Figuring out the character of darkish matter, the invisible substance that makes up a lot of the mass in our universe, is likely one of the best puzzles in physics. New outcomes from the world’s most delicate darkish matter detector, LUX-ZEPLIN (LZ), have narrowed down the probabilities for one of many main darkish matter candidates: weakly interacting large particles (WIMPs).
“Whereas we at all times hope to find a brand new particle, it is necessary for particle physics that we’re in a position to set bounds on what the darkish matter would possibly really be,” stated UC Santa Barbara experimental physicist Hugh Lippincott. Scientists have suspected the existence of darkish matter for many years, nevertheless it stays a mysterious substance — one which however performs a elementary function within the construction of the universe.
LZ hunts for darkish matter from a cavern practically one mile underground on the Sanford Underground Analysis Facility (SURF) in South Dakota. The experiment’s new outcomes discover weaker darkish matter interactions than ever searched earlier than and additional restrict what WIMPs might be. The outcomes analyze 280 days’ value of knowledge: a brand new set of 220 days (collected between March 2023 and April 2024) mixed with 60 earlier days from LZ’s first run. The experiment plans to gather 1,000 days’ value of knowledge earlier than it ends in 2028.
The inside portion of the LZ detector consists of two nested titanium tanks crammed with ten tonnes of clear pure liquid xenon, which is so dense it creates a extremely remoted setting, free from the “noise” of the surface world and excellent for capturing the faintest of faint alerts that might be indicative of a WIMP. The hope is for a WIMP to knock right into a xenon nucleus, inflicting it to maneuver, very similar to a success from a cue ball in a recreation of pool. By amassing the sunshine and electrons emitted throughout interactions, LZ captures potential WIMP alerts alongside different information. This liquid xenon core is surrounded by a a lot bigger Outer Detector (OD) — acrylic tanks crammed with gadolinium-loaded liquid scintillator.
LZ’s sensitivity comes from the myriad methods the detector can cut back backgrounds, the false alerts that may impersonate or disguise a darkish matter interplay. Deep underground, the detector is shielded from cosmic rays coming from area. To cut back pure radiation from on a regular basis objects, LZ was constructed from hundreds of ultraclean, low-radiation components. The detector is constructed like an onion, with every layer both blocking outdoors radiation or monitoring particle interactions to rule out darkish matter mimics. And, subtle new evaluation strategies assist rule out background interactions.
UCSB was one of many founding teams in LZ, led by UCSB physicist Harry Nelson, who hosted the primary LZ assembly at UCSB in 2012. The crew presently consists of college members Lippincott and Nelson, postdoctoral researchers Chami Amarasinghe and TJ Whitis, and graduate college students Jeonghwa Kim, Makayla Trask, Lindsey Weeldreyer, and Jordan Thomas. Different contributors to the end result embody latest Ph.D. recipient Jack Bargemann, now a postdoctoral researcher at Pacific Northwest Nationwide Laboratory, and former undergraduate researcher; Tarun Advaith Kumar, now a graduate pupil on the Perimeter Institute. The physics coordinator for the end result was Scott Haselschwardt, who acquired his Ph.D. from UCSB in 2018 and is now an assistant professor on the College of Michigan.
Neutrons, subatomic particles that exist in each atom save hydrogen, are among the many most typical confounders of WIMP alerts. Nelson and UCSB led the design, fabrication, and commissioning of the OD, the important element that enables the collaboration to rule out these particles and allow an actual discovery.
“The tough factor about neutrons is that additionally they work together with the xenon nuclei, giving off a sign an identical to what we count on from WIMPs,” Trask stated. “The OD is great at detecting neutrons, and confirms a WIMP detection by not having any response.” Presence of a pulse within the OD can veto an in any other case good candidate for a WIMP detection.
Radon can be a WIMP mimic, for which the scientists have to be vigilant. “Radon undergoes a specific sequence of decays, a few of which might be mistaken for WIMPs,” Bargemann stated. “One of many issues we have been in a position to do on this run was look out for the entire set of decays within the detector to determine the radon and keep away from complicated them for WIMPs.”
To allow a powerful end result and eradicate unconscious bias, the LZ collaboration utilized a method referred to as “salting,” which provides faux WIMP alerts throughout information assortment. By camouflaging the actual information till “unsalting” on the very finish, researchers can keep away from unconscious bias and maintain from overly decoding or altering their evaluation.
“We’re pushing the boundary right into a regime the place folks haven’t seemed for darkish matter earlier than,” stated Haselschwardt. “There is a human tendency to need to see patterns in information, so it is actually vital once you enter this new regime that no bias wanders in. Should you make a discovery, you need to get it proper.”
With these outcomes, the sector of prospects for what WIMPs could also be has narrowed dramatically, permitting all scientists looking for darkish matter to higher focus their searches and reject incorrect fashions of how the universe operates. It is a lengthy recreation, with extra information assortment sooner or later and one that may do greater than speed up the seek for darkish matter.
“Our experiment can be delicate to uncommon occasions with roots in various areas of physics,” Amarasinghe stated. “Some examples are photo voltaic neutrinos, the fascinating decays of sure xenon isotopes, and even different varieties of darkish matter. With the depth of this end result behind us, I am very excited to spend extra time on these searches.”
“The us Physics Division has an extended historical past of devising searches for darkish matter, beginning with one of many first printed outcomes of a search in 1988,” Nelson stated. Earlier college members embody David Caldwell (now deceased), and Michael Witherell, now director of the Lawrence Berkeley Laboratory. David Hale (now retired) pioneered most of the strategies for suppressing faux darkish matter alerts which are actually employed all through the sector of darkish matter searches. “UCSB, by the Physics Division, the Faculty of Letters and Science, the administration, and thru non-public donations, has strongly supported the darkish matter effort for many years, and made substantial contributions to LZ.”
LZ is a collaboration of roughly 250 scientists from 38 establishments in america, United Kingdom, Portugal, Switzerland, South Korea, and Australia; a lot of the work constructing, working, and analyzing the record-setting experiment is finished by early profession researchers. The collaboration is already trying ahead to analyzing the following information set and increasing our information evaluation strategies to hunt alerts from lower-mass darkish matter. Scientists are additionally considering by potential upgrades to additional enhance LZ, and planning for a next-generation darkish matter detector referred to as XLZD.
LZ is supported by the U.S. Division of Power, Workplace of Science, Workplace of Excessive Power Physics and the Nationwide Power Analysis Scientific Computing Middle, a DOE Workplace of Science consumer facility. LZ can be supported by the Science & Expertise Services Council of the UK; the Portuguese Basis for Science and Expertise; the Swiss Nationwide Science Basis, and the Institute for Fundamental Science, Korea. Greater than 38 establishments of upper training and superior analysis offered help to LZ. The help of the Sanford Underground Analysis Facility has always been important for UCSB efforts to LZ.
