  The fate of thymine upon excitation by ultraviolet radiation has been the
subject of intense debate over the past three decades. Today, it is widely
believed that its ultrafast excited state decay stems from a radiationless
transition from the bright ${\pi}{\pi}^*$ state to a dark $n{\pi}^*$ state.
However, conflicting theoretical predictions have made the experimental data
difficult to interpret. Here we simulate the ultrafast dynamics in thymine at
the highest level of theory to date, performing wavepacket dynamics with a new
coupled cluster method. Our simulation confirms an ultrafast ${\pi}{\pi}^*$ to
$n{\pi}^*$ transition (${\tau} = 41 \pm 14$ fs). Furthermore, the predicted
oxygen-edge X-ray absorption spectra agree quantitatively with the experimental
results. Our simulation also predicts an as-yet uncharacterized photochemical
pathway: a ${\pi}{\sigma}^*$ channel that leads to hydrogen dissociation at one
of the two N-H bonds in thymine. Similar behavior has been identified in other
heteroaromatic compounds, including adenine, and several authors have
speculated that a similar pathway may exist in thymine. However, this was never
confirmed theoretically or experimentally. This prediction calls for renewed
efforts to experimentally identify or exclude the presence of this channel.
