New measurement of the relative scintillation efficiency of xenon nuclear recoils below 10 keV

Abstract

Liquid xenon is an important detection medium in direct dark matter experiments, which search for low-energy nuclear recoils produced by the elastic scattering of WIMPs with quarks. The two existing measurements of the relative scintillation efficiency of nuclear recoils below 20 keV lead to inconsistent extrapolations at lower energies. This results in a different energy scale and thus sensitivity reach of liquid xenon dark matter detectors. We report a new measurement of the relative scintillation efficiency below 10 keV performed with a liquid xenon scintillation detector, optimized for maximum light collection. Greater than 95% of the interior surface of this detector was instrumented with photomultiplier tubes, giving a scintillation yield of 19.6 photoelectrons/keV electron equivalent for 122-keV γ rays. We find that the relative scintillation efficiency for nuclear recoils of 5 keV is 0.14, staying constant around this value up to 10 keV. For higher energy recoils we measure a value of 0.21, consistent with previously reported data. In light of this new measurement, the XENON10 experiment's upper limits on spin-independent WIMP-nucleon cross section, which were calculated assuming a constant 0.19 relative scintillation efficiency, change from 8.8×10-44 cm2 to 9.9×10-44 cm2 for WIMPs of mass 100 GeV/c2, and from 4.5×10-44 cm2 to 5.6×10-44 cm2 for WIMPs of mass 30 GeV/c2.

Liquid noble gas detectors for low energy particle physics

“…Most experimental measurements of the scintillation efficiency for nuclear recoils to date were carried out with different setups but using the same method -elastic scattering of mono-energetic ∼MeV neutrons off a LXe target [61,123,124,125,126,127,128]. The scattered neutrons are detected at a fixed angle, thus allowing the energy transferred to Xe atoms to be determined kinematically.…”

“…Figure 12. Energy-dependent relative scintillation yield for nuclear recoils in liquid xenon; data from monoenergetic neutron beam measurements are labelled by the following markers: ( ) [61]; (•) [127]; ( ) [126]; ( ) [123]; ( ) [124]; (•) [128]; ( ) [125]. Indirect measurements from Monte Carlo: ( ) [66]; the two lines are the indirect measurements obtained from the first and second runs of ZEPLIN-III, with 68% CL bands shown in green (\\\) and blue (///), respectively [67].…”

Singlet-Doublet Dark Matter

“…Based on the fact that the σ SI limits have improved by approximately a factor of 10 between XENON10 [30] and XENON100, we use the XENON10 spin-dependent limits to project that the σ SD limits will be O (10 −3 pb) and O (10 −2 pb) for scattering off of neutrons SD for m ν 1 200 GeV (for higher masses, the significantly stronger limits from IceCube become relevant). While at first glance it may appear that much of the parameter space is out of the reach of both present or near future direct detection, it is important to consider the correlation between σ SI and σ SD .…”

WIMP astronomy and particle physics with liquid-noble and cryogenic direct-detection experiments

“…Energy errors for the current germanium-based experiments are negligible for parameter-estimation purposes [43], but are potentially a major issue for liquid-noble experiments. For example, there is currently a large systematic error on the inferred nuclear recoil energies based on the scintillation light observed in xenon-based experiments [24,[73][74][75][76]. Experiments are underway to better characterize the relation between the energy seen in experiments and the nuclear recoil spectrum, so it is likely that the energy resolution, systematics, and background sources will be far better characterized in the future than they are now.…”