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Register a new userA Search for Neutral Baryon Resonances Below Pion Threshold
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The reaction p(e,e^{prime}pi^+)X^0 was studied with two high resolution magnetic spectrometers to search for narrow baryon resonances. A missing mass resolution of 2.0 MeV was achieved. A search for structures in the mass region of 0.97 < M_{X^0} < 1.06 GeV yielded no significant signal. The yield ratio of p(e,e^{prime}pi^+)X^0/p(e,e^{prime}pi^+)n was determined to be (-0.35 +/- 0.35) times 10^{-3} at 1.004 GeV and (0.34 +/- 0.42) times 10^{-3} at 1.044 GeV.
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In two series of high-resolution coincidence experiments at the three-spectrometer facility at MAMI, the H(e,epi^+)X and D(e,ep)X reactions were studied to search for narrow nucleon resonances below pion threshold. The missing-mass resolution was 0.6 to 1.6 MeV/c^2 (FWHM) in the proton experiment and 0.9 to 1.3 MeV/c^2 in the deuteron experiment. The experiments covered the missing-mass region from the neutron mass up to about 1050 and 1100 MeV/c^2, respectively. None of our measurements showed a signal for narrow resonances to a level of down to 10^-4 with respect to the neutron peak in the missing-mass spectra.
A hypothesis is proposed herein, suggesting that a pion-nuclear resonance may be observed in the $alpha+dto{}^6mathrm{Li}(3.563)+pi^0$ reaction. The resonance has a $pi NNalpha$ structure, containing $alpha NN$ and $pi NN$ subsystems. The former corresponds to the $A=6$ isotriplet ($^6mathrm{He}_text{g.s.}$, $^6mathrm{Li}(3.563)$, $^6mathrm{Be}_text{g.s.}$), whereas the latter is a hypothetical $NN$-decoupled dibaryon. We propose an experiment to search for this resonance using the $^7mathrm{Li}(p,d)$ reaction.
We report the measurement of near threshold neutral pion electroproduction cross sections and the extraction of the associated structure functions on the proton in the kinematic range $Q^2$ from 2 to 4.5 GeV$^2$ and $W$ from 1.08 to 1.16 GeV. These measurements allow us to access the dominant pion-nucleon s-wave multipoles $E_{0+}$ and $S_{0+}$ in the near-threshold region. In the light-cone sum-rule framework (LCSR), these multipoles are related to the generalized form factors $G_1^{pi^0 p}(Q^2)$ and $G_2^{pi^0 p}(Q^2)$. The data are compared to these generalized form factors and the results for $G_1^{pi^0 p}(Q^2)$ are found to be in good agreement with the LCSR predictions, but the level of agreement with $G_2^{pi^0 p}(Q^2)$ is poor.
The total cross sections were measured for coherent double neutral-pion photoproduction on the deuteron at incident energies below 0.9 GeV for the first time. No clear resonance-like behavior is observed in the excitation function for $W_{gamma d}=2.38$--2.61 GeV, where the $d^*(2380)$ dibaryon resonance observed at COSY is expected to appear. The measured excitation function is consistent with the existing theoretical calculation for this reaction. The upper limit of the total cross section is found to be $0.034$~$mu$b for the dibaryon resonance at $W_{gamma d}=2.37$~GeV (90% confidence level) in the $gamma d to pi^0pi^0 d$ reaction.
The E815 (NuTeV) neutrino experiment has performed a search for a 33.9 MeV/c^2 weakly-interacting neutral particle produced in pion decay. Such a particle may be responsible for an anomaly in the timing distribution of neutrino interactions in the KARMEN experiment. E815 has searched for this particles decays in an instrumented decay region; no evidence for this particle was found. The search is sensitive to pion branching ratios as low as 10^-13.
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