 %% introduction.tex, Mike's new version, received Jan.9


\section{Introduction\label{intro}}

Recent results from the SNO collaboration~\cite{snolatest} coupled with data
from the SuperK collaboration~\cite{superk} have provided convincing
evidence that neutrinos oscillate and that they very likely do so among the
three known neutrino species. Experiments currently under way or planned in
the near future will shed further light on the nature of these mixings among
neutrino species and the magnitudes of the mass differences between them.
Neutrino oscillations and the implied non-zero masses and mixings represent
the first experimental evidence of effects beyond the Standard Model, and as
such are worthy of vigorous scientific study.

This document indicates our progress along a path toward establishing an
ongoing program of research in accelerator and experimental physics based on
muon beams, and neutrino beams derived therefrom, that can proceed in an
incremental fashion. At each step, new physics vistas open, leading
eventually to a Neutrino Factory and possibly a Muon Collider. This concept
has aroused significant interest throughout the world scientific community.
In the U.S., a formal collaboration of some 110 scientists, the Neutrino
Factory and Muon Collider Collaboration, 
also known as the Muon Collaboration (MC)~\cite{EPP:collaboration}, has
undertaken the study of designing a Neutrino Factory, along with R\&D
activities in support of a Muon Collider design. The MC comprises three
sponsoring national laboratories (BNL, FNAL, LBNL) along with groups from
other U.S. national laboratories and universities and individual members
from non-U.S. institutions.

One of the first steps toward a Neutrino Factory is a proton driver that can
be used to provide intense beams of conventional neutrinos in addition to
providing the intense source of low energy muons (from pion decay) that must
first be ``cooled'' before being accelerated and stored. 
Our vision is that
while a proton driver is being constructed, R\&D on collecting and cooling
muons would continue. A source of intense cold muons could be immediately
used for physics measurements, such as determining the electric and magnetic
dipole moments of the muon to higher precision, muonium-antimuonium
oscillations, muon spin rotation experiments and rare muon decays. 
Once the capability of cooling and
accelerating muons is fully developed, a storage ring for such muons would
serve as the first Neutrino Factory. Its specific beam energy and its
distance from the long-baseline experiment will be chosen using the
knowledge of neutrino oscillation parameters gleaned from the present
generation of solar and accelerator experiments (Homestake, Kamiokande,
SuperKamiokande, SAGE, GALLEX, K2K, SNO), the next generation experiments
(MiniBooNE, MINOS, CNGS, KamLAND, Borexino), and the high-intensity
conventional beam experiments that would already have taken place.

A Neutrino Factory provides both $\nu _{\mu }$ and $\anti\nu _{e}$ beams of
equal intensity from a stored $\mu ^{-}$ beam, and their charge-conjugate
beams for a stored $\mu ^{+}$ beam. Beams from a Neutrino Factory are
intense compared with today's neutrino sources. In addition, they have
smaller divergence than conventional neutrino beams of comparable energy.
These properties permit the study of non-oscillation physics at near
detectors, and the measurement of structure functions and associated
parameters in non-oscillation physics, to unprecedented accuracy. Likewise,
they permit long-baseline experiments that can determine oscillation
parameters to unprecedented accuracy.

Depending on the value of the parameter $\sin ^{2}2\theta _{13}$ in the
three-neutrino oscillation formalism, the oscillation $\nu _{e}\rightarrow
\nu _{\mu }$ is expected to be measurable. By comparing the rates for this
channel with its charge-conjugate channel $\anti\nu _{e}\rightarrow \anti\nu
_{\mu }$, the sign of the leading mass difference in neutrinos, $\delta
m_{32}^{2}$, can be determined by observing the passage through matter of
the neutrinos in a long-baseline experiment. Such experiments can also shed
light on the CP-violating phase, $\delta $, in the lepton mixing matrix and
enable the study of CP violation in the lepton sector. (It is known that CP
violation in the quark sector is insufficient to explain the baryon
asymmetry of the Universe; lepton sector CP violation possibly played a
crucial role in creating this asymmetry during the initial phases of the Big
Bang.)

While the Neutrino Factory is being constructed, R\&D aimed at making the
Muon Collider a reality would be performed.
The Muon Collider will require muon beams that are more intensely cooled 
and have generally more challenging properties than those for a 
Neutrino Factory, so the latter forms a practical goal en route to the former.
A Muon Collider, if realized,
provides a tool to explore Higgs-like objects by direct $s$-channel fusion,
much as LEP explored the $Z$. It also provides a potential means to reach
higher energies (3--4~TeV in the center of mass) using relatively compact
collider rings.

