Organizing the masses at MINOS

By combining its neutrino and antineutrino data sets, MINOS has provided first constraints on the spectrum of neutrino masses (represented by the sign of Δm2), the CP-violating phase δ, and whether muon or tau neutrinos are more strongly mixed with the so-called ν3 mass state (indicated by θ23). The relative goodness of each scenario is given along the vertical axis in terms of a difference of log-likelihoods. The parameter Δm2 and the angles θ13 and θ23 relate to the relative masses of the neutrinos and to how quantum mechanically “mixed” the three types are.

Over a decade ago the evidence became clear that neutrinos, which come in three varieties, can morph from one type to another as they travel, a phenomenon known as neutrino oscillation. By tallying how often this transformation happens under various conditions—different neutrino energies, different distances of travel—one can tease out a number of fundamental properties of neutrinos, for example, their relative masses. The MINOS collaboration has been doing exactly this by sending an intense beam of muon-type neutrinos from Fermilab to northern Minnesota, where a 5-kiloton detector lies in wait deep underground.

In this new result, MINOS has observed the rare case of muon-type neutrinos changing into electron-type neutrinos. This transformation is governed by a parameter known as θ13, and the MINOS data provide new constraints on θ13 using different experimental techniques than previous measurements. MINOS also collected data with an antineutrino beam, and the real excitement comes in when combining the antineutrino and neutrino data sets. Differences between the rates of this particular oscillation mode between neutrinos and antineutrinos would point to a violation of something called CP symmetry. While physicists know that CP symmetry is violated by quarks, it remains unknown whether the same is true for neutrinos. A new source of CP violation is required to explain why the universe began with more particles than antiparticles, and neutrinos could hold the key. (If the universe began with equal numbers of particles and antiparticles, they would have subsequently annihilated away, leaving nothing left over to make the stars and galaxies we have today.)

The neutrino-antineutrino oscillation comparison is not as straightforward as it could be, however. As MINOS’ beam travels through the Earth’s crust en route to Minnesota, the electrons present in the crust influence the traveling neutrinos and antineutrinos differently, inducing an asymmetry between them that has nothing to do with the fundamental one mentioned above. The size of this extra asymmetry depends on which neutrino is the heaviest. This is currently unknown, so comparing these oscillation rates actually provides information both on CP violation and on the spectrum of the neutrino masses.

While further data will be needed to bring the answers into sharper focus, MINOS is the first to use this accelerator-based neutrino-antineutrino technique to probe such deep questions in the neutrino sector, paving the way for the next round of measurements.

—Ryan Patterson, Caltech