The series of measurements made after the initial observation of the Cosmic Microwave Background (CMB) Radiation has helped transform observational cosmology into an extremely predictive science. Many of the parameters of the cosmological concordance model have now been measured and further exploration of these parameters over the next few decades are likely to reveal even more hidden aspects of our universe. Although many of the predictions from cosmology have now been realized, there are others that remain unobserved. The direct detection of the Cosmic Neutrino Background produced from the primordial Big Bang stands as one of the fundamental challenges in both neutrino physics and cosmology. Like their photon counterparts, the existence of relic neutrinos in the cosmos is expected; yet, its direct observation remains elusive. Observation of the cosmic relic neutrinos or, conversely, the absence of such, stands as an important verification of the model.
The direct observation of relic neutrinos is an extremely difficult challenge from an experimental perspective. The neutrino temperature, T, is related directly to the CMB temperature, and is expected to be T=1.95 K, or 0.17 meV in energy. Most conventional methods of detecting neutrinos rely on interactions that have some threshold for the energy of the incoming neutrino which is often many orders of magnitude larger than this predicted relic neutrino energy. Fortunately, there exists a good candidate by which such low energy neutrinos can be detected: neutrino capture on radioactive nuclei. The signal of the neutrino capture process is unique: a monoenergetic peak above the endpoint energy of the beta decay energy spectrum. With a detector of sufficient resolution and target mass, detection of the Cosmic Neutrino Background appears, at least in principle, to be possible. This EAGER award is for an exploratory, high-risk, high-payoff effort to build a small novel experiment to test this high precision detection technique.
For Broader Impacts, this project addresses a search for the direct detection of the Cosmic Neutrino Background. If successful, it will have significant overlap with particle physics, nuclear physics and astrophysics. The technique envisioned for this measurement has potential ramifications for other disciplines with significant interest in low energy electron metrology.
This research was directed at exploring a possible mechanism by which neutrinos created in the primordial Big Bang could be detected by terrestrial experiments directly. Although standard cosmology firmly predicts that, like photons, neutrinos are produced in the early universe, their existence has not been directly observed. Since their energies are extremely small, such direct detection is extremely difficult experimentally. Fortunately, there does exist a unique mechanism that could allow for their detection, known as neutrino capture on radioactive nuclei. Of all proposed techniques, neutrino capture appears to be the most viable approach. However, to make the mechanism at all feasible, it requires (a) a large target of radioactive material (tririum) available and (b) a highly resolved method to measure the kinetic energy of the particles emitted in the process. The MIT group has proposed a novel technique to measure the kinetic energy of charged particles from radioactive decays, an essential stepping stone to detect the process of neutrino capture. The method uses radio-frequency techniques to make these well-resolved energy measurements, an approach previously not used in this field of research. The technique also allows for exploration of the absolute mass scale of neutrinos, also an open question in the scientific community. Thanks to this grant, two milestones have been achieved in the past 12 months: (1) A phenomenological study on the sensitivity of current neutrino capture experiments has been performed. The study was recently published in Physical Review Letters. (2) A prototype experiment intended to demonstrate the RF technique has been fully assembled and constructed and has begun to take data. Though such a prototype has no sensitivity to neutrino capture per se, it plays an important role in determining whether this technique can be expanded to eventually make a neutrino capture (and neutrino mass) measurement.
