How long does a neutron live?
Although physics can describe the structure of atomic nuclei very well, there is still great uncertainty about one fundamental value: the lifespan of the neutron. With a new numerical method, Jülich researchers have now come one step closer to solving this mystery.
All atomic nuclei that make up our universe consist of protons and neutrons – in varying numbers, depending on the type of atom. If neutrons or protons occur individually, they differ in one important respect: while the free proton can exist forever or at least for an unimaginably long amount of time, a free neutron lives a comparably short life before it decays. Its average lifespan lasts only about 15 minutes. Physicists cannot say how long exactly – which is unusual for the otherwise very precise scientists.
One reason is different observations: if physicists look at the disappearance of the neutrons, they get a different lifespan than if they record the occurrence of the decay products. The two measuring methods are very precise in themselves, but their results differ by well over eight seconds. For physicists, that’s an eternity. This difference is so far not only inexplicable, but also unsatisfactory. Knowing the exact lifespan of the neutron would help to better understand the laws of the universe and to verify theoretical ideas of the big bang.
Computer simulations are intended to help in this respect; but: “How long a single neutron will exist cannot be predicted. Quantum mechanics, which describes this process, merely provides probabilities,” says Dr. Evan Berkowitz from the Nuclear Physics Institute/Institute for Advanced Simulation (IKP-3/IAS-4), describing the problem so far. However, Berkowitz and his colleagues have brought research an important step forward: With the help of supercomputers, they calculated, for the first time directly and as precisely as never before, how great this probability is.
“We have determined the so-called coupling constant for this purpose. It describes how easily neutrons decay due to the weak interaction, the force that can cause conversions from one type of particle to another,” explains Berkowitz. “This constant is difficult to calculate with the standard model of particle physics.”
In a nutshell
The new mathematical methods used by the researchers improve the algorithms previously used in part of the Standard Model, known as quantum chromodynamics (QCD). “In these methods, space and time are represented by points on a grid,” explains Berkowitz. “Through this construction, a calculation of the relationship between the elementary particles is fundamentally possible – but only with the aid of powerful supercomputers.”
The method also points the way to further improvements that may clarify the discrepancy between measurements of neutron lifespan. For this, though, the accuracy of the calculations would have to be improved further. Evan Berkowitz says that this is basically no problem: “If someone is willing to pay the electricity bill for our complex calculations on supercomputers, we can continue to reduce the uncertainty of our response. The electricity bill will not be cheap, however,” he warns.
“If someone is willing to pay the electricity bill for our complex calculations on supercomputers, we can continue to reduce the uncertainty of our response.”
Mini universe simulated
Neutrons and protons consist of so-called quarks – the smallest building blocks of matter. The neutron is composed of two down quarks and one up quark, the proton of one down quark and two up quarks. The quarks are held together by the gluons, a kind of particle adhesive.
When a free neutron decays, further elementary particles are formed on the one hand – an electron and an antineutrino – and on the other hand, the neutron is transformed into a proton. In the world of quarks, this means that a down quark turns into an up quark. Jülich researchers have calculated the frequency of this process in order to determine the probability of neutron decay. To do so, they have used the part of the standard model of particle physics that describes how quarks and gluons interact with each other, which is known as quantum chromodynamics (QCD). The calculation is extremely complex, which is why the researchers only simulated a tiny model universe with a single neutron consisting of the quarks. Even for this simplified system, they needed a powerful supercomputer.
Another problem: quantum chromodynamics says that down quarks always remain down quarks and up quarks always remain up quarks. Accordingly, a calculation with the help of QCD usually shows that neutrons are stable. “QCD is a simplification of reality, however, because it only describes the strong interaction between the particles. We must take into account that there are also small effects of the weak interaction between down quarks and up quarks – the force that allows particles to transform. That’s why we added the transformation of a down quark into an up quark to our simulation,” explains Evan Berkowitz. This allows the researchers to determine the coupling constant and the probability that a neutron will decay.
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