\subsection{History}

The concept of a Muon Collider was first proposed by Budker~\cite
{PREFACE:budker} and by Skrinsky~\cite{PREFACE:skrinsky} in the 60s and
early 70s. However, additional  substance to the concept had to wait 
until the idea
of ionization cooling was developed by Skrinsky and Parkhomchuk~\cite
{INTRO:ref3}. The ionization cooling approach was expanded by Neuffer~\cite
{INTRO:ref4} and then by Palmer~\cite{PREFACE:palmer}, whose work led to the
formation of the Neutrino Factory and Muon Collider Collaboration (MC)~\cite
{EPP:collaboration} in 1995\footnote{
A good summary of the Muon Collider concept can be found in the Status
Report of 1999~\cite{INTRO:ref5}; an earlier document~\cite{INTRO:ref6},
prepared for Snowmass-1996, is also useful reading. MC Notes prepared by the
Collaboration are available on the web~\cite{INTRO:ref11}}.

The concept of a neutrino source based on a pion storage ring was originally
considered by Koshkarev~\cite{INTRO:ref7}. However, the intensity of the
muons created within the ring from pion decay was too low to provide a
useful neutrino source. The Muon Collider concept provided a way to produce
a very intense muon source. The physics potential of neutrino beams produced
by high-intensity  muon storage rings was briefly investigated in 1994 by King~\cite{king94}and in more detail by Geer in 1997 at a Fermilab
workshop~\cite{rajageer,geer} where it became evident that the neutrino
beams produced by muon storage rings needed for the Muon Collider were
exciting in their own right. As a result, the MC realized that a Neutrino
Factory could be an important first step toward a Muon Collider. With this
in mind, the MC has shifted its primary emphasis toward the issues relevant
to a Neutrino Factory. The Neutrino Factory concept quickly captured the
imagination of the particle physics community, driven in large part by the
exciting atmospheric neutrino deficit results from the SuperKamiokande
experiment. The utility of non-oscillation neutrino physics from neutrinos 
produced by muon storage rings has been studied in detail from 1997 
onwards~\cite{non-osc}.

There is also considerable international activity on Neutrino
Factories, with international conferences held at Lyon in
1999~\cite{INTRO:ref13}, Monterey in 2000~\cite{INTRO:ref14}, Tsukuba
in 2001~\cite{nufact01}, London in 2002~\cite{nufact02} and another
planned in New York in 2003~\cite{nufact03}. There are also efforts in Europe~\cite{europenf} and Japan~\cite{japannf} to study different approaches to realizing the neutrino factory.
Recently a proposal has
been submitted to perform an International Muon Ionization Cooling
Experiment (MICE) to the Rutherford Appleton
Laboratories~\cite{mice-prop}.


\subsection{Feasibility Studies}

Complementing the MC experimental and theoretical R\&D program, which
includes work on targetry, cooling, rf hardware (both normal conducting and
superconducting), high-field solenoids, liquid hydrogen absorber design, 
muon scattering experiments, theory,
simulations, parameter studies, and emittance exchange~\cite{INTRO:ref12},
the Collaboration has participated in several paper studies of a complete
Neutrino Factory design.

In the fall of 1999, Fermilab, with help from the MC, undertook a
Feasibility Study (``Study-I'') of an entry-level Neutrino Factory~\cite
{INTRO:ref1}. Study-I showed that the evolution of the Fermilab accelerator
complex into a Neutrino Factory was clearly possible. The performance
reached in Study-I, characterized in terms of the number of 50-GeV muon
decays aimed at a detector located 3000 km away from the muon storage ring,
was $N$ = 2 $\times $ 10$^{19}$ decays per ``Snowmass year'' (10$^{7}$ s)
per MW of protons on target.

Simultaneously, Fermilab launched a study of the physics that might be
addressed by such a facility~\cite{INTRO:ref9} and, more recently, initiated
a study to compare the physics reach of a Neutrino Factory with that of
conventional neutrino beams~\cite{superbeams} powered by a high-intensity
proton driver (referred to as ``superbeams''). As will be described later in
this paper, a steady and diverse physics program will result from following
the evolutionary path from a superbeam to a full-fledged Neutrino Factory.

