The CKM matrix is a 3-by-3 matrix determining the ability of different flavors of quark to interact with each other. To the extent that the standard model is correct, the matrix must be unitary. In particular, this means that the sum of the squares of the top-row elements should equal 1. The current best estimate for that sum is merely 0.9985 +- 0.0005.
This question asks whether that discrepancy is real, or will disappear with better statistics and analysis. This question resolves YES if the discrepancy in the 3x3 CKM matrix grows to 5 sigma. This question resolves NO if the estimate and (2-sigma) error bars are updated to lie entirely above 0.9995.
For the sake of intuition, a simple way in which the CKM matrix could fail to be unitary is if there is an additional, fourth generation of quark. In such a world, electroweak interactions between the quarks are determined by a unitary 4x4 matrix, of which CKM is just a (generically non-unitary) 3x3 block. The top row of the 4x4 matrix will sum to 1, but the top row of CKM will generally not.
I do not expect this to require much of a judgement call, so I'll be betting freely in this market.
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