Neutrinos are the least understood of the currently known elementary particles. They have a nearly negligible mass and they rarely interact, yet they are extremely abundant and have had a significant effect on the evolution of our universe. Neutrinos come in three flavour states (electron, muon and tau), and since the 1990s we have known that a neutrino can change flavour as it travels. This astonishing phenomenon is due to quantum mechanics: neutrinos are created or detected in flavour states, but these are superpositions of mass states. The neutrinos propagate in different mass states with different frequencies, the interference between them causing oscillations in the flavour components. The discovery of neutrino oscillations was awarded the Nobel Prize in Physics in 2015.
There are currently a few unanswered questions in the neutrino sector, which have major implications for our understanding of the laws of nature. Some of these are:
- Do neutrinos and antineutrinos oscillate in the same way?
- Which neutrino mass state is the lightest?
- Are neutrinos their own antiparticles?
Answering the above questions will allow us to understand why the universe evolved to be dominated by matter rather than antimatter; whether the four fundamental forces can be unified in a single manifestation of all of them; and what roles Majorana particles play in nature.
On top of holding the keys to some of the most interesting questions in science, neutrinos can also be used as a tool to explore the cosmos. Being able to travel extremely long distances completely undisturbed, neutrinos can bring us vital information from the most remote corners of the universe. Before 2018, the only detection of neutrinos from outside our galaxy occurred in 1987, when neutrinos emitted by a star going supernova in the Large Magellanic Cloud arrived on Earth. Of the 24 neutrinos detected more than 1000 papers have been written! In 2018 the IceCube neutrino observatory detected the first neutrino flux coming from a blazar. These neutrinos travelled 3.7 billion light-years to Earth: this means that they left the astrophysical source around about the same time as the Earth was formed! This groundbreaking discovery started the new era of neutrino astronomy!
At Queen Mary, we work on different experiments to tackle these mysteries and more:
NOvA (link to NOvA subpage): is a long-baseline neutrino oscillation experiment. An intense beam of muon neutrinos is produced at the Fermi National Accelerator Laboratory (Fermilab), near Chicago, and directed towards a detector 810 km away in Minnesota. NOvA uses a near detector to measure the flavour and energy spectrum of the neutrinos produced at Fermilab, and a far detector to measure the flavour and energy spectrum of the neutrinos that arrive in Minnesota. Neutrino oscillations are measured by comparing the flavour and energy spectra measured at the two detector locations. NOvA has made the best measurement of the muon neutrino disappearance oscillation parameters to date, and is the first experiment to have observed muon antineutrinos changing into electron antineutrinos.
DUNE (link to DUNE subpage): will be the future world flagship experiment for neutrino oscillation measurements. It is approved by the US Department of Energy, fully funded, and currently in the construction phase. DUNE will use the intense muon neutrino beam from Fermilab directed towards a detector 1300 km away in South Dakota. Compared to NOvA, DUNE will use a more powerful beam (up to 4 MW vs 0.75 MW), a much bigger far detector (40 kton vs 14 kton), and more advanced detector technology. Using liquid argon as a detection medium, DUNE will have a spatial resolution 1000 times better than NOvA, facilitating a huge improvement in the understanding of neutrino interactions.
ANITA/PUEO (link to ANITA/PUEO subpage): are NASA-funded long-duration balloon experiments based in Antarctica. The ANtarctic Impulsive Transient Antenna (ANITA) is a radio detector attached to a giant helium balloon that periodically flies above Antarctica looking for ultra-high energy neutrinos and cosmic rays. The last ANITA flight was in 2016. The Payload for Ultra-high Energy Observation (PUEO) is an upgrade of the ANITA detector and is expected to make its first flight in mid 2020s.
We are also involved in phenomenology work in collaboration with the University of Southampton, and the University of London through the NExT network.
In the past our group was also involved in the following experiments:
T2K is a long-baseline neutrino experiment in Japan, from J-PARC to Super-Kamiokande 295 Km away. It started data-taking in 2010, and it presented the first indication of neutrino electron appearance from a muon neutrino beam, that corresponds to the measurement of the theta13 angle in the PMNS matrix. The current goals, apart from consolidating the measurement of the electron neutrino appearance and improving the error on the measurement of the muon neutrino disappearance, are the measurement of the neutrino mass hierarchy, solving the theta23 degeneracy and set limits on the CP violation parameter. The near detector ND280, 280 m away from the neutrino source, also aims to measure neutrino cross sections at energies below 1 GeV.
Multipurpose low energy neutrino experiment aiming primarily at the measurement of neutrino-less double beta decays, that is the golden channel to measure whether neutrinos are Dirac or Majorana particles. The experiment aims also to measure for the first time very low energy solar neutrinos coming from the pep nuclear interactions. Other important measurements are the anti-neutrinos from nearby nuclear reactors and geoneutrinos produced by radioactive decays in the Earth's crust and mantle, plus supernova neutrinos, if a supernova explosion happens during the lifetime of the experiment. It will start data taking in 2014 studying neutrino-less double beta decays.
The Hyper-Kamiokande detector is the detector that aims to replace Super-Kamiokande in the next decade as the far detector for the T2K upgrade. The Hyper-Kamiokande detector is 25 times larger than Super-Kamiokande. This will be complemented by an increased beam power in the MW range. The prime physics objective of this experiment are to measure the CP violation parameter for the first time using beam and atmospheric neutrinos.