By Hamish Johnston
The particle physicist Nigel Lockyer will take over as director of Fermilab in September this year. Lockyer is currently in charge of TRIUMF in Vancouver, Canada. He will succeed Pier Oddone, who is stepping down after heading Fermilab for eight years.
Much-debated results suggesting the existence of a fourth kind of neutrino, described as sterile, are to be put to the test in a new experiment under Italy’s Gran Sasso mountain. The physicists who have devised the experiment say that by using an existing solar-neutrino detector they can carry out an inexpensive yet thorough search for the hypothetical sterile neutrino.
Neutrinos are chargeless, almost massless subatomic particles that interact with ordinary matter only via the weak nuclear force. As a result they can pass through vast amounts of material undisturbed. To study them, physicists build huge detectors – the idea being that a large number of target nuclei will result in a few neutrino collisions that can be detected.
If they exist, then sterile neutrinos would be even more difficult to detect because they probably would not interact with ordinary matter at all – only with other neutrinos. They would do so via “oscillation”, a well-established phenomenon in which ordinary neutrinos transform and re-transform continually from one of three flavours – electron, muon and tau – to another as they travel. Likewise, ordinary neutrinos would oscillate into sterile neutrinos and back again but probably over much shorter distances than those typical of normal neutrino oscillation.
The existence of sterile neutrinos is suggested by a number of lines of evidence. These include results from experiments studying the oscillation of ordinary neutrinos and from recent calculations showing that the numbers of neutrinos captured by detectors placed close to nuclear reactors are lower than expected, given all of the different ways that neutrinos can be produced inside those reactors.
If these hints turned out to be real, then the implications would be “enormous”, according Marco Pallavicini of the University of Genoa, in Italy, who points out that sterile neutrinos would be the first fundamental particles discovered to lie outside the Standard Model of particle physics. They may also have played a significant role in the evolution of the universe, he adds. Like many physicists, however, he remains sceptical of the particles’ existence.
Pallavicini is leading an international collaboration that will search for sterile neutrinos using the Borexino detector at Italy’s Gran Sasso National Laboratory, which is used mainly to measure neutrinos emitted by the Sun. The detector contains an array of photomultiplier tubes that record the light emitted when neutrinos interact with electrons inside a 300-tonne sphere of a hydrocarbon scintillator. The new experiment is known as Short Distance Neutrino Oscillations with BoreXino (SOX) and will intercept neutrinos from an intense radioactive source placed several metres away.
SOX will establish exactly where each of the source-induced neutrino interactions takes place within the hydrocarbon by recording the precise time that the associated light emission reaches several of the photomultiplier tubes. If sterile neutrinos exist, then the number of interactions taking place as a function of distance from the source would show a small but distinct quasi-sinusoidal variation, with a wavelength on the scale of metres – far too short to be caused by normal neutrino oscillation.
SOX will use one of two radioactive sources. One is chromium-51, which emits electron neutrinos and will be placed in a pit just below the detector; the other is cerium-144, an electron–antineutrino emitter that will be positioned inside Borexino’s water shield. The researchers would prefer to use cerium because it would allow the search for sterile neutrinos over a slightly wider range of masses and “mixing angles” (a parameter that determines the strength of oscillations) but they will probably use chromium because it has been used successfully in two previous experiments, albeit it at lower intensities.
Having recently secured the bulk of the funding needed for SOX from the European Research Council – €3.5m – the researchers’ next task is to find a supplier of the necessary radioactive material and then obtain the relevant licences to deliver that material to the lab. Pallavicini estimates that the experiment will start taking data in late 2015 and that the first results will appear in 2016. Further funding permitting, the collaboration will carry out a second – and possibly third – round of tests. The latter being the most challenging because it would put a cerium-144 source at the centre of the detector.
The researchers have calculated that even using chromium-51 SOX would almost certainly be sensitive enough to rule the existing reactor anomalies either in or out. However, the experiment will not see any sterile neutrinos if they happen to be particularly light or mix very weakly with standard neutrinos. Pallavicini also admits that other physicists would be sceptical if his collaboration were simply to record a lower-than-expected overall number of neutrinos, which could be caused by sterile oscillations but which might raise suspicions about poor intensity calibrations or low detector efficiency. However, he says that if the data reveal the telltale oscillatory signal, then he would “challenge anyone to come up with a different explanation” to that of sterile neutrinos.
