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Register a new userLimits on the abundance of millicharged particles bound to matter
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Millicharged particles (mCPs) are hypothesized particles possessing an electric charge that is a fraction of the charge of the electron. We report a search for mCPs with charges $gtrsim 10^{-4}~e$ that improves sensitivity to their abundance in matter by roughly two orders of magnitude relative to previous searches. This search is sensitive to such particles over a wide range of masses and charges for which they can form stable bound states with matter, corresponding to a gap in parameter space that is beyond the reach of previous searches from accelerators, colliders, cosmic-ray experiments, and cosmological constraints.
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A search for millicharged particles, a simple extension of the standard model, has been performed with the ArgoNeuT detector exposed to the Neutrinos at the Main Injector beam at Fermilab. The ArgoNeuT Liquid Argon Time Projection Chamber detector enables a search for millicharged particles through the detection of visible electron recoils. We search for an event signature with two soft hits (MeV-scale energy depositions) aligned with the upstream target. For an exposure of the detector of $1.0$ $times$ $10^{20}$ protons on target, one candidate event has been observed, compatible with the expected background. This search is sensitive to millicharged particles with charges between $10^{-3}e$ and $10^{-1}e$ and with masses in the range from $0.1$ GeV to $3$ GeV. This measurement provides leading constraints on millicharged particles in this large unexplored parameter space region.
We study the production of exotic millicharged particles (MCPs) from cosmic ray-atmosphere collisions which constitutes a permanent MCP production source for all terrestrial experiments Our calculation of the MCP flux can be used to reinterpret existing limits from experiments such as MACRO and Majorana on an ambient flux of ionizing particles. Large-scale underground neutrino detectors are particularly favorable targets for the resulting MCPs. Using available data from the Super-K experiment, we set new limits on MCPs, which are the best in sensitivity reach for the mass range $0.1 lesssim m_{chi} lesssim 0.5$ GeV, and which are competitive with accelerator-based searches for masses up to 1.5 GeV. Applying these constraints to models where a sub-dominant component of dark matter (DM) is fractionally charged allows us to probe parts of the parameter space that are challenging for conventional direct-detection DM experiments, independently of any assumptions about the DM abundance. These results can be further improved with the next generation of large-scale neutrino detectors.
We propose the use of trapped ions for detection of millicharged dark matter. Millicharged particles will scatter off the ions, giving a signal either in individual events or in the overall heating rate of the ions. Ion traps have several properties which make them ideal detectors for such a signal. First, ion traps have demonstrated significant isolation of the ions from the environment, greatly reducing the background heating and event rates. Second, ion traps can have low thresholds for detection of energy deposition, down to $sim text{neV}$. Third, since the ions are charged, they naturally have large cross sections for scattering with the millicharged particles, further enhanced by the low velocities of the thermalized millicharges. Despite ion-trap setups being optimized for other goals, we find that existing measurements put new constraints on millicharged dark matter which are many orders of magnitude beyond previous bounds. For example, for a millicharge dark matter mass $m_Q=10~textrm{GeV}$ and charge $10^{-3}$ of the electron charge, ion traps limit the local density to be $n_Q lesssim 1 , textrm{cm}^{-3}$, a factor $sim 10^8$ better than current constraints. Future dedicated ion trap experiments could reach even further into unexplored parameter space.
We investigate the cosmological stability of light bosonic dark matter carrying a tiny electric charge. In the wave-like regime of high occupation numbers, annihilation into gauge bosons can be drastically enhanced by parametric resonance. The millicharged particle can either be minimally coupled to photons or its electromagnetic interaction can be mediated via kinetic mixing with a massless hidden photon. In the case of a direct coupling current observational constraints on the millicharge are stronger than those arising from parametric resonance. For the (theoretically preferred) case of kinetic mixing large regions of parameter space are affected by the parametric resonance leading at least to a fragmentation of the dark matter field if not its outright destruction.
Particles with electric charge q < 10^(-3)e and masses in the range 1--100 MeV/c^2 are not excluded by present experiments. An experiment uniquely suited to the production and detection of such millicharged particles has been carried out at SLAC. This experiment is sensitive to the infrequent excitation and ionization of matter expected from the passage of such a particle. Analysis of the data rules out a region of mass and charge, establishing, for example, a 95%-confidence upper limit on electric charge of 4.1X10^(-5)e for millicharged particles of mass 1 MeV/c^2 and 5.8X10^(-4)e for mass 100 MeV/c^2.
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