  We report precise radial velocity (RV) observations of HD 212657 (= K2-167),
a star shown by K2 to host a transiting sub-Neptune-sized planet in a 10 day
orbit. Using Transiting Exoplanet Survey Satellite (TESS) photometry, we
refined the planet parameters, especially the orbital period. We collected 74
precise RVs with the HARPS-N spectrograph between August 2015 and October 2016.
Although this planet was first found to transit in 2015 and validated in 2018,
excess RV scatter originally limited mass measurements. Here, we measure a mass
by taking advantage of reductions in scatter from updates to the HARPS-N Data
Reduction System (2.3.5) and our new activity mitigation method called CCF
Activity Linear Model (CALM), which uses activity-induced line shape changes in
the spectra without requiring timing information. Using the CALM framework, we
performed a joint fit with RVs and transits using EXOFASTv2 and find $M_p =
6.3_{-1.4}^{+1.4}$ $M_{\oplus}$ and $R_p = 2.33^{+0.17}_{-0.15}$ $R_{\oplus}$,
which places K2-167 b at the upper edge of the radius valley. We also find
hints of a secondary companion at a $\sim$ 22 day period, but confirmation
requires additional RVs. Although characterizing lower-mass planets like K2-167
b is often impeded by stellar variability, these systems especially help probe
the formation physics (i.e. photoevaporation, core-powered mass loss) of the
radius valley. In the future, CALM or similar techniques could be widely
applied to FGK-type stars, help characterize a population of exoplanets
surrounding the radius valley, and further our understanding of their
formation.
