Pomerons

Pomerons are concepts in Regge theory used to describe the leading contribution to high-energy hadronic and photoproduction scattering. They are modeled as exchanges in the t-channel carrying vacuum quantum numbers, with isospin I=0 and positive C- and P-parity. The term honors Isaak Pomeranchuk, and the trajectory was introduced to explain the slow rise of total cross sections and the prevalence of diffractive processes at high energies.

In Regge theory, the energy dependence of scattering is governed by the Pomeron trajectory alpha_P(t). The total cross section scales roughly as s^(alpha_P(0)-1). The soft Pomeron, used for non-perturbative, low-momentum-transfer processes, has an intercept alpha_P(0) ≈ 1.08 and a small slope alpha'_P ≈ 0.25 GeV^-2, which yields a gradual rise with energy and a modest t-dependence.

In quantum chromodynamics, the Pomeron is described as a color-singlet exchange of gluons. The simplest realization is a two-gluon exchange or a gluon ladder; a more complete description comes from the Balitsky-Fadin-Kuraev-Lipatov (BFKL) formalism, which yields a hard Pomeron with a larger effective intercept (often quoted around 1.2–1.3 after higher-order corrections). Many analyses employ a two-Pomeron framework, with a soft Pomeron governing non-perturbative regimes and a hard Pomeron relevant at high momentum transfer or small x.

Experimentally, Pomeron exchange underpins diffractive scattering, rapidity gaps, and the energy dependence of cross sections in hadron-hadron and photon-induced processes. Observables include diffractive structure functions and vector-meson production at small momentum transfer, studied at HERA and the LHC. The Odderon, a C = −1 counterpart, remains less definitively established.