Measurement of θ 13 in Double Chooz using neutron captures on hydrogen with novel background rejection techniques

Abstract

The Double Chooz collaboration presents a measurement of the neutrino mixing angle θ 13 using reactor ν e observed via the inverse beta decay reaction in which the neutron is captured on hydrogen. This measurement is based on 462.72 live days data, approximately twice as much data as in the previous such analysis, collected with a detector positioned at an average distance of 1050 m from two reactor cores. Several novel techniques have been developed to achieve significant reductions of the backgrounds and systematic uncertainties. Accidental coincidences, the dominant background in this analysis, are suppressed by more than an order of magnitude with respect to our previous publication by a multi-variate analysis. These improvements demonstrate the capability of precise measurement of reactor ν e without gadolinium loading. Spectral distortions from the ν e reactor flux predictions previously reported with the neutron capture on gadolinium events are confirmed in the independent data sample presented here. A value of sin 2 2θ 13 = 0.095 +0.038 −0.039 (stat+syst) is obtained from a fit to the observed event rate as a function of the reactor power, a method insensitive to the energy spectrum shape. A simultaneous fit of the hydrogen capture events and of the gadolinium capture events yields a measurement of sin 2 2θ 13 = 0.088 ± 0.033(stat+syst).

Pinning down neutrino oscillation parameters in the 2–3 sector with a magnetised atmospheric neutrino detector: a new study

“…The sensitivity to the mass hierarchy is directly proportional to the value of sin 2 2θ 13 , which is quite precisely known [12][13][14][15][16][17]. It also depends on the ability of ICAL to separate neutrino and antineutrino events which is possible since ICAL is magnetised.…”

New realization of the conversion calculation for reactor antineutrino fluxes

“…3 will be employed. This realization has been widely used in previous calculations [16,25] but its reliability is challenged by the recent observed reactor rate and spectral anomalies [18][19][20][21][22][23]. Thus a new realization of the conversion calculation will be presented in Sec.…”

“…One can either employ the ab initio method [4][5][6][7][8][9][10][11][12] by a direct summation of the antineutrino energy spectrum in the beta decay of each fission fragment from the state-of-the-art nuclear database, or apply the effective method with a conversion procedure [13][14][15][16][17] based on the measurement of the integral electron energy spectrum for the main fissionable isotopes. However, the reliability and accuracy of the isotopic flux calculations should be carefully examined in order to resolve the reactor antineutrino rate and spectral anomalies [18][19][20][21][22][23] and serve for the neutrino mass hierarchy measurement in future reactor antineutrino experiments [24].…”

“…In previous publications, the reliability and accuracy of the conversion method have been tested with the assumption of allowed beta decay transitions for all the virtual branches [16,25]. However, this simple treatment has been challenged by the incompatibility of the total inverse-beta-decay rate and energy spectrum between the theoretical calculations and experimental measurements [18][19][20][21][22][23]. Moreover, according to the current nuclear database the beta decay branches of the first forbidden type contribute to around 30% of the total branches [26,27] in each fission isotope and they may induce a large uncertainty [26][27][28][29][30] in the antineutrino flux predictions.…”

A sterile neutrino search at compact materials irradiation facility

“…During the past few years, various neutrino experiments with different sources and detection concepts have been running to directly explore the favored parameter space for the sterile neutrino and the relevant parameter space is restricted by several null results [19][20][21][22][23][24][25]. Recently, evidence for one of these anomalies, the reactor anomaly, has been weakened by the Daya Bay's reactor fuel cycle measurements [26] and also by the observed shoulder at 5 MeV in the reactor neutrino spectrum [27][28][29], which is in contradiction with the theoretical reactor flux prediction of Ref. [30].…”