Cornering the Planck Alens tension with future CMB data

The precise measurements of cosmic microwave background (CMB) anisotropy angular power spectra made by the Planck satellite show an anomalous value for the lensing amplitude, defined by the parameter Alens, at about 2 standard deviations (2.6 standard deviations when cosmic shear data are included). Moreover, considering Alens brings the values of the cosmological parameters determined by Planck in better agreement with those found by pre-Planck data sets. In this paper, after discussing the current status of the anomaly, we quantify the potential of future CMB measurements in confirming/falsifying the Alens tension. We find that a space-based experiment such as LiteBIRD could falsify the current Alens tension at the level of 5 standard deviations. Similar constraints can be achieved by a stage-III experiment assuming an external prior on the reionization optical depth of τ=0.055±0.010 as already provided by the Planck satellite. A stage-IV experiment could further test the Alens tension at the level of 10 standard deviations. A comparison between temperature and polarization measurements made at different frequencies could further identify possible systematics responsible for Alens>1. We show that, in the case of the CMB-S4 experiment, polarization data alone have the potential of falsifying the current Alens anomaly at more than 5 standard deviations and to strongly bound its frequency dependence. We also evaluate the future constraints on a possible scale dependence for Alens.