Measurement of subjet multiplicities in neutral current deep inelastic scattering at HERA and determination of alpha(s)

The subjet multiplicity has been measured in neutral current e(+) p interactions at Q(2) > 125 GeV2 with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb(-1). Jets were identified in the laboratory frame using the longitudinally invariant k(T) cluster algorithm. The number of jet-like substructures within jets, known as the subjet multiplicity, is defined as the number of clusters resolved in a jet by reapplying the jet algorithm at a smaller resolution scale y(cut). Measurements of the mean subjet multiplicity, <n(sbj)>, for jets with transverse energies E-T,E-jet > 15 GeV are presented. Next-to-leading-order perturbative QCD calculations describe the measurements well. The value of alpha(s) (M-Z), determined from <n(sbj)> at y(cut) = 10(-2) for jets with 25 < E-T,E-jet < 71 GeV, is alpha(s) (M-Z) = 0.1187 +/- 0.0017(stat.)(-0.0009)(+0.0024)(syst.)(-0.0076)(+0.0093)(th.).