Researchers continue search for elusive new particles at CERN

Sung-Won Lee, an assistant professor of physics at Texas Tech and a member of the university’s High Energy Physics Group, said researchers have not given up finding any possible hints of new physics, which could add more subatomic particles to theStandard Modelof particle physics.

Their findings were published recently inPhysical Review Letters. Their results are the first of the“new physics” research papers produced from theCMS experimentat LHC.

“So far, we have not yet found any hint of the new particles with early LHC data, but we set the world’s most stringent limits on the existence of several theorized new types of particles,” said Lee, who co-led the analysis team searching for these new particles.

Currently, the Standard Model of physics only explains about 5 percent of the universe, Lee said.

“The Standard Model of particle physics has been enormously successful, but it leaves many important questions unanswered,” Lee said.“Also, it is widely acknowledged that, from the theoretical standpoint, the Standard Model must be part of a larger theory, known as‘beyond the Standard Model,’ which is yet to be experimentally confirmed.”

Finding evidence of new particles could open the door to whole new realms of physics that researchers believe could be there, such as string theory, which posits that subatomic particles such as electrons and quarks are not zero-dimensional objects, but rather one-dimensional lines, or“strings.” It could also help prove space-time-matter theory, which requires the existence of several extra spatial dimensions to the universe as well as length, width, height and time.

One of the most popular suggestions for the‘beyond the Standard Model’ theory is Supersymmetry, which introduces a new symmetry between fundamental particles, he said. Supersymmetry signals are of particular interest, as they provide a natural explanation for the“dark matter” known to pervade our universe and help us to understand the fundamental connection between particle physics and cosmology.

Furthermore there are a large number of important theoretical models that make strong cases for looking for new physics at the LHC.

“Basically, we’re looking for the door to new theories such as string theory, extra dimensions and black holes,” Lee said.“None of the rich new spectrum ofparticlespredicted by these models has yet been found within the kinematic regime reachable at the present experiments. The LHC will increase this range dramatically after several years of running at the highest energy and luminosity.

“I believe that, with our extensive research experience, Texas Tech’s High Energy Physics Group can contribute to making such discoveries.”

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Scientists crash lead nuclei together to create the hottest and densest nuclear material ever

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On December 2 several scientists at theCERNlaboratory in Geneva, Switzerland reported the first results of an experiment in which the nuclei ofleadatoms were shot around the 17 mile racetrack called theLarge Hadron Colliderand then smashed into each other to create, for an instant, a speck of matter at a temperature of trillions of degrees.

Although the miniature fireballs that occur at the lead-lead collision points only last a fleeting moment -- about a trillionth of a trillionth of a second -- the immense detectors poised nearby are designed to act rapidly and sort through the myriad debris particles streaming outwards.

"This is the hottest nuclear matter ever created in a lab,"said Bolek Wyslouch of the Ecole Polytechnique near Paris who spoke at the CERN gathering. He is a representative of the Compact Muon Solenoid collaboration, which uses one of the giant detectors at LHC to observe the lead-lead collisions.

"I like to call this the Little Bang,"said Juergen Schukraft, also speaking at the CERN colloquium, suggesting that the violent collisions of heavy ions at the LHC were smaller cousins of the Big Bang explosion that ushered in the visible universe some 14 billion years ago. Indeed, the conditions of the mini-fireballs at LHC resemble theearly universeas it was only microseconds after the Big Bang in terms of energy density and temperature. Schukraft represented a second CERN detector group called Alice.

Never before has so much energy -- in this case hundreds of trillions of electron volts abbreviated as TeV -- been deliberately deposited in a volume of space only a few times the size of a proton. A proton is one of the constituents of the nucleus inside each atom, and is some 10,000 times smaller than the atom itself. Scientists who work at accelerators often use the electron volt as their unit of energy since it is precisely the energy gained by an electron accelerated by an electric force difference of one volt.

What happens when two lead nuclei containing hundreds of protons and neutrons, each of which have an energy of 1.4 TeV, smash into each other in an almost head-on collision? As they meet and interact the protons and neutrons melt into even more basic constituents, called quarks and gluons. What you get is a seething liquid of hundreds of strongly interacting particles, called by physicists a quark-gluon plasma.

Earlier this year scientists at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in New York reported on RHIC's collision measurements from a quark-gluon plasma made by colliding gold nuclei. They reported the temperature of the plasma to be 4 trillion degrees, the hottest temperature ever carefully measured in an experiment.

The LHC scientists haven't yet directly measured the temperature of their quark plasma. Schukraft said that since theenergy densityof the collisions is some three times larger at LHC than at RHIC, the temperatures will be higher also.

