Elusive Higgs slips from sight again

Now you see it, now you don't. Rather like a conjurer's white rabbit, the elusive Higgs boson may have slipped from sight again.

A recent report hinted at a glimpse of the long-sought particle at a major detector at the Large Hadron Collider (LHC) at CERN near Geneva, Switzerland. But a second detector has now checked its own data and found no corroborating sign of the particle.

The Higgs boson is thought to endow other particles with mass, but has yet to be observed. Four physicists associated with the LHC's ATLAS detector claimed to have found an anomalous "bump" in its data, possibly due to Higgs particles decaying into pairs of photons. An abstract of their study was leaked online in April.

Bump, what bump? 

Now physicists working on the LHC's other main detector, CMS, have come up empty in an initial search for a similar bump in their data, according to a document shown to New Scientist. So ATLAS's bump may not be due to Higgs particles, after all, but instead down to something mundane, such as an error in the analysis.

The internal CMS document has not been released to the public, so the result is still preliminary, as was the news of the original ATLAS bump, for that matter, which was leaked before it was reviewed or endorsed by the ATLAS collaboration.

Both leaks are a testament to the excitement surrounding the Higgs. With a result this hot on the horizon, expect more fits and starts in the months to come.

Source New Scientist

Solar power, with a side of hot running water

New system for flat-panel solar power could be combined with hot water systems for greater performance.

Doctoral student Daniel Kraemer, right, and Professor Gang Chen display a prototype of a flat-panel solar-thermoelectric generating device.
Photo: Melanie Gonick

MIT researchers and their collaborators have come up with an unusual, high performance and possibly less expensive way of turning the sun’s heat into electricity.

Their system, described in a paper published online in the journal Nature Materials on May 1, produces power with an efficiency roughly eight times higher than ever previously reported for a solar thermoelectric device — one that produces electricity from solar heat. It does so by generating and harnessing a temperature difference of about 200 degrees Celsius between the interior of the device and the ambient air.

The concept “is very radical,” says Gang Chen, MIT’s Carl Richard Soderberg Professor in Power Engineering and director of the Pappalardo Micro and Nano Engineering Laboratories, who co-authored the new paper with MIT doctoral student Daniel Kraemer and collaborators from Boston College and GMZ Energy. The work is funded by the Solid-State Solar-Thermal Energy Conversion Center, an Energy Frontier Research Center at the U.S. Department of Energy.

While solar thermal electricity systems aren’t a new idea, they typically involve vast arrays of movable mirrors that track the sun and focus its rays on a small area. The new approach uses flat, stationary panels similar to traditional solar panels, eliminating the need for tracking systems.

Like the silicon photovoltaic cells that produce electricity when struck by sunlight, Chen’s system is a solid-state device with no moving parts. A thermoelectric generator, placed inside a vacuum chamber made of glass, is covered with a black plate of copper that absorbs sunlight but does not re-radiate it as heat. The other side of the generator is in contact with ambient temperatures. Placed in the sun, the entire unit heats up quickly, even without facing the sun directly.

The device requires much less material than conventional photovoltaic panels, and could therefore be much less expensive to produce. It can also be integrated into solar hot water systems, allowing the expenses of the structure and installation to serve two functions at once. Such solar water heaters are rarely seen in the United States, but are already a highly successful mass-market product in China and Europe, where they provide households with hot water and in some cases space heating as well.

The materials used to build such solar thermoelectric generators, made through a nanostructured process, were developed jointly a few years ago in Chen’s lab at MIT and in co-author Zhifeng Ren’s lab at Boston College. Their teams have continued to work on improving these materials and integrating them into complete systems.

Chen points out that the U.S. Department of Energy has programs to develop thermoelectric systems, mostly geared toward harnessing waste heat from car and truck engines. He says that solar applications for such devices also can “have an important role to play” in reducing carbon emissions. “Hopefully we can prove that,” he adds.

Li Shi, associate professor of mechanical engineering at the University of Texas at Austin, says this approach to solar power is “very novel, simple, and easy for low-cost implementation.” The efficiency level they have demonstrated so far, at 4.6 percent, is “already quite impressive,” he says.

