A LN2-based cooling system for a next-generation liquid xenon dark matter detector

In recent years, cooling technology for liquid xenon(LXe) detectors has advanced driven by the development of dark matter(DM) detectors with target mass in the 100–1000 kg range. The next generation of DM detectors based on LXe will be in the 50,000 kg(50 t)range requiring more than 1 k W of cooling power. Most of the prior cooling methods become impractical at this level.For cooling a 50 t scale LXe detector, a method is proposed in which liquid nitrogen(LN2) in a small local reservoir cools the xenon gas via a cold finger. The cold finger incorporates a heating unit to provide temperature regulation. The proposed cooling method is simple, reliable, and suitable for the required long-term operation for a rare event search. The device can be easily integrated into present cooling systems, for example the ‘‘Cooling Bus’ ’employed for the Panda X I and II experiments. It is still possible to cool indirectly with no part of the cooling or temperature control system getting in direct contact with the clean xenon in the detector. Also, the cooling device can be mounted at a large distance, i.e., the detector is cooled remotely from a distance of 5–10 m. The method was tested in a laboratory setup at Columbia University to carry out different measurements with a small LXe detector and behaved exactly as predicted.

Ultrasonic positioning system for the calibration of central detector

A thorough detector response calibration using radioactive sources is necessary for the Jiangmen Underground Neutrino Observatory. Herein, we discuss the design of a source positioning system based on ultrasonic technology, aiming for a 3-cm precision over the entire35-m diameter detector sphere. A prototype system is constructed and demonstrated for the experiment.

Screener3D:a gaseous time projection chamber for ultra-low radioactive material screening

In experiments searching for rare signals,background events from the detector itself are some of the major factors limiting search sensitivity.Screening for ultra-low radioactive detector materials is becoming ever more essential.We propose to develop a gaseous time projection chamber(TPC)with a Micromegas readout for radio screening.The TPC records three-dimensional trajectories of charged particles emitted from a flat sample placed in the active volume of the detector.The detector can distinguish the origin of an event and identify the particle types with information from trajectories,which significantly increases the screening sensitivity.For a particles from the sample surface,we observe that our proposed detector can reach a sensitivity higher than 100 l Bq m-2 within two days.

Searching for neutrino-less nouble neta necay of 136Xe with PandaX-Ⅱ liquid xenon detector

We report the Neutrino-less Double Beta Decay（NLDBD） search results from PandaX-Ⅱ dual-phase liquid xenon time projection chamber.The total live time used in this analysis is 403.1 days from June 2016 to August2018.With NLDBD-optimized event selection criteria,we obtain a fiducial mass of 219 kg of natural xenon.The accumulated xenon exposure is 242 kg yr,or equivalently 22.2 kg yr of 136Xe exposure.At the region around 136Xe decay Q-value of 2458 keV,the energy resolution of PandaX-Ⅱ is 4.2%.We find no evidence of NLDBD in PandaX-Ⅱand establish a lower limit for decay half-life of 2.1×1023yr at the 90% confidence level,which corresponds to an effective Majorana neutrino mass mββ <（1.4-3.7） eV.This is the first NLDBD result reported from a dual-phase xenon experiment.

Transition edge sensor-based detector:from X-ray to γ-ray

A transition edge sensor(TES)is extremely sensitive to changes in temperature,and combined with a high-Z metal of a certain thickness,it can realize high-energy resolution measurements of particles such as X-rays.X-rays with energies below 10 keV have a weak penetrating ability,hence,only gold or bismuth of a few micrometers in thickness can guarantee a quantum efficiency higher than 70%.Therefore,the entire structure of the TES X-ray detector in this energy range can be realized using a microfabrication process.However,for X-rays or γ-rays from 10 keV to 200 keV,submillimeter absorber layers are required,which cannot be realized using the microfabrication process.This paper first briefly introduces a set of TES X-ray detectors and their auxiliary systems,and then focuses on the introduction of the TES γ-ray detector with an absorber based on a submillimeter lead-tin alloy sphere.The detector achieved a quantum efficiency above 70% near 100 keV and an energy resolution of approximately 161.5 eV at 59.5 keV.

