Quantum Chromodynamics.

Astrid Morréale

Eyes on pQCD


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1900: Looking inside matter:

There exists today more than one hundred chemical elements in nature, all neatly classified in Mendeleev's table of elements. The original table created by the chemist Dmitri Mendeleev was done before the discovery of subatomic structure of matter.

Mendeleev's 1869 periodic table. Photo from Wikipedia.

The intuition of the scientists was not dormant however: before the formulation of quantum mechanics was made, elements were already listed in order of increasing atomic number.

The arrival of the proton:

Experiments in 1918 done by Ernest Rutherford had determined that when α particles were shot into nitrogen gas, signatures of hydrogen nuclei could be observed.
α particles are subatomic fragments consisting to two protons and two neutrons. These particles are ejected in the form of ionising radiation which comes from the nucleai of certain unstable atoms.

Rutherford proposed that the hydrogen nuclei which he observed were elementary particles.
This led to the discovery of the proton.

The discovery of the neutron followed soon after that of the proton some decades later when James Chadwick made several experiments trying to understand (or rather disprove!) the γ theory of the strong radiation which was observed by Bothe and Becker and detailed later by Irene Joliot-Curie and her husband F. Joliot-Curie.

The radiation under study occured when very energetic α particles emitted from polonium fell on certain light elements.

Thanks to all activity in the 1900's, it was well established that an atom is made up of protons and neutrons.

The proton is not an elementary particle: here come the partons:

It turned out that nucleons (the collective name for protons and neutrons) also have structure, they are not point like.

Nucleons are composed of quarks and gluons whose interactions, as well the interactions of other fundamental particles, have helped us understand matter at its most fundamental level.

How do we know quarks and gluons exist?

We know from experimental data. I will get to this later.

A proton with its confined colored quarks bounded by gluons.

There are four types of forces distinguishable in nature: gravitation , which becomes relevant in densely aggregated matter, electromagnetism, the weak force, and the strong force.
The strong force has charge: the strong color charge, with 3 charges and eight carriers.

The Strong Force

In nature, the strong force holds atomic nuclei together despite strong electrostatic repulsion.
It is a force that dominates the nucleons, and one that is responsible for strong binding of the quarks and gluons (called partons collectively) in order to form the proton and the neutron.
However, as experiments in the late 1960's have shown, the distance at which this force is exerted is tiny: about one Fermi, a distance of about 1/1000000000000000 meters (10-15 m).

Data from deep inelastic scattering (DIS) of electrons on protons have indicated that, after being hit by an energetic electron, one of the quarks from the proton propagates freely appearing to barely interact with the other quarks.


Little Girl Plays with the Gauge Field (oil painting, gift to Nicole d'Hose)

To be continued

A quark anti-quark pair.


you can e-mail me at : astrid.morreale@cern.ch