Is it Wave or is it Particle?
By Karol Lang and Sacha Kopp
The MINOS experiment has published an article in
Physical Review Letters to show that the small particles called neutrinos oscillate from one type to another, as would be expected for wave packets. The experiment, which starts with a beam of neutrino particles produced at Fermilab, sends the beam down into the ground so that it will re-emerge from the Earth in northern Minnesota. Neutrinos are detected in the 5,400-ton experiment which is located in the historic Soudan iron mine. MINOS in 2006 confirmed data from the Super Kamiokande experiment that neutrinos disappear in flight (as then reported on Fox News, “The Federal Government has Lost Some Neutrinos!”). With this new publication, the MINOS experiment has now shown that the neutrinos‘ disappearance seems to arise from the wave phenomenon of oscillations.
MINOS is a large-scale replay of Thomas Young‘s famous 1801 “double slit” experiment. Young was then in a debate with Newton: Is light a wave or a particle? Particles, when added together, should give us more particles. Waves, when added together, can actually give us nothing—that is, they can cancel one another out. (Think of noise-cancelling headsets or anti-reflective coatings on eyeglasses.)
Young‘s experiment was to pass light through slits in a pair of barriers (see figure). The first barrier (“a”) gave a source of light. The second barrier had two slits (“b”) which selected two light beams from that source. If light is a wave, those light beams, when combined together, could either add to or cancel one another, depending upon the relative paths that the beams had to follow in order to arrive at a point along a screen. Thus, an observer at a remote location (“d”) might see either bright light or no light at all.
The neutrinos in the MINOS experiment show a similar wave behavior to Young‘s experiment. Neutrinos produced or detected in experiments are one of three types, the electron-neutrino
νe, muon neutrino νμ, or tau-neutrino ντ
because these are the types of neutrinos present in the weak nuclear force, which allows neutrinos to interact with the rest of the world (detectors, beamlines, stars, supernovae, etc.). The MINOS experiment begins with a source of pure muon neutrinos.
The neutrino “slits” occur because another way to describe these obscure particles is by how massive they are. If neutrinos have mass, they could also be labeled as m1, m2, m3 (light, middle, heavy). These mass states cannot be detected in nature, and any muon neutrino produced in the beam is really a superposition of all three mass types. By the time the neutrinos arrive at the MINOS detector in Minnesota they again recombine as either νe, νμ, or ντ. Because the MINOS experiment records mostly the muon neutrinos, those neutrino wave packets that recombine as either electron- or tau-neutrinos appear to be “lost.”
In its new article, MINOS observed 848 events in the far-away detector. While that‘s a lot, it falls short of the 1065 events expected if all neutrinos from the beam arrived undeterred. Further, the energy spectrum of those surviving neutrinos (see figure) bears a resemblance to Young‘s double-slit experiment: neutrinos at some energies survive the 735 km trip just fine, while others at specific energies have cancelled one another. The observed deficit, furthermore, is now precisely measured to the point that other explanations for neutrinos disappearing in flight appear less likely (some physicists had conjectured that the neutrinos simply decay to other new, more exotic particles, for example). The MINOS data provide the world‘s most precise measurement of the difference in masses between the states
m2 &
m3, a factor of two better than previous measurements by the Super-Kamiokande or K2K experiments in Japan.
It‘s been over 200 years since Thomas Young thought he settled the score on whether light is a wave or a particle. With the advent of quantum mechanics, many phenomena have to be described both ways. With the recent MINOS article, neutrino particles are shown to have both particle and wave properties, and the experiment is using that “wave particle duality” to reveal the mass of nature‘s most ubiquitous particle.
The group from UT Austin participating on the MINOS experiment consists of Professors Sacha Kopp and Karol Lang, postdoctoral fellows Parker Cravens and Rashid Medheyev, and PhD students Laura Loiacono, Jasmine Ma, Rustem Ospanov, and Zarko Pavlovic. Kopp and students built several components of the neutrino beam at Fermilab, while Lang and his students developed optical sensing photomultiplier tubes for the MINOS detector.
