  The spin decomposition of the proton is a long-standing topic of much
interest in hadronic physics. Lattice QCD has had much success in calculating
the connected contributions to the quark spin. However, complete calculations,
which necessarily involve gluonic and strange-quark contributions, still
present some challenges. These "disconnected" contributions typically involve
small signals hidden against large statistical backgrounds and rely on
computationally intensive stochastic techniques. In this work we demonstrate
how a Feynman-Hellmann approach may be used to calculate such quantities, by
measuring shifts in the proton energy arising from artificial modifications to
the QCD action. We find a statistically significant non-zero result for the
disconnected quark spin contribution to the proton of about -5% at a pion mass
of 470 MeV.
