1. RESULTS FROM DOUBLE CHOOZ
S. PERASSO
Laboratoire APC, Universit´e Paris 7 Diderot,
10 rue Alice Domon et L´eonie Duquet, 75013, Paris, France
The Double Chooz experiment has measured the neutrino oscillation angle θ13 observing the
¯νe flux produced in the two reactors of the Chooz nuclear power plant, via inverse beta decay
(IBD) in a 10.3 m3
Gd-loaded LS detector, with a 1050 m baseline. The observation of ¯νe via
neutron absorption on Gd gives a selection of 8249 candidate events in 227.93 live days with
a 33.71 GW-ton-years exposure, the expectation in case of θ13 = 0 being 8937. The deficit is
interpreted as evidence of ¯νe disappearance. The Rate+Shape analysis gives sin2
2θ13 = 0.109
± 0.030 (stat.) ± 0.025 (syst.) (no oscillation hypothesis excluded at 99.8% CL, 2.9σ). The
observation of the ¯νe via neutron absorption on H exploits a three times larger fiducial volume,
with an exposure of 113.1 GW-ton-years. The data set is distinct from the Gd analysis and
the systematic uncertainties are largely independent. A combined Rate+Shape analysis gives
sin2
2θ13 = 0.97 ± 0.034 (stat) ± 0.034 (syst), compatible with the result from Gd analysis
and excluding the no oscillation hypothesis at 2σ.
1 Introduction
The neutrino flavor oscillation angle θ13 has been recently measured by accelerator and reactor
experiments. While the measurement at accelerator experiments is limited by the MSW matter
effect, the lack of knowledge of the δCP phase and the degeneracy on θ23, reactor experiments
provide a clean and direct measurement of θ13. Nuclear reactors are copious sources of ¯νe (∼1020
¯νe/sec at 1 GWth) with a spectrum below 10 MeV. At these energies, the survival probability
exhibits a minimum at a distance of the order of 1 km from the reactor, which depends exclusively
on θ13.
The Double Chooz experiment observes the ¯νe flux from the two 4.25 GWth N4 type (water
pressurized) reactors of the Chooz power plant in a two-detector configuration: a Near Detector
(400 m baseline) for the measure of the unoscillated flux, and a Far Detector (1050 m baseline)
for observing the oscillation. Currently, the measurement is performed with the Far Detector
only. The Near Detector is under construction, the start of data taking in the two-detector
configuration being foreseen for Spring 2014.
The geometry of the two detectors, identical to each other, features a series of concentric
cylindrical volumes (Fig. ??). The external cylinder, the Inner Veto (IV), contains 90 m3 of
liquid scintillator and allows to veto cosmic muons with an efficiency of 95% and to tag the fast
neutrons entering the detector. It houses the Inner Detector (ID), from which it is optically
separated by a stainless-steel cylinder. The ID is partitioned into three volumes: the Neutrino
Target (NT), the innermost volume filled with 10.3 m3 of liquid scintillator doped with Gd to
enhance the neutron capture; the Gamma Catcher (GC), 22 m3 of Gd-free liquid scintillator
aiming at fully contain the gammas from neutron capture on Gd escaping the NT; the Buffer
volume, 110 m3 of non-scintillating mineral oil, a shield for the inner volumes from the external
radioactivity.

2. Figure 1: Section of the Far Detector. The Near
Detector geometry is identical in order to suppress
the systematics.
Figure 2: Top plot: prompt spectrum measured in the
n-Gd analysis, along with the expected spectrum and to
the best fit. The background spectrum is resolved into its
components in the inset. Center plot: data to prediction
ratio. Bottom plot: difference between data and predic-
tion.
Anti-neutrinos are detected via the inverse beta decay (IBD), p+ ¯νe → e+ +n. The two-fold
coincidence of the positron signal and the neutron capture allows to abate the background. The
oscillation analysis relies on the event rate deficit and the spectral shape distortion at the Far
Detector. The analyses of the two data sets identified by the neutron capture on Gd and on H
are reported here.
