Two approaches yield different results
Neutrons can spontaneously decay, transforming into a proton, electron, and antineutrino, a process predicted by quantum mechanics. While neutrons inside atomic nuclei can be stable, free neutrons are more prone to decay. Measuring the average lifetime of free neutrons, however, has proven challenging. "For nearly thirty years, physicists have been perplexed by conflicting results in this area," stated Benjamin Koch from TU Wien's Institute of Theoretical Physics, who collaborated with Felix Hummel on analyzing the discrepancies. Their research is closely integrated with the work of Hartmut Abele's neutron research team at TU Wien's Atomic Institute.
"Typically, a nuclear reactor serves as the neutron source for such experiments," explained Koch. "Free neutrons emerge from radioactive decay within the reactor and are directed into a neutron beam for precise measurement." By counting the neutrons in the beam and the protons produced during decay, researchers can calculate the average lifetime of the beam neutrons.
Alternatively, neutrons can be confined using magnetic fields in a setup known as a "bottle" experiment. According to Koch, "Neutrons from the beam live approximately eight seconds longer than those measured in a bottle." Given the average neutron lifespan of just under 900 seconds, this variance is too large to be attributed to simple measurement errors.
The possibility of a new neutron state
Koch and Hummel's hypothesis suggests that the discrepancies could be explained if neutrons possess excited states - higher-energy configurations not previously observed. While such states are well-documented for atoms and play a crucial role in technologies like lasers, calculating similar states for neutrons is more complex. "We can estimate the properties needed to account for the varying neutron lifetime measurements," Koch noted.
The researchers propose that when neutrons emerge from decay, they may initially exist in a combination of states: some in a typical ground state and others in an excited state. Over time, excited neutrons would transition to the ground state. "Imagine a bubble bath," said Hummel. "If you add energy, the bubbles represent an excited state, but they eventually pop, returning the bath to its original state."
In neutron beam experiments, multiple neutron states may be present, while bottle experiments would primarily measure ground-state neutrons due to the time taken for neutrons to cool and be trapped.
"Our model suggests that a neutron's decay likelihood is strongly influenced by its state," Hummel said, resulting in differing average lifetimes for beam versus bottle measurements.
Further investigation required
"Our model delineates the parameter range for the search," Koch stated. The excited state's lifetime would need to be between 5 milliseconds and 300 seconds to account for the discrepancies. Re-evaluation of past experimental data could provide initial insights, though new experiments will be essential for conclusive evidence.
Collaborations with TU Wien's Institute for Atomic and Subatomic Physics are underway, utilizing the PERC and PERKEO experiments to test the theory. Additionally, research groups from Switzerland and Los Alamos in the U.S. have expressed interest in employing their techniques to examine the hypothesis. With the technical and conceptual groundwork already in place, these efforts may soon clarify whether this new theory resolves a longstanding puzzle in physics.
Research Report:Exciting hint toward the solution of the neutron lifetime puzzle
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