Neutrinos: NOvA and T2K reduce uncertainty on oscillation parameters
The T2K experiment in Japan, in which INFN is heavily involved, and NOvA in the United States have conducted their first joint analysis, providing some of the most precise measurements ever obtain ed of ne utrino oscillations. The results, published today in Nature, combine ten years of T2K data (collected since 2010) and six years of NOvA data (collected since 2014) and reduce the uncertainty in the differences between squared neutrino mass to less than 2%, also placing strong constraints on the CP symmetry violation (a difference in behavior between particles and antiparticles). This milestone represents an important step towards understanding the matter-antimatter asymmetry in the universe and demonstrates the relevance of collaboration between "competing" but complementary experiments.
"The joint analysis work has benefited both collaborations," commented Patricia Vahle, co-spokesperson for the NOvA scientific collaboration. "We have gained a much better mutual understanding of the strengths and challenges of the different experimental apparatuses and analysis techniques."
Both T2K and NOvA are long-baseline neutrino oscillation experiments: each sends an intense neutrino beam that passes through a near and a far detector. T2K sends its neutrino beam for 295 kilometers, from Tokai to Kamioka (hence the name "T2K"). Tokai hosts the J-PARC accelerator complex, while Kamioka is home to the Super-Kamiokande neutrino detector, a huge tank containing 50,000 tons of ultra-pure water located one kilometer underground. NOvA (NuMI Off-axis νe Appearance), on the other hand, sends a neutrino beam for 810 kilometers from Fermilab, near Chicago, to a 14,000-ton liquid scintillator detector in Ash River, Minnesota. By leveraging the substantial differences in oscillation distance and average neutrino beam energy, the two experiments were able to obtain more comprehensive information on neutrino behavior.
"The T2K-NOvA analysis required an intense and virtuous exchange between the communities. It was necessary to thoroughly understand the data from both experiments and analyze them in a common framework that has appropriately accounted for systematic uncertainties in the measurements," observed Andrea Longhin, professor at the University of Padua, INFN associate, and country representative for Italy of the T2K collaboration. "INFN plays a leading role in managing T2K's near detector, dealing with the TPC (Time Projection Chambers) detectors, with which it is possible to measure the energy and nature of particles generated by neutrino interactions."
Neutrinos, extremely abundant yet extremely difficult to detect, change type, or "flavor," as they propagate through space. Electronic, muonic, and tauonic: each of the three flavors is not associated with a well-defined mass, but rather a mixture of the three possible "mass states" of neutrinos, and it is precisely this mixing that causes a neutrino to "oscillate" from one flavor to another during motion. One of the great mysteries of neutrino physics is determining the mass ordering of these three states. There are two possibilities, conventionally referred to as "normal" ordering and "inverted" ordering. In normal ordering, two mass states are light and one is heavy; in inverted ordering, two are heavy and one is light. In the normal case, muon neutrinos are more likely to oscillate into electron neutrinos, but muon antineutrinos are less likely to oscillate into electron antineutrinos. In the inverted ordering, the opposite occurs. However, the difference in behavior betw een neut rinos and antineutrinos depends not only on the mass ordering but on intrinsic differences between neutrinos and antineutrinos, which in technical jargon is called a violation of CP (charge-parity) symmetry. This violation implies that neutrinos do not behave like their antiparticles and, if confirmed, could help explain why, after the Big Bang, matter prevailed over antimatter, giving rise to the universe as we know it.
The combined results of NOvA and T2K do not favor either ordering. If the ordering turns out to be normal, the current results would not fully clarify the issue of CP symmetry, requiring further data. If, on the other hand, the ordering was the “inverted” one, the results would provide evidence of CP symmetry violation.
"Neutrino physics is a strange field. It's very difficult to isolate the effects," explained Kendall Mahn, co-spokesperson for the T2K scientific collaboration. "Combining the analyses allows us to isolate one of these effects, and that's in progress."
The combined analysis provided one of the most precise measurements of the mass difference between neutrino mass states, a quantity called Δm²₃₂. With an uncertainty of less than 2%, this new value will allow precise comparison of results from other experiments and verification of whether the theory of neutrino oscillations is complete. In the future, in addition to NOvA and T2K – the only long-distance neutrino experiments currently operating – the Deep Underground Neutrino Experiment (DUNE), under construction in the United States, and Hyper-Kamiokande, under construction in Japan, will also try to answer the still open questions, thanks to, respectively, greater sensitivity to neutrino mass ordering and high-statistics measurements on CP symmetry violation. INFN has an active and leading role in both projects.
"INFN is already at the forefront of the Hyper-Kamiokande experiment. It has recently led the construction of a new version of the TPCs (High-Angle TPCs) for the near detector – already operational and destined to play a key role at the start of Hyper-Kamiokande in 2028 – and is engaged in activities of essential importance also for the far detector, including the development of readout electronics, multi-PMT photo-detectors, and a significant part of the computing resources," concluded Longhin.
