• Physics 15, 120
Lengthy-baseline neutrino experiments are paving the way in which for the answer of two excellent puzzles in neutrino physics—mass ordering and charge-parity violation.
In 1998, researchers found that neutrinos can change their “taste” as they journey. This conduct is just doable if neutrinos have a mass—opposite to the preliminary assumption of the usual mannequin of particle physics. The invention of this beyond-standard-model conduct, acknowledged by the 2015 Nobel Prize in Physics, drove intense efforts to characterize neutrino oscillations by more and more correct experiments. One such effort, the NOvA experiment at Fermilab, now stories the evaluation of oscillation knowledge collected between 2014 and 2020 , delivering a number of the most correct estimates to this point of parameters describing neutrino oscillations and offering necessary hints on two necessary elements of neutrino physics—the ordering of neutrino lots and the diploma of charge-parity (CP) violation. The outcomes bode effectively for the following era of “long-baseline” experiments, which is able to dramatically enhance our means to probe elusive elements of neutrino physics.
When neutrinos of a given sort, or taste, journey over a long way, they will swap their taste with a chance relying on distance and on neutrino power. This oscillation could be defined by assuming that there are three neutrino mass eigenstates that blend to kind three taste eigenstates (electron, muon, and tau) amongst which oscillations happen. Such conduct is parametrized (that’s, mathematically described by a restricted set of measurable numbers) by the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix. As usually happens in physics, precision measurements of a phenomenological parametrization can ship hints to new physics—which may imply creating an easier mannequin linked to a smaller set of parameters and even discovering a extra elementary idea, solely primarily based on symmetries, that describes observations.
At the moment’s experiments purpose to deal with, specifically, two essential open questions. First, are neutrinos ordered in mass in an identical manner (“regular ordering”) as their charged-lepton companions, the well-known electron, muon, and tau particles? A naive analogy would recommend that that is the case—however discovering an “inverted ordering” can be an thrilling outcome that would information theoretical developments. Second, do neutrinos oscillate in the identical manner as their antiparticles (antineutrinos), that’s, do they obey CP symmetry (Fig. 1)? If not, we might set up, for the primary time, CP violation by leptons (the particle sector that features neutrinos, electrons, muons, and taus). CP violation is on the coronary heart of one in every of physics’ best puzzles—matter’s dominance over antimatter within the Universe. Understanding CP violation in several particle sectors, together with leptons, may assist in fixing this puzzle.
The invention of neutrino oscillations was enabled by two pure sources of neutrinos: cosmic rays hitting Earth’s environment, and nuclear reactions within the Solar. At the moment, synthetic neutrino sources, akin to particle accelerators, enable researchers to higher management the flavour and power of the produced neutrinos and the gap, or “baseline,” over which these particles journey earlier than they’re detected. Particle accelerators can work in neutrino-beam and antineutrino-beam modes, producing separate fluxes of neutrinos and antineutrinos, which is essential for CP-violation searches. Presently, two accelerator experiments have lengthy sufficient baselines to watch PMNS neutrino oscillations: T2K in Japan and NOvA within the US, with baselines of 295 and 810 km, respectively.
In 2020 the 2 collaborations introduced outcomes that indicated an intriguing, but gentle, stress [2, 3]. Each experiments barely favored regular ordering over inverted ordering, though NOvA, by advantage of its longer baseline, has a way more pronounced sensitivity to mass ordering. The T2K experiment steered that neutrinos oscillate quicker than antineutrinos—implying a big CP violation. The diploma of CP violation extracted from NOvA’s outcomes, nevertheless, trusted mass ordering. For regular ordering, NOvA favored a small CP violation, whereas for inverted ordering, NOvA’s outcomes can be suitable with T2K’s. Statistical fluctuations are essentially the most economical rationalization for these variations, however the stress is a chance for a deep investigation of the systematic uncertainties affecting these measurements. Extra unique explanations for the stress, together with “nonstandard” interactions of neutrinos, have been additionally proposed.
