# PROJECT TITLE: Off-Axis Neutrino Detector

**Executive Summary:**

The discovery of neutrino oscillations in atmospheric and solar neutrino
experiments has catapulted neutrino physics to the forefront of high
energy physics. Exploration this unexpected sector promises a new
program at least as rich and informative as the heavy quarks physics
measurements of the last 20 years. A non-zero mass of the neutrino
implied by the observation of neutrino oscillations is the only
evidence, so far, of phenomena beyond the Standard Model. The magnitude
of the neutrino mass may be related to the Grand Unification scale.
Unraveling of the pattern of neutrino masses will provide further clues
to the puzzle of mass generation.

Neutrinos have mass, the weak eigenstates are a combination of mass
eigenstates. The key to the further understanding is the observation of
ν~μ~ to ν~e~ transition at a frequency corresponding to the
'atmospheric' mass difference; such a measurement would provide the
determination of the mixing angle θ~13~ and may lead to the discovery of
CP non-conservation in the neutrino sector. The latter may be related to
the matter-antimatter asymmetry of our Universe.

The NuMI neutrino beam line represents a major investment of US High
Energy Physics in the area of neutrino physics. Initially designed to
verify the existence of ν~μ~ disappearance, this beam also offers an
opportunity to explore a whole new area of neutrino oscillation physics.

An Off-axis Neutrino Detector, by virtue of its position, would be
exposed to a narrow band neutrino beam with low energy and would be
ideally suited to search for ν~μ~ to ν~e~ oscillations. Such a facility
would enable the US to take a lead in the exploration of the neutrino
sector. The results of the proposed experiments could include:

-   Observation of ν~μ~ to ν~e~ transition and measurement of the mixing
    angle θ~13~

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-   Potential discovery of the CP violation in the neutrino sector and a
    first measurement of the CP violating phase in the neutrino mixing
    matrix

-   Determination, via matter effects, of the pattern of neutrino mass
    hierarchy, depending on the actual parameter values

The facility includes a new detector, optimized for ν~e~ detection, with
a fiducial mass of about 50 kton exposed successively to neutrino and
antineutrino beams. In a five year run its sensitivity to ν~μ~ to ν~e~
oscillations will be at least a factor of ten beyond the current limit.

The neutrino detector will be a state-of-the art device constructed
using existing technology. The primary challenge is the engineering of a
very large detector volume. Detailed optimization of the detector design
is currently underway and the construction of the detector can be
started within three years after approval.. Several suitable sites in
northern Minnesota and southern Ontario have been identified as possible
locations for the new experiment.

The physics potential of the facility depends directly on the product of
the detector mass and proton intensity delivered onto the NuMI target.
Initial studies indicate several avenues of possible upgrades to the
Fermilab accelerator complex and we envisage that such upgrades will be
an integral part of the 50kton program.

This program could be the natural progenitor of a Neutrino Superbeam
(Proton Driver) facility, which will further improve the precision of
the measured parameters or extend the search capabilities for the ν~μ~
to ν~e~ oscillations.

# PROJECT TITLE: Off-Axis Neutrino Detector

# On the importance of neutrino physics

Neutrinos are ghostly elementary particles that are everywhere in our
Universe. Trillions of them from the Sun go through our body every
second. Even in the seemingly empty regions of the Universe, there are
about 300 of them per cubic centimeter. Yet they interact very little
and we normally do not notice them. Since Pauli proposed their existence
to solve a mystery of apparent energy non-conservation in subatomic
physics in 1930, the progress in understanding their nature had been
slow. The so-called Standard Model of particle physics was built
assuming that neutrinos are exactly massless. Only in the past few
years, have we come to the conclusion that they do have a mass,
requiring revision of the Standard Model.

However many mysteries still remain. What exactly are their masses? We
have only detected *differences* of masses because we had to use the
interferometric technique called neutrino oscillation. Are there
right-handed neutrinos? We have so far observed only left-handed ones,
but the finite mass implies the existence of both right-handed and
left-handed states. Are neutrinos and anti-neutrinos distinct or the
same? Such a basic questions currently do not have answers.

