a Search for Low-Mass Long-Lived States Coupling to Positron-Electron

A search for long-lived (tau _sp{~}{>} 10^ {-12} sec) low-mass (1500 _sp {~}{<} M_{X ^0} _sp{~}{< } 1850 keV) states coupling to e^+e ^- has been performed at Brookhaven National Laboratory. The experimental search in this mass-lifetime region has been motivated by the observation of anomalous monoenergetic e^+e^- emission in heavy-ion collisions near the Coulomb barrier at GSI Darmstadt. An apparatus was constructed at Brookhaven National Laboratory for these measurements. A monoenergetic positron beam is obtained from the moderation of positrons obtained from a ^{22}Na source in a thin W foil. The source and moderator assembly are housed in the terminal of a 3 MV electrostatic accelerator. The detector system consists of an array of MWPC's and plastic scintillators for the complete measurement of the e ^+e^- scattering kinematics. The extension of the experimental sensitivity to this previously inaccessible lifetime region is accomplished with the use of an active target system. The active target consists of a layered arrangement of Be-plastic scintillator -Platinum-plastic scintillator. The Be serves as a production target, while the Pt stops the beam and eliminates the non-resonant (Bhabha) e^+e^- scattering background. Neutral non-interacting objects produced in the Be which live sufficiently long may decay outside the active target. The active target allows a range in lifetimes to be investigated which is determined by the active target thickness and the fiducial volume of the detector apparatus. Data are presented from two active target systems of total thicknesses 3 mm and 2 mm. Excitation functions have been obtained spanning the mass range 1500 _sp{~}{<} M_ {X^0} _sp{~} {<} 1850 keV. The data and complementary Monte-Carlo studies have been used to obtain lifetime limits on neutral non-interacting e^+e^ - states. The possibility of absorption of the neutral object in matter has also been considered. Results are presented which rule out non-interacting states coupling to e^+e^- in the lifetime range 8times10^{-13} _sp{~}{<} tau _sp{~}{<} 6times10^{-10} sec throughout the mass range 1500 _sp{~} {<} M_{X^0} _sp{~}{<} 1850 keV.