Neutrinos are mysterious particles that are produced in nuclear reactions like those that occur inside stars. Unlike other particles, neutrinos barely interact with normal matter, allowing them to pass through entire planets as if they weren’t there. But researchers have discovered that neutrinos can change – a phenomenon called neutrino oscillation. This unexpected behavior has provided important clues to help explain some of the biggest mysteries in physics.
What Are Neutrinos?
Neutrinos are neutral subatomic particles that have very little mass. There are three known types or “flavors” of neutrinos: electron neutrinos, muon neutrinos, and tau neutrinos. Each neutrino flavor is associated with a different type of charged particle:
- Electron neutrinos: Associated with electrons
- Muon neutrinos: Associated with muons
- Tau neutrinos: Associated with taus
Neutrinos are constantly streaming through the universe in incredibly huge numbers. For example, trillions of neutrinos from the Sun pass through your body every second. But you don’t feel a thing because neutrinos interact so rarely with matter. It takes an entire lightyear of lead to block half of the neutrinos passing through it.
This “ghostly” property makes neutrinos very difficult to study. Giant facilities are required just to detect a few neutrinos now and then. Rare neutrino interactions in these detectors have revealed that neutrinos have the strange ability to transform from one flavor into another. This phenomenon is what’s known as neutrino oscillation.
The Discovery of Neutrino Oscillation
The story of neutrino oscillations begins with a strange finding from early neutrino detectors. These detectors observed far fewer solar electron neutrinos than theoretical models predicted. This mismatch became known as the “solar neutrino problem.”
To try and explain the discrepancy, physicists proposed that electron neutrinos from the Sun might be oscillating into muon and tau neutrinos on their way to Earth. This could reduce the solar electron neutrino flux since the detectors were only designed to observe electron neutrinos.
The theory was validated in 1998 at the Super-Kamiokande detector in Japan. This instrument was able to confirm that muon and tau neutrinos were indeed coming from the Sun, clearing up the solar neutrino problem. The discovery proved that neutrino oscillation was real, and earned researchers Takaaki Kajita and Arthur B. McDonald the 2015 Nobel Prize in Physics.
What Causes Neutrino Oscillation?
Neutrino oscillation arises due to a quantum mechanical effect involving the neutrino’s flavor and mass states. It’s easiest to understand by looking at just two neutrino flavors.
A neutrino produced in a specific flavor state (like an electron neutrino) is really a mixture of two mass states, one lighter and one heavier. As the neutrino propagates through space, the two mass states accumulate different phases based on their different masses, causing the neutrino’s flavor to oscillate between the two possibilities (in this case, electron and muon neutrino).
But neutrinos don’t necessarily have definite masses like other fundamental particles. Due to an effect known as mass mixing, the electron, muon and tau neutrino flavors are each a superposition of three possible mass states, labeled 1, 2 and 3. The relative abundances of the three mass states differ for each neutrino flavor.
Together, the three mass states and three flavored states allow for neutrino oscillations between all three flavors. The following table summarizes the flavor content of each neutrino mass state:
| Mass State | Electron Neutrino | Muon Neutrino | Tau Neutrino |
|---|---|---|---|
| 1 | 35% | 35% | 35% |
| 2 | 40% | 30% | 40% |
| 3 | 25% | 35% | 25% |
As an example, say a muon neutrino is produced in the upper atmosphere from a cosmic ray collision. After traveling some distance it may transform into an electron neutrino. This is because the muon neutrino state is a mixture of the three mass states. As they propagate, their phases change relative to each other causing the probability of measuring the electron or muon neutrino flavor to oscillate.
Matter Effects on Neutrino Oscillation
Neutrino oscillation is also affected by the presence of matter. As neutrinos pass through matter, their oscillation patterns can be dramatically modified through an effect called the Mikheyev–Smirnov–Wolfenstein (MSW) mechanism.
This occurs because of the weak force interactions neutrinos experience with particles in matter. Electron neutrinos can interact via charged current interactions that other neutrino flavors don’t experience. This gives electron neutrinos an additional matter-dependent phase shift as they propagate.
The result is that the oscillation probabilities of neutrinos passing through dense matter can be quite different. Experiments have observed this “matter effect” by studying neutrinos passing through the Earth. This provides yet another confirmation of neutrino flavor change and gives insight into neutrino mass differences.
What Neutrino Oscillations Reveal
The behavior of neutrino oscillations depends on some key neutrino properties including the differences in neutrino masses and the mixing angle parameters.
By studying neutrino oscillations carefully, physicists can try to work backwards to determine the values of these unknown neutrino properties. The hope is that mapping out the neutrino mass and mixing parameters will help unlock some of the deeper mysteries neutrinos may hold.
Neutrino oscillation experiments have shown that:
- Neutrinos have tiny but non-zero masses.
- The three neutrino mass states are separated by very small mass differences.
- Neutrinos have large mixing angles compared to quarks.
Some important unresolved questions include:
- What causes the different masses of the three neutrino mass states?
- Does the 3-neutrino model fully explain all observations or are additional sterile neutrinos required?
- Do neutrinos act differently from antineutrinos?
- Could neutrinos break symmetry laws in certain processes?
