First Observation of Beam Spin Asymmetries in $K^+$ SIDIS

Beam spin asymmetry (BSA) measurements of semi-inclusive deeply inelastic scattering (SIDIS) for charged and neutral pions have already shown that $A_{LU}^{\sin\phi_h}$ is not consistent with zero at JLab 6 GeV kinematics.
It is presently not known if this observation holds for heavier mesons.
This study presents the BSA for positively charged kaons ($K^+$) for the first time obtained by analyzing data measured by the CLAS collaboration in 2003 ($E_{beam} = 5.5 \; GeV$).
A parameter estimation of $A_{LU}^{\sin\phi_h}$ is performed over a wide range of momentum transfer ($1.0 \leq Q^2 \leq 4.5 \; GeV^2/c^2$).
The results are sufficiently precise to definitively claim that the BSA is non-zero for the measured values of $x$, $Q^2$, $z_h$, and $P_T$.
When comparing with previous results reported for $\pi^+$, one observes a qualitative agreement in shape and magnitude.
Based on this non-zero observation of $K^+$ BSA in SIDIS it is possible to conclude that twist-three effects are significant contributors over the measured kinematics.