  Pulsar Timing Arrays (PTAs) have reported evidence for a stochastic
gravitational wave (GW) background at nHz frequencies, possibly originating in
the early Universe. We show that the spectral shape of the low-frequency
(causality) tail of GW signals sourced at temperatures around $T\gtrsim 1$ GeV
is distinctively affected by confinement of strong interactions (QCD), due to
the corresponding sharp decrease in the number of relativistic species.
Bayesian analyses in the NANOGrav 15 years and the previous International PTA
datasets reveal a significant improvement in the fit with respect to cubic
power-law spectra, previously employed for the causality tail. This suggests
that the inclusion of Standard Model effects on GWs can have a potentially
decisive impact on model selection.
