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Galaxy survey casts doubt on cold dark matter

The physical properties of most galaxies in the universe can be explained in terms of just a single parameter. That’s the controversial conclusion of a team of astronomers in the UK and US, who have studied some 200 galaxies using radio and optical telescopes. The team believes that their discovery could mean that cold dark matter — an invisible substance that some astrophysicists have invoked to explain the formation and motion of galaxies — does not exist. However, not all astrophysicists are convinced.

One of the most important questions in cosmology is how galaxies emerged from the primordial fireball that followed the Big Bang. Physicists believe that ordinary matter — the stuff that makes up stars — would have been distributed evenly throughout the universe by the intense radiation present after the Big Bang. That makes it very unlikely that dense patches of matter would form and rapidly grow into galaxies, which is what seems to have happened less than a billion years after the Big Bang.

The solution to this puzzle could lie with cold dark matter (CDM) — invisible stuff that interacts via gravity but not electromagnetic radiation. CDM, astrophysicists argue, would initially not have been evenly spread by radiation but would instead have been pulled together into clumps by gravity. These denser regions of CDM would then evolve into galaxies by dragging in surrounding matter and CDM. This view is backed up by indirect observations of dark matter, which suggest that it accounts for most of the mass of a typical galaxy.

Many astrophysicists believe that this process can be described by the “hierarchical theory of galaxy formation”, in which progressively larger clumps of matter — and ultimately galaxies — attract each other and merge in often violent collisions. But because of this chaotic formation process, there should therefore be a wide variety of galaxy types in the Universe. That view appeared to be backed up with the observation that galaxies have various properties — such as their radii, mass, rate of rotation and luminosity — that can not be related in a simple way.

Six into one

However, in the last decade or so, astronomers have found that there are correlations between some properties of galaxies. The radius of a galaxy, for example, can be predicted by measuring its luminosity. Now, though, Mike Disney at Cardiff University and colleagues claim to have shown that six important properties of galaxies are controlled by a single parameter. Although the team has yet to identify this parameter, they believe that it is related to the mass of the galaxies (Nature 455 1082).

According to Disney, the discovery makes it very unlikely that galaxies formed according to the hierarchical theory of galaxy formation. Indeed, he goes one step further and says that the team’s results put the very existence of CDM into question — a statement that is certain to rile many astrophysicists.

The team used observational data from the Parkes radio telescope in Australia to identify about 300 objects that looked like they could be galaxies from the radio waves given off by neutral hydrogen. Julianne Dalcanton and colleagues at the University of Washington then searched through optical data from the Sloan Digital Sky Survey (SDSS) for the same objects and found that 200 of them were indeed galaxies.

Single parameter

Using data from both telescopes, the team classified the galaxies in terms of six independent properties. These were two optical radii (which define the sizes of the regions of a galaxy that emit 50% and 90% of the object’s light); the luminosity; the mass of neutral hydrogen in the galaxy; the dynamical mass (which includes dark matter); and the colour of the galaxy.

The team then carried out a statistical analysis of the data and found five correlations between these six properties — leading them to conclude that the structure of these galaxies is controlled by just one parameter. Although the team was unable to conclude exactly what this parameter is, Disney says that it seems to have a strong relationship with the mass of the galaxies.

Disney argues that the team’s finding is at odds with the hierarchical theory of galaxy formation, according to which the structure of a galaxy would be strongly influenced by the nature of the collisions that formed it. “If this were the case we would have expected to see 4-5 independent parameters”, he says. And because the hierarchical theory has CDM at its root, Disney believes that the team’s survey provides strong evidence that CDM does not exist. “Maybe our observations could be explained by CDM, but I wouldn’t bet on it”, he said.

Not everyone is convinced

Not surprisingly, not everyone agrees with Disney. Richard Bower of the University of Durham in the UK told physicsworld.com that CDM advocates are aware of correlations between galaxy properties and are trying to explain them. “The theory has been tested successfully against a number of individual correlations”, said Bower, but admitted that CDM advocates have yet to demonstrate that the theory can deal with this method of analysis.

