Session C12: Dark Matter II

The Expected Signal Response of Neutron-Induced Migdal Effect in the LZ Experiment

The Migdal effect is an atomic process in which electrons are excited or ionized due to the perturbation of the electron cloud from a recoiling nucleus. In noble-liquid direct-detection dark matter experiments, this can be utilized to improve sensitivity to low-mass dark matter through the elevated signal response of the primary nuclear recoil (NR) due to the secondary electronic recoil (ER) of the Migdal effect. To test the current theoretical predictions of this pathology, the LUX-ZEPLIN (LZ) dark matter experiment has used a deuterium-deuterium (DD) neutron generator to produce a high rate of NRs via monoenergetic 2.45 MeV neutrons. We construct a model for the combined response of the secondary ERs with the standard NRs of neutron-xenon interactions in the LZ detector using GEANT and NEST. In this talk, I will review the signal model used for this rare-event search, and outline the relevant backgrounds in the Migdal effect region of interest.   

Migdal Search in LUX-ZEPLIN Dark Matter Experiment

When a xenon atom's nucleus recoils from a dark matter particle or any other incident radiation, the atom's electron cloud is expected to fall behind, resulting in possible ionization and excitation. This phenomenon is called the Migdal effect and is attracting attention as it can improve the sensitivity of direct dark matter search in the sub-GeV/c2 regime. In a liquid xenon detector like LUX-ZEPLIN Experiment, it is expected that the inelastic component from the Migdal effect enhances the nuclear recoil signals. To search for such enhancement, monoenergetic 2.45 MeV neutrons from Adelphi Technologies' DD 109 neutron generator were used to make high-rate nuclear recoil events. In this talk, I will discuss our efforts to confirm and calibrate the Migdal effect using neutron-xenon scattering data from the LUX-ZEPLIN Experiment, focusing on the Migdal effects with electrons emitted from L and M shells.   

Experimental search for the Migdal Effect in a compact liquid xenon TPC

Direct dark matter searches have reported dramatically increased sensitivity to sub-GeV parameter space by taking into account the "Migdal Effect": a predicted inelastic process in which a particle scattering with a nucleus occasionally ejects a bound electron from the recoiling atom. The resulting atomic relaxation can emit energy at the ~keV level even for very low-energy nuclear recoils, enabling the detection of nuclear scattering events with recoil energies well below the detectors' ionization or scintillation thresholds. However, the Migdal Effect has never been experimentally observed in a scattering process, and should be confirmed and characterized before a potential dark matter signal in this channel can be reliably discovered. In this talk, we report on a dedicated experimental campaign to search for the Migdal Effect using neutron scattering in a small liquid xenon detector at Lawrence Livermore National Laboratory. Scattered neutrons (14.1 MeV incident energy) are detected by a ring of liquid scintillator detectors placed at an angle of 15 degrees from the neutron beam axis, resulting in a 300,000-event sample of 7-9 keV nuclear recoils in the liquid xenon. We make use of the two-dimensional ionization vs. scintillation phase space to search for nuclear recoil events with an electronic recoil component consistent with atomic excitation from the Migdal Effect.   

Background Modeling for the First Science Run of the LUX-ZEPLIN Dark Matter Experiment

LUX-ZEPLIN (LZ) is a dark matter experiment located at the Sanford Underground Research Facility in South Dakota, USA employing a 7 tonne active volume of liquid xenon in a dual-phase time projection chamber (TPC). It’s surrounded by an instrumented xenon “skin” region and a gadolinium-loaded liquid scintillator outer detector all contained within an ultra-pure water tank, which primarily serve as active vetoes for gamma-ray and neutron backgrounds, respectively. A comprehensive material assay and selection campaign for detector components, along with a xenon purification campaign, have further ensured an ultra-low background environment. These mitigations have allowed LZ to achieve a background rate of (6.3 ± 0.5) x 10⁻⁵ events/kg/day/keVee in the low energy region (< 15 keVee), approximately 60 times lower than that of its predecessor LUX experiment. In this low background region, LZ has recently set new world-leading limits for the spin-independent elastic scattering of nuclear recoils of Weakly Interacting Massive Particles (WIMPs) with masses above 9 GeV/c² using an exposure of 60 live days and a fiducial mass of 5.5 tonnes. This talk will provide an overview of the backgrounds present in the detector with a specific emphasis on the background model used in LZ’s first results and how these backgrounds were constrained in situ. 

