Enhancement of NEST capabilities for simulating low-energy recoils in liquid xenon

Abstract

The Noble Element Simulation Technique (NEST) is an exhaustive collection of models explaining both the scintillation light and ionization yields of noble elements as a function of particle type (nuclear recoil, electron recoil, alphas), electric field, and incident energy or energy loss (dE/dx). It is packaged as C++ code for Geant4 that implements said models, overriding the default model which does not account for certain complexities, such as the reduction in yields for nuclear recoils (NR) compared to electron recoils (ER). We present here improvements to the existing NEST models and updates to the code which make the package even more realistic and turn it into a more full-fledged Monte Carlo simulation. All available liquid xenon data on NR and ER to date have been taken into consideration in arriving at the current models. Furthermore, NEST addresses the question of the magnitude of the light and charge yields of nuclear recoils, including their electric field dependence, thereby shedding light on the possibility of detection or exclusion of a low-mass dark matter WIMP by liquid xenon detectors.

Tritium calibration of the LUX dark matter experiment

“…At low energy beta particles and Compton electrons will lead to similar track lengths and are expected to produce similar event characteristics [11]. The comparison of Fig.…”

“…83m Kr, which emits 9.4 keV and 32.1 keV internal conversion electrons, is well suited for routine monitoring and for correcting the spatial and temporal variations of the S1 and S2 signals, but is less useful for studies of the S2/S1 ER discrimination variable because both conversion electrons are above the dark matter energy range, and because the S2 signals from the two electrons generally overlap in the detector due to the short half-life of the intermediate state (154 ns). We note that the most important background in LUX is due to Compton scatters, and such events are expected to have similar properties to beta decays in the tritium energy range [11].…”

“…The result is shown for 180 V/cm and 105 V/cm in Fig. 7, along with NEST v0.98 model predictions at each field [11]. For these plots a small correction has been applied to the data to account for smearing of tritium events across energy bins due to the energy resolution and the spectral shape [22] [23].…”

“…It is worth noting that Refs. [24] and [5] use Compton scatters as the source of ER events, while in tritium data the ER source is a [11]. The NEST curves are solid red and dashed green for 180 and 105 V/cm respectively, with triangle markers spaced every one keV.…”

X-ray ionization yields and energy spectra in liquid argon

“…In particular in liquid Xe, at further energy decrease a minimum is predicted first, and then the growth of the yield [19], [20]. Moreover, such a behaviour is definitely observed in liquid Ar when the higher energy data are combined with those of the lower energy: see Fig.…”