Evidence for a resonance in positron-electron scattering at 810 keV centre-of-mass Energy

Abstract

A monoenergetic positron (e § beam (A T/T< 5.10 -4) from the Stuttgart pelletron accelerator and a 4.6mg/cm 2 Be target has been employed to investigate e § e-scattering in the MeV region in a large solid angle with high statistics by means of a novel positionsensitive detector system. Superimposed on the Bhabha scattering, a structure has been found at an e § kinetic energy of 2263 keV (810keV excitation energy in the e+e -rest frame). From the energy-integrated resonance cross-section of ares' Fr;~ e-_____ 30 b. eV (c.m.) and the standard resonance cross-section we estimate partial resonance widths Fr~ e-~-72 meV or 24 meV for total angular momenta J = 0 or J = i, respectively. The structure, which has not been predicted within the framework of quantum electrodynamics, coincides with one of the sum energies of the correlated e § e-lines observed in heavy-ion collisions. 13.10.+q; 12.20.Fv; 14.80.Pb PACS: ExperimentThe detection of momentum-and energy-correlated e § e-pairs in heavy-ion collisions [-1, 2] has initiated a search for corresponding structures in elastic e § escattering [3][4][5][6][7][8]. Continuing our experiments on e § e-scattering at MeV energies [5], we have investigated the region around the structure observed at 810 keV excitation energy in heavy-ion collisions [2] with improved set-up and higher statistics.Positrons from a 26 mCi 22Na source, thermalized in a 5 gm tungsten moderator and accelerated in the Stuttgart pelletron [9], were deflected by 20 ~ in a large (if5 =40 cm) iron-free coil and focused onto a 4.6 mg/cm 2 polycrystalline beryllium target (thickness 25 ~m) or onto an aligned Be single crystal of 30 gm thickness (cf. Fig. 1). Position and divergence of the e + beam were monitored steadily by four active scintillator slits mounted 50 cm upstream of the target and leaving a 4 x 4 mm 2 aperture. Together with slits of 3.5 x 3.5 mm 2 aperture at the bending coil (4 m upstream) they defined size and divergence of the e § beam. By reversing the current of the coil, we could deflect the e § beam into the opposite direction and implant it into a window-less intrinsic Ge planar detector. This procedure enabled us to monitor the beam energy Tbe,m on-line within _+0.5keV. From the comparison with a 228Th source with its 2.615 MeV y line and the corresponding single and double escape photolines, we estimate that the quoted positron energies are accurate within _+ 4 keV on an absolute scale. The stability of the beam energy during a run of typically two hours was better than +_ 0.6 keV. The possibility to determine the beam energy at any time in a simple manner (without having to readjust the beam) allowed us to reproduce any beam energy for repeating the measurements.The detection system (cf.[5]) has been optimized

New particles in strong fields?

“…Surprisingly a clear signal was claimed by the Stuttgart group [32]. A consecutive run with increased sensitivity showed, however, that at least this original claim was wrong.…”

“…This ratio implies that to be sensitive to the whole range of uXe down to lo-" one needs an energy resolution of the order of 0.1 eV. In the experiments done so far- [28][29][30][31][32][33] using a positron beam and a fixed low-Z target A E was about 20 keV so that the interesting range of uXe could not be tested.…”

Limits for two-photon ande + e ? decay widths of positron-electron scattering resonances for?s=1.78 to 1.92 MeV

“…In an early experiment at the Stuttgart pelletron [11], at = 1.83 MeV a structure on the 5 ~-level was found with a width of ~8 keV (FWHM in the e+e-c.m, system), which fitted the Compton profile of the bound electrons of the Be target used in this experiment. Repetitions of this measurement with much improved statistical accuracy, however, did not reproduce the structure observed previously [15,17].…”

Bhabha to M�ller cross section ratios in the energy range 2.15?2.45 MeV

“…Subsequently, the present, more precise experiment was performed in the energy range above 2 MeV using thicker detectors (2000 gin). Meanwhile a discrepancy arose between results of the groups from Stuttgart [16] and Grenoble [10]. The Stuttgart group claimed the evidence of a resonance at an energy of 2263 keV with an intrinsic width of 18 meV.…”

“…A second generation of more precise Bhabha scattering experiments was performed using positron beams of higher intensity, low-Z-targets like Beryllium or hydrocarbon compounds and more sophisticated experimental techniques by groups at Grenoble [10][11][12], Groningen [13], Munich [14], Stuttgart [15,16] and by our group at Giessen [17,18]. Our first experiment [17] in the energy range 1.2-1.8 MeV was performed 1987 with the same experimental set-up as described later in this article with the exception of using semiconductor detectors of 1000 gm thickness and a different data acquisition system.…”