K.B. Luk: Daya Bay
The final presentation of today’s session was given by Kay-Biu Luk (see picture below), about the Daya Bay experiment.
There are two ways to measure the theta_13 parameter of the neutrino mixing matrix. One is to use a nuclear reactor, for disappearance of electron antineutrinos. The other is an accelerator-based electron-neutrino appearance experiment. A neutrino reactor can be an extremely pure source; however the resultant energy spectrum of the neutrino is known with some percent uncertainty. Also, the spectrum may vary with time.
The easiest way to detect reactor antineutrino is to use the inverse beta-decay reaction in a liquid scintillator, whereby the interaction with a proton produces a positron and a neutron, and the neutrin can be captured by gadolinium, yielding a bunch of gamma rays after some 30 microseconds. The energy of the antineutrinos can be determined from the detected energy.
The first search aiming at theta_13 was done by Chooz in the 90ies. It was a 5-ton liquid scintillator detector, with 0.1% load of gadolinium. In 2003 the first meeting to launch the Daya Bay experiment took place in November 28, in Hong Kong.
Luk explained how the construction of Daya bay was complicated by issues with the access tunnel availability at the detector site. The detector design envisioned a 4-meter acrylic tank sandwiched between top and bottom reflectors, going for the largest detector they could afford.
In 2011 the first two active detectors were operated. Then four more joined. After a shutdown, in October 2012 the experiment could start with all eight detectors. The uncertainty in the relative antineutrino detection efficiency was found to be of 0.2%, significantly better than the 0.38% value of design. In 2012 the first indications of a non-zero theta_13 arrived from other experiments, and this gave motivation to move very quickly. With just 55 days of data they could discover the disappearance of reactor antineutrinos at short baseline. This was confirmed by Reno a month later.
While collecting more data they spent more time trying to understand very accurately the energy response of their detector. At high statistics they could see a defect in the far detector, a distortion in the total rate. Analysis of neutron capture on Hydrogen with 217 days of data taking from 6 active detectors could reduce the systematics. Since 2011 Daya Bay has remained the most precise experiment in the determination of theta_13. This is shown clearly by the graph on the left above.