Search for Highly Interacting Fractionally Charged Particles at Pep.

We have searched for fractionally charged, highly interacting particles produced in e('+)e('-) annihilation at 29 GeV c.m. energy. Our experiment was performed at the positron electron storage ring (PEP) at the Stanford Linear Accelerator Center (SLAC). The search used the innermost part of the two-arm Free Quark Search (FQS) detector. This part was called the Thin Front End (TFE) and it covered 1/3 of the full solid angle. Each of its arms consisted of five multiwire proportional chambers (MWPC's), used for tracking and dE/dx measurement, and three hodoscopes of 0.16 cm thick Pilot F scintillator. The total thickness of the five MWPC's and the beam pipe was 0.007 hadronic collision lengths ((lamda)(,c)). We found no candidates for fractionally charged particles. Our upper limits on R(,qq) = (sigma)(e('+)e('-)(--->)qq)/(sigma)(e('+)e('-)( --->)(mu)(mu)) are between: (1) 0.7% to 7% for quark interaction lengths ((lamda)(,q)) equal to (lamda)(,c) and between 3% and 33% for (lamda)(,q) = 100(lamda)(,c) for Q = 1/3e quark charge and for quark masses up to 13 GeV/c('2); (2) 2% to 38% for (lamda)(,q) = (lamda)(,c) and from 7% to 160% for (lamda)(,q) = 100(lamda)(,c) for Q = 2/3e quark charge and for masses up to 8 GeV/c('2). In the inclusive production channel our upper limits on R(,q) = (sigma)(e('+)e(' -)(--->)qqX)/(sigma)(e('+)e('-)(--->)(mu)(mu)) are for charge 1/3e only. R(,q) varies from 2% to 11% for (lamda)(,q) = (lamda)(,c) and from 3% to 16% for (lamda)(,q) = 100(lamda)(,c) and for quark masses up to 6.5 GeV/c('2). These are the first limits on the production of fractionally charged particles with (lamda)(,q) = 100(lamda)(,c).