Bower says that the merging of galaxies is now thought to be less important than it was in earlier versions of hierarchical theory. As a result, he is confident that galaxy formation can be explained using CDM. Bower also questioned the significance of the dominance of the mass parameter over the others, noting that the masses of galaxies varied over a much wider dynamic range than the other parameters so it was not surprising that the mass correlation was the strongest.

Disney is now exploring theories of galaxy formation that don’t involve CDM, but rather more conventional matter such as “hydrogen snow”.

Quantum robots are a 'brand-new paradigm'

By Hamish Johnston

If you are like me and you struggle with the basics of quantum computing you might want to stop reading now because this blog entry is about “quantum robotics”.

According to Daoyi Dong and colleagues at the University of Science and Technology of China, “A quantum robot is essentially a complex quantum system which generally consists of three fundamental components: multi-quantum computing units (MQCU), quantum controller/actuator, and information acquisition units”.

And they should know, because they have just published a preprint that “proposes a brand-new paradigm of robots”.

So why would you want a quantum robot?

According to Dong and Co, they are better at learning than classical robots because they would use “quantum reinforcement learning algorithms”.

Of course, physicists are still struggling to make a just one practical quantum computing device, let alone a MQCU. So I’m guessing that you might have to wait a while for a robot that uses quantum mechanics to learn how to iron your shirts.

UPDATE 27 October: This preprint has been withdrawn by the authors.

Spin segregation puzzles physicists

Physicists in the US are the first to segregate a Fermi gas of ultracold atoms according to their spin — with “spin-up” and “spin-down” atoms moving to opposite sides of the optical trap in which they were contained.

John Thomas and colleagues at Duke University found that about 60% of the lithium-6 atoms became segregated and that the spin-up and spin-down atoms remained apart for several seconds. However, they are puzzled as to why the segregation lasts much longer, and is more intense, than predicted by theory (Phys. Rev. Lett. 101 150401).

Spin segregation is interesting for physicists because it plays an important role in “spintronics”, where it is used to create currents of spin-polarized electrons. As lithium-6 atoms and electrons are both fermions — that is, particles with half-integer spin — the new system could therefore be used as a “quantum simulator” of spintronic devices.

Although Eric Cornell and colleagues at the University of Colorado have previously been able to segregate an ultracold atomic gas of bosons (rubidium atoms with integer spin) in terms of spin, such a system has not been useful for simulating interactions between electrons in solids because fermions and bosons behave so differently.

Hit with a radiofrequency pulse

Thomas’s team used a Fermi gas of ultracold lithium-6 atoms that are trapped in the centre of a vacuum chamber by a laser beam. The team begin their experiment with all the atoms in the spin-up state. The gas is then hit with a radiofrequency pulse that puts each atom into a coherent superposition of spin-up and spin-down states. This means that until, a measurement is made on the spin of an atom, it is both spin-up and spin-down.

The team then switch on a magnetic field that varies in strength across the trap and causes the atoms to migrate to the centre. Occasionally, two atoms collide leaving one atom “spin-up” with respect to the magnetic field and the other “spin-down”. The atoms then move away from the centre with velocities that are correlated to their spins — spin-up electrons moving in one direction and spin-down electrons in the other.

The team found that this process took about 200 ms to reach a maximum spin segregation of 60% — and that this segregation endured for about 5 s. By contrast Cornell’s bosons remained segregated for a mere 200 ms — something that has been successfully described by a theory that assumes that the atoms interact strongly with each other.

However, Thomas told physicsworld.com that this “theory would predict that our spins would segregate in 7 ms and relax back to equilibrium in about 7 ms”.

Freedom to move

Thomas believes that his atoms remained segregated for seconds because they are much freer to move about the trap than were Cornell’s bosons — which couldn’t travel very far without colliding with each other. By carefully selecting the applied magnetic field, Thomas and colleagues we able to minimize the interaction strength between fermions, which means that the rarely collide. Because segregation and its subsequent decay are both driven by collisions Thomas believes that this is why these processes too much longer in his system.

As well as simulating the behaviour of electrons in solids, Thomas believes the system could be used to gain a better understanding of why spin segregation endures for such a long time could help physicists create new types of entangled spin-states.

