Researchers See Kelvin Wave on Quantum “Tornado” for First Time

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Draining the water from a bathtub causes a spinning tornado to appear. The downward flow of water into the drain causes the water to rotate, and as the rotation speeds up, a vortex forms that obeys the laws of classical mechanics. However, if the water is extremely cold liquid helium, the fluid will swirl around an invisible line to form a vortex that obeys the laws of quantum mechanics. Sometimes, two of these quantum tornadoes flex into curved lines, cross over one another to form a letter X shape, swap ends, and then violently retract from one another—a process called reconnection.

Computer simulations have suggested that after the vortexes snap away from each other, they develop ripples called “Kelvin waves” to quickly get rid of the energy caused by the connection and relax the system. However, the existence of these waves had never been experimentally proven.

Now, for the first time, researchers provide visual evidence confirming that the reconnection of quantum vortexes launches Kelvin waves. The study, which was conducted at the University of Maryland, will be published the week of March 24, 2014 in the online early edition of the journal Proceedings of the National Academy of Sciences. The research was supported by the National Science Foundation.

“We weren’t surprised to see the Kelvin waves on the quantum vortex, but we were excited to see them because they had never been seen before,” said Daniel Lathrop, a UMD physics professor. “Seeing the Kelvin waves provided the first experimental evidence that previous theories predicting they would be launched from vortex reconnection were correct.”

Understanding turbulence in quantum fluids, such as ultracold liquid helium, may offer clues to neutron stars, trapped atom systems and superconductors. Superconductors, which are materials that conduct electricity without resistance below certain temperatures, develop quantized vortices. Understanding the behavior of the vortices may help researchers develop superconductors that remain superconducting at higher current densities.

Physicists Richard Feynman and Lars Onsager predicted the existence of quantum vortices more than a half-century ago. However, no one had seen quantum vortices until 2006. In Lathrop’s laboratory at UMD, researchers prepared a cylinder of supercold helium—at 2 degrees Celsius above absolute zero—injected with frozen tracer particles made from atmospheric air and helium gases. When they shined a laser into the cylinder, the researchers saw the particles trapped on the vortices like dew drops on a spider web.

“Kelvin waves on quantized vortices had been predicted, but the experiments were challenging because we had to conduct them at lower temperatures than our previous experiments,” explained Lathrop.

Since 2006, the researchers have used the same technique to further examine quantum vortexes. During an experiment in February 2012, they witnessed a unique reconnection event. One vortex reconnected with another and a wave propagated down the vortex. To quantitatively study the wave’s motion, the researchers tracked the position of the particles on the vortex. The resulting waveforms agreed generally with theories of Kelvin waves propagating from quantum vortexes.

“These first observations of Kelvin waves will surely lead to exciting new experiments that push the limits of our knowledge of these exotic quantum motions,” added Lathrop.

In the future, Lathrop plans to use florescent nanoparticles to investigate what happens near the transition to the superfluid state.

Lathrop conducted the current study with David Meichle, a UMD physics graduate student; Enrico Fonda, who was a research scholar at UMD and graduate student at the University of Trieste when the study was performed and is now a postdoctoral researcher at New York University; Nicholas Ouellette, who was a visiting assistant professor at UMD when the study was performed and is now an associate professor in mechanical engineering & materials science at Yale University; and Sahand Hormoz, a postdoctoral researcher at the University of California, Santa Barbara’s Kavli Institute for Theoretical Physics.

This research was supported by the National Science Foundation (NSF) under Award No. DMR-0906109. The content of this article does not necessarily reflect the views of the NSF.

The research paper, “Direct observation of Kelvin waves excited by quantized vortex reconnections,” Enrico Fonda, David P. Meichle, Nicholas T. Ouellette, Sahand Hormoz, and Daniel P. Lathrop, published the week of March 24, 2014 in the online early edition of the journal Proceedings of the National Academy of Sciences.


Researchers Demonstrate Long-Lived High-Power Optical Waveguides in Air

In a recent experiment, published in Physical Review X, Howard Milchberg and his colleagues in the intense laser-matter interactions group demonstrate that femtosecond filaments can set up an extended and robust thermal waveguide structure in air with a lifetime of several milliseconds, making possible the very-long-range guiding and distant projection of high-energy laser pulses and high-average power beams. This is the subject of articles that appear in the latest issues of APS Viewpoint, Physics Today and Science News.


APS/Alan Stonebraker
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The Era of Neutrino Astronomy Has Begun

Astrophysicists using a telescope embedded in Antarctic ice have succeeded in a quest to detect and record the mysterious phenomena known as cosmic neutrinos – nearly massless particles that stream to Earth at the speed of light from outside our solar system, striking the surface in a burst of energy that can be as powerful as a baseball pitcher’s fastball. Next they hope to build on the early success of the IceCube Neutrino Observatory to detect the source of these high-energy particles, said Physics Professor Gregory Sullivan, who led the University of Maryland’s 12-person team of contributors to the IceCube Collaboration.

