The Nu-SNS proposal - ORNL Physics Division - Oak Ridge ...

Figure 2.2 SNS neutrino production mechanism.
The time structure of the SNS beam is particularly advantageous for neutrino studies. Time
correlations between candidate events and the SNS proton beam pulse will greatly reduce
background rates and may provide sensitivity to neutral current events. As shown in the left
panel of Figure 2.3, all neutrinos will arrive at the ν-SNS location within several microseconds
of the 60 Hz proton beam pulses. As a result, background events resulting from cosmic rays will
be suppressed by a factor of ~2000 by ignoring events which occur too long after a beam pulse.
At the beginning of the beam spill the neutrino flux is dominated by muon neutrinos resulting
from pion decay, perhaps making it possible to isolate pure neutral-current events. This exciting
possibility is quite challenging due to the presence of a large number of high energy neutrons,
the most important source of beam-induced background from the SNS. Investigations of the
achievable time resolution and consultations with SNS operations staff about minimizing beam
losses (both necessary to meet this challenge) will be a part of our proposed R&D effort in the
next two years. However, charged-current measurements can be made essentially backgroundfree
because the neutron backgrounds are greatly suppressed for t > ~1 µs after the start of the
beam spill while the neutrino production, governed by the muon lifetime
(τ µ ~ 2.2 µs), proceeds for several microseconds. This time structure presents a great advantage
over a continuous-beam facility such as the Los Alamos Neutron Science Center (LANSCE),
where the LSND experiment was located.
The energy spectra of SNS neutrinos are shown in the right hand panel of Figure 2.3. These
spectra are known because almost all neutrinos come from decay-at-rest processes in which the
kinematics are well defined. The decay of stopped pions produces monoenergetic muon
neutrinos at 30 MeV. The subsequent 3-body muon decay produces a spectrum of electron
neutrinos and muon antineutrinos with energies up to 52.6 MeV. This energy range overlaps
extremely well with the range of neutrino energies in supernovae (see Figure 2.4). Cross sections
for neutrino interactions at these energies are crucial for understanding supernova dynamics,
nucleosynthesis, and the response of terrestrial supernova neutrino detectors, as discussed in the
previous section.
ν-SNS Proposal 8 8/4/2005