First dark matter search results from Coherent CAPTAIN-Mills

This paper describes the operation of the Coherent CAPTAIN-Mills (CCM) detector located at the Los Alamos Neutron Science Center at Los Alamos National Laboratory. CCM is a 10-ton liquid argon detector located 20 meters from a high flux neutron/neutrino source and is designed to search for sterile neutrinos (${\nu }_s$’s) and light dark matter (LDM). An engineering run was performed in fall 2019 to study the characteristics of the CCM120 detector by searching for coherent scattering signals consistent with ${\nu }_s$’s and LDM resulting from the production and decays of ${\pi }^{+}$ and ${\pi }^0$ in the tungsten target. New parameter space in a leptophobic dark matter (DM) model was excluded for DM masses between $\sim 2.0$ and 30 MeV. The lessons learned from this run have guided the development and construction of the new CCM200 detector that will begin operations in 2021 and significantly improve on these searches.

Figure 1

An 800-MeV proton beam strikes a tungsten target that produces (a) charged pions that decay into muon neutrinos, and (b) neutral pions that may decay into sub-GeV dark matter $\chi$. The neutrinos and dark matter particles travel to the CCM detector where they are detected by (c) $\mathrm{CE}\nu \mathrm{NS}$ , (d) quasielastic nucleon scattering, or (e) elastic electron scattering. Active neutrinos from ${\nu }_{\mu }$ oscillations produce nuclear recoils ($<50\mathrm{keV}$), while ${\nu }_s$’s do not. Likewise, the LDM particles ($\chi$’s) will produce energetic nuclear recoils ($>50\mathrm{keV}$).

Figure 2

The time profile of an individual proton bunch incident on the Lujan tungsten production target as measured by a pickup coil placed just upstream of the target.

Figure 3

Energy spectra for individual neutrino species (left) resulting from ${\pi }^{+}$ and ${\mu }^{+}$ decays, and simulated time profiles (right) of the neutrinos from an ideal stopped-pion neutrino source.

Figure 4

The relative prompt neutrino and DM timing spectra are shown and compared to an exponential energy spectrum of neutrons for the case where the detector is placed 20 m from the Lujan target with a 280-ns-wide (at the base) beam. Time-of-flight measurements outside CCM120 determined the fastest neutrons were about 20 MeV. A beam time profile of 100 ns, a planned near-term PSR upgrade, would yield a region that is only populated by monoenergetic 30-MeV stopped-pion neutrinos and potential DM.

Figure 5

The CCM detector in the ER2 region of the LANSCE Lujan facility with the concrete/poly shielding around it.

Figure 6

The inside of the CCM120 detector. The 120 inner PMTs are placed around the cylinder barrel, 96 coated, 24 uncoated, and TPB painted reflective foils are also shown.

Figure 7

The ${\pi }^0$ (a) angular and (b) momentum distributions produced at the Lujan target assuming $\mathrm{POT}=2.71\times {10}^{21}$. The angular production is mostly isotropic, with a significant peak in the production parallel to the beam. The momentum distribution peaks between 100 and 120 MeV, with a mean momentum of 145 MeV. The CCM120 detector is 90° with respect to the beam direction.

Figure 8

The $\chi$ momentum distribution expected by the CCM detector, assuming $\mathrm{POT}=2.71\times {10}^{21}$ and (a) $m_{\chi }=10\mathrm{MeV}$ and (b) $m_{\chi }=50\mathrm{MeV}$, where the dependence on $ϵ$ has been removed.

Figure 9

The process for finding pulses from a PMT waveform. (Top row) The raw waveform coming from the CAEN digitizers. (Second row) Exponential average of the raw waveform. (Third row) Running average or smoothed waveform of the exponential average. (Bottom row) The derivative of the waveform after all smoothing. The start (green) and stop (red) of each pulse, after a 5-ADC integral threshold is applied, are marked to point out where single photoelectrons (SPEs) are observed. Note how the noise is reduced as subsequent smoothing techniques are applied.

Figure 10

Examples of an accumulated waveform where each PMT is represented by a different color (nine colors for 120 PMTs). The pulses from the PMTs are represented by a triangle with an area equal to the integral of the pulse. The threshold is indicated by a long-dashed line. The start, 90 ns, and end of an event are represented by dashed-dot-dot, dot, and dashed lines respectively. Only events that passed source calibration cuts have start, 90 ns, and end lines.

Figure 11

A comparison between with and without the $3\text{−}\mu \mathrm{Ci}$ ${}^{22}\mathrm{Na}$ or the $10\text{−}\mu \mathrm{Ci}$ ${}^{57}\mathrm{Co}$ is shown for sources in the center of the detector. (Lower) The excess event rate for each source.

