= IceCube starting-event veto
{c}
The 2013 selection rejected early boundary light. Tagged <muons> checked leakage; <air shower> companion <muons> also vetoed downward <atmospheric neutrinos>.<Evidence for High-Energy Extraterrestrial Neutrinos at the IceCube Detector by IceCube Collaboration>[Event Selection; Atmospheric Muon Background]{ref}
\Image[/icecube-contained-event-veto.png]
{title=Entering tracks, contained interactions and an accompanying-muon veto}
{description=Original schematic, not to scale or measured data. Dots: optical modules. Dashed arrows: unseen neutrinos; solid arrows: muons.}
A simple original <conditional probability> model explains why a veto changes the selected mixture. Let $A$ mean that an <atmospheric neutrino> interaction would pass all other cuts, $M$ that an accompanying <muon> reaches the veto, and $V$ that the veto fires. Set $a=P(M\mid A)$ and $\epsilon=P(V\mid M,A)$, and neglect unrelated veto hits. The <law of total probability> gives
$$
P(\text{passes veto}\mid A)=1-a\epsilon.
$$
For chosen $a=0.8$ and $\epsilon=0.95$, the passing fraction is $0.24$. Neither this value nor either input is an <IceCube> measurement. With equal incident candidate populations, this mechanism alone reduces the atmospheric contribution relative to a hypothetical population having $a=0$. It does not make every surviving event astronomical: $a=0$ or $\epsilon=0$ gives no suppression, while $a=\epsilon=1$ removes the entire chosen atmospheric population. If unrelated light vetoes a surviving event independently with <probability> $u$, its passing fraction becomes $(1-u)(1-a\epsilon)$; independence is an additional assumption.
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