Page 1325 – Physics World

Geophysics Texas style

By Hamish Johnston at the APS March Meeting in Dallas

I’m here in Dallas for the March Meeting of the American Physical Society, which this year will be celebrating the 100th anniversary of the discovery of superconductivity and the 25th anniversary of high-temperature superconductivity.

Eager to get stuck into superconductivity I went to a session on Sunday on industrial physics that, among other things, asked what we could be doing to get more practical use out of superconductors.

And in case you are wondering when physicists will get round to finding a theory for high-T_c superconductivity, Seamus Davis of Cornell University predicted that it will arrive next year – although I think he was being slightly tongue-in-cheek.

On Saturday I did some sightseeing in Fort Worth. After seeing the cowpokes at the stockyard and having some tasty Texas BBQ, we went to the Fort Worth Museum of Science and History.

One physics-related highlight was an exhibit that looked at the geophysics of natural gas exploration. The impressive vehicle above is used to vibrate the ground, sending soundwaves deep into the Earth – a process called vibroseis. These waves reflect off various geological features and are then picked up by an array of microphones back on the surface. After some impressive computer processing, the data are rendered as a 3D image.

Some of the display was focused on the controversial process of hydraulic fracturing or “fracking”. This involves pumping water and a small amount of sand into gas-bearing rock – the pressure of the fluid fractures the rock and the sand props open the cracks. Gas can then escape along the borehole to the surface.

What the exhibit didn’t seem to discuss is the growing controversy surrounding the process. There seemed to be no mention of the concern of some people that some toxic additives to the fracking water could end up seeping into local groundwater, and ultimately into drinking water. That was the subject of a recent documentary film called GasLand.

Other highlights of the museum include a real Sputnik satellite – apparently the Soviets made lots of them, but didn’t get round to sending them into space – and a letter from Albert Einstein to Fort Worth schoolchildren that you can read below.

einstein letter2.jpg

Japan quake triggers nuclear rethink

Governments around the globe are planning to review their nuclear programmes in light of the ongoing crisis at the Fukushima Daiichi nuclear-power plant. The situation follows the 8.9 magnitude earthquake (on the Richter scale) and subsequent tsunami that struck Japan last Friday, leaving thousands dead and causing significant damage to the nation’s infrastructure. Today, Japanese authorities have upgraded the emergency at Fukushima from four to five on a seven-point scale, placing it on a par with 1979’s Three Mile Island accident in the US.

Early reports suggest that the emergency at Fukushima stemmed from a failure of cooling systems associated with the plant’s six reactors. When the earthquake struck, damage to power supplies meant that cooling water could no longer be circulated within the reactor core, causing fuel rods to overheat and their metal casings to partially melt. This released chemicals that reacted with water vapour to produce hydrogen, which escaped and exploded, damaging the reactor buildings.

As an emergency response, Japanese authorities drenched the reactor compound with seawater but there are still fears that further explosions could release dangerous levels of radioactive substances into the local environment. On Tuesday the UK government’s chief scientific officer John Beddington responded to concerns that the Japanese authorities were unwilling to release information about the developing emergency. “What they’re putting out is pretty comprehensive and it’s going to the appropriate international organizations,” he told the British embassy in Toyko. “In fact we are getting information through the international energy agencies and we do have pretty detailed knowledge of what these plants are like.”

Safety reviews

However, the UK government has already commissioned its nuclear inspector, Mike Weightman, to conduct a review into the implications of events at Japanese nuclear reactors on existing and new plants in the UK. An interim report is expected by mid-May and a final report within six months. “Safety is and will continue to be the number one priority for existing nuclear sites and for any new power stations. I want to ensure that any lessons learned from Mike Weightman’s report are applied to the UK’s new-build programme,” says Chris Huhne, the UK energy secretary.

When we see a crisis like the one in Japan, we have a responsibility to learn from this event, and to draw from those lessons to ensure the safety and security of our people Barack Obama

Germany has taken its seven oldest reactors offline until at least June and put on hold plans to extend their lives. While France, the number two producer of nuclear power behind the US, has announced that it will conduct tests on security systems at the country’s 58 nuclear reactors, and the results would be made public. President Barack Obama has also requested a comprehensive review of US nuclear facilities, which will be carried out by the US nuclear regulatory commission. Announcing the review during a speech to journalists at the White House, Obama also restated his support for nuclear power.

