A new fine-tuning problem in standard model: quantum chromodynamics trilemma

We find that a serious fine-tuning exists in quantum chromodynamics (QCD) at physical point, which involves three fundamental quantities essential for the QCD vacuum structure: susceptibility functions for the chiral symmetry, axial symmetry, and the topological charge. The fine-tuning is unavoidably driven when the balance in magnitude among them (dobbed the QCD trilemma) is relaxed. It turns out that QCD is actually unnatural at vacuum, and even in a whole low-temperature regime including the chiral crossover epoch. It may be challenging to resolve this unnaturalness in a whole thermal history of Universe even going beyond the framework of the standard model, because it is tightly tagged with the anomalous Ward identity for chiral and axial symmetries of the up, down and strange quarks. A supercooling first-order electroweak phase-transition might be one possibility to realize the naturalness at higher temperatures.