Subsequently, BNL organized a follow-on study 
(``Study-II'')~\cite{EPP:studyii} on a
high-performance Neutrino Factory, again in collaboration with the MC.
Study-II demonstrated that BNL was likewise a suitable site for a Neutrino
Factory. Based on the improvements in Study-II, the number of 20-GeV muon
decays aimed at a detector located 3000 km away from the muon storage ring,
was $N$ = 1.2 $\times $ 10$^{20}$ decays per Snowmass year per MW of protons
on target. Thus, with an upgraded 4 MW proton driver, the muon decay
intensity would increase to 4.8 $\times $ $10^{20}$ decays per Snowmass
year. (R\&D to develop a target capable of handling this beam power would be
needed.) Though these numbers of neutrinos are potentially available for
experiments, in the current storage-ring design the angular divergence at
both ends of the production straight section is higher than desirable for
the physics program. In any case, we anticipate that storage-ring designs
are feasible that would allow 30--40\% of the muon decays to provide useful
neutrinos.

Both Study-I and -II are site specific in that each has a few site-dependent
aspects; otherwise, they are generic. In particular, Study-I assumed a new
Fermilab booster to achieve its beam intensities and an underground storage
ring. Study-II assumed BNL site-specific proton driver specifications
corresponding to an upgrade of the 24-GeV AGS complex and a BNL-specific
layout of the storage ring, which is housed in an above-ground berm to avoid
penetrating the local water table. The primary substantive difference
between the two studies is that Study-II aimed at a lower muon energy (20
GeV), but higher intensity (for physics reach) than Study-I. Taking the two
Feasibility Studies together, we conclude that a high-performance Neutrino
Factory could easily be sited at either BNL or Fermilab. Figure \ref
{studycomp} shows a comparison of the performance of the Neutrino Factory
designs in Study-I and Study-II~\cite{INTRO:ref9} with the physics
requirements.

\begin{figure}[tbh]
\centerline{\includegraphics[width=4.0in]{studyIvsII.eps}}
\caption[Muon decays in a straight section \textit{vs.} muon energy]{ Muon
decays in a straight section per $10^{7}\,$s \textit{vs.} muon energy, with
fluxes required for different physics searches assuming a 50~kT detector.
Simulated performance of the two studies is indicated.}
\label{studycomp}
\end{figure}

To put the above performance figures in context, it is important to note
that a $\mu ^{+}$ storage ring with an average neutrino energy of 15~GeV and 
$2\times 10^{20}$ useful muon decays would yield (in the absence of
oscillations) $\approx $30,000 charged-current events in the $\nu _{e}$
channel per kiloton-year in a detector located 732~km away. In comparison, a
1.6~MW superbeam~\cite{superbeams} from the Fermilab Main Injector with an
average neutrino energy of 15~GeV would yield only $\approx $13,000 $\nu_\mu$ charged-current events per kiloton-year. In addition to having
lower intensity than a Neutrino Factory beam, a superbeam would have
significant $\nu_e$ contamination, which will be the major background in 
$\nu_\mu\rightarrow \nu_e$ appearance searches. That is, it will be
much easier to detect the oscillation $\nu_e\rightarrow \nu_\mu$ from
a muon storage ring neutrino beam than to detect the oscillation $\nu_\mu
\rightarrow \nu_e$ from a conventional neutrino beam, because the
electron final state from the conventional beam has significant background
contribution from $\pi^{0}$'s produced in the events.

\subsection{Neutrino Factory Description\label{NFsection}}

The muons we use result from decays of pions produced when an intense proton
beam bombards a high-power production target. The target and downstream
transport channel are surrounded by superconducting solenoids to contain the
pions and muons, which are produced with a larger spread of transverse and
longitudinal momenta than can be conveniently transported through an
acceleration system. To prepare a beam suitable for subsequent acceleration,
we first perform a ``phase rotation,'' during which the initial large energy
spread and small time spread are interchanged using induction linacs. Next,
to reduce the transverse momentum spread, the resulting long bunch, with an
average momentum of about 250 MeV/c, is bunched into a 201.25-MHz bunch
train and sent through an ionization cooling channel consisting of LH$_{2}$
energy absorbers interspersed with rf cavities to replenish the energy lost
in the absorbers. The resulting beam is then accelerated to its final energy
using a superconducting linac to make the beam relativistic, followed by one
or more recirculating linear accelerators (RLAs). Finally, the muons are
stored in a racetrack-shaped ring with one long straight section aimed at a
detector located at a distance of roughly 3000 km. A schematic layout is
shown in Fig.~\ref{nufact-scheme-bnl}.

\begin{figure}[tbh]
\centerline{\includegraphics[width=4.0in,angle=-90]{nufact_scheme_bnl.ps}}
\caption[Schematic of the Neutrino Factory Study-II version]{Schematic of
the Neutrino Factory Study-II version.}
\label{nufact-scheme-bnl}
\end{figure}

\subsection{Detector}

Specifications for the long-baseline Neutrino Factory detector are rather
typical for an accelerator-based neutrino experiment. However, because of
the need to maintain a high neutrino rate at these long distances ($\approx $%
3000 km), the detectors considered here are 3--10 times more massive than
those in current neutrino experiments.