The interest in neutrinos goes well beyond their own nature. Having
discovered the finite mass of neutrinos, and so many of them in our
Universe, they could contribute to the energy budget of the Universe.
Neutrinos may account for anywhere between 0.3% to 10% of the Universe.
At the upper end of this range , they could have affected the way
galaxies and eventually stars formed.

The most interesting role of neutrinos is to help us understand why we
exist at all. At the time of the Big Bang, an equal amount of matter and
anti-matter was created. Had it remained so, all matter and anti-matter
would have annihilated each other and we (matter) could not have
existed. There must have been some process which changed the balance
slightly between matter and anti-matter, making approximately one out of
ten billion matter particles survive. Theories suggest that neutrino
mass may well have been responsible for changing this balance. If this
were the case, neutrinos must have distinguished matter and anti-matter
in a subtle way. This is CP violation. It would be of great interest to
observe the effect of CP violation in the neutrino sector.

# Neutrino oscillations studies

We now know that neutrinos have mass and that, similarly to quarks,
their weak eigenstates are linear superpositions of their mass
eigenstates. This mixing can be parameterized in terms of three mixing
angles θ~12~, θ~13~, θ~23~, one complex phase δ  and three mass
differences ∆m^2^~12~, ∆m^2^~13~ and ∆m^2^~23~ (only two of them being
independent).

Neutrino oscillations, where the beam of neutrinos of a given flavor
evolves with time, or distance, into a mix of all three flavors, is the
only mechanism available to study the neutrino mixing and determine
their mass differences. The oscillation frequency is determined by the
mass differences between the neutrino mass eigenstates and by the
experimentally controlled ratio L/E, with L being the baseline of the
experiment and E being the neutrino energy.

The long-standing solar neutrino puzzle has been resolved by a series of
experiments, the most recent ones being SNO and KamLAND. It is by now
demonstrated that the solar  mass difference is of the order of  10^-4^
eV^2^ and the corresponding mixing angle is large, sin^2^2θ~12~\~0.8.
The SuperK and K2K experiments have shown that there is a second
oscillation frequency corresponding the the mass difference about
2-3x10^-3^ eV^2^ and the corresponding mixing angle is nearly maximal,
sin^2^2θ~23~\~1.

Surprisingly, two of the mixing angles are very large ( \~ 45^o^) and
yet the third one, θ~13, ~ is constrained by the reactor experiments to
be smaller than \~9^o^.

A measurement of the two remaining elements of the leptonic mixing
matrix: θ~13~ and the CP phase δ are the principal experimental
challenges for the future neutrino experiments. Determination of these
elements requires a measurement of the strength of ν~μ~ to ν~e~
transition for neutrinos and antineutrinos for different kinematic (i.e.
L and/or E) configurations. The NuMI beam provides an ideal laboratory
for such measurements.

In addition, there are two other important issues to settle:

-   The mixing angle θ~23~ is very close to, and consistent with its
    maximal value of 45^o^. This is very puzzling. If it is indeed
    maximal, it may indicate some new, hereto unknown, symmetry of
    nature. It is possible that a small deviation from the maximal value
    may be related to the breaking of this symmetry

-   The neutrino spectrum apparently consists of two closely spaced
    states 1 and 2. The mass of the third state is significantly
    different, by neutrino standards. It may be higher (normal
    hierarchy) or lower (inverted hierarchy). The determination of the
    pattern of neutrino masses may shed some light on the origin of the
    masses of particles in general. Electron neutrinos are composed
    primarily of states 1 and 2, hence determination of the pattern of
    neutrino masses is of fundamental importance for the future tritium
    beta decay and double beta decay experiments.

> Both issues can be addressed with the NuMI beam. Significant matter
> effects over a long baseline characteristic for the NuMI experiment
> allow the determination of the pattern of the mass hierarchy, whereas
> a narrow spectrum beam with energy close to the oscillation maximum
> provides a good sensitivity to a small deviation of the mixing angle
> θ~23 ~from its maximal value.

