  High-resolution cross-correlation spectroscopy (HRCCS) combined with adaptive
optics has been enormously successful in advancing our knowledge of exoplanet
atmospheres, from chemistry to rotation and atmospheric dynamics. This powerful
technique now drives major science cases for ELT instrumentation including
METIS/ELT, GMTNIRS/GMT and MICHI/TMT, targeting biosignatures on rocky planets
at 3-5 $\mu$m, but remains untested beyond 3.5 $\mu$m where the sky thermal
background begins to provide the dominant contribution to the noise. We present
3.51-5.21 $\mu$m M-band CRIRES+/VLT observations of the archetypal young
directly imaged gas giant $\beta$ Pictoris b, detecting CO absorption at S/N =
6.6 at 4.73 $\mu$m and H$_2$O at S/N = 5.7, and thus extending the use of HRCCS
into the thermal background noise dominated infrared. Using this novel spectral
range to search for more diverse chemistry we report marginal evidence of SiO
at S/N = 4.3, potentially indicative that previously proposed
magnesium-silicate clouds in the atmosphere are either patchy, transparent at
M-band wavelengths, or possibly absent on the planetary hemisphere observed.
The molecular detections are rotationally broadened by the spin of $\beta$ Pic
b, and we infer a planetary rotation velocity of $v$sin(i) = 22$\pm$2 km
s$^{-1}$ from the cross-correlation with the H$_2$O model template, consistent
with previous K-band studies. We discuss the observational challenges posed by
the thermal background and telluric contamination in the M-band, the custom
analysis procedures required to mitigate these issues, and the opportunities to
exploit this new infrared window for HRCCS using existing and next-generation
instrumentation.
