  The study of temperate sub-Neptunes is the new frontier in exoplanetary
science. A major development in the past year has been the first detection of
carbon-bearing molecules in the atmosphere of a temperate sub-Neptune, K2-18 b,
a possible Hycean world, with the James Webb Space Telescope (JWST). The JWST
is poised to characterise the atmospheres of several other such planets with
important implications for planetary processes in the temperate regime.
Meanwhile, ground-based high-resolution spectroscopy has been highly successful
in detecting chemical signatures of giant exoplanets, though low-mass planets
have remained elusive. In the present work, we report the atmospheric
reconnaissance of a temperate sub-Neptune using ground-based high-resolution
transmission spectroscopy. The long orbital period and the low systemic
velocity results in a low planetary radial velocity during transit, making this
system a valuable testbed for high-resolution spectroscopy of temperate
sub-Neptunes. We observe high-resolution time-series spectroscopy in the H- and
K-bands during the planetary transit with the IGRINS instrument (R$\sim$45,000)
on Gemini-South. Using observations from a single transit we find marginal
evidence (2.2$\sigma$) for the presence of methane (CH$_4$) in the atmosphere
and no evidence for ammonia (NH$_3$) despite its strong detectability for a
cloud-free H$_2$-rich atmosphere. We assess our findings using injection tests
with different atmospheric scenarios, and find them to be consistent with a
high CH$_4$/NH$_3$ ratio and/or the presence of high-altitude clouds. Our
results demonstrate the capability of Gemini-S/IGRINS for atmospheric
characterization of temperate sub-Neptunes, and the complementarity between
space- and ground-based facilities in this planetary regime.
