“Bigger Than LIGO’s Detection of Gravitational Waves?” –The Discovery of Dark Matter, Argue Scientists





The discovery of dark matter, argued Carlos Frenk, Director of the Institute for Computational Cosmology, at Durham University's world-renowned theoretical cosmology research group, at the annual meeting of the American Association for the Advancement of Science, would be more important than the detection of gravitational waves, warped spacetime detected this week by LIGO scientists predicted by Einstein, born of black holes colliding.

Scientists are confident that dark matter exists because the effects of its gravity can be seen in the rotation of galaxies and in the way light bends as it travels through the universe. WIMPs, or weakly interacting massive particles, which are among the leading candidates for dark matter. Because WIMPs are thought to interact with other matter only on very rare occasions, they have yet to be detected directly.

“In a certain way we’re still going through an existential crisis,” said Tim Andeen, one of the hundreds of scientists who helped find the Higgs boson particle exactly as they’d hoped to in 2012, at the Large Hadron Collider in Geneva. “We had a thing to go and search for, and we got it,” he said. “Things would’ve gotten really weird if we hadn’t – we would’ve observed all kinds of things in the detector.”

The work for new discoveries continue apace, for signatures of supersymmetry, extra dimensions, dark matter, observed Andeen, now at the University of Texas at Austin, “We don’t have a Higgs boson to look for anymore, but we do know the Higgs boson can’t be the end of the story.”

"And so the search continues," says Dan McKinsey, a UC Berkeley physics professor and co-spokesperson for LUX who is also an affiliate with Berkeley Lab."LUX, The Large Underground Xenon dark matter experiment, is once again in dark matter detection mode at Sanford Lab. The latest run began in late 2014 and is expected to continue until June 2016. This run will represent an increase in exposure of more than four times compared to our previous 2013 run. We will be very excited to see if any dark matter particles have shown themselves in the new data."

LUX operates nearly a mile underground at the Sanford Underground Research Facility (SURF) in the Black Hills of South Dakota has proven itself to be the most sensitive detector in the hunt for dark matter, the unseen stuff believed to account for most of the matter in the universe. Now, a new set of calibration techniques employed by LUX scientists has again dramatically improved the detector's sensitivity.

Researchers with LUX are looking for WIMPs. "We have improved the sensitivity of LUX by more than a factor of 20 for low-mass dark matter particles, significantly enhancing our ability to look for WIMPs," said Rick Gaitskell, professor of physics at Brown University and co-spokesperson for the LUX experiment. "It is vital that we continue to push the capabilities of our detector in the search for the elusive dark matter particles," Gaitskell said. A view inside the LUX detector shown below. (Photo by Matthew Kapust/Sanford Underground Research Facility).




LUX improvements, coupled to advanced computer simulations at the U.S. Department of Energy's Lawrence Berkeley National Laboratory's (Berkeley Lab) National Energy Research Scientific Computing Center (NERSC) and Brown University's Center for Computation and Visualization (CCV), have allowed scientists to test additional particle models of dark matter that now can be excluded from the search. NERSC also stores large volumes of LUX data–measured in trillions of bytes, or terabytes–and Berkeley Lab has a growing role in the LUX collaboration.

The new research is described in a paper submitted to Physical Review Letters. The work reexamines data collected during LUX's first three-month run in 2013 and helps to rule out the possibility of dark matter detections at low-mass ranges where other experiments had previously reported potential detections.

LUX consists of one-third ton of liquid xenon surrounded with sensitive light detectors. It is designed to identify the very rare occasions when a dark matter particle collides with a xenon atom inside the detector. When a collision happens, a xenon atom will recoil and emit a tiny flash of light, which is detected by LUX's light sensors. The detector's location at Sanford Lab beneath a mile of rock helps to shield it from cosmic rays and other radiation that would interfere with a dark matter signal.

So far LUX hasn't detected a dark matter signal, but its exquisite sensitivity has allowed scientists to all but rule out vast mass ranges where dark matter particles might exist. These new calibrations increase that sensitivity even further.

One calibration technique used neutrons as stand-ins for dark matter particles. Bouncing neutrons off the xenon atoms allows scientists to quantify how the LUX detector responds to the recoiling process.

"It is like a giant game of pool with a neutron as the cue ball and the xenon atoms as the stripes and solids," Gaitskell said. "We can track the neutron to deduce the details of the xenon recoil, and calibrate the response of LUX better than anything previously possible."

The nature of the interaction between neutrons and xenon atoms is thought to be very similar to the interaction between dark matter and xenon. "It's just that dark matter particles interact very much more weakly–about a million-million-million-million times more weakly," Gaitskell said.