NOvA’s new report  affords an in depth description of the outcomes introduced in 2020. With respect to the earlier evaluation printed by NOvA in 2019 , the brand new one contains about 50% extra knowledge in neutrino-beam mode and the reanalysis of all knowledge taken since 2014. The reanalysis is additional optimized to use the truth that NOvA’s close to and much detectors are primarily based on the identical expertise, utilizing a process that partly cancels out systematic uncertainties. NOvA principally produces muon neutrinos or antineutrinos and makes use of as fundamental observables the quantity and power of muon neutrinos and antineutrinos that survive the journey to the far detector and the quantity and power of electron neutrinos that seem on the far detector. From these measured observables, the evaluation extracts estimates of oscillation parameters, of mass ordering, and of the diploma of CP violation.
Because the 2020 announcement, a lot of researchers have already carried out “joint suits” to the units of knowledge coming from NOvA, T2K, and different experiments (together with these utilizing neutrinos produced by nuclear reactors, by the Solar, and by cosmic rays) [5–7]. These joint suits favor regular ordering, with a level of CP violation mendacity between NOvA’s and T2K’s. The desire for regular ordering is extremely influenced by knowledge from Tremendous-Kamiokande—an observatory in Japan that measures cosmic-ray-produced neutrinos—which is delicate to mass ordering. When T2K and NOvA outcomes are included within the suits, such desire diminishes as a result of inverted ordering would launch the stress between these experiments seen for regular ordering. Clearly, there may be extra work to be performed. These joint suits, nevertheless, can’t account for correlations of the systematic uncertainties between NOvA and T2K. Happily, the 2 experiments are cooperating to supply a brand new joint match that may make clear the doable influence of such correlations when combining their measurements.
So, what does the long run maintain in retailer? A brand new era of long-baseline experiments beneath building, akin to Hyper-Kamiokande in Japan and the Deep Underground Neutrino Experiment (DUNE) within the US, will enhance the obtainable statistics by greater than an element of 20. At that time, an unprecedented management over the systematic uncertainties might be wanted. Essentially the most advanced of these uncertainties—related to the modeling of neutrino manufacturing and of neutrino-nucleus interactions—contact on deep nuclear-physics issues that require a detailed collaboration with the nuclear-physics group. The statistics enhance is prone to enable researchers to make some straightforward progress: establishing the mass hierarchy and the diploma of CP violation. However with the flurry of knowledge turning into obtainable in the long term, we might have to have a look at neutrino oscillation with a extra open thoughts, probably enjoyable a number of the restrictive assumptions of the present paradigm, akin to a unitary PMNS matrix and a minimal situation involving solely three neutrino flavors and solely customary interactions.
To regulate systematic uncertainties and permit for a extra model-independent interpretation of the info, the mix of complementary experiments with completely different baselines and neutrino energies might be essential. T2K and NOvA are exhibiting how highly effective these synergies could be. A guiding instance for neutrino searches might come from essentially the most celebrated success of particle physics—the invention of the Higgs boson. Such success was constructed on a number of generations of experiments that continuously improved their efficiency and refined the basic understanding of the electroweak sector, in addition to on the mix of the 2 main Higgs-searching experiments—ATLAS and CMS.
- M. A. Acero et al., “Improved measurement of neutrino oscillation parameters by the NOvA experiment,” Phys. Rev. D 106, 032004 (2022).
- A. Himmel (NOvA Collaboration), “New oscillation outcomes from the NOvA experiment,” Neutrino 2020 (2020), Zenodo.
- P. Dunne (T2K Collaboration), “Newest neutrino oscillation outcomes from T2K,” Neutrino 2020 (2020), Zenodo.
- M. A. Acero et al. (NOvA Collaboration), “First measurement of neutrino oscillation parameters utilizing neutrinos and antineutrinos by NOvA,” Phys. Rev. Lett. 123, 151803 (2019).
- P. F. de Salas et al., “2020 international reassessment of the neutrino oscillation image,” J. Excessive Energ. Phys. 2021, 71 (2021).
- F. Capozzi et al., “Unfinished cloth of the three neutrino paradigm,” Phys. Rev. D 104, 083031 (2021).
- I. Esteban et al., “The destiny of hints: Up to date international evaluation of three-flavor neutrino oscillations,” J. Excessive Energ. Phys. 2020, 178 (2020).