updating the xray paper

r15a_xray/tex/intro.tex

@@ -1,11 +1,46 @@
 \section{Introduction}
-Why are we even doing this measurement?
-\begin{itemize}
-  \item targets for mu-e conversion experiments
-  \item why did we measure \ce{W}, \ce{H_2O}, \ldots: background for Xrays of
-    interest in Mu2e
-  \item existing data? focused on nuclear charge radii, did not report muonic
-    X-ray yields. This is true for \ce{^{nat}Ti}~\cite{Wohlfahrt1981}
+The next generation of charged leption flavor violation (CLFV) experiments
+(COMET at J-PARC and Mu2e at Fermilab) are going to stop about \num{e18} muons
+in their targets to search for new physics beyond the Standard Model. Knowing
+accurately number of stopped muons in the targets is important as it is the
+denominator for the branching ratio of the coherent muon decay to electron
+without a neutrino process these experiments looking for. The proposed way to
+measure the actual number of stopped muons is infering that from number of
+charateristic muonic X-rays, and gamma rays emitted from excited nuclei after
+muon capture. 
 
-\end{itemize}
+Both COMET and Mu2e will use pulsed proton beam to produce pions which decay in
+flight to muons. The primary beam line of Mu2e would deliver proton pulses
+\SI{1.7}{\micro\second} apart, each pulse contains about \num{4E7} protons.
+Electrons from decays of pions would hit the muon stopping target about
+\SI{100}{\nano\second} earlier than muons do, and produce an intense
+``beam~flash'' (about \SI{51}{\mega\hertz\per\square\centi\meter} of
+bremsstrahlungs with average energy of
+\SI{1.4}{\mega\electronvolt})~\cite{mu2etdr}. It is therefore challenging to
+measure the X-rays and gamma rays mentioned above. The situation in COMET Phase-I
+is similar~\cite{comettdr}.
+
+There are two proposed target materials for COMET and Mu2e, namely aluminum and
+titanium. Most prominent aluminum muonic X-rays at \SI{346.8}{\keV}
+(\twoPoneS~transition) and \SI{412.8}{\keV} (\threePoneS~transition) were
+measured precisely by Measday et
+al.~\cite{Measday2007}. Observing these X-rays in a highly intense pulsed beam
+experiment like COMET and Mu2e would be difficult as they might be buried by
+the ``beam~flash'' described above. A gamma ray of \SI{1808.7}{\keV} from
+\ce{^{26}Mg^*} would provide a better proxy to the number of stopped muons for
+it has a lifetime of muon in aluminium, therefore can be measured out of the
+``beam flash''. The emission rate of this gamma was measured at
+\SI{10}{\percent} uncertainty in~\cite{Measday2007}. 
+
+Knowledge about muonic X-rays and gammas after muon capture on titanium is less
+prehensive. Measurements of titanium were mostly done in context of  either nuclear
+charge radii~\cite{Wohlfahrt1981}, or neutrinoless double beta
+decay~\cite{Zinatulina2019}, and did not report X-ray yeilds. 
+
+To have a more completed picture of the situation, AlCap has carried out
+measurements of photons after muon capture on aluminum and titanium. The goals
+are emission rates of charateristic muonic X-rays from titanium, and improvement
+on the rate of the \SI{1808.7}{\keV} gamma from aluminum. In addion, we have
+measured photons from other materials where muons would stop in the experiments
+to learn about potential background around the gammas and X-rays of interest.