Liquid xenon scintillation measurements and pulse shape discrimination in the LUX dark matter detector

Abstract

Weakly interacting massive particles (WIMPs) are a leading candidate for dark matter and are expected to produce nuclear recoil (NR) events within liquid xenon time-projection chambers. We present a measurement of the scintillation timing characteristics of liquid xenon in the LUX dark matter detector and develop a pulse shape discriminant to be used for particle identification. To accurately measure the timing characteristics, we develop a template-fitting method to reconstruct the detection times of photons. Analyzing calibration data collected during the 2013-2016 LUX WIMP search, we provide a new measurement of the singlet-to-triplet scintillation ratio for electron recoils (ER) below 46 keV, and we make, to our knowledge, a first-ever measurement of the NR singlet-to-triplet ratio at recoil energies below 74 keV. We exploit the difference of the photon time spectra for NR and ER events by using a prompt fraction discrimination parameter, which is optimized using calibration data to have the least number of ER events that occur in a 50% NR acceptance region. We then demonstrate how this discriminant can be used in conjunction with the charge-to-light discrimination to possibly improve the signal-to-noise ratio for nuclear recoils.

A measurement of the scintillation decay time constant of nuclear recoils in liquid xenon with the XMASS-I detector

“…The singlet fraction obtained in this work agrees with the results of Ref. [13,14], however, the τ T is about 5 ns longer than those values. The τ T discrepancy might stem from a time delay introduced by the recombination process, which is suppressed under an electric field.…”

“…While the recombination process has a longer decay time constant of more than 30 ns, measured with 1 MeV electrons from a 207 Bi source [10,11]. In the case of neutron induced NR events, the decay time constant of the triplet state was reported to be ∼20 ns both with an applied electric field (0.1-0.5 kV/cm) [13,14] and without [15,19]. The decay time constants of the singlet and triplet states depend weakly on the density of the excited species, whereas the ratio of the singlet to triplet state as well as the recombination time depends on the deposited energy density [20].…”

“…The scintillation timing information can be used for position reconstruction of an event in the detector [6] as well as for particle identification [7]. Studies on the scintillation process in LXe has been conducted in various experiments [8][9][10][11][12][13][14][15][16][17][18][19]. A scintillation photon is produced by two mechanisms.…”

Field dependence of electronic recoil signals in a dual-phase liquid xenon time projection chamber

“…This is most notable for low ionization density recoils and low electric fields [23]. Rather than constructing a detailed model of recombination, the scintillation pulse shape is usually described by an effective model, using a single exponential distribution [24,25] or absorbing the delay due to recombination into an effective lifetime τ eff t [26,27]. The normalized photon emission time distribution then becomes We use the model from equation 3.6 and fit this to the average pulse shape observed for all measured fields.…”

Multi-channel direct detection of light dark matter: theoretical framework

“…Direct detection has been playing a central role in the quest for the particle nature of dark matter (DM). Over the past few decades, tremendous progress has been made at a range of experiments focused on nuclear recoil signals, including ANAIS [1], CDMSlite [2], CRESST [3,4], DAMA/LIBRA [5], DAMIC [6], DM-Ice [7], KIMS [8], LUX [9], SABRE [10], SuperCDMS [11,12], and Xenon1T [13]. While these experiments have excluded much of the parameter space for DM heavier than roughly a GeV, much less is known about lighter DM.…”