User:Sophie/RPC

Channel Description

An important background to the Mu2e experiment is that originating from negatively charged pions produced at the primary production target. These pions can provide two potential sources of backgrounds to both signal channels:

\begin{equation} \mu^{-} \rightarrow e^{-}\end{equation} and \begin{equation}\mu^{-} \rightarrow e^{+}\end{equation} if they stop on the Al Stopping Target and a radiative nuclear capture process occurs. %There are two sources of pion backgrounds to both the \begin{equation}\mu^{-} \rightarrow e^{-}\end{equation} and $\mu^{-} \rightarrow e^{+}\end{equation} conversion channels. In the case of the \textbf{External Radiative Pion Capture (RPC)} process the outgoing (real) photon converts externally to an \begin{equation}e^{+}e^{-}\end{equation} pair, most likely in one of the Al foils:

\begin{equation} \pi^{-} + N(A,Z) \rightarrow \gamma + N(A, Z-1) \rightarrow N(A, Z-1) + e^{+} + e^{-}, \end{equation}

where $(A, Z - 1)$ is the daughter nucleus of the process. \textbf{Internal Conversion} is related, but quantum mechanically independent:

\begin{equation} \pi^{-} + N(A,Z) \rightarrow N(A, Z-1) + e^{+} + e^{-}. \end{equation}

Here the $e^{+}e^{-}$ pair is produced by an internal, virtual, photon. External Conversions and Internal Conversions can be categorized collectively as pion-capture backgrounds to the $\mu^{-} \rightarrow e^{-}$ if the pair-produced electron has an energy consistent with a conversion-electron signature. Likewise, the outgoing positron can pose a background to the lepton number violating $\mu^{-} \rightarrow e^{+}$ channel.

Although pion-capture backgrounds are sub-dominant they are the driving force behind the livegate optimization. In this study, the expected background contributions from both Internal Conversion and External Conversion, in both channels, will be presented along with detailed discussion of the associated uncertainties.