# Challenges of a ν~e~ appearance  experiment

The challenge for the experiment is to observe ν~μ~ to ν~e~ oscillations
down to the level of a few parts per thousand

Charged current ν~e~ interactions can be identified by the presence of
an electron in the final state. The experimental backgrounds to the ν~μ~
to ν~e~ oscillation signals arise from two general sources. There are
genuine events with electrons resulting mostly from the intrinsic ν~e~
component in the beam. In addition there are potentially mis-identified
neutral current (NC) and charged current (CC) ν~μ~ events in which one
or more π^0^\'s in the final state simulate an electron.

The experimental challenge can be separated into two parts:

-   reducing these two backgrounds as much as possible

-   measuring them as accurately as possible so that the principal
    ultimate uncertainty comes from the statistical fluctuations in the
    event sample of interest

The background from ν~e~\'s can only be reduced by a detector with good
energy resolution since the ν~e~\'s from background sources have a
broader energy spectrum than the potential signal. The NC background,
however, must be reduced by a well designed detector. The challenge is
to suppress it to a level comparable to or lower than the intrinsic ν~e~
level without significantly degrading the signal detection efficiency.

A proven technology for identification of ν~e~ interactions has been the
use of low Z calorimeters (eg CHARM II and the BNL oscillation
experiment) which allow identification of the electron by tracking. This
is a result of the long radiation length compared to the interaction
length in such calorimeters.

In a highly segmented detector electrons can be distinguished from
π^0^\'s by utilizing several experimental characteristics:

-   finite separation between the vertex and conversion points of the γ
    from the π^0^,

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-   two electromagnetic showers (for π^0^) vs one (for electrons),

-   double pulse height right after the conversion of a γ.

Success of the separation based on these criteria requires fine
segmentation: longitudinally, less than a radiation length, X~0~;
transversely, finer than the spatial separation of the two γ\'s from the
π^0^ decay. The transverse segmentation also has to be such that
individual tracks in the final state can be separated from each other.

Besides the need to distinguish electrons from π^0^\'s, one must also
distinguish electrons from hadrons. This is harder in a low Z material
and relies on the absence of hadronic interactions (for electrons) and a
generally broader track from electrons due to the accompanying
bremsstrahlung.

#  The NuMI beam 

The NuMI beam facility transports 120 GeV protons from the Booster and
the newly constructed Main Injector towards a target located in a NuMI
target hall. The secondary pions and kaons are captured and focused by
two parabolic magnetic horns and subsequently decay in an evacuated
region 675 m in length and 2 m in diameter to yield a very pure beam of
muon neutrinos.

The unique feature of the NuMI neutrino beam is its flexibility. The
neutrino energy spectrum can be selected by adjusting the relative
positions of the focusing horns and the target.

The NuMI beam line is designed for a proton intensity in excess of
2.5x10^13^ protons per pulse every 1.9 sec - roughly \~0.25MW. At this
intensity NuMI should collect 2.5x10^20^ protons per year, which, in the
absence of oscillations, will produce 300 neutrino interactions per year
per kton of detector mass, on axis, at the far MINOS detector location
in the lowest-energy beam configuration.

Every neutrino beam is accompanied by a halo of lower energy neutrinos.
The kinematics of two-body decay ensures that these neutrinos have a
narrow energy spectrum at a given angle with respect to the nominal beam
direction. It is therefore possible to position a detector at a site
corresponding to the value of L/E of choice by selecting the distance of
the detector from the beam axis. Intensity of such off-axis neutrino
beams reaches useable levels only for relatively low energies, below
10-15 GeV, of parent pions and kaons.

