First moments of the nucleon transverse quark spin densities using lattice QCD

TL;DR

This paper uses lattice QCD to calculate the first moments of transverse quark spin densities in the nucleon, revealing a distortion linked to transversity form factors and confirming a large Boer-Mulders function.

Contribution

It provides the first lattice QCD calculation of the Mellin moments of transverse quark spin densities using physical quark masses and multiple lattice spacings.

Findings

Distortion in transversely polarized quark densities observed.

Isovector tensor anomalous magnetic moment measured as 1.051(94).

Results support a negative, large Boer-Mulders function in the nucleon.

Abstract

We present a calculation of the Mellin moments of the transverse quark spin densities in the nucleon using lattice QCD. The densities are extracted from the unpolarized and transversity generalized form factors extrapolated to the continuum limit using three $N_f=2+1+1$ twisted mass fermion gauge ensembles simulated with physical quark masses and spanning three lattice spacings. The first moment of transversely polarized quarks in an unpolarized nucleon shows an interesting distortion, which can be traced back to the sharp falloff of the transversity generalized form factor $\bar{B}_{Tn0}(t)$. The isovector tensor anomalous magnetic moment is determined to be $\kappa_T=1.051(94)$, which confirms a negative and large Boer-Mulders function, $h_1^{\perp}$, in the nucleon.