Quark contribution to the proton spin from 2+1+1-flavor lattice QCD

We present the first high-statistics chiral-continuum extrapolated up, down and strange quark spin contribution to the proton spin using lattice QCD.  We use up to eleven ensembles of 2+1+1-flavors of Highly Improved Staggered Quarks (HISQ) generated by the MILC collaboration, which  covers 4 lattice spacings covering \{0.15,0.12,0.09,0.06\} fm and two the physical pion mass.  High-statistics estimates on each ensemble allow us to quantify systematic uncertainties and perform a simultaneous chiral-continnum extrapolation in the lattice spacing and light-quark masses.  Our final results are $\Delta u = 0.777(25)$, $\Delta d  = -0.438(18)$, and $\Delta s = -0.053(8)$, adding up to a total  quark contribution to proton spin of $\sum_{q=u,d,s} (\frac{1}{2}  \Delta q) = 0.143(31)(29)$.