Dark Matter Search Probes Xenon Excitation After LZ Event
A new study analyzes the potential for detecting inelastic xenon excitation signals in the LZ dark matter experiment, following its report of a high-energy
The LUX-ZEPLIN (LZ) dark matter experiment recently reported a single event consistent with a high-energy nuclear recoil of 248 keV, plus or minus 23 keV in statistical and systematic uncertainties. A new theoretical paper, published on the arXiv preprint server, investigates whether a companion signal from inelastic dark matter interactions could be seen in the same detector.
Researchers Shao-Song Tang and colleagues calculated the expected rates for a process where a dark matter particle, denoted as chi, strikes a xenon atom in the LZ detector and excites it to a higher energy state. This inelastic channel, written as χ + Xe → χ + Xe*, provides a complementary test for dark matter interpretations of the unusual LZ event.
Framework and Signal Simulation
The team used the non-relativistic effective field theory (NREFT) framework to model the interactions. They incorporated inputs from a nuclear shell model to describe the transitions within the xenon nucleus. The study simulated the detector's dual-phase time projection chamber response, specifically the S1 (primary scintillation) and S2 (ionization) signals, for these inelastic events. Their detector modeling was validated against publicly available LZ calibration data.
A key finding is that the nuclear de-excitation following the initial strike adds an electromagnetic component to the signal. This shifts the event's signature away from the typical nuclear-recoil band and toward the electronic-recoil band used to distinguish potential dark matter signals from background noise. In some scenarios, this shift could push the signal into regions with clustered backgrounds from radioactive isotopes.
Backgrounds and Statistical Analysis
Given this potential overlap with backgrounds, the authors constructed a simplified, physics-motivated background model for the extended-energy search region. They then performed a statistical analysis to assess the observability of the inelastic-xenon signal against this background. The analysis considered several representative dark matter scenarios, including models with inelastic dark matter and accelerated dark matter populations.
For many benchmark models tested, the rate of inelastic-xenon events was found to be on the order of 0.1 times the rate of elastic nuclear recoils, or even lower. The paper states, "substantially larger exposures are required before this companion channel becomes observable."
Implications for Future Searches
The results illustrate the significant challenge posed by background structures from isotopes in the extended-energy region of detectors like LZ. The study extends the theoretical phenomenology of inelastic-xenon signatures to a wider range of possible dark matter scenarios beyond the simplest models. It shows that while the inelastic channel offers a valuable independent check, current experimental exposures are likely insufficient to detect it, even if the high-recoil LZ event were caused by dark matter. The work provides a framework for future, more sensitive searches to include this complementary signature in their analyses.
