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High Energy / Nuclear Theory / RIKEN seminars

[CFNS Seminar] Small-x contributions to the proton spin puzzle

by Yuri Kovchegov (Ohio State University)


An integral part of the proton spin puzzle is the contribution to the proton spin coming from quarks and gluons having small very values of the Bjorken x variable. This contribution is mostly beyond the reach of current experiments and is very hard to calculate numerically on the lattice. It appears that theoretical understanding of quark and gluon helicity distributions at small x is needed to assess the amount of proton spin coming from this region. In my talk I will describe the recent theoretical work aimed at finding the small-x asymptotics of the quark and gluon helicity distributions, along with their orbital angular momenta (OAM). I will derive small-x evolution equations for helicity and solve them to find the small-x asymptotics of the parton helicity distributions and OAM. The results of this work can be compared to the data to be collected at the upcoming Electron-Ion Collider (EIC) in order to extrapolate the small-x helicity distributions to be measured at EIC to even smaller values of x, thus completely constraining the proton spin coming from small x and helping to resolve the proton spin puzzle.