Sticky tape takes X-ray images

Adhesive tape is turning out to be an unexpected scientific tool. It is already used to make graphene — a layer of carbon one atom thick that has remarkable electronic properties — by peeling the surface off a piece of graphite using sticky tape. Now, researchers in the US have used this household item to generate nanosecond-length X-ray bursts. They show that simply peeling the tape from its roll can create flashes of X-rays intense enough to record images of a human finger.

The phenomenon of generating visible light by moving contacting surfaces relative to one another — known as triboluminescence — has been known for hundreds of years. Indeed, English scholar Francis Bacon noted in the seventeenth century that sugar sparkles when broken or scraped in the dark. Then in the 1950s and 1980s Russian groups showed that peeling tape could generate high energy electrons and X-rays, although these results were never reproduced in the West.

The latest research has been carried out by Seth Putterman and colleagues at the University of California, Los Angeles (Nature 455 1089).

To be honest we didn’t believe that peeling tape would generate X-rays Seth Putterman, UCLA

Putterman and co-workers placed an off-the-shelf roll of Scotch tape on a ball bearing mounted on stiff spring leaves and attached the free end of the tape to a cylinder turned by an electric motor. They then used a solid-state detector and a radiofrequency antenna to measure any X-rays and radio waves given off from the point at which the tape left the roll, and calculated the force needed to peel the tape via induction measurements of the leaves’ displacement. The whole set up was placed in a vacuum chamber.

Sticky tape X-ray

Putterman’s group recorded X-ray emission in the form of intense bursts some billionth of a second long (with the width of the X-ray pulses calibrated using the well characterized radiowaves), and found that these bursts are correlated with very slight slippages (i.e. reductions in force) in the otherwise smooth removal of the tape from its reel.

Eureka moment

The researchers then used their set up to record images of a number of objects, including the finger of a group member using 20 second long exposures. “To be honest, we didn’t believe that peeling tape would generate X-rays. The glow of the X-ray scintillator was a eureka experience for me,” said Putterman, referring to the group’s initial detection of X-rays from their tape made using a zinc-sulphide scintillating screen.

The group believes that as the tape peels the acrylic adhesive on the exposed tape becomes positively charged and the outer surface of the remaining polyethylene roll acquires a negative charge. This causes electric fields to build up to values that trigger discharges.

A ‘new inspiring mystery’

The researchers say that at the reduced pressure in the experiment — about one millionth of an atmosphere — the discharges accelerate the electrons to energies that generate X-rays when they suddenly decelerate in the positive side of the tape. However, the researchers remain stumped as to how the diffuse mechanical energy needed to peel the tape is focused to the extent that it can produce X-rays, and even more strangely how it can do so in the form of nanosecond pulses. This, says Putterman, gives us a “new inspiring mystery to dig into”.

The research also suggests one or two practical applications. Putterman has applied for a patent for the imaging potential of the technique, pointing out that it could lead to a portable X-ray device that would not require a high voltage power supply. In addition, he says, humble sticky tape might one day become a new energy source. He points out that X-ray emission from peeling tape is similar to sonoluminescence — the production of light using sound waves to collapse bubbles very rapidly — in that both phenomena involve enormous multiplication of energy densities.

There has been much controversy over whether sonoluminescence can generate energy via fusion reactions, but Putterman believes that it is a line of research worth pursuing.

Party time at CERN

Sharp suits, limos and armed police officers do not normally feature at the CERN particle-physics lab near Geneva. But yesterday dignitaries from some 40 countries descended on the lab to officially inaugurate the Large Hadron Collider (LHC), which circulated its first protons on 10 September. The mammoth security operation saw police — complete with dog — surprising physicists in the control room of the ATLAS detector at 4.15 a.m. on Tuesday, well before the 334 delegates, 130 journalists and more than 500 invited physicists turned up for the afternoon event.

CERN pressed ahead with the inauguration, the largest official delegation it has hosted, despite the LHC having been out of action for the past month. Deep beneath the security tents and cordoned-off roads, the 27 km circumference machine was subject to ongoing investigations into an electrical fault that occurred during testing on 19 September. The incident knocked several 15 m long, 35 tonne dipole bending magnets out of position and released six tonnes of liquid helium into the LHC’s tunnel, which will put the collider out of action until May or June next year.