Current values of ∆m^2^~23~ indicated by the SuperKamiokande experiment
suggest that a beam energy of about 2 GeV and a distance of 700-900 km
from Fermilab would be a suitable choice of experimental parameters to
maximize sensitivity to oscillations. At these distances a detector
located about 10 km off the nominal neutrino beam axis would be exposed
to a narrow band beam of just such an energy. With the NuMI beam
operating in a medium energy mode about 100 ν~μ~ CC events per year per
kiloton would expected, in the absence of oscillations.

The off-axis NuMI neutrino beam offers very attractive features, well
matched to the requirements of the ν~e~ appearance experiment:

-   The intrinsic ν~e~ component of the beam, an irreducible background,
    is relatively low, on the order of 0.5%. This background can be
    experimentally determined.

-   The narrow ν~μ  ~energy spectrum will enable to reduce the
    background induced by neutral current interactions to levels below
    the ν~e~ component of the beam

-   The geography of northern Minnesota and southern Ontario allows for
    a choice of a detector site optimized for the physics capabilities
    of the experiment

-   The beam is designed for both polarities of the horn current, thus
    allowing neutrino and antineutrino running.

Simultaneous operation of the MINOS detector providing an independent
measurement of the oscillation parameters will provide an additional
constraint for the analysis of the neutrino oscillations.

# The ν~e~ detector

The detector must be capable of detecting and identifying ν~e~ charged
current interactions. Such a detector must meet several requirements:

-   it must have fine granularity in order to identify the final state
    electrons

-   it must have a very large mass to provide maximal sensitivity to the
    oscillation amplitude

-   it must have an acceptable cost per unit mass

The detector should be optimized for the neutrino energy range of 1--3
GeV. The detector must be massive enough to provide a sufficient number
of neutrino interactions while at the same time be of fine enough
granularity to allow the measurement and identification of electrons and
photons. These conflicting requirements can be met by a fine grained
calorimeter with a sampling frequency of ½ of a radiation length, or
better. Constructing the absorber out of a low-Z material will minimize
the number of detector planes, and therefore the cost, of the detector
for a given overall mass.

The required transverse granularity of the detector is related to the
local particle density on one hand and to the Moliere radius on the
other hand. Hadron and electron showers develop over large volumes in a
low density detector, hence the requirements on the transverse
granularity of the detector will be relatively modest. The optimal
transverse granularity of the active detectors needs to be studied in
detail, but initial investigations indicate that a readout with a pitch
of 3 cm is adequate.

Such a detector will provide more detailed information on individual
neutrino interactions than the large water Cerenkov detectors used or
proposed for neutrino experiments.

A direct consequence of the low density of the detector is a necessity
of a fiducial cut of the order of 1.5-2 meter away from the edges of the
detector. To minimize the loss in acceptance due to this cut the
transverse dimensions of the detector must be as large as possible.

With such a detector we estimate that the signal detection efficiency
will be about 40% while keeping the NC background at the same level as
the intrinsic ν~e~ background.

# Readiness

Several possible detector designs using existing technologies can meet
these requirements. Cheap materials such as particle board or water can
be used as absorber. Active detector planes can be constructed out of
solid or liquid scintillator counters or, alternatively, Glass Resistive
Plate Chambers. Initial studies indicate that the detector can be
constructed at a cost of about \$1-2M per kiloton, or \$50-100M for a 50
kiloton detector.

The design of the detector needs to be validated by detailed engineering
studies. Preliminary calculations indicate that absorber planes of
dimensions approaching 20x20 m^2^ can be constructed in a
self-supporting manner, not requiring external support structures. An
attractive alternative is to construct detectors using shipping
containers as the fundamental structural elements and using standard
industrial techniques for stacking them into a very large volume.

We expect the detector to operate on the surface, possibly with a small
overburden. Cosmic ray-induced background is estimated to be very small;
this conjecture is being verified experimentally

It is expected that the decision on the detector technology will be
taken in summer 2003 thus enabling complete engineering studies to be
completed by summer 2004.The review and approval process could start
early in 2004. Given the simplicity of the detector, and the fact that
an existing, well understood, technology will be used it is expected
that construction can start relatively soon after approval.

