Spin: Identity Card Of Elementary Particles

Artist's impression of the spin diode created by researchers at MIT. Atoms with clockwise spin can only move in one direction, while atoms with anticlockwise spin move in the opposite direction. (Courtesy: Christine Daniloff) - Source

A few days ago, a reader asked me some information about muons. I thought I'd start with an article about spin, a physical quantity that is identity card of elementary particles.

This article is intended to non-expert users.

Nucleons, i.e. protons and neutrons, constitute 99.9% of known matter, while the remaining 0.1% is given by electrons.There is a long-established idea that electron is an elementary particle without a substructure while nucleons are essentially made up of quarks, point particles without internal structure.
Many properties of nucleons can be derived by combining characteristics of quarks composing them. This is not the case for spin and this has created, for years, strong doubts in physicists about validity of the so far formulated model to explain the structure of matter.

But what is spin? Together with quantities such as electric charge or mass, spin is one of the few fundamental quantities that form "identity card" of elementary particles. Unlike the first ones, the last is a purely "quantic" quantity  hardly relatable to everyday experience and shows that fundamental building blocks of matter have an "intrinsic angular momentum". If you want to have a naive interpretation of angular moment you can think of that is equivalent to a rotation.

The Earth, for example, has an orbital angular momentum due to its revolution around the Sun and an intrinsic angular momentum due to rotation around its axis. By extending this concept from macroscopic world to microscopic one, it is as if all particles were like mini spinning tops. All known matter is made up of objects in rotation.
Proton and all its components can be summarized as so many small spinning tops.
Quantum mechanics teaches us, however, that spin can only take on certain values: integer (0, 1, 2, ...) or half-integer (1/2, 3/2, ...). This is an important datum just because it determines behavior of a particle and thus of entire Universe. Particles such as quarks, the ones made of quarks, or electrons have half-integer spin and are considered as the building blocks of the Universe.

Particles like photons or gluons have integer spin and are mediators of fundamental forces. Consequently, otherwise from mass or electric charge, spin is a physical quantity that "specializes" behavior of particles.
Theoretical formulation of spin was made in the '20s by Austrian physicist Wolfgang Pauli, but, as mentioned earlier, experimental measurement of spin has given uncertainties for years for the fact that total spin of a particle did not correspond to total spin of its elementary constituents.

Only in recent years the complicated puzzle has been solved thanks to difficult high-precision measurements of HERMES (*) experiment at DESY laboratory in Hamburg, in collaboration with physicists at the National Institute of Nuclear Physics laboratory in Frascati.

Nucleons spin is given by sum of quarks spin, gluons spin (mediating particles that are exchanged between quarks) and all particles and anti-particles, which the latter ones can form inside nucleon, in a never-ending dance of evanescent particles in continuous creation and annihilation.

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 (*)HERMES is an experiment investigating the quark-gluon structure of matter: we study the spin structure of the nucleon. HERMES data-taking started in 1995 and ended in summer 2007.

Via|http://www.vivavoceonline.it/index.php
Via|http://en.wikipedia.org/wiki/Spin_%28physics%29