# Daya Bay Reactor Neutrino Experiment

One of the Daya Bay detectors.

The Daya Bay Reactor Neutrino Experiment is a China-based multinational particle physics project studying neutrinos. The multinational collaboration includes researchers from China, Chile, the United States, Taiwan, Russia, and the Czech Republic. The US side of the project is funded by the US Department of Energy's Office of High Energy Physics.

It is situated at Daya Bay, approximately 52 kilometers northeast of Hong Kong and 45 kilometers east of Shenzhen. There is an affiliated project in the Aberdeen Tunnel Underground Laboratory in Hong Kong. It measures the neutrons produced by cosmic muons which may affect the Daya Bay Reactor Neutrino Experiment.

The experiment consists of eight antineutrino detectors, clustered in three locations within 1.9 km (1.2 mi) of six nuclear reactors. Each detector consists of 20 t of liquid scintillator (linear alkylbenzene doped with gadolinium) surrounded by photomultiplier tubes and shielding.[1]

A much larger follow-up is in development in the form of the Jiangmen Underground Neutrino Observatory (JUNO) in Kaiping,[2] which will use an acrylic sphere filled with 20,000 t of liquid scintillator to detect reactor antineutrinos. Groundbreaking began January 10, 2015, with operation expected in 2020.[3]

## Neutrino oscillations

The experiment studies neutrino oscillations and is designed to measure the mixing angle θ13 using antineutrinos produced by the reactors of the Daya Bay Nuclear Power Plant and the Ling Ao Nuclear Power Plant. Scientists are also interested in whether neutrinos are CP violators.

On 8 March 2012, the Daya Bay collaboration announced[4][5][6] a 5.2σ discovery of θ13 ≠ 0, with

${\displaystyle \sin ^{2}(2\theta _{13})=0.092\pm 0.016\,\mathrm {(stat)} \pm 0.005\,\mathrm {(syst)} .}$

This significant result represents a new type of oscillation and is surprisingly large.[7] It is consistent with earlier, less significant results by T2K, MINOS and Double Chooz. With θ13 so large, NOνA has about a 50% probability of being sensitive to the neutrino mass hierarchy. Experiments may also be able to probe CP violation among neutrinos.

The collaboration produced an updated analysis of their results in 2014,[8] which used the energy spectrum to improve the bounds on the mixing angle:

${\displaystyle \sin ^{2}(2\theta _{13})=0.090_{-0.009}^{+0.008}}$

An independent measurement was also published using events from neutrons captured on hydrogen:[9] ${\displaystyle \sin ^{2}(2\theta _{13})=0.083\pm 0.018}$.

Daya Bay has used its data to search for signals of a light sterile neutrino, resulting in exclusions of some previous unexplored mass regions.[10]

At the Moriond 2015 physics conference a new best fit for mixing angle and mass difference was presented:[11]

${\displaystyle \sin ^{2}(2\theta _{13})=0.084\pm 0.005,\quad |\Delta m_{ee}^{2}|=2.44_{-11}^{+10}\times 10^{-3}{\rm {eV}}^{2}}$

## Antineutrino spectrum

Daya Bay Collaboration measured the anti-neutrino energy spectrum, and found that anti-neutrinos at an energy of around 5 MeV are in excess relative to theoretical expectations. This unexpected disagreement between observation and predictions suggested that the current theory needs improvement.[12]

## References

1. ^ Daya Bay collaboration (22 May 2012). "A side-by-side comparison of Daya Bay antineutrino detectors" (PDF). Nuclear Instruments and Methods in Physics Research A. 685 (1): 78–97. arXiv:. Bibcode:2012NIMPA.685...78A. doi:10.1016/j.nima.2012.05.030.
2. ^ Li, Xiaonan (August 2013). Daya Bay II: Jiangmen Underground Neutrino Observatory (JUNO) (PDF). Windows On the Universe.
3. ^ "Groundbreaking at JUNO". Interactions NewsWire. 10 January 2015. Retrieved 2015-01-12.
4. ^ Daya Bay Collaboration (2012). "Observation of electron-antineutrino disappearance at Daya Bay". Physical Review Letters. 108 (17): 171803. arXiv:. Bibcode:2012PhRvL.108q1803A. doi:10.1103/PhysRevLett.108.171803. PMID 22680853.
5. ^ Adrian Cho (8 March 2012). "Physicists in China Nail a Key Neutrino Measurement". ScienceNow.
6. ^ Eugenie Samuel Reich (8 March 2012). "Neutrino oscillations measured with record precision". Nature. doi:.
7. ^ "Announcing the First Results from Daya Bay: Discovery of a New Kind of Neutrino Transformation" (Press release). University of California, Berkeley. 8 March 2012.
8. ^ Daya Bay Collaboration (10 February 2014). "Spectral measurement of electron antineutrino oscillation amplitude and frequency at Daya Bay". Physical Review Letters. 112 (6): 061801. arXiv:. Bibcode:2014PhRvL.112f1801A. doi:10.1103/PhysRevLett.112.061801. PMID 24580686.
9. ^ Daya Bay Collaboration (3 October 2014). "Independent measurement of the neutrino mixing angle θ13 via neutron capture on hydrogen at Daya Bay". Phys. Rev. D 2014. 90 (7): 071101. arXiv:. Bibcode:2014PhRvD..90g1101A. doi:10.1103/PhysRevD.90.071101.
10. ^ Daya Bay Collaboration (1 October 2014). "Search for a Light Sterile Neutrino at Daya Bay". Phys. Rev. Lett. 113 (14): 141802. arXiv:. Bibcode:2014PhRvL.113n1802A. doi:10.1103/PhysRevLett.113.141802.
11. ^ Bei-Zhen Hu; et al. (14 May 2015). "Recent Results from Daya Bay Reactor Neutrino Experiment". arXiv: [hep-ex].
12. ^ An, F. P.; et al. (Daya Bay Collaboration) (2016-02-12). "Measurement of the Reactor Antineutrino Flux and Spectrum at Daya Bay". Physical Review Letters. 116 (6): 061801. arXiv:. Bibcode:2016PhRvL.116f1801A. doi:10.1103/PhysRevLett.116.061801. PMID 26918980.