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Close encounters of the muon kind

Photo of g-2 magnet — G-2 electromagnet at the Hidden Lake Forest Preserve. (Courtesy: Reidar Hahn/Fermilab)

By James Dacey

Don’t worry, the aliens haven’t landed. The people in this photo are watching with excitement shortly before this giant electromagnet completed its 5000 km journey on Friday to arrive at Fermi National Accelerator Laboratory just outside Chicago. The 15 m-wide ring that weighs more than 15,000 kg has been travelling for the past five weeks by land and sea from its previous home on Long Island in New York State.

The giant electromagnet has served as part of the Muon g-2 experiment at Brookhaven National Laboratory. This experiment – to describe it crudely – is designed to measure how muons wobble in a magnetic field, as many believe this will provide clues to new physics beyond the Standard Model. This experiment is now relocating to Fermilab, which offers a more intense and pure beam of muons than the Brookhaven lab.

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How to make zeptosecond X-ray pulses

A technique for producing radiation pulses that endure for less than one attosecond (10^–18 s) has been proposed by researchers in Spain and the US. If the technique can be realized in the lab, then it could produce X-ray flashes brief enough to capture the movement of an atom’s inner electrons or perhaps even look directly at the movement of protons and neutrons during nuclear fission or fusion.

Ultrashort radiation pulses are the mainstay of pump–probe spectroscopy, which is used to study fast processes such as the motion of electrons taking part in chemical reactions. In this technique, a short and highly energetic laser pulse (the pump) provides the activation energy needed to kick-start a chemical reaction. A moment later, a second pulse (the probe) hits the reacting particles and is then detected. Through careful study of the detected pulse, the instantaneous state of the particles at the time of collision can be deduced. By varying the time delay between the pump and probe pulses, scientists can reconstruct the behaviour of electrons in atoms and molecules during a chemical reaction. In the future, the technique could be used to optimize the conditions for desired reactions or even to manipulate the reactions directly.

The shorter the pulses used, the faster the chemical processes that can be studied and manipulated. Pulses of a few hundred attoseconds can be produced by irradiating an atomic gas cloud with a powerful infrared laser. As the laser radiation is coherent, the electric field experienced by all the atoms in the cloud oscillates near-simultaneously. When the electric field of the radiation points in one direction, some of the negatively charged electrons are pulled a few nanometres away from the nuclei of their atoms. When the phase of the radiation changes and the electric field reverses, they are then pushed back towards the parent atoms. Some of these electrons collide with the nuclei and give up their kinetic energy in a sudden burst of broadband, coherent X-ray radiation.

Using wasted electrons

Many electrons, however, shoot straight past the nuclei without colliding and emitting radiation. Now, a new way to use these wasted electrons has been proposed by Carlos Hernández-García and colleagues at the University of Salamanca and the Centre for Pulsed Lasers, both in Spain, and the University of Colorado at Boulder in the US. What is more, the team has shown theoretically that these electrons could be used to produce sub-attosecond pulses.

The idea is that electrons that shoot straight past a nucleus the first time can be pulled back for a second time in the subsequent oscillation of the field. This gives the electrons a second opportunity to collide. The catch, however, is that these electrons produce X-rays at a slightly different frequency from those that collide the first time round.

When two adjacent notes on a musical instrument sound simultaneously, the resulting harmony produces a distressing sensation because the volume oscillates rapidly as the frequencies move in and out of phase. Similarly, the researchers’ model shows that the intensity of a pulse should fluctuate as the two X-ray frequencies move in and out of phase. The attosecond pulse could be split into a pulse train with each pulse lasting just a few hundred zeptoseconds (a zeptosecond is 10^–21 s). “In the paper we show how to get 800 zs, but this is scalable,” says team member Tenio Popmintchev of the University of Colorado, “we don’t know yet how short we can go.”

Finding an appropriate laser

The team believes that the biggest obstacle to realizing this scheme in practice is the development of a suitable infrared laser – paradoxically, shorter pulses require a longer-wavelength driving laser. The scientists have received two research grants and are currently working to develop such a device.

Jon Marangos, an expert on laser–matter interactions at Imperial College London, believes that it should be possible to develop a suitable infrared laser and that an experiment to test the principle could be conducted using currently available equipment. He says that the work is one of several interesting, recent proposals to produce sub-attosecond pulses, notably one in March at the University of Strathclyde in Glasgow to do so using a free-electron laser, but that they all need to address a key issue: a pulse train would be problematic in experiments because there would be no way to attribute a signal to a specific pulse.

