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Register a new userEvidence for Narrow S=+1 Baryon Resonance in Photo-production from Neutron
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The gamma n -> K+ K- n reaction on 12C has been studied by measuring both K+ and K- at forward angles. A sharp baryon resonance peak was observed at 1.54 +- 0.01 GeV with a width smaller than 25 MeV and a Gaussian significance of 4.6 sigma. The strangeness quantum number (S) of the baryon resonance is +1. It can be interpreted as a molecular meson-baryon resonance or alternatively as an exotic 5-quark state (uudd{s_bar}) that decays into a K+ and a neutron. The resonance is consistent with the lowest member of an anti-decuplet of baryons predicted by the chiral soliton model.
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The first study of quasi-free Compton scattering on the neutron in the energy range of $E_{gamma}=0.75 - 1.5$ GeV is presented. The data reveals a narrow peak at $Wsim 1.685$ GeV. This result, being considered in conjunction with the recent evidence for a narrow structure at $Wsim 1.68$GeV in the $eta$ photoproduction on the neutron, suggests the existence of a new nucleon resonance with unusual properties: the mass $Msim 1.685$GeV, the narrow width $Gamma leq 30$MeV, and the much stronger photoexcitation on the neutron than on the proton.
Exclusive and kinematically complete high-statistics measurements of quasifree polarized $vec{n}p$ scattering have been performed in the energy region of the narrow resonance structure $d^*$ with $I(J^P) = 0(3^+)$, $M approx$ 2380 MeV/$c^2$ and $Gamma approx$ 70 MeV observed recently in the double-pionic fusion channels $pn to dpi^0pi^0$ and $pn to dpi^+pi^-$. The experiment was carried out with the WASA detector setup at COSY having a polarized deuteron beam impinged on the hydrogen pellet target and utilizing the quasifree process $vec{d}p to np + p_{spectator}$. That way the $np$ analyzing power $A_y$ was measured over a large angular range. The obtained $A_y$ angular distributions deviate systematically from the current SAID SP07 NN partial-wave solution. Incorporating the new $A_y$ data into the SAID analysis produces a pole in the $^3D_3 - ^3G_3$ waves as expected from the $d^*$ resonance hypothesis.
Revised analysis of $Sigma$ beam asymmetry for $eta$ photoproduction on the free proton reveals a resonant structure at $Wsim 1.69$ GeV. Comparison of experimental data with multipole decomposition based on the E429 solution of the SAID partial wave analysis and including narrow states, suggests a narrow ($Gamma leq 15$ MeV) resonance. Possible candidates are $P_{11}$, $P_{13}$, or $D_{13}$ resonances. The result is considered in conjunction with the recent evidence for a bump-like structure at $Wsim 1.67 - 1.68$ GeV in quasi-free $eta$ photoproduction on the neutron.
New results on quasi-free $eta$ photoproduction on the neutron and proton bound in a deuteron target are presented. The $gamma n to eta n$ quasi-free cross section reveals a bump-like structure which is not seen in the cross section on the proton. This structure may signal the existence of a relatively narrow ($Msim 1.68$ GeV, $Gamma leq 30$ MeV) baryon state.
All the time since its discovery the N$^*$(1440) baryon state, commonly known as Roper resonance, has been a state with many question marks - despite of its 4-star ranking in the particle data book. One reason is that it does not produce any explicit resonance-like structures in the observables of $pi$N or $gamma$N reactions. Only in partial wave analyses of $pi$N scattering data a clear resonance strcuture gets obvious in the $P_{11}$ partial wave. Very recent measurements of the J/$Psi$ decay by the BES collaboration and of the $pp to nppi^+$ reaction at 1.3 GeV by the CELSIUS-WASA collaboration show for the first time a clear resonance structure in the invariant $npi^+$ mass spectrum for the Roper resonance at M $approx$ 1360 MeV with a width of about 150 MeV. These values agree very favorably with the pole position results of recent $pi$N phase shift analyses. In consequence of this very low-lying pole postion, which is roughly 100 MeV below the nominal value, the decay properties have to be reinvestigated. From our two-pion production data we see that the decay mainly proceeds via N$^* to $N$sigma$, i.e. a monopole transition as expected for the breathing mode of the nucleon.
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