In following weeks, a series of specific results from the LHC heavy ions will appear in scientific journals. Scientists from the Atlas collaboration -- which operates a third large detector at LHC -- report on their observations of huge jets emerging sideways from the collisions. A jet is a powerful cone of energy, in the form of flying particles that emerges from the fireball shortly after the collision. Scientists expect that if a powerful jet shoots out of the collision on one side, there should be a complementary jet on the other side that balances momentum.

In many collision events, however, only one jet is observed. In an article about to appear in the journalPhysical Review Letters, the Atlas scientists report the first such examples of the imbalance between jets in the lead-lead collisions. But what happened to the missing jet?

Brian Cole, speaking at CERN on behalf of the Atlas team, said that the quark-gluon plasma itself is probably absorbing part or all of the jets on their way outwards. This process doesn't have to be symmetric.

"The more central the collision,"Cole said, referring to how head-on the collision,"the more asymmetric the jets are."

Another Atlas scientist, Peter Steinberg, said that scientists expected that some of the jet energy would be absorbed, but were surprised that in some events the jet seemed to be completely absorbed.

The asymmetric appearance of jets, the scientists hope, can be used to understand the unprecedented nature of this densest matter ever observed in a lab.

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Farmers slowed down by hunter-gatherers: Our ancestors' fight for space

Research published today, Friday, 3 December 2010, inNew Journal of Physics, details a physical model, which can potentially explain how the spreading of Neolithic farmers was slowed down by thepopulationdensity of hunter-gatherers.

The researchers from Girona, in Catalonia, Spain, use a reaction-diffusion model, which explains the relation between population growth and available space, taking into account the directional space dependency of the established Mesolithic population density.

The findings confirm archeological data, which shows that the slowdown in the spreading of farming communities was not, as often assumed, the result of crops needing to adapt to chillier climates, but indeed a consequence of the struggle for space with prevalent hunter-gatherer communities.

In the future, the researchers' model could be used for further physical modeling of socioeconomic transitions in the history of humanity. As the researchers write,"Themodelpresented in this work could be applied to many examples of invasion fronts in which the indigenous population and the invasive one compete for space in a single biological niche, both in natural habitats and in microbiological assays."

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Researchers create high performance infrared camera based on type-II InAs/GaSb superlattices

Created by Manijeh Razeghi, Walter P. Murphy Professor of Electrical Engineering and Computer Science, and researchers in the Center for Quantum Devices in the McCormick School of Engineering and Applied Science, the long wavelength infrared focal plane array camera provides a 16-fold increase in the number of pixels in the image and can provideinfrared imagesin the dark. Their results were recently published in the journalApplied Physics Letters, Volume 97, Issue 19, 193505 (2010).

The goal of the research is to offer a better alternative to existing long wavelengthinfrared radiation(LWIR) cameras, which, with their thermal imaging capabilities, are used in everything from electrical inspections to security and nighttime surveillance. Current LWIR cameras are based on mercurycadmium telluride(MCT) materials, but the Type-II superlattice is mercury-free, more robust, and can be deposited with better uniformity. This will significantly increase yield and reduce camera cost once the technology goes commercial.

"Not only does it prove Type-II superlattices as a viable alternative to MCT, but also it widens the field of applications for infrared cameras,"Razeghi said."The importance of this work is similar to that of the realization of mega-pixel visible cameras in the last decade, which shaped the world's favor for digital cameras."

Type-II InAs/GaSb superlattices were first invented by Nobel laureate Leo Esaki in the 1970s, but it has taken time for the material to mature. The LWIR detection mechanism relies on quantum size effects in a completely artificial layer sequence to tune the wavelength sensitivity and demonstrate high efficiency. Razeghi's group has been instrumental in pioneering the recent development of Type-II superlattices, having demonstrated the world's first Type-II–based 256×256infrared camerajust a few years ago.

"Type-II is a very interesting and promising new material for infrared detection,"Razeghi said."Everything is there to support its future: the beautiful physics, the practicality of experimental realization of the material. It has just taken time to prove itself, but now, the time has come."

Tremendous obstacles, especially in the fabrication process, had to be overcome to ensure that the 1024×1024 Type-II superlattice–based camera would have equivalent performance as the previously realized 320×256 cameras. Operating at 81 K, the new camera can collect 78 percent of the light and is capable of showing temperature differences as small as 0.02° C.

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Magnetic switching under pressure

Now scientists at the U.S. Department of Energy’s (DOE’s) Argonne National Laboratory, along with a collaborator from Eastern Washington University, have harnessed the power of extreme high pressure and discovered a novel approach to predictably tune the switching of a promising new family of next-generation magnetic materials. Their results were published in the online version ofAngewandte Chemie International Editionon September 10, 2010.

