July 10, 2002
Adam Para
MS 220
Dear Adam,
The Fermilab Physics Advisory Committee considered the Letter of Intent to Build an Off-axis Detector to Study nm ® ne Oscillations with the NuMI Neutrino Beam (P-929) at its June meeting in Aspen CO. This stimulated a broad discussion of neutrino physics potential at Fermilab. Following are the Committee's comments on the neutrino program in general:
Tremendous progress in neutrino
physics during the past few years has led to a completely new paradigm. The old assumptions that neutrinos are
massless and that lepton flavor is conserved have been overturned; neutrinos
evidently do have mass, and the mass eigenstates are not diagonal in flavor,
leading to oscillations. In analogy
with the CKM matrix for quark flavor mixing, we have the MNS matrix Uai for
neutrinos:
na
= [ Uai ] ni
where
a = e, m, t
are the flavor indices and i = 1, 2, 3 are the mass indices. It is informative to write this matrix as
the product of three two-generation matrices,
where cij
= cos qij , sij
= sin qij and qij
is the mixing angle.
Recent results from SNO using solar
neutrinos, combined with all previous solar neutrino data, strongly favor a
large mixing angle solution, q12 ~
p/6 and m2 2 – m12 = +(2-10)
´
10-5 eV2 at 90% CL.
In the next year this solution will be tested by KamLAND; if KamLAND
confirms this result, they can improve the precision to better than ±1 ´ 10–5 eV2. The LSND experiment sees evidence for oscillations
at a much higher mass scale, 0.3 – 2 eV2 . The LSND result will be definitively tested by MiniBooNE, which
will begin data-taking this summer.
Our best present knowledge for q23 comes from atmospheric
neutrino experiments, especially SuperKamiokande which favors maximal mixing,
sin22q23 ~ 1 and
|Dm232| =
|m3 2 – m22| = (1.6 - 3.9) ´ 10-3
eV2 at 90% CL. (Note that the sign of Dm232 is not determined.) The NuMI/MINOS experiment, expected to come on line in early
2005, will significantly improve the precision on these numbers.
In a striking analog to quark-flavor
mixing, it appears that first-to-third generation mixing is suppressed in the
neutrino sector. In contrast to the
relatively large values favored for q12
and q23 , the best limits, from the
Chooz experiment, indicate that sin22q13 < 0.11
(for Dm232 =
2.5 ´
10-3 eV2). Chooz was a reactor experiment that searched
for ne disappearance;
a significant improvement of the Chooz limit will probably require a search for
ne appearance in a nm beam. NuMI/MINOS
can improve these limits by approximately a factor of two. A future generation of neutrino experiments
to measure sin22q13 is now being formulated. Extensions of these experiments may be
sensitive to matter effects, and hence be able to determine the sign of Dm232. If sin22q13 is not too small, it may also be possible to observe CP violation
in the neutrino sector and measure its phase, d. This is an especially
tantalizing prospect.
The extended Fermilab community has
been actively engaged in possibilities for future neutrino experiments for some
time. The PAC believes that the
question of future Fermilab experiments in neutrino physics is a timely one. This program naturally divides into two stages,
an initial phase using the NuMI beam to measure or further constrain sin22q13 and a second stage aimed at measuring CP violation and the sign of
Dm232,
which would require a new Proton Driver.
The Committee was pleased to learn that a
well-attended workshop on future neutrino experiments utilizing the NuMI beam
was held at Fermilab in May. Prior to
its June meeting, the Committee received a Letter of Intent for an experiment
using the off-axis NuMI neutrino beam to search for nm ®
ne oscillations and measure or significantly
constrain q13, an Expression of Interest for a near
off-axis NuMI detector to measure neutrino cross sections and characterize the
off-axis neutrino beam, and a report on “Physics Potential at FNAL with
Stronger Proton Sources”. The Committee
is also aware of other LOIs or EOIs that are in preparation, as well as
proposals to search for
nm ® ne oscillations using neutrino beams at Brookhaven, at the
Japanese Hadron Facility (JHF), and at CERN.
Given the exciting recent results,
the eagerly anticipated results from the present and near future program, and
the worldwide interest in future experiments, it is clear that the field of
neutrino physics is rapidly evolving.
Fermilab is already well positioned to contribute through its investment
in MiniBooNE and NuMI/MINOS. Beyond
this, the significant investment made by the Laboratory in NuMI could be
further exploited to play an important role in the elucidation of q13 and the exciting possibility of observing CP violation in the
neutrino sector. The Committee
encourages the Laboratory to continue to engage with the neutrino community
through workshops and colloquia in an ongoing exploration of the experimental
possibilities utilizing Fermilab's unique resources. The Committee anticipates that the Laboratory may want to issue a
Call for Proposals in a year or two if a compelling role for Fermilab is
identified.
