Dark Matter Search Results from a One Tonne$\times$Year Exposure of XENON1T (1805.12562v2)

Abstract: We report on a search for Weakly Interacting Massive Particles (WIMPs) using 278.8 days of data collected with the XENON1T experiment at LNGS. XENON1T utilizes a liquid xenon time projection chamber with a fiducial mass of $(1.30 \pm 0.01)$ t, resulting in a 1.0 t$\times$yr exposure. The energy region of interest, [1.4, 10.6] $\mathrm{keV_{ee}}$ ([4.9, 40.9] $\mathrm{keV_{nr}}$), exhibits an ultra-low electron recoil background rate of $(82\substack{+5 \ -3}\textrm{ (sys)}\pm3\textrm{ (stat)})$ events/$(\mathrm{t}\times\mathrm{yr}\times\mathrm{keV_{ee}})$. No significant excess over background is found and a profile likelihood analysis parameterized in spatial and energy dimensions excludes new parameter space for the WIMP-nucleon spin-independent elastic scatter cross-section for WIMP masses above 6 GeV/c${}^2$, with a minimum of $4.1\times10^{-47}$ cm$^2$ at 30 GeV/c${}^2$ and 90% confidence level.

• The paper establishes a new upper limit of ~4.1×10⁻⁴⁷ cm² for 30 GeV/c² WIMPs at 90% confidence using profile likelihood analysis.
• It employs a two-phase liquid xenon TPC to effectively discriminate between nuclear and electronic recoils under ultra-low background conditions.
• Detailed calibration and an active muon veto enhance detection precision, setting the stage for future upgrades like the XENONnT experiment.

Dark Matter Search Results from a One Tonne-Year Exposure of XENON1T

The paper "Dark Matter Search Results from a One Tonne-Year Exposure of XENON1T" presents a comprehensive search for Weakly Interacting Massive Particles (WIMPs) using the XENON1T experiment, which is situated at LNGS. This research reports on results obtained from 278.8 days of data accumulation utilizing a liquid xenon (LXe) time projection chamber (TPC) with a fiducial mass of 1.3 tonnes, leading to a total exposure of approximately 1 tonne-year.

Methodology and Technology

XENON1T employs a meticulously engineered LXe TPC to detect both scintillation and ionization signals resulting from particle interactions. The detector encompasses a 2-tonne active LXe volume within a PTFE-lined cylindrical chamber, precisely monitored by arrays of Hamamatsu R11410-21 photo-multiplier tubes (PMTs). Particle interactions yield prompt scintillation photons (S1 signal) and separated ionization electrons, the latter of which, once extracted into gaseous xenon, generate secondary scintillation light (S2 signal) through electroluminescence.

This two-phase TPC enables significant discrimination capability between nuclear recoils (indicative of WIMP interactions) and electronic recoils (primarily from beta and gamma interactions), based on the ratio of S2 to S1 signal sizes. A crucial element of this setup is the active water Cherenkov muon veto system that surrounds the detector, ensuring further background suppression.

Data Collection and Calibration

The research encompasses data from two distinct science runs, SR0 and SR1, with a notable cathode voltage-induced drift field differentiation. The XENON1T experiment meticulously calibrates signal yields using various internal and external radioactive sources, such as krypton and radon isotopes, as well as neutron generators, ensuring precise model validation and accurate event reconstruction.

Results and Analysis

The analysis confirms no observable excess in terms of background in the energy region of interest [1.4, 10.6] keV (ER energy) or [4.9, 40.9] keV (NR energy). The background levels were found to be extraordinarily low at +5 events/tonne/day. A profile likelihood analysis on the spatial and energy parameter space refines the constraints on WIMP-nucleon spin-independent elastic scattering, achieving an upper limit of approximately $4.1 \times 10^{-47}$ cm$^2$ for a 30 GeV/c$^2$ WIMP at a 90% confidence level.

Interpretations and Implications

These findings advance the constraints on WIMP properties, meaningfully narrowing the parameter space for dark matter candidates. The advancement in detection sensitivity is attributed to low background rates and improved event detection efficiencies, proving the efficacy of LXe TPC technology in establishing limits on WIMP interactions with baryonic matter.

Future Directions

The research underlines the potential enhancement of dark matter detection sensitivity with the forthcoming XENONnT upgrade, scheduled to increase the target mass to approximately 5.9 tonnes. The implications of these developments extend beyond present physical theories, encouraging theoretical innovations and fostering new experimental pursuits in the field of dark matter research.

This paper exemplifies a rigorous approach to investigating dark matter, demonstrating the alignment between experimental innovations and foundational astrophysical questions. By setting new stringent limits on WIMP cross-sections, this research affirms the role of large-scale LXe detectors as cornerstones in unraveling the enigmatic nature of dark matter.