Speaker
Description
Extra-dimensional scenarios for the QCD axion are well motivated to address the quality problem. However, like other pre-inflationary axion theories, they suffer from the axion isocurvature problem where QCD axion dark matter is incompatible with high scale inflation due to CMB isocurvature bounds. We tackle this problem by utilizing an additional ingredient generically predicted by extra dimensional theories: a scalar modulus field in the 4D EFT that controls the size of the extra dimensions and the axion’s decay constant. During an epoch of modulus kination between inflation and reheating, the axion decay constant dynamically decreases, permitting a higher inflationary energy scale while remaining consistent with isocurvature bounds. However, modulus kination tends to suffer from the overshoot problem, leading to decompactification of the extra dimensions. In this work, we investigate how to trap the modulus field with generic extra-dimensional field contents and determine the associated change in the QCD axion decay constant. Towards this end, we calculate scalar and gauge field particle production during the inflation—kination transition and comment on analogies to the tunneling problem in quantum mechanics.