Abstract
We derive limits on millicharged dark states, as well as particles with electric or magnetic dipole moments, from the number of observed forward electron scattering events at the Big European Bubble Chamber in the 1982 CERN-WA-066 beam dump experiment. The dark states are produced by the 400~GeV proton beam primarily through the decays of mesons produced in the beam dump, and the lack of excess events places bounds extending up to GeV masses. These improve on bounds from all other experiments, in particular CHARM~II.
Highlights
Photon is massless, the model accommodates particles with a millicharge, allowing an apparent circumvention of charge quantisation
The coupling may be through operators of mass dimension higher than four; the dimension-five operators involving two fermions and a photon are magnetic dipole moments (MDMs) and electric dipole moments (EDMs)
Data from the Big European Bubble Chamber (BEBC) WA66 beam dump experiment [24] was used some years ago to carry out a dedicated search for a MDM of the tau neutrino [25]
Summary
We detail the calculation of the number of dark states entering the detector and their subsequent scattering. We focus on the dominant production channels, which are meson decays, with Drell-Yan forming a highly subdominant component. The dark state flux is handled primarily by the MADGRAPH plugin MADDUMP [32, 33], which provides the distribution of scattered electrons in the detector differential in both energy and angle.
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