Neutrinos and Dark Matter

In July 2017, the DEAP-3600 collaboration published its first paper reporting on the search for Weakly Interacting Massive Particles (WIMPs) using four days of data collected during the commissioning phase. While the data showed no interactions between the liquid argon atoms and WIMPs (as expected based on the short exposure time), the result showed the true power of DEAP’s Pulse-Shape Discrimination (PSD), a methodology that differentiates electronic interactions (backgrounds) to nuclear interactions (signal). In Physics Review Letters, the collaboration showed that the experiment is performing to specifications and PSD can discriminate electronic recoils from nuclear recoils with an unprecedented measured power of 1.2x10^-7.

In August 2016, the DEAP-3600 experiment was finally completed with the filling of the acrylic vessel with 3.6 tonnes of liquid argon. This milestone marked the beginning of the physics operation for this experiment. The DAQ and the Science & Technology groups at TRIUMF worked extensively to ensure that the data acquisition system was ready for the data intake upon filling. One of the biggest challenges for the scientists at TRIUMF who drove this part of the project was to ensure that the experiment could keep up with the high rate of the intrinsic argon beta-emitter ³⁹Ar, of the order of ~3.6 kHz, while never missing any possible dark matter-argon interaction. The commissioning period was brief and smooth, and the data acquisition system operated to specification due primarily to the efforts of the TRIUMF team.

To listen for interactions between DEAP’s argon detector and dark matter, the collaboration had to create one of the most radiogenically clean environments in the entire universe. To do this, the collaboration fabricated its inner detector, a sphere of radius 85 cm, from ultra-pure acrylic, and designed and implemented an 18 feet tall sanding robot (the Resurface) capable of removing the innermost layer of the acrylic sphere in a contained environment (with the aim of removing any contamination introducted during installation). This never-before-attempted, large-scale robot, a true experiment within the experiment, was deployed at the center of the DEAP vessel in October 2014 and after a month of operation, had successfully removed 500 microns uniformly from the detector surface, thus returning the purity levels of the acrylic back to production standards.

Constraints on new phenomena via Higgs boson couplings and invisible decays: A crucial question in particle physics is whether the Higgs boson discovered in 2012 is truly the fundamental scalar predicted by the Standard Model (SM). Strong theoretical arguments suggest that the SM is only an approximation to a more fundamental theory such as supersymmetry or composite Higgs models, which predict modified properties of the Higgs with respect to SM expectations. As published in the Journal of High Energy Physics (2015), the results of several analyses of production and decay rates of the Higgs boson in different channels were combined to determine how the couplings scale with mass and hence put constraints on various extensions of the SM. Vector boson processes and associated WH/ZH production set an upper limit on the Higgs boson decay branching ratio to invisible particles, such as dark matter, of 25%.    

Elusive dark matter and other exotic phenomena: A number of astrophysical measurements point to the existence of a new form of matter. For instance, the rotational speed of stars and observation of gravitational lensing effects strongly indicate the presence of so-called dark matter, in addition to our ordinary matter, that would compose a large fraction of our universe. Dark matter particles can be directly produced at the Large Hadron Collider, and one striking event signature would be the presence of an energetic jet of ordinary particles (called a monojet) and large missing energy due to dark matter particles escaping the ATLAS detector. As reported in the Journal of High Energy Physics (2018), a monojet final state constitutes a distinctive signature of beyond-Standard Model physics, and is also used to search for extra spatial dimensions and supersymmetry. Constraints have been set on various models.  

Sensitivity and discovery potential of the proposed nEXO experiment: nEXO is a future 0nbb decay experiment searching for this weak process in the decay of ¹³⁶Xe. nEXO is building on the experience gained and the success of the EXO-200. As reported in Physical Review C (2018), the sensitivity of nEXO, which is anticipated to deploy 5x10³ kg of liquid xenon enriched in the isotope ¹³⁶Xe to 90%, has been investigated under demonstrated and realizable background rates. The projected sensitivity of nEXO after 10 years of operation reaches 10²⁸ years, which is an improvement of almost two orders of magnitude compared to current experiments.    

Searching for new dark forces at lower-energy experiments: New forces may exist beyond those described by the Standard Model. The characteristic energy of a new force can be much smaller than that of the weak force provided it interacts only very feebly with regular matter. One of the most promising ways to discover such new dark forces is in lower-energy beam dump experiments in which a very intense beam is directed at a material target. As reported in the Journal of High Energy Physics (2014), TRIUMF theorists investigated new ways to apply existing beam dump experiments (usually designed for other scientific objectives) to look for dark forces and new particles associated with them. They also connected theories of dark forces with limits from cosmology and direct searches for dark matter.    

Dark matter from a new dark strong force: Most of the matter in the Universe seems to be a new form that gives off very little light, called dark matter (DM).  While the evidence for DM is very strong, very little is known about what it is made of.  As reported in Physical Review D, TRIUMF theorists showed that DM can arise from a new strong force that interacts only very feebly with regular matter. In this realization, DM consists of glueballs consisting of a conglomeration of the mediators of the new force. Implications of glueball dark matter and decays on the formation of light elements in the early Universe, the cosmic microwave background radiation, and astronomical gamma rays seen today were also studied.

The Case of the Missing Neutrinos: After 15 years of solar-neutrino measurements, 13% of the theoretically expected flux, or number, of neutrinos is unobserved. This is leading physicists to explore a variety of possible reasons, from the underlying nuclear physics to detector design. One proposed reason is the energetic cost of the detector material to capture a neutrino. To explore this, TITAN deployed its high-accuracy, high-precision Penning trap, the only one in the world coupled to a charge breeder. The charge breeder's removal of electrons boosted the precision of the measurements with gallium and germanium isotopes and allowed for a novel radioactive-beam purification. As reported in Physical Letters B (2013), TITAN measurements validate the final piece of the nuclear physics underpinning the predicted neutrino flux, and thus the cause of the missing neutrinos remains an open case.