Figure 12

The detector response to a nuclear recoil where (a) is the total efficiency as a function of the true recoil energy, and (b) is the smearing matrix from true recoil energy to reconstructed energy where each column is normalized so the sum in the color equals 1. Both distributions were constructed after DM selection criteria were applied.

Figure 13

The solid line shows the figure-of-merit calculation for the length cut. The dashed line is the efficiency for keeping nuclear recoil events and the dotted line is the efficiency for keeping electron recoil events from ${}^{39}\mathrm{Ar}$ decay. The long-dashed lines (horizontal) help to compare the efficiencies at the maximum figure of merit.

Figure 14

Comparing the flight path 3 (FP3) and CCM event turn-on to determine neutrino arrival time in CCM. (Top) The FP3 (solid thick) and CCM (dotted) accumulated waveforms as a function of time in the DAQ window. Both distributions are normalized to 1. (Bottom) The derivative of the accumulated waveforms. The FP3 turn-on is represented by the leftmost dot short-dashed line. The CCM turn-on is represented by the rightmost dot short-dashed line. The thin solid, dashed, and long-dashed lines show the beam pulse, $\nu$ from $\pi$-decay, and $\nu$ from $\mu$-decay respectively, each starting at the FP3 turn-on. LDM from ${\pi }^0$ decay, with a quick lifetime of $8.5\times {10}^{-8}\mathrm{ns}$, is represented by the beam pulse line.

Figure 15

Progression of event selection criteria for both (a) DAQ time and (b) reconstructed energy. The region to the left of the short-dashed vertical line is the beam-out-of-time region and the region between the thick long-dashed vertical lines is the beam ROI. The reconstructed energy plot is for the beam-out-of-time region only.

Figure 16

The top frame shows the reconstructed energy distribution after all selection criteria are applied. The background prediction based off the beam-out-of-time window is the shaded region, and measured data in the beam signal region of interest are the solid lines. The bottom frame shows the background subtracted distribution along with a blue line that is arbitrarily normalized to show the shape of the expected DM distribution. The thickness of the blue line shows the variation due to 383 different $m_V$, $m_{\chi }$ mass combinations.

Figure 17

For the vector portal model (top) the number of predicted DM events for $m_V=3m_{\chi }$, $m_V=5m_{\chi }$, and $m_V=7m_{\chi }$ represented by solid, dashed, and dotted lines respectively. The predicted mean (middle) and rms (bottom) PE distributions are displayed for each of the $m_V/m_{\chi }$ ratios shown on top. Each plot was made with ${ϵ}^4{\alpha }_D=5\times {10}^{-13}$, and after all selection criteria were applied.

Figure 18

The median sensitivity (dashed) and 90% confidence limit (solid) results from CCM120 compared to current limits. (a) Vector portal model with limits from previous neutrino experiments, LSND, E137, and MiniBooNE, are displayed to show current limits from other experiments that are sensitive to ${ϵ}^4{\alpha }_D$. See Refs. [24, 51, 52, 53, 54, 55, 56, 57, 58] for an explanation of the previous limits. (b) Leptophobic model. See Refs. [26, 37, 38, 39, 51, 52, 59] for an explanation of the previous limits.

Figure 19

CCM200 expected improvements in efficiency from simulations. The biggest effect is from the improved attenuation length from the clean argon. The pink (DM) and orange (${}^{39}\mathrm{Ar}$) lines are for clean/filtered LAr expected with CCM200, while the black (DM) and blue (${}^{39}\mathrm{Ar}$) lines are with the contaminated LAr in CCM120. When convoluting this with the expected spectrum, the CCM200 energy integrated DM efficiency is about 25% while only 1% for ${}^{39}\mathrm{Ar}$ decay.

Figure 20

The blue line is the CCM200 DM full fit sensitivity with 10-keV threshold for a three-year run ($2.25\times {10}^{22}\mathrm{POT}$) and is based on the most recent CCM200 simulation with clean LAr and improved instrumentation. The background model is taken from the measured CCM120 rates and shape that are adjusted for the effects of the assumed improvements. The orange CCM200 sensitivity line includes the effects of upgraded shielding. The pink line could be achieved by using isotopically pure LAr to further reduce the ${}^{39}\mathrm{Ar}$ backgrounds, or significant background rejection with improved analysis techniques. Limits from previous neutrino experiments are shown in Fig. 18.

Figure 21

Comparing the beam-out-of-time DAQ window rates from when the beam is on to when the beam is off. The beam-on rate is about 3.5 times higher than the beam-off rate.

Figure 22

The gamma-ray spectra for both beam-on and beam-off conditions were recorded near the CCM120 detector. The spectra were normalized to 17.25 hours of live time so comparisons could be made. Approximately 19 photopeaks were observed between 50 and 2700 keV with the same rate for beam on and beam off. However, above 2700 keV, the beam-on to beam-off ratio was $\sim 3$:1, consistent with the observations found in Fig. 21.