Dear Committee on the Physics of the Universe:

Historically, the intersection between neutrino physics and astronomy
has led to much understanding.  For example, the observation of solar
neutrinos confirms that nuclear fusion is the power source for the
Sun.  The comparison of the neutrino flux with solar model predictions
provides evidence for neutrino oscillations.  Also, the cosmological
abundances of the elements provides a prediction for the number of
neutrino species.

A group of us hosted by Fermilab are investigating possible future
neutrino experiments using new conventional beams and, ultimately, a
neutrino factory based on a muon storage ring.  We are writing this
letter to point out that these experiments might provide an
opportunity for further understanding of the universe as well as for
particle physics.  Also, construction of the facilities and detectors
may involve agencies for particle physics, nuclear physics, and
astrophysics.

A study of a possible new proton source at Fermilab [1] has recently
been completed.  Such a source could provide for higher intensity
conventional neutrino beams, for which the physics potential has been
discussed in Ref. [2].  More ambitiously, such a source would provide
a front end for a neutrino factory based on a muon storage ring [3]
for which the physics potential has been discussed in Ref. [4].

The physics goals for these projects include the study of neutrino
oscillations to obtain:
1) The neutrino mixing matrix
2) The study of matter effects to obtain the sign of dm**2 for
   atmospheric neutrino oscillations.  Given the evidence for a 
   hierarchical neutrino mass spectrum, this will tell us if two 
   neutrinos are heavier, or if two are lighter.
3) CP violation in the lepton sector.
4) Study of eV-scale sterile neutrinos, if verified by MiniBooNE,
   or a further search for such neutrinos.

Neutrino oscillations, including oscillations to sterile neutrinos,
have implications for nucleosynthesis in supernovae and the early
universe.  Comparisons of measured neutrino mixing angles with
requirements of astrophysical and cosmological models may provide a
revealing test of our models [5].

Neutrino oscillation measurements only measure the dm**2 splitting
between neutrino species.  Measurements of the tritium decay spectrum,
or of double beta decay, may help establish the absolute mass scale.
Many cosmological observables are also sensitive to the neutrino mass.
For example, Hu et al. [6] have shown that the distribution of
galaxies may be sensitive to neutrino masses well below 1 eV.  Thus,
the interplay of neutrino physics and cosmology may establish the
absolute spectrum of neutrino masses.

The matter-antimatter asymmetry of the universe implies CP violation
in particle interactions.  CP violation in the standard model is not
able to explain this asymmetry.  Studies of CP violation in the lepton
sector may provide some insight into this question, and may involve
unexpected phenomena.  For example, the existence of eV-scale sterile
neutrinos would require a dramatic extension of the standard model.
The experiments we envision would be sensitive to CP asymmetries in
oscillations to these sterile neutrinos.

The large detectors required to detect accelerator-generated neutrinos
may also be used for other purposes such as proton decay, solar
neutrino studies, and observation of supernovae.  The neutrino beams
may also be useful for nuclear physics.  Thus, there may be
multi-agency involvement in the development of the facilities and the
detectors.

In conclusion, we envision a possible long-term program of neutrino
physics that will provide insights into particle physics as well as
the physics of the universe.  We suggest that this program might be
part of the set of projects to realize the list of opportunities at
the intersection of physics and astronomy.

[1] R. Alber et al., fermilab-tm-2136, 
http://www-bd.fnal.gov/pdriver/reports.html

[2] V. Barger et al., fermilab-fn-703, hep-ex/0103052

[3] T. Anderson et al., 
http://www.fnal.gov/projects/muon_collider/nu/study/report/machine_report/

[4] C. Albright et al., fermilab-fn-692, hep-ex/0008064

[5] For a review, see D. O. Caldwell, Int. J. Mod. Phys. A13, 4409 (1998)
[hep-ph/9804367].

[6] W. Hu, D. J. Eisenstein, and M. Tegmark
Phys. Rev. Lett. 80, 5255 (1998) [astro-ph/9712057].

Signed,

Maury Goodman
Argonne National Laboratory

Fritz DeJongh, Deborah Harris, Jorge Morfin, Andrea Romanino, Ray Stefanski
Fermi National Accelerator Laboratory

Bing-Lin Young
Iowa State University

Paul Nienaber
Marquette University

Heidi Schellman
Northwestern University

Kirk McDonald
Princeton University

David Casper
University of California, Irvine

Jeff Nelson
University of Minnesota

Contact Person:
Fritz DeJongh (fritzd@fnal.gov)