  Pulsar timing arrays (PTAs) provide a way to detect gravitational waves at
nanohertz frequencies. In this band, the most likely signals are stochastic,
with a power spectrum that rises steeply at lower frequencies. Indeed, the
observation of a common red noise process in pulsar-timing data suggests that
the first credible detection of nanohertz-frequency gravitational waves could
take place within the next few years. The detection process is complicated by
the nature of the signals and the noise: the first observational claims will be
statistical inferences drawn at the threshold of detectability. To demonstrate
that gravitational waves are creating some of the noise in the pulsar-timing
data sets, observations must exhibit the Hellings and Downs curve -- the
angular correlation function associated with gravitational waves -- as well as
demonstrating that there are no other reasonable explanations. To ensure that
detection claims are credible, the International Pulsar Timing Array (IPTA) has
a formal process to vet results prior to publication. This includes internal
sharing of data and processing pipelines between different PTAs, enabling
independent cross-checks and validation of results. To oversee and validate any
detection claim, the IPTA has also created an eight-member Detection Committee
(DC) which includes four independent external members. IPTA members will only
publish their results after a formal review process has concluded. This
document is the initial DC checklist, describing some of the conditions that
should be fulfilled by a credible detection. At the present time none of the
PTAs have a detection claim; therefore this document serves as a road map for
the future.
