Requiem for a fractionally charged, massive particle

Fractionally charged massive particles (FCHAMPs) appear in extensions of the standard model, especially those with superstring constructions. The lightest FCHAMP would be absolutely stable and any of them produced during the early evolution of the Universe would be present today. The thermal production, annihilation and, survival of an FCHAMP, a lepton L with electroweak (i.e., U(1)_Y) but no strong interactions, of mass m_L and charge Q_L (in units of the charge on the electron) are explored. The FCHAMP relic abundance is determined by the total annihilation cross section which depends on m_L, Q_L and on the available annihilation channels. Since massive (m_L≳1GeV) charged particles (Q _L≳0.01) behave like baryons (heavy ions), primordial nucleosynthesis and the cosmic background radiation temperature anisotropies limit the FCHAMP relic density. Requiring that Ω_L≲Ω _B/5 leads to a constraint on the Q_L-m_L relation. Further constraints on Q_L and m_L are provided by the invisible width of the Z (Q_L>0.16 for m_L≤M _Z/2) and by accelerator searches for massive, charged particles. Our key result is to exploit the fact that in the early Universe, after L ^± freeze-out but prior to e^± recombination, the negatively charged L^- will combine with the more abundant alpha particles and protons to form tightly bound, positively charged states (fractionally charged heavy ions). The Coulomb barriers between these positively charged L^-α and L^-p (L^-pp, L ^-αα,⋯) bound states and the free L⁺ suppresses late-time FCHAMP annihilation in the interstellar medium (ISM) of the Galaxy and on Earth, limiting significantly the late-time reduction of the FCHAMP abundance compared to its relic value. The surviving FCHAMP abundance on Earth is orders of magnitude higher than the limits from terrestrial searches for fractionally charged particles, closing the window on FCHAMPs with Q _L≳0.01. However, as Q_L approaches an integer (e.g., |Q_L-n|≲0.25) these searches become increasingly insensitive, leaving some potentially unconstrained "islands" in the Q _L-m_L plane which may be explored by searching for FCHAMPs in the cosmic rays.