    

The LUX-ZEPLIN Outer Detector PMT System and its performance during Science Run I

The LUX-ZEPLIN (LZ) experiment is centered on a liquid xenon time projection chamber (LXe-TPC) searching for nuclear recoils induced by interactions with Weakly Interacting Massive Particles (WIMPs). One of the most important backgrounds is neutrons, as they also result in nuclear recoils in the TPC. Surrounding the liquid xenon cryostat is an Outer Detector veto system with the primary aim of vetoing neutron single-scatter events in the liquid xenon. The Outer Detector consists of approximately 17t of gadolinium-loaded liquid scintillator confined to acrylic tanks surrounding the cryostat and 238t of high purity water as the outermost layer. The volume is monitored by 120 Hamamatsu R5912 photomultiplier tubes (PMTs). The PMTs are calibrated and monitored using an Optical Calibration System (OCS) which is situated within the array. I will present an overview of the two systems and a review of their performance during LZ's first science run. I will also describe the operating parameters chosen during commissioning and the impact of environmental interference on the PMT system.   

The LUX-ZEPLIN Skin Detector

The LUX-ZEPLIN (LZ) experiment is a dark matter detector centered on a dual-phase xenon time projection chamber operating at the Sanford Underground Research Facility in Lead, South Dakota, USA. Immediately surrounding LZ's 7-ton TPC is a 2-ton liquid xenon 'skin'. This acts as an HV standoff between the TPC and cryostat, and is instrumented with 131 PMTs to act as an anticoincidence detector for additional background rejection. The skin is a powerful tool to understand and characterize LZ's backgrounds. In particular, the atomic de-excitations following Xe127 electron capture (EC) decays in the TPC can be tagged when the associated gamma(s) deposit energy in the skin. This allows us to confirm LZ's sensitivity down to 1keV, and inform our modeling of Xe124 ECEC decays, another background of xenon detectors such as LZ and one of the rarest decays ever measured.   

Single Photoelectron Detection Efficiency in the LUX-ZEPLIN Experiment's PMT Systems

The LUX-ZEPLIN (LZ) experiment is a dark matter detector deployed at the Sanford Underground Research Facility in Lead, South Dakota. LZ is instrumented with 745 photomultiplier tubes (PMTs) in three sub-detectors. To be sensitive to low-energy nuclear recoil (NR) events, LZ requires a high single photoelectron (SPHE) detection efficiency in the liquid xenon time projection chamber (TPC). We characterize the SPHE detection efficiency using an LED calibration system. These calibrations are vital to monitoring the health of PMTs and the LZ signal chain.

During commissioning, the high SPHE detection efficiency of the as-installed LZ experiment allowed us to explore reducing PMT gain. These studies indicated that the low-noise environment of the signal chain permitted a reduction of PMT gain while maintaining sensitivity to our science objectives. It was found that reducing PMT gain allowed us to resolve pathological behavior exhibited by some PMTs.

In this work, we motivate our study of SPHE detection efficiency by quantifying the impact on low-energy NR events. We present an overview of how LZ's SPHE detection efficiency is measured. Finally, we show the time evolution of SPHE detection efficiency in each of LZ's sub-detectors.   

Application of a novel method to characterize the performance of zero suppression in the LUX-ZEPLIN experiment

The LUX-ZEPLIN (LZ) experiment, located at Sanford Underground Research Facility, is a direct detection dark matter experiment optimized to search for WIMPs. The LZ data acquisition (DAQ) system digitizes signals from 745 PMTs in three detector volumes. Zero suppression (ZS) is implemented to remove periods containing only baseline noise. To verify ZS we used a novel DAQ operation mode, for which both the raw and ZS waveforms are captured simultaneously.

 

Three independent studies were carried out to evaluate the performance of ZS: 1) The bit consistency is checked by comparing each sample of the ZS data with the raw waveforms. 2) The zero suppression algorithm implemented on the FPGA is simulated in Python, applied to the raw waveforms, and compared with the ZS data. 3) Python-based tools are utilized to look for peaks in the raw waveforms, and the results of this analysis are used to validate the ZS algorithm.

This DAQ mode provides the powerful ability to carry out cross-checks on real data with the hardware in-situ. These studies were extremely valuable while commissioning the LZ DAQ and are used biannually to monitor the performance of the signal and DAQ chain. We will present details of these studies and their results.   

Cold trap mass spectrometry techniques and application for the LUX-ZEPLIN experiment

I explore the methods, usage, and contributions of cold trap mass spectrometry sampling systems to the LZ (LUX-ZEPLIN) liquid xenon dark matter experiment. Mass spectrometry sampling can reveal information about the purity of the xenon in the LZ experiment. Adding a cold trap to common mass spectrometry techniques can improve sensitivity, enabling us to see krypton at a level of tens of parts per quadrillion (ppq) i.e. one part in 10^14 (g/g). The technique has been used to determine a krypton concentration of 144 +/- 22 ppq in the LZ detector immediately after being filled in September of 2021. I will review this and other measurements made by cold trap mass spectrometry sampling systems for the LZ experiment in this talk.