
\chapter{Executive Summary}

\label{summary}
 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 our utmost attention.

This document points the way towards establishing an ongoing program of
research in accelerator and experimental physics that can be implemented in
an incremental fashion. At each step, one opens up new physics vistas,
leading eventually to a Neutrino Factory and a Muon Collider. 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 be cooled to be
accelerated and stored. While the proton driver is being constructed, we
will simultaneously engage in R\&D on collecting and cooling muons. A source
of intense cold muons can be immediately used to do physics on such items as
measuring the electric and magnetic dipole moments of the muon to higher
precision, muonium-antimuonium oscillations, rare muon decays and so on.
Once we develop the capability of cooling and accelerating muons, the
storage ring for such muons will be the first Neutrino Factory. Its precise
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 for stored $\mu ^{-}$ beams and their charge conjugate beams
for stored $\mu ^{+}$ beams. Beams from a Neutrino Factory are intense. 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.
They also 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, one can expect to measure the oscillation $\nu _{e}\rightarrow
\nu _{\mu }$. By comparing the rates for this channel with its
charge-conjugate channel $\anti\nu _{e}\rightarrow \anti\nu _{\mu }$ , one
can determine the sign of the leading mass difference in neutrinos, $\delta
m_{32}^{2}$, by making use of their passage through matter in a
long-baseline experiment. Such experiments can also shed light on the CP
violating phase, $\delta $, in the lepton mixing matrix and enable us to
study 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. Perhaps the lepton sector CP violation plays a crucial role in
creating this asymmetry during the initial phases of the Big Bang.

While the Neutrino Factory is being constructed, R\&D can be performed to
make the Muon Collider a reality. This would require orders of magnitude
more cooling. Muon Colliders, if realized, provide a tool to explore
Higgs-like objects by direct $s$-channel fusion, much as LEP explored the $Z$
. They also provide a means to reach higher energies (3--4~TeV in the center
of mass) using compact collider rings.

These concepts and ideas have aroused significant interest throughout the
world scientific community. In the U.S., a formal collaboration of some 140
scientists, the Neutrino Factory and Muon Collider 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.

\section{Feasibility Studies}

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}. One aim of Study-I was to assess the extent to which the
Fermilab accelerator complex could be made to evolve into a Neutrino
Factory. Study-I showed that such an evolution was clearly possible. The
performance reached in Study-I, characterized in terms of the number of 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''). It was determined that a
steady and diverse physics program will result from following the
evolutionary path from a superbeam to a full-fledged Neutrino Factory.

After the completion of Study-I, BNL organized a follow-on study
(``Study-II'') on a high-performance Neutrino Factory sited at BNL, also in
collaboration with the MC. An important goal of Study-II was to evaluate
whether BNL was a suitable site for a Neutrino Factory. Study-II has
recently answered that question affirmatively. A second goal of Study-II was
to examine various site-independent means of enhancing the performance of a
Neutrino Factory. Based on the improvements in Study-II, the number of muons
delivered to the storage ring per Snowmass year from a 1-MW proton driver
would be:

\begin{eqnarray*}
\mu /\text{year} &=&10^{14}\text{ ppp}\times 2.5\text{ Hz}\times 10^{7}\text{
s/year}\times 0.17\text{ }\mu /\text{p}\times 0.81 \\
&=&3.4\times 10^{20}
\end{eqnarray*}

\noindent where the last factor (0.81) is the estimated efficiency of the
acceleration system. For the case of an upgraded 4 MW proton driver, the
muon production would increase to 1.4 $\times $ $10^{21}$ $\mu $ /year.
(R\&D to develop a target capable of handling this beam power would be
needed.)

The number of muons decaying in the production straight section per Snowmass
year would be 35\% of this number, or 1.2 $\times $ 10$^{20}$ decays for a 1
MW proton driver (4.8 $\times $ 10$^{20}$ decays for a 4 MW proton driver).
Though these 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. This can be improved in a straightforward manner and we are
confident that storage ring designs allowing 30--40\% of useful muon decays
are feasible.

Both Study-I and -II are site specific in that each has a few site-dependent
aspects; otherwise, they are generic. In particular, Study-II uses 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. Study-I uses a new Fermilab booster to achieve its beam intensities
and an underground storage ring. The primary substantive difference between
the two studies is that Study-II is aimed at a lower muon energy (20 GeV),
but higher intensity (for physics reach). Taking the two Feasibility Studies
together, we conclude that a high-performance Neutrino Factory could easily
be sited at either BNL or Fermilab.

