How a particle's spin is decided?

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• phoenix95
In summary, particle physicists can make theoretical predictions or directly measure the spin of particles. For stable or long-living charged particles, the spin can be measured by how they react to electromagnetic radiation. For shorter-lived particles, the spin can be determined by studying the angular and energy distributions of their decay products. The spin of the Higgs boson, for example, was measured using this method. The graviton, which has a spin of 2, has never been directly observed but its existence is inferred from the polarization type seen in gravitational waves. The theory of spin is based on how particles behave when rotated, with different particles having different rotation requirements due to their spins. More information on this topic can be found in particle physics and quantum
phoenix95
Gold Member
I'm taking a course in particle physics. One of the features different from particle to particle is the spin. For example Higgs boson has spin 0, muon and electron have spin 1/2, graviton has spin 2, and so on. How is this decided upon? Does it occur in the experiments or is it based on theoretical calculations?

You can make theoretical predictions but you can also measure it. For stable or long-living charged particles you can directly measure how the particle react to electromagnetic radiation (e.g. for protons and electrons). For particles that live too short for this you can study the angular distributions and energy distributions of decay products as they depend on the particle spin. This is how the spin of the Higgs boson has been measured for example.

The graviton has never been seen as individual particle, but particles that lead to the polarization type we see with gravitational waves have to have spin 2.

phoenix95
mfb said:
The graviton has never been seen as individual particle, but particles that lead to the polarization type we see with gravitational waves have to have spin 2.
So I assume theoretical calculation of spin follows from the polarisation of the waves? How?

If you rotate an electromagnetic wave around its propagation direction you have to rotate it by 360 degrees (2pi) until it matches the original wave. It has spin 1.
If you rotate the wave function of an electron in an analog way you have to rotate it by 720 degrees (4pi) until it matches the original wave. It has spin 1/2.
If you rotate a gravitational wave you have to rotate it by 180 degrees (pi) until it matches the original wave. It has spin 2.

Every particle physics textbook and good textbooks on quantum mechanics should cover that.

phoenix95

1. What is particle spin?

Particle spin is a quantum mechanical property that describes the angular momentum of a particle. It is independent of its actual physical rotation and can have values of either +1/2 or -1/2.

2. How is a particle's spin determined?

A particle's spin is determined by its intrinsic properties, such as its mass and charge. It is an inherent property of the particle and does not change over time.

3. What role does spin play in quantum mechanics?

Spin plays a crucial role in quantum mechanics as it is one of the fundamental properties used to describe the behavior of particles. It is also used to explain various phenomena, such as the Pauli exclusion principle and the behavior of magnetic materials.

4. Can a particle's spin be changed or manipulated?

No, a particle's spin cannot be changed or manipulated as it is an intrinsic property of the particle. However, its spin state can be measured and observed through certain experiments.

5. How is spin measured or observed in experiments?

Spin can be measured or observed through various experiments, such as the Stern-Gerlach experiment or electron spin resonance. These experiments involve subjecting particles to external magnetic fields and observing the deflection or absorption of particles based on their spin states.

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