Grant: Subatomic Physics Envelope - Individual

The nature of the missing matter in the Universe is one of the most important and mysterious unknown aspects in the physics field today (see for example the report by the P5 committee in the US, setting policy for the 2014-2023 period). Considerable effort toward the detection of missing matter has been expended by many research groups without success thus far in the search for this elusive component of the Universe. The proposed research explores a direct dark matter search through the construction of a novel 10kg Scintillating Bubble Chamber (SBC) detector at Queen's University, to be ultimately located at SNOLAB outside of Sudbury, Ontario. The SBC collaboration builds on the world-leading research of the PICO collaboration [1] and is an extension of the work with xenon bubble chambers [2], expanding the sensitivity to lower mass dark matter, a region of particular interest in the community [3].  As with the majority of dark matter projects, limitations are placed on the PICO results due to the inability to reliably separate background events from the nuclear recoils which are the signature of the dark matter interaction. The proposed detector improves dramatically on particularly the electron recoil discrimination, providing an excellent opportunity to further investigate low mass candidates. Operation of the SBC is similar to that of any bubble chamber - a recoiling nucleus deposits a critical amount of heat within a defined radius (with both of those parameters controllable through control of the temperature and pressure) and a phase transition (boiling) is incited.  Current superheated liquid detectors, while having excellent rejection of electron recoils at typical operating thresholds, become limited by this background when exploring lower energies. Evidence from operation of a xenon bubble chamber at Northwestern University in Illinois indicates that this is not the case for the SBC. Data taken from this prototype indicates that the rejection factor for electron recoils is at least as good as that achieved in others, with indications that it is improved significantly. In addition, the energy threshold of the detector has been lowered considerably without any impact on stable operation. With this evidence in hand, the next steps for this experiment call for increased statistics in order to properly quantify the achievable threshold and therefore the ability to study dark matter, as well as topics such as sterile neutrinos, the neutrino magnetic moment, and non-standard interactions. The addition of the scintillation measurement to the more traditional bubble chambers being operated for dark matter detection supplements the project not only by improving the results and leading the world.  The requested HQP working on this project will also be exposed to novel technologies and their use in a cutting-edge project.  This project is then an ideal training ground for future scientists including the five sponsored by this grant request.