My Thoughts on Physics Issues of High Priority that can be probed at
DUSEL

Dear Friends of DUSEL,

Since I am unable to participate at the Washington meeting of Nov. 2,
Milind Diwan and Hank Sobel asked me to jot down my thoughts on the
physics issues of high priority that can be probed at the forthcoming
DUSEL. I am indeed happy to do so. First I would like to congratulate
the National Science Foundation for deciding on the site for the DUSEL
to be Homestake and the leaders of Homestake/DUSEL for bringing the idea
of the underground Laboratory in the U.S. to a promising stage.

At present there appear to be quite a few deserving issues, such as
searches for neutrinoless double beta decay, direct detection of dark
matter, n-n-bar oscillation and others, which could be and should be
studied at DUSEL, with the building of appropriate detectors. Among
these, however, I think a Large Size Detector (eqivalent in efficiency
to that of a 1000KT water Cherenkov detector) would have a unique role.
This is because such a detector can simultaneously serve to achieve
three major goals:

-   To search more sensitively into proton decay which is a crucial
    prediction of grand unification;

-   To probe into some crucial aspects of neutrino oscillations, with
    linkage to a long baseline neutrino facility; and

-   To study supernova neutrinos sensitively, which is of fundamental
    significance to particle physics, nuclear physics and astrophysics.

It is indeed a lucky coincidence that one and the same detector would be
sensitive to all three processes. It is proven that a good proton decay
detector is also a good neutrino detector. This is the unique advantage
of such a Large Size Detector.

Physics Significance

Improved studies of proton decay and neutrino oscillations would enable
us to probe nature at the shortest distance (\~ 10^-30^ cm) , or
equivalently at the highest energy scale of 10^16^ GeV\--something that
would not be possible by any other means.

Neutrino Oscillations have been discovered, but there is a whole world
of new physics lying buried in neutrinos. These pertain to the questions
of leptonic CP violation, measurement of the crucial angle theta_13,
accurate measurements of theta_23; the nature of the neutrino
mass-hierarchy (inverted or normal), and others. A large size detector
with linkage to a long baseline facility would enable us to probe into
these issues of fundamental significance.

The discovery of proton decay would have profound significance on
unification ideas. With the discovery of coupling unification at a scale
of 10^16^ GeV, proton decay should in fact occur at accessible rates
(with a lifetime of about 10^35^ years for the positron+neutral
pion-decay mode and less than 10^34^ years for the nu-bar + K-plus
mode), within a well-motivated set of ideas on \"Grand Unification\",
which attempt to unify the three basic forces of nature\--the weak, the
electromagnetic, and the strong nuclear force. The Grand Unification
idea has been spectacularly successful so far as its predictions for (a)
coupling unification, (b) neutrino oscillations with mass scales just of
the type observed, and (c) generation of the observed baryon excess of
the universe via lepton excess. In this sense, proton decay is [the
missing piece]{.underline} of grand unification. Unless the successes
mentioned above are mere coincidences, proton decay should be discovered
with improvement in the current sensitivity by a factor of 5 to 10. This
is why the search for proton decay is now most pressing. This is only
possible with a large size detector. Proton decay, if found, would no
doubt constitute a landmark discovery in particle physics.

Supernova neutrinos can be studied sensitively only at a large size
detector. They are a sure bonus from nature that we can not afford to
miss.

To sum up, a large size detector is desperately needed to address a set
of fundamental issues in particle physics. The discovery potential of
such a detector would be high in all three realms mentioned above. With
its unique multi-purpose value and its physics significance, it would
surely be a stellar physics facility for the U.S. and the world. Thus in
my opinion it should deserve the highest priority from the beginning.

Jogesh Pati,

SLAC, Stanford University

November 1, 2007