“The era of neutrino astronomy has begun,” Sullivan said as the IceCube Collaboration announced the observation of 28 very high-energy particle events that constitute the first solid evidence for astrophysical neutrinos from cosmic sources.

By studying the neutrinos that IceCube detects, scientists can learn about the nature of astrophysical phenomena occurring millions, or even billions of light years from Earth, Sullivan said. “The sources of neutrinos, and the question of what could accelerate these particles, has been a mystery for more than 100 years. Now we have an instrument that can detect astrophysical neutrinos. It’s working beautifully, and we expect it to run for another 20 years.”

The collaboration’s report on the first cosmic neutrino records from the IceCube Neutrino Observatory, collected from instruments embedded in one cubic kilometer of ice at the South Pole, was published Nov. 22 on the cover of Science.

“This is the first indication of very high-energy neutrinos coming from outside our solar system,” said University of Wisconsin-Madison Physics Professor Francis Halzen, principal investigator of IceCube. “It is gratifying to finally see what we have been looking for. This is the dawn of a new age of astronomy.”

“Neutrinos are one of the basic building blocks of our universe,” said UMD Physics Associate Professor and Director Kara Hoffman, an IceCube team member. Billions of them pass through our bodies unnoticed every second. These extremely high-energy particles maintain their speed and direction unaffected by magnetic fields. The vast majority of neutrinos originate either in the sun or in Earth’s own atmosphere. Far more rare are astrophysical neutrinos, which come from the outer reaches of our galaxy or beyond.

The origin and cause of astrophysical neutrinos are unknown, though gamma ray bursts, active galactic nuclei and black holes are potential sources. Better understanding of these neutrinos is critically important in particle physics, astrophysics and astronomy, and scientists have worked for more than 50 years to design and build a high-energy neutrino detector of this type.

IceCube was designed to accomplish two major scientific goals: measure the flux, or rate, of high-energy neutrinos and try to identify some of their sources. The neutrino observatory was built and is operated by an international collaboration of more than 250 physicists and engineers. UMD physicists have been key collaborators on IceCube since 2002, when its unique design was devised and construction began.

IceCube is made up of 5,160 digital optical modules suspended along 86 strings embedded in ice beneath the South Pole. The National Science Foundation-supported observatory detects neutrinos through the tiny flashes of blue light, called Cherenkov light, produced when neutrinos interact in the ice. Computers at the IceCube laboratory collect near-real-time data from the optical sensors and send information about interesting events north via satellite. The UMD team designed the data collection system and much of IceCube's analytic software. Construction took nearly a decade, and the completed detector began gathering data in May 2011.

“IceCube is a wonderful and unique astrophysical telescope – it is deployed deep in the Antarctic ice but looks over the entire Universe, detecting neutrinos coming through the Earth from the northern skies, as well as from around the southern skies,” said Vladimir Papitashvili of the National Science Foundation (NSF) Division of Polar Programs.

In April 2012 IceCube detected two high-energy events above 1 PeV, nicknamed Bert and Ernie, the first astrophysical neutrinos definitively recorded by a terrestrial detector. After Bert and Ernie were discovered, the IceCube team searched their records from May 2010 to May 2012 of events that fell slightly below the energy level of their original search. They discovered 26 more high-energy events, all at levels of 30 teraelectronvolts (TeV) or higher, indicative of astrophysical neutrinos. Preliminary results of this analysis were presented May 15 at the IceCube Particle Astrophysics Symposium at UW–Madison. The analysis presented in Science reveals a highly statistically significant signal (more than 4 sigma), providing solid evidence that IceCube has successfully detected high-energy extraterrestrial neutrinos, said UMD’s Sullivan.

Since astrophysical neutrinos move in straight lines unimpeded by outside forces, they can act as pointers to the place in the galaxy where they originated. The 28 events recorded so far are too few to point to any one location, Sullivan said. Over the coming years, the IceCube team will watch, “like waiting for a long exposure photograph,” as more measurements fill in a picture that may reveal the point of origin of these intriguing phenomena.

New detection systems for astrophysical neutrinos are also in the works. Hoffman is leading the development of the Askaryan Radio Array, a neutrino telescope that uses radio frequency, which transmits best through very cold ice, to detect the particles. Plans are underway for 37 subsurface clusters of radio antennae

The IceCube Neutrino Observatory was built under a NSF Major Research Equipment and Facilities Construction grant, with assistance from partner funding agencies around the world. The NSF's Division of Polar Programs and Physics Division continue to support the project with a Maintenance and Operations grant, along with international support from participating institutes and their funding agencies.