“You really need to work out a way of generating isolated attosecond pulses,” he says, “because then you can use them in a pump–probe experiment with no ambiguity about when the pump and the probe events occurred.”

The research described in Phys. Rev. Lett. 111 033002.

There is more about why ultrashort radiation pulses are used to study electron motion in the Physics World video “Can we see the motion of electrons on the atomic scale?”.

Nobel laureate Andre Geim profiled on BBC radio

Nobel laureate Andre Geim. (Courtesy: University of Manchester)

By Hamish Johnston

I thoroughly enjoyed a recent BBC Radio 4 profile of Andre Geim of the University of Manchester, who shared the 2010 Nobel Prize for Physics. In the 13 minute broadcast, which is available for download, Geim and several admirers talk about the passion for doing quirky fundamental research that led to his co-discovery of graphene.

There is even the bold suggestion from one of Geim’s colleagues that there might be another Nobel in the Russian-born physicist.

LHCb and CMS see rare decay of the strange B meson

A strange B meson decay event as seen by CMS (Courtesy: CMS) — A strange B meson decay event as seen by CMS. (Courtesy: CERN)

By Hamish Johnston

It’s a story with a hint of both “man bites dog” and “dog bites man” about it.

Physicists working on the CMS and LHCb experiments at CERN have independently seen an incredibly rare decay of a particle – a strange B meson decaying into two muons. The odds of this meson decaying in this particular way is about one in a billion, making the joint discovery a triumph of experimental particle physics. And it is officially a discovery. That’s because when data from the two experiments are combined the observation has a statistical significance of greater than 5σ, which is the gold standard in particle physics.

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Van Allen electrons are accelerated from within

The debate surrounding how ultra-relativistic electrons trapped in the Earth’s Van Allen radiation belts are accelerated has finally been settled. Recent data from NASA’s Van Allen Probes mission suggest that the electrons are accelerated locally through wave–particle interactions, rather than by radial transport of electrons from outside the belts. The study clearly distinguishes between the two types of acceleration. Knowing where the acceleration occurs is essential to making more accurate space-weather predictions, as changes in the radiation belts can cause satellites in geostationary orbits to malfunction or breakdown.

The Van Allen radiation belts are two concentric, doughnut-shaped rings that are made up of high-energy electrons that vary in intensity. The belts are confined within the Earth’s magnetosphere and extend from about 1000 to 60,000 km above the Earth’s surface. Although discovered by American physicist James Van Allen more than 50 years ago, the Van Allen belts are not yet fully understood. According to the lead researcher of the new study, Geoffrey Reeves at the Los Alamos National Laboratory, New Mexico in the US, previous observational data taken in the 1990s did not fit the conventional theories of the time, leading researchers to question and debate over what processes really control the intensity of the radiation belts.

Capricious processes

For example, it was thought that all solar storms intensified the radiation belts but a study that Reeves and others carried out in 2003 showed that only half the storms intensified the belts. More surprisingly, they found that about a quarter of storms depleted the belts, making them less intense. Reeves says that the Van Allen Probes mission, launched on 30 August 2012, was designed specifically to fly straight through the belts and to discover which processes control how intense the belts are and how they change.

Previous theories suggested that two possible mechanisms were driving the electrons in the belts – local acceleration or radial acceleration. In radial acceleration, the electrons were thought to be transported perpendicular to the Earth’s magnetic field, from areas of low magnetic strength far from Earth to areas of high magnetic strength nearer Earth. This would cause acceleration as electrons being transported would speed up as the magnetic field strength increases. On the other hand, the local acceleration theory purports that low-energy electrons gain energy in situ, from a source at the heart of the belts.

Reeves told physicsworld.com that previous satellite missions had provided tantalizing evidence for local acceleration, but there were always limitations. “Essentially, neither side of the argument could convince the other side that local acceleration did or did not happen,” he says. He goes on to explain that Van Allen Probes have three features that make the latest observations unique, including “the right instruments spanning a broad range of energies with amazing sensitivity, an equatorial orbit that cuts through the belts at different altitudes and two satellites that can unambiguously resolve whether something is changing in time or in space or both”.