“Here at home, nuclear power is an important part of our own energy future, along with renewable sources like wind, solar, natural gas and clean coal,” said Obama. “But when we see a crisis like the one in Japan, we have a responsibility to learn from this event, and to draw from those lessons to ensure the safety and security of our people.” India, China and Pakistan also intend to conduct reviews of their nuclear safety.

Infrastructure damage

The earthquake and tsunami have also affected some of Japan’s major scientific facilities, although the country’s strict building codes managed to restrict major damage. A preliminary inspection of the massive new $1.5bn Japan Proton Accelerator Research Complex (J-PARC), which lies around 200 km south of the region hit worst by the quake, revealed it had come off relatively unscathed, although at the lab’s neutron spallation source, the mercury target used to produce neutrons had moved around 30 cm. It is expected to take more than six months for the source to return to normal. “We are confident to recover the machine within reasonable time,” says J-PARC director Shoji Nagamiya.

Not so fortunate, however, is the Photon Factory experiment, a national synchrotron radiation facility based at the KEK particle-physics lab in Tsukuba, some 50 km north-east of Tokyo. The experiment’s director, Soichi Wakatsuki has reported that the facility’s linear accelerator has suffered “substantial damages”, including the displacement of three radio frequency modules by about 10 cm and one magnet that fell onto the floor.

MESSENGER spacecraft enters Mercury orbit

The first spacecraft to orbit Mercury began circling the solar system’s smallest and least-understood planet early this morning, in what mission scientists hailed as the “historic” conclusion to a six-and-a-half year, 7.9 bn kilometre journey.

At 12.45 a.m. GMT on Friday 18 March the main thrusters on NASA’s MESSENGER spacecraft began firing, slowing it down by 0.862 km/s so that it could be “captured” by Mercury, which has an escape velocity of 4.25 km/s.

After the 15-minute “burn” finished and the spacecraft repositioned itself for transmitting signals back to Earth, mission operators at the Johns Hopkins University Applied Physics Lab (APL) spent another anxious 10 minutes analysing the anticipated radiometric signals confirming nominal “burn shutdown” before declaring that MESSENGER had been successfully inserted into orbit around Mercury.

By 1.45 a.m. GMT the craft had rotated back to the Earth and started transmitting data.

“Achieving Mercury orbit was by far the biggest milestone since MESSENGER was launched more than six and a half years ago,” says MESSENGER project manager Peter Bedini from APL. “This accomplishment is the fruit of a tremendous amount of labour on the part of the navigation, guidance and control, and mission operations teams, who shepherded the spacecraft through its journey.”

The fun starts here

At 6:47 a.m. MESSENGER began its first full orbit around Mercury, tracing out an elliptical path that brings it within 200 km of the planet’s scorched and cratered surface before swooping out to 15,193 km, where the reflected heat from the surface is less intense. The craft’s instruments are due to be activated on 23 March, with the primary “science phase” of the mission beginning on 4 April. Over the next year, the craft will complete one such revolution every 12 Earth-hours, racking up 730 laps before the mission’s scheduled end.

During this time, instruments on the half-tonne, $446m craft will collect unprecedented amounts of data about Mercury’s surface features and composition, as well as its magnetic field and tenuous atmosphere, or exosphere. According to MESSENGER principal investigator Sean Solomon from the Carnegie Institution for Science in Washington, DC, these data will yield new information on some of Mercury’s biggest mysteries – including the intriguing possibility that our solar system’s innermost planet, where surface temperatures can exceed 700 K, may harbour small amounts of water ice in its polar regions.

01:58

00:00

Mysterious world

Infrequent visitors

MESSENGER is the first spacecraft to visit Mercury since the mid-1970s, when the Mariner 10 probe flew past three times. That mission notched up several intriguing discoveries – including Mercury’s magnetic field and exosphere – but it mapped only 45% of the planet’s surface and left many questions unanswered.

MESSENGER had already added to this body of knowledge before today’s rendezvous, thanks to its complex trajectory. To reach the correct velocity and position for entering Mercury orbit, the spacecraft flew past the planet three times in 2008 and 2009, using the planet’s gravity to tweak its path each time. During these fly-bys, MESSENGER’s camera imaged most of what Mariner 10 missed, while spectrometers collected data on Mercury’s composition and a magnetometer sketched out the geometry of the planetary magnetic field.

