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SU(1), SU(2), SU(3) – Unitary Groups – QCD


In particle physics, SU(1), SU(2), and SU(3) are special unitary groups that play a fundamental role in describing the symmetries of elementary particles and their interactions.

These groups are associated with different types of quantum fields and have important implications in the theory of quantum chromodynamics (QCD) and the electroweak theory.


  • SU(1)

SU(1) is the simplest special unitary group. It corresponds to the phase transformations of a single complex scalar field. The mathematical expression for SU(1) can be written as:

SU(1) = $\left\{ U \in \text{Mat}(1 \times 1, \mathbb{C}) \;|\; UU^\dagger = I, \; \det(U) = 1 \right\}$

where $\text{Mat}(1 \times 1, \mathbb{C})$ represents the set of $1 \times 1$ complex matrices, $U$ is a unitary matrix, $U^\dagger$ is the conjugate transpose of $U$, and $I$ is the identity matrix.

However, it is important to note that SU(1) is trivial since it contains only the identity element, and it does not provide any interesting symmetry beyond the phase transformations of the complex scalar field.


  • SU(2)

SU(2) is a more interesting group that appears in the electroweak theory, describing the unified electroweak interactions. It is associated with the weak isospin symmetry. The mathematical expression for SU(2) can be written as:

SU(2) = $\left\{ U \in \text{Mat}(2 \times 2, \mathbb{C}) \;|\; UU^\dagger = I, \; \det(U) = 1 \right\}$

where $\text{Mat}(2 \times 2, \mathbb{C})$ represents the set of $2 \times 2$ complex matrices.

The SU(2) group contains three generators, corresponding to the three weak isospin states. These generators are represented by $2 \times 2$ matrices, typically denoted as $\tau_i$, where $i = 1, 2, 3$.

The commutation relations of the generators satisfy the SU(2) Lie algebra.

The SU(2) group is non-Abelian, meaning that the order in which transformations are applied affects the result.

This non-Abelian nature leads to interesting phenomena in the electroweak theory, such as the spontaneous symmetry breaking and the generation of masses for the weak bosons.


  • SU(3)

SU(3) is a crucial group in the theory of quantum chromodynamics (QCD), describing the strong interaction between quarks and gluons.

It is associated with the color charge of quarks and the gauge symmetry of the strong force. The mathematical expression for SU(3) can be written as:

SU(3) = $\left\{ U \in \text{Mat}(3 \times 3, \mathbb{C}) \;|\; UU^\dagger = I, \; \det(U) = 1 \right\}$

where $\text{Mat}(3 \times 3, \mathbb{C})$ represents the set of $3 \times 3$ complex matrices.

The SU(3) group contains eight generators, corresponding to the eight colors of QCD. These generators are represented by $3 \times 3$ matrices, typically denoted as $\lambda_a$, where $a = 1, 2, \ldots, 8$.

The commutation relations of the generators satisfy the SU(3) Lie algebra.

Similar to SU(2), SU(3) is a non-Abelian group, leading to rich phenomena in QCD, such as color confinement and the formation of bound states called hadrons.



In this overview, we discussed the significance of SU(1), SU(2), and SU(3) in particle physics.

While SU(1) is trivial, SU(2) plays a crucial role in the electroweak theory, and SU(3) is fundamental to the theory of quantum chromodynamics.

These groups and their associated symmetries provide the foundation for understanding the interactions of elementary particles and the structure of the universe.


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