Chiral symmetry is a type of symmetry that distinguishes between left-handed and right-handed versions of particles or fields, especially in the context of quantum field theory and particle physics.
The term “chiral” comes from the Greek word cheir, meaning hand, and refers to objects or systems that cannot be superimposed on their mirror images—just like your left and right hands.
In particle physics, chirality refers to the intrinsic “handedness” of particles:
- Left-handed particles spin in the opposite direction of their motion.
- Right-handed particles spin in the same direction as their motion.
Key Aspects:
- Chiral symmetry is especially relevant in theories involving massless fermions, such as in Quantum Chromodynamics (QCD).
- For massless particles, chirality and helicity (spin direction relative to motion) are the same.
- When particles acquire mass, chiral symmetry is explicitly or spontaneously broken.
Applications and Importance:
- In QCD, chiral symmetry and its spontaneous breaking explain the appearance of pions as light pseudo-Goldstone bosons.
- In the Standard Model, the weak interaction violates chiral symmetry by only interacting with left-handed fermions, leading to parity violation.
- The Higgs mechanism breaks chiral symmetry by giving mass to fermions through Yukawa couplings.
In condensed matter physics:
- Chiral symmetry plays a role in graphene, topological insulators, and chiral superconductors, where it affects electronic and transport properties.