Definition
Chiral color is a theoretical extension of the quantum chromodynamics (QCD) sector of the Standard Model of particle physics, in which the conventional vector-like colour gauge group SU(3)₍c₎ is replaced by a chiral product group SU(3)₍L₎ × SU(3)₍R₎. The symmetry is spontaneously broken to the diagonal subgroup identified with ordinary colour, resulting in additional massive colour‑octet gauge bosons known as axigluons.
Overview
The chiral colour framework was proposed in the mid‑1980s as a possible avenue for addressing unanswered questions in the Standard Model, such as the origin of parity violation in the strong interaction and the hierarchy of fermion masses. By assigning left‑handed and right‑handed quarks to different colour gauge groups, the model introduces a new source of chiral asymmetry in the strong sector. After spontaneous symmetry breaking, the low‑energy phenomenology reproduces standard QCD, while the heavy axigluons could, in principle, manifest in high‑energy collisions as resonances in dijet or top‑antitop production.
Etymology/Origin
The term combines “chiral”, referring to the handedness (left‑ or right‑handed) of particle fields, with “color”, the quantum number associated with the SU(3) gauge symmetry of the strong interaction. The concept was first articulated in papers by Frampton and Glashow (1987) and subsequent works that explored chiral extensions of the colour gauge group.
Characteristics
| Feature | Description |
|---|---|
| Gauge structure | Initial gauge group SU(3)₍L₎ × SU(3)₍R₎; broken to diagonal SU(3)₍c₎. |
| Symmetry breaking | Achieved via a scalar field transforming as a bi‑fundamental (3, $\bar{3}$) under the two SU(3) factors, acquiring a vacuum expectation value. |
| New particles | Massive colour‑octet axial gauge bosons (axigluons) with parity‑odd couplings to quarks. |
| Couplings | Axigluons couple with opposite sign to left‑handed and right‑handed quarks, leading to chiral‑asymmetric interactions. |
| Phenomenology | Potential signatures include: • Resonant structures in dijet invariant‑mass spectra. • Modifications to the forward‑backward asymmetry in top‑quark pair production. • Contributions to flavour‑changing neutral currents, constrained by precision measurements. |
| Experimental status | No confirmed observation of axigluons; collider searches at the Tevatron and the LHC have placed lower mass limits typically in the multi‑TeV range, depending on model parameters. |
Related Topics
- Quantum Chromodynamics (QCD) – the established vector‑like SU(3)₍c₎ gauge theory of the strong interaction.
- Axigluon – the massive colour‑octet gauge boson predicted by chiral colour models.
- Left‑Right Symmetric Models – theories extending the electroweak sector with SU(2)₍L₎ × SU(2)₍R₎, sharing the chiral gauge‑group concept.
- Grand Unified Theories (GUTs) – frameworks that often embed chiral colour structures within larger gauge groups (e.g., SU(6) or E₆).
- Parity Violation in Strong Interactions – a motivation for chiral extensions, though standard QCD is parity‑conserving.
Chiral colour remains a speculative but well‑documented proposal within theoretical high‑energy physics, subject to ongoing experimental tests at high‑energy particle colliders.