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HK: Hadronen und Kerne
HK 62: Hadronic Probes VI, Resonances, Reactions
HK 62.2: Gruppenbericht
Donnerstag, 19. März 1998, 14:30–15:00, C
Search for the Dibaryon d′ in Pionic Double Charge Exchange on Helium — •R. Meier, R. Bilger, H. Clement, J. Gräter, J. Pätzold, and G. J. Wagner — Physikalisches Institut, Universität Tübingen, Auf der Morgenstelle 14, D-72076 Tübingen* - for the CHAOS-Collaboration.
The hypothesis of a resonance in the πNN system, the so-called d′, has been suggested as the explanation for the behaviour of low energy pionic double charge exchange (DCX) on nuclei [1]. The properties attributed to the d′ according to an analysis of low energy DCX on nuclei are I(JP) = even (0−), mass m ≈ 2.06 GeV/c2 and width Γπ NN ≈ 0.5 MeV/c2 [1]. It is desirable to produce the d′ outside the nuclear medium as a proof of its existence. Searches are in progress in various reaction channels, e. g. in the two pion production in proton-proton scattering, in photoproduction on the deuteron, and in proton-deuteron scattering. Recently a narrow signal consistent with the d′ hypothesis has been observed in the invariant mass spectrum Mppπ− in a measurement of the reaction pp→ppπ+π− by the WASA/PROMICE-collaboration at Celsius [2].
Free production of the d′ should also be possible in the pionic double charge exchange reaction on helium isotopes. Here, in contrast to the measurements of DCX on nuclei to discrete states, the final state consists of unbound identical nucleons. Three experiments (E725, E719 and E785) looking for d′ signatures in DCX on helium isotopes have been carried out at TRIUMF using the CHAOS spectrometer [3]. E725 searched for a signature in the inclusive total π+ 4He DCX cross section. In the region just above the d′ threshold, d′ production should dominate the total DCX cross section and should produce a visible shoulder [4]. Such a shoulder was found in the data of experiment E725. In a semiexclusive measurement, E719 investigated the invariant mass Mppπ− in the exit channel of DCX on 4He in an energy region about 30 MeV above the d′ threshold, searching for a peak at the predicted d′ mass. E785 extended the measurements of E725 and E719 to DCX on 3He. Status and results of these experiments will be reported.
[1] R. Bilger et al., Z. Phys. A 343, 491 (1992); Phys. Rev. Lett. 71, 42 (1993); 72, 2972 (1994); K. Föhl et al., Phys. Rev. Lett. 79 (1997).
[2] W. Brodowski et al., Z. Phys. A 355, 5 (1996).
[3] G. R. Smith et al., Nucl. Instr. Meth. A 362, 349 (1995).
[4] H. Clement, M. Schepkin, G.J. Wagner, O. Zaboronsky, Phys. Lett. B 337, 43 (1994).
*supported by BMBF (06 TU 886) and DFG (MU 705/3).