Search for sterile neutrinos and measurement of neutrino-argon interactions at the SBN programme

There are three known flavours of neutrinos corresponding to the three different
flavours of charged leptons. The number of neutrinos may be determined by observing the decay
of the Z boson, since its lifetime is dependent on how many different flavours there are. The
lifetime of the Z boson has been found to be consistent with a three neutrino model . There
have, however, been a number of experimental results which are not consistent with oscillations
in a three neutrino model. The three main results are the reactor anomaly which refers to the
apparent lack of anti-electron neutrinos observed from nuclear reactors, the Gallium anomaly
which refers to the apparent lack of electron neutrinos observed from radioactive sources placed
in the Gallium based solar neutrino experiments, SAGE and GALLEX and the apparent excess
of electron neutrinos observed by the LSND and MiniBooNE experiments from a predominantly
muon (anti)-neutrino beam . For the reactor anomaly, the ratio of the number of observed anti-
electron neutrinos to the predict number was 0.938 _ 0.023. Similarly, the corresponding ratio
for the Gallium anomaly was 0.86 _ 0.05. Both of these ratios equate to about a 2.7_ deviation
from unity and both anomalies were observed in the case of low energy electron neutrinos and
over short baselines of a few meters. The LSND experiment was performed over a baseline of
30 m with a beam of muon anti-neutrinos with an energy range of 20-53 MeV. The number of
events observed corresponded to a 3.8_ excess. The MiniBooNE experiment was performed with
a baseline of 540 m and with a beam energy of 700 MeV. The experiment was performed with a
beam of muon neutrinos and muon anti-neutrinos. A 3.4_ and 2.8_ excess of events were observed
respectively. These results may be explained by oscillations from at least one additional neutrino
state with a mass splitting. This fourth neutrino state may be interpreted as a
sterile neutrino (as opposed to the active neutrinos. Unlike the active neutrinos, a sterile
neutrino would only interact via gravity and not the weak interaction. As a result of this, a sterile
neutrino would not contribute to the rate of decay of the Z boson and would not result in any sort
of a signal when passing through a detector .
The short baseline neutrino (SBN) programme aims to address the anomalies in the experimental results by establishing whether sterile neutrinos exist. The SBN programme consists of
three separate liquid argon time projection chambers (LArTPC); the Short Baseline Near Detector
(SBND), MicroBooNE and ICARUS. The three detectors are located along the axis of the booster
neutrino beam (BNB) at 110 m, 470 m and 600 m respectively from the neutrino beam source.
The BNB initially consist of close to a 100% muon neutrinos and one of the primary goals of
the programme is to look for e appearance and disappearance. As the near detector, SBND
will measure the unoscillated flavour content of the BNB, determining its exact characteristics.
Additionally, the close proximity of SBND to the beam source allows for the measurement of many
neutrino-argon interactions and thus with such a large data sample the study of these interactions
may be performed to new levels of precision. The MicroBooNE detector is purposely positioned
close to its predecessor, the MiniBooNe detector, and is attempting to replicate the excess of events
observed there. The ICARUS detector will focus on measuring the flavour content of the beam
and comparing it with the results of SBND to determine if any discrepancies in the data may be
as a result of a sterile neutrino.
LArTPC's allow images of particle trajectories and interactions to be recorded. In the case
of the SBN, this happens when a neutrino interacts in the detector, producing particles. If the
produced particles are charged, they will in turn create ionisation track, but if they are neutral,
they will travel through the detector unnotice