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 $20÷60$ 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.

arXiv comment19 pages, 7 figures, 3 tables

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