UMD contributors to the IceCube collaboration include Sullivan and Hoffman; faculty and staff members Erik Blaufuss, John Felde, Jordan Goodman, Henrike Wissing, Alex Olivas, Donald La Dieu, and Torsten Schmidt; and graduate students Elim Cheung, Robert Hellauer, Ryan Maunu, and Michael Richman.

First Results from LUX

World’s Most Sensitive Dark Matter Detector
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After its first run of more than three months, operating a mile underground in the Black Hills of South Dakota, a new experiment named LUX has proven itself the most sensitive dark matter detector in the world.

“This result shows that LUX is working quite well”, says Carter Hall, associate professor of physics at the University of Maryland and leader of the Maryland LUX group. “With just a fraction of our final dataset in hand, we’ve produced an important science result, and we’ve demonstrated the promise of the full dataset which we’ll be collecting in the coming years.”

LUX stands for Large Underground Xenon experiment. The scientific collaboration, which is supported by the National Science Foundation and DOE, includes 17 research universities and national laboratories in the United States, the United Kingdom, and Portugal. Researchers from the University of Maryland, including Prof. Hall and graduate students Attila Dobi, Richard Knoche and Jon Balajthy, have played a prominent role in the design, construction, and operation of the detector as well as the recent analysis of the dark matter search data.

Dark matter, so far observed only by its gravitational effects on galaxies and clusters of galaxies, is the predominant form of matter in the universe. Weakly interacting massive particles, or WIMPs – so-called because they rarely interact with ordinary matter except through gravity – are the leading theoretical candidates for dark matter. Theories and results from other experiments suggest that WIMPs could be either “high mass” or “low mass.”

Assuming a high-mass WIMP with a mass of 35 GeV/c2 , LUX has a sensitivity that is more than two times better than any other experiment to directly detect dark matter. (Physicists express the mass of subatomic particles in electron volts (eV) divided by the speed of light squared (c2 ) A giga-electron volt (GeV) is a billion electron volts). LUX also has greatly enhanced sensitivity to low-mass WIMPs, whose possible detection has been suggested by other experiments. Three candidate WIMP events recently reported in ultra-cold silicon detectors, however, would have produced more than 1,600 events in LUX’s much larger detector, or one every 80 minutes in the recent run. No such signals were seen.

“This is only the beginning for LUX,” Dan McKinsey of Yale University says. “Now that we understand the instrument and its backgrounds, we will continue to take data, testing for more and more elusive candidates for dark matter.”

In both theory and practice, collisions between WIMPs and normal matter are rare and extremely difficult to detect, especially because a constant rain of cosmic radiation from space can drown out the faint signals. That’s why LUX is searching for WIMPs 4,850 feet underground in the Sanford Lab, where few cosmic ray particles can penetrate. The detector is further protected from background radiation from the surrounding rock by immersion in a tank of ultra-pure water.

At the heart of the experiment is a 6-foot-tall titanium tank filled with almost a third of a ton of liquid xenon, cooled to minus 150 degrees Fahrenheit. If a WIMP strikes a xenon atom it recoils from other xenon atoms and emits photons (light) and electrons. The electrons are drawn upward by an electrical field and interact with a thin layer of xenon gas at the top of the tank, releasing more photons.

Light detectors in the top and bottom of the tank are each capable of detecting a single photon, so the locations of the two photon signals – one at the collision point, the other at the top of the tank – can be pinpointed to within a few millimeters. The energy of the interaction can be precisely measured from the brightness of the signals.

“LUX is a complex instrument,” says McKinsey, “but it insures that each WIMP event’s unique signature of position and energy will be precisely recorded.”

LUX’s biggest advantage as a dark matter detector is its size, a large xenon target whose outer regions further shield the interior from gamma rays and neutrons. Installed in the Sanford Lab in the summer of 2012, the experiment was filled with liquid xenon in February, and its first run of three months was conducted this spring and summer, followed by intensive analysis of the data. The dark matter search will continue through the next two years.

"The universe's mysterious dark sector presents us with two of the most thrilling challenges in all of physics," says Saul Perlmutter of DOE’s Lawrence Berkeley National Laboratory (Berkeley Lab), a winner of the 2011 Nobel Prize in Physics for discovering the accelerating expansion of the universe. "We call it the dark sector precisely because we don't know what accounts for most of the energy and mass in the universe. Dark energy is one challenge, and as for the other, the LUX experiment's first data now take the lead in the hunt for the dark matter component of the dark sector."

