Second, do neutrinos oscillate in the same way as their antiparticles (antineutrinos), that is, do they obey CP symmetry (Fig. First, are neutrinos ordered in mass in a similar way (“normal ordering”) as their charged-lepton partners, the well-known electron, muon, and tau particles? A naive analogy would suggest that this is the case-but finding an “inverted ordering” would be an exciting result that could guide theoretical developments. Today’s experiments aim to tackle, in particular, two crucial open questions. As often occurs in physics, precision measurements of a phenomenological parametrization can deliver hints to new physics-which could mean developing a simpler model connected to a smaller set of parameters or even discovering a more fundamental theory, solely based on symmetries, that describes observations. Such behavior is parametrized (that is, mathematically described by a limited set of measurable numbers) by the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix. This oscillation can be explained by assuming that there are three neutrino mass eigenstates that mix to form three flavor eigenstates (electron, muon, and tau) among which oscillations occur. When neutrinos of a given type, or flavor, travel over some distance, they can switch their flavor with a probability depending on distance and on neutrino energy. The results bode well for the next generation of “long-baseline” experiments, which will dramatically boost our ability to probe elusive aspects of neutrino physics. One such effort, the NOvA experiment at Fermilab, now reports the analysis of oscillation data collected between 20, delivering some of the most accurate estimates to date of parameters describing neutrino oscillations and providing important hints on two important aspects of neutrino physics-the ordering of neutrino masses and the degree of charge-parity (CP) violation. The discovery of this beyond-standard-model behavior, recognized by the 2015 Nobel Prize in Physics, drove intense efforts to characterize neutrino oscillations through increasingly accurate experiments. This behavior is only possible if neutrinos have a mass-contrary to the initial assumption of the standard model of particle physics. In 1998, researchers discovered that neutrinos can change their “flavor” as they travel.
Experiments such as NOvA seek to spot these differences by comparing how neutrinos and antineutrinos change their flavor, or “oscillate,” over long distances. APS/Carin Cain Figure 1: Neutrinos may behave differently from their “mirror” antiparticle counterparts.
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