotes a pronounced low-mass enhancement in the ππ -invariant mass spectrum. Whereas the resonance structure got its explanation by the d ⁎ ( 2380 ) dibaryonic resonance, a satisfactory explanation for the ABC effect is still pending. In this paper we discuss possible explanations of the ABC effect and their consequences for the internal structure of the d ⁎ dibaryon. To this end we examine and review a variety of proposed explanations for the ABC effect, add a new hypothesis and confront all of them with the experimental results for the n p → d π 0 π 0 and n p → n p π 0 π 0 reactions, which are the most challenging ones for this topic. [ABSTRACT FROM AUTHOR]

Exclusive measurements of the quasi-free n p → n p π 0 π 0 reaction have been performed by means of dp collisions at T d = 2.27 GeV using the WASA detector setup at COSY. Total and differential cross sections have been obtained covering the energy region 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 Fäldt and Wilkin as well with those of Albadajedo and Oset. The ABC effect, i.e. the low-mass enhancement in the π 0 π 0 -invariant mass spectrum, is found to be very modest – if present at all, which might pose a problem to some of its interpretations. [ABSTRACT FROM AUTHOR]

Abstract: The two-pion production in nucleon–nucleon collisions has been investigated systematically by exclusive and kinematically complete measurements at CELSIUS/WASA and more recently at COSY-TOF and particularly WASA@COSY. The data for all -induced isovector production channels in the energy range from threshold up to = 1.4 GeV can be well understood by -channel Roper, and possibly excitation. In contrast to that the purely isoscalar reaction channel exhibits a narrow resonance-like structure with and in the total cross section, which is correlated with an intriguing low-mass enhancement in the invariant mass spectrum (ABC effect) as well as with the formation of a system in the intermediate state. From the angular distributions the spin and parity of this structure have been derived resulting in . The consequences of the resonance hypothesis for other reaction channels are discussed. The fact that also the double-pionic fusion reactions to 3He and 4He exhibit a corresponding resonance-like structure in the total cross section correlated with the ABC effect suggests that the underlying conjectured resonance is obviously robust enough to survive in nuclei. [Copyright &y& Elsevier]