Direct Measurement of the Gluon Polarisation in the Nucleon via Charmed Meson Production

Abstract

We present the first measurement of the gluon polarisation in the nucleon based on the photon-gluon fusion process tagged by charmed meson production and decay to charged K and pi. The data were collected in polarised muon scattering off a polarised deuteron target by the COMPASS collaboration at CERN during 2002-2004. The result of this LO analysis is <Delta g/g>_x = -0.47 +- 0.44 (stat) +- 0.15 (syst) at <x> ~= 0.11 and a scale mu^2 ~ 13 (GeV/c)^2.

Figures (3)

• Figure 1: Invariant mass distributions of the $K\pi$ pairs tagged with the $D^*$ decay ($D^*$ sample, lower) and of the non-tagged $K\pi$ pairs ($D^0$ sample, upper). The curves represent fits with functional form described in the text.
• Figure 2: Correlation between generated and reconstructed values of $a_{\rm LL}$.
• Figure 3: Compilation of the $\left< {\Delta g}/{g} \right>_{x}$ measurements from open charm and high $p_{\rm T}$ hadron pair production by COMPASS compass_highpt_lowq, SMC SMC_highpt and HERMES hermes_highpt as a function of $x$. Horizontal bars mark the range in $x$ for each measurement, vertical ones give the statistical precision and the total errors (if available). The open charm measurement is at a scale of about 13 (GeV/$c$)$^2$, other measurements at 3 (GeV/$c$)$^2$. The curves display parameterizations from a NLO QCD analysis in the $\overline{\rm MS}$ scheme at 3 (GeV/$c$)$^2$, compass: fits with $\Delta G > 0$ (broken line) and with $\Delta G < 0$ (dotted line).