We’ve gone far enough to know that the LHC works as expected Lyn Evans, CERN

“On 10 September we showed the world what a beautiful machine the LHC is,” project manager Lyn Evans told a 1500-strong audience seated in a giant metal shed off the main CERN site where up to 24 dipole magnets may soon be jostling for repair. “We’ve gone far enough to know that the LHC works as expected.”

Heaping praise

That the LHC should have produced its first proton collisions by now did not appear to dampen the mood of delegates, who heaped high praise on the LHC and CERN as shining examples of human achievement and of European and international collaboration. French prime minister Francois Fillon and Swiss president Pascal Couchepin were joined by ministers and ambassadors from CERN’s 18 other European member states, including the UK’s new science and innovation minister Paul Drayson. The global economic situation, which reached crisis point the very week after the LHC broke down, had not helped efforts to secure heads of state, admitted some CERN officials.

Drayson, a former biotechnology entrepreneur on his first visit to CERN, said he was really impressed. “You get a real sense of the collaborative energy here,” he told Physics World. “We have to continue to support fundamental research like this, which addresses questions about the very nature of matter, whatever the economic conditions.”

You have to have faith that if you try and answer the very big questions then good will come out of it Paul Drayson, UK science and innovation minister

Having earlier been prised from a conversation with a CERN physicist about the LHC’s superconducting technology by his entourage for a prearranged armchair chat with German science minister Annette Schavan, Drayson said that economic return was not all that mattered in science. “You have to have faith that if you try and answer the very big questions then good will come out of it,” he said. “History tells us that strategically you can’t predict discoveries or innovation, and CERN is a living example of that.”

‘Molecular gastronomy’

Ettore Bocchia is a pioneer of “molecular gastronomy”

As the official delegation departed, full of the delights of a 16,000 piece physics-themed buffet of “molecular gastronomy” created by pioneering Italian chef Ettore Bocchia and pondering the debut performance of a 20 minute audiovisual concert by composer Philip Glass and photographer Frans Lanting called “Origins”, more than 2000 CERN physicists arrived by the busload for their own celebration, dubbed “LHCfest”.

Although food and drink flowed rather less freely than it had earlier in the day, during which ranks of safety-glass sporting CERN volunteers dished out ice cream freshly prepared using flasks of liquid nitrogen, LHC-festers were treated to a concert by the inimitable CERN rock group Les Horribles Cernettes and to a live performance of YouTube hit “The Large Hadron Rap”, which saw Evans take to the stage wearing a baseball cap greeted with claps and cheers — a performance that he later described as “a very interesting experience.”

India launches first lunar mission

India has launched its first mission to the Moon in a bid to create the highest resolution 3D maps of the lunar surface and provide a complete chemical mapping of the Moon’s soil. The unmanned Chandryaan-1 spacecraft successfully launched from Sriharikota, an island off the coast of the southern state of Andhra Pradesh at 06:22 local time and is expected to arrive in the Moon’s orbit on 27 October.

The Chandryaan-1 craft, was commissioned in 2003 by the Indian Space Research Organisation (ISRO) at a cost of $83m. Chandrayaan — Sanskrit for ‘mooncraft’ — is expected to last two years and will orbit the Moon 100 km above the surface to perform remote experiments at visible, infrared and X-ray frequencies.

The craft weighs over 1300 kg and carries 11 instruments — five scientific payloads from India and six from outside space agencies. Chandryaan-1 will also carry a 25 kg impactor — bearing the Indian flag — that will hit the surface and throw up lunar dust, which will be instantly analyzed by an Indian built mass spectrometer . The two main scientific objectives of the mission are to outline the surface of the Moon with a spatial resolution of 5–10 m and to map the entire lunar surface for elements such as magnesium, iron, uranium and thorium with a resolution of 25 km.

Chandrayaan-1 shows the maturity and vision of the Indian space programme Bernard Foing, European Space Agency

The other four India-based payloads are a stereo camera to map the lunar surface in visible wavelengths, a laser interferometer to measure the lunar topography and a spectral imager to measure the chemical composition of the crater regions on the Moon. A high-energy X-ray spectrometer will study the radioactive decay of uranium-238 and thorium-232 in the lunar soil. “Chandrayaan-1 shows the maturity and vision of the Indian space programme,” says Bernard Foing, executive director of the international lunar exploration working group of the European Space Agency (ESA).

