Category: Physics Page 10 of 11

Leading nuclear physicists to speak at Duke

On March 7, 2012

In Lecture, Physics, Science Communication & Education

By Ashley Yeager

Physics grad student Georgios Laskaris, left, and Haiyan Gao, the chair of Duke's physics department, right, work on an experiment to look for a new force of nature. Credit: Megan Morr, Duke Photography.

More than 20 years after Haiyan Gao began her work on the neutron, she is hosting the Symposium on Electroweak Nuclear Physics at Duke to celebrate Caltech physicist Robert McKeown’s influence on her and others in nuclear physics.

The event will also honor his achievements in the field and celebrate his sixtieth birthday. The conference will be held March 8-9 in the French Family Science Center. Click here for a full schedule of the talks. Physicist Steve Koonin will also give a public lecture on addressing the nation’s energy issues as part of the symposium.

Gao will present her latest research, describing how she and her collaborators are identifying the factors that cause a neutron to spin. Other leading scientists will present their research on protons, neutrinos, dark matter and more exotic particles, such as free quarks and dark photons.

You can read more about Gao’s work in Duke Today.

Physicist's thesis now 'famous'

By Ashley Yeager

On February 22, 2012

In Faculty, Physics, Students

By Ashley Yeager

Physicist Ashutosh Kotwal's thesis paper is now famous in the high-energy physics field. Image courtesy of Ashutosh Kotwal.

Physicist Ashutosh Kotwal spent his days in graduate school making particles scatter. Little did he know that his 1996 thesis on protons, deuterons and muons would become famous.

Kotwal, now a full professor at Duke, recently learned from a former mentor, Northwestern physicist Heidi Schellman,that his Ph.D thesis earned “famous paper” distinction in his field of high-energy physics. The honor is based on the number of times other researchers have mentioned the work in the scientific literature. Schellman had been the leader for Kotwal’s thesis research collaboration.

The high-energy physics community has maintained its own database of publications and citations at the Stanford Linear Accelerator Center since the 1980s. Kotwal’s thesis paper has 252 citations, to date.

His paper is now ranked number 99 among experimental high-energy physics famous papers from the last 15 years.

According to the stats, Kotwal is also co-author on five other “famous” papers and two “renowned” articles, which each have more than 500 citations. Kotwal said he hopes his thesis paper’s distinction “can provide some inspiration” for students interested in physics research.

Steve Koonin to speak March 8

By Ashley Yeager

On February 15, 2012

In Climate/Global Change, Lecture, News Release, Physics, Science Communication & Education

Official portrait of Steven E. Koonin, former Under-Secretary for Science of the United States Department of Energy. Credit: DOE.

Want to know what we should do to address America’s energy challenges?

Come hear the ideas of Steve Koonin, a former chief scientist at British Petroleum and more recently the Under Secretary for Science at the Department of Energy. He’ll speak at Duke at 5:15 p.m. on March 8, 2012. The lecture will be held in room 2231 of the French Family Science Center.

Koonin, an MIT-minted theoretical physicist and currently a researcher at the Institute for Defense Analyses in Washington D.C., will talk about strategies to incorporate alternative and renewable energy sources into our energy profile. The lecture is free and open to the public.

The talk is part of the university-hosted Symposium on Electroweak Nuclear Physics, a two-day science conference to explore the latest experiments and ideas on matter and how it behaves at ordinary temperatures — quite the opposite of what’s being studied at the Large Hadron Collider and other high-energy particle accelerators.

The symposium is being held in honor of Jefferson Lab deputy director for science R. D. McKeown’s sixtieth birthday.

Quarks make their world turn

By Ashley Yeager

On February 3, 2012

In Physics

By Ashley Yeager

This cartoon shows the axis rotation and the intrisic spin, from mass or charge, of the protons and neutrons in a He-3 atom. Credit: Yi Qiang, Jefferson Lab.

It’s good that things spin.

If the Earth didn’t, we wouldn’t have night and day. If wheels didn’t, we wouldn’t have quick ways to get around. And, if the protons and neutrons in atoms didn’t, well, it’s not clear what would happen, but we definitely wouldn’t have medical tests like MRIs.

Even though spinning particles play a large role in our everyday lives, physicists aren’t sure where the rotation comes from. They certainly have some ideas, but actually observing the origin of protons’ and neutrons’ spin has been challenging.