The Main Injector exists and the NuMI beamline will start operation in
early 2005. If upgrades are secured they could take place over the
course of subsequent years.

# Signal and backgrounds

The goal of the experiment is to detect ν~e~ CC interactions resulting
from neutrino oscillations on top of the background due to the intrinsic
ν~e~ component of the beam and the NC-induced background.

The signal is proportional to the unknown parameter, to be measured,
sin^2^2θ~13~ . \[In real life the situation is more complicated as the
unknown CP phase δ may lead to a very large, a factor of 2 or more,
change of the signal. In the following we will adopt a customary
convention of ignoring the CP violating effects.\] It is also
proportional to the identification efficiency of the ν~e~ CC
interactions. This efficiency depends on the details of the detector
design, but it is expected to be at least 40%.

The intrinsic ν~e~ component of the beam is about 0.5% of the ν~μ~ flux.
It can be calculated with a precision of few percent once the inclusive
particle production cross sections are measured in experiment E907. This
background will be further constrained by the measurement of ν~μ~ and
anti-ν~μ~ events in the near MINOS detector. It is, therefore, expected
that the dominant uncertainty of this background will be of statistical
nature.

The NC-induced background is reduced to its minimum in the off-axis beam
configuration through the use of a narrow energy spectrum and absence of
significant tails at higher energies. At large angles with respect to
the beam axis high energy neutrinos are kinematically forbidden, the
residual tail being produced by decays of unfocused particles in the
target area. The NC-induced background is reduced further by requiring
the presence of electromagnetic energy in the event. The ultimate
rejection power depends on the details of the final detector design but
very conservative initial studies indicate that the NC-induced
backgrounds is at or below the level of the intrinsic ν~e~ background.
Nevertheless it will be very important to measure it experimentally
using a small size near detector of the same design. Background due to
ν~μ ~CC events is negligible as the  ν~μ  ~flux is greatly reduced by
oscillations. It is important to notice that the most of ν~τ  ~resulting
from the oscillations are below the threshold for τ production, hence
this source of the potential background is not very important.

The physics potential of the proposed experiment can be simply
summarized by the table of identified ν~e~ CC candidate events. These
numbers depend somewhat on the location of the experiment. As an example
we take a detector located 735 km from Fermilab and 10 km away from the
beam axis. We assume 8 years exposure with the nominal 4x10^20^ protons
on target per year and a 50 kiloton detector with 40% detection and
identification efficiency. The 'signal' column corresponds to the
current 90% CL exclusion limit from CHOOZ, sin^2^2θ~13~=0.1.

  --------------------- ---------------------- ----------------- ---------
  ν~μ~ (no osc.)        ν~e~ (intrinsic)       NC-induced        signal

  30,000                25                     25                214
  --------------------- ---------------------- ----------------- ---------

The reach of the experiment can be expressed as:

-   30 σ discovery if the mixing is at the present limit

-   3 σ discovery if the mixing angle is at 1/10 of its present limit

-   20-fold improvement of the limit on the mixing angle,
    sin^2^2θ~13~\<0.005, if no signal is observed

# Beam intensity upgrades

The Fermilab accelerator complex, including the Booster and Main
Injector are currently operating and providing protons for the Tevatron
and for the MiniBOONE experiment. The NuMI neutrino beam is under
construction and should start operating at the beginning of 2005. It is
expected that the initial beam intensity delivered to NuMI will be
2.5x10^13^ protons per extraction, thus giving 2.5x10^20^ protons on
target per year. The beamline is designed to transport 4x10^13^ protons
per pulse.

The number of neutrino events detected is proportional to the product of
the detector mass and the proton intensity delivered onto the neutrino
target. Optimization of the experiment for a given total cost must,
therefore, include a balance between the detector size and possible
improvements of the beam intensity.

Initial studies of the accelerator complex indicate that there are
several possible improvements of the Booster and Main Injector, which
can overcome some of the present limitations. In the Booster these
improvements will help to stabilize the beam, hence reduce proton
losses, and permit a higher repetition rate. In the Main Injector it is
expected that additional magnet power supplies and RF power will result
in the reduction of the cycle time down to 1 sec, hence nearly doubling
the average delivered proton intensity.