01:32

00:00

Long and winding road

Despite these early successes, for many scientists this morning marked the real beginning of MESSENGER’s mission. “The three successful fly-bys of MESSENGER past Mercury have already rewritten the textbooks about the Sun’s nearest neighbour,” said Daniel Baker, a co-investigator on the mission, speaking ahead of the orbit insertion. “But we think there is so much more to learn – we’ve probably just scratched the surface.”

Solving Mercury’s mysteries

One of the biggest remaining puzzles is Mercury’s magnetic field, which has been a “great mystery” ever since Mariner 10 discovered it, Solomon told physicsworld.com. Mercury’s intrinsic field is weak, with a dipole strength almost 1000 times less than Earth’s. However, since larger planets like Mars and Venus have no intrinsic dipolar field at all, the existence of even a weak field on Mercury is surprising.

Another related question is Mercury’s extraordinarily high density, which at 5.3 g/cm³ is the biggest of any planet in the solar system, after gravitational compression is factored out. To achieve such a high density, Mercury’s heavy, metal-rich core must account for 60% of its mass, compared with 30% for the cores of Earth, Mars and Venus. The presence of a magnetic field also suggests that this core is at least partially molten. However, the structure and dynamics of Mercury’s core are poorly understood, and there are competing theories for why it is so large relative to the rest of the planet.

01:21

00:00

Rocky road

MESSENGER’s suite of seven scientific instruments – which includes an altimeter as well as the camera, magnetometer and four spectrometers – should begin to answer these questions on 4 April, after the spacecraft has passed a series of checks to determine that it is operating well in Mercury’s harsh thermal environment.

Project scientists are used to being patient. When MESSENGER launched in 2004, “this milestone seemed like it was a long, long way away,” says mission co-investigator Bill McClintock of the University of Colorado’s Laboratory for Atmospheric and Space Physics. “But here we are at last, poised to help solve some of the many tantalizing mysteries about Mercury.”

CERN's colourful characters caught on film

Mural of the inner workings of the ATLAS detector by Josef Kristofoletti

By James Dacey

I'm on a bit of a comedown today after spending the past two days whizzing around CERN with a film crew to record a series of short videos for physicsworld.com.

It was my second time at the particle-physics lab and I was inspired yet again by the enthusiasm of researchers and the way they discuss profound questions of nature as if they were chatting about last night's football or the weather.

My previous CERN visit was last March just after the Large Hadron Collider (LHC) had achieved its first collisions at 7 TeV and the physics programme was finally under way.

A year down the line and researchers across CERN's various experiments have collected plenty of data to analyse and big physics results could be just around the corner.

One of our videos will look at the hunt for the Higgs boson and for physics beyond the Standard Model. We interviewed scientists from the LHC's two general purpose detectors, CMS and ATLAS and both seemed fairly confident that they will have collected enough data by the end of 2012 (when the LHC will be shutdown for at least a year) to have a decent crack at finding the elusive Higgs. If indeed it exists!

Both scientists were also excited by the prospect of discovering SUSY particles to advance the theory of supersymmetry. This is the idea that every boson has a partner fermion with identical mass and internal quantum numbers.

A separate video will look at the LHC's ALICE experiment, which is designed to recreate the moments that existed just picoseconds after the Big Bang. I was particularly excited about this project because it meant I could interview my old university tutor, David Evans, who is head of the UK team at ALICE.

Evans was clearly really proud of his "girl" who apparently showed excellent performance recording data at the end of last year when the LHC collided lead ions instead of protons. The idea behind this is to create and study a state of matter known as quark-gluon plasma that may have existed at the very beginning of the universe.

But while news from CERN tends to be dominated these days by the LHC and its search for fundamental particles, there is plenty of other science going on there. Another video will look at one of these experiments, known as CLOUD, which is designed to study processes in the Earth's atmosphere and the effect these could have on the climate.

Then in our final video we change direction again by exploring the CERN arts programme, which is seeing something of a renaissance under the leadership of Ariane Koek. Ariane, who has been employed to develop a cultural policy for CERN, explained to me that her big idea is to break down the barriers between art and science by creating opportunities for artists to come to CERN to collaborate with scientists.