Now, experiments led by physicist Haiyan Gao and her colleagues are providing a more detailed picture of what makes sub-atomic particles turn. The team is using high-energy electrons and a rare form of helium at the Jefferson Lab to look at how quarks, the particles within protons and neutrons, spin.

The experiment seems simple – slam the electrons into helium-3, or He-3, atoms. But, there’s a bit more complexity. The scientists have to carefully manipulate the rotation of all of the particles involved. The neutrons of He-3, for example, must turn perpendicular to the direction of the incoming electrons. And, the electrons have to turn either in the same direction as they are moving or in the exact opposite direction.

The physicists manipulate the direction the particles spin using polarized lasers.

Getting these spins correct is critical to figuring out if the quarks within protons and neutrons spin, and if so, in what direction. In the experiment, the incoming high-energy electrons slam into the cores of the helium-3 atoms and penetrate deep into their neutrons. The electrons strike the quarks inside, knocking them loose.

The physicists found that the struck quarks turn on imaginary axes just like Earth does. And, they turn in the same direction as their parent particle. This is the first successful experiment to measure the alignment between the spin of a quark and the direction its neutron is moving, a discovery that gives scientists hints about where the rotations of neutrons and protons originate, Gao says.

This cartoon shows the rotation of an incoming electron compared to the spin of a helium-3 neutron and the ejected down quark. Credit: Jin Huang, LANL.

Past experiments have shown that the quarks’ intrinsic rotation (from mass or charge) makes up about a third of a proton’s or neutron’s overall spin. The results of the new experiment, which appear in the Feb. 3 Physical Review Letters, suggest that the remaining two-thirds of a parent particle’s rotation comes from orbital motion of the particle’s quarks.

The conclusion, however, is far from certain, Gao cautions. This new experiment is like a snapshot of how the quarks turn. Physicists do not yet have the full picture of the orbital motions of these particles, she says.

To get a more complete picture, she and her team are working on a more sensitive detector and other upgrades to the Jefferson Lab electron beam that they will use to study even more carefully how quarks spin.

And that too, is a good thing, though we may not yet understand – in terms of everyday applications – why.

Citation: “Beam-Target Double-Spin Asymmetry ALT in Charged Pion Production from Deep Inelastic Scattering on a Transversely Polarized 3He Target at 1.4<Q2<2.7 GeV2.” J. Huang et al. (2012). Physical Review Letters. 108: 5.
DOI: 10.1103/PhysRevLett.108.052001

Double-walled nanotubes shine, sometimes

By Ashley Yeager

On January 13, 2012

In Physics, Visualization

By Ashley Yeager

This montage shows modeled and imaged double-walled carbon nanotubes. Courtesy of: Morinobu Endo, Shinshu University.

Nanotubes are tiny, and they can give off light. Those properties make the carbon constructions promising for looking at cells inside our bodies and also making small electrons that can capture and manipulate light.

But recent research suggests that not all nanotubes shine as chemists thought, a discovery that ends a debate in the field about which type of tubes to use for applications relying on their ability to emitted light.

The debate pitted single-walled carbon nanotubes against double-walled ones. Some scientists thought only single-walled tubes could give off light and be used in light-related applications. But other scientists showed that double-walled nanotubes could also emit light and possibly replace their single-walled cousins.

Now, Sungwoo Yang, a former chemistry graduate student at Duke, and his colleagues in Jie Liu’s lab have shown that both single and double-walled carbon nanotubes shine when hit with lasers. But, in the double-walled tubes, only the inner wall emits light and only a small range of diameters of the inner tube could get their light to the outside world. The ones outside of this range gave off light, but it got doused on its way through the outer layer.

Bottom line: Some double-walled carbon nanotubes do emit light but most don’t, if you’re looking for the light outside of the tube. That discovery makes both camps in the nanotube debate correct, depending on the diameter of tube being considered.

CITATION: “Photoluminescence from Inner Walls in Double-Walled Carbon Nanotubes: Some Do, Some Do Not.” Sungwoo Yang, Ashley Parks, Stacey Saba, P. Lee Ferguson, and Jie Liu. Nano Lett., 2011, 11:10, 4405–4410
DOI: 10.1021/nl2025745

Chaos puts a path on nanoparticles

By Ashley Yeager

On January 13, 2012

In Biology, Cancer, Physics

By Ashley Yeager

Shaquille O'Neal towers over Ralph Stefanelli, the doctor who delivered him. Credit: Rich Krauss.

