Abstract
We present a first calculation of the heavy flavor contribution to the longitudinally polarized deep-inelastic scattering structure function $g_1^{Q}$, differential in the transverse momentum or the rapidity of the observed heavy quark $Q$ or antiquark $\overline Q$. All results are obtained at next-to-leading order accuracy in QCD within the framework of a newly developed parton-level Monte Carlo generator that also allows one to study observables associated with the produced heavy quark pair such as its invariant mass distribution or its correlation in azimuthal angle. First phenomenological studies are carried out for various heavy quark distributions in a kinematic regime relevant for a future Electron-Ion Collider with a particular emphasis on the expected size of the corresponding double-spin asymmetries and their sensitivity to the still poorly constrained helicity gluon distribution. Theoretical uncertainties associated with the choice of the factorization scale are discussed for selected observables.
Highlights
AND MOTIVATIONHeavy flavors, more precisely their contributions to deep-inelastic scattering (DIS) processes or their production in hadron-hadron collisions, have played a rather minor role so far in constraining longitudinally polarized parton distributions (PDFs) [1,2] despite their sensitivity to the elusive gluon helicity density
In this paper we have presented a new parton-level Monte Carlo program at next-to-leading order accuracy in QCD that allows one to study heavy flavor production in longitudinally polarized deep-inelastic scattering in terms of any infrared-safe differential distribution for the structure function gQ1
The full heavy flavor mass dependence is retained throughout all calculations which makes this code suited for phenomenological studies at the future Electron-Ion Collider which can explore charm electroproduction at small-to-medium momentum fractions and photon virtualities not much larger than the charm quark mass
Summary
More precisely their contributions to deep-inelastic scattering (DIS) processes or their production in hadron-hadron collisions, have played a rather minor role so far in constraining longitudinally polarized parton distributions (PDFs) [1,2] despite their sensitivity to the elusive gluon helicity density. The presence of different scales, at least m and Q, significantly complicates all calculations involving HQs in DIS already at the level of next-to-leading order (NLO) accuracy, in particular, the evaluation of the necessary virtual corrections and phase space integrals This is even more so the case if one considers, in addition, the polarization of the initial-state partons because of the well-known complications due to the presence of γ5 and the Levi-Civita tensor in the helicity projection operators in n ≠ 4 dimensions [10]. We wish to remark that for phenomenological applications at a future EIC at small x, and for Q not much larger than the HQ mass m, HQ production is most likely best described by retaining the full mass dependence as has been done in our largely analytical calculation [15] and for all numerical results presented here This implies that HQs can be only produced extrinsically, for instance, through the PGF mechanism and that the notion of a massless HQ PDF makes no sense..
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