Petr Vogel at the California Institute of Technology in the US, describes the experiment as “challenging, but doable” and believes that it would “give a very strong indication of whether sterile neutrinos exist or not” if the researchers can collect data using both the chromium and cerium.
Also enthusiastic is William Louis of the Los Alamos National Laboratory in New Mexico, US. “[SOX] will help test and resolve the present evidence for sterile neutrinos,” he says, “assuming that the proper radioactive sources are delivered.” However, he believes that the experiment on its own will not be able to completely prove or disprove the existence of sterile neutrinos, arguing that definitive proof will require the results of several experiments, recording different kinds of oscillation and spanning different energy scales.
SOX is described in a preprint on arXiv.
By Hamish Johnston
Earlier this week my colleague James Dacey was filming in Grenoble, which is home to two major facilities used by condensed-matter physicists: the European Synchrotron Radiation Facility (ESRF) and the Institut Laue-Langevin (ILL). His trip has inspired this week’s poll question, which pits coherent photons against thermal neutrons.
The ESRF and other synchrotron facilities produce coherent beams of synchrotron radiation. These have proven particularly useful to physicists studying soft and biological materials as well as the electronic, atomic and molecular structures of hard materials such as crystals. Although I don’t think that any Nobel prizes have been awarded for condensed-matter physics done at a synchrotron, the 2009 chemistry Nobel was shared by Venkatraman Ramakrishnan, who used synchrotron radiation to work out the structure of ribosomes.
By James Dacey
This photo looks a little bit like we were filming the moment that I got down on my knee and popped the big question to Andrew Harrison, the Director General of the Institut Laue-Langevin (ILL). While that would certainly make for an intriguing story worthy of a blog entry, the truth is that earlier this week we were interviewing Harrison for a short film about his international research facility. In case you are still wondering, the reason I am kneeling is so that we could frame our shot to include the dome that houses the ILL’s nuclear reactor, where neutrons are generated.
The first system that can produce atom–photon entangled pairs on demand has been built by physicists in the US. The set-up uses a 1D optical lattice of ultracold atoms to emit a single photon. However, unlike previous experiments, in which the quantum state of the atoms is destroyed immediately after photon emission, the atomic state endures and is entangled with the photon. Another important feature of the system is that it is deterministic, producing entangled states on demand. Together, these properties could be exploited to create networks for processing and distributing quantum information.
Entanglement is a purely quantum-mechanical phenomenon that allows two particles to have a much closer relationship than is allowed by classical physics. This relationship can be exploited in quantum-information systems and physicists are keen to create entangled systems that could be used in practical applications. In particular, researchers would like to entangle optical photons – which can carry quantum information over hundreds of kilometres – with stable stationary quantum bits (qubits) such as trapped atoms. The problem, however, is that it can be very difficult to get photons to interact with such stationary qubits in a reliable way.
Rydberg states in ensembles of ultracold atoms offer a way forward because these highly excited states interact very strongly with light. They are much like a Rydberg atom, in which one electron is promoted to a very high energy state. However, in this case the electron is not associated with one specific atom but rather with the entire ensemble. Thanks to an effect called the “Rydberg excitation blockade”, only one Rydberg state at a time can occur in the ensemble, which ensures that only a single photon is emitted or absorbed.
While physicists had previously been able to store and emit photons using Rydberg states, any entanglement did not endure in the atoms for long enough to be measured. Now, Alex Kuzmich and colleagues at the Georgia Institute of Technology have come up with a way to entangle photons and Rydberg states for relatively long periods of time.
The experiment begins by cooling a gas of rubidium-87 atoms to microkelvin temperatures. A laser field is then switched on to create a 1D optical lattice containing about 500 atoms in a line. This ensemble is then placed in a collective Rydberg state by zapping it with laser pulses. More pulses are then fired at the ensemble to produce an entangled state of a photon and a “spin-wave” state that remains in the atomic ensemble.
The phase of the emitted photon is then measured. To confirm that the photon is entangled with the atomic ensemble, more pulses are fired at the atoms, causing them to emit a second photon that is characteristic of the spin-wave state.
The phase of this second photon is measured and by repeating this process many times, the correlation between the phases of the two photons is determined. The team found that the correlation is greater than that allowed by classical physics and therefore the photon and atomic ensemble are entangled.