At just over seven feet tall, Shaquille O’Neal is easy to spot in crowd. But the individual virus structures that give him, and us, a cold aren’t so easy to see.

They’re harder to spot because they, unlike Shaq, are much smaller than the wavelengths of light we use to see them. But, physicists at Duke are now designing a technique that could give scientists a way to sense these tiny viruses and other nanoparticles and even capture their path when they’re on the go.

The team describes the new sensing system in the Dec. 16 issue of Physical Review Letters.

The new detector will compliment the few existing ways to track tiny objects, says physics graduate student Seth Cohen. He says that currently detectors can sense the presence of nanoparticles and that biomedical researchers have used virus-sized objects to tag specific parts of human cells.

“Ultimately, I would like to see this new system used to map out the dynamics inside of a living cell, using nanoparticle tags on the cell’s internal structures,” he says.

That can’t be done right now because the internal structures and other nanoparticles are smaller than 100 nanometers. Our eyes see wavelengths of light between 400 to 700 nanometers, and Shaq, by comparison, is 2.16 billion nanometers. We see Shaq because he has a lot more nanometers than those found in the wavelengths of light we use to see. But, we can’t see virus or cell structures because they’re size is far below that limit.

To overcome the problem, Cohen and his colleagues based their new sensing system on two physical properties, wave chaos and nonlinear dynamics. The proposed apparatus is shaped like a stadium, where multiple reflections of radio-frequency waves fill the entire cavity and take many different paths, a form of wave chaos. The stadium-shaped cavity and multiple-path reflections also allow the physicists to suspend the information flowing through the system, forming a nonlinear delayed feedback loop.

The virus structure, shown here, that causes the common cold is only 20 nanometers across. Credit: Michael Taylor, TurboSquid.com

In initial experiments, the team used a pair of stationary broadband antennas to track a small container filled with water, which also scatters the radio waves broadcast into the cavity. Working out the geometry of the cavity, along with all of the possible reflection angles of light, the team was able to pinpoint the object in the cavity and track it as it moved. In other words, by combining wave chaos and nonlinear dynamics, the physicists could track an object that was much smaller than the wavelengths scientists used to see it.

The team is now designing a new version of the system using laser light and a microcavity, which is only a few hundred micrometers in size and can be constructed on silicon chips. The system, which will work in the visible wavelengths of light, will provide a new and simple technique for tracking small nano-scale objects, such as viruses. Cohen will report on his progress on this sensing system in May at the Experimental Chaos and Complexity Conference in Ann Arbor, Michigan.

Citation: Subwavelength Position Sensing Using Nonlinear Feedback and Wave Chaos. Seth D. Cohen, Hugo L. D. de S. Cavalcante, and Daniel J. Gauthier. Phys. Rev. Lett. 107, 254103 (2011). DOI: 10.1103/PhysRevLett.107.254103

Particles of light overcome their lack of attraction

By Ashley Yeager

On December 5, 2011

In Faculty, Nanotech, Physics, Students, Visualization

By Ashley Yeager

This waveguide shows an electric field moving from right to left. Credit: Setreset, Wikimedia Commons.

Electrons typically repulse each other. But sometimes they can actually attract each other and pair up, which is why superconductors exist. The particles that make up light, on the other hand, have no charge and rarely appeal to or repel one other.

Now, Duke theoretical physicist Harold Baranger and his collaborators think it’s possible to get these particles, called photons, to pair up, and stay that way, as they travel through space.

The new idea could help with the development of quantum communications, and possibly quantum computing in the future, Baranger says.

Particles of light do not have electric charge so they’ve got no attraction to the particles around them. That lack of attraction gives photons the ability to travel long distances without losing information, a good trait for building quantum networks.

But, the photons’ reserve has a drawback. It makes it more difficult for scientists to control each particle and retrieve the information it carries. To rein in the seemingly aloof photons, physicists have designed cavities that trap them and boost their contact with individual atoms.

In these cavities, the particles pass one by one through an atom in a phenomenon called a photon blockade. But, the cavities trap the photons for a really long time, so they pass through the cavities really slowly, which isn’t good for networking — especially in this age of instant information.

