Thepp→ppπ+π−rea

Abstract

Exclusive measurements of the pp → ppπ + π − 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 σ and N * → ∆π → N σ their energy-dependent relative branching ratio is determined.

On the history of dibaryons and their final observation

“…Starting first with the WASA/PROMICE detector setup, followed by the completed WASA detector at CELSIUS and finally at COSY all ppinduced, i.e. isovector two-pion production channels from threshold up to √ s = 2.5 GeV have been studied [323,324,325,326,327,328,329,330,331] complemented by a corresponding program with polarized beam at COSY-TOF [314,332]. The results for the total cross sections are shown in Fig.…”

“…By fine adjustments of the resonance parameters in the Valencia model calculations all differential distributions could be quantitatively described by the conventional process of t-channel meson exchange leading to the excitation of the Roper resonance close to threshold followed at higher energies by the excitation of the ∆∆ system. It has been demonstrated that the Roper excitation and its decay via the interfering routes N * → N σ → N ππ and N * → ∆π → N ππ lead to very characteristic patterns in the differential distributions, especially in the ππ invariant-mass distribution [314,323,325,326]. In the ∆∆ region it is in addition the pπ invariant mass distributions, which exhibit a characteristic pattern [329].…”

“…As a result it was found [350] that there is, indeed, a highly visible low-mass enhancement but no high-mass enhancement as seen in previous σ 0 π + π pn → pp Figure 9: Energy dependence of the pp-induced total cross sections for two-pion production and their isospin decomposition. Open symbols denote results from bubble-chamber measurements [317,318,319,320,321,322], solid symbols represent measurements from WASA/PROMICE (triangles) [323,324,325], CELSIUS/WASA (circles) [326,327,328,329,330] and COSY-TOF (squares) [314]. At the top the reaction channels pp → ppπ 0 π 0 (left) and pp → nnπ + π + (right) are shown together with their isospin decomposition.…”

The legacy of the experimental hadron physics programme at COSY

“…Shown in Fig. 54 are the points obtained by the COSY-TOF collaboration [179] and values from an early version of WASA at CELSIUS [180]. These data seem to behave more or less like phase space, whose dependence is shown by the shaded area.…”

“…Total cross section for the pp → ppπ + π − reaction as a function of the proton beam energy. Closed (red) circles are WASA data taken at CELSIUS [180] whereas the (blue) squares were obtained by the collaboration at COSY. A collection of older data is also shown [181].…”