Multinational science programme

Unlike China’s lunar orbitor Chang’e-1, which launched October last year, Chandryaan-1 features instruments built by foreign agencies. These include a soft X-ray spectrometer constructed by the Rutherford Appleton Laboratory in the UK and an infra-red spectrometer developed by the Max Planck Institute for Solar System Research in Lindau. Both these projects are supported by ESA and will look for traces of elements such as magnesium, aluminium and iron in the lunar soil.

Chandryaan-1 will also try to bury existing controversies such as the presence of water ice in the permanently shadowed regions of the lunar poles. A miniature imaging radar built at the Applied Physics Laboratory at Johns Hopkins University in collaboration with NASA will study the reflection of radio waves from the poles to look for signatures of water ice.

Chandryaan-1 provides a new national thrust in scientific research in space science Parameswaran Sreekumar, Indian Space Research Organisation

According to Parameswaran Sreekumar head of space astronomy and Instrumentation at the ISRO satellite centre in Bangalore, many of the instruments onboard Chandrayaan-1 will be unique compared to other lunar missions. “Chandryaan-1 provides a new national thrust in scientific research in space science,” says Sreekumar. “It will energise the public and enthuse the academic community to take up planetary sciences.”

Asian space race

India’s launch of Chandryaan-1 comes hot on the heels of both Japan and China’s launch of lunar orbitors late last year. Chang’e-1 had eight instruments onboard for its year long trip which is due to end next month, while SELENE launched in September 2007 for one year had thirteen scientific payloads.

However, India’s plans for the moon don’t end with Chandrayaan-1. India plans a robotic Moon lander — Chandryaan-2 — by 2012 that has recently been given the go-ahead by the government. “The collaboration in instrument development [for Chandryaan-1] will open the way for future collaborations on landers,” says Foing. A manned mission in space is also planned by 2015 similar to the Chinese mission last month and a manned mission to the Moon by 2025.

Physicists get closer to metallic hydrogen

Physicists in Sweden and France are the first to calculate the crystal structure of the metallic phase of silane — a normally gaseous compound of silicon and hydrogen that becomes a solid metal and then a superconductor when put under increasingly high pressure.

The result could be an important step towards understanding the structure of metallic hydrogen — an elusive material that is the “holy grail” of high-pressure physics because it could provide researchers with a new system to study high-temperature superconductivity.

A glance at the periodic table suggests that hydrogen should be a metal at sufficiently high densities because it is in the same column as the alkali metals — an idea that was proposed over 70 years ago by Eugene Wigner. Calculations done four years ago by Cornell University’s Neil Ashcroft also suggest that metallic hydrogen could be a superconductor at temperatures well above 30 K.

However, physicists haven’t had much luck in creating the metallic phase — solid hydrogen has been compressed to pressures as high as 320 GPa only to see it remain a semiconductor. This is about three million times atmospheric pressure and just about the limit of the diamond-anvil pressure cells used in such experiments. Scientists have had better luck with liquid hydrogen, which was shown to become a metal when compressed at 140 GPa by a shockwave — but this only lasted about 100 ns.

Hydrogen-rich materials

Instead of struggling to make metallic hydrogen, some physicists have turned their attention to the metallic phases of hydrogen-rich materials, such as methane (CH₄) and silane (SiH₄), which can be produced at much lower pressures. Such materials have electron densities similar to solid hydrogen and therefore could provide insights into the physics of metallic hydrogen.

Earlier this year, two independent teams of physicists compressed samples of silane and discovered that it became a metal at pressures greater than about 50 GPa — and became a superconductor when the pressure reached about 100 GPa. Although the positions of the silicon atoms in the metal was determined using X-ray diffraction, the precise crystal structure of the material could not be determined because it is almost impossible to “see” hydrogen atoms using X-rays.

Now, using state-of-the-art quantum mechanical calculations and a process of elimination, DuckYoung Kim of Uppsala University in Sweden and colleagues claim to have clearly identified the crystal structure of the metallic silane phase (PNAS 105 16454).