Rapid rise

On 9 October 2012 Reeves and his team observed a rapid energy increase in the energies of electrons in the belts that lasted for about 12 hours. If the acceleration was due to radial transport, the effects would first be measured further away from Earth and be seen moving inward. Instead, their measurements revealed an increase in electron energy that started in the middle of the belts and gradually spread both inward and outward, implying a local acceleration source.

Thanks to solar processes that affect the Earth’s magnetosphere and the belts, there is a peak in intensity of the belts. The local acceleration is produced thanks to resonant interaction of radiation-belt electrons with naturally occurring electromagnetic waves. If an electromagnetic wave spirals around the magnetic field at the same speed as that of an electron, the wave gives the electron a series of well-timed pushes that increase the electron’s speed. Once the electrons are accelerated locally they spread out (diffuse) both inward and outward but when electrons diffuse outward they lose energy. “So, diffusion still happens but it is not the cause of the acceleration,” says Reeves.

Electrons with these mega-electronvolt energies can easily penetrate satellites and their electronics, causing them to malfunction or fail completely. Knowing where these super-energetic electrons come from and what gives them their energy is a key step in predicting hazards to satellites, says Reeves. He feels that “there’s tonnes more exciting stuff to look at with the Van Allen Probes data”, some of which will be things that the satellites were designed to look for and some completely unexpected.

The research is published in Science 10.1126/science.1237743.

‘Electronic skin’ lights up when touched

Researchers at the University of California at Berkeley have integrated three distinct electronic components to create touch-sensitive “electronic skin” or e-skin. The new technology combines semiconducting carbon-nanotube transistors, pressure-sensitive polymer sensors and organic light-emitting diodes (OLEDs) – which are integrated over large areas on a single plastic substrate. The result is a mechanically flexible sensor network that responds to a finger touch by immediately lighting up. And the harder it is touched, the brighter the light.

The researchers believe that the technology could help enhance the sense of touch in robots of the future and even find use in applications such as touchscreen wallpapers. Medical applications, such as “e-bandages” that monitor a patient’s health in real time, might also be possible.

Led by Ali Javey, the team made the new e-skin by first spin coating a polymer sheet just 25 µm thick on top of a silicon-wafer substrate and subsequently hardening the plastic by baking it in an oven at 300 °C. The electronic components were then vertically built on top of the plastic surface using conventional microfabrication processes. Once the electronics were stacked, the plastic backing layer was peeled away leaving a free-standing film with the sensor network embedded within it.

Active matrix

Each pixel in the active matrix of the device contains a nanotube transistor with its drain electrode connected to the anode of an OLED. A pressure-sensitive polymer is laminated on top of the OLED and it is in electrical contact with the cathode of the OLED at each pixel. The top surface of the polymer is made conducting by coating it with silver ink and acts as the ground contact. When the device is touched, current flows through the polymer layer and switches the OLED on.

“Our e-skin is the first flexible system that responds to pressure stimuli of varying intensities and provides a real-time response by emitting light through the integrated OLED display,” team member Chuan Wang says. “In the system, OLEDs are turned on only where the surface is touched and the intensity of the emitted light depends on the amount of pressure applied. This basically allows us to visualize the applied pressure.”

The e-skin can be laminated on a variety of surfaces, curved or otherwise, he adds. Potential applications include robot skin, interactive wallpaper and interactive in-vehicle dashboards. “I can also imagine things like e-bandages applied to a person’s arm that would continuously monitor blood pressure and pulse rates, for example, while providing real-time feedback.”

The Berkeley team is now busy integrating additional sensing capabilities – such as those that respond to thermal and light stimuli – into its e-skin system. “We are also experimenting with the possibility of having the whole system built using roll-to-roll printing processes for large-scale, low-cost fabrication of the sensor networks, reveals Wang.

The e-skin is described in Nature Materials 10.1038/nmat3711.

How to survive earthquakes and noisy neighbours

By Jon Cartwright

The past few years has seen a steady stream of proposals for cloaking objects, whether it’s from light, heat, water waves, magnetic fields or even time. Now, physicist Sang-Hoon Kim at the Mokpo National Maritime University in Korea is adding to this list, first off with a cloak that could protect buildings from earthquakes.

