Complex sentence: For instance, certain formulas in the Standard Model require that neutrinos be massless; however, experiments in the past few decades have shown that neutrinos possess a small, but non-zero mass. Thus, some modification to the Standard Model becomes necessary.
Split into simpler sentences: For instance, certain formulas in the Standard Model require that neutrinos be massless. However since its formulation, experiments have shown that neutrinos possess a small, but non-zero mass. Thus, some modification to the Standard Model becomes necessary.