Two-pion production in proton–proton collisions near threshold

Abstract

Two-pion production reactions in proton-proton collisions have been studied using the PROMICE/WASA detector and an internal cluster gas-jet target at the CELSIUS storage ring in Uppsala. Three out of the four isospin-independent reaction channels have been measured at several energies in the intermediate and near threshold energy region. Important p a r t s o f the analysis include the identi cation of neutral pions from the invariant mass of the decay gammas, the identi cation of positive pions with the delayed pulse technique and the use of Monte Carlo simulations to understand the detector response. The total cross sections for the pp!pp + , the pp!pp 0 0 and the pp!pn + 0 reactions are presented at beam energies ranging from 650 to 775 MeV.The production mechanism for two-pion production near threshold seems to be dominated by resonance production. The contribution from the non-resonant terms alone can not reproduce the total cross sections. In most models, two-pion production is governed by the a n d the N resonances in either one or both of the participating nucleons. The N (1440)!N( ) T=0 S wave transition has been suggested as the dominating production mechanism for two-pion production in proton-proton collisions. However, the total cross sections presented in this thesis show that other production mechanisms also must give large contributions.

From CELSIUS to COSY: on the observation of a dibaryon resonance

“…As a result of these kinematically complete and exclusive two-pion production measurements at CELSIUS and subsequently at COSY it was shown that all pp induced, i.e. all isovector channels, behave as expected from conventional t-channel excitation of Roper, ∆∆ (mutual excitation of the colliding nucleons into their first excited state ∆(1232)) and ∆(1600) resonances [8,9,10,11,12,13,14,15,16,17].…”

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.…”

“…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 NUMEN project: NUclear Matrix Elements for Neutrinoless double beta decay

“…Overall, the process scrutinized here corresponds on the target side to a nn → pp + ππreaction. In free space, the corresponding charge-conjugated reaction pp → nn + π + π + reaction and other double-pion production channels were in fact already investigated experimentally at CELSIUS and COSY [153], [154], [155], [156], [157], [158], [159], [160], [161], [162], [163] and later also at HADES [164]. Thus these correlations are expected to be present also in more involved nuclear reactions.…”