It is worthwhile noting that a $\mu ^{+}$ storage ring with an average
neutrino energy of 15~GeV and $2\times 10^{20}$ useful muon decays will
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 will yield $
\approx $13,000 $\nu _{\mu }$ charged-current events per kiloton-year.
However, a superbeam has a significant $\nu _{e}$ contamination, which will
be the major background in $\nu _{\mu }\rightarrow \nu _{e}$ appearance
searches. It is much easier to detect the oscillation $\nu _{e}\rightarrow
\nu _{\mu }$ from muon storage rings than the oscillation $\nu _{\mu
}\rightarrow \nu _{e}$ from conventional neutrino beams, since the electron
final state from conventional beams has significant background contribution
from $\pi ^{0}$'s produced in the events.

\section{Neutrino Factory Description}

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 list of the main ingredients of a Neutrino Factory is given below. Details
of the design described here are based on the specific scenario of sending a
neutrino beam from Brookhaven to a detector in Carlsbad, New Mexico. More
generally, however, the design exemplifies a Neutrino Factory for which the
two Feasibility Studies demonstrated technical feasibility (provided the
challenging component specifications are met), established a cost baseline,
and established the expected range of physics performance.

\begin{itemize}
\item  \textbf{Proton Driver:} Provides 1--4 MW of protons on target from an
upgraded AGS; a new booster at Fermilab would perform equivalently.

\item  \textbf{Target and Capture:} A high-power target immersed in a 20-T
superconducting solenoidal field to capture pions produced in proton-nucleus
interactions.

\item  \textbf{Decay and Phase Rotation:} Three induction linacs, with
internal superconducting solenoidal focusing to contain the muons from pion
decays, that provide nearly non-distorting phase rotation; a
``mini-cooling'' absorber section is included after the first induction
linac to reduce the beam emittance and lower the beam energy to match the
cooling channel acceptance.

\item  \textbf{Bunching and Cooling:} A solenoidal focusing channel, with
high-gradient rf cavities and liquid-hydrogen absorbers, that bunches the
250~MeV/$c$ muons into 201.25-MHz rf buckets and cools their transverse
normalized emittance from 12 mm$\cdot $rad to 2.7 mm$\cdot $rad.

\item  \textbf{Acceleration:} A superconducting linac with solenoidal
focusing to raise the muon beam energy to 2.48 GeV, followed by a four-pass
superconducting RLA to provide a 20 GeV muon beam; a second RLA could
optionally be added to reach 50 GeV, if the physics requires this.

\item  \textbf{Storage Ring:} A compact racetrack-shaped superconducting
storage ring in which $\approx $35\% of the stored muons decay toward a
detector located about 3000 km from the ring.
\end{itemize}

\section{Detector}

The Neutrino Factory plus its long-baseline detector will have a physics
program that is a logical continuation of current and near-future neutrino
oscillation experiments in the U.S., Japan and Europe. Moreover, detector
facilities located in experimental areas near the neutrino source will have
access to integrated neutrino intensities $10^{4}$--$10^{5}$ times larger
than previously available ($10^{20}$ neutrinos per year compared with $
10^{15}$--$10^{16}$).

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 are possible for the far detector:

\begin{itemize}
\item  A 50 kton steel--scintillator--proportional-drift-tube (PDT)
detector. The PDT detector would resemble MINOS. A detector with dimensions $
8~\text{m}\times 8~\text{m}\times 150$~m would record up to $4\times 10^{4}$ 
$\nu _{\mu }$ events per year.

\item  A large water-Cherenkov detector, similar to SuperKamiokande but with
either a magnetized water volume or toroids separating smaller water tanks.
This could be the UNO detector~\cite{DET:uno}, currently proposed to study
both proton decay and cosmic neutrinos. UNO would be a 650-kton
water-Cherenkov detector segmented into a minimum of three tanks. It would
have an active fiducial mass of 440~kton and would record up to $3\,\times
\,10^{5}$ $\nu _{\mu }$ events per year from the Neutrino Factory beam.

\item  A massive liquid-argon magnetized detector~\cite{landd} that would
attempt to detect proton decay, detect solar and supernova neutrinos, and
also serve as a Neutrino Factory detector.
\end{itemize}

For the near detector, a compact liquid-argon TPC (similar to the ICARUS
detector~\cite{ICARUS}) could be used. It would be cylindrically shaped with
a radius of 0.5 m and a length of 1~m, would have an active volume of $10^{3}
$ kg, and would provide a neutrino event rate \textsl{O}(10~Hz). The TPC
could be combined with a downstream magnetic spectrometer for muon and
hadron momentum measurements. At these neutrino intensities, it is even
possible to envision an experiment with a relatively thin Pb target (1~$
L_{rad}$~), followed by a standard fixed-target spectrometer containing
tracking chambers, time-of-flight and calorimetry, with an event rate 
\textsl{O}(1~Hz).

