MAP

Thursday, 23 October 2014

AUSTRALIA’S FIRST DARK MATTER EXPERIMENT A proposed dark matter experiment would use two underground detectors, one in each hemisphere - Glenn Roberts Jr.

Physicists are hoping to hit pay dirt with a proposed experiment—the first of its kind in the Southern Hemisphere—that would search for traces of dark matter more than a half mile below ground in Victoria, Australia.
The current plan, now being explored by an international team, is for two new, identical dark matter experiments to be installed and operated in parallel—one at an underground site at Grand Sasso National Laboratory in Italy, and the other at the Stawell Gold Mine in Australia.
“An experiment of this significance could ultimately lead to the discovery of dark matter,” says Elisabetta Barberio of the ARC Centre of Excellence for Particle Physics at the Terascale (CoEPP) and the University of Melbourne, who is Australian project leader for the proposed experiment.
The experiment proposal was discussed during a two-day workshop on dark matter in September. Work could begin on the project as soon as 2015 if it gathers enough support. “We’re looking at logistics and funding sources,” Barberio says.
The experiments would be modeled after the DAMA experiment at Gran Sasso, now called DAMA/LIBRA, which in 1998 found a possible sign of dark matter.
DAMA/LIBRA looks for seasonal modulation, an ebb and flow in the amount of potential dark matter signals it sees depending on the time of year.
If the Milky Way is surrounded by a halo of dark matter particles, then the sun is constantly moving through it, as is the Earth. The Earth’s rotation around the sun causes the two to spend half of the year moving in the same direction and the other half moving in opposite directions. During the six months in which the Earth and sun are cooperating, a dark matter detector on the Earth will move faster through the dark matter particles, giving it more opportunities to catch them.
This seasonal difference appears in the data from DAMA/LIBRA, but no other experiment has been able to confirm this as a sign of dark matter.
For one thing, the changes in the signal could be caused on other factors that change by the season.
“There are environmental effects—different characteristics of the atmosphere—in winter and summer that are clearly reversed if you go from the Northern to the Southern hemisphere,” says Antonio Masiero, vice president for the Italian National Institute of Nuclear Physics (INFN) and a member of the Italian delegation collaborating on the proposal, which also includes Gran Sasso Director Stefano Ragazzi. If the results matched up at both sites at the same time of year, that would help to rule out such effects.
The Australian mine hosting the proposed experiment could also house scientific experiments from different fields.
“It wouldn’t be limited to particle physics and could include experiments involving biology, geosciences and engineering,” Barberio says. “These could include neutrino detection, nuclear astrophysics, geothermal energy extraction and carbon sequestration, and subsurface imaging and sensing.”
Preliminary testing has begun at the mine site down to depths of about 880 meters, about 200 meters above the proposed experimental site. Regular mining operations are scheduled to cease at Stawell in the next few years.

The ARC Centre of Excellence for All-Sky Astrophysics (CAASTRO), the local government in the Victoria area, and the mine operators have joined forces with COEPP and INFN to support the proposal.

Thursday, 16 October 2014

STUDY OF ELECTRONS IN SPACE COULD HELP WEATHER FORECASTING [phys.org] Oct 14, 2014

Researchers have discovered a formerly undetected impact of space weather on the polar atmosphere, which may explain some previously unexplained variations in winter weather patterns. Their results, published today (Tuesday 14 October), in the journalNature Communications could have important implications for seasonal weather forecasting.
The international team from the Finnish Meteorological Institute, Otago University and British Antarctic Survey (BAS) studied data from three different satellites over an 11 year period. They found that energetic electrons (highly charged particles created by the sun) from the outer radiation belt hitting the Earth's atmosphere cause ozone loss high above the Earth.
Vast quantities of energetic electrons are found in the Earth's radiation belts, trapped there by the Earth's magnetic field. During magnetic storms, which are driven by the solar wind, the electrons accelerate to high speeds and 'rain' into the atmosphere at the poles. The temporary, but frequent, ozone loss occurring as a result of these 'rains' may explain changes in wind patterns which affect regional winter temperatures in the Northern Hemisphere by a maximum of plus or minus 5 degrees centigrade. The variation in temperature is only seen during winter because of the complex linkages from space through to the Earth's surface.

BAS co-author Dr Mark Clilverd says, "This is an exciting piece of research because it shows us a key part of the chain-reaction of how electrons affect ozone at higher latitudes and can ultimately affect weather systems – a link that we didn't truly understand before. It could contribute to data for weather forecasting, for instance to show us when Europe is likely to experience an especially cold winter."

