Neutrino detectors around the world seem to see evidence that these weakly interacting, little-understood particles are not really massless,
as was thought so far. Not only do they have non-zero masses, different species (or flavours ) of neutrinos seem to mix and oscillate
into one another as they traverse through the cosmos. If this is true, this is not only one of the first pieces of evidence for physics beyond
the so-called Standard Model of Particle Physics but would also have great impact on diverse fields such as nuclear and particle physics,
astrophysics and cosmology. It is thus imperative to study the details of the interactions of these particles. The best option of course is to
have a lab in order to do so. In order to maximize the sensitivity to the interactions of these weakly interacting particles, such a neutrino lab
is necessarily placed underground.
The India-based Neutrino Observatory (INO) is now in the feasibility study stage. More than 50 scientists from about 15 Institutes and
Universities in India have come together to form the National Neutrino Collaboration group (NNCG). This group has the task of detailing various
aspects related to INO activity and come up with a proposal for an underground neutrino laboratory.
It is possible that INO may ultimately house more than one type of detector. This is especially so because of the vast range of energy over
which interesting neutrino physics questions are being asked (from MeVs of neutrinos from the sun and supernovae to GeVs of neutrinos from
Earth's atmosphere and accelerators to many 1000s of GeVs of neutrinos from ultra high energy cosmic rays).
After much discussion, and keeping in mind the open issues related to neutrino physics research and the expertise available, it was decided that
the best option in the first phase at least, would be to study the so-called atmospheric neutrinos produced by interactions of cosmic rays in
the Earth's atmosphere. Both neutrinos and antineutrinos of different species (flavours) are produced here.
Certain interesting physical phenomena that neutrinos (and antineutrinos) may undergo if indeed they mix requires that neutrino and antineutrino
interactions be separately identified. Charged-current interactions of neutrinos and antineutrinos with atomic nuclei produce
leptons (such as negatively charged electrons and muons) and antileptons (such as positrons and mu-plus) respectively. The interactions of the
neutrinos and antineutrinos in the detector is thus identified by the track of this charged particle.
These will be detected by means of an iron calorimeter (ICAL) which will be constructed in horizontal layers. These layers will be sandwiched with
the detector material that will trigger whenever a charged particle passes through it. The direction and the energy of the original incoming neutrino
that caused the interaction can then be determined. By winding coils solenoidally around the iron plates, and passing current through them, a
uniform magnetic field can be created inside the detector. The charged particles bend in these magnetic field, with oppositely charged particles
bending in opposite directions. This will not only allow an identification of the charge of the emitted particle, but also provides a good
measurement of its energy and momentum.
An underground neutrino facility in India offers the unique possibility of locating a neutrino detector near the Earths equator. This can have some
very interesting consequences for solar neutrinos which would then pass through the core of the Earth in their passage to the detector from the
Sun at night.
The INO collaboration has now decided on a site in Bodi West Hills(BWH) region near Pottipuram village in Theni district of Tamilnadu (110KM
from the temple town of Madurai).
For this, the traditional response is to study the problem via simulations. The geometry of the detector is simulated on a computer
and the interactions are fed in. Different processes (interactions of the neutrinos with the detector material) are studied and this is used to
determine which are the most promising processes to study.
There are many long-term options. One is to branch out into solar and supernova studies, another is to try and use the ICAL to try and detect
neutrinos in a beam shot out from an accelerator far away (such as in Japan or CERN in Geneva). All options need to be carefully studied with
respect to the physics rewards as well as to their feasibility. The importance of this endeavour derives not only from the breadth of
scientific questions but also from benefits that may acrue from the rich interaction between ideas and techniques. In short, we believe that the
future of neutrino physics will be very exciting!
From data collected around the world, especially from the Super-Kamiokande and SNO experiments, it seems clear that different flavours of neutrinos
mix and oscillate into each other. So far, all these detectors have only seen a depletion in their expected spectrum. Recently Super-Kamiokande
has improved this analysis and discounts non-oscillation scenarios at the 98% confidence level.
To clinch further the evidence for oscillation, it is necessary to see a depletion as well as an enhancement (for example, if we are observing
one flavour of neutrino and it has oscillated into another species, we will see a depletion in the expected event rate, but depending on
the oscillation parameters, the extent of oscillation into the other flavours may also be seen). In short, a clear oscillation pattern should
be established to ensure that it is oscillations that are responsible for the observed effects. ICAL should be able to study the spectra of
muons from the atmospheric neutrinos and establish this "trough and peak" effect, thereby determining the oscillation parameters to good accuracy.
Also, since ICAL can distinguish neutrino events from anti-neutrino events (by detecting muons of negative or positive sign in the detector,
from a charge-current interaction), it can also study Earth-matter effects. This will enable it to study certain oscillation parameters
which are not well-known.