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

Successful construction of a muon storage ring to provide a copious source
of neutrinos requires many novel approaches to be developed and
demonstrated. To construct a high-luminosity Muon Collider is an even
greater extrapolation of the present state of accelerator design. Thus,
reaching the full facility performance in either case requires an extensive
R\&D program.

Each of the major systems has significant issues that must be addressed by
R\&D activities, including a mix of theoretical, simulation, modeling, and
experimental studies, as appropriate. Component specifications need to be
verified. For example, the cooling channel assumes a normal conducting rf
(NCRF) cavity gradient of 17 MV/m at 201.25 MHz, and the acceleration
section demands similar performance from superconducting rf (SCRF) cavities
at this frequency. In both cases, the requirements are beyond the
performance reached to date for cavities in this frequency range. The
ability of the induction linac units to coexist with their internal SC
solenoids must be verified, and the ability of the target to withstand a
proton beam power of up to 4 MW must be tested. Finally, some sort of
cooling demonstration experiment should be undertaken to validate the
implementation of the cooling channel.

To make progress on the R\&D program in a timely way, the required support
level is about \$15M per year. At present, the MC is getting only about \$8M
per year, so R\&D progress is less rapid than it could be.

\section{Cost Estimate}

As part of the Study, we have specified each system in sufficient detail to
obtain a ``top-down'' cost estimate for it. Clearly this estimate is not the
complete and detailed cost estimate that would come from preparing a full
Conceptual Design Report (CDR). However, there is considerable experience in
designing and building accelerators with similar components, so we have a
substantial knowledge base from which costs can be derived. With this
caveat, we find that the cost of such a facility is about \$1.9$\,$B in FY01
dollars. This value represents only direct costs, not including overhead or
contingency allowances.

It should be noted that the current design has erred on the side of
feasibility rather than costs. Thus, we do not yet have a fully
cost-optimized design, nor one that has been reviewed from the standpoint of
``value engineering.'' \ In that sense, there is hope that a detailed design
study will \textit{reduce} the costs compared with what we indicate here.

\section{Staging Scenario}

If desired by the particle physics community, a fast-track plan leading
directly to a Neutrino Factory could be executed. This would be done by
beginning now to create the required Proton Driver (see Stage 1 below),
using well-understood technology, while working in parallel on the R\&D
needed to complete a CDR for the Neutrino Factory facility. We estimate
that, with adequate R\&D support (see Section \ref{RDprog}), we could
complete a CDR in 2006 and be ready for construction in 2007. 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). Below we list possible
stages for the evolution of a muon beam facility and give an indication of
incremental costs. These cost increments represent only machine-related
items and do not include detector costs.

\begin{description}
\item  \textbf{Stage 1:} \$250--330M (1 MW) or \$330--410M (4 MW)

\item  \qquad We envision a Proton Driver and a Target Facility. The Driver
could have a 1 MW beam level or be designed from the outset to reach 4 MW.
The Target Facility is built initially to accommodate a 4 MW beam. \ A 1 MW
beam would provide about $1.2\times 10^{14}$ $\mu $/s ($1.2\times 10^{21}$ $
\mu $/year) and a 4 MW beam about $5\times 10^{14}$ $\mu $/s ($5\times
10^{21}$ $\mu $/year) into a solenoid channel. Costs for this stage depend
on site-specific choices, e.g., beam energy. This stage could be
accomplished within the next 4--5 years if the particle physics community
considers it a high priority.

\item  \textbf{Stage 2:} \$660--840M

\item  \qquad We envision a muon beam that has been phase rotated and
transversely cooled. This provides a muon beam with a central momentum of
about 200 MeV/$c$, a transverse (normalized) emittance of 2.7 mm-rad and an
rms energy spread of about 4.5\%. The intensity of the beam would be about $
4\times 10^{13}$ $\mu $/s ($4\times 10^{20}$ $\mu $/year) at 1 MW, or $
1.7\times 10^{14}$ $\mu $/s ($1.7\times 10^{21}$ $\mu $/year) at 4 MW. The 
\textit{incremental} cost of this option is \$840M, based on taking the
cooling channel length adopted in Study-II. If more intensity were needed,
and if less cooling could be tolerated, the length of the cooling channel
could be reduced. \ Accepting twice the transverse emittance would reduce
the incremental cost by about \$180M. At this stage, physics with intense
cold muon beams can start and continue to the stage when the muons are
accelerated.

