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New Opportunities in $h\to 4\ell$

The Higgs decay $h\to 4\ell$ has played an important role in discovering the Higgs and measuring its mass thanks to low background and excellent resolution. Current cuts in this channel have been optimized for Higgs discovery via the dominant tree level $ZZ$ contribution arising from electroweak symmetry breaking. Going forward, one of the primary objectives of this sensitive channel will be to probe other Higgs couplings and search for new physics on top of the tree level $ZZ$ `background'. Thanks to interference between these small couplings and the large tree level contribution to $ZZ$, the $h\to 4\ell$ decay is uniquely capable of probing the magnitude \emph{and} CP phases of the Higgs couplings to $γγ$ and $Zγ$ as well as, to a lesser extent, $ZZ$ couplings arising from higher dimensional operators. With this in mind we examine how much relaxing current cuts can enhance the sensitivity while also accounting for the dominant non-Higgs continuum $q\bar{q}\to4\ell$ background. We find the largest enhancement in sensitivity for the $hZγ$ couplings ($\gtrsim 100\%$) followed by $hγγ$ ($\gtrsim 40\%$) and less so for the higher dimensional $hZZ$ couplings (a few percent). With these enhancements, we show that couplings of order Standard Model values for $hγγ$ may optimistically be probed by end of Run-II at the LHC while for $hZγ$ perhaps towards the end of a high luminosity LHC. Thus an appropriately optimized $h\to4\ell$ analysis can complement direct decays of the Higgs to on-shell $γγ$ and $Zγ$ pairs giving a unique opportunity to directly access the CP properties of these couplings.