We present the results from combining machine learning with the profile likelihood fit procedure, using data from the Large Underground Xenon (LUX) dark matter experiment. This approach demonstrates ...reduction in computation time by a factor of 30 when compared with the previous approach, without loss of performance on real data. We establish its flexibility to capture non-linear correlations between variables (such as smearing in light and charge signals due to position variation) by achieving equal performance using pulse areas with and without position-corrections applied. Its efficiency and scalability furthermore enables searching for dark matter using additional variables without significant computational burden. We demonstrate this by including a light signal pulse shape variable alongside more traditional inputs such as light and charge signal strengths. This technique can be exploited by future dark matter experiments to make use of additional information, reduce computational resources needed for signal searches and simulations, and make inclusion of physical nuisance parameters in fits tractable.
Dual-phase xenon time projection chamber (TPC) detectors have demonstrated superior search sensitivities to dark matter over a wide range of particle masses. To extend their sensitivity to include ...low-mass dark matter interactions, it is critical to characterize both the light and charge responses of liquid xenon to sub-keV nuclear recoils. In this work, we report a new nuclear recoil calibration in the LUX detector \(\textit{in situ}\) using neutron events from a pulsed Adelphi Deuterium-Deuterium neutron generator. We demonstrate direct measurements of light and charge yields down to 0.45 keV (1.4 scintillation photons) and 0.27 keV (1.3 ionization electrons), respectively, approaching the physical limit of liquid xenon detectors. We discuss the implication of these new measurements on the physics reach of dual-phase xenon TPCs for nuclear-recoil-based low-mass dark matter detection.
We estimate the amount of \(^{37}\)Ar produced in natural xenon via cosmic ray-induced spallation, an inevitable consequence of the transportation and storage of xenon on the Earth's surface. We then ...calculate the resulting \(^{37}\)Ar concentration in a 10-tonne payload~(similar to that of the LUX-ZEPLIN experiment) assuming a representative schedule of xenon purification, storage and delivery to the underground facility. Using the spallation model by Silberberg and Tsao, the sea level production rate of \(^{37}\)Ar in natural xenon is estimated to be 0.024~atoms/kg/day. Assuming the xenon is successively purified to remove radioactive contaminants in 1-tonne batches at a rate of 1~tonne/month, the average \(^{37}\)Ar activity after 10~tonnes are purified and transported underground is 0.058--0.090~\(\mu\)Bq/kg, depending on the degree of argon removal during above-ground purification. Such cosmogenic \(^{37}\)Ar will appear as a noticeable background in the early science data, while decaying with a 35~day half-life. This newly-noticed production mechanism of \(^{37}\)Ar should be considered when planning for future liquid xenon-based experiments.
The projected sensitivity of the LUX-ZEPLIN (LZ) experiment to two-neutrino and neutrinoless double beta decay of \(^{134}\)Xe is presented. LZ is a 10-tonne xenon time projection chamber optimized ...for the detection of dark matter particles, that is expected to start operating in 2021 at Sanford Underground Research Facility, USA. Its large mass of natural xenon provides an exceptional opportunity to search for the double beta decay of \(^{134}\)Xe, for which xenon detectors enriched in \(^{136}\)Xe are less effective. For the two-neutrino decay mode, LZ is predicted to exclude values of the half-life up to 1.7\(\times\)10\(^{24}\) years at 90% confidence level (CL), and has a three-sigma observation potential of 8.7\(\times\)10\(^{23}\) years, approaching the predictions of nuclear models. For the neutrinoless decay mode LZ, is projected to exclude values of the half-life up to 7.3\(\times\)10\(^{24}\) years at 90% CL.
We report here the results of an Effective Field Theory (EFT) WIMP search analysis using LUX data. We build upon previous LUX analyses by extending the search window to include nuclear recoil ...energies up to \(\sim\)180 keV\(_{nr}\), requiring a reassessment of data quality cuts and background models. In order to use a binned Profile Likelihood statistical framework, the development of new analysis techniques to account for higher-energy backgrounds was required. With a 3.14\(\times10^4\) kg\(\cdot\)day exposure using data collected between 2014 and 2016, we set 90\% C.L. exclusion limits on non-relativistic EFT WIMP couplings to neutrons and protons, providing the most stringent constraints on a significant fraction of the possible EFT WIMP interactions. Additionally, we report world-leading exclusion limits on inelastic EFT WIMP-nucleon recoils.
We report here methods and techniques for creating and improving a model that reproduces the scintillation and ionization response of a dual-phase liquid and gaseous xenon time-projection chamber. ...Starting with the recent release of the Noble Element Simulation Technique (NEST v2.0), electronic recoil data from the β decays of 3H and 14C in the Large Underground Xenon (LUX) detector were used to tune the model, in addition to external data sets that allow for extrapolation beyond the LUX data-taking conditions. This paper also presents techniques used for modeling complicated temporal and spatial detector pathologies that can adversely affect data using a simplified model framework. The methods outlined in this report show an example of the robust applications possible with NEST v2.0, while also providing the final electronic recoil model and detector parameters that will used in the new analysis package, the LUX Legacy Analysis Monte Carlo Application (LLAMA), for accurate reproduction of the LUX data. As accurate background reproduction is crucial for the success of rare-event searches, such as dark matter direct detection experiments, the techniques outlined here can be used in other single-phase and dual-phase xenon detectors to assist with accurate ER background reproduction.
