martedì 18 giugno 2013

Spin: The 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 Eu Vergara asked me some informations about the muons. I thought I'd start with an article about the spin, a physical quantity that is the identity card of elementary particles.

The article is intended  to a non-expert users.

The nucleons, ie protons and neutrons, constitute 99.9% of  known matter, while the remaining 0.1% is given by the electrons. Is consolidated the idea that the electron is an elementary particle without a substructure while the nucleons are essentially made up of quarks, point particles without internal structure.

Many properties of nucleons can be derived by combining the characteristics of the quarks composing them. This is not the case for the spin and this has created, for years, strong doubts in the physicists  about the validity of the so far formulated model  to explain the structure of matter.

But what is the spin? Together with quantities such as electric charge or mass, the spin is one of the few fundamental quantities that form the "identity card" of elementary particles. Unlike the first ones, the last is a purely "quantic" quantity  hardly relatable to everyday experience and shows that the fundamental building blocks of matter have an "intrinsic angular momentum". If you want to have a naive interpretation of the 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 the rotation around its axis. By extending this concept from the macroscopic to the microscopic, it is as if all the particles were like mini spinning tops. All the known matter is made up of objects in rotation.
The proton and all its components can be summarized as so many small spinning tops.
Quantum mechanics teaches us, however, that the 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 the behavior of a particle and thus of the entire Universe. The particles such as quarks, those made from quarks, or electrons have half-integer spin and are considered the building blocks of the Universe.

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

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

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


 (*)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.


2 commenti:

  1. Annarita, dovrò fare un corso di inglese:))
    Bacio ciao

    1. Rosaria, un corso di inglese sarebbe utile a prescindere;)

      Un bacione. Ciao!


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