Theoretical ‘detective story’

The researchers have shown that a structure with space group P4/nbm should be metallic at pressures of more than about 60 GPa, which is in line with experimental results. Kim and colleagues’ work resembles a theoretical “detective story”: the scientists investigated a pool of plausible candidate structures and ruled out each structure that did not fulfil certain criteria that the true structure of metallic silane must possess. “This approach left us with a single structure by way of elimination, which we concluded to be the correct crystal structure,” explained Kim.

“Understanding metallic hydrogen-rich materials, such as silane, represents a stepping stone towards understanding metallic hydrogen itself,” he told physicsworld.com. The team, which also includes researchers from Henri Poincaré University in Vandoeuvre-lès-Nancy and the University of Rennes and University of Strasbourg, now plans to focus more specifically on superconductivity in silane.

IBEX blasts off to map edge of the solar system

NASA’s IBEX satellite blasted off today to begin its mission to map the outer reaches of the solar system. The spacecraft was launched from an aircraft high above the South Pacific and will soon settle into a very high altitude Earth orbit, where it will study how ions in the solar wind interact with the plasma from interstellar space.

Scientists hope that IBEX will tell us more about the shape of the solar system’s protective “bubble”, which is created by the solar wind and shields us from harmful galactic cosmic rays.

The solar wind is a stream of charged particles emitted from the Sun in all directions and forms the protective region around the Sun known as the heliosphere. Without this bubble, radiation from outer space would make manned space flight extremely dangerous.

The solar wind collides with plasma in the interstellar medium some 10–20bn km from the Sun at the so-called termination shock. Beyond this, in a region known as the heliosheath, the solar wind slows down dramatically, becomes heated and turbulent. The heliosheath extends for several billion kilometres further from the Sun and its end marks the edge of the solar system.

A ‘global view’ of the heliosphere

Over the past four years the Voyager 1 and Voyager 2 spacecraft have both crossed the termination shock and sent back spectacular data on this region. However, as IBEX principal investigator David McComas explains, these data tell us only about the specific points at which the spacecraft crossed the termination shock. “It’s like having two excellent weather stations that provide detailed reports of the weather in their areas, but not having the satellite data that tell you how the weather fronts are changing,” he says. “For this global view we need IBEX.”

A ‘low-cost’ mission

IBEX, being one of NASA’s small, low-cost Explorer missions, will be launched by a rocket released from the underside of an aircraft. The rocket will climb to about 200 km above the Earth’s surface, at which point an additional rocket will fire and send IBEX into a high-altitude orbit. Taking IBEX some 5/6 of the way to the Moon, this orbit will allow the satellite to spend much of its time outside of the Earth’s magnetosphere and therefore free from potential interference.

Once in position, IBEX will use two sensors to collect neutral hydrogen atoms that form when protons from the solar wind combine with electrons from interstellar neutral atoms drifting into the heliosphere, with some portion of these hydrogen atoms deflected back towards the Sun by their motion about the solar magnetic field lines. Because the satellite spins as it moves along its orbit, it will, over the course of six months, collect particles from every part of the sky. By measuring the direction from which the particles arrive, their time of arrival, their mass and their energy, McComas and colleagues will be able to build up a full-sky map showing the distribution of these particles and their energies.

How does the solar wind slow down?

Analysing this map will then allow the researchers to work out the interaction between the solar wind and the interstellar medium along the full length of the termination shock. For example, it will tell them whether the slowing down of the solar wind is fairly uniform in all directions or whether there are some places where it occurs abruptly and others where it takes place more slowly. It will also reveal the overall shape of the heliosphere, which might be affected by density fluctuations and field lines in the interstellar medium.

The mission has a nominal lifetime of two years but all of its components are designed to last for four and could last even longer, says McComas.

Did a Chinese calligrapher use 'fractal expression'?

Huai Su’s fractal calligraphy

By Hamish Johnston

In the scientific world, fractals were first identified in the mid-1970s by the mathematican Benoît Mandelbrot.

However, it’s possible that artists and artisans have long been using the fragmented shapes in their work.

In 1999, two Australian physicists famously showed that the “paint-drip” canvasses of Jackson Pollock could be dated by computing their fractal dimension — which tended to increase as Pollock matured as an artist.

Now, Yuelin Li of Argonne National Lab in the US has posted a paper on the arXiv preprint server claiming that calligraphy done by the “maniac Buddist monk” Huai Su more than 1200 years ago contains fractals. Li analysed a request for “bitter bamboo shoots and tea” written by the monk and found that it can be characterized by two different fractal dimensions.

