A reevaluation of evidence for light neutral bosons in nuclear emulsions

Electron–positron pair production data obtained by bombardment of emulsion detectors with either cosmic rays or projectiles with masses between 1 and 207 amu and kinetic energies between 18 GeV and 32 TeV have been reanalyzed using a consistent and conservative model of the background from electromagnetic pair conversion. The combined data yield a spectrum of putative neutral bosons decaying to e+e- pairs, with masses between 3 and 20 MeV/c2 and femtosecond lifetimes. The statistical significance against background for these "X-bosons" varies between 2σ and 8σ. The cross-section for direct production of X-bosons increases slowly with projectile energy, remaining over 1,000 times smaller than the pion production cross-section. (author)