Sterile-neutrino

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“Sterile" Neutrino search

Standard neutrinos come in three flavors, each of which is associated with a particular charged lepton: electron, muon or tau. While the three-flavor neutrino standard model seems working fine to explain the data from solar, atmospheric, accelerator and reactor experiments, hint of the fourth flavor of neutrino, coming from LSND anomaly is worth to pursue. There is a number of experiments being planned to test this. If “sterile” neutrino is indeed existed, it might play an important role in the evolution of Universe.

Hints for “sterile" neutrino existence.

There are scattering indication of the “sterile” neutrino existence. It can be found in following [[todo: include plots for data excess]]:

LSND (Liquid Scintillator Neutrino Detector) anomaly: This detector observed electron antineutrinos appearing in a beam of muon antineutrinos [1]
MiniBooNE (Mini boosted Neutrino Experiment) found excesses of both electron neutrinos and antineutrinos [2]
Reactor anomaly: the anomalous disappearance of electron antineutrinos [3]

How to hunt for “sterile" neutrinos?

The effect of “sterile” neutrino existence can be searched via a deviation from the standard neutrino oscillation paradigm since they can mix with the standard neutrinos and thus change the amplitude of oscillation probability of standard neutrinos. In the simplest model of sterile neutrino, a fourth mass state with mass m₄ is added into the mixing matrix of the standard neutrinos. Based on the LSND and MiniBooNE results, a mass range of “sterile” neutrino is predicted. The neutrino oscillation cannot tell the absolute mass of neutrinos but the mass squared difference between two flavor neutrinos. Three standards neutrinos have two mass squared differences of ~7.5x10^-5 eV² and 2.4x10^-3 eV². The sterile neutrino mass is hypothesized much heavier than the standard neutrinos and the mass squared difference of mixing between a sterile neutrino and a normal neutrino to be 0.1-10 eV².

So far, there are two methods to search “sterile” neutrino

Search deviation in the oscillation probability by fitting with the modified oscillation probability with “sterile” neutrino taken into account
Search for the disappearance of neutral-current neutrinos. In principle, the neutral-current neutrinos are not under standard oscillation, only the charged-current neutrinos do. Thus a deficit in total number of neutral-current neutrinos comparing to the predicted one might indicate the existence of sterile neutrino.

Recent results in “sterile" neutrino search

Recent result published by both MINOS and IceCube collaborations doesn’t show any evidence. However the search is still going on.

From MINOS: Limits on Active to Sterile Neutrino Oscillations from Disappearance Searches in the MINOS, Daya Bay, and Bugey-3 Experiments, Physical Review Letters (2016). DOI: 10.1103/PhysRevLett.117.151801
From ICECUBE: “Searches for Sterile Neutrinos with the IceCube Detector,” Phys. Rev. Lett. 117, 071801 (2016).

[1] “Evidence for Neutrino Oscillations from the Observation of 𝜈̄ eAppearance in a 𝜈̄ 𝜇 Beam,” Phys. Rev. D 64, 112007 (2001).

[2] “Improved Search for 𝜈̄ 𝜇→𝜈̄ e Oscillations in the MiniBooNE Experiment,” Phys. Rev. Lett. 110, 161801 (2013).

[3] “Reactor Antineutrino Anomaly,” Phys. Rev. D 83, 073006 (2011).