What characteristic regarding spin describes fundamental particles?

The characteristic of spin that describes fundamental particles is that it can take on positive or negative values, which refers to the intrinsic angular momentum of the particles. Specifically, in quantum mechanics, spin is quantified in units of reduced Planck's constant (ħ) and can be described as a half-integer or integer value. For example, electrons, which are fundamental particles, have a spin of 1/2, described as being "up" or "down" in terms of its orientation.

This option correctly emphasizes the duality of spin values. In quantum mechanics, the concept of spin is not limited to positive values; rather, it can represent a range of orientations such as "spin up" or "spin down," related to the magnetic moment of the particle in an external magnetic field.

The other options miss the nuances of spin in particle physics: asserting that it is always positive disregards the nature of half-integer spins; claiming that it does not exist in elementary particles contradicts established principles since spin is a fundamental property; and suggesting that it is irrelevant in particle physics undermines the crucial role spin plays in determining the behavior of particles and their interactions. Understanding this property is vital for delving into particle interactions and the fundamental structure of matter