New Determination of the ¹²C(α, γ)¹⁶O Reaction Rate and Its Impact on the Black-hole Mass Gap

We present a precise measurement of the asymptotic normalization coefficient (ANC) for the ¹⁶O ground state (GS) through the ¹²C(¹¹B, ⁷Li)¹⁶O transfer reaction using the Quadrupole‐3‐Dipole (Q3D) magnetic spectrograph. The present work sheds light on the existing discrepancy of more than 2 orders of magnitude between the previously reported GS ANC values. This ANC is believed to have a strong effect on the ¹²C(α, γ)¹⁶O reaction rate by constraining the external capture to the ¹⁶O ground state, which can interfere with the high-energy tail of the 2⁺ subthreshold state. Based on the new ANC, we determine the astrophysical S-factor and the stellar rate of the ¹²C(α, γ)¹⁶O reaction. An increase of up to 21% in the total reaction rate is found within the temperature range of astrophysical relevance compared with the previous recommendation of a recent review. Finally, we evaluate the impact of our new rate on the pair-instability mass gap for black holes (BH) by evolving massive helium core stars using the MESA stellar evolution code. The updated ¹²C(α, γ)¹⁶O reaction rate decreases the lower and upper edges of the BH gap about 12% and 5%, respectively.