A search for low-mass short-lived states coupling to e{sup +}e{sup {minus}}
Following the discovery of correlated positron-electron peaks in heavy ion experiments, the existence of a previously undetected particle coupling to e{sup +}e{sup {minus}} was postulated. A new experiment with improved sensitivity was suggested to detect this particle. Monoenergetic positrons would be accelerated onto electrons in a lithium target and the kinematics of the e{sup +}e{sup {minus}} scattering events measured by energy and position sensitive detectors. The experiment was carried out at Brookhaven National Laboratory where a 3 MV Dynamitron electrostatic accelerator was converted to a positron accelerator. The monoenergetic positrons were provided by a {sup 22}Na source in conjunction with a thin single crystal tungsten moderator, and accelerated by the Dynamitron to a tunable energy between 1.0 and 2.5 MeV with keV resolution. The beam was transported through a new beam line to the target chamber where it could be focused down to 1 mm spot size. The energy of the interactions of the positrons with the lithium target was measured by plastic scintillators, and the position of the scattered particles was recorded by a set of four multi-wire proportional chambers capable of sub millimeter resolution. In a first experiment, the positron beam was scanned between 2150 and 2350 keV in 5 keV steps onto a 1.5 mg/cm{sup 2} lithium target with at least 60,000 coincidences recorded per energy point. The full kinematics of each positron-electron scattering event was reconstructed. The positron-nucleus scattering events were also detected and served as a normalization to eliminate all beam and target effects. A normalized e{sup +}-e{sup {minus}} scattering excitation function was derived, leading to an upper limit (90% CL) to the cross section for pointlike particles of {sigma} {approx} 1-5 mb. This translates to lower limits on the lifetime of {tau} {ge} 0.5 to 7 x 10{sup 13} sec.
- Research Organization:
- Yale Univ., New Haven, CT (United States)
- OSTI ID:
- 111267
- Resource Relation:
- Other Information: TH: Thesis (Ph.D.); PBD: 1993
- Country of Publication:
- United States
- Language:
- English
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