Abstract
Using the Crystal Ball detector at the DORIS II e +e − storage ring, we measure the ηπ 0 π 0 mass spectrum in the reaction e +e −→e +e − ηπ 0 π 0→e +e −6 γ from the ηπ 0 π 0 threshold to about 2500 MeV/ c 2. The spectrum is dominated by the η′, for which we determine Γ γγ ( η′)BR( η'→6 γ)=0.36±0.02±0.03 keV. In addition, we observe an enhancement in the cross section near 1900 MeV/ c 2 which we attribute to the two-photon production of a new resonance X(1900). The angular distributions and the invariant mass distribution of the ηπ 0 subsystem are consistent with those expected for the hypothetical η 2 meson with J PC =2 −+. For this J P assignment the resonance parameters are M(X) = 1876 ±35±50 MeV/ c 2, Γ tot(X)=228±90±34 MeV/ c 2 and Γ γγ (X)BR(X→ ηππ) = 0.9±0.2±0.3 keV.
Highlights
We report on an investigation of the reaction 27,~n°n °, observed in the six-photon final state ~t
We improve the measurement of FvrOI' )BR0q'-,6y) and report for the first time on an enhancement in the cross section of "y~rln°n ° at an invariant mass of about 1900 M e V / c 2. We interpret it as the production of a new resonance X(1900)
The X(1900) is the first new resonance to be discovered in 27 collisions
Summary
We report on an investigation of the reaction 27-,~n°n °, observed in the six-photon final state ~t. Candidate events for the six-photon final state are selected by requiring six energy depositions in the calorimeter.
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