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Si stellar reaction rate are discussed.

We report on a search for a short-lived neutral particle produced in the decay of the 9.17-MeV J r,r = 2 + state in 14 N. The experiment is sensitive to decays into an e+e~ pair with T^.< 10"" U S. For m^= 1.7 MeV we place a limit on the branching ratio of r^/r y^4 x 10~4 at the 90% confidence level.PACS numbers: 14.80. Gt, 23.90. +w Anomalous narrow peaks have been observed in the spectra of positrons emitted in heavy-ion collisions in several recent experiments at Gesellschaft fur Schwerionenforschung Darmstadt (GSI). 1 " 3 In addition, a new experiment 4 has revealed that the positrons associated with these peaks are correlated with electrons whose energy spectrum also contains a narrow peak at the same energy as the positron peaks. One explanation 4 " 6 for these peaks is the production and subsequent decay of a previously unobserved neutral particle of mass -1.7 MeV. Monte Carlo simulations 4 of such a particle decay, assuming the particle to be produced at rest in the center-of-mass system and then to decay isotropically into ane + e" pair, have produced results in good agreement with the experimental spectra. Less exotic explanations 4,7 ' 8 have also been considered, but none so far has succeeded in explaining all the features of the data.Schafer et al s and Balantekin et al. 6 have considered the results of the GSI experiments and concluded that a neutral particle could have gone undetected in previous experiments only if it were pseudoscalar with a lifetime in the range 10"* 9 to 10"" 13 s. There have been many searches for light pseudoscalar particles ("axions") in nuclear 9 and particle 10 physics, at beam dumps, 11 and at reactors. 12 However, none of these is sensitive to a particle with a lifetime less than -10"" n -10~1 2 s. 13 If the has a nonzero coupling to quarks and a mass less than the energy of a nuclear transition then it could be produced in nuclear decay. The spin and parity of such a particle constrain its emission to obey the same selection rules as a magnetic y-ray transition, 14 and decay is thus expected to compete with the y decay of excited nuclear states if the transition is predominantly magnetic. In this Letter we report on a search for a neutral particle with a mass between 1.02 and 2.2 MeV and a lifetime between ~-10" n and -10"" 19 s emitted in a nuclear Ml transition. 15 The observed positron peaks in the heavy-ion exper-iments, if attributed to this new neutral particle, indicate a large branching ratio for decay to e + e~. To observe these pairs emitted from a particle produced in nuclear decay we must separate them from the ordinary internal pairs produced in the electromagnetic decay of the nuclear state (a virtual photon converts internally to e + e~). It has been suggested previously 16 that pairs produced from the decay of a pseudoscalar particle can be distinguished from internal pairs by their angular correlation. Previous detailed studies 17 of the angular correlation of nuclear pairs, which were used to extract transition multipolarities, indic...

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We present the results of a search for scalar and pseudoscalar particles produced in nuclear deexcitation. Measurements of the angular correlation of e + epairs emitted from excited states of I4N, 160, and '~e provided sensitivity to particles with lifetimes I O p l 9 < r < 10p" s within the mass range 2m, < Mx < 5 MeV. We find no evidence for such particles and set upper limits on the branching ratio to electromagnetic decay.