Further studies are in progress, but it is important to realize the
potential scale of upgrades and associated improvement in the machine
performance. It is estimated that a \$15M investment would be necessary
to bring the Main Injector proton intensity to the value of 4x10^20^ per
year. A more substantial investment of \~\$50M could result in a further
doubling the proton intensity to 8x10^20^ per year. Such an intense beam
would be equivalent to doubling the detector mass and would improve the
sensitivity of the experiment by a factor of 1.8, compared to the
numbers shown in the previous section.

If a new proton driver is built at Fermilab, the Main Injector intensity
could reach 1.6x10^21^ protons per year, with an upgrade potential to
2.5x10^21^ per year. A simultaneous increase of the detector mass by a
factor of 5 would give a further increase of the sensitivity of the
experiment by a factor of 5-7.

# Relation of the NuMI off-axis project to other proposed facilities

New experiments and facilities are proposed in several regions of the
world. They include:

-   a neutrino beam at the Japanese Hadron Facility in conjunction with
    the SuperK detector

-   an off-axis detector in the Gulf of Taranto in theCERN-to-Grand
    Sasso beam

-   a new Superconducting Proton Linac at CERN in conjunction with a
    megaton-class water Cherenkov detector in Frejus

-   a wide-band neutrino beam from Brookhaven to a future proton decay
    experiment in an underground laboratory

The most advanced project is the one involving the Japanese Hadron
Facility (JHF).

The JHF, a 50 GeV proton accelerator with very high intensity, has been
approved and is under construction. This facility is due to be completed
around 2007 and has a first-stage design beam power of 0.77 MW. There
are now rather detailed plans to construct a neutrino beam, most likely
an off-axis beam at 2 deg with an energy centered around 0.7 GeV, aimed
at the existing Super-Kamiokande detector 295 km away. Even though the
neutrino program at the JHF is not officially approved as yet, it is
anticipated that the formal approval will come within a year.

The JHF-SuperK experiment will have very similar goals to the experiment
described here. The difference in the experimental conditions, primarily
in different baselines resulting in different matters effects, makes
these two experiments very complementary. Measurements of the ν~e~ and
anti-ν~e~ appearance rates in, respectively, neutrino and antineutrino
beams under different combinations of kinematical conditions are
necessary to disentangle the fundamental physics parameters. However a
comparison of the ν~e~ appearance rate at NuMI and at JHF would already
be a powerful tool to identify the pattern of neutrino masses.

A direct comparison of these two experiments involves the following
factors:

-   both experiments plan to operate near the oscillation maximum. The
    expected event rate, in the absence of oscillations, is of the order
    of 100 events per kiloton per year (despite the difference in
    distance). The fiducial mass of the NuMI detector is more that twice
    that of the SuperK, thus giving a numerical advantage to NuMI. This
    advantage is potentially doubled by the proposed upgrade to the
    accelerator complex.

-   Geography of northern Minnesota and southern Ontario allows for an
    optimization of the experiment baseline and/or beam energy. It is
    even possible to imagine a detector which can be moved from one
    location to another, should the physics require such an action. It
    is also possible to build several, smaller, detectors measuring the
    oscillation probability under different kinematical conditions. The
    existing far and near MINOS detectors operating in the same beam
    will provide additional constraints for the oscillations
    measurements. The JHF-SuperK experiment is far more constrained by
    the utilization of the existing detector. Some flexibility may be
    provided by constructing a more adjustable neutrino beam.

# Conclusions

A large "off-axis" neutrino detector would augment and complement the
current accelerator-based neutrino physics program and further
capitalize on the investment in the NuMI facility. Such a facility would
enable theUS to assume a leading role in the investigation of the very
rapidly evolving field of neutrino oscillations.

# Reference materials

http://www-off-axis.fnal.gov/