Long-lived tracks in emulsions: New hadrons or background?

Abstract

We discuss the observation of long-lived particle tracks in emulsion chambers in the context of the recent developments in particle physics that hint at the existence of particles with new quantum numbers. Both cosmic ray ($E\simeq {10}^4$ GeV) and accelerator ($E=205\mathrm{}$ GeV) events have been reported. We address the following questions: What is the nature of the evidence? What do the observations imply for the production cross sections of new hadrons? What are the backgrounds and how do some of them depend on the accuracy of coplanarity measurements? We reach the following conclusions: Hyperon decay and elastic scattering or diffraction dissociation of a secondary track in the emulsion, mimicking the decay of a long-lived particle, constitute backgrounds at the few- to 50-percent level. Therefore events in which only a single candidate is seen are unlikely to be conclusive; in candidate events in which both members of a possible associated pair of new particles are seen, these backgrounds are suppressed to the 0.01-percent level. If both of the candidates for associated production observed in the cosmic ray energy range are real, we can estimate a production cross section for new particles with hadron beams at the 10-${10}^2$-μb level at ${10}^4$ GeV. From threshold behavior we estimate a production cross section at the 1-10-μb level at Fermilab energies.