Panda X-III： Searching for neutrinoless double beta decay with high pressure ^(136)Xe gas time projection chambers

Searching for the neutrinoless double beta decay(NLDBD)is now regarded as the topmost promising technique to explore the nature of neutrinos after the discovery of neutrino masses in oscillation experiments.Panda X-III(particle and astrophysical xenon experiment III)will search for the NLDBD of136Xe at the China Jin Ping Underground Laboratory(CJPL).In the first phase of the experiment,a high pressure gas Time Projection Chamber(TPC)will contain 200 kg,90%136Xe enriched gas operated at10 bar.Fine pitch micro-pattern gas detector(Microbulk Micromegas)will be used at both ends of the TPC for the charge readout with a cathode in the middle.Charge signals can be used to reconstruct the electron tracks of the NLDBD events and provide good energy and spatial resolution.The detector will be immersed in a large water tank to ensure～5 m of water shielding in all directions.The second phase,a ton-scale experiment,will consist of five TPCs in the same water tank,with improved energy resolution and better control over backgrounds.

Search for electron-antineutrinos associated with gravitational-wave events GW150914,GW151012,GW151226,GW170104,GW170608,GW170814,and GW170817 at Daya Bay

The establishment of a possible connection between neutrino emission and gravitational-wave(GW)bursts is important to our understanding of the physical processes that occur when black holes or neutron stars merge.In the Daya Bay experiment,using the data collected from December 2011 to August 2017,a search was per-formed for electron-antineutrino signals that coincided with detected GW events,including GW150914,GW151012,GW151226,GW170104,GW170608,GW 170814,and GW 170817.We used three time windows of±10,±500,and±1000 s relative to the occurrence of the GW events and a neutrino energy range of 1.8 to 100 MeV to search for correlated neutrino candidates.The detected electron-antineutrino candidates were consistent with the expected background rates for all the three time windows.Assuming monochromatic spectra,we found upper limits(90%confidence level)of the electron-antineutrino fluence of(1.13-2.44)×10^(11)cm^(-2)at 5 MeV to 8.0×10^(7)cm^(-2)at 100 MeV for the three time w indows.Under the assumption of a Fermi-Dirac spectrum,the upper limits were found to be(5.4-7.0)×10^(9)cm^(2)for the three time windows.

Development of a 6D Kalman filter for charged particle tracking in time projection chamber without magnetic field

Background Track reconstruction is necessary for time projection chamber(TPC),because TPCs usually face the measure-ment error that impedes gaining precise spacial and angular resolution.Purpose Kalman filter is a well-performed and applicable algorithm to denoise and reconstruct the event track.Methods In this paper,we develop a six-dimensional Kalman filter to reconstruct the particle track in high-energy physics experiments,while the most common form of Kalman filter used in many research fields is four-dimensional.The modelisation is based on a gaseous TPC,and the whole reconstruction process is first tested by a toy Monte Carlo simulation.Results The results show the Kalman filter can effectively reduce the noise and improve the detector resolution.Then,the performance of the Kalman filter is also verified with the data produced by the Geant4 toolkit.

Unambiguously Resolving the Potential Neutrino Magnetic Moment Signal at Large Liquid Scintillator Detectors

Non-vanishing electromagnetic properties of neutrinos have been predicted by many theories beyond the Standard Model, and an enhanced neutrino magnetic moment can have profound implications for fundamental physics. The XENON1T experiment recently detected an excess of electron recoil events in the 1–7 keV energy range, which can be compatible with solar neutrino magnetic moment interaction at a most probable value of μ_(v) = 2.1 × 10^(-11)μ_(B).However, tritium backgrounds or solar axion interaction in this energy window are equally plausible causes.Upcoming multi-tonne noble liquid detectors will test these scenarios more in depth, but will continue to face similar ambiguity. We report a unique capability of future large liquid scintillator detectors to help resolve the potential neutrino magnetic moment scenario. With O(100) kton·year exposure of liquid scintillator to solar neutrinos, a sensitivity of μ_(v) < 10^(-11)μ_(B) can be reached at an energy threshold greater than 40 keV, where no tritium or solar axion events but only neutrino magnetic moment signal is still present.

A gaseous time projection chamber with Micromegas readout for low-radioactive material screening

Purpose Low-radioactive material screening is becoming essential for rare event search experiments,such as neutrinoless double beta decay and dark matter searches in underground laboratories.A gaseous time projection chamber(TPC)can be used for such purposes with large active areas and high efficiency.Methods A gaseous TPC with a Micromegas readout plane of approximately 20×20 cm^(2)is successfully constructed for surface alpha contamination measurements.Results We have characterized the energy resolution,gain stability,and tracking capability with calibration sources.Conclusion With the unique track-related background suppression cuts of the gaseous TPC,we have established that the alpha background rate of the TPC is(0.13±0.03)×10^(−6)Bq/cm^(2),comparable to the leading commercial solutions.

