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

The ${\text{K}}^{-}$ over ${\text{K}}^{+}$ 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 ⁶LiD target. The regime of deep-inelastic scattering is ensured by requiring $Q^2>1{(\text{GeV}/c)}^2$ for the photon virtuality and $W>5\text{GeV}/c^2$ 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.