Fractionally charged particles not yet seen

Fractionally charged particles not yet seen. Quarks have fractional charges ±e/3 and ±2e/3. But the force that binds them into hadrons is so strong that quarks cannot be free particles. Nor has any other fractionally charged particle been seen to date. But there’s no obvious theoretical impediment to the existence of fractionally charged free particles, if they’re immune to the strong nuclear force. And indeed, fractionally charged massive particles (FCHAMPs) with no strong interactions are anticipated by some extensions of particle theory’s standard model. Those proposed extensions predict, as a function of the putative FCHAMP’s mass and charge, the production rate for oppositely charged pairs of them in collisions between high-energy protons. The telltale signature of an FCHAMP would be its anomalously low energy loss by ionization of materials in a detector. Now the collaboration that runs the gargantuan CMS detector (shown in the photo) at CERN’s Large Hadron Collider reports that it has found no evidence of FCHAMP production in the debris from almost 10¹⁴ p–p collisions during the LHC’s 2011 run. The more massive the FCHAMP, the lower should be its production rate. Given the null result, the collaboration assigns a lower mass limit (at 95% confidence) of about 400 proton masses (m_p) if the FCHAMP charges are ±2e/3, and 260 m_p if they are ±e/3. The theories under scrutiny don’t limit fractional charges to those values, but the production rate would increase with increasing charge. (CMS collaboration, http://arxiv.org/abs/1210.2311 .)