The weak nuclear force is the mechanism of interaction between subatomic particles. These are responsible for the radioactive decay of atoms. The weak interaction participates in nuclear fission. The theory describing its behavior and effects is called quantum flavourdynamics although it is also called an electroweak theory.

Its effective range of weak force is limited to subatomic distance. Which is less than the diameter of a proton.

Standard Model

weak interaction

The standard model of particle physics gives a uniform framework for understanding strong, weak, and electromagnetic interactions. These interactions occur when two particles exchange integer-spin, force-carrying bosons. The fermions( A fermion is a category of elementary particles. They are very small and light. They are thought of as the building blocks of matter. As atoms are made up of fermions. Paul Dirac named them fermions in honor of the famous scientist Enrico Fermi.) involved in such exchanges can either be electrons/quarks also known as elementary or protons/neutrons also known as composite. The deepest levels of all weak interactions untimely are between elementary particles. Fermions can exchange three types of force carriers named W(+), W(-), and Z bosons.

The mass of these bosons is greater than the mass of protons or neutrons. It is consistent with a short range of the weak force. This force is termed weak because its field strength over a given distance is normally several orders of magnitude less than that of the strong nuclear force or electromagnetic force.

What are Quarks

Quarks mainly composite particles like neutrons and protons, and they come in six “flavors”- up, down, strange, charm, top, and bottom. These give those composite particles their properties. The weak interaction is unique as it allows quarks to swap their flavor for another. The swapping of those properties is mediated by the force carrier bosons.

History of Weak Nuclear Force

weak nuclear force

In 1993, Enrico Fermi proposed the first theory of weak interaction. It was known as Ferni’s interaction. He suggested that beta decay could be explained by a four-fermion interaction. As it involves a contact force with no range. However, it is better described as a non-contact force field having a finite range. In the 1960s Abdus Salam, Steven Weinberg, Sheldon Glashow unified the electromagnetic force and the weak interaction by showing them to be two aspects of a single force. The existence of W and Z particles was not confirmed until 1983.

On 4th July 2014, experimental teams at the Large Hadron Collider announced that they had confirmed the formal discovery of a previously unknown boson of mass between 125 and 127 GeV/c2, whose behavior is so far consistent with the Higgs Boson, it was done with an added note that the advanced data and analyses are needed before positively identifying the new boson as being a Higgs boson of some type. By 14th March 2013, a Higgs Boson particle was tentatively confirmed to exist.

Symmetry violated

The laws of nature were long thought to remain the same under mirror reflection. The results of an experiment viewed via a mirror were expected to be identical to the results of a mirror-reflected copy of the experimental apparatus. The law was known as parity conservation was known to be respected by classical gravitation, electromagnetism, and the strong interaction; it was assumed to be a universal law. But in the year the 1950s two Chinese scientists suggested that the weak interaction might violate this law. Due to this, they earned the Nobel prize in Physics in the year 1957.

Although the weak interaction was once described by Fermi’s theory. The discovery of parity violation and renormalization theory suggested that a new approach was needed. In 1957 Robert Marshak and George Sudarshan and, somewhat later, Richard Feynman and Murray Gell-Mann proposed a new theory. In which the weak interaction acts only on left-handed particles (and right anti-particles). Since the mirror reflection of a left-handed particle is right-handed, this explains the maximal violation of parity. The V-A theory was developed before the discovery of the Z boson. So it did not include the right-handed fields that enter in the neutral current interaction.

Further continuation of Violation of Symmetry

nuclear force

However, this theory allowed a compound symmetry to be observed. CP combines parity P (switching left to right) with charge conjugation C (switching particles with antiparticles). Physicists were again surprised when 1964, James Cronin and Val Fitch provided clear evidence in Kaon’s decays that CP symmetry could be broken too, winning them the Nobel Prize in Physics in 1980. In 1973, Makoto Kobayashi and Toshihide Maskawa showed that CP violation in the weak interaction required more than two generation particles, effectively predicting the existence of a then-unknown third generation. This discovery earned them half of the 2008 Nobel Prize in Physics.

Unlike parity violation, CP violation occurs only in limited circumstances. Despite its rarity, it is widely believed to be the reason that there is much more matter than antimatter in the universe, and thus forms one of Andrei Sakharov’s three conditions for Baryogenesis.

~Nikshan


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