Observation of an “ABC” Effect in Proton-Proton Collisions

Abstract

The cross section for inclusive multipion production in the pp → ppX reaction was measured at COSY-ANKE at four beam energies, 0.8, 1.1, 1.4, and 2.0 GeV, for low excitation energy in the final pp system, such that the diproton quasi-particle is in the 1 S0 state. At the three higher energies the missing mass MX spectra show a strong enhancement at low MX , corresponding to an ABC effect that moves steadily to larger values as the energy is increased. Despite the missing-mass structure looking very different at 0.8 GeV, the variation with MX and beam energy are consistent with two-pion production being mediated through the excitation of two ∆(1232) isobars, coupled to S-and D-states of the initial pp system. The ABC effect is a sharp enhancement of the twopion invariant mass spectrum near its threshold that has puzzled hadron physicists for almost fifty years. First observed in a pd → 3 HeX missing-mass experiment [1], and subsequently confirmed using a deuteron beam [2], it was seen most spectacularly in the dd → 4 HeX reaction [3,4]. The ABC typically manifests itself as a peak at an invariant mass M X ≈ 310 − 330 MeV/c 2 and it was first even speculated that it might be a scalar meson with isospin I = 0. However, the fact that the mass and width varied with experimental conditions [2] suggests that the ABC is a kinematic enhancement. This is consistent with the smooth behavior of the isoscalar ππ phase shifts at low energies. To understand the nature of the ABC it is necessary to investigate the effect in simpler systems.The ABC showed up very clearly in the zero-degree momentum spectrum of the deuteron from np → dX [5].Here it leads to two peaks, i.e. forward and backward production in the c.m. system, though the momentum spread of the neutron beam degraded the missing-mass resolution. This has recently been overcome by measuring the quasi-free pd → dπ 0 π 0 p sp reaction semi-exclusively at beam energies of T p = 1.03 and 1.35 GeV, with just the spectator proton p sp escaping detection [6]. The only published study of the ABC effect in proton-proton collisions was carried out at T p = 1.52 and 1.81 GeV [7], though this concentrated on possible substructure.Studies of the energy dependence of the pd → 3 HeX and dd → 4 HeX cross sections showed that the ABC effect is most prominent for energies where the maximum missing mass is about 600 MeV/c 2 [3]. Since this corresponds to the mass difference between two ∆(1232) isobars and two nucleons, it is tempting to suppose that the two-pion production is mediated by the excitation and decay of two separate ∆ isobars. Such a model with π [8] and then π + ρ exchange [9] had semi-quantitative success in describing the existing np → dX data. The dominant contribution to the cross section arises when the two ∆ are in a relative S-wave. For the production of an I ππ = 0 pion pair, isospin conservation requires I ∆∆ = 0. It follows from the generalized Pauli principle that the two isobars must then couple to a total spin of S ∆∆ = 1 or 3. In contrast, for pp → ∆∆ → ppππ, I...

Abashian-Booth-Crowe Effect in Basic Double-Pionic Fusion: A New Resonance?

“…NN no evidence for a significant ABC effect is found [9][10][11]-including the limiting case of a fusion to a quasibound pp pair in an s wave [11,12]. In all these cases the observables are well accounted for by Roper and t-channel ÁÁ processes.…”

Resonance-like coherent production of a pion pair in the reaction $pd \rightarrow pd \pi\pi$pd→pdππ in the GeV region

“…The experimental setup, technique, and conditions in this work are similar to those used in several experiments performed at ANKE and described in detail before [30][31][32][33][34][35]. So sect.…”

The legacy of the experimental hadron physics programme at COSY

“…In some ways the pp → {pp} s π 0 reaction is an ideal one to study at COSY because the 1 S 0 diproton configuration means that there are no spin degrees of freedom in the final state that have to be determined through a doublescattering measurement. The differential cross section was measured at 800 MeV at ANKE [132] by detecting both protons in the Forward Detector, with the π 0 being recognized from the peak in the missing-mass spectrum. However, this restricted the angular coverage to diproton c.m.…”

“…Such experiments are only possible at ANKE close to threshold where the spectrometer acceptance covers a large fraction of the solid angle. The programme involved the measurement of the differential cross section and analyzing power in pp → {pp} s π 0 [120], the differential cross section and analyzing power in quasi-free pn → {pp} s π − with a deuterium target [137], and the analyzing powers and spincorrelations in quasi-free n p → {pp} s π − with a polarized deuteron beam and a polarized hydrogen cell target [125].…”

“…The pp → {pp} s π 0 study was carried out in a similar manner to the earlier π 0 experiments at ANKE [132,133], but it is important to realize that the differential cross section is symmetric about 90 • and the analyzing power is antisymmetric. This feature provides a useful extension to the limited ANKE acceptance.…”