An earthquake cloak has been proposed before using – as is common in invisibility cloaks – elaborately structured “metamaterials” to guide seismic waves safely around a building. However, Kim, together with Mukunda Das at the Australian National University in Canberra, has put forward a different approach: a metamaterial barrier that dissipates seismic energy as sound and heat. The idea is that many buildings could hide in the “shadow zone” of the barrier. This could be a boon for city planners, who would not have to make cloaks for individual buildings. Kim and Das’s paper has been accepted for publication in Modern Physics Letters B and is available as a preprint entitled “Artificial seismic shadow zone by acoustic metamaterials“.

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B-mode polarization spotted in cosmic microwave background

The South Pole Telescope (SPT) has made the first detection of a subtle twist in light from the cosmic microwave background (CMB), known as B-mode polarization. The signal, the existence of which has been long predicted, paves the way for a definitive test of inflation – a key theory in the Big Bang model of the universe.

“While this effect was fully expected, its detection is a milestone event in the use of the CMB to probe our universe,” says Chuck Bennett, a leading expert in CMB observation based at Johns Hopkins University in Maryland, US, who was not involved with the study. “It is solid research and I believe the result.”

Often called the afterglow of the Big Bang, the CMB is thought to have originated some 380,000 years into the life of the universe when neutral atoms first formed and space became transparent to light. Roughly speaking, it consists of microwaves with a temperature of about three kelvin, but it also contains details that have helped to refine our understanding of the early universe. The most noticeable of these details are variations in temperature of about 100 μK, which reveal density fluctuations in the early universe – the seeds of the stars and galaxies that we see today.

Polarized by scattering

The CMB does not only contain variations in temperature, however. Its radiation was scattered towards us from the universe’s earliest atoms in the same way that blue light is scattered towards us from the atoms in the sky. And in the same way that the blue light from the sky is polarized – a fact you can check by wearing polarized sunglasses – so too is the light from the CMB polarized. Variations in CMB polarization were first detected in 2002 by the DASI interferometer in Antarctica and helped cosmologists understand the dynamics of the early universe.

These polarization variations were known as E-mode or gradient variations because they describe how the magnitude of polarization changes over the CMB. But there are even subtler variations known as B-mode variations, which describe the rotation or “curl” of CMB polarization. The majority of B-mode polarization is produced by galaxies acting as gravitational lenses, twisting the E-polarized light on its 14-billion-year journey from the other side of the observable universe. It is incredibly faint, producing temperature variations of about 0.4 μK and accounting for just one part in 10 million in the CMB temperature distribution. “B-mode polarization is very difficult to measure,” says Duncan Hanson, a member of the SPT team who is based at McGill University in Canada.

The SPT has managed to detect B-mode polarization largely thanks to improvements in detector technology. Although the detection will probably have little application, it opens new doors in experimental cosmology. With more precision, B-mode signals could help cosmologists place tougher constraints on neutrino masses, which cannot be predicted in the Standard Model of particle physics.

Gargantuan ripples

But the biggest prize would be using B-mode signals to uncover evidence of primordial gravitational waves – gargantuan ripples in space–time. Such ripples are predicted to have been generated in inflation, a brief period prior to the formation of the CMB when the universe is thought to have undergone rapid expansion and given birth to large-scale structures.

Although most cosmologists today believe in inflation, the theory lacks crucial details such as how it started and stopped and there has been no way to test it. A detection of primordial gravitational waves would be strong evidence for the existence of inflation, which was first proposed back in 1980 by the American physicist Alan Guth.

“This possibility of detecting B modes from gravitational waves is a remarkable enough possibility that it is driving numerous experimental efforts,” says cosmologist Arthur Kosowsky at the University of Pittsburgh in Pennsylvania, US. “SPT is the first to detect any B modes, [and now] several other experiments are in hot pursuit, so this is the first leg in what is shaping up to be an exciting race to the finish line over the next decade.”

Others are looking

Gravitational-wave B modes could be detected by the European Space Agency’s Planck observatory, which orbits the Earth, although the toughest competition will come from the BICEP telescope, which sits alongside the SPT, or the POLARBEAR or ACT telescopes in northern Chile. If the discovery is made by one of the ground-based telescopes it would continue the tradition of ground-based experimental cosmology firsts that began with the discovery of the CMB, made by the American astronomers Arno Penzias and Robert Wilson with the horn antenna at Bell Labs in Holmdel Township in New Jersey, US, in 1964.