The magnetic materials the team studied are made from copper ions and simple building blocks, including water and fluoride. Their simple structures are held together by strong hydrogen bonds, making very robust molecular networks. Each copper ion, which sits at the corner of a molecular cube, contains one unpaired electron. These spins are disordered at normal temperatures, but begin to align in opposite directions at low temperatures, creating the magnetic state called antiferromagnetism.

Using state-of-the-art high-pressure equipment and the high-energy, highly focused x-ray beams from the X-ray Science Division 1-BM beamline at the Argonne Advanced Photon Source, the scientists observed a series of structural transitions as they exerted pressure on the material. These rearrangements abruptly reoriented the magnetic spins of the material, creating a reversible magnetic switch effect.

High-pressure science has traditionally been the domain of Earth scientists and, until now, has not been routinely used to study molecular systems. Such studies promise to greatly improve our understanding of the often complex way materials’ functional properties are related to their molecular structures—a key step in realizing the diverse potential of molecular materials.

“Molecule-based materials are much softer than traditional solid-state materials, like oxides and minerals,” said Argonne scientist Gregory Halder.“As such, we can expect to induce dramatic changes to their structures and functional properties at relatively low, industrially relevant pressures.”

Next up for this team of researchers: Studying a targeted series of molecular networkmaterialsunder pressure to understand the broader implications of this new phenomenon.

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A step toward fusion power: MIT advance helps remove contaminants that slow fusion reactions

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The new experiments have revealed a set of operating parameters for the reactor— a so-called“mode” of operation— that may provide a solution to a longstanding operational problem: How to keep heat tightly confined within the hot charged gas (called plasma) inside the reactor, while allowing contaminating particles, which can interfere with the fusion reaction, to escape and be removed from the chamber.

Most of the world’s experimental fusion reactors, like the one at MIT’s Plasma Science and Fusion Center, are of a type called tokamaks, in which powerful magnetic fields are used to trap the hot plasma inside a doughnut-shaped (or toroidal) chamber. Typically, depending on how the strength and shape of the magnetic field are set, both heat and particles can constantly leak out of the plasma (in a setup called L-mode, for low-confinement) or can be held more tightly in place (called H-mode, for high-confinement).

Now, after some 30 years of tests using the Alcator series of reactors (which have evolved over the years), the MIT researchers have found another mode of operation, which they call I-mode (for improved), in which the heat stays tightly confined, but the particles, including contaminants, can leak away. This should prevent these contaminants from“poisoning” the fusion reaction.“This is very exciting,” says Dennis Whyte, professor in the MIT Department of Nuclear Science and Engineering and coauthor of some recent papers that describe more than 100 experiments testing the new mode. Whyte presented the results in October at the International Atomic Energy Agency International Fusion Conference in South Korea.“It really looks distinct” from the previously known modes, he says.

While in previous experiments in tokamaks the degree of confinement of heat and particles always changed in unison,“we’ve at last proved that they don’t have to go together,” says Amanda Hubbard, a principal research scientist at MIT’s Plasma Science and Fusion Center and coauthor of the reports. Hubbard presented the latest results in an invited talk at the November meeting of the American Physical Society’s Division of Plasma Physics, and says the findings“attracted a lot of attention.” But, she added,“we’re still trying to figure out why” the new mode works as it does. The work is funded by the U.S. Department of Energy.

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Alcator C-Mod, shown here, is the most powerful university-based fusion device in the world. Recent findings there could help point the way to power-producing fusion reactors. Image: Plasma Science and Fusion Center

The fuel in planned tokamaks, which comprises the hydrogen isotopes deuterium and tritium, is heated to up to more than 100 million degrees Celsius (although in present reactors like Alcator C-Mod, tritium is not used, and the temperatures are usually somewhat lower). This hot plasma is confined inside a doughnut-shaped magnetic“bottle” that keeps it from touching— and melting— the chamber’s walls. Nevertheless, its proximity to those walls and the occasional leakage of some hot plasma causes a small number of particles from the walls to mix with the plasma, producing one kind of contaminant. The other kind of expected contaminant is a product of the fusion reactions themselves: helium atoms, created by the fusing of hydrogen atoms, but which are not capable of further fusion under the same conditions.

When a fusion reactor operates, the impurities accumulate. Whyte says there have been various experimental observations and theoretical proposals for removing them at intervals after they accumulate. Now, he says,“We seem to have discovered a completely different flushing mechanism… so they don’t build up in the first place.”

One of the keys to triggering the new mode was to configure the magnetic fields inside the tokamak in a way that is essentially upside-down from the usual H-mode setup, Hubbard says.