LUX-ZEPLIN (LZ) is a dark matter detector expected to obtain world-leading sensitivity to weakly interacting massive particles (WIMPs) interacting via nuclear recoils with a ~7-tonne xenon target ...mass. This manuscript presents sensitivity projections to several low-energy signals of the complementary electron recoil signal type: 1) an effective neutrino magnetic moment and 2) an effective neutrino millicharge, both for pp-chain solar neutrinos, 3) an axion flux generated by the Sun, 4) axion-like particles forming the galactic dark matter, 5) hidden photons, 6) mirror dark matter, and 7) leptophilic dark matter. World-leading sensitivities are expected in each case, a result of the large 5.6t 1000d exposure and low expected rate of electron recoil backgrounds in the \(<\)100keV energy regime. A consistent signal generation, background model and profile-likelihood analysis framework is used throughout.
We present a comprehensive analysis of electronic recoil vs. nuclear recoil discrimination in liquid/gas xenon time projection chambers, using calibration data from the 2013 and 2014-16 runs of the ...Large Underground Xenon (LUX) experiment. We observe strong charge-to-light discrimination enhancement with increased event energy. For events with S1 = 120 detected photons, i.e. equivalent to a nuclear recoil energy of \(\sim\)100 keV, we observe an electronic recoil background acceptance of \(<10^{-5}\) at a nuclear recoil signal acceptance of 50%. We also observe modest electric field dependence of the discrimination power, which peaks at a field of around 300 V/cm over the range of fields explored in this study (50-500 V/cm). In the WIMP search region of S1 = 1-80 phd, the minimum electronic recoil leakage we observe is \({(7.3\pm0.6)\times10^{-4}}\), which is obtained for a drift field of 240-290 V/cm. Pulse shape discrimination is utilized to improve our results, and we find that, at low energies and low fields, there is an additional reduction in background leakage by a factor of up to 3. We develop an empirical model for recombination fluctuations which, when used alongside the Noble Element Scintillation Technique (NEST) simulation package, correctly reproduces the skewness of the electronic recoil data. We use this updated simulation to study the width of the electronic recoil band, finding that its dominant contribution comes from electron-ion recombination fluctuations, followed in magnitude of contribution by fluctuations in the S1 signal, fluctuations in the S2 signal, and fluctuations in the total number of quanta produced for a given energy deposition.
Two-phase xenon detectors, such as that at the core of the forthcoming LZ dark matter experiment, use photomultiplier tubes to sense the primary (S1) and secondary (S2) scintillation signals ...resulting from particle interactions in their liquid xenon target. This paper describes a simulation study exploring two techniques to lower the energy threshold of LZ to gain sensitivity to low-mass dark matter and astrophysical neutrinos, which will be applicable to other liquid xenon detectors. The energy threshold is determined by the number of detected S1 photons; typically, these must be recorded in three or more photomultiplier channels to avoid dark count coincidences that mimic real signals. To lower this threshold: a) we take advantage of the double photoelectron emission effect, whereby a single vacuum ultraviolet photon has a \(\sim20\%\) probability of ejecting two photoelectrons from a photomultiplier tube photocathode; and b) we drop the requirement of an S1 signal altogether, and use only the ionization signal, which can be detected more efficiently. For both techniques we develop signal and background models for the nominal exposure, and explore accompanying systematic effects, including the dependence on the free electron lifetime in the liquid xenon. When incorporating double photoelectron signals, we predict a factor of \(\sim 4\) sensitivity improvement to the dark matter-nucleon scattering cross-section at \(2.5\) GeV/c\(^2\), and a factor of \(\sim1.6\) increase in the solar \(^8\)B neutrino detection rate. Dropping the S1 requirement may allow sensitivity gains of two orders of magnitude in both cases. Finally, we apply these techniques to even lower masses by taking into account the atomic Migdal effect; this could lower the dark matter particle mass threshold to \(80\) MeV/c\(^2\).
This paper presents a novel technique for mitigating electrode backgrounds that limit the sensitivity of searches for low-mass dark matter (DM) using xenon time projection chambers. In the LUX ...detector, signatures of low-mass DM interactions would be very low energy (\(\sim\)keV) scatters in the active target that ionize only a few xenon atoms and seldom produce detectable scintillation signals. In this regime, extra precaution is required to reject a complex set of low-energy electron backgrounds that have long been observed in this class of detector. Noticing backgrounds from the wire grid electrodes near the top and bottom of the active target are particularly pernicious, we develop a machine learning technique based on ionization pulse shape to identify and reject these events. We demonstrate the technique can improve Poisson limits on low-mass DM interactions by a factor of \(2\)-\(7\) with improvement depending heavily on the size of ionization signals. We use the technique on events in an effective \(5\) tonne\(\cdot\)day exposure from LUX's 2013 science operation to place strong limits on low-mass DM particles with masses in the range \(m_{\chi}\in0.15\)-\(10\) GeV. This machine learning technique is expected to be useful for near-future experiments, such as LZ and XENONnT, which hope to perform low-mass DM searches with the stringent background control necessary to make a discovery.