Several detector options could be considered for the far detector:

\begin{itemize}
\item  A 50 kton steel--scintillator--proportional-drift-tube (PDT) detector

\item  A large water-Cherenkov detector, similar to SuperKamiokande but with
either a magnetized water volume or toroids separating smaller water tanks~\cite{DET:uno}.

\item  A massive liquid-argon magnetized detector~\cite{landd}.
\end{itemize}

For the near detector, a compact liquid-argon TPC (similar to the ICARUS
detector~\cite{ICARUS}) could be used. An experiment with a relatively thin
Pb target (1~$L_{rad}$), followed by a standard fixed-target spectrometer
could also be considered.

\subsection{Staging Scenario}

If desired by the particle physics community, a fast-track plan leading
directly to a Neutrino Factory could be executed. On the other hand, the
Neutrino Factory offers the distinct advantage that it can be built in
stages. This could satisfy both programmatic and cost constraints by
allowing an ongoing physics program while reducing the annual construction
funding needs. Depending on the results of our technical studies and the
results of ongoing searches for the Higgs boson, it is hoped that the
Neutrino Factory is really the penultimate stage, to be followed later by a
Muon Collider (e.g., a Higgs Factory). Such a collider offers the potential
of bringing the energy frontier in particle physics within reach of a
moderate-sized machine. Possible stages for the evolution of a muon beam
facility are described in Section~\ref{StagingOps}.

\subsection{R\&D Program\label{RDprog}}

Successful construction of a muon storage ring to provide a copious source
of neutrinos requires development of many novel approaches; construction of
a high-luminosity Muon Collider requires even more. It was clear from the
outset that the breadth of R\&D issues to be dealt with would be beyond the
resources available at any single national laboratory or university. For
this reason, in 1995, interested members of the high-energy physics and
accelerator physics communities formed the MC to coordinate the required
R\&D efforts nationally. The task of the MC is to define and carry out R\&D
needed to assess the technical feasibility of constructing initially a muon
storage ring that will provide intense neutrino beams aimed at detectors
located many thousands of kilometers from the accelerator site, and
ultimately a $\mu ^{+}\mu ^{-}$ collider that will carry out fundamental
experiments at the energy frontier in high-energy physics.

The MC also serves to coordinate muon-related R\&D activities of the
NSF-sponsored University Consortium (UC) and the state-sponsored Illinois
Consortium for Accelerator Research (ICAR), and is the focal point for
defining the needs of muon-related R\&D to the managements of the sponsoring
national laboratories and to the funding agencies (both DOE and NSF). As
already noted, though the MC was formed initially to carry out R\&D that
might lead eventually to the construction of a Muon Collider, more recently
its focus has shifted mainly, but not exclusively, to a Neutrino Factory.

The MC maintains close contact with parallel R\&D efforts under way in
Europe (centered at CERN) and in Japan (centered at KEK). Through its
international members, the MC also fosters coordination of the international
muon-beam R\&D effort. Two major initiatives, a Targetry Experiment (E951)
in operation at BNL and a Muon Cooling R\&D program (MUCOOL), have been
launched by the MC. In addition, the Collaboration, working in conjunction
with the UC and ICAR in some areas, coordinates substantial efforts in
accelerator physics and component R\&D to define and assess parameters for
feasible designs of muon-beam facilities.

\subsection{Outline of Report}

In what follows, we give the motivation and a scenario for a staged approach
to constructing a Neutrino Factory and eventually a Muon Collider. 
Section~\ref{physics} discusses the physics opportunities, starting from
conventional ``superbeams'' and going to cold muon beams, then a Neutrino
Factory with its near and far detectors, and finally a Muon Collider. In
Section~\ref{neufact}, we describe the components of a Neutrino Factory,
based on the Study-II design, and indicate a scientifically productive
staged path for reaching it. Section~\ref{higgsfact} covers our present
concept of an entry-level Higgs Factory Muon Collider. In support of the
construction of a Neutrino Factory, an R\&D program is already under way to
address various technical issues. A description of the status and plans for
this program is presented in Section~\ref{r_and_d}. Section~\ref{mice}
describes current thinking about a cooling demonstration experiment that
would be carried out as an international effort. Finally, in Section~\ref{Summary} we provide a brief summary of our work.


