Comment on “Reevaluation of the parton distribution of strange quarks in the nucleon”

The HERMES collaboration in Phys. Rev. D 89, 097101 (2014) extracted information about the strange quark density in the nucleon. One of the main results is an observation that the shape of the extracted density is very different from the shapes of the strange quark density from global QCD fits and also from that of the light antiquarks. In this paper systematic studies on the HERMES published multiplicity of pion and kaon data are presented. It is shown that the conclusions concerning the strange quark distribution in the nucleon reached in Phys. Rev. D 89, 097101 (2014) are at the moment premature.

Figure 1

Comparison of $\pi$ and K multiplicity sums obtained from [3], left and right panels, respectively. Striking and not expected similarities in shape are observed.

Figure 2

Left panel: A tentative of extraction of the strange quark contribution in the nucleon using a certain combination of $K^{+}$ and $K^{-}$ multiplicities $(K^{\prime })$, where the contribution from $D_u^{K^{+}}$ cancels. A drop of the multiplicity at low $x$ is observed instead of the expected rise. The points correspond to the $K^{\prime }$ extraction using MSTW08 LO, and black band on the bottom marks systematic uncertainty of data. The $K^{\prime }$ was also extracted using NNPDF30L set, the gray band it the $\pm 1\sigma$ uncertainty of NNPDF30L. Finally, a set of lines corresponds to the LO pQCD prediction for $K^{\prime }$, where on top of extracted from HERMES $S(x)\int D_{\mathrm{str}}$ distribution (divided by $Q$) a $10D_{\mathrm{unf}}^{\mathrm{DSS}}$ is added with additional scaling factors applied at the reference scale. Right panel: Comparison of the kaon multiplicity difference in the HERMES data multiplied by $(5Q+2S)/4(u_v+d_v)$ with the DSS expectation.