All of these short films have now entered the post-production phase, but watch this space because they will be appearing on physicsworld.com over the next few weeks.

Laser beam could nudge space junk away

Satellites in orbit around the Earth are at risk of collision with space-based objects, which have either been discarded by space missions or created in satellite collisions. But now a team of researchers at NASA believes it may have found a relatively cheap solution for dealing with this "space junk" – aim a medium-powered laser into space and nudge any objects on a collision course out of harm's way.

Earth is surrounded by thousands of discarded man-made objects, from spent rocket stages to flecks of paint. Even very small items pose a danger to working satellites because they travel at tens of thousands of kilometres an hour. In 2009 the threat to satellites hit the headlines when a defunct Russian satellite destroyed an Iridium telecommunications satellite – creating even more junk.

More and more rubbish

Computer models suggest that in the most popular low-Earth orbits (altitudes of about 800–1000 km) the rate at which new debris is created through collisions exceeds the rate at which debris drops back down to Earth. This means that even without any new missions launched the amount of space junk is destined to keep on increasing. This is the "Kessler syndrome", first identified in 1978 by NASA scientist Don Kessler.

Proposals to remove individual items of space junk involve moving them into progressively lower orbits. Ideas include robots that push objects closer to the Earth and ground-based laser beams that impart recoil to an object by vaporizing its surface material. Each proposal could cost tens of millions of dollars or more to implement and given the still relatively low risk that space junk poses to satellites, none currently deliver value for money.

Giving junk a nudge

Now, a team headed by James Mason of the NASA Ames Research Center and the Universities Space Research Association in California has come up with an alternative approach. It points out that knocking an object out of orbit requires a laser with a power output of millions of watts. Instead, they propose using a laser of a few kilowatts to change the velocity of an orbiting object just enough to move it out of the path of a second, oncoming object. The change would occur simply by the momentum of the photons in the beam.

The scheme would involve continually evaluating the chances of a collision between any two items of space junk with a diameter of 5 cm or more, using radar data provided by the US Space Surveillance Network. Objects on a collision course would then be tracked by an optical telescope and, with one of the objects locked on, the laser beam would be sent along the telescope's main optical path into space.

Using a computer model, Mason and colleagues calculate that the risk of slightly more than half of all potential collisions in low-Earth orbit could be significantly reduced by using just one 5 kW commercially available laser mounted on a 1.5 m telescope somewhere close to the poles (since most of the objects are in roughly polar orbits). Likely to cost no more than about $10m, including instrumentation for adaptive optics, they say this approach could represent a much cheaper alternative to the direct removal of space debris, and they calculate that the far lower rate of collisions should lead to a reversal of the Kessler syndrome within a few decades

Testing needed

However, team member William Marshall, also of NASA Ames and the Universities Space Research Association, points out that there are a number of uncertainties in the model, one of which is the precise nature of the objects in orbit. Proving the validity of the scheme will involve rigging up a suitable laser and directing it through an existing telescope, as well as running space-debris models to see whether the scheme really can reverse the Kessler syndrome in the long term. The researchers say that it would be desirable to get international backing for the implementation of their scheme, given possible safety concerns about firing laser beams into space.

Kessler is now a private consultant on space debris and said of this latest work: "it is encouraging to see new ideas to prevent the growth in the orbital debris population". But he points out that a kilowatt-power laser could only alter the path of quite small objects and that it would therefore be of no use in preventing the most destructive collisions – those involving two large, intact items. "This system might reduce the number of required removals," he says, "but it would not be a total solution".

The work is described on arXiv.

Stranger than fiction

By Margaret Harris

Quick question: are time travel, invisibility cloaks and "tractor beams" that lift objects science fact, or science fiction?

I'd argue that all three are closer to science fact than science fiction. True, Britain's streets are not lined with time-travelling Tardises (or is it Tardii?), but physical theories of the quantum vacuum indicate that subatomic particles are constantly popping back and forth in time. And although invisibility cloaks aren't exactly at the Harry Potter stage, negative-refractive index materials are serious science – we've written several stories on them, including one just last week. As for tractor beams, the Bristol physicist Mervyn Miles may not be reeling in alien spacecraft, but this amazing video of his work shows that it's perfectly possible to manipulate small objects with a laser beam.

