  The LHCb experiment measured the time-dependent CP asymmetries $C_{KK}$ and
$S_{KK}$ in $B_s\to K^+K^-$ decay. Combining with the corresponding CP
asymmetries $C_{\pi\pi}$ and $S_{\pi\pi}$ in $B\to \pi^+\pi^-$ decay, we find
that the size of $U$-spin breaking in this system is of order $20\%$. Moreover,
the data suggest that these effects are dominated by factorizable
contributions. We further study the constraints on new physics contributions to
$b\to u\bar uq$ ($q=s,d$). New physics that is minimally flavor violating (MFV)
cannot be distinguished from the Standard Model (SM) in these decays. However,
new physics that is not MFV can mimic large $U$-spin breaking. Requiring that
the $U$-spin breaking parameters remain below the size implied by the data
leads to a lower bound of $5-10$ TeV on the scale of generic new physics. If
the new physics is subject to the selection rules that follow from the
Froggatt-Nielsen (FN) mechanism or from General Minimal Flavor Violation
(GMFV), the bound is relaxed to 2 TeV.