“Results come out from space, and there’s lots of press and beautiful results, and the ground-based work tends to get forgotten,” says John Carlstrom, the principal investigator on the SPT team who is based at the University of Chicago in Illinois, US. “But the ground-based telescopes, balloons and short-duration flights are an extremely important part of the [experimental] program and have led the way consistently since the beginning. And they still do.”

The discovery is described in the preprint arXiv:1307.5830.

What is the lifetime of a photon?

The photon – the quantum of light or other electromagnetic radiation – is normally considered to have zero mass. But some theories allow photons to have a small rest mass and one consequence of that would be that photons could then decay into lighter elementary particles. So if such a decay were possible, what are the limits on the lifetime of a photon? That is the question asked by a physicist in Germany, who has calculated the lower limit for the lifetime of the photon to be three years in the photon’s frame of reference. This translates to about one billion billion (10¹⁸) years in our frame of reference.

An issue of mass

The idea that photons have a finite lifespan, and therefore mass, is difficult to imagine. Indeed, astronomers looking at distant cosmic objects regularly detect photons that are billions of years old. But some theories suggest that photons could have a non-zero rest mass, albeit a small one – the upper limit for the mass of the photon is constrained to 10^–18 eV or 10^–54 kg thanks to experiments with electric and magnetic fields. And with this small mass, a photon could decay into other lighter elementary particles, such as a pair of the lightest neutrino and an antineutrino, or even particles that are currently unknown and beyond the Standard Model of particle physics.

Now, Julian Heeck of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, has turned to cosmological observations for signs of this photon decay (Phys. Rev. Lett. 111 021801). He looked at the cosmic microwave background (CMB), a remnant of the Big Bang that came into being when the universe was very young – only about 380,000 years old.

Background glow

Before that time, matter and radiation were intrinsically linked. But as the universe underwent a period of extreme growth known as “inflation” and expanded, the hot plasma of electrons and light nuclei cooled enough to allow neutral atoms to form. This “decoupling” of matter and radiation suddenly allowed photons to travel freely across the universe. Over time, their wavelengths were stretched by the expansion of the universe to leave a faint glow of radiation in the microwave region of the spectrum – an emission of uniform, black-body thermal energy – in every direction that we can detect today.

More than 100 experiments have studied the CMB since it was first discovered, including NASA’s Cosmic Background Explorer (COBE) satellite, its Wilkinson Microwave Anisotropy Probe (WMAP) and more recently the European Space Agency’s Planck mission, all of which have made increasingly precise measurements of this radiation. In fact, the CMB spectrum is the most precisely measured black-body spectrum in nature.

A long lifetime

It is this spectrum that Heeck used as a constraint for his calculations – he used extremely accurate data from the COBE mission and compared it to his calculated spectrum, which included the photon decay.

If the photon has mass and is decaying into lighter particles, then the number density of photons in the CMB should decrease as the photons travel. But this in turn would mean that the CMB spectrum would no longer fit the near-perfect thermal curve that is observed. Heeck reasons that as the CMB is an almost a perfect black body, very few photons, if any, will have decayed during the 13.8-billion-year existence of the universe and so the CMB measurements can constrain the photon’s lifetime.

Using a combination of the mass and CMB constraints, Heeck calculates the photon’s lifetime within its own rest frame to be three years. But as these photons with tiny mass travel at nearly the speed of light, time dilatation must be accounted for to obtain their lifetime in our frame of reference, for visible light – and this was calculated to be 10¹⁸ or a billion billion years. Improving this limit might be difficult until new studies can probe the early universe further.

The research is published in Physical Review Letters.

Carbon map of Panama leads the way

By Madeleine Fowler, who is doing a work experience placement at Physics World

Panama is a country of diverse ecosystems and complex landscapes, with vegetation ranging from grasslands and scrublands to dense forests. This makes it the perfect location for scientists to experiment with different methods of measuring above-ground carbon density – carbon that is locked up in vegetation.

Scientists have now mapped the above-ground carbon density of the entire country, which is a first in the world of carbon mapping. Field data and satellite data were integrated with high-resolution airborne light detection and ranging (LiDAR) data. This made it possible to create the first carbon map that could quantify carbon stocks in a local area as small as one hectare. What’s more, it can do this over millions of hectares.

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Close encounters of the muon kind