Tuesday, 14 October 2014

How Was Uranus Formed? by Nola Taylor Redd, SPACE.com Contributor   |   November 30, 2012 12:18pm ET

There are two theories as to how planets in the solar system were created. The first and most widely accepted, core accretion, works well with the formation of the terrestrial planets but has problems with giant planets such as Uranus. The second, the disk instability method, may account for the creation of giant planets.
The core accretion model
Approximately 4.6 billion years ago, the solar system was a cloud of dust and gas known as a solar nebula. Gravity collapsed the material in on itself as it began to spin, forming the sun in the center of the nebula.
With the rise of the sun, the remaining material began to clump together. Small particles drew together, bound by the force of gravity, into larger particles. The solar wind swept away lighter elements, such as hydrogen and helium, from the closer regions, leaving only heavy, rocky materials to create terrestrial worlds. But farther away, the solar winds had less impact on lighter elements, allowing them to coalesce into gas giants such as Uranus. In this way, asteroidscomets, planets, and moons were created.
Unlike most gas giants, Uranus has a core that is rocky rather than gaseous. The core likely formed first, and then gathered up the hydrogen, helium, and methane that make up the planet's atmosphere. Heat from the core drives Uranus' temperature and weather, overpowering the heat coming from the distant sun, which is almost two billion miles away.
The disk instability model
But the need for a rapid formation for the giant gas planets is one of the problems of core accretion. According to models, the process takes several million years, longer than the light gases were available in the early solar system. At the same time, the core accretion model faces a migration issue, as the baby planets are likely to spiral into the sun in a short amount of time.
According to a relatively new theory, disk instability, clumps of dust and gas are bound together early in the life of the solar system. Over time, these clumps slowly compact into a giant planet. These planets can form faster than their core accretion rivals, sometimes in as little as a thousand years, allowing them to trap the rapidly-vanishing lighter gases. They also quickly reach an orbit-stabilizing mass that keeps them from death-marching into the sun.
As scientists continue to study planets inside of the solar system, as well as around other stars, they will better understand how Uranus and its siblings formed.
A dangerous youth
The early solar system was a time of violent collisions, and Uranus was not exempt. While the surface of the moon and Mercury both show evidence of bombardment by smaller rocks and asteroids, Uranus apparently suffered a significant collision with an Earth-size proto planet. As a result, Uranus is tipped on its side, with one pole pointing toward the sun for half the year.
Uranus is the smallest of the gas giants, perhaps in part because it lost some of its mass during the impact.
— Nola Taylor Redd, SPACE.com Contributor

A COLD-ATOM AMMETER A super fluid current is only as strong as its weak link - October 8, 2014