By varying the delay between the emissions of the two photons, the team was able to work out how long the entanglement endured in the atomic ensemble – which the researchers calculate to be a few microseconds. While this does not sound long, it is enough to allow the photon to travel a few hundred metres, which means that several such atomic ensembles could be connected together in a lab to create a simple circuit for processing quantum information.
An important feature of the system is that the entanglement is deterministic – at least in principle. This means that the process would create an entangled pair every time if it could be implemented using a perfect experimental set-up. This is different to many other schemes for creating entanglement, which are inherently probabilistic and cannot deliver an entangled state every time. Another important feature of the system is that it can generate as many as 5000 entangled photons per second – a thousand times more than previous systems using Rydberg states.
Together, these features suggest that several such sources could be connected to create a multinode network for processing quantum information. “Such atom–light entanglement might be useful for future work on the realization of distributed quantum systems, which may be applied to quantum communication, computation or similar tasks,” says Kuzmich.
Charles Adams of Durham University in the UK describes the work as “groundbreaking” and “a key advance towards the realization of a quantum network”.
Kuzmich and colleagues are now working on ways to improve the trap used to hold the atoms, which should increase the time for which the entanglement endures.
The experiment is described in Nature.
In less than 100 seconds, Nicola Bowler draws on her own experiences to describe the key to a successful physics class.
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Can you obtain the dimensions of a darkened room by clapping your hands and listening to the echoes? Bats, dolphins and some other animals navigate using echoes and some blind humans have trained themselves to do this. Now engineers in Switzerland and the US have worked out a way to calculate the dimensions of a room using a single loudspeaker and four arbitrarily placed microphones. They believe this could find applications in building design, audio forensics and much more.
In his famous 1966 paper entitled “Can one hear the shape of a drum?” the Polish mathematician Mark Kac asked whether or not a listener could uniquely identify the shape of a vibrating membrane after hearing its resonant frequencies. In 1992 the American mathematician Carolyn Gordon and colleagues showed that the answer is no. In theory, however, it should be possible to hear the shape of a room by producing a sound and measuring the time taken for the echoes to arrive at particular points. But doing this in practice is not easy.
Several methods to calculate a room’s geometry from its echoes have all had significant limitations. Some have worked only in 2D. Others need an array of microphones that are placed sufficiently close together that the sound only has time to make one round trip to the walls and back, which may not always be possible if the sound source is close to one of the walls.
Now researchers at the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland and Harvard University in the US have developed an algorithm that uses sound to work out the dimensions of any room with flat, protrusion-free walls. The system uses a single loudspeaker to create sound and four microphones placed anywhere in the room to capture the echoes.
The algorithm that was created by the team only deals with first-order echoes, which means they exclude echoes of echoes. Lead researcher Ivan Dokmanić explains that this makes the system more practicable, as higher-order echoes can be too faint to isolate from background noise. However, isolating first-order echoes is also a challenge because it is not possible to say from a single recorded sound how many walls it has bounced off before reaching the microphone. The algorithm begins, therefore, by looking at the sounds recorded by the four microphones together and working out which sounds come from the same wall. The researchers propose two mathematical methods for doing this. “Once we know which echoes have to be grouped together,” says Dokmanić, “then from there we can compute which signals are first order and which are second order.” Using the arrival time at each microphone of all the first-order echoes, the researchers calculate the size and shape of the room.
They tested their algorithm by calculating the dimensions of a lecture theatre at EPFL and comparing the results of their echo calculations with the actual values. They found the two were remarkably close: the 7.08 m distance between two walls, for example, was calculated as 7.01 m – a discrepancy of less than 1%. They decided to find out how well their algorithm would perform when the requirement that the walls be flat and protrusion-free was not satisfied by placing their set-up in the portal of Lausanne Cathedral, which has a domed ceiling and numerous protrusions such as pillars and large statues. Even here, their system calculated the distances between flat surfaces accurately.
The researchers foresee a variety of possible applications for their technology. Most obviously, it could be used by architects and sound engineers designing a building in which echoes are important, such as a concert hall, to ensure that the room has the desired acoustics. Another potential application that the researchers are currently investigating, says Dokmanić, is to measure the echoes that a sound produces within a building where the geometry is known and to use the information to work out where in that building the sound was emitted. This could be used in forensics, for example.
