Phenomenological Estimate for the Longitudinal Unpolarized Structure Function in Semi-inclusive Deep Inelastic Scattering

We present a phenomenological study of the longitudinal unpolarized structure function $F_{UU,L}$ in semi-inclusive deep inelastic scattering (SIDIS) and its ratio with the transverse structure function, $R_{\mathrm{S}\mathrm{I}\mathrm{D}\mathrm{I}\mathrm{S}}=F_{UU,L}\mathrm{/}{F}_{UU,T}$. We study their transverse-momentum dependence, combining for the first time low- and high-transverse-momentum contributions across the kinematic range relevant to SIDIS measurements. At low transverse momentum, $F_{UU,L}$ is power suppressed, making it sensitive to next-to-next-to-leading-power dynamics in the unpolarized sector. We find that this contribution can be sizable, with $R_{\mathrm{S}\mathrm{I}\mathrm{D}\mathrm{I}\mathrm{S}}\simeq 0.3$ at low transverse momentum and hard scale, producing a correction of the order of $10\mathrm{\%}$ to transverse-momentum-integrated observables. Although these estimates remain subject to theoretical uncertainties, our results indicate that accounting for $F_{UU,L}$ is likely to be important for the interpretation of SIDIS multiplicities and motivate dedicated measurements of $R_{\mathrm{S}\mathrm{I}\mathrm{D}\mathrm{I}\mathrm{S}}$ to better constrain the longitudinal contribution.