In certain exotic situations, a collection of atoms can transition to a superfluid state, flouting the normal rules of liquid behavior. Unlike a normal, viscous fluid, the atoms in a superfluid flow unhindered by friction. This remarkable free motion is similar to the movement of electron pairs in a superconductor, the prefix ‘super’ in both cases describing the phenomenon of resistanceless flow. Harnessing this effect is of particular interest in the field of atomtronics, since superfluid atom circuits can recreate the functionality of superconductor circuits, with atoms zipping about instead of electrons. Now, JQI scientists have added an important technique to the atomtronics arsenal, a method for analyzing a superfluid circuit component known as a ‘weak link’. The result, detailed in the online journal Physical Review X, is the first direct measurement of the current-phase relationship of a weak link in a cold atom system.
“What we have done is invented a way to characterize a particular circuit element [in a superfluid atomtronic circuit],” says Stephen Eckel, lead author of the paper. “This is similar to characterizing a component in an ordinary electrical circuit, where one measures the current that flows through the component vs. the voltage across it.”
Properly designing an electronic circuit means knowing how each component in the circuit affects the flow of electrons. Otherwise, your circuit won’t function as expected, and at worst case will torch your components into uselessness. This is similar to the plumbing in a house, where the shower, sink, toilet, etc. all need the proper amount of water and water pressure to operate. Measuring the current-voltage relationship, or how the flow of current changes based on a voltage change, is an important way to characterize a circuit element. For instance, a resistor will have a different current-voltage relationship than a diode or capacitor. In a superfluid atom circuit, an analogous measurement of interest is the current-phase relationship, basically how a particular atomtronic element changes the flow of atoms.
Interferometric Investigations
The experiment, which took place at a JQI lab on the NIST-Gaithersburg campus, involves cooling roughly 800,000 sodium atoms down to an extremely low temperature, around a decidedly chilly hundred billionths of a degree above absolute zero. At these temperatures, the atoms behave as matter waves, overlapping to form something called a Bose-Einstein condensate (BEC). The scientists confine the condensate between a sheet-like horizontal laser and a target shaped vertical laser. This creates two distinct clouds, the inner one shaped like a disc and the outer shaped like a ring. The scientists then apply another laser to the outer condensate, slicing the ring vertically. This laser imparts a repulsive force to the atoms, driving them apart and creating a low density region known as a weak link (Related article on this group's research set-up).
The weak link used in the experiment is like the thin neck between reservoirs of sand in an hourglass, constricting the flow of atoms across it. Naturally, you might expect that a constriction would create resistance. Consider pouring syrup through a straw instead of a bottle -- this would be a very impractical method of syrup delivery. However, due to the special properties of the weak link, the atoms can flow freely across the link, preserving superfluidity. This doesn’t mean the link has no influence: when rotated around the ring, the weak link acts kind of like a laser ‘spoon’, ‘stirring’ the atoms and driving an atom current.
After stirring the ring of atoms, the scientists turn off all the lasers, allowing the two BECs to expand towards each other. Like ripples on a pond, these clouds interfere both constructively and destructively, forming intensity peaks and valleys. The researchers can use the resulting interference pattern to discern features of the system, a process called interferometry.
Gleaning useful data from an interference pattern means having a reference wave. In this case, the inner BEC serves as a phase reference. A way to think of phase is in the arrival of a new day. A person who lives on the other side of the planet from you experiences a new day at the same frequency as you do, once every 24 hours. However, the arrival of the day is offset in time, that is to say there is a phase difference between your day and the other person's day.
As the two BECs interfere, the position of the interference fringes (peaks in the wave) depends on the relative phase between the two condensates. If a current is present in the outer ring-shaped BEC, the relative phase is changing as a function of the position of the ring, and the interference fringes assume a spiral pattern. By tracing a single arm of the spiral a full 360 degrees and measuring the radial difference between the beginning and end of the trace, the researchers can extract the magnitude of the superfluid current present in the ring.
They now know the current, so what about the phase across the weak link? The same interferometry process can be applied to the two sides of the weak link, again yielding a phase difference. When coupled with the measured current, the scientists now have a measure of how much current flows through the weak link as a function of the phase difference across the link, the current-phase relationship. For their system, the group found this dependence to be roughly linear (in agreement with their model).
A different scenario, where the weak link has a smaller profile, might produce a different current response, one where non-linear effects play a larger role. Extending the same methods makes it possible to characterize these weak links as well, and could be used to verify a type of weak link called a Josephson junction, an important superconducting element, in a cold atom system. Characterizing the current-phase relationship of other atomtronic components should also be possible, broadening the capabilities of researchers to analyze and design new atomtronic systems.
This same lab, led by JQI fellow Gretchen Campbell, had recently employed a weak link to demonstrate hysteresis, an important property of many electronic systems, in a cold atom circuit. Better characterizing the weak link itself may help realize more complex circuits.  “We’re very excited about this technique,” Campbell says, “and hope that it will help us to design and understand more complicated systems in the future."
This article was written by S. Kelley/JQI.

- See more at: http://jqi.umd.edu/news/cold-atom-ammeter#sthash.WQAji039.dpuf

RESEARCHERS MAKE FIRST OBSERVATION OF ATOMS MOVING INSIDE BULK MATERIAL - 23 hours ago

Researchers at the Department of Energy's Oak Ridge National Laboratory have obtained the first direct observations of atomic diffusion inside a bulk material. The research, which could be used to give unprecedented insight into the lifespan and properties of new materials, is published in the journal Physical Review Letters.
"This is the first time that anyone has directly imaged single dopant atoms moving around inside a material," said Rohan Mishra of Vanderbilt University who is also a visiting scientist in ORNL's Materials Science and Technology Division.
Semiconductors, which form the basis of modern electronics, are "doped" by adding a small number of impure atoms to tune their properties for specific applications. The study of the dopant atoms and how they move or "diffuse" inside a host lattice is a fundamental issue in materials research.
Traditionally, diffusion of atoms has been studied through indirect macroscopic methods or through theoretical calculations. Diffusion of single atoms has previously been directly observed only on the surface of materials.
The experiment also allowed the researchers to test a surprising prediction: Theory-based calculations for dopant motion in aluminum nitride predicted faster diffusion for cerium atoms than for manganese atoms. This prediction is surprising as cerium atoms are larger than manganese atoms.
"It's completely counterintuitive that a bigger, heavier atom would move faster than a smaller, lighter atom," said the Material Science and Technology Division's Andrew Lupini, a coauthor of the paper.
In the study, the researchers used a scanning transmission electron microscope to observe the diffusion processes of cerium and manganese dopant atoms. The images they captured showed that the larger cerium atoms readily diffused through the material, while the smaller manganese atoms remained fixed in place.
The team's work could be directly applied in basic material design and technologies such as energy-saving LED lights where dopants can affect color and atom movement can determine the failure modes.