Fabio Antonacci of the Image and Sound Processing group at Milan Polytechnic, an expert on this subject who was not involved in the present research, says, “One of the trickiest technical points in the estimation of room geometry is the assignment of echoes to the wall that generated them. Dokmanić [and colleagues] propose two elegant and innovative techniques for this purpose. This is the core of the article and, I believe, the most promising feature for follow-up.”
The research is published in Proceedings of the National Academy of Sciences.
By Hamish Johnston
Venus is a breezy planet. Planetary scientists have known for some time that its clouds zip along at hundreds of kilometres per hour – speeds on par with Earth’s high-velocity jet stream.
But now a team of researchers looking at data from the European Space Agency (ESA) Venus Express mission have noticed that the winds appear to have accelerated by about 33% over the past six years.
In less than 100 seconds, Nicola Bowler explains how physicists can help to keep aircraft-structures healthy, safe and reliable.
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Physicists working independently at two different particle-physics labs have found tantalizing evidence for a new and mysterious hadron. Dubbed Zc(3900), the particle seems to be a “charged charmonium” and is made from quarks assembled in a way that has possibly never been seen before. Further studies of Zc(3900) could provide important new information about the strong force that glues together quarks in hadrons.
Charmonia, which are heavy mesons, contain a charm and anti-charm quark. Because they are a composite particle, they can exist in a number of different energy states – the most famous being the first excited state called the J/ψ particle. Discovered in 1974, the J/ψ particle made physicists realize for the first time that quarks are real. Although physicists have learned much about quarks over the past four decades, current theories are still not good enough to predict which of the many possible combinations of quarks will form stable mesons.
Zc(3900) was spotted independently by physicists on the BESIII experiment in Beijing and the Belle experiment in Tsukuba, Japan. Both teams focused on the mysterious Y(4260) particle, which was discovered in 2005 at the BaBar experiment in the US. Y(4260) is perhaps the most puzzling of the “XYZ” particles, which have been produced over the past decade at BaBar, Belle, BESIII and elsewhere.
Although they are believed to be combinations of quarks, the XYZ particles have so far defied explanation. At first glance Y(4260), which has a mass of about 4.260 GeV/c2, appears to be a charmonium meson. However, closer inspection suggests that its properties cannot be explained simply in terms of a charm and anti-charm quark bound together by the strong force.
One possible explanation is that Y(4260) is part of a new family of “hybrid charmonium” particles in which the gluons that mediate the strong force exist in excited states. Alternatively, Y(4260) could contain four quarks rather than just two (a tetraquark structure) – and could even resemble a “molecule” made of two mesons bound together.
To gain a better understanding of Y(4260), the BESIII and Belle teams therefore created large numbers of them by colliding electrons and positrons together. While the Y(4260) is so short-lived that it cannot be detected directly, its signature turns up in the energy spectrum of pions and J/ψ particles produced in the collision.
But both teams found more than they bargained for – evidence of an unexpected particle Zc(3900) with a mass around 3.9 GeV/c2. This new particle is even more mysterious than Y(4260) because it appears to decay to an electrically charged pion plus an electrically neutral J/ψ. This means that Zc(3900) must carry electric charge, therefore not simply comprising charm and anti-charm quarks.
One explanation for this behaviour is that the new charged particle is a molecule comprising two D mesons that are somehow bound together – something that is predicted by some models of how quarks interact. Another, more tantalizing possibility, is that Zc(3900) is a tetraquark comprising a charm/anti-charm pair plus an up quark and an anti-down quark. If the latter proves to be true, the number of possible hadrons allowed by nature could be much greater than physicists had thought – and by studying these new particles, important new insights into low-energy quark interactions could be gleaned.
“With [BESIII], we can accumulate a lot more data that will permit more comprehensive investigations of this unusual electrically charged charmonium state,” says Yifang Wang who is director of the Institute of High Energy Physics in Beijing and a member of the BESIII team. “When all of these results are used as inputs to theory, we may begin to open the door toward a fuller understanding of the XYZ particles discovered in recent years.”
The discoveries are described in two papers in Physical Review Letters (Phys. Rev. Lett. 110 252001 and Phys. Rev. Lett. 110 252002).
