Double Chooz

Experimental techniques

Neutrino mixing

Neutrinos are electrically neutral, extremely light particles that only interact weakly, meaning they can travel vast distances without ever being noticed. One of the properties of neutrinos is that as the propagate they have a chance to oscillate from one flavor (${\displaystyle e,\mu ,\tau }$) to another, and this is the principle under which the experiment operates. The goal of Double Chooz is to more tightly constrain the value for the ${\displaystyle {\theta }_{13}}$ mixing angle.

The Chooz experiment, performed in the 1990s, found that the ${\displaystyle {\theta }_{13}}$ mixing angle is constrained by

${\displaystyle {\sin }^2(2{\theta }_{13})<0.2}$

which was the best experimental upper limit for over a decade. The goal of the Double Chooz experiment is to continue to explore the ${\displaystyle {\theta }_{13}}$ angle by probing an even smaller region

${\displaystyle 0.03<{\sin }^2(2{\theta }_{13})<0.2}$

Observations of the mixing angle are accomplished by observing the ${\displaystyle {\overline{\nu }}_e}$ flux that comes off of the reactors during their fission reactions. Because one of the neutrino mass-squared differences is much smaller than the other, the Double Chooz experiment only needs to consider a two-flavor oscillation. In the two-flavor model the survival probability of any given neutrino is modelled by

${\displaystyle P=1-{\sin }^2(2{\theta }_{13}){\sin }^2\left(\frac{1.27\mathrm{\Delta }{m}_{31}^{2}L}{E_{\nu }}\right)(\mathrm{i}\mathrm{n}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{s}).}$

Here ${\displaystyle L}$ is the length in meters the neutrino has travelled and ${\displaystyle E_{\nu }}$ is the energy of the ${\displaystyle {\overline{\nu }}_e}$ particle. From this the value of the mixing angle can be measured from the oscillation amplitude in reactor neutrino oscillations.

Observations

The neutrinos from the reactor are observed via the inverse beta decay (IBD) process

${\displaystyle {\overline{\nu }}_e+p\to e^{+}+n.}$

Since there are backgrounds to consider, candidates for (IBD) are determined by the following: visible energy from the prompt signal must be between 0.5 and 20 MeV; the delayed signal must have an energy between 4 and 10 MeV; the time difference between those two signals must be between 0.5 and 150 microseconds; the distance between the vertices of the two signals should be less than 100 cm; and no other signals (except for the delayed signal) are found 200 microseconds before or 600 microseconds after the prompt signal. Detection of the prompt signal has reached nearly 100% efficiency, however it is not as easy to detect the delayed signal due to issues such as Gd-concentration and neutron scattering models.

Results

Mixing angle

In November 2011, first results of the experiment, using 228 days of data, were presented at the LowNu conference in Seoul, hinting at a non-zero value of θ₁₃, followed by an article submitted to arXiv in December 2011. In the PRL article (published in 2012), the zero θ₁₃ oscillation hypothesis was excluded at 2.9 sigma by combining the Double Chooz experiment disappearance data and the T2K experiment appearance data, that had been released only some months before. This result became both the most important evidence at the time and the first accurate measurement of the amplitude of θ₁₃. Shortly after, the Daya Bay experiment provided its confirming measurement with ≥5σ significance. The central values of both Double Chooz and Daya Bay experiments were in excellent agreement and has remained so (within ≤2σ) so far. A similar analysis combination technique as done by the Double Chooz experiment in 2012 has been employed by the T2K experiment to yield the first constraints on the non-zero CP-violation phase in 2020.

Neutron capture on hydrogen was used to produce independent data, which was analysed to yield a separate measurement in 2013:

${\displaystyle {\sin }^2(2{\theta }_{13})=0.097\pm 0.034(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 0.034(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}).}$

Using reactor-off data, a background-independent measurement was published July 2014 in Physics Letters B:

${\displaystyle {\sin }^2(2{\theta }_{13})=0.102\pm 0.028(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 0.033(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}).}$

An improved measurement with reduced background and systematic uncertainties after 467.90 days of data was published in the Journal of High Energy Physics in 2014:

${\displaystyle {\sin }^2(2{\theta }_{13})={0.090}_{-0.029}^{+0.032}.}$

Other results

Double Chooz was able to identify positronium formation in their detector, which delays positron annihilation and distorts the scintillation signal. A tagging algorithm was developed that could be used in neutrino detectors for improved background rejection, which was similarly done by Borexino for cosmogenic ¹¹C background. An ortho-positronium lifetime of 3.68±0.15 ns was measured, compatible with other dedicated setups.

Limits on Lorentz violation parameters were also set.