The neutron experiments help to calibrate the detector for interactions with the xenon nucleus. But LUX scientists have also calibrated the detector's response to the deposition of small amounts of energy by struck atomic electrons. That's done by injecting tritiated methane–a radioactive gas–into the detector.

"In a typical science run, most of what LUX sees are background electron recoil events," said Carter Hall a University of Maryland professor. "Tritiated methane is a convenient source of similar events, and we've now studied hundreds of thousands of its decays in LUX. This gives us confidence that we won't mistake these garden-variety events for dark matter."

Another radioactive gas, krypton, was injected to help scientists distinguish between signals produced by ambient radioactivity and a potential dark matter signal.

"The krypton mixes uniformly in the liquid xenon and emits radiation with a known, specific energy, but then quickly decays away to a stable, non-radioactive form," said Dan McKinsey. By precisely measuring the light and charge produced by this interaction, researchers can effectively filter out background events from their search.

"And so the search continues," McKinsey said. "LUX is once again in dark matter detection mode at Sanford Lab. The latest run began in late 2014 and is expected to continue until June 2016. This run will represent an increase in exposure of more than four times compared to our previous 2013 run. We will be very excited to see if any dark matter particles have shown themselves in the new data." McKinsey, formerly at Yale University, joined UC Berkeley and Berkeley Lab in July, accompanied by members of his research team.

The Sanford Lab is a South Dakota-owned facility. Homestake Mining Co. donated its gold mine in Lead to the South Dakota Science and Technology Authority (SDSTA), which reopened the facility in 2007 with $40 million in funding from the South Dakota State Legislature and a $70 million donation from philanthropist T. Denny Sanford. The U.S. Department of Energy (DOE) supports Sanford Lab's operations.

The LUX scientific collaboration, which is supported by the DOE and National Science Foundation (NSF), includes 19 research universities and national laboratories in the United States, the United Kingdom and Portugal.

Planning for the next-generation dark matter experiment at Sanford Lab is already under way. In late 2016 LUX will be decommissioned to make way for a new, much larger xenon detector, known as the LUX-ZEPLIN (LZ) experiment. LZ would have a 10-ton liquid xenon target, which will fit inside the same 72,000-gallon tank of pure water used by LUX. Berkeley Lab scientists will have major leadership roles in the LZ collaboration.

"The innovations of the LUX experiment form the foundation for the LZ experiment, which is planned to achieve over 100 times the sensitivity of LUX. The LZ experiment is so sensitive that it should begin to detect a type of neutrino originating in the Sun that even Ray Davis' Nobel Prize-winning experiment at the Homestake mine was unable to detect," according to Harry Nelson of UC Santa Barbara, spokesperson for LZ.

Dark matter has often been regarded as a totally new exotic form of matter, such as a particle moving in extra dimensions of space or its quantum version, super-symmetry.

"We have seen this kind of particle before. It has the same properties – same type of mass, the same type of interactions, in the same type of theory of strong interactions that gave forth the ordinary pions, which are responsible for binding atomic nuclei together. It is incredibly exciting that we may finally understand why we came to exist," said Hitoshi Murayama this past July. He's Professor of Physics at the University of California, Berkeley, and Director of the Kavli Institute for the Physics and Mathematics of the Universe at the University of Tokyo.




The image above is an artist's impression of dark matter distribution. Left image assumes conventional dark matter theories, where dark matter would be highly peaked in small area in galaxy center. Right image assumes SIMPs, where dark matter in galaxy would spread out from the center.

The new theory predicts dark matter is likely to interact with itself within galaxies or clusters of galaxies, possibly modifying the predicted mass distributions. "It can resolve outstanding discrepancies between data and computer simulations," says Eric Kuflik, a postdoctoral researcher at Cornell University. University of California, Berkeley postdoctoral researcher Yonit Hochberg adds, "The key differences in these properties between this new class of dark matter theories and previous ideas have profound implications on how dark matter can be discovered in upcoming experimental searches."

The next step will be to put this theory to the test using experiments such as CERN's Large Hadron Collider and the new SuperKEK-B, and a proposed experiment SHiP.

The image at the top of the page from the NASA/ESA Hubble Space Telescope shows the rich galaxy cluster Abell 3827. The strange blue structures surrounding the central galaxies are gravitationally lensed views of a much more distant galaxy behind the cluster. Observations of the central four merging galaxies have provided hints that the dark matter around one of the galaxies is not moving with the galaxy itself, possibly implying dark matter-dark matter interactions of an unknown nature are occurring.

The Daily Galaxy via Kavli IPMU, DOE/Lawrence Berkeley National Laboratory, and theguardian.com

Image credits: Kavli IPMU – Kavli IPMU modified this figure based on the image credited by NASA, STScI; Top of page Nasa/ESA/European Southern Observatory

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