46 lines
2.7 KiB
TeX
46 lines
2.7 KiB
TeX
\section{Introduction}
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The next generation of charged leption flavor violation (CLFV) experiments,
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COMET at J-PARC and Mu2e at Fermilab, are going to stop about \num{e18} muons
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in their targets to search for new physics beyond the Standard Model.
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Knowing accurately number of stopped muons in the targets is important as it is
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the denominator for the branching ratio of the CLFV process these experiments
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are looking for. The proposed way to determine the number of stopped muons
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is measuring the numbers of charateristic muonic X-rays, and gamma rays
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emitted from cascading muons, and excited nuclei after muon capture,
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respectively.
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Both COMET and Mu2e will use pulsed proton beam to produce pions which decay in
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flight to muons. The primary beam line of Mu2e would deliver proton pulses
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\SI{1.7}{\micro\second} apart, each pulse contains about \num{4E7} protons.
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Electrons from decays of pions would hit the muon stopping target about
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\SI{100}{\nano\second} earlier than muons do, and produce an intense
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``beam~flash'' (about \SI{51}{\mega\hertz\per\square\centi\meter} of
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bremsstrahlungs with average energy of
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\SI{1.4}{\mega\electronvolt})~\cite{mu2etdr}. It is therefore challenging to
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measure the X-rays and gamma rays mentioned above. The situation in COMET Phase-I
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is similar~\cite{comettdr}.
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There are two proposed target materials for COMET and Mu2e, namely aluminum and
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titanium. Most prominent aluminum muonic X-rays at \SI{346.8}{\keV}
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(\twoPoneS~transition) and \SI{412.8}{\keV} (\threePoneS~transition) were
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measured precisely by Measday et
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al.~\cite{Measday2007}. Observing these X-rays in a highly intense pulsed beam
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experiment like COMET and Mu2e will be difficult as they might be buried by
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the ``beam~flash''. A gamma ray of \SI{1808.7}{\keV} from
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\ce{^{26}Mg^*} would provide a better proxy to the number of stopped muons for
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it has a lifetime of muon in aluminium, therefore can be measured out of the
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``beam~flash''. The emission rate of this gamma was measured at
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\SI{10}{\percent} uncertainty in~\cite{Measday2007}.
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Knowledge about muonic X-rays and gammas after muon capture on titanium is less
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comprehensive. Measurements of titanium were mostly done in context of either nuclear
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charge radii~\cite{Wohlfahrt1981}, or neutrinoless double beta
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decay~\cite{Zinatulina2019}, and did not report X-ray yeilds.
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To have a more completed picture of the situation, AlCap has carried out
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measurements of photons after muon capture on aluminum and titanium. The goals
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are emission rates of charateristic muonic X-rays from titanium, and improvement
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on the rate of the \SI{1808.7}{\keV} gamma from aluminum. In addion, we have
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measured photons from other materials where muons would stop in the experiments
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to learn about potential background around the gammas and X-rays of interest.
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