Using pencil and paper, Baranger, his student Huaixiu Zheng and colleague Daniel Gauthier, figured out that they could avoid the problems with the cavities if the photons instead went into a one-dimensional structure, called a waveguide, which also channels the photons past an atom one by one.

In the waveguide, a control atom acts as an intermediary between the incoming particles. The first photon passes through the atom and changes its state.

This diagram shows a photon blockade in a waveguide. Multiple photons (left, yellow) pass by the control atom in a one-by-one manner, ending with a train of single-photon pulses and empty pulses (gray). Courtesy of Harold Baranger, Duke.

The atom then interacts differently with the next photons, which ultimately causes the two particles of light to interact. What’s surprising, Baranger says, is that the photons are still bound to each other for a long time, even as they move away from the atom that paired them.

These bound states also end up producing a photon blockade much like in a cavity, but through a completely different mechanism, and the photons move a lot faster, he says.

The work, which appears online in Physical Review Letters, “paves the way” for experimentalists who want to try to build quantum networks without using cavities, Baranger says. He says experiments in this area may be done at Duke in the next few years.

Right now, he plans to work out what happens to the photons if more than one atom sits in the waveguide. The photons will be interacting in a lot of different places, and “one can imagine that there could even be a quantum phase transition, giving rise to some new quantum state,” he says. “But, that’s just a hope at this point.”

CITATION: Cavity-Free Photon Blockade Induced by Many-Body Bound States. Zheng, H., Gauthier, D., and Baranger, H. Phys. Rev. Lett. 107, 223601 (2011).

DOI: 10.1103/PhysRevLett.107.223601

Atomic core, shaken not stirred

By Ashley Yeager

On December 5, 2011

In Physics

By Ashley Yeager

When hit right, protons and neutrons can have a distinct ring, like a hammer hitting a bell. Credit: Geo Martinez, 123rf.com.

When struck just right, protons and neutrons ring. The sub-atomic particles don’t jingle like when a hammer hits a bell. But they do jiggle in an odd dance where the protons move in one direction and the neutrons move the other way.

Studying this particular particle movement has been difficult because of other motion within an atom’s nucleus. But now, researchers using the High Intensity Gamma Ray Source, or HIGS, located on Duke’s campus, say they have gotten the best look yet at a nucleus as it starts its complex, resonant dance.

In the experiment, the researchers slammed a chunk of bismuth-209, which had 83 protons and 126 neutrons, with a polarized beam of high-energy photons.

The team then recorded all the gamma rays knocked from the nucleus after the collision. Based on the direction the gamma rays were traveling, the scientists were able to figure out the energy, width and strength of the resonance at which the particles’ rang.

The results, which appear in the Nov. 25 issue of Physical Review Letters, along with future studies on the vibrations of other nuclei planned at the HIGS facility will help theorists write a clear, accurate mathematical description of how protons and neutrons behave and also for the state of matter in black holes, neutrons stars and even in star explosions called supernovae.

CITATION: New Method for Precise Determination of the Isovector Giant Quadrupole Resonances in Nuclei. Henshaw,S., Ahmed, M., Feldman, G., Nathan, A., and Weller, H. PRL 107, 222501 (2011).

DOI: 10.1103/PhysRevLett.107.222501

Particles on Ice

By Ashley Yeager

On November 10, 2011

In Field Research, Lecture, Physics

By Ashley Yeager

Protons, neutrinos and photons take different paths toward Earth. Physicists have to figure out their paths to determine where the particles came from. Credit: Deutsches Elektronen-Synchrotron.

Imagine nabbing the cover of the journal Science. Now, imagine doing it without making a single discovery. For University of Wisconsin physicist Francis Halzen, those dreams came true in 2007.

“We hadn’t done anything by then,” he said during a Nov. 9 physics colloquium as he described the experiment called IceCube.

IceCube is a neutrino detector buried deep in the ice of Antarctica. It did not begin full scientific operations until May 2011. So why did it claim prime Science real estate four years earlier?

Because Halzen and his team were the clear winners in a race, which began in the 1970s, to build a kilometer-wide bucket to capture particles called neutrinos.

To cover just over a half mile of Antarctica with such a detector, Halzen’s team engineered 5,000 neutrino buckets to sit on 86 electronic wires, a bit like strings of Christmas lights. The team then fed each string a mile and a half into the ice near the South Pole.

