Determination of the Polarization Observables $${{\varvec{C}}}_{{\varvec{x}}}$$, $${{\varvec{C}}}_{{\varvec{z}}}$$, and $${{\varvec{P}}}_{{\varvec{y}}}$$ for the Quasi-Free Mechanism in the Reaction $${\varvec{\overrightarrow{\gamma }}}{\varvec{d}}\varvec{\rightarrow } {\varvec{K}}^{\varvec{+}}{\varvec{\overrightarrow{\varLambda }}} {{\varvec{n}}}$$

Abstract

The study of the excited-nucleon ( $$N^*$$ ) spectrum is fundamental for the quest to understand non-perturbative quantum chromodynamics. Measured kinematical evolutions of experimental observables, such as polarizations and unpolarized cross sections, for exclusive meson photoproduction off proton and neutron, are the foundation of theoretical and phenomenological analyses that identify the $$N^*$$ states excited in these reactions and determine their parameters. While free proton targets are readily available, experiments typically use bound neutrons, such as in the deuteron, to estimate observables for photoproduction off the free neutron. Even though exclusivity allows to select event samples dominated by quasi-free scattering (QF), it is not clear if the obtained observables are unbiased estimates of the observables for photoproduction off the free neutron. Studies assessing the possible sources of bias, such as the neutron binding and final-state interactions (FSI), are sparse and primarily focused on the unpolarized cross sections. The comparison between observables for production off the free and off the bound proton in high-statistics reactions, seems to be the natural way to study and to quantify these effects experimentally. Alternatively, one can map the evolution of the QF observables with target-neutron momentum and potentially extrapolate it to the “free”-neutron point of zero momentum. In this work, we determine the recoil polarization $$P_y$$ of the $$\varLambda $$ hyperon and the polarization transfers $$C_x$$ and $$C_z$$ from circularly-polarized photons to the $$\varLambda $$ for the quasi-free mechanism of the reaction $$\overrightarrow{\gamma }d\rightarrow K^+\overrightarrow{\varLambda } n$$ , i.e., for photoproduction off the bound proton. The data were taken with the CLAS detector at the Thomas Jefferson National accelerator facility (JLab) during the E06-103 (g13a) experiment and cover $$E_{\gamma }$$ between 0.9 and 2.6 GeV and $$\cos \theta _{KCM}$$ between $$-\,0.3$$ and 0.8. For several kinematic bins, the dependence of the polarizations on the target-proton momentum is studied. Our work can be used as a benchmark to develop analysis strategies to reduce potential bias due to FSI and the binding of the neutron in the deuteron in reported polarization observables for quasi-free meson photoproduction off the bound neutron.