Longitudinal spin transfer to the $\Lambda$ hyperon in semiinclusive deep-inelastic scattering

The transfer of polarization from a high-energy positron to a ${\Lambda }^0$ hyperon produced in semiinclusive deep-inelastic scattering has been measured. The data have been obtained by the HERMES experiment at DESY using the 27.6 GeV longitudinally polarized positron beam of the HERA collider and unpolarized gas targets internal to the positron (electron) storage ring. The longitudinal spin-transfer coefficient is found to be $D_{L{L}^{\prime }}^{\Lambda }=0.11\pm 0.10(\mathrm{stat})\pm 0.03(\mathrm{syst})$ at an average fractional energy carried by the ${\Lambda }^0$ hyperon $⟨z⟩=0.45$. The dependence of $D_{L{L}^{\prime }}^{\Lambda }$ on both the fractional energy $z$ and the fractional longitudinal momentum $x_F$ is presented.

Figure 1

The single-quark scattering mechanism leading to ${\Lambda }^0$ production in polarized deep-inelastic positron scattering.

Figure 2

Purities for ${\Lambda }^0$ production from the proton target within the HERMES acceptance, calculated separately for quarks and antiquarks of various flavors at $x_F>0$ with $⟨Q^2⟩=2.41{\mathrm{GeV}}^2$.

Figure 3

The yield of semiinclusively produced ${\Lambda }^0$ hyperons in deep-inelastic scattering. The left (right) panel shows the invariant-mass spectrum before (after) the application of background-suppression cuts. The vertical lines show the boundaries at $\pm 3.3\sigma$. The spectra include essentially all data recorded from unpolarized targets by the HERMES spectrometer in the years 1996–1997 and 1999–2000, corresponding to a yield of $30.3\times {10}^6$ inclusive deep-inelastic scattering events.

Figure 4

Dependence of the longitudinal spin-transfer coefficient $D_{L{L}^{\prime }}^{\Lambda }$ on $z$, for $x_F>0$. The curves represent the phenomenological model calculations of Refs. [5, 30], as described in the text. Error bars are statistical only.

Figure 5

Dependence of the longitudinal spin-transfer coefficient $D_{L{L}^{\prime }}^{\Lambda }$ on $x_F$. The HERMES measurements are represented by the solid circles, while the open symbols represent data from NOMAD [13] (squares) and E665 [15] (circles). Error bars are statistical only. As explained in the text, for the neutrino-induced NOMAD data the quantity plotted is $-P_{\Lambda }^{\nu }$.