2 Measurement of θ13 from Neutron Capture on Gd
The neutron capture on Gd has a characteristic mean time of ∼30 µs. The net product is the
emission of a set of gammas of ∼8 MeV total energy. In the framework of this analysis?, the IBD
candidate selection requires a prompt event in the energy range [0.7, 12.2] MeV, followed by a
delayed event in the range [6, 12] MeV, with the time delay falling between 2 and 100 µs. A
multiplicity cut rejects the IBD candidates containing additional events between 100 µs before
the prompt event and 400 µs after the delayed. The cosmogenic background is reduced rejecting
all the events within a 1 ms (500 ms) after a muon of energy lower (larger) than 600 MeV.
The instrumental noise consisting in light emission from the bottom of the PMTs, namely the
Light Noise, is rejected on the basis of the light pulse time profile and of the light distribution
homogeneity over the PMTs. No spatial cuts are applied in order to reduce the systematic
uncertainty. The selected IBD candidates amount to 8249, in a live time of 227.93 days at 33.71
GW/(ton·year).
The background surviving the two-fold coincidence has an uncorrelated and a correlated
components. In most cases, the former consists in accidental coincidences between a radioactive
decay and a neutron capture. This kind of background is very well constrained by means of an
off-time window analysis, which finds an accidental rate of 0.261 ± 0.002 events per day.
The sources of correlated background include fast neutrons, stopping muons and spallation
products of cosmic muons. Fast neutrons reproduce the IBD topology as the fast neutron
scatters on H producing a proton recoil and is captured on Gd after thermalizing. Fast neutrons
from the detector surroundings are identified with simultaneous IV and ID triggers. Also muons

3. decaying in the NT can be selected as IBD candidates, as the muon energy released in the NT
and the Michel electron can mimic prompt and delayed event respectively. Cosmic muons can
enter the NT without being vetoed by passing through the chimney connecting the ID to the
above glove box (Fig. ??). As their prompt spectrum is expected to be constant in both cases,
fast neutrons and stopping muons are studied in a combined analysis, in which a global rate of
0.67 ± 0.20 events per day is found.
Cosmogenic 8He and 9Li are the most critical background as they both undergo a βn-decay
with lifetimes of 172 ms and 257 ms respectively. Their rate is extracted from the distribution of
the prompt event distance in time from the previous muon, via an exponential fit which indicates
a global βn-decay rate of 1.25 ± 0.54 events per day. The total measured background rate is
compatible with the result from the Reactor Rate Modulation Analysis (see next section) and
is corroborated by the data taken with both reactors off.
The oscillation analysis is performed taking into account both the event rate and spectral
shape. In the current Far Detector-only configuration, the expected flux is computed on the
basis of the reactor dynamics. The cross section per fission has been anchored to the Bugey4
measurement? in order to reduce the uncertainty. The IBD candidate data set is divided in
two integration periods based on the reactor power to exploit the different signal to background
ratio. The measure of θ13 is obtained by fitting the data in a two-neutrino oscillation scenario,
with the mass splitting at the MINOS value of (2.32 ± 0.12) × 10−3 eV2, in a χ2 minimization
approach. The best fit gives
sin2
2θ13 = 0.109 ± 0.030 (stat.) ± 0.025 (syst.) (1)
with a χ2/n.d.f. of 42.1/35. The systematic uncertainty is dominated by the uncertainties on the
reactor flux and on the cosmogenic background. Fig. ?? illustrates the fit result on the prompt
energy spectrum from the whole IBD candidate data set. An analysis based only on the compari-
son of observed and expected rate gives a best fit of sin2
2θ13 = 0.170 ± 0.035 (stat.) ± 0.040 (syst.)
with a χ2/n.d.f. of 0.50/1. The no-oscillation hypothesis is excluded at 99.8% (2.9σ).
