Quantum Chromodynamics

A Physicist named Murray Gell-Mann introduced the term ‘quark.’ Here, quark is a type of fundamental particle and a constituent of matter. The interaction between quarks is possible by a subatomic particle or a glue called a gluon.

Now, talking about chromodynamics, the aforementioned statement about the QCD discusses the strong interaction in terms of an interaction between quarks mediated or transmitted by gluons, where both quarks and gluons are assigned a quantum number called ‘colour.’

On this page, we will understand QCD quantum chromodynamics, and lattice quantum chromodynamics in detail.

Quantum Chromodynamics Definition

A quantum field talks of the following two theoretical theories:

• QED Quantum Electrodynamics

• QCD Quantum Chromodynamics

Quantum electrodynamics talks about the electric charge; however, quantum chromodynamics classifies the interaction between quarks and gluon in terms of colour. It means QCD Quantum Chromodynamics is analogous to QED Quantum Electrodynamics.

In the nutshell, theoretical Physics talks a lot about QCD or quantum chromodynamics. QCD is the interaction between quarks and gluon. Quarks and gluons make up the composite particles, like protons, neutrons, and pions. Therefore, the interaction between these particles is allocated a quantum number, known as colour.

Point to Note:

In QCD, gluons evolve the theory all around, as it the force carrier of QCD, like photons are for the electromagnetic force in QED theory. 

History of Quantum Chromodynamics

In 1973 the concept of colour because the source of a “strong field” was developed into the idea of QCD by European physicists Harald Fritzsch and Heinrich Leutwyler, alongside American physicist Gell-Mann. 

They used a general field theory developed by Chen Ning Yang and Mills in the 1950s when the carrier particles of a force could themselves radiate further carrier particles.

Properties of Quantum Chromodynamics

Quantum Chromodynamics for dummies is extensively expressed in the two following properties:

• Colour confinement, and

• Asymptotic freedom

Colour Confinement

This property is seen as a consequence of the constant force transfer between two coloured electric charges during their separation: A large amount of energy is required to increase the separation between two quarks within a hadron, as they are very tightly bound in their lattice.

Since we discussed the “lattice,” thing here. It indicates that quarks are fixed in their lattice points, and we need excess energy to set two quarks apart. In the nutshell, QCD is called lattice quantum chromodynamics.

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So, what happens further is, this energy added to the system intensifies on spontaneously producing a quark-antiquark pair, turns the initial hadron into a pair of hadrons rather than producing an isolated coloured charge. 

Do you know?

Though the above analysis on colour confinement is just theoretical; however, this theory is well established from lattice QCD calculations and decades of experiments. 

This is the mere reason, we call the above analysis as lattice quantum chromodynamics.

Asymptotic Freedom

Asymptotic meaning is a straight line that recurrently reaches a given curve but hardly meets the curve at a finite distance.

The asymptotic freedom property of QCD describes a steady decrease in the magnitude of interactions between quarks and gluons, as the energy scale of those interactions increases with the decrease in the scale length.

Point to Note:

Asymptotic freedom is the second property of quantum chromodynamics. It was discovered in 1973 by two American theoretical physicists named David Jonathan Gross and Frank Wilczek, and independently by another American theoretical physicist Hugh David Politzer in the same year. For this work, all three shared the 2004 Physics Nobel Prize.

Point to Note:

In the nutshell, asymptotic freedom is large energy that corresponds to short distances - it infers that there is no interaction between the particles. 

Do You Know?

Every particle physics theory is affirmed on certain natural symmetries whose existence is deduced from observations. These are often called local and global symmetries, the definition of these are as follows:

• Local symmetries are the symmetries that act independently at each point in spacetime. Here, each symmetry is the basis of a gauge theory and requires the introduction of its gauge bosons.

• Global symmetries functions must be applied to all or any points of spacetime at the same time.

However, QCD may be a non-abelian gauge theory (or Yang-Mills theory) of the SU(3) gauge group that was obtained by taking the colour charge to define an area symmetry.

Here, non-abelian is sometimes called non-commutative during which there exists a minimum of one pair of elements: a and b of a group (G, *), such a ∗ b ≠ b ∗ a.

Since the strong force cannot differentiate among various flavours of quark, QCD has approximate flavour symmetry, which is broken by the differing quark masses.

Unsolved Problems in Quantum Chromodynamics

There are the two following questions, each on the property of QCD that need to be answered:

1. Confinement

The QCD equations are yet unsolvable at energy scales relevant for describing atomic nuclei. 

A query comes across that how does QCD produce the physics of nuclei and nuclear constituents?

2. Quark Matter

The equations of QCD assume that plasma/soup of quarks and gluons should be formed at heat and density. 

But the properties of matter at this phase still creates a big question mark.