Longitudinal Spin Transfer to $Λ$ Hyperons in Semi-Inclusive Deep Inelastic Scattering with CLAS12

Abstract

The polarization of $Λ$ hyperons is preserved in the angular distribution of their decay products. This property allows one to study the spin structure of the $Λ$. In Semi-Inclusive Deep Inelastic Scattering where a high energy lepton interacts with a nucleon target and one or more hadrons and the scattered lepton are detected in the final state, the probability for a struck quark to impart the polarization of the lepton to the $Λ$ may be measured. In particular, in electron-proton scattering this quantity may be related to the longitudinal light quark polarization of the $Λ$. Currently, limited experimental data cannot discriminate between different models of $Λ$ spin structure. This work reports on the measurement of the longitudinal spin transfer $D^Λ_{LL'}$ to the $Λ$ using data taken by the CLAS12 spectrometer at Jefferson Lab with a $10.6$ GeV longitudinally polarized electron beam and an unpolarized hydrogen target. This measurement is the most precise to date, and, in comparison with theory predictions, it offers valuable insight into the relative dominance of current and target fragmentation in $Λ$ production.

Figures (10)

• Figure 1: The $\Lambda \rightarrow p\pi^{-}$ decay in the $\Lambda$ rest frame is shown with the $\Lambda$ spin axis $\vec{P}_{\Lambda}$ retained from the $\gamma^*N$ CM frame.
• Figure 2: Fits to the invariant mass spectrum $M_{p\pi^{-}}$ of reconstructed $p\pi^{-}$ pairs in data (a) and MC (b) are similar up to scale factors. These distributions are from the middle bin $3$ in our $z_{p\pi^{-}}$ bin scheme. The histogram counts over the fitted background are shown in the black histograms at the bottom of each plot for comparison with the fitted signal function.
• Figure 3: Raw spin transfer results obtained with the HB method, before background correction, as a function of the mass spectrum $M_{p\pi^{-}}$ over the entire dataset. The subpanels show results for $\Lambda$ polarization along the $\Lambda$ momentum (a) and along the virtual photon $\gamma^*$ momentum (b) in the $\gamma^*N$ CM frame. Vertical uncertainties are statistical and horizontal uncertainties are given by the standard deviation within each bin.
• Figure 4: Kinematic coverage in $Q^2$, $x$, and $W$ of our dataset.
• Figure 5: Results for $D_{LL'}^{\Lambda}$ binned in $z_{p\pi^{-}}$. The subpanels show results for $\Lambda$ polarization along the $\Lambda$ momentum (a) and along the virtual photon $\gamma^*$ momentum (b) in the $\gamma^*N$ CM frame. Vertical uncertainties are statistical and horizontal uncertainties are given by the standard deviation within each bin. Gray boxes show the systematic uncertainties.