engleski

Measurement of pi^-p -> pi^0pi^0 n from threshold to p_pi^- = 750 MeV/c

Reaction pi^- p pi^0pi^0n has been measured with high statistics in the beam momentum range 270 - 750 MeV/c. The data were obtained using the Crystal Ball multiphoton spectrometer, which has 93% of 4pi solid angle coverage. The dynamics of the pi^- p pi^0pi^0n reaction and the dependence on the beam energy are displayed in total cross sections, Dalitz plots, invariant-mass spectra, and production angular distributions. Special attention is paid to the evaluation of the acceptance that is needed for the precision determination of the total cross section sigma_t(pi^- p pi^0pi^0n). The energy dependence of sigma_t(pi- p pi^0pi^0n) shows a shoulder at the Roper resonance [i.e., the N(1440)1/2^+], and there is also a maximum near the N(1520)3/2-. It illustrates the importance of these two resonances to the pi^0pi^0 production process. The Dalitz plots are highly nonuniform ; they indicate that the pi^0pi^0n final state is dominantly produced via the pi^0Delta^0(1232) intermediate state. The invariant-mass spectra differ much from the phase-space distributions. The production angular distributions are also different from the isotropic distribution, and their structure depends on the beam energy. For beam momenta above 550 MeV/c, the density distribution in the Dalitz plots strongly depends on the angle of the outgoing dipion system (or equivalently on the neutron angle). The role of the f_0(600) meson (also known as the sigma) in pi^0pi^0n production remains controversial.

total cross sections; invariant-mass spectra; Rope resonance

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