[Catalyst 2017]
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neutrinos. The results showed a lower-thanexpected<br />
probability of oscillation. 6 These<br />
results highly suggest either an oscillation<br />
to another neutrino flavor. A subsequent<br />
experiment at Fermilab called the mini<br />
Booster Neutrino Experiment (MiniBooNE)<br />
again saw a discrepancy between predicted<br />
and observed values of ve appearance with<br />
an excess of v e<br />
events. 7 All of these results<br />
have a low probability of fit when compared to<br />
the standard model of particle physics, which<br />
gives more plausibility to the hypothesis of the<br />
existence of more than three neutrino flavors.<br />
GALLEX, an experiment measuring neutrino<br />
emissions from the sun and chromium-51<br />
neutrino sources, as well as reactor neutrino<br />
experiments gave inconsistent data that<br />
did not coincide with the standard model’s<br />
predictions for neutrinos. This evidence<br />
merely suggests the presence of these new<br />
particles, but does not provide conclusive<br />
evidence for their existence. 4,5 Thus, scientists<br />
designed a new project at the South Pole to<br />
search specifically for newly hypothesized<br />
sterile neutrinos.<br />
IceCube Studies<br />
IceCube, a particle physics laboratory, was<br />
designed specifically for collecting data<br />
concerning sterile neutrinos. In order to collect<br />
conclusive data about the neutrinos, IceCube’s<br />
vast resources and acute precision allow it to<br />
detect and register a large number of trials<br />
quickly. Neutrinos that come into contact with<br />
IceCube’s detectors are upgoing atmospheric<br />
neutrinos and thus have already traversed the<br />
Earth. This allows a fraction of the neutrinos<br />
to pass through the Earth’s core. If sterile<br />
neutrinos exist, then the large gravitational<br />
force of the Earth’s core should cause some<br />
muon neutrinos that traverse it to oscillate<br />
into sterile neutrinos, resulting in fewer muon<br />
neutrinos detected than expected in a model<br />
containing only three standard mass states,<br />
and confirming the existence of a fourth<br />
flavor. 3<br />
For these particles that pass upward through<br />
IceCube’s detectors, the Earth filters out<br />
the charged subatomic particle background<br />
noise, allowing only the detection of muons<br />
(the particles of interest) from neutrino<br />
interactions. The small fraction of upgoing<br />
atmospheric neutrinos that enter the ice<br />
surrounding the detector site will undergo<br />
reactions with the bedrock and ice to produce<br />
muons. These newly created muons then<br />
traverse the ice and react again to produce<br />
Cherenkov light, a type of electromagnetic<br />
radiation, that is finally able to be detected by<br />
the Digital Optical Modules (DOMs) of IceCube.<br />
This radiation is produced when a particle<br />
having mass passes through a substance<br />
faster than light can pass through that same<br />
substance. 8<br />
In 2011-2012, a study using data from<br />
the full range of DOMs, rather than just a<br />
portion, was conducted. 8 This data, along<br />
with other previous data, were examined<br />
in order to search for conclusive evidence<br />
of sterile neutrino oscillations in samples of<br />
atmospheric neutrinos. Experimental data<br />
were compared to a Monte Carlo simulation.<br />
For each hypothesis of the makeup of<br />
the sterile neutrino, the Poissonian log<br />
likelihood, a probability function that finds<br />
the best correlation of experimental data to<br />
a hypothetical model, was calculated. Based<br />
on the results shown in Figure 2, no evidence<br />
points towards sterile neutrinos. 8<br />
Conclusion<br />
Other studies have also been conducted at<br />
IceCube, and have also found no indication<br />
of sterile neutrinos. Although there is strong<br />
evidence against the existence of sterile<br />
neutrinos, this does not completely rule out<br />
their existence. These experiments have<br />
focused only on certain mixing angles and<br />
may have different results for different<br />
mixing angles. Also, if sterile neutrinos are<br />
conclusively found to be nonexistent by<br />
IceCube, there is still the question of why<br />
Fig 2: This graph shows the experimental results<br />
compared to the MC simulations. It shows the<br />
close correlation and how the experimental results<br />
highly correspond to a no sterile neutrino<br />
hypothesis. 8<br />
the anomalous data appeared at LSND and<br />
MiniBooNE. Thus, IceCube will continue sterile<br />
neutrino experiments at variable mixing<br />
angles to search for an explanation to the<br />
anomalies observed in the previous neutrino<br />
experiments.<br />
WORKS CITED<br />
[1] Fukuda, Y. et al. Evidence for Oscillation of<br />
Atmospheric Neutrinos. Phys. Rev. Lett. 1998, 81, 1562.<br />
[2] Beringer, J. et al. Review of Particle Physics. Phys. Rev.<br />
D. 2012, 86, 010001.<br />
[3] Schmitz, D. W. Viewpoint: Hunting the Sterile<br />
Neutrino. Physics. [Online] 2016, 9, 94. https://physics.<br />
aps.org/articles/pdf/10.1103/Physics.9.94<br />
[4] Hampel, W. et al. Final Results of the 51Cr Neutrino<br />
Source Experiments in GALLEX. Phys. Rev. B. 1998, 420,<br />
114.<br />
[5] Mention, G. et al. Reactor Antineutrino Anomaly. Phys.<br />
Rev. D. 2011, 83, 073006.<br />
[6] Aguilar-Arevalo, A. A. et al. Evidence for Neutrino<br />
Oscillations for the Observation of ve Appearance in a vμ<br />
Beam. Phys. Rev. D. 2001, 64, 122007.<br />
[7] Aguilar-Arevalo, A. A. et al. Phys. Rev. Lett. 2013, 110,<br />
161801.<br />
[8] Aartsen, M. G. et al. Searches for Sterile Neutrinos<br />
with the IceCube Detector. Phys. Rev. Lett. 2016, 117,<br />
071801.<br />
DESIGN BY Vidya Giri<br />
EDITED BY Shaurey Vetsa<br />
CATALYST | 61