Chooz (experiment)

Detector

The Chooz detector was underground, with a 300- meter water equivalent overburden to reduce cosmic ray backgrounds. The detector itself was a cylinder 5.5 m in diameter and 5.5 m tall. The detector was composed of three regions. The innermost region (region I) contained 5 tons of scintillator doped with gadolinium in a Plexiglas container. The gadolinium quickly captured the neutrons produced in the inverse beta decay. The second region (region II) contained 17 tons of undoped scintillator to capture the electromagnetic energy from the inverse beta decay (≈99%) and the photons from the neutron capture in the Gd (>95%). The outer surface of region II contained 192 inward facing photomultiplier tubes (PMT) held in an opaque plastic structure.

The outermost region (region III) contained ninety tons of the same undoped scintillator as region II and vetoed cosmic ray events using 48 PMTs.

The Plexiglas layer between regions I and II was clear, to allow scintillation light from region I to be observed by the PMTs in region II. The inner surface of the region II container was painted black to avoid reflections, which would degrade position measurements. The outer surface of the region II container and the inner surface of the region III container were painted white to maximize the veto signals.

Data sample

Chooz collected data for a total of 8210 hours: 3420 hours with neither reactor running, 3250 hours with one reactor running, and 1540 hours with both reactors running. A total of 2991 neutrino candidates were observed during this period, with 287 of the candidates occurring during the reactor off periods. The correlated background was 1.01 events per day, and the uncorrelated background was 0.42 events per day. The neutrino yield was also studied as a function of reactor power, fuel burnup, and for each reactor separately.

The energy spectrum of the positrons and the calculated direction of the incoming neutrino were also calculated. All distributions agreed with predictions.

Neutrino oscillation searches

Three analysis techniques were used to search for neutrino oscillations. The most powerful method used a global χ² fit of seven positron energy bins for each reactor, for a total of fourteen bins. The χ² was calculated for various (θ, δm²) combinations. The full 14 × 14 covariance matrix was used to account for correlations among the bins. The χ² statistic also includes a term for the overall neutrino normalization (with an uncertainty of 2.7%) and a term for the energy calibration (with an uncertainty of 1.1%). The global minimum, with a χ² probability of 96%, corresponds to sin²(2θ) = 0.23, δm² = 8.1×10⁻⁴ eV², overall neutrino normalization = 1.012, and energy scale = 1.006. The no-oscillation hypothesis is also has a high χ² probability of 93%, corresponding to an overall neutrino normalization = 1.008 and energy scale = 1.011.

Interpretation of results

The Chooz results place limits on the PMNS matrix element U_e3². Two possibilities exist, U_e3² < 0.03 or U_e3² > 0.97. The solar neutrino problem excludes the second inequality, therefore U_e3² is restricted to small values. The CHOOZ data also indicate a strong preference for the ν
_μ → ν
_τ maximum mixing hypothesis.

Note on naming

The Chooz collaboration is not consistent in its capitalization. The experiment sometimes appears as Chooz and sometimes as CHOOZ. However, it is not an acronym.