"Diffusion governs how dopants get inside a material and how they move," said Lupini. "Our study gives a strategy for choosing which dopants will lead to a longer device lifetime.”

Monday, 22 September 2014

PARTICLE DETECTOR FINDS HINTS OF DARK MATTER IN SPACE - Sep 19, 2014 by Jennifer Chu

Researchers at MIT's Laboratory for Nuclear Science have released new measurements that promise to shed light on the origin of dark matter.

 The MIT group leads an international collaboration of scientists that analyzed two and a half years' worth of data taken by the Alpha Magnetic Spectrometer (AMS)—a large particle detector mounted on the exterior of the International Space Station—that captures incoming cosmic rays from all over the galaxy.
Among 41 billion cosmic ray events—instances of cosmic particles entering the detector—the researchers identified 10 million electrons and positrons, stable antiparticles of electrons. Positrons can exist in relatively small numbers within the cosmic ray flux.
An excess of these particles has been observed by previous experiments—suggesting that they may not originate from cosmic rays, but come instead from a new source. In 2013, the AMS collaboration, for the first time, accurately measured the onset of this excess.
The new AMS results may ultimately help scientists narrow in on the origin and features of dark matter—whose collisions may give rise to positrons.
The team reports the observed positron fraction—the ratio of the number of positrons to the combined number of positrons and electrons—within a wider energy range than previously reported. From the data, the researchers observed that this positron fraction increases quickly at low energies, after which it slows and eventually levels off at much higher energies.
The team reports that this is the first experimental observation of the positron fraction maximum—at 243 to 307 gigaelectronvolts (GeV)—after half a century of cosmic ray experiments.
"The new AMS results show unambiguously that a new source of positrons is active in the galaxy," says Paolo Zuccon, an assistant professor of physics at MIT. "We do not know yet if these positrons are coming from dark matter collisions, or from astrophysical sources such as pulsars. But measurements are underway by AMS that may discriminate between the two hypotheses."

 The new measurements,
Zuccon adds, are compatible with a dark matter particle with mass on the order of 1 teraelectronvolt (TeV)—about 1,000 times the mass of a proton.
Zuccon and his colleagues, including AMS's principal investigator, Samuel Ting, the Thomas D. Cabot Professor of Physics at MIT, detail their results in two papers published today in the journal Physical Review Letters and in a third, forthcoming publication.
Catching a galactic stream
Nearly 85 percent of the universe is made of dark matter—matter that somehow does not emit or reflect light, and is therefore invisible to modern telescopes. For decades, astronomers have observed only the effects of dark matter, in the form of mysterious gravitational forces that seem to hold together clusters of galaxy that would otherwise fly apart. Such observations eventually led to the theory of an invisible, stabilizing source of gravitational mass, or dark matter.
The AMS experiment aboard the International Space Station aims to identify the origins of dark matter. The detector takes in a constant flux of cosmic rays, which Zuccon describes as "streams of the universe that bring with them everything they can catch around the galaxy."
Presumably, this cosmic stream includes leftovers from the violent collisions between dark matter particles.
According to theoretical predictions, when two dark matter particles collide, they annihilate, releasing a certain amount of energy that depends on the mass of the original particles. When the particles annihilate, they produce ordinary particles that eventually decay into stable particles, including electrons, protons, antiprotons, and positrons.
As the visible matter in the universe consists of protons and electrons, the researchers reasoned that the contribution of these same particles from dark matter collisions would be negligible. However, positrons and antiprotons are much rarer in the universe; any detection of these particles above the very small expected background would likely come from a new source. The features of this excess—and in particular its onset, maximum position, and offset—will help scientists determine whether positrons arise from astrophysical sources such as pulsars, or from dark matter.
After continuously collecting data since 2011, the AMS team analyzed 41 billion incoming particles and identified 10 million positrons and electrons with energies ranging from 0.5 to 500 GeV—a wider energy range than previously measured.
The researchers studied the positron fraction versus energy, and found an excess of positrons starting at lower energies (8 GeV), suggesting a source for the particles other than the cosmic rays themselves. The positron fraction then slowed and peaked at 275 GeV, indicating that the data may be compatible with a dark matter source of positrons.
"Dark matter is there," Zuccon says. "We just don't know what it is. AMS has the possibility to shine a light on its features. We see some hint now, and it is within our possibility to say if that hint is true."
If it turns out that the AMS results are due to dark matter, the experiment could establish that dark matter is a new kind of particle, says Barry Barish, a professor emeritus of physics and high-energy physics at the California Institute of Technology.

"The new phenomena could be evidence for the long-sought dark matter in the universe, or it could be due to some other equally exciting new science," says Barish, who was not involved in the experiments. "In either case, the observation in itself is what is exciting; the scientific explanation will come with further experimentation.”