Statistical Data Analysis in the DANSS Experiment Including Antineutrino Relative Count Rate Data as a Function of Dista
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istical Data Analysis in the DANSS Experiment Including Antineutrino Relative Count Rate Data as a Function of Distance N. A. Skrobovaa, b a
Lebedev Physical Institute, Russian Academy of Sciences, Moscow, 119991 Russia Alikhanov Institute for Theoretical and Experimental Physics NRC “Kurchatov Institute,” Moscow, 117218 Russia e-mail: [email protected]
b
Received March 25, 2020; revised June 25, 2020; accepted June 26, 2020
Abstract–The DANSS detector, i.e., a scintillation detector with a sensitive volume of 1 m3, is installed at the Kalinin Nuclear Power Plant under the reactor on a moving stage. The distance from the detector center to the reactor core center can be varied from 10.9 to 12.9 m. The detector records about 5000 events of inverse beta decay per day with a background level from cosmic muons of the order of two percent. An analysis on the search for the sterile neutrino, based on only the ratio of spectra shapes at various distances is compared with an analysis which, along with the shape, accounts for changes in relative count rates. The effect of systematic errors contributing to the analysis using relative count rates is discussed. The analysis considering three detector positions is described. Keywords: neutrino oscillations, sterile neutrinos, nuclear reactor DOI: 10.3103/S1068335620090067
1. INTRODUCTION Currently, there are a number of indications (see, e.g., the recent review [1]) for the existence of the sterile neutrino which, in contrast to three known neutrino types, is not produced in Z-boson decays. The sterile neutrinos in the DANSS experiment are searched in the model with three active and one sterile neutrino. In the case of the existences of the sterile neutrino, a fraction of ν e will transform to sterile neutrinos, which will lead to oscillations in the number of ν e depending on the distance. For ν e , the probability of not to oscillate (to survive) at short distances is described by the formula [2] 2 1.27Δm14 [eV 2 ]L [m] p = 1 − sin 2 2θ14 sin 2 , Eν [MeV]
(1)
2 where Δm14 = m42 − m12 is the difference between squared mass eigenstates of neutrinos, sin 2 2θ14 is the mixing parameter, Eν is the ν e energy, and L is the distance from the production point to the detection 2 point. Since Δm14 is expected to be rather high (~1 eV2) in comparison with already known differences of squared masses, known neutrino oscillations at short distances can be neglected. Thus, the existence of the sterile neutrino state would introduce distance-dependent distortions into the energy spectrum of reactor ν e . Therewith, both the spectrum shape and the total count change. In measuring the spectrum at two distances by the same detector, the results obtained are independent of the absolute detector efficiency or theoretical predictions on ν e fluxes. The DANSS detector [3] installed at the Kalinin Nuclear Power Plant measures the spectrum at distances from 10.9 m (up position) to 12.9 m (down position) from the reactor core center, and then the ratio of spectra is compar
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