Antiproton over proton and K${}^{-}$ over K${}^{+}$ multiplicity ratios at high $z$ in DIS

Abstract

The $\bar{\mathrm{p}}$ over p multiplicity ratio is measured in deep-inelastic scattering for the first time using (anti-) protons carrying a large fraction of the virtual-photon energy, $z>0.5$. The data were obtained by the COMPASS Collaboration using a 160 GeV muon beam impinging on 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. The range in Bjorken-$x$ is restricted to $0.01<x<0.40$. Protons and antiprotons are identified in the momentum range $20\tilde{A}\cdot 60$ GeV/$c$. In the whole studied $z$-region, the $\bar{\mathrm{p}}$ over p multiplicity ratio is found to be below the lower limit expected from calculations based on leading-order perturbative Quantum Chromodynamics (pQCD). Extending our earlier analysis of the K${}^{-}$ over K${}^{+}$ multiplicity ratio by including now events with larger virtual-photon energies, this ratio becomes closer to the expectation of next-to-leading order pQCD. The results of both analyses strengthen our earlier conclusion that the phase space available for hadronisation should be taken into account in the pQCD formalism.

19 pages, 7 figures, 3 tables