“With the use of other or new thermoelectric materials that can operate at a higher temperature,” Shi adds, “the efficiency may be improved further to be competitive with that for state-of-the-art amorphous silicon solar cells. This can potentially provide a different approach to realizing the $1-per-watt goal for solar-electricity conversion.”

The new system wouldn’t be a substitute for solar photovoltaics, Chen says, but offers “another way” of tapping into the enormous amount of solar energy that bathes the Earth every day. And because it can be piggybacked onto the existing solar hot-water industry, the thermoelectric device could be a relatively inexpensive addition, with “no subsidies required,” Chen suggests. “It can be a game-changing thing,” he says.

Source MIT News

Study helps explain behavior of latest high-temp superconductors

Rice University, Los Alamos physicists explain similar behavior by dissimilar compounds

HOUSTON -- (May 3, 2011) -- A Rice University-led team of physicists this week offered up one of the first theoretical explanations of how two dissimilar types of high-temperature superconductors behave in similar ways.

The research appears online this week in the journal Physical Review Letters. It describes how the magnetic properties of electrons in two dissimilar families of iron-based materials called "pnictides" (pronounced: NICK-tides) could give rise to superconductivity. One of the parent families of pnictides is a metal and was discovered in 2008; the other is an insulator and was discovered in late 2010. Experiments have shown that each material, if prepared in a particular way, can become a superconductor at roughly the same temperature. This has left theoretical physicists scrambling to determine what might account for the similar behavior between such different compounds.

Rice physicist Qimiao Si, the lead researcher on the new paper, said the explanation is tied to subtle differences in the way iron atoms are arranged in each material. The pnictides are laminates that contain layers of iron separated by layers of other compounds. In the newest family of insulating materials, Chinese scientists found a way to selectively remove iron atoms and leave an orderly pattern of "vacancies" in the iron layers.
Si, who learned about the discovery of the new insulating compounds during a visit to China in late December, suspected that the explanation for the similar behavior between the new and old compounds could lie in the collective way that electrons behave in each as they are cooled to the point of superconductivity. His prior work had shown that the arrangement of the iron atoms in the older materials could give rise to collective behavior of the magnetic moments, or "spins," of electrons. These collective behaviors, or "quasi-localizations," have been linked to high-temperature superconductivity in both pnictides and other high-temperature superconductors.

"The reason we got there first is we were in a position to really quickly incorporate the effect of vacancies in our model," Si said. "Intuitively, on my flight back (from China last Christmas), I was thinking through the calculations we should begin doing."
Si conducted the calculations and analyses with co-authors Rong Yu, postdoctoral research associate at Rice, and Jian-Xin Zhu, staff scientist at Los Alamos National Laboratory.
"We found that ordered vacancies enhance the tendency of the electrons to lock themselves some distance away from their neighbors in a pattern that physicists call 'Mott localization,' which gives rise to an insulating state," Yu said. "This is an entirely new route toward Mott localization."
By showing that merely creating ordered vacancies can prevent the material from being electrical conductors like their relatives, the researchers concluded that even the metallic parents of the iron pnictides are close to Mott localization.

"What we are learning by comparing the new materials with the older ones is that these quasi-localized spins and the interactions among them are crucial for superconductivity, and that's a lesson that can be potentially applied to tell experimentalists what is good for raising the transition temperature in new families of compounds," Zhu said.
Superconductivity occurs when electrons pair up and flow freely through a material without any loss of energy due to resistance. This most often occurs at extremely low temperatures, but compounds like the pnictides and others become superconductors at higher temperatures -- close to or above the temperature of liquid nitrogen -- which creates the possibility that they could be used on an industrial scale. One impediment to their broader use has been the struggle to precisely explain what causes them to become superconductors in the first place. The race to find that has been called the biggest mystery in modern physics.
"The new superconductors are arguably the most important iron-based materials that have been discovered since the initial discovery of iron pnictide high-temperature superconductors in 2008," Si said. "Our theoretical results provide a natural link between the new and old iron-based superconductors, thereby suggesting a universal origin of the superconductivity in these materials."

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The research was funded by the National Science Foundation, the Robert A. Welch Foundation and the Department of Energy. It was facilitated by the International Collaborative Center on Quantum Matter, a collaborative entity Rice formed with partner institutions from China, Germany and United Kingdom.

Source EurekaAlert!