K− over K+ multiplicity ratio for kaons produced in DIS with a large fraction of the virtual-photon energy

Abstract The K − over K + multiplicity ratio is measured in deep-inelastic scattering, for the first time for kaons carrying a large fraction z of the virtual-photon energy. The data were obtained by the COMPASS collaboration using a 160 GeV muon beam and an isoscalar 6 LiD target. The regime of deep-inelastic scattering is ensured by requiring Q 2 > 1 ( GeV / c ) 2 for the photon virtuality and W > 5 GeV / c 2 for the invariant mass of the produced hadronic system. Kaons are identified in the momentum range from 12 GeV/ c to 40 GeV/ c , thereby restricting the range in Bjorken- x to 0.01 x 0.40 . The z -dependence of the multiplicity ratio is studied for z > 0.75 . For very large values of z , i.e. z > 0.8 , we observe the kaon multiplicity ratio to fall below the lower limits expected from calculations based on leading and next-to-leading order perturbative quantum chromodynamics. Also, the kaon multiplicity ratio shows a strong dependence on the missing mass of the single-kaon production process. This suggests that within the perturbative quantum chromodynamics formalism an additional correction may be required, which takes into account the phase space available for hadronisation.