T-2, Nuclear and Particle Physics, Astrophysics and Cosmology

# Light-front models and constraints for the heavy flavor structure of the nucleon

Timothy Hobbs
University of Washington

The push to uncover physics beyond the Standard Model and map the proton's internal flavor structure has in recent years placed a premium on improved understanding of nonperturbative strange and charm. Despite a variety of relevant measurements in different channels and using diverse mechanisms, however, considerable theoretical ambiguities remain as to the detailed size and momentum dependence of the nucleon's strange/charm content. To make progress, we present several recent calculations on the light-front that enable us to place novel constraints on the magnitude of nucleon strangeness and intrinsic charm (IC). For the former, we develop proton light-front wave functions that simultaneously permit the computation of the strange contributions to the nucleon's electroweak form factors and DIS structure function, finding that current DIS data place heightened limits on elastic strangeness that outstrip current experimental sensitivities. For IC, we derive a convolution model consisting of hadronic and quark-level distributions, which we then subject to a global QCD analysis. Relative to other recent calculations, our fitting technology enjoys several enhancements (among them, improved treatments of higher twist effects and nuclear corrections) that empower an extended reach to lower $Q^2$ and $W^2$ --- precisely the regions at which IC is expected to predominate. Treating an expanded data set with this improved fitting regime, we obtain strong constraints to the overall momentum carried by IC --- $\langle x \rangle_{_{\rm IC}} \leq 0.5\%$ at the 4$\sigma$ level.