Physics reach of the XENON1T dark matter experiment

Abstract: The XENON1T experiment is currently in the commissioning phase at the Laboratori Nazionali del Gran Sasso, Italy. In this article we study the experiment's expected sensitivity to the spin-independent WIMP-nucleon interaction cross section, based on Monte Carlo predictions of the electronic and nuclear recoil backgrounds. The total electronic recoil background in $1$ tonne fiducial volume and ($1$, $12$) keV electronic recoil equivalent energy region, before applying any selection to discriminate between electronic and nuclear recoils, is $(1.80 \pm 0.15) \cdot 10^{-4}$ ($\rm{kg} \cdot day \cdot keV)^{-1}$, mainly due to the decay of $^{222}\rm{Rn}$ daughters inside the xenon target. The nuclear recoil background in the corresponding nuclear recoil equivalent energy region ($4$, $50$) keV, is composed of $(0.6 \pm 0.1)$ ($\rm{t} \cdot y)^{-1}$ from radiogenic neutrons, $(1.8 \pm 0.3) \cdot 10^{-2}$ ($\rm{t} \cdot y)^{-1}$ from coherent scattering of neutrinos, and less than $0.01$ ($\rm{t} \cdot y)^{-1}$ from muon-induced neutrons. The sensitivity of XENON1T is calculated with the Profile Likelihood Ratio method, after converting the deposited energy of electronic and nuclear recoils into the scintillation and ionization signals seen in the detector. We take into account the systematic uncertainties on the photon and electron emission model, and on the estimation of the backgrounds, treated as nuisance parameters. The main contribution comes from the relative scintillation efficiency $\mathcal{L}\mathrm{eff}$, which affects both the signal from WIMPs and the nuclear recoil backgrounds. After a $2$ y measurement in $1$ t fiducial volume, the sensitivity reaches a minimum cross section of $1.6 \cdot 10^{-47}$ cm$^2$ at m$\chi$=$50$ GeV/$c^2$.

• The paper demonstrates XENON1T’s improved physics reach with a sensitivity of 1.6×10⁻⁴⁷ cm² for 50 GeV/c² WIMPs over a 2-year exposure.
• It employs a liquid xenon TPC with enhanced light collection and advanced PMT technology to significantly reduce electronic and nuclear recoil backgrounds.
• The study paves the way for future dark matter searches, informing the design of the upgraded XENONnT experiment with increased target mass and improved sensitivity.

Analysis of the Physics Reach of the XENON1T Dark Matter Experiment

The XENON1T experiment is a pivotal endeavor in the field of direct dark matter detection, as detailed in the paper submitted to JCAP. With its development by the XENON collaboration, XENON1T aims to enhance the sensitivity to spin-independent Weakly Interacting Massive Particle (WIMP)-nucleon interactions. This research focuses on background predictions and the experiment's capability in detecting potential dark matter candidates.

The experiment utilizes a liquid xenon (LXe) time projection chamber (TPC) located at the Laboratori Nazionali del Gran Sasso, Italy. The detector's construction phase began in 2013, with first science runs expected around 2016. Its design marks a significant improvement over its predecessor, XENON100, by increasing the target mass by a factor of 32 and reducing background rates significantly.

Background and Sensitivity Analysis

Background Sources: The background predictions for XENON1T categorize into electronic (ER) and nuclear recoils (NR). ER backgrounds primarily arise from radioactive contamination within detector materials, intrinsic impurities in LXe, and solar neutrinos. The total ER background rate, assuming a uniform distribution of radon at 10 Bq/kg, comes out to approximately 1.80 × 10⁻⁴ kg⁻¹ day⁻¹ keV⁻¹. Meanwhile, NR backgrounds primarily stem from radiogenic and muon-induced neutrons and coherent neutrino-nucleus scattering (CNNS). CNNS emerges as a fundamental limitation due to its indistinguishability from WIMP signals.

Sensitivity Enhancement: A novel aspect of XENON1T is its improved light collection efficiency and photodetection capabilities, achieved through the incorporation of PTFE reflectors and enhanced photomultiplier tube (PMT) technology. The LXe response is carefully calibrated, incorporating advanced knowledge on the relative scintillation efficiency and electron yield. The predicted spin-independent WIMP-nucleon cross-section sensitivity reaches values as low as 1.6 × 10⁻⁴⁷ cm² at a WIMP mass of 50 GeV/c² after a 2-year exposure of 1 ton of fiducial volume.

Implications and Future Directions

The implications of XENON1T's sensitivity projections are profound, particularly in testing the parameter space models predicting WIMP dark matter. The projected significant reduction in background rates and enhanced sensitivity places XENON1T in a pivotal position among direct dark matter detection experiments.

As part of the experiment's future directions, the XENON collaboration plans an eventual upgrade to XENONnT, which could incorporate up to 7 tons of LXe. This upgrade forecasts an order of magnitude improvement in sensitivity, further mitigating the impact of intrinsic backgrounds.

In conclusion, XENON1T presents a robust framework for advancing our capabilities in detecting dark matter. Its design improvements illustrate the collaboration's strategy to push the boundaries of experimental physics closer to probing the most theoretically favorable regions for WIMP dark matter. As such, XENON1T stands as a formidable next step in the pursuit of understanding the dark sector of our universe. Future integrations and efforts will likely continue to center on enhancing detection efficiency, reducing background influences, and refining theoretical models to align with empirical evidence gathered by XENON1T and its future iterations.