pseudoscalars

Pseudoscalars are quantities or particles that behave as scalars under proper rotations but change sign under parity transformation. In field theory, a pseudoscalar field φ(x) transforms as φ'(t, -x) = -φ(t, x), in contrast to a true scalar field which remains unchanged. This parity-odd behavior makes pseudoscalar interactions distinguishable from scalar ones, and it plays a central role in how these fields couple to other particles.

In quantum chromodynamics (QCD) and particle physics, pseudoscalar mesons are spin-0 states with negative parity, written as J^P = 0^-. They include the lightest mesons: the three pions (π^+, π^0, π^-), the kaon doublets (K^+, K^0 and their antiparticles), and the heavier eta and eta-prime (η, η'). These mesons form an octet and a singlet under flavor SU(3). The pions are associated with the approximate chiral symmetry of light quarks and are interpreted as pseudo-Goldstone bosons that acquire mass through explicit chiral symmetry breaking by quark masses.

Phenomenologically, pseudoscalar mesons often decay into two photons, as seen in π^0 → γγ, a process tied to the axial anomaly. They can also couple to fermions via Yukawa-like interactions involving γ^5, reflecting their parity properties. Masses, decay constants, and mixing among η and η′ provide important tests of QCD in the nonperturbative regime.

Beyond hadrons, the term pseudoscalar also appears for hypothetical fields such as axions and axion-like particles, which are pseudoscalar candidates for new physics with weak couplings to photons and matter, relevant to the strong CP problem and dark matter searches.