| Neutrino physics has been becoming a hot spot in current researches as a new cross-disciplinary point of particle physics, astrophysics and cosmology. In the Standard Model, neutrinos are neutral weakly interacting fermions with zero mass. However, a long series of experiments in recent decades have demonstrated that neutrinos can mix and oscillate as a result of non-zero neutrino masses. In the 3-flavor mixing framework, the three neutrino flavor eigenstates ve, vμ and vτ can be written as unitary linear combi-nations of the three massive eigenstates v1, v2,v3. With parameterization, the neutrino oscillation probability can be described by three mixing angle θ1,θ13,θ23. two indepen-dent mass squared differences △m212, △m322 and a CP phases δCP. Among numerous experiments which measure the oscillation parameters, LSND was designed to search for oscillations at higher mass splittings around 1 eV2, because at that time the solar and atmospheric mass splittings were not well determined and there were some dark matter models postulating the existence of a neutrino with a mass greater than 1 eV. LSND observed an excess of signal events above background, which was interpreted as an os-cillation signal and gave a best fit value of Am2= 1.2 eV2. However, now we have measured the two mass splittings in the 3-flavor mixing framework as △m212～10-5 eV2 and |△m312|～10-3 eV2, respectively. Therefore, the result of LSND cannot be ac-commodated in the 3-flavor mixing framework. And this result first attracted attention of the researchers to the idea of the sterile neutrino with a mass about 1 eV. Some subse-quent experimental anomalies from accelerator, reactor and solar neutrino experiments, even from the cosmological data, also provide hints for the existence of the sterile neu-trino. However, apart from those hints for sterile neutrino, there are several experiments which do not show any evidence for possible sterile neutrino oscillation.The results of the global fits to different data sets prefer a mass splitting value of about 1 eV2. Considering there are some tensions existing in the global fits, we are not sure whether the sterile neutrino exists. We are facing an intriguing accumulation of hints for the existence of sterile neutrino at the eV scale. To confirm or refute the observed anomalies, multiple experiments looking at different oscillation channels and covering a wide range of L/E regions are required in the future. This makes it possible to search for sterile neutrinos with the Daya Bay antineutrino detectors.The Daya Bay Reactor Neutrino Experiment is a China-based multinational par-ticle physics project. It was designed to measure the last unknown neutrino mixing angle θ13 with a sensitivity of 0.01 or better in sin2 2θ13 at the 90% C.L. through a mea-surement of the relative rates and energy spectra of reactor antineutrinos at different baselines. In March 2012, the Daya Bay collaboration first excluded the no-oscillation hypothesis at 5.2σ, and found sin2 2θ13= 0.092±0.016(stat.)+0.005(syst.). This result was named as one of the ten major scientific breakthrough of the year by Science, and won the Panofsky Prize in 2015. Daya Bay experiment is planning to stop data taking in a few years. It would be a pity if various experimental devices are abandoned after then. So we lay out a proposal of searching sterile neutrinos with the Daya Bay antineutrino detectors, by transporting them to some short baseline (< 100 m) locations near the reactors. The proposal would have higher probability to probe the favored pa-rameter space in global fits, and may supply more valuable information of the reactor antineutrino spectrum, as well as our understanding of the detectors. This dissertation aims to investigate the feasibility of the proposal.To obtain the experimental sensitivity, we adjust some current configurations, take some assumptions and consturct a χ2 function. We have discussed the influences from the dimension effect and the arrangement of detectors. It was found that the uncertainty of neutrino flight length plays a significant role in probing large △m412 values above 1 eV2, and the room for improvement in this region is limited because of the big size of the commercial reactor. It was also found that better sensitivity can be obtained in the 10-2 eV2(?)△m412(?)1 eV2 region. If we aim to probe △m412～1 eV2, the distance between the near AD and the near reactor should be less than 30 m and the distance between ADs should be a small value on the order of a few meters. If we aim to probe △m412～several×0.1 eV2, the symmetrical arrangement is recommended in order to avoid factitious insensitivity to some △m412, and the optimized distance between ADs varies with the true △m412 value.Different with the currently running Day a Bay experiment, when we move the ADs to somewhere at short baselines, the overburden will decrease, resulting in an increas-ing of muon and cosmic ray-induced backgrounds, which may significantly assail the detection of the neutrino and degrade the sensitivity. To estimate the muon rate and related background level, we develop a simulation code with the Geant4 toolkit. A shielding structure with water, lead and concrete is assumed at first. It was found that the muon rate in the veto system and scintillator region can be up to about 15 kHz and 2 kHz with the assumed detector construction. The level of the muon-induced correlat-ed background is determined as 0.2 Hz conservatively by applying a 20μs muon veto time and a multiplicity cut which requires no additional>0.7 MeV signal in the time range (tp-20μs, td+20μs), where tp and td are the time of the prompt and delayed signals in a neutrino event, respectively. Actually, the 0.2 Hz correlated backgrounds are all from the un-vetoed signals after muons. This is quite different with the current-ly running Daya Bay experiment, in which the muon-induced correlated backgrounds are mainly from 9Li/8He and fast neutron. According to the experience in Daya Bay simulation, we know that it is impossible to accumulate enough statistics of 9Li/8He and fast neutron in our case. Thus we estimate the background level by utilizing the Power Law, and finally give the result of 0(10-3) Hz for 9Li/8 He and O(10-2) Hz for fast neutron. We also estimate the accidental background level (<0.04 Hz) by using the simulated 11 Hz muon-induced prompt like signal rate and 3 Hz muon-induced delayed like signal rate, together with the 50 Hz radioactivity singles rate from the Daya Bay Project Technical Design Report. Additional discussion of the cosmic neutron-induced background level (<0.001 Hz) shows that the assumed shielding structure can attenu-ate neutrons significantly. As a result, we think it is possible to obtain an acceptable signa-to-background ratio in our proposal. For example, if the AD could be located in the middle of the two reactors, the neutrino event rate would be around 0.6 Hz, which makes the signa-to-background ratio better than 1.Taking into account the study of the sensitivity and background level, we finally supply the experimental requirements for our proposal through selecting two available positions near the reactors. It was found that as long as the bin-to-bin uncertainty σdb(?) (1)% and background event rate Rbkg(?)(10)Hz, it is possible to test for oscillations in the 10-2 eV2(?)△Am412(?)3 eV2 region. The requirement of σdb(?)(1)% could be easily fulfilled referring to previous experiments, thus it should not be a problem. As to the other requirement of Rbkg(?)(10)Hz, we think it should be easily fulfilled too by a simulation based on an assumed shielding structure with plastic-scintillator, polyethylene and iron. In conclusion, the proposal of searching sterile neutrinos with the Daya Bay antineutrino detectors is feasible if we aim to probe 10-2 eV2(?)△m412(?) 3 eV2 oscillation parameter space. |
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