Transverse polarization of $\Lambda$ and $\overline{\Lambda }$ hyperons in quasireal photoproduction

The HERMES experiment has measured the transverse polarization of $\Lambda$ and $\overline{\Lambda }$ hyperons produced inclusively in quasireal photoproduction at a positron beam energy of 27.6 GeV. The transverse polarization $P_{\mathrm{n}}^{\Lambda }$ of the $\Lambda$ hyperon is found to be positive while the observed $\overline{\Lambda }$ polarization is compatible with zero. The values averaged over the kinematic acceptance of HERMES are $P_{\mathrm{n}}^{\Lambda }=0.078\pm 0.006(\mathrm{stat})\pm 0.012(\mathrm{syst})$ and $P_{\mathrm{n}}^{\overline{\Lambda }}=-0.025\pm 0.015(\mathrm{stat})\pm 0.018(\mathrm{syst})$ for $\Lambda$ and $\overline{\Lambda }$, respectively. The dependences of $P_{\mathrm{n}}^{\Lambda }$ and $P_{\mathrm{n}}^{\overline{\Lambda }}$ on the fraction $\zeta$ of the beam’s light-cone momentum carried by the hyperon and on the hyperon’s transverse momentum $p_{\mathrm{T}}$ were investigated. The measured $\Lambda$ polarization rises linearly with $p_{\mathrm{T}}$ and exhibits a different behavior for low and high values of $\zeta$, which approximately correspond to the backward and forward regions in the center-of-mass frame of the ${\gamma }^{*}N$ reaction.

Figure 1

Schematic diagram of inclusive $\Lambda$ production and decay. The angle ${\theta }_p$ of the decay proton with respect to the normal $\widehat{n}$ to the production plane is defined in the $\Lambda$ rest frame.

Figure 2

Invariant mass distributions for $\Lambda$ and $\overline{\Lambda }$ events. The central region was used for the determination of the $\Lambda$ ($\overline{\Lambda }$) polarization. The shaded areas indicate the invariant mass intervals used for the determination of the background polarization.

Figure 3

Correlation between $x_{\mathrm{F}}$, evaluated in the ${\gamma }^{*}N$ system, and the light-cone fraction $\zeta$ determined in the $eN$ system, as determined from a PYTHIA Monte Carlo simulation.

Figure 4

Ratio of $\Lambda$ to $\overline{\Lambda }$ yields versus light-cone fraction $\zeta$ observed in the data, after background subtraction.

Figure 5

Transverse polarizations $P_{\mathrm{n}}^{\Lambda }$ and $P_{\mathrm{n}}^{\overline{\Lambda }}$ (upper panel) and mean $⟨p_{\mathrm{T}}⟩$ (lower panel) as functions of $\zeta =(E_{\Lambda }+p_{z\Lambda })/(E_e+p_e)$. The inner error bars represent the statistical uncertainties, and the outer error bars represent the statistical and systematic uncertainties added in quadrature.

Figure 6

Transverse polarizations $P_{\mathrm{n}}^{\Lambda }$ and $P_{\mathrm{n}}^{\overline{\Lambda }}$ as a function of $p_{\mathrm{T}}$ for hyperons from the region $\zeta <0.25$ (upper panel) and the region $\zeta >0.25$ (lower panel). The inner error bars represent the statistical uncertainties, and the outer error bars represent the statistical and systematic uncertainties added in quadrature.