In addition, the Committee had the following comments specific to your Letter of Intent:
The Committee thanks the proponents
for their Letter of Intent for an experiment in the off-axis NuMI beam and
appreciates this effort to flesh out an optimum experiment to measure q13. Such a measurement is
the crucial next step towards the long-range goal of observing CP violation in
neutrino oscillations. The Committee
encourages continued discussion within the neutrino community on how best to
achieve these ambitious goals. More
detailed discussion of the off-axis experiment, which was also discussed in the
proton-driver report, is given below.
As has already been discussed, the
next important problem in the study of neutrino mixing is to measure q13. It is especially interesting to search for q13 in the parameter range within
about a factor of 10 below the Chooz limit, because this is the region in which
it may be feasible to detect CP violation in neutrino mixing with conventional nm beams without
having to build a muon storage ring.
However, the Committee notes that
the measurement of q13 in
an off-axis experiment using the currently planned NuMI beam with 2.5´1020
protons/year is very challenging. For
example, a 20 kton experiment would only observe 1 signal event per year if sin22q13 = 0.01, and a comparable number of
background events.
The total number of protons
available to NuMI is an important constraint on this program. Both the LOI and the near-term program in
the proton driver report assumed that the total number of protons delivered to
NuMI would be 20´1020 for a five-year program. However, the current accelerator complex can
only provide 2.5´1020 protons/year in dedicated running for
NuMI. The proton driver report states
that this level could be raised to 4´1020
protons/year with a modest program of accelerator improvements. The Committee would appreciate a report at
the fall PAC meeting from the Beams Division on the Proton Driver project and
on possible adiabatic accelerator improvements, including more detailed cost
estimates. The Committee also suggests
that the Laboratory issue guidance on the maximum proton flux that could be
available to NuMI without replacing the Booster.
To help the PAC evaluate the prospects
for an off-axis experiment, the Committee would appreciate answers to the
following questions, which need to be addressed in any
future submissions:
1.
Can one confidently pick a location of the off-axis
experiment today? What is the
flexibility in optimizing the location of the detector once Dm223 is known better?
2.
What is the optimum detector technology, for a fixed
cost, to measure q13?
3.
What is the discovery reach (as well as the 90%CL
limit) in q13 and the
achievable precision in such a measurement?
4.
What supporting measurements are critical to understand
the background? In particular, can the
nature and magnitude of all important backgrounds be determined experimentally?
5.
If the MINOS on-axis near detector is the only one
available to characterize the beam, how well can the flux at the off-axis far
detector be understood?
6.
How does the detector proposed fit into a longer term
program to measure CP violation? Should
one adopt in the first stage the technology most appropriate for the later
stages?
7.
In view of the low signal rates, how significant are
the cosmic-ray backgrounds, and is it convincing that the detector can be on or
near the surface? Is there other
compelling physics that an underground version of the same detector could do?
8.
Are there other important measurements that a 20 kton
detector optimized for electron ID could perform?
The Committee understands that an
off-axis NuMI experiment would be complementary, in its sensitivity to matter
and CP violation effects, to other proposed experiments utilizing different
baselines and/or neutrino energies. The
Committee would like to understand whether there are unique or complementary
aspects for the first phase NuMI experiment to measure q13. More generally, as the
scale of neutrino mixing experiments increases, the Committee encourages the
members of the Fermilab neutrino community to plan globally in collaboration
with other laboratories.
I agree with the PAC's comments that Fermilab could be a focal point for future neutrino experiments and, indeed, an ongoing healthy neutrino program would be very natural given the investments in the Main Injector and the NuMI Project. We appreciate the initiative taken by you and your collaborators. Through this letter, I would like to encourage a vigorous and broad exploration of this potential with a view to eventual convergence on a broadly supported proposal. To this end, we envisage supporting/sponsoring one or two workshops over the next twelve months.
Finally, while not all the signatories of the Letter of Intent are MINOS collaborators, it is important to recognize that the overarching prerequisite for a strong, long-baseline and long-term neutrino program at Fermilab is the successful mounting and completion of the NuMI/MINOS project which deserves the unstinting effort of all.
Sincerely,
Michael Witherell
cc: K. Stanfield
M. Shaevitz
H. Montgomery
S. Holmes
T. Yamanouchi