Exploring rapidity regularization schemes at low $x$ with the DIS longitudinal structure function

We propose three possible rapidity regulators for higher-order calculations in low $x$ QCD with gluon saturation, as alternatives to the usual lower cut-off for the integrals over the light-cone momentum $k^+$. These rapidity regulators are closely related to the $\eta$ regulator and to the pure rapidity regulator, which have been used primarily in studies of transverse-momentum-dependent (TMD) factorization within the soft-collinear effective theory (SCET). By choosing one of the three rapidity regulators that we propose, formulated in terms of $k^+$, $k^-$ or rapidity respectively, one can set from the start of the calculation in which of these three variables one wishes to formulate the low $x$ evolution equations, which is one of the main advantages of our approach. As a test of the viability of these rapidity regulators and of their practical implementation in higher order calculations with gluon saturation effects, we use them to revisit the calculation of the NLO corrections to the dipole factorization of the $F_L$ structure function in inclusive DIS at low $x$.