How Physicists Tested the Pauli Exclusion Principle Using Carbon express
In a carbon atom's nucleus, particles occupy their own 'apartments'—like residents who cannot share the same room. Physicists call this rule the Pauli exclusion principle: two identical inhabitants of a nucleus cannot be in the same state. But what if the rule is sometimes broken? Then one 'resident' could move into an already occupied room, with the excess energy escaping as a flash of light or an ejected particle.
The Borexino detector, buried deep beneath the Italian Alps, watched a huge tank of liquid scintillator, a substance that flashes when particles hit it, for fourteen years. The scientists were looking for exactly such 'illegal moves' in carbon nuclei: transitions where a particle occupies an already filled spot. But over the entire observation period, the detector registered not a single such event.
The absence of a signal is also a result. It allowed them to determine that a carbon nucleus lives at least 10 to the 32nd power years before violating the Pauli exclusion principle. That's a number with thirty-two zeros — billions of billions of times the age of the Universe. In other words, if the Pauli exclusion principle is ever violated, it happens so rarely that in the entire history of the cosmos, no carbon nucleus in the detector has ever done it.
The Standard Model of particle physics, which incorporates the Pauli exclusion principle, has passed another test. Now we have the strongest evidence yet that this rule is not just a convenient invention but a strict law of nature.
Original abstract
as written by the authors arXiv:2604.08950
The Pauli exclusion principle (PEP) was tested for nucleons in $$\rm{^{12}C}$$ nuclei using the Borexino dataset from 2007 to 2021. %the complete Borexino detector data. The approach consists of searching for $$\gamma$$-quanta, neutrons, protons, as well as electrons and positrons emitted in non-Paulian transitions of nucleons from the $$1P_{3/2}$$ shell to the filled $$1S_{1/2}$$ shell. Due to the uniquely low background level, the large mass, and long measurement time of the Borexino detector, the most stringent experimental constraints to date on the lifetime of the $$\rm{^{12}C}$$ nucleus with respect to PEP-forbidden transitions were obtained: $$\tau({^{12}\rm{C}}\rightarrow{^{12}\widetilde{\rm{C}}}+\gamma) \geq {1.1\times10^{32}}$$ y, $$\tau({^{12}\rm{C}}\rightarrow{^{11}\widetilde{\rm{B}}}+ p) \geq {1.0\times10^{31}}$$ y, $$\tau({^{12}\rm{C}}\rightarrow{^{11}\widetilde{\rm{C}}}+ n) \geq 2.0 \times 10^{31}$$ y, $$\tau({^{12}\rm{C}}\rightarrow{^{12}\widetilde{\rm{N}}}+ e^- + \widetilde{ u_e}) \geq 6.4 \times 10^{30}$$ y and $$\tau({^{12}\rm{C}}\rightarrow{^{12}\widetilde{\rm{B}}}+ e^+ + u_e) \geq 6.6 \times 10^{30}$$ y (90\% C.L.). The upper limits on the relative strengths for the non-Paulian electromagnetic, strong, and weak transitions have been obtained: $$\delta^2_{\gamma}\leq 1.0\times 10^{-57}$$, $$\delta^2_{N}\leq 7.0\times 10^{-61}$$ and $$\delta^2_{\beta}\leq 9.6\times 10^{-36}$$, all at 90\% C.L..
The Pauli exclusion principle was formulated by Wolfgang Pauli in 1925, when he was just 25 years old. For this discovery, he received the Nobel Prize in 1945.