Observation of the strange sea in the proton via inclusive phi-meson production in neutral current deep inelastic scattering at HERA

Abstract Inclusive φ (1020)-meson production in neutral current deep inelastic e + p scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 45 pb −1 . The φ mesons were studied in the range 10 Q 2 2 , where Q 2 is the virtuality of the exchanged photon, and in restricted kinematic regions in the transverse momentum, p T , pseudorapidity, η , and the scaled momentum in the Breit frame, x p . Monte Carlo models with the strangeness-suppression factor as determined by analyses of e + e − annihilation events overestimate the cross sections. A smaller value of the strangeness-suppression factor reduces the predicted cross sections, but fails to reproduce the shapes of the measured differential cross sections. High-momentum φ mesons in the current region of the Breit frame give the first direct evidence for the strange sea in the proton at low x .