Date: Thu, 27 Apr 2000 19:48:21 -0500
From: Heidi Schellman <schellman@fnal.gov>
Subject: [Fwd: Revised executive summary]
To: schellma
Message-id: <3908DFD5.E5E2568B@fnal.gov>
MIME-version: 1.0
Content-type: text/plain; charset=us-ascii
Content-transfer-encoding: 7bit


-------- Original Message --------
Subject: Revised executive summary
Date: Fri, 21 Apr 2000 11:52:49 -0600 (CST)
From: Chris Quigg <quigg@fnal.gov>
To: schellman@fnal.gov

\documentclass[12pt]{article}
\usepackage{latexsym}

\def\ltap{\mathop{\raisebox{-.4ex}{\rlap{$\sim$}} \raisebox{.4ex}{$<$}}}
\def\gtap{\mathop{\raisebox{-.4ex}{\rlap{$\sim$}} \raisebox{.4ex}{$>$}}}

\begin{document}

\section*{Executive Summary}
In the Fall of 1999, the Fermilab Directorate chartered a study group 
to investigate the physics motivation for a \textit{neutrino factory} 
based on a muon storage ring that would operate in the era beyond the 
current set of neutrino-oscillation experiments.  We were charged to 
evaluate the prospective physics program as a function of the stored 
muon energy (up to $50\hbox{ GeV}$), the number of useful muon decays 
per year (in the range from $10^{19}$ to $10^{21}$ decays per year), 
and distance from neutrino source to detector; and to assess the value 
of muon polarization within the storage ring.  A companion study 
evaluated the technical feasibility of a neutrino factory and 
identified an R\&D program that would lead to a detailed design.

The principal motivation for a neutrino factory is to provide the 
intense, controlled, high-energy beams that will make possible 
incisive experiments to pursue the mounting evidence for neutrino 
oscillations.  The composition and spectra of intense neutrino beams 
from a muon storage ring will be determined by the charge, momentum, 
and polarization of the stored muons, through the decays $\mu^{-} 
\rightarrow e^{-}\nu_{\mu}\bar{\nu}_{e}$ or $\mu^{+} \rightarrow 
e^{+}\bar{\nu}_{\mu}\nu_{e}$.  There is no comparable source of 
electron neutrinos and antineutrinos.  The neutrino beam also offers 
unprecedented opportunities for precise measurements of nucleon 
structure and of electroweak parameters.  The intense muon source 
needed for the neutrino factory would make possible exquisitely 
sensitive searches for muon-electron conversion and other rare 
processes.

Experiments carried out at a neutrino factory within the next decade 
can add compelling new information to our understanding of neutrino 
oscillations, provided that the number of useful muon decays exceeds 
$10^{19}$ per year and the muon energy is at least $20\hbox{ GeV}$.  
By studying the oscillations of $\nu_{\mu}$, $\nu_{e}$, 
$\bar{\nu}_{\mu}$, and $\bar{\nu}_{e}$, it will be possible to 
measure, or put stringent limits on, all of the appearance modes 
$\nu_e \rightarrow \nu_\tau$, $\nu_e \rightarrow \nu_\mu$, and 
$\nu_\mu \rightarrow \nu_\tau$; to determine precisely (or place 
stringent limits on) all of the leading oscillation parameters; to 
infer the pattern of neutrino masses; and, under the right 
circumstances, to observe \textsf{CP} violation in the lepton sector.  
Baselines greater than about 2000~km will enable a quantitative study 
of matter effects and a determination of the mass hierarchy.  If the 
Mini\textsc{BooNE} experiment confirms the $\nu_{\mu} \leftrightarrow 
\nu_{e}$ effect reported by the LSND experiment, experiments with 
rather short baselines (a few tens of km) could be extremely 
rewarding.

The physics program at detectors located close to the neutrino factory
is also very interesting.  The neutrino fluxes are four orders of magnitude
higher than those from existing beams. Such intense beams make experiments
with high precision detectors and low mass targets feasible for the first
time.  The  spin and flavor structure of the weak interactions 
allow unique studies of nucleon structure and of the electro-weak couplings
themselves.  

\subsection*{Recommendations}
The physics program we have explored for a neutrino factory is highly 
promising.  We recommend a sustained effort to study both the physics 
opportunities and the machine realities.
\begin{description}
\item{(i)} We encourage support for the R\&D needed 
to learn whether a neutrino factory can be a real option in the next
decade. 
\item{(ii)} We propose continued studies of oscillation physics 
to better understand how to build very massive detectors for
long--baseline 
neutrino oscillation physics at a neutrino factory. These studies 
should identify the detector R\&D that is required to realize detectors 
with masses of a few times 10~kt or more that are able to detect and 
measure wrong--sign muons, and detectors of a few kt or more able to 
observe tau--lepton appearance with high efficiency.  It is also 
desirable to identify and measure the charge of electrons.
Because of the size of such detectors both the detector technologies
themselves and the civil engineering issues of building such massive
detectors need to be addressed.

\item{(iii)} We have not completed the optimization of physics 
performance in terms of baseline.  More work is also needed to 
understand the benefits of polarization.  These are appropriate topics 
for a continuing study.

\item{(iv)} This study concentrated on the muon storage ring as a
 neutrino source and did not cover the additional physics  programs
 which would use the proton driver and the high intensity muon beams. 
 A further study directed at these other facets of physics at a muon 
storage ring facility would be very useful.
\end{description}


\end{document}