Li believes that the fractal nature of some artworks “can be attributed to the artist’s pursuit of the hidden order of [the] fractal”.

LHC report confirms electrical fault

The cause of the huge magnet warm-up or “quench” at the Large Hadron Collider (LHC) a month ago that released masses of helium coolant into its tunnel was a faulty electrical connection, an official report has confirmed.

The report, which was completed by the CERN lab near Geneva on Wednesday and released publicly yesterday evening, reveals that the fault resulted in the loss of some six tonnes of liquid helium, broken anchors in the concrete floor and damaged “jumper” connections in the cryogenic distribution line. It also states that up to 29 magnets will have to be repaired.

But the report adds that there are enough spare components to allow the LHC to restart in 2009 at the full energy of 7 TeV after the accelerator’s standard winter maintenance period.

Robert Aymar, the director-general of CERN, said in a statement, “The incident was unforeseen, but I am now confident that we can make the necessary repairs, ensure that a similar incident cannot happen in the future and move forward to achieving our research objectives.”

We are taking this process slowly and carefully to ensure that we do not miss anything that could help in the final analysis Paul Collier, CERN

Resistive zone

The report describes in detail the sequence of events that lead to the 19 September quench, which was first revealed by physicsworld.com. It began soon after 11 a.m. CET (10 a.m. BST) when the operations team was commissioning the final eighth of the LHC, sector 3–4, to an energy of 5.5 TeV. To do this, the team had to ramp up the current in the circuits of the “dipole” magnets, which steer the machine’s proton beams. Unfortunately — and for reasons yet unknown — the higher current was met by resistance in an electrical link between a dipole magnet and a neighbouring “quadrupole”, or focusing, magnet.

This resistance put an unwanted load on the power supply. In response, the power supply switched off and triggered a batch of resistors in the circuit to curb the high current. It also triggered quenches in many nearby magnets — an automatic safety system that is in place to distribute excess energy over a wide area.

Meanwhile, an electrical arc sprung from the fault and punctured a hole in the magnets’ cooling enclosure, allowing liquid helium to escape into the outer, thermally insulating vacuum of the cryostat. Relief valves in the cryostat opened to let out the helium into the tunnel — initially about two tonnes, though over time another four tonnes. However, the pressure of the helium was so great that several cryostats shifted and broke their anchors in the concrete floor. It was this movement that was responsible for the damaged the cryogenic jumper connections, which exist every 107 m along the sector.

‘Safe powering in the future’

According to the report, engineers will have to remove at most five quadrupole and 24 dipole magnets for repair. But it adds that the engineers may be forced to lift even more magnets from the 100 m-deep tunnel to the surface for cleaning because of a “soot-like” dust that was sprinkled down the beam pipes.

CERN now plans to improve the pressure release devices and cryostat anchors, and add more measurements to the early-warning systems before the technicians attempt to feed in any more high currents.

The technical parameters of the LHC are beyond precedent, and the energy stored in the superconducting magnets huge Official CERN report

“The technical parameters of the LHC are beyond precedent, and the energy stored in the superconducting magnets huge,” the report concludes. “Consequently, operation of this machine will always comprise a certain technical risk. We are however convinced that the repair actions underway and the improved protection systems to be implemented will ensure safe powering in the future.”

Paul Collier, the head of the accelerator operations team, told physicsworld.com that those at CERN are still in the process of warming up the entire sector, though they have begun inspecting the interconnects between magnets to gather more information. “We are taking this process slowly and carefully to ensure that we do not miss anything that could help in the final analysis,” he says.

“We have not yet fully disconnected any magnets,” he continues. “This will happen in the next week or so. Once disconnected, the first magnets — initially [those in the] short straight sections — will be removed from the machine for further study and the repair process started.”

Aside from the collateral damage it caused, the problem on 19 September was a blow to CERN scientists who had only recently celebrated a highly successful “switch on” and who were just days from being able to collide proton beams at 5 TeV. While further analysis of the incident continues over the winter, the operations team will have to find ways to rekindle its morale.

• You can read the full copy of the CERN report here.

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