Within the Standard Model, xs (the mixing parameter in the Bs−Bˉs system) is constrained to the range 7≤xs≤40. We point out that if New Physics contributes significantly to xd (the mixing parameter in the Bd−Bˉd system), then 2≤xs≤7 is possible without any fine-tuned cancellations between the Standard Model and the New Physics contributions.
Cite
@article{arxiv.hep-ph/9402348,
title = {Naturally Small $x_s$?},
author = {Yosef Nir},
journal= {arXiv preprint arXiv:hep-ph/9402348},
year = {2014}
}