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Massive stars die an explosive death as a core-collapse supernova (CCSN). Theexact physical processes that cause the collapsing star to rebound into anexplosion are not well-understood, and the key in resolving this issue may liein the measurement of the shape of CCSNe ejecta. Spectropolarimetry is the onlyway to perform this measurement for CCSNe outside of the Milky Way andMagellanic Clouds. We present an infrared (IR) spectropolarimetric detection ofa CCSN, enabled by the new highly sensitive WIRC+Pol instrument at PalomarObservatory, that can observe CCSNe (M = -17 mags) out to 20 Mpc to ~0.1%polarimetric precision. IR spectropolarimetry is less affected than optical bydust scattering in the circumstellar and interstellar media, thereby providinga more unbiased probe of the intrinsic geometry of the SN ejecta. SN 2018hna, aSN 1987A-like explosion, shows 2.0+-0.3% continuum polarization in the J bandoriented at ~160 degree on-sky at 182 d after the explosion. Assuming prolategeometry like in SN 1987A, we infer an ejecta axis ratio of <0.48 with the axisof symmetry pointing at 70 degree position angle. The axis ratio is similar tothat of SN 1987A suggesting that they may share intrinsic geometry andinclination angle. Our data do not rule out oblate ejecta. We also observe oneother core-collapse and two thermonuclear SNe in the J band. SN 2020oi, astripped-envelope Type Ic SN in Messier 100 has p = 0.37+-0.09% at peak light,indicative of either a 10% asymmetry or host interstellar polarization. The SNeIa, 2019ein and 2020ue have p < 0.33% and < 1.08% near peak light, indicativeof asymmetries of less than 10% and 20%, respectively.

Infrared Spectropolarimetric Detection of Intrinsic Polarization from a Core-Collapse Supernova