Only integer changes of spin are observed in particle interactions

Tommaso Dorigo expressed this physics law in many of his blog posts in many different ways. This expression is from one of his recent posts:

“all reactions among particles involve only the integer changes of the total spin of initial and final state” (source)

Here’s another one:

“In any particle-particle or particle decay interaction the total spin before and total spin after can differ only by integer changes”.

He also reminded readers on various occasions that fermions cannot be cloned because of this law. Fermions (electrons, quarks, neutrinos) carry 1/2 unit of angular momentum. If we could clone them the final state would have 1 unit of angular momentum implying a spin change of 1/2. The law prevents such a process.

Reminder: “cloning” is also known as “point-like self-coupling”.

Tommaso Dorigo is an experimental physicist. A theoretical physicist would explain the same law like this:

Interactions among elementary particles are mediated by force-carrying quanta known as bosons. All force-carrying bosons carry 1 unit of spin angular momentum. Therefore, no matter how complicated the interaction is or no matter how many interactions take place in the process the spin difference between the initial and the final states will be either a multiple of 1 or zero.

Note 1: The “boson” in the name of Higgs boson” is a misnomer because Higgs boson is the only particle in nature that carries zero angular momentum. Higgs boson is a special case.

Note 2: The theorized entity known as the “graviton” is the force-carrying quantum of the gravitational force. Graviton is supposed to carry 2 units of spin. The physics law mentioned in the beginning still applies. The spin change between the initial and the final states will be an integer in the processes involving gravitons as well.

Note 3: The concept of “cloning” is also discussed in the context of Quantum Mechanics, in more general terms. There is a fundamental principle in Quantum Mechanics, called no-cloning: it is impossible to make a copy of an arbitrary quantum state without destroying the original state. There is a process known as “quantum teleportation” that seems to copy a quantum state but there is no cloning. In quantum teleportation the copied state is destroyed. This general principle (no-cloning of a quantum state) seems to apply to all elementary particles including fermions (electrons, quarks, neutrinos) as well.