Measurement of single-spin azimuthal asymmetries in semi-inclusive electroproduction of pions and kaons on a longitudinally polarised deuterium target

Abstract

Single-spin asymmetries have been measured for semi-inclusive electroproduction of $π^+$, $π^-$, $π^0$ and $K^+$ mesons in deep-inelastic scattering off a longitudinally polarised deuterium target. The asymmetries appear in the distribution of the hadrons in the azimuthal angle $φ$ around the virtual photon direction, relative to the lepton scattering plane. The corresponding analysing powers in the $\sin φ$ moment of the cross section are $0.012 \pm 0.002 {(stat.)} \pm 0.002 {(syst.)}$ for $π^+$, $0.006 \pm 0.003 {(stat.)} \pm 0.002 {(syst.)}$ for $π^-$, $0.021 \pm 0.005 {(stat.)} \pm 0.003 {(syst.)}$ for $π^0$ and $0.013 \pm 0.006 {(stat.)} \pm 0.003 {(syst.)}$ for $K^+$. The $\sin 2φ$ moments are compatible with zero for all particles.

Figures (7)

• Figure 1: Kinematic planes for meson production in semi-inclusive deep-inelastic scattering: Lepton scattering plane (white) and the meson production plane (shaded).
• Figure 2: Invariant mass ($M_{\gamma\gamma}$) spectrum of photon pairs measured in the electromagnetic calorimeter. A fit to the data using a Gaussian function for the peak plus a second order polynomial (solid curve) for the background of uncorrelated photons is shown as the dotted curve. The two vertical lines embrace the invariant mass interval used for $\pi^0$ identification.
• Figure 3: Target spin asymmetries $A_\mathrm{UL}(\phi)$ for electroproduction of $\pi^+$, $\pi^0$, $\pi^-$ and $K^+$ mesons. Fits of the form $P_0+P_1 \sin\phi$(solid line) and $P_0+P_1 \sin\phi+P_2 \sin2\phi$(dashed line) are also displayed in the figure. The error bars give the statistical uncertainties of the measurements. The values of the coefficients $P_0$ are all compatible with zero and the coefficients $P_1$ and $P_2$ for the various hadrons and their statistical uncertainties are listed in each panel.
• Figure 4: Target spin analysing powers $A_\mathrm{UL}^\mathrm{\sin\phi}$ for semi-inclusive $\pi^+$, $\pi^0$, $\pi^-$ and $K^+$ production on the deuteron (filled circles) and on the proton (open squares). The latter are taken from Refs. hermes_pi+_ssa:1999hermes_pi0_ssa:2001. The data are shown as a function of one of the kinematic variables $x$, $P_\perp$ and $z$ while integrating over the other variables. The error bars give the statistical uncertainties of the measurements and the bands in the lower parts of each panel give the systematic uncertainties for the deuteron (hashed band) and for the proton measurement (open band).
• Figure 5: Comparison of the measured analysing powers $A_\mathrm{UL}^\mathrm{\sin\phi}$ on the deuteron for $\pi^+$, $\pi^0$, $\pi^-$ and $K^+$ production with predictions from theoretical calculations in the chiral quark soliton model ($\chi$QSM, solid linesEfremov+:2002), the quark-diquark model (QdQ, dashed linesMa+:2002) and a perturbative QCD model (pQCD, dotted linesMa+:2002. The shown curves refer to "approach 2" of the models in Ref. Ma+:2002. The error bars give the statistical uncertainties of the measurements, and the bands in the lower part of the panels show the systematic uncertainties of the measurements.