2.1 Reactor Rate Modulation Analysis
The same data set, with the inclusion of additional data taken with both reactors off, has been
analyzed also by comparing the expected and the observed rate at different reactor activities.
The data set has been split into 7 subsets, each corresponding to a different reactor activity
interval, and for each group the expected and the observed rates have been computed. The
distribution of the observed rate as a function of the expected rate is fitted to the straight
line, whose intercept B describes the total background, while the angular coefficient depends on
sin2 2θ13. The fit results are
sin2
2θ13 = 0.10 ± 0.04 (stat. + syst.) (2)
B = 1.1 ± 0.5 day−1
(3)
The value of θ13 is in excellent agreement with the result from the Gd-Analysis in terms of both
the central value and the accuracy. In addition, the total background measured here agrees with
the background measurements described in the previous section.
3 Measurement of θ13 from Neutron Capture on H
A second measurement of θ13 comes from the analysis of the IBD candidates followed by neutron
capture on H?. In this analysis, the active volume can be expanded to include the GC, in addition
to the Neutrino Target, thus providing a 3 times larger fiducial volume.
The n-H and the n-Gd analyses are carried out over the same time period of data. The
selection criteria are the same in the two analyses, but as the capture on H occurs in ∼200 µs

4. Figure 3: Observed vs Expected rate at different
reactor powers. The results from the linear fit are
also indicated.
Figure 4: Top plot: background-subtracted spectrum in
the n-H analysis, along with prediction and best fit. Cen-
ter plot: data to prediction ratio. Bottom plot: difference
between data and prediction.
with the emission of a 2.2 MeV gamma, in the n-H analysis the delayed event is selected in the
range [1.5, 3] MeV with a delay time between 10 and 600 µs. The multiplicity cut is applied
to a window from 600 µs before the prompt to 1 s after it. A spacial cut is introduced here
to reject the accidental background, the distance between prompt and delayed limited to be
within 90 cm. The total IBD candidate data set amounts to 36284 events, with a live time of
240.1 days and at 113.1 GW/(ton·year). The candidates distribution in space differs from the
n-Gd analysis, as 95% of the IBD candidates are in the GC, due to its larger volume and to the
presence of Gd in the NT (only 13% of neutron captures in the NT occurs on H).
The background components are the same as the n-Gd analysis, with a small additional
contribution from the Light Noise (0.32 ± 0.07 events per day). The signal to background
ratio is ∼1, due to accidental coincidences accounting for ∼48% of IBD candidates. Their
rate is measured to be 73.45 ± 0.16 events per day by means of an off-time window analysis.
The correlated background is given by fast neutrons and cosmogenic radioisotopes, with no
contribution from the stopping muons, efficiently rejected by the selection cuts. As in the n-
Gd analysis, fast neutrons are identified by simultaneous triggers from the IV and the ID and
cosmogenic 9Li is measured exploiting the correlation to the previous muon. The measured rates
are 2.50 ± 0.47 and 2.8 ± 0.1 events per day respectively.
The IBD candidate data set is analyzed in a single integration period, comparing rate and
spectral shape to the expectations. The best fit gives
sin2
2θ13 = 0.097 ± 0.034 (stat.) ± 0.034 (syst.) (4)
with χ2/n.d.f = 38.9/30, the mass splitting being fixed at the MINOS value. An analysis based
on the rate only gives as best fit sin2
2θ13 = 0.044 ± 0.022 (stat.) ± 0.056 (syst.), the smaller
central value being motivated by an underestimation of the 9Li background. The no-oscillation
hypothesis is excluded at 97.4% (2.0σ).
References
1. Y. Abe et al., Phys. Rev. D 86, 052008 (2012)
2. Y. Declais et al., Phys. Lett. B 338, 383 (1994)
3. Y. Abe et al., Phys. Lett. B 723, 66 (2013)