When the Kondo metal URu₂Si₂ is cooled below 17.5 K, it undergoes a second order phase transition marked by a λ-anomaly in the specific heat, a jump in resistivity as well as a jump in the thermal conductivity. However, the emerging low temperature state does not display any obvious order and hence, this transition has been referred to as a 'hidden order' transition. A wealth of experimental data has been accumulated with the consensus being that at the transition an energy gap opens up across the entire Brillouin zone, but the mechanism behind this opening of the gap has not been identified. 

We show that the mechanism is fully attributable to the number of quasi-particles present, with the number increasing rapidly upon approaching the transition from below. The increased number of quasi-particles present results in a sharp decrease in their lifetimes according to the Landau-Khalatnikov damping mechanism, and causes the excitations to transition from being propagating (gapped) below the transition to becoming overdamped (non-gapped) above the transition. We show the validity of this scenario by mapping the hidden-order transition onto the normal fluid to superfluid transition in ⁴He. We demonstrate that the entropy and damping rate curves of the two systems display an identical temperature evolution upon approaching their respective transitions. Since ⁴He is a relatively simple system with only one type of excitations present that become overdamped at the λ-transition because of collisions between quasi-particles, this demonstrates that the gap in URu₂Si₂ opens up spontaneously when the number of thermally excited quasi-particles drops below a critical value. We argue that this scenario also accounts for the sharp increases in resistivity and thermal conductivity, provided the ground state and first excited state are magnetic singlets.