However, according to this quiz on the BBC website, I'm wrong on two out of the three: time travel and invisibility cloaks are classed as "science fiction", while Miles's "tractor beams" are deemed "science fact". Go figure.

The "news peg" for the quiz is a recent survey conducted by Birmingham Science City which purports to show that many Britons have trouble distinguishing between science fact and fiction. Reports on the survey have mostly heralded its results as proof of the British public's ignorance, mocking the 30% of respondents who believed that time travel was possible and the 22% who thought that invisibility cloaks were the real deal.

But maybe the real story is that 70% and 78% thought – incorrectly – that they weren't.

Single spins flipped in optical lattice

Physicists in Germany are the first to flip individual atomic spins in an optical lattice. The researchers, who are based at the Max Planck Institute for Quantum Optics in Garching, used a combination of laser light and microwaves to address individual rubidium atoms arranged in a state known as a "Mott insulator". Their method could be used for making quantum computers and also for simulating the behaviour of electrons in solids – especially superconductors.

This newfound ability is just the latest example of the progress that physicists have made in understanding quantum interactions by studying ultracold atoms in optical lattices of crisscrossing laser beams. By adjusting the laser light and applied magnetic fields, scientists can "tune" the interactions between atoms and simulate the behaviour of electrons in crystalline solids. Although an atom in an optical lattice can normally tunnel from one lattice site to a neighbouring site, in a Mott insulator all the sites are occupied, which means that the energy cost of tunnelling is too great and the atoms are frozen in place.

Each of these frozen atoms, however, could make an excellent quantum bit (qubit) in a quantum memory because they are highly isolated from the surrounding environment. And as each atom has a magnetic spin, optical Mott insulators could be used to simulate the effect of spin on electronic properties such as conduction. However, physicists had been unable to adjust the value of individual spins, limiting the usefulness of optical Mott insulators.

Flipping a spin

What Stefan Kuhr, Immanuel Bloch and colleagues have now done is to devise a way of flipping the spin of an individual atom without affecting the rest of the lattice. The team began with a cloud of about one billion rubidium-87 atoms, which were cooled to less than 100 nK. As this process involves atoms continually leaving the cloud, the team was left with just a few hundred individual ultracold atoms. The crisscrossing lasers were then switched on to create a 2D square lattice and the parameters of the lattice were tweaked to transform the system from a conducting superfluid to a Mott insulator with a lattice spacing of 532 nm.

All of the atoms in the lattice are initially in the "0" spin state. To flip an atom, it is first illuminated with laser light. The beam is tightly focused so that nearly all of the light falls on one lattice site, where it modifies the energy difference between the "0" and "1" spin states of that atom alone. The entire lattice is then bathed in microwaves at the modified energy difference, which flips the spin of the illuminated atom but no others.

Microscope images of patterns in Mott insulators

Drawing spin symbols in a Mott insulator

The process can then be repeated at different lattice sites and the team managed to write several patterns in the Mott insulator, including the Greek symbol ψ (see figure). While it is not possible to measure the spin of individual atoms without destroying the Mott insulator, the physicists verified the technique by firing a laser that is tuned to only eject atoms in the "1" state. An optical microscope image is then taken of the remaining "0" atoms, revealing the pattern.

Crucial step

Peter Zoller, a quantum physicist at the University of Innsbruck in Austria, thinks the work is "a seminal step forward in experiments with optical lattices". Henning Moritz of the University of Hamburg, meanwhile, says that the team's ability to address individual atoms "is a crucial step on the path toward quantum computing with ultracold atoms". Indeed, Zoller thinks that quantum-computing devices based on atoms could soon catch up with those using trapped ions – which currently lead the pack in terms of performance.

However, there is more work to be done. If the Mott insulator is used as a quantum computer, atoms need to be put into a quantum superposition of the "0" and "1" state. Kuhr told physicsworld.com that requires some better control over the experimental parameters and that is on the team's "to do list".

Quantum computers also require quantum gates (and entanglement) between pairs of atoms. This could be realized by putting atoms into "Rydberg states" – in which the atom's outer electron shell extends a great distance from the atomic nucleus and overlaps with that of its neighbour.

