DarkSide-LowMass

Technical Drawing of the DarkSide-LowMass by undergraduate student Sarah Howick

DarkSide-LowMass (DS-LM): DS-LM is a 1.5 tonne dual-phase LAr TPC viewed by two photosensor arrays in a cryostat filled with underground argon (UAr). LAr is transparent to electrons and photons and can be made exceptionally pure, allowing LAr TPCs to be scaled to large sizes needed to search for dark matter (DM) down to the limit where coherent nuclear scattering of solar neutrinos becomes a limiting background (the "neutrino fog")

DS-LM aims to enhance S2 and minimize backgrounds that produce <3 keV electron-equivalent signals, optimized for electron-counting analyses. The inner detector is a dual-phase TPC with ultra-pure acrylic and UAr depleted of 39Ar (using Aria -- the world's tallest distillation column (350 m-tall, taller than the Eiffle Tower), in Sardinia, Italy).

When DM scatters in the UAr, recoiling nuclei ionize nearby atoms. Ionizatoin electrons drift in an electric field to a gas pocket at the top and accelerates them through GAr, producing a measurable electroluminescence signal (S2). This light is detected by two arrays of silicon photomultipliers (SiPMs): one 10 cm above the TPC and one 10 cm below. This system is immersed in a 170 cm-diameter stainless steel cryostat filled with UAr.

A two-fold veto system surrounds the TPC, providing anti-coincidence signals and passive buffers. The two dominate γ-ray sources are the cryostat stainless steel and the PDM electronics. When γ-rays come from these sources, they often first scatter in the UAr buffers that separate from the TPC, allowing them to be tagged. The 10 cm distance between each plane and the vessel allows for cm-scale spatial resolution and plays the role of a buffer for the associated hardware and the γ-rays emitted by the PDMs. PDMs on the walls of the 4.5t cryostat act as another γ-ray veto and have a 28cm buffer to shield against γ-rays from the cryostat and their backgrounds. In order to shield against external radiation, the entire 4.5t cryostat is submerged in a 8-10 cm diameter water tank with the ability to detect Cherenkov light if cosmic-ray veto is needed.

[Sensitivity projections for a dual-phase argon TPC optimized for light dark matter searches throughout the ionization channel. https://journals.aps.org/prd/abstract/10.1103/PhysRevD.107.112006]

Top: Illustrates DM spectra for masses of 2.5, 5, and 10 GeV/c2. It also compares backgrounds after selection cuts from Darkside-50 to those from 39Ar, CEνNS, and γ-rays. Depending on the final 39Ar activitiy, DS-LM can reach ultra-low level backgrounds that are dominated by coherent scattering of solar neutrinos, an exciting benchmark that has never before been reached. This estimate does not include contributions from spurious electron backgrounds, which appear in S2-only searches like DS-LM will perform, and are poorly understood. These will likely dominate the lowest few Ne- bins of the spectrum but require more R&D to understand and mitigate. This is one of the exciting R&D topics we are now exploring in the lab. We are also looking at other ways, such as doping LAr, to increase the number of electrons we detect after some energy is deposited.

Bottom: Illustrates the upper limits of a median 90% confidence level and 1σ expectation band based for 5 GeV/c2 DM masses at varying 39Ar activity. One of the most concrete ways to improve DS-LM's sensitivity is by decreasing the 39Ar concentration in the UAr. While argon is the third most abundant element in the atmosphere, cosmic-ray interactions activate the radioisotope 39Ar, and so UAr, which is shielded from cosmic-rays by the Earth, is much more radiopure. The Aria cryogenic distillation column will be capable of further reducing 39Ar activity by a factor of 10 reducing it from 73 µBq/kg to 7.3 µBq/kg in a single pass at a throughput close to 10 kg/day.

[Sensitivity projections for a dual-phase argon TPC optimized for light dark matter searches throughout the ionization channel. https://journals.aps.org/prd/abstract/10.1103/PhysRevD.107.112006]

Overview: These plots show the projected sensitivity of DS-LM for studying light dark matter under various potential scenarios. Lower background rates are expected to improve sensitivity at all masses. Reducing the 39Ar activity and SE backgrounds are illustrated too. Multiple projections that assume a 39Ar activity level of 73 µBq/kg along with binominal quenching fluctuations. The index n reveals the neutrino fog in LAr, below which interactions with solar neutrinos impede further sensitivity (gray area). Projections are compared to current limits for DarkSide-50, CREEST-III, and XENON1T. All graphs below illustrate the the upper limits of a projected 90% confidence level on the spin independent DM-neutron scattering cross section for a 1 t per year exposure.

Both with and without binominal fluctuations Varying background rates and thresholds

Inclusion of the Migdal Effect Attempted modeling of SE backgrounds

[Sensitivity projections for a dual-phase argon TPC optimized for light dark matter searches throughout the ionization channel. https://journals.aps.org/prd/abstract/10.1103/PhysRevD.107.112006]

Bottom Left: Some scattering DM might transfer additional energy to the target atom due to inelastic atomic effects. This will add a X-ray and Auger electron cascade to the nuclear recoil. This effect, known as the "Migdal effect", allows for the creation of higher energy signals than possible for a pure nuclear recoil by light DM. Through the exploitiation of this effect, significant sensitivity to DM masses as low as 30 MeV/c2. However, this effect has never been observed before, making it tricky to use to exclude dark matter -- one of our R&D goals is to perform low-energy calibration of LArTPCs to try to measure this effect for the first time.

Overview: The plots on the left show DS-LM's projected sensitivity for light DM with spin-independent nuclear couplings. The gray regions represent the neutrino fog in LAr, and n represents the impediment to a 3σ DM observation. The following detectors and limits are accounted for in the model: Darkside-50, CRESST-III, XENON1T. Projections are shown for: NEWS-G, DAMIC-1K, and SuperCDMS.

Top: The 90% confidence level exclusion curves with 73 µBq/kg of 39Ar for the spin independent DM-nucleon scattering sensitivity, compared to the projected limits (dashed lines) and current limits (solid lines).

Bottom: 3σ significance evidence contours with a 2e− threshold represented by dashed lines and a 4e− threshold represented by solid lines. 73 (thick) or 7.3 (thin) µBq/kg of 39Ar is included as well.

[Sensitivity projections for a dual-phase argon TPC optimized for light dark matter searches throughout the ionization channel. https://journals.aps.org/prd/abstract/10.1103/PhysRevD.107.112006]

Overview: The projected 3σ evidence contours 90% CL exclusion curves with a 1 t per year exposure rate at 7.3–73 µBq/kg of 39Ar. Thick gray lines illustrate the cross section that gives the observed relic DM abundance through freeze-in and freeze-out production mechanisms. Existing limits are for Darkside 50, SENSEI, XENON10, and XENON1T.

90% C.L. exclusion curves for light mediators 3σ significance evidence projections for light mediators

90% C.L. exclusion curve for heavy mediators 3σ significance evidence projections for heavy mediators

[Sensitivity projections for a dual-phase argon TPC optimized for light dark matter searches throughout the ionization channel. https://journals.aps.org/prd/abstract/10.1103/PhysRevD.107.112006]

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