South Dakota Gov. Dennis Daugaard says his state is proud to play a role in this important research. Homestake Mining Co. donated its gold mine in Lead to the South Dakota Science and Technology Authority, which reopened it in 2007 with funding from the state Legislature and a $70 million donation from philanthropist T. Denny Sanford. “We congratulate the LUX researchers, and we look forward to working with dark matter scientists and other partners in the years to come,” Daugaard says.

The LUX announcement is major step forward for the Sanford Lab’s science program, which Laboratory Director Mike Headley points out has its roots in a famous physics experiment installed in the same experiment hall in the 1960s. “These are the first physics results achieved at Homestake since the Ray Davis solar neutrino experiment, which earned him a Nobel Prize for Physics,” Headley says. “I’m very proud of our staff’s work to help LUX reach this major milestone.”

UMD Higgs hunters celebrate Nobel Prize in Physics

UMD physicists worked on an experiment that led to the Higgs boson and the Nobel Prize in Physics

The Royal Swedish Academy of Sciences today awarded the Nobel Prize in Physics 2013 to François Englert and Peter Higgs to recognize their work developing the theory of what is now known as the Higgs field, which scientists say gives mass to subatomic and atomic particles, thus making possible the universe and everything in it. The Nobel Committee noted that the ideas of Englert and Higgs “were confirmed by the discovery of a so called Higgs particle at the CERN laboratory outside Geneva in Switzerland.”

University of Maryland scientists played a significant role in the world-wide scientific collaboration that culminated in 2012, when two multi-national research teams generated and detected the long-sought Higgs particle, or Higgs boson, which scientists say confirms the theory of the Higgs field, an invisible energy plane that exists throughout the universe.

“It is fitting that the Nobel Committee has recognized these theorists,” said University of Maryland Physics Professor Nicholas Hadley, chair of the U.S. collaboration board for one of the two experimental teams. “And it is an honor that I and 21 other UMD scientists have been part of the historic international particle accelerator experiments that proved them correct. I congratulate the winners, the particle physics community, and my Maryland colleagues.”

Englert and Higgs and colleagues first proposed the existence of the Higgs field in three scientific papers published in 1964. A key concept held that as particles pass through the Higgs field, they interact with a fundamental particle, the Higgs boson, that endows them with mass. Without mass, particles would not be attracted to one another, and would simply float freely around the universe at light speed.

To test the theory, researchers worked for decades to plan and conduct experiments at the world’s largest particle accelerator, the Large Hadron Collider at CERN near Geneva, Switzerland. On July 4, 2012, members of the two teams, known by the acronyms ATLAS and CMS, announced that they had independently found a subatomic particle that fit the criteria for the Higgs boson.

“Without some kind of Higgs-like field, there really wouldn't be a universe at all,” said Hadley. “Because the particles would have no mass, and if everything were massless, there wouldn't be atoms, there wouldn't be planets, there wouldn't be stars and there wouldn't be people. The great question has been did Higgs, Englert and colleagues get it right with their particular model? And now it appears the answer is yes.”

UMD’s 22 scientists are among nearly 1,300 U.S. researchers from 89 U.S. universities and seven U.S. Department of Energy laboratories who participate in the two ongoing Large Hadron Collider experiments. Maryland’s team helped to build the CMS particle detector and analyzed the masses of data - many times greater than the contents of all the books in the Library of Congress - generated by the experiment, thus helping to confirm the discovery of the Higgs boson particle.

"To find the Higgs boson, we used a collider to smash together protons traveling just a gnat's eyebrow below the speed of light," said UMD Physics Professor and Chair Andrew Baden. "We reconstructed these tremendously high-energy collisions, which recreate the conditions that existed when the universe was about one-billionth of a second old, and tried to find evidence of a new particle, a Higgs boson. And we found it."

The majority of U.S. scientists participating in Large Hadron Collider experiments do so from their home institutions, remotely accessing and analyzing data through high-capacity networks and grid computing. The United States plays an important role in this distributed computing system, providing 23 percent of the computing power for ATLAS and 40 percent for CMS. Maryland’s researchers also helped to build the very high speed electronics transmitting data for CMS.

University of Maryland participants in the CMS experiments included:

Baden, Drew; Bard, Robert; Calvert, Brian; Eno, Sarah Catherine; Ferencek, Dinko; Gomez, Jaime; Grassi, Tullio; Hadley, Nicholas John; Kellogg, Richard G; Kirn, Malina; Kolberg, Ted; Lu, Ying; Marionneau, Matthieu ; Mignerey, Alice; Pedro, Kevin; Peterman, Alison; Rossato, Kenneth; Rumerio, Paolo; Santanastasio, Francesco; Skuja, Andris; Temple, Jeffrey; Tonjes, Marguerite; Tonwar, Suresh C; Toole, Terrence; and Twedt, Elizabeth.