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LUX-ZEPLIN (LZ Collaboration)

BHSU is a member of the LZ Collaboration

Dark matter Hunt with LUX-ZEPLIN Transcript

Narrator

People have always been fascinated by the beauty and richness of the night sky, and scientists have been studying the cosmos for many centuries. But we still don't know a whole lot about what the universe is made of. In fact, 85% of all the matter in the universe, called dark matter, can't be seen even with our most advanced scientific instruments. Scientists know it's there because it bends light and affects how galaxies rotate. It's also a crucial part of galaxy formation, and our world wouldn't exist without it. Identifying the building blocks of dark matter is one of the highest priorities of modern physics. Most scientists believe it's made of ghostly particles that rarely bump into their surroundings. That's why billions of dark matter particles might zip right through our bodies every second without us even noticing. Leading candidates for dark matter particles are WIMPS, or weakly interacting massive particles. Now, SLAC is helping to build and test one of the biggest and most sensitive detectors ever designed to catch a WIMP- the LUX-ZEPLIN, or LZ detector. LZ's heart is a tank filled with ten tons of liquid xenon. If a particle streaks through the tank and strikes a xenon nucleus, two things will happen: the xenon will emit a flash of light, and it will also release electrons, which drift in an electric field to the top of the tank, where they produce a second flash of light. A WIMP will produce a characteristic combination of flashes, which will be detected with nearly 500 light-sensitive tubes at the top and bottom of the tank. But to have any hope of seeing a potential WIMP signal, LZ scientists will have to deal with a lot of background noise, such as unwanted signals from cosmic rays from spaces, and gamma rays and neutrons released in natural radioactive decay in the environment and detector materials. Therefore, the detector core will be surrounded by several layers that eliminate this noise as much as possible. First, the researchers will purify the xenon to an incredible degree, to get rid of traces of radioactive krypton. The goal is less than one krypton atom per 100 trillion xenon atoms. Next, they'll use only the innermost 80% of the purified xenon in the tank to detect dark matter. The outer 20% will serve as a radiation shield. Another think layer of xenon, called the xenon skin, will reject signals from gamma rays and neutrons. The tank that holds the xenon will be made of ultraclean, medical-grade titanium that generates very few background signals. The titanium tank will sit inside a bigger tank filled with a liquid that, like the xenon skin, detects gamma rays and neutrons. And both of those tanks will be inside a third tank, filled with 70,000 gallons of water the outer radiation shield. All of this will be located nearly a mile underground at the Sanford Underground Research Facility in South Dakota, where it's protected from cosmic rays. With LZ going online in early 2020, researchers hope for a breakthrough in the search for dark matter to learn more about ourselves, our galactic home, and the entire universe.

Dark matter constitutes about 85% of the mass in the universe [1-4], but its composition and nature remain unknown. Leading theories continue to postulate that dark matter is formed of Weakly Interacting Massive Particles (WIMPs) with masses between ~ 2 and at least 1000 GeV/c ². The 2023 P5 recommendations highlighted the significance of identifying dark matter for the Office of Science High Energy Physics program. Recommendation 1 emphasizes the importance of the LUX-ZEPLIN (LZ) experiment as it has entered its primary WIMP discovery phase; stating “as one of the highest priorities independent of budget scenario”, that the LZ detector at the Sanford Underground Research Facility (SURF) [5-7] in Lead, SD be one of the experiments to continue to receive support from DOE for “ongoing experiments and research to enable maximum science[8].” Additionally, “an ultimate Generation 3 (G3) dark matter direct detection experiment reaching the neutrino fog, in coordination with international partners and preferably sited in the US” was also recommended by the P5 panel.

The LZ collaboration is a merging of the LUX [9] and ZEPLIN [10] experiments and is searching for the elusive WIMP signal [11-14]. The experiment is located in SURF’s Davis cavern on the 4850L, which provides 4300 meter water equivalent shielding.

The LZ detector consists of a 7-tonne active mass liquid xenon target in a dual-phase Time Projection Chamber (TPC), see Figure 1.

