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.

HAWC Gamma Ray Observatory Begins Operations

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The High-Altitude Water Cherenkov (HAWC) Gamma Ray Observatory formally began operations on August 1. This is the culmination of four years of work by a persevering team of scientists and technicians from Mexico and the United States. Researchers from the University of Maryland, including Jordan Goodman, Andrew Smith, Brian Baughman, Jim Braun and Josh Wood, are playing a very prominent role in the construction of HAWC and will help lead in the analysis of data from the observatory.

“HAWC will be the world’s premier wide-field TeV gamma-ray observatory with between 10 and 15 times the sensitivity of previous generation wide-field detectors,” said Goodman, UMD professor and principal investigator for the project.

HAWC is located at an altitude of 4100 meters (13,500’) on the slope of the volcanoes Sierra Negra and Pico de Orizaba at the border between the states of Puebla and Veracruz. The observatory, which is still under construction, uses an array of Cherenkov detectors to observe high-energy cosmic rays and gamma rays. Currently 111 out of 300 Cherenkov detectors are deployed and taking data. Each Cherenkov detector consists of 180,000 liters (40,000 gallons) of extra-pure water stored inside an enormous tank (5 meters high and 7.3 meters in diameter) with four highly sensitive light sensors fixed to the bottom of the tank.

The HAWC array, operating with 111 Cherenkov detectors since August 1 and growing each week, will be sensitive to of high-energy particles and radiation between 100 GeV and 100 TeV, energy equivalent to billion times the energy of visible light.

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In 2009, HAWC was identified as the Mexican astronomical project with the highest expected impact on high-energy astrophysics and in the US was endorsed by a joint NSF – DOE panel. Shortly thereafter a test array with three Cherenkov detectors was installed at the volcano Sierra Negra and successfully observed cosmic rays and gamma rays. Following these early tests, a prototype array of seven Cherenkov detectors was built in 2009 to test the tank design, simulate real data-taking, and study the logistics of deploying a large-scale observatory in this remote location. In 2012, the first 30 of 300 HAWC detectors were deployed, and since that time have been operated nearly continuously. The 30-detector stage of HAWC permitted calibration of the observatory via the observation of the shadow of the moon as it blocked cosmic rays.


The Most Energetic Particles in the Known Universe

Gamma rays (electromagnetic radiation of very-high frequency) and cosmic rays (subatomic particles of very-high energy) are products of the most energetic and cataclysmic events in the known universe. These phenomena include the collisions of two neutron stars, the explosions of supernovae, binary systems of stars with stellar accretion, and active galactic nuclei which host black holes millions of times more massive than the Sun.

When high-energy cosmic rays and gamma rays reach the Earth, they interact with air molecules in the upper atmosphere. Gamma rays, for example, are converted into pairs of charged matter and anti-matter particles (mainly electrons and positrons). These particles rapidly interact with other air molecules, producing additional gamma rays of reduced energy which then create further charged particle pairs. This chain reaction proceeds until a large cascade of particles and radiation reaches ground level, where it can be recorded in the HAWC detectors.

When the charged particle cascade from an extra-terrestrial gamma ray passes through a Cherenkov detector, its particles are traveling faster than the speed of light in water. The resulting effect is similar to the shock wave produced in the atmosphere by a supersonic airplane (a "sonic boom"), but instead of producing sound the particles produce a visible cone of blue light. The flash of light, called Cherenkov radiation, is measured by the light sensor fixed to the bottom of each detector in HAWC. By combining the light signal observed in many tanks with fast electronics and high precision computing equipment, it is possible for scientists to determine the time of arrival, energy, and direction of the original extraterrestrial gamma ray or cosmic ray."

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Finding Unconventional Superconductors by Visual Inspection

Researchers, including Steve Anlage, have discovered a new and simple experimental test to see if a superconductor is ‘unconventional’ and unusual. Many of the high temperature superconductors discovered over the past 30 years are thought to be unconventional in nature. One strategy for discovering new higher temperature superconductors is to create even more exotic unconventional superconductors. The experiment creates a clear graphical image of the properties of the superconductor in different directions, as shown below. An anisotropic image is a clear sign of the unconventional nature of the superconducting state. This image shows the four nodal directions of the d-wave superconductor YBa2Cu3O7- as enhanced response (tall, white areas). The technique can now be applied to newly discovered superconductors to test them for unconventional properties, and will help guide the search for even higher temperature superconductors.

The results appeared in a spring issue of Physical Review Letters.


JQI and CNAM Researchers Uncover New Exotic Insulator

Topological insulators have recently emerged as a new and exciting form of quantum matter, in which topologies of the system (rather than symmetries) dictate the physical properties much like the situation found for the quantum Hall effect. To date, the majority of research has focused on non-interacting electron materials such as the bismuth-dichalcogenides, where the nearly free-electron model is sufficient to explain the non-trivial electronic structures that give rise to the observed topological states. An extension of this approach to strongly-correlated electron systems is challenging owing to the difficulty in determining the correct band structure by calculation, and remains as the next grand challenge in condensed matter physics.

A pioneering advance in this direction was made in 2009 by the team of Victor Galitski, who showed that a particularly simple type of heavy-electron material called a Kondo insulator, in which a filled band of heavy quasiparticles gives rise to a narrow band insulator, can host a three-dimensional topological insulating phase analogous to the non-interacting systems such as Bi2Se3. They developed a topological classification scheme for the emergent band structures of these systems, and predicted that at least one compound, samarium hexaboride, may be an ideal candidate.

Recently, this work and experiments performed in the Center for Nanophysics and Advanced Materials by the teams of Johnpierre Paglione and Richard Greene were highlighted in Nature [vol. 492, 165 (2012)] and Scientific American [December 12, 2012]. The experiments, using a technique called point-contact spectroscopy, explored the nature of the Kondo insulator SmB6 and found that the material indeed hosts an unusual metallic surface state as predicted by Dzero and co-workers, even though the bulk of the material becomes insulating upon cooling. Confirmed by other groups studying the properties of the surface state transport, this observation paves the way for the discovery of a new class of strongly-correlated topological insulator materials.

December 14, 2012