The findings could be significant in enabling the next step forward in fusion energy, where fusion reactions and power are sustained mostly by“self-heating” without requiring a larger constant addition of outside power. Researchers expect to achieve this milestone, referred to as“fusion burn,” in a new international collaboration on a reactor called ITER, currently being built in France. The findings from MIT“almost certainly could be applied” to the very similar design of the ITER reactor, Whyte says.

Patrick Diamond PhD’79, professor of plasma physics at the University of California at San Diego, says,“The findings are potentially of great importance,” because they could solve a key problem facing the design of next-generationfusionreactors: the occurrence of unpredictable bursts of heat from the edge of the confined plasma, which can“fry” some of the tokamak’s internal parts.“The I-mode eliminates or greatly reduces” these bursts of heat,“because it allows a steep temperature gradient— which is what you want— but does not allow a steep density gradient, which we don’t really need,” he says.

Diamond adds that theorists will have their work cut out to explain this mode.“Why do heat and particle transport behave differently? This is a really fundamental question, since most theories would predict a strong coupling between the two,” he says.“It’s a real challenge to us theorists— and important conceptually as well as practically.”

Rich Hawryluk, a researcher at the Princeton Plasma Physics Laboratory, says this is a"significant advance"which has generated considerable international interest and that other groups are now planning to follow up on these results. One area of research will be whether it is possible to"reliably operate in the I-mode and not go into the H-mode, which might have these violent edge instabilities. The operating conditions and the control requirements to stay in I-mode need to be better understood."

Hubbard explained that one of the key differences that made it possible to discover this phenomenon in MIT’s Alcator C-Mod was that this relatively small reactor, though large enough to produce results relevant to future reactors such as ITER, has great operational flexibility and can easily follow up on new findings. While larger reactors typically plan all their tests up to two years in advance, she says,“with this smaller machine, we have the ability to try new things when they appear. This ability to explore has been a key.”

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This story is republished courtesy of MIT News (http://web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and teaching.

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Dark matter could transfer energy in the Sun

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"We assume that thedark matter particlesinteract weakly with the Sun's atoms, and what we have done is calculate at what level these interactions can occur, in order to better describe the structure and evolution of the Sun", Marco Taoso, researcher at the IFIC, a combined centre of the Spanish National Research Council and the University of Valencia, explains to SINC.

The astrophysical observations suggest that our galaxy is situated in a halo of dark matter particles. According to the models, some of these particles, the WIMPs (Weakly Interacting Massive Particles) interact weakly with other normal ones, such as atoms, and could be building up on the inside of stars. The study, recently published in the journalPhysical Review D, carries out an in-depth study of the case of the Sun in particular.

"When the WIMPs pass through the Sun they can break up the atoms of our star and lose energy. This prevents them from escaping the gravitational force of the Sun which captures them, and they become trapped, orbiting inside it, with no way of escaping", the researcher points out.

The dark matter cools down the Sun's core

Scientists believe that the majority of the dark matter particles gather together in the centre of the Sun, but in their elliptic orbits they also travel to the outer part, interacting and exchanging with the solar atoms. In this way, the WIMPs transport the energy from the burning central core to the cooler peripheral parts.

"This effect produces a cooling down of the core, the region from where the neutrinos originate due to the nuclear reactions of the Sun", Taoso points out."And this corresponds to a reduction in the flux of solar neutrinos, since these depend greatly on the temperature of the core".

The neutrinos that reach the Earth can be measured by means of different techniques. These data can be used to detect the modifications of the solar temperature caused by the WIMPs. The transport of energy by these particles depends on the likelihood of them interacting with the atoms, and the"size"of these interactions is related to the reduction in the neutrino flux.

"As a result, current data about solar neutrinos can be used to put limits on the extent of the interactions between dark matter andatoms, and using numerical codes we have proved that certain values correspond to a reduction in the flux of solar neutrinos and clash with the measurements", the scientist reveals.

The team has applied their calculations to better understand the effects of low massdark matterparticles (between 4 and 10 gigaelectronvolts). At this level we find models that attempt to explain the results of experiments such as DAMA (beneath an Italian mountain) or CoGent (in a mine in the USA), which look for dark material using"scintillators"or WIMP detectors.

Debate about WIMP and solar composition

This year another study by scientists from Oxford University (United Kingdom) also appeared. It states that WIMPs not only reduce the fluxes of solarneutrinos, but also, furthermore, modify the structure of the Sun and can explain its composition.

"Our calculations, however, show that the modifications of the star's structure are too small to support this claim and that theWIMPscannot explain the problem of the composition of thesun", Taoso concludes.

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