Figure 1 Rendering of the LZ experiment, showing the major detector subsystems. At the center is the liquid xenon TPC (1), monitored by two arrays of PMTs and serviced by various cable and fluid conduits (upper and lower). The TPC is contained in a double-walled vacuum insulated titanium cryostat and surrounded on all sides by a GdLS Outer Detector (2). The cathode high voltage connection is made horizontally at the lower left (5). The GdLS is observed by a suite of 8” PMTs (3) standing in the water (4) which provides shielding for the detector. The pitched conduit on the right (6) allows for neutron calibration sources to illuminate the detector [15].

Interactions occurring within the TPC fiducial volume will create two bursts of light, each at separate times, which reach the Photomultiplier tubes (PMTs) located at the top and the bottom of the TPC. The first signal, S1, is the prompt scintillation signal from the original interaction between a particle and the xenon. The second signal, S2, is created when electrons from the original interaction are drifted to the gas phase of the TPC through an applied uniform electric field, where they produce a second scintillation signal through electroluminescence. These two signals allow for three-dimensional reconstruction of the location of the event as well as the determination of its energy. Interactions with the xenon nucleus (nuclear recoils, NR) are differentiated from interactions with the xenon electron cloud (electron recoil, ER) through the ratio of these S1 and S2 signals.

LZ’s TPC is surrounded by a thin layer of instrumented xenon within the titanium cryostat, which is called the Skin. The cryostat is then surrounded by the outer detector (OD), which are large acrylic tanks filled with gadolinium-doped liquid scintillator. Both the Skin and OD have dedicated PMTs. The entire system is then contained in a water tank. The combination of the Skin and the OD form a system which detects gamma and neutron emissions from within the detector materials themselves, which allows these background signals to be vetoed.

Construction was completed in 2021 and the first results were published in 2023 (see Figure 2 and [16]). Data-taking is ongoing! The LZ experiment will achieve detection limits close to the neutrino fog.

Figure 2: The 90 % confidence limit (black line) for the spin independent WIMP cross section vs. WIMP mass. The green and yellow bands are the 1σ and 2σ sensitivity bands. The dotted line shows the median of the sensitivity projection. Also shown are the PandaX-4T [17], XENON1T[18], LUX[19], and DEAP-3600[20] limits [16].

References

References

[1] Y N. Aghanim et al. (Planck Collaboration), Astron. Astrophys. 641, A6 (2020).
[2] Y. Sofue and V. Rubin, Annu. Rev. Astron. Astrophys. 39, 137 (2001).
[3] D. Harvey, R. Massey, T. Kitching, A. Taylor, and E. Tittley, Science 347, 1462 (2015).
[4] A. Arbey and F. Mahmoudi, Prog. Part. Nucl. Phys. 119, 103865 (2021).
[5] J Heise, J. Phys.: Conf. Ser. 606 012015 (2015).
[6] J Heise, J. Phys.: Conf. Ser. 2156 012172 (2021).
[7] J Heise, arXiv:2203.08293 [hep-ex] (2022).
[8] 2023 P5 Report Pathways to Innovation and Discovery in Particle Physics. https://www.usparticlephysics.org/2023-p5-report/
[9] D.S. Akerib, (LUX Collaboration), Phys. Rev. Lett. 118, 012303 (2017).
[10] V.N. Lebedenko, et al. Phys. Rev. D 80, 052010 (2009).
[11] D.S. Akerib, et al., (LZ Collaboration), arXiv:1509.02910 [physics.ins-det] (2015).
[12] J. Aalbers, et al, (LZ Collaboration), Phys.Rev.D 108, 7 (2023).
[13] J. Aalbers et al (LZ Collaboration), Phys.Rev.D 108, 1 (2023).
[14] D. Akerib, et al, (LZ Collaboration), Astropart.Phys. 125 102480 (2021).
[15] D. Akerib et al. (LZ Collaboration) Nucl.Instrum.Meth.A 953, 163047 (2020).
[16] J. Aalbers et al. (LZ Collaboration) Phys.Rev.Lett. 131, 4 (2023).
[17] Y.Meng et al. (PandaX-4T Collaboration), Phys.Rev. Lett.127, 261802(2021).
[18] E.Aprile et al.(XENON Collaboration),Phys.Rev.Lett. 121,111302 (2018).
[19] D.S. Akerib, (LUX Collaboration), Phys. Rev. Lett. 118, 012303 (2017).
[20] R.Ajaj et al.(DEAP),Phys.Rev.D100,022004 (2019).

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