\item  \textbf{Stage 3:} \$220--250M

\item  \qquad We envision using the pre-acceleration Linac to raise the beam
energy to roughly 2.5 GeV. \ The incremental cost of this option is about
\$220M. At this juncture, it may be appropriate to consider a small storage
ring, comparable to the $g-2$ ring at BNL, to be used, perhaps, for the next
round of muon $g-2$ experiments. No cost estimate has been made for this
ring, but it would be expected to cost roughly \$30M.

\item  \textbf{Stage 4:} \$550M (20 GeV) or \$1250--1350M (50 GeV)

\item  \qquad We envision having a complete Neutrino Factory. For a 20 GeV
beam energy, the incremental cost of this stage, which includes the RLA and
the storage ring, is \$550M. If it were necessary to provide a 50 GeV muon
beam for physics reasons, an additional RLA and a larger storage ring would
be needed. \ The incremental cost would then increase by \$700--800M.

\item  \textbf{Stage 5}

\item  \qquad We envision an entry-level Muon Collider to operate as a Higgs
Factory. No cost estimate has yet been prepared for this stage, so we
mention here only the obvious ``cost drivers''---the additional cooling and
the additional acceleration. Future work will define the system requirements
better and permit a cost estimate of the same type provided for Studies-I
and -II.
\end{description}

\section{Muon Collider}

As is clear from the above discussion, a Neutrino Factory facility can be
viewed as a first critical step on the path toward an eventual high-energy
Muon Collider. Such a collider offers the potential of bringing the energy
frontier in particle physics within reach of a moderate sized machine. The
very fortuitous situation of having an intermediate step along this path
that offers a powerful and exciting physics program in its own right
presents an ideal opportunity, and it is hoped that the particle physics
community will have the resources to take advantage of it.

To reach the feasibility study stage, we must find robust technical
solutions to longitudinal emittance cooling, issues related to the high
bunch charges, techniques for cooling to the required final emittances, and
the design of a very low $\beta $* collider ring. We are confident that
solutions exist along the lines we have been investigating. We in the MC are
eager to advance to the stage of building a Muon Collider on the earliest
possible time scale. However, for that to happen there is an urgent need to
increase support for our R\&D so that we can address the vital issues.
Unless and until we obtain such support, it is hard to predict how long it
will take to solve the longitudinal emittance cooling and other
collider-specific problems.
%
\section{International Activities}

Work on Neutrino Factory R\&D is being carried out both in Europe and in
Japan. Communication between these groups and the MC is good. In addition to
having members of the MC Executive Board from these regions, there are
annual NUFACT workshops held to disseminate information. These meetings,
which rotate through the three regions, have been held in Lyon (1999), in
Monterey (2000), and in Tsukuba (2001); the next meeting will be held in
London.

Activities in Europe are centered at CERN but involve many European
universities and labs. Their concept for a Neutrino Factory is analogous to
that of the MC, but the implementation details differ. The European Proton
Driver is based on a 2.2-GeV superconducting proton linac that makes use of
the LEP rf cavity infrastructure. Phase rotation and cooling are based on rf
cavities operating at 44 and 88 MHz, along with appropriate LH$_{2}$
absorbers. R\&D on the rf cavities is in progress. CERN has mounted the HARP
experiment to measure particle yields in the energy regime of interest to
them (about 2 GeV). The CERN group is participating actively in the E951
Targetry experiment at BNL, and has provided some of the mercury-jet
apparatus that was tested successfully. European groups are also heavily
involved in the MUSCAT experiment at TRIUMF, where they play a lead role.

Activities in Japan have concentrated on the development of Fixed-Field
Alternating Gradient (FFAG) accelerators. These have very large transverse
and longitudinal acceptance, and thus have the potential of giving a
Neutrino Factory that does not require cooling. They are pursuing this
scheme. A proof-of-principle FFAG giving 500-keV protons has already been
built and tested, and plans exist for a 150 MeV version.  A 50-GeV 1-MW
Proton Driver is approved for construction in Japan, with a six-year
schedule. A collaboration with the MC on LH$_{2}$ absorber design is under
way, using U.S.-Japan funds.

On a global note, the three regions are in the process of developing a joint
proposal for an international Cooling Demonstration Experiment that could
begin in 2004. A Steering Committee has been set up for this purpose, with
representatives from all three regions.


\section{Conclusions}

In summary, the Muon Collaboration is developing the knowledge and ability
to create, manipulate, and accelerate muon beams. Our R\&D program will
position the HEP community such that, when it requires a Neutrino Factory or
a Muon Collider, we shall be in a position to provide it. A staged plan for
the deployment of a Neutrino Factory has been developed that provides an
active neutrino and muon physics program at each stage.
The requisite R\&D program, diversified over laboratories and
universities and having international participation, is currently
supported at the \$8M level, but requires of the order of \$15M per year
to make progress in a timely way.



