Transverse polarization of $\mathrm{\Lambda }$ hyperons from quasireal photoproduction on nuclei

The transverse polarization of $\mathrm{\Lambda }$ hyperons was measured in inclusive quasireal photoproduction for various target nuclei ranging from hydrogen to xenon. The data were obtained by the HERMES experiment at HERA using the 27.6 GeV lepton beam and nuclear gas targets internal to the lepton storage ring. The polarization observed is positive for light target nuclei and is compatible with zero for krypton and xenon.

Figure 1

Invariant-mass distributions for $\mathrm{\Lambda }$ events obtained with hydrogen and deuterium targets (top panel) and with krypton and xenon targets (bottom panel). The vertical lines indicate the invariant-mass interval used for the determination of the $\mathrm{\Lambda }$ polarization. The quantities given in the legends are the number of analyzed $\mathrm{\Lambda }$ events, $N^{\mathrm{\Lambda }}$, in the selected invariant-mass window after subtraction of background events, the reconstructed $\mathrm{\Lambda }$ mass ${\mathrm{M}}^{\mathrm{\Lambda }}$, the resolution $\sigma$ of the invariant-mass distribution, and the fraction $\eta$ of $\mathrm{\Lambda }$ events in this mass window.

Figure 2

Sketch of $\mathrm{\Lambda }$ production and decay. The polarization vector ${\overrightarrow{P}}_{\mathrm{n}}^{\mathrm{\Lambda }}$ is directed along the normal $\widehat{n}$ to the $\mathrm{\Lambda }$ production plane; $\theta$ is the angle between the momentum of the decay proton and $\widehat{n}$ in the rest frame of the $\mathrm{\Lambda }$ hyperon.

Figure 3

Dependence of the transverse polarization $P_{\mathrm{n}}^{\mathrm{\Lambda }}$ on the atomic-mass number $A$ of the target nuclei. The inner error bars represent the statistical uncertainties; the full error bars represent the total uncertainties, evaluated as the sum in quadrature of statistical and systematic uncertainties.

Figure 4

Dependence of the transverse polarization $P_{\mathrm{n}}^{\mathrm{\Lambda }}$ for the combined hydrogen and deuterium data (closed symbols) and the combined krypton and xenon data (open symbols) on the variable $\zeta$. The error bars represent statistical uncertainties only; the systematic uncertainties are not shown, since they are strongly correlated for the kinematic dependences. The values of $⟨p_T⟩$ for each $\zeta$ bin are shown in the lower panel.

Figure 5

Dependence of the transverse polarization $P_{\mathrm{n}}^{\mathrm{\Lambda }}$ on the transverse $\mathrm{\Lambda }$ momentum $p_T$. Closed circles (squares) represent the combined hydrogen and deuterium data for the region $\zeta <0.2$ ($\zeta >0.3$). The combined krypton and xenon data (open triangles) are shown for the full $\zeta$ range. The error bars represent the statistical uncertainty. The values of $⟨\zeta ⟩$ for each $p_T$ bin are shown in the lower panel.