Answering these questions may help explain why neutrino masses are so much smaller than other particles. And it may provide critical clues about the matter-antimatter asymmetry observed in the universe.
Oscillation Experiments
Many experiments have been conducted over the years to reveal the secrets of neutrino oscillations. Some highlights include:
Solar Neutrino Experiments
Experiments using radiochemical detectors observed fewer electron neutrinos from the Sun than expected. This was resolved by proposing neutrino flavor change.
- Homestake Experiment: First experiment to detect solar neutrinos. Saw 1/3 expected electron neutrino flux.
- GALLEX/GNO: Detected pp fusion neutrinos from the Sun, proving the core fusion process was working.
- SAGE: Also detected pp neutrinos, confirming the neutrino deficit.
- Super-Kamiokande: A large water Cherenkov detector in Japan. Provided strong evidence that solar electron neutrinos were oscillating into muon/tau neutrinos.
- SNO: Used heavy water to show solar neutrinos converts to all three flavors, not just electron neutrinos.
Atmospheric Neutrino Experiments
Studies of neutrinos created by cosmic ray interactions in the atmosphere also revealed anomalous neutrino oscillations.
- IMB: One of the first detectors observing anomalous atmospheric neutrino ratios.
- Super-Kamiokande: Helped confirm atmospheric muon neutrinos oscillating into tau neutrinos.
Short-baseline Reactor Experiments
Observations of antineutrino fluxes from nuclear reactors helped probe neutrino oscillation parameters.
- KamLAND: Detected disappearance of reactor antineutrinos over long distances, confirming solar neutrino results.
- Daya Bay: Measured non-zero theta-13 mixing angle from reactor antineutrinos.
Long-baseline Experiments
Long-baseline experiments use neutrino beams directed between 200 to 800 km to probe specific oscillation effects.
- T2K: Finds muon neutrino to electron neutrino transitions over 295 km in Japan.
- NOvA: Studies muon to electron neutrino oscillations over 810 km.
- MINOS: Used Fermilab neutrino beam directed 735 km to the Soudan Mine in Minnesota.
Each generation of experiments probes neutrino properties and oscillations with increasing precision. Upcoming experiments hope to determine the neutrino mass ordering and whether CP symmetry is violated in the neutrino sector.
Major Unknowns in Neutrino Physics
While great progress has been made, neutrinos still remain quite mysterious. Some major questions still to be addressed by future neutrino experiments include:
Neutrino Mass Ordering
The ordering of the three neutrino mass states is not known. The possibilities are:
- Normal Ordering – m1 is lightest, m3 is heaviest
- Inverted Ordering – m3 is lightest, m1 is heaviest
Determining the mass ordering is a major goal for current experiments. The ordering has implications for the origin of neutrino mass and leptogenesis theories.
CP Violation in the Lepton Sector
It is unknown whether neutrinos break CP symmetry, which relates particles and antimatter. Observing CP violation in neutrinos could help explain the matter-antimatter asymmetry of the universe.
Absolute Neutrino Mass Scale
Oscillation experiments only constrain the differences between neutrino masses, not the absolute values. The total scale of neutrino masses impacts theories of dark matter, structure formation, and the cosmic neutrino background.
Sterile Neutrinos?
Some anomalies hint at the possible existence of “sterile” neutrinos that don’t have the usual weak interactions. If confirmed, these would require extending the standard neutrino model.
Dirac vs Majorana Neutrinos
It’s unknown whether neutrinos are Dirac or Majorana particles. Majorana neutrinos would imply neutrinos are their own antiparticle and could lead to processes like neutrinoless double beta decay.
Additional Neutrino Interactions?
There are open questions about whether neutrinos have additional interactions beyond the known weak force, possibly related to dark matter or a hidden sector.
The Future of Neutrino Physics
Upcoming neutrino experiments will probe these mysteries using powerful new neutrino sources and detection technologies. These include:
- DUNE: Using high-intensity LBNF beamline and large liquid argon detector.
- Hyper-Kamiokande: A megaton water Cherenkov detector in Japan.
- JUNO: 20 kiloton liquid scintillator detector in China.
- KATRIN: Directly measuring neutrino mass via tritium beta decay.
- nEXO: Searching for neutrinoless double beta decay.
- IceCube: Detecting cosmic neutrinos with Antarctic ice.
Understanding the true nature of neutrinos will have profound implications across particle physics, astrophysics, and cosmology. Neutrino experiments continue to push the boundaries of particle physics, sometimes challenging established theory. The continuing study of neutrino oscillations remains one of the most exciting frontiers in physics today.
Conclusion
The discovery of neutrino oscillation opened a new window into physics beyond the standard model. Neutrinos clearly exhibit quantum mechanical behavior and properties that are unlike any other particle. Upcoming experiments aim to complete our understanding of neutrino masses, mixing parameters, and any CP symmetries they may violate.
Solving these mysteries could help answer some of the biggest open questions in physics like the origin of mass and the matter-antimatter asymmetry. For a particle that is notoriously difficult to detect, neutrinos have provided an astonishing amount of insight with far-reaching implications across many areas of physics. Their subtle transitions between flavors have illuminated some of the most perplexing phenomena in nature.