The physicists are also interested in flipping a spin and then watching how the spin excitation moves though the lattice, thus simulating quantum magnetism and transport phenomena in solids.

The work is reported in Nature 471 319.

Reading discs with fewer photons

The intensity of light required to read data from an optical disc can be reduced dramatically by using entangled photons – according to a physicist in the UK. The concept, which has yet to be verified by experiment, could allow more data to be stored on CD or DVDs and lead to new types of rewritable optical storage media.

Entanglement is a quantum-mechanical property that allows particles to have a much closer relationship than permitted by classical physics. A famous example of this is the Einstein-Podolsky-Rosen (EPR) correlation between the position and momentum of pairs of photons. This is unlike the laser light used to read conventional optical discs, which does not have such strong correlations between photons.

EPR light can be created in the lab and Stefano Pirandola from the University of York, UK has calculated that it could offer a new way of reading data from optical discs. Pirandola came up with the idea when considering a memory comprising a collection of cells, each with two possible reflectivities. Higher reflectivity represents a "1", and lower reflectivity a "0".

Measuring intensities

In his proposed system, light hits the memory cell and a detector records the intensity of reflected light. Light is also sent directly from source to detector, creating ancillary "idler" modes that may improve the reading of cells by exploiting possible correlations within the signals (see figure below). "We do not know if the idler mode is necessary or not," admitted Pirandola.

He believes that the gain in information – the difference in information extracted by an EPR source and the best classical one – can be almost 100%. "When it is equal to 100%, it means that the EPR source retrieves all the information perfectly, while all classical sources cannot read the memory."

How to read a disc

In most situations, creating a practical system based on entangled light is extremely difficult because interacting with the environment destroys the entanglement. According to Pirandola's analysis, his system should not suffer that fate. Calculations reveal that measurements of cell reflectivities are not impaired by stray photons within the system that hit the detector after being scattered by the environment.

Putting theory into practice

The biggest challenge to building a real system based on Pirandola's calculations is making a suitable EPR source. But this hurdle is not overwhelming, given that such sources have already been created in many quantum optics labs. This is done in a process called parametric down conversion, whereby light from a pump laser hits a special "nonlinear" crystal to yield entangled photon pairs.

Pirandola thinks that a practical system could employ just a few tens of photons to read each cell. However, this does not mean that expensive, single-photon-counting detectors are needed. Instead, the signal reflected from the cells can be combined with that from the pump laser, before being separated into two parts, each impinging on their own photodetector. "Thanks to this set-up, the input signal is amplified into a macroscopic one before being measured," explains Pirandola.

However, even if such a system is shown to be far better at reading CDs and DVDs, its size and cost make it impractical for this application. The biggest barrier is the realization of small, efficient sources of EPR light. "One promising technology is two-photon emission from semiconductors," says Pirandola. "[Such a source] can generate correlated photons at a very high rate, and be miniaturized as well."

Unexpected results

One researcher who is surprised by the results of Pirandola's calculations is Seth Lloyd from MIT: "The scheme considered is very close to quantum illumination, and we verified that quantum illumination could not do significantly better for detection than classical schemes."

He says that Pirandola's work is very important, providing a rare example of a quantum mechanical measurement that is significantly superior to a classical one.

Pirandola's work is described in Phys. Rev. Lett. 106 090504.

The flip-flop world of research

By Louise Mayor

Life in research involves a turbulent rollercoaster of emotions. But often the only glimpse we see is the success and jubilation of when things work out and results get published.

This new video report (below) offers a behind-the-scenes look into the whole research process, from the long hours spent working in the lab to that day when the results finally get accepted for publication in a journal. It features researchers at Nottingham University achieving a breakthrough in part of their broader aim: to construct 3D objects on surfaces, atom by atom, using scanning probes. “The novel aspect of this video is not so much the science but the fact that we've filmed the entire research process over the course of a year or so,” says Philip Moriarty, the main protagonist in this adventure.

The joy that results when experiments go well comes across nicely when, while being filmed in the lab, Moriarty breaks off mid-sentence to throw his fists in the air and exclaim “yes!” However, he reveals that the groundwork preceding what looks so effortless has been 18-months-plus in the making and has sometimes involved 24- and even 36-hour shifts.

