Free neutron decay

A free neutron is a subatomic particle that is not bound to an atomic nucleus. While neutrons inside stable atoms do not decay, free neutrons are unstable. They undergo a process called beta decay. This is one of the simplest examples of decay in particle physics.

This process is important for understanding how the universe works. It helps scientists study the weak interaction and the early history of our cosmos.

In the Standard Model of physics, this decay is driven by the weak interaction. This interaction is mediated by a particle called a W- boson. Inside the neutron, a single down quark transforms into an up quark. This change in quarks turns the neutron into a proton. The W- boson then quickly decays into two other particles. These are an electron and an electron antineutrino.

The decay is energetically favorable because the mass of the neutron is higher than the mass of the products. This mass difference is released as kinetic energy.

Most neutron decays follow the same basic path. However, there are rare variations to this process. In about 1% of cases, a photon is also released. This is known as internal bremsstrahlung. It happens when the emitted electron interacts with the proton's charge. Another rare event occurs in about four out of every million decays. This is called a two-body decay. In this case, the electron cannot escape the proton. Instead, it stays bound to it to form a neutral hydrogen atom.

The history of this discovery began in 1932 when the neutron was first found. In 1935, James Chadwick and Maurice Goldhaber speculated that free neutrons would be unstable. They believed this because the neutron's mass is larger than that of a hydrogen atom. Scientists first observed free neutron decay in 1948. The first measurement of its lifetime happened in 1950. By 2011, researchers had conducted more than 20 different experiments to measure this lifetime.

Measuring the exact lifetime of a neutron is difficult. A free neutron has a mean lifetime of nearly 15 minutes. This corresponds to a half-life of approximately 608 seconds. Scientists use different techniques to find this number. The beam method uses a beam of neutrons in a high vacuum. The bottle method stores ultracold neutrons in a material chamber. Another way is magnetic trapping, which uses a neutron's magnetic dipole moment to hold them.

Currently, there is a major disagreement in these measurements. This is known as the neutron lifetime puzzle. As of 2014, the difference between measurement methods was about 9 seconds. For example, a 2013 beam method experiment found a lifetime of 888 seconds. However, a 2021 bottle method experiment found a lifetime of 879 seconds. There is also a 3.9 standard deviation disagreement between material storage and magnetic trap results. Some scientists suggest this could be caused by new, unknown physics, such as decay into dark matter.

This decay has massive implications for the field of cosmology. The decay lifetime affects models of Big Bang nucleosynthesis. It specifically influences the ratio of protons to neutrons in the early universe. In astrophysics, we see related processes in extreme environments. In neutron stars, the reverse process of electron capture can happen. This can turn a proton and an electron back into a neutron. The study of neutron decay provides vital data about the weak interaction at low energies.