First exclusion of CP conservation in neutrino oscillation at 90% confidence level: The observation of muon to electron-neutrino oscillation opens the possibility of using neutrino oscillation to study CP violation by comparing neutrino flavour transformations between neutrinos and anti-neutrinos. CP violation has been observed in quarks but not yet measured in leptons (particles, including neutrinos, that do not undergo strong interactions). The size of quark CP violation is not enough to explain the matter anti-matter asymmetry of the universe, and as such, leptons' role may be significant. As reported in Physical Review Letters (2017) as an Editor's Suggestion, T2K  performed a combined analysis of neutrino and anti-neutrino oscillations and notably excluded CP conservation at the 90% confidence level.

Experimental observation of neutrino flavour transformation: As reported in Physical Review Letters (2014) as an Editor's Suggestion and since then cited almost 600 times, the T2K experiment for the first time experimentally observed neutrino flavour transformation. Muon neutrinos were produced at the J-PARC facility on Japan's east coast and sent 295 km through the ground to the Super-Kamiokande (SK) neutrino detector in western Japan. SK recorded a decreased number of muon neutrinos and the appearance of electron neutrinos, demonstrating flavour transformation. The discovery of an explicit oscillation confirms the neutrino oscillation mechanism (first observed with solar neutrinos) and opens the door to the use of neutrino oscillation as a probe of CP symmetry violation in leptons.

A successful SNO+ start-up: SNO+ has successfully completed the first water phase of the experiment. During this phase the expected levels of background were verified; in particular, the goals for uranium and thorium contamination in the water were met, and all sources of external backgrounds were measured. The external background measurements are directly transferable to the upcoming scintillator and tellurium-loaded second and third phases of the experiment. An underwater camera system was deployed and commissioned, used to monitor the hold-down rope system of the acrylic vessel, and the location of calibration sources inside the vessel. The SNO+ team also commissioned the new electronics and data acquisition systems, upgraded to handle the higher event rates in SNO+.

Direct measurement of spin polarization of decaying atoms: In 2016, TRINAT demonstrated a direct atomic-physics probe of the spin polarization of potassium-37 (³⁷K) nuclei as they decayed. Many spin-polarized decay experiments disturb the polarization or must measure it separately. As reported in the New Journal of Physics (2016), the high accuracy of the polarization achieved (99.1% +/- 0.1%) enabled a sensitive search for wrong-handed neutrinos. The result also demonstrates the potential to measure the polarization to a level of precision which would be competitive in searches for new physics. The paper was selected as one of the journal's 2016 Highlights.  

HALO development: The HALO supernova neutrino detector in SNOLAB is unique among neutrino detectors in that it is primarily sensitive to electron-type neutrinos, rather than electron-type antineutrinos.  As such, it has an important role to play in understanding the neutrino flavour content of the neutrino burst from the next galactic core-collapse supernova. The period 2013-2018 had several highlights for HALO:

1. The completion of the construction of the HALO detector and its water shielding
2. The calibration of the detector with a neutron source inserted at multiple locations within the lead matrix
3. HALO joining the international SuperNova Early Warning System (SNEWS) in October 2015

The HALO collaboration has recently expanded to become the HALO-1kT collaboration, to include new collaborators from the USA and Italy. The collaboration is currently designing a much larger detector, using the 1000 tonnes of lead from the decommissioned OPERA detector at the Gran Sasso laboratory in Italy, and making plans to measure the neutrino-lead cross section at the Spallation Neutron Source at Oak Ridge National Laboratory, Tennessee. TRIUMF's role in the construction of the detector: TRIUMF's electronics shop fabricated the electrical cables that supply the neutron detectors with high voltage, and carry the signals from the neutron detectors to the amplifiers and then onto the data acquisition computers. TRIUMF's machine shop built the test stand that was used to test the neutron detectors one by one before installation inside the lead matrix of HALO. A substantial part of the manpower for design and assembly of the detector was supplied by TRIUMF. TRIUMF's role in the calibration of the detector: The neutron detectors count the number of neutrons emitted when supernova neutrinos hit the mass of lead. But not all neutrons are counted; some are absorbed by the lead itself, and others escape out the surface of the lead mass. To relate the number of neutrinos arriving from the supernova to the number of neutrons counted, it is necessary to know the efficiency with which neutrons are counted.  We do this by inserted a radioactive source that injects a known number of neutrons at various locations in the lead matrix, and compare this with the number of neutrons that are counted).

Electron drift velocity and transverse diffusion measurements in liquid Xe with EXO-200: EXO-200 is using a liquid xenon (LXe) time projection chamber to search for 0nbb. This measurement relies on modeling the transport of charge deposits produced by interactions in the LXe to allow discrimination between signal and background events. As reported in Physical Review C (2017), by varying the electric field of the EXO-200 TPC, we measured the transverse diffusion constant and drift velocity of electrons at drift fields between 20 V/cm and 615 V/cm using EXO-200 data. At the operating field of 380 V/cm EXO-200 measures a drift velocity of 1.705^+0.014_−0.010mm/μs and a transverse diffusion coefficient of 55±4cm²/s. This measurement provide information for liquid xenon dark matter detectors such as LZ and XENON, as well as for the future nEXO detector.