Test of the TMD gluon density in a proton with the longitudinal structure function $F_L(x,Q^2)$

We investigate the dependence of the deep inelastic proton structure function $F_L(x,Q^2)$ on different forms of the transverse momentum dependent (TMD, or unintegrated) gluon distribution. We present a comparison of theoretical results with the latest experimental data taken by the H1 and ZEUS Collaborations at HERA. We demonstrate that these data, despite of having quite large uncertainties, can test different kinds of the gluon TMD in a proton. Moreover, different phenomenological models at low $x$ could be tested by future experiments on deep inelastic scattering.