But research is rarely over once you've got that crucial result: there are then the highs and lows of trying to get the work published in as prestigious a journal as possible. Moriarty highlights that there's a definite hierarchy of journals to which physicists submit papers. In this case their work was rejected from both Nature and Science before finally being accepted in Physical Review Letters.

Film-maker Brady Haran really digs deep with a frank set of questions that would make many less-composed subjects squirm, such as: “Why is this impressive?”; “What you've written...looks really hard to read and really boring – who’s this for?”; and “If only you and a select number of people in the world can understand that paper, how is it doing the world any good?”

The up-and-coming Haran highlights this video on his blog as a great example of what he hopes to achieve with science films. Haran is the mastermind behind the Test Tube project where this video is featured alongside a veritable trove of other gems, as well as The Periodic Table of Videos and Sixty Symbols.

Tevatron tightens its grip on the Higgs

The latest results from the Tevatron collider at Fermilab near Chicago suggest that the Higgs boson is on the light side – which means that it could be harder to detect than a heavier particle. Predicted by the Standard Model of particle physics, the Higgs, if discovered, would provide an explanation for how elementary particles acquire mass. The Standard Model does not, however, say anything about what the exact mass of the Higgs boson ought to be and its eventual detection would be a massive achievement in high-energy physics.

Also in the race to detect the Higgs is the Large Hadron Collider (LHC) at CERN in Switzerland, which yesterday announced its first 7 TeV proton–proton collisions of the year. Although the LHC is expected to run continually until the end of 2012 before a year-long upgrade, researchers at Fermilab are keen to steal a march on their European rivals. Time is running out, though, as the Tevatron is due to close for good in September.

This makes the Tevatron the frontrunner in the hunt for the Standard Model Higgs boson Rob Roser, Fermilab

The new analysis of data from Tevatron's CDF and D0 experiments – along with earlier results – adds spice to that race, ruling out a Higgs mass of 156–183 GeV/C². Much of this region is excluded to 95% confidence, with some excluded to 90%. The new analysis extends Tevatron's previous Higgs exclusion zone of 158–175 GeV/c² (95%), which was reported in July 2010. "This makes the Tevatron the frontrunner in the hunt for the Standard Model Higgs boson," claims Fermilab physicist Rob Roser, who works on the CDF experiment.

When combined with previous searches for the Higgs and constraints imposed by the Standard Model, the Higgs mass is most likely to be in either a small sliver at about 183–185 GeV/C² or somewhere between 114 and 156 GeV/C² (see figure).

Lurking in the background

The ease with which the LHC can look for the Higgs depends partly on the particle's mass. If the Higgs is heavier than about 140 GeV/c², it is more likely to decay into pairs of Z or W bosons, which would cause a distinct signal in the LHC's detectors. A lighter Higgs, in contrast, would favour a decay to b-quarks, which would be harder to see against the background of other events. Indeed, this difficulty is the reason why the Tevatron has not yet been able to extend its exclusion zone to lower Higgs masses.

Although it's the toughest region, the [LHC] experiments have been designed to do the job Greg Heath, University of Bristol, UK

If the Higgs weighs in towards the bottom of the theoretical range it could prove very difficult for the LHC to find the particle. However, Greg Heath of the University of Bristol in the UK, who works on the LHC's CMS experiment, points out that the collider is equipped for the job.

"In the LHC experiments we have a range of strategies for looking in the low-mass region, not all of which are available at the Tevatron because the LHC detectors are more powerful," he says. "Although it's the toughest region, the experiments have been designed to do the job there and we have a good chance of seeing at least the first signs with this year's data sample."

As for a higher mass Higgs, Heath points out that the LHC will also be looking at masses above 180 GeV/c².

Colliders race for the Higgs

Despite the collider's imminent closure, researchers at Fermilab will continue operating the Tevatron until September 2011 in what is shaping up to be a race for the Higgs. "In the coming months our collaborations will focus on both the high-mass and low-mass scenarios and optimize our analysis techniques for the entire Higgs mass range," says CDF physicist Giovanni Punzi, of the University of Pisa and the National Institute of Nuclear Physics in Italy.

Browse articles by content type

Geophysics Texas style