The idea was that the instrument would detect blue light coming from the reaction of a single neutrino crashing into an ice atom. From that reaction, the physicists could start to tease apart where neutrinos and other high-energy showers of particles, called cosmic rays, come from. Now, the team has also begun adding to the instrument so it can probe what dark matter and dark energy are.

This diagram, superimposed on an aerial photo of the South Pole Station, shows where the 86 detector strands sit. Courtesy of: Tom Gaisser, Univ. of Delaware.

Halzen said studying neutrinos is a lot like taking an X-ray, rather than a normal photo, of the galaxy and the universe. He added that even though there are some promising data points in IceCube’s preliminary scans, no one should get too excited. The neutrino buckets haven’t really seen anything, yet.

It’s a bit like the Large Hadron Collider not seeing the Higgs boson, he said. If scientists don’t see what they are looking for, it will be really interesting and may call for a re-write of the physics textbooks.

But, Halzen said, “I can tell you, we want to see something,” and, doing it before next year, when the discovery of cosmic rays turns a century old, would fulfill another of his physics dreams.

Prescription lens brings spinning black holes into focus

By Ashley Yeager

On November 4, 2011

In Physics, Visualization

This computer generated image highlights how strange space would look if you could fly right up to a black hole. The effect of gravity on light causes some very unusual visual distortions. Credit & Copyright: Alain Riazuelo.

By Ashley Yeager

If a black hole is the eye of a galaxy, then Duke mathematician Arlie Petters is its optometrist.

Petters along with his colleagues, visiting scholar Amir Aazami and Rutgers astronomer Charles Keeton, have written the prescription, or mathematical equation, to describe the lens of a spinning black hole.

The new equation provides astronomers with an easier way of calculating what’s going on around a spinning black hole, says Harvard astrophysicist Avi Loeb, who was not involved in the research.

Astronomers typically classify black holes into two types, static or spinning.

Static black holes are easier to describe mathematically, which is why most previous studies describing a black hole’s action on light did not include a spin variable.

In reality, though, everything is in motion. Stars, planets, even black holes, spin. “As scientists, we need to add that spin into the equation if we are going to try to explain spinning black holes as an element of nature and how they work on a grand scale,” Petters says.

To describe black holes mathematically, Petters and his team had to first consider how elements of nature distort light. On Earth, air, water, glass and even our eyes alter how we interpret patterns of light.

In the case of our eyes, doctors can describe the distortion with a “lensmakers equation,” which underlies how they write precise prescriptions for our contacts or glasses.

In space, it’s gravity that bends light. Black holes have so much gravity due to their extreme mass that they can pull particles of light onto new paths. That bending and pulling of light acts as a cosmic lens creating cosmic mirages like Einstein rings.

The mirages or effects of the lensing can convey a lot of information about the universe, such as its age and the nature of dark matter. They also reveal details about the black holes themselves, Petters says.

This Hubble Space Telescope image shows a double Einstein ring. Credit: NASA

But, to the decode the mirages, astronomers need a precise prescription of the lenses creating them, just like we need prescription lenses to see our world more clearly.

In the past, astronomers would calculate the characteristics of a black hole lens using the equation for a static black hole. Or, they would use heavy-duty computer simulations or other painstakingly difficult methods to track particle trajectories and describe the lensing effects.

The new prescription Petters and his team has written, however, allows astronomers to calculate certain characteristics of a black hole by observing it and recording its mass and lensing effects. The researchers can then solve the lensmakers equation for the spin of the black hole. Petters and his colleagues describe the equation in two papers published in the Journal of Mathematical Physics.

Aside from making it easier to study black holes, the new equation also gives scientists another way to test Albert Einstein’s theory of gravity.

It is important to test Einstein, just as scientists continued to test Newton’s theory of gravity, Petters says. “We need to find any discrepancies in Einstein’s theory in order to push beyond it and to continue to comprehend and to appreciate the structure of the universe around us.”

Citations

A. B. Aazami, C. R. Keeton, and A. O. Petters. Lensing by Kerr Black Holes. I. General Lens Equation and Magnification Formula. J. Math. Phys., vol 52, (2011). doi:10.1063/1.3642614

A. B. Aazami, C. R. Keeton, and A. O. Petters. Lensing by Kerr Black Holes. II. Analytical Study of Quasi-Equatorial Lensing Observables J. Math. Phys., vol 52, (2011). doi:10.1063/1.3642616