Naturalness (particle physics)

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In particle physics, the assumption of naturalness means that unless a more detailed explanation exists, all conceivable terms in the effective action that preserve the required symmetries should appear in this effective action with natural coefficients. Natural coefficients have the form

h = cΛ^{4 − d}

where d is the dimension of the operator, Λ is the cutoff scale - energy, above which the effective field theory breaks down - and c is a "random" number comparable to one.

However, many parameters in the effective actions we know seem to have far smaller coefficients than required by naturalness. See

• hierarchy problem
• fine-tuning
• cosmological constant problem

among other examples. These surprisingly small values seem to contradict naturalness.