Signal-background discrimination with convolutional neural networks in the PandaX-Ⅲ experiment using MC simulation

The PandaX-Ⅲ experiment will search for neutrinoless double beta decay of 136Xe with high pressure gaseous time projection chambers at the China Jin-Ping underground Laboratory. The tracking feature of gaseous detectors helps suppress the background level, resulting in the improvement of the detection sensitivity. We study a method based on the convolutional neural networks to discriminate double beta decay signals against the background from high energy gammas generated by 214Bi and 2^208 T1 decays based on detailed Monte Carlo simulation. Using the 2-dimensional projections of recorded tracks on two planes, the method successfully suppresses the background level by a factor larger than 100 with a high signal efficiency. An improvement of 62% on the efficiency ratio of Еs/√Еb is achieved in comparison with the baseline in the PandaX-Ⅲ conceptual design report.

An improved evaluation of the neutron background in the PandaX-Ⅱ experiment

In dark matter direct detection experiments,neutron is a serious source of background,which can mimic the dark matter-nucleus scattering signals.In this paper,we present an improved evaluation of the neutron background in the PandaX-II dark matter experiment by a novel approach.Instead of fully relying on the Monte Carlo simulation,the overall neutron background is determined from the neutron-induced high energy signals in the data.In addition,the probability of producing a dark-matter-like background per neutron is evaluated with a complete Monte Carlo generator,where the correlated emission of neutron(s)andγ(s)in the(α,n)reactions and spontaneous fissions is taken into consideration.With this method,the neutron backgrounds in the Run 9(26-ton-day)and Run 10(28-ton-day)data sets of PandaX-II are estimated to be(0.66±0.24)and(0.47±0.25)events,respectively.

Improved measurement of the reactor antineutrino flux and spectrum at Daya Bay

A new measurement of the reactor antineutrino flux and energy spectrum by the Daya Bay reactor neutrino experiment is reported. The antineutrinos were generated by six 2.9 GWth nuclear reactors and detected by eight antineutrino detectors deployed in two near（560 m and 600 m flux-weighted baselines） and one far（1640 m flux-weighted baseline） underground experimental halls. With 621 days of data, more than 1.2 million inverse beta decay（IBD） candidates were detected. The IBD yield in the eight detectors was measured, and the ratio of measured to predicted flux was found to be 0.946±0.020（0.992±0.021） for the Huber＋Mueller（ILL＋Vogel） model. A 2.9σ deviation was found in the measured IBD positron energy spectrum compared to the predictions. In particular, an excess of events in the region of 4–6 MeV was found in the measured spectrum, with a local significance of 4.4σ. A reactor antineutrino spectrum weighted by the IBD cross section is extracted for model-independent predictions.

Antineutrino energy spectrum unfolding based on the Daya Bay measurement and its applications

The prediction of reactor antineutrino spectra will play a crucial role as reactor experiments enter the precision era.The positron energy spectrum of 3.5 million antineutrino inverse beta decay reactions observed by the Daya Bay experiment,in combination with the fission rates of fissile isotopes in the reactor,is used to extract the positron energy spectra resulting from the fission of specific isotopes.This information can be used to produce a precise,data-based prediction of the antineutrino energy spectrum in other reactor antineutrino experiments with different fission fractions than Daya Bay.The positron energy spectra are unfolded to obtain the antineutrino energy spectra by removing the contribution from detector response with the Wiener-SVD unfolding method.Consistent results are obtained with other unfolding methods.A technique to construct a data-based prediction of the reactor antineutrino energy spectrum is proposed and investigated.Given the reactor fission fractions,the technique can predict the energy spectrum to a 2%precision.In addition,we illustrate how to perform a rigorous comparison between the unfolded antineutrino spectrum and a theoretical model prediction that avoids the input model bias of the unfolding method.

Enhanced search sensitivity to the double beta decay of ^(136)Xe to excited states with topological signatures

The double beta decay of ^(136)Xe to excited states of 136Ba(DBD-ES)has not yet been discovered experimentally.The experimental signature of such decays,one or two gamma rays following the beta signals,can be identified more effectively in a gaseous detector with the help of topological signatures.We have investigated key parameters of particle trajectories of DBD-ES with Monte Carlo simulation data of the proposed PandaX-III detector as an example.The background rates can be reduced by about one order of magnitude while keeping more than half of signals with topological analysis.The estimated half-life sensitivity of DBD-ES can be improved by 1.8 times to 4.1×10^(23) year(90%C.L.).Similarly,the half-life sensitivity of neutrinoless double beta decay of ^(136)Xe to excited states of 136Ba can be improved by a factor of 4.8 with topological signatures.