Measurement of the pn → ppπ0π− Reaction in Search of the ABC Resonance

With pd collisions at T_p = 1.2 GeV exclusive measurements of the quasi-free pn -> pppi^0pi^- reaction have been carried out. Using the WASA detector setup at COSY total and differential cross sections have been obtained for the energy region &radics = 2.35 - 2.48 GeV. Though this includes the region of the ABC effect and its associated resonance structure, no low-mass enhancement (ABC effect) is found in the pi^0pi^--invariant mass spectrum - in agreement with the constraint from Bose statistics demanding the isovector pion pair to be in relative p-wave. Conventional calculations including t-channel processes for Roper, Delta(1600) and DeltaDelta excitations and their decays, which are well-known from the study of the two-pion production in pp collisions, provide a reasonable description of the data at high energies, but fall low at low energies. From this we conclude that a large contribution from a so far unknown isoscalar low-energy process is missed in the calculations. Inclusion of the ABC resonance at m = 2.37 GeV with Gamma = 70 MeV and I(J^P) = 0(3^+), which was recently observed in the pn -> dpi^0pi^0 reaction, leads to a much improved description of the data at low energies.


Introduction
Recent data on the basic double-pionic fusion reaction pn → dπ 0 π 0 demonstrate that the so-called ABC effect is tightly correlated with a narrow resonance structure in the total cross section of this reaction [1,2]. The ABC effect denoting a huge low-mass enhancement in the ππ invariant mass spectrum is observed to happen, if the initial nucleons or light nuclei fuse to a bound final nuclear system and if the produced pion pair is isoscalar. Since as of yet no quantitative understanding of this phenomenon has been available, it has been named after the initials of Abashian, Booth and Crowe, who first observed it in the inclusive measurement of the pd → 3 HeX reaction more than fifty years ago [3].
In exclusive high-statistics measurements of the isoscalar pn → dπ 0 π 0 reaction it has recently been shown that this ABC effect is correlated with a pronounced resonance structure in the total cross section at √ s = 2.37 GeV with a width of only 70 MeV and quantum numbers I(J P ) = 0(3 + ) [1]. This structure is situated about 90 MeV below √ s = 2m ∆ , the peak position of the conventional tchannel ∆∆ process, and has a width, which is about three times narrower than this process. From the Dalitz plots of the pn → dπ 0 π 0 reaction it is concluded that this resonance decays nevertheless via the intermediate ∆ + ∆ 0 system into its final dπ 0 π 0 state. If this scenario is correct, then also the pn → ppπ 0 π − reaction should be affected by this resonance, since this channel may proceed via the same intermediate ∆ +

Experiment and Results
We have investigated this question experimentally by using a pd run taken at T p = 1.2 GeV with the WASA detector facility at COSY [4,5]. The data have been analyzed for the reaction pd → ppπ 0 π − + p spectator utilizing quasifree kinematics. That way we obtain a data sample spanning the energy region 2.35 GeV ≤ √ s ≤ 2.45 GeV and thus also covering the region of the ABC effect and its associated resonance structure.
The observed differential cross sections are reasonably well described by conventional t-channel resonance processes [6]. In particular the π 0 π − invariant mass spectra show no ABC effect -in agreement with the fact that the isovector pion pair must be in relative p-wave and hence strongly suppresses 09033-p.2 any low-mass enhancement. Preliminary results for the total cross section are shown in Fig. 1 together with previous data from Bubble chamber measurements [7][8][9]. They exhibit a smoothly and monotonically rising total cross section with some indication of a flat top in the energy region of the ABC effect. As already observed in other two-pion production channels [10,16] the Gatchina bubble-chamber data [9] (open squares in Fig. 1) lie high compared to all other data. Since there appears to be a systematic problem with these data, we omit them in the subsequent discussion.
Original Valencia model calculations [10], which are based on resonant and non-resonant t-channel processes for the two-pion production are depicted by the dash-dotted line with their largest contributions originating from Roper excitation and its decay route N * → ∆π (dotted) as well as from the ∆∆ process, the mutual excitation of two nucleons into the ∆(1232) state (dashed). Modified calculations of the Valencia model, which are tuned to describe the isovector pp-induced channels quantitatively by modifications of the Roper decay process [11][12][13][14], the ∆∆ excitation process [15] as well as inclusion of the∆(1600) resonance [16,17], give a somewhat larger cross section and lead to agreement with the data at high energies. However, at low energies they still under-predict the observed cross section severely. This failure points to an isoscalar reaction component, not included in the t-channel treatment of two-pion production.
It is intriguing that this failure appears to be largest in the energy region, where the ABC effect has been observed in the other isoscalar channel, the dπ 0 π 0 channel [1,2]. Indeed, adding an amplitude for the ABC resonance at m = 2.37 GeV with Γ = 70 MeV, I(J P ) = 0(3 + ) and fitting the strength of its contribution to the data leads to a reasonable description of the total cross section also at low energies (solid line in Fig. 1). However, a simultaneous good description of the differential data is only achieved, if the phenomenologically introduced [1] form factor at the ∆∆ decay vertex is omitted. Considerations to overcome this problem are worked on [6,18].