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Posts Tagged ‘neutrinos’

Dark Matter: A New Hope

Monday, December 7th, 2015

[Apologies for the title, couldn’t resist the temptation to work in a bit of Star Wars hype]

To call the direct detection of dark matter “difficult” is a monumental understatement. To date, we have had no definite, direct detection on Earth of this elusive particle that we suspect should be all around us. This seems somewhat of a paradox when our best astronomical observations indicate that there’s about five times more dark matter in the universe than the ordinary, visible matter that appears to make up the world we see. So what’s the catch? Why is it so tricky to find?

An enhanced image of the “Bullet Cluster”: two colliding galaxies are observed with ordinary “baryonic” matter (coloured red) interacting as expected and the dark matter from each galaxy, inferred from gravitational lensing (coloured blue), passing straight through one another. Source: NASA Astronomy Picture of the Day 24/08/2006

The difficulty lies in the fact that dark matter does not interact with light (that is, electromagnetically) or noticeably with atoms as we know them (that is, with the strong force, which holds together atomic nuclei). In fact, the only reason we know it exists is because of how it interacts gravitationally. We see galaxies rotate much faster than they would without the presence of some unseen “dark matter”, amongst other things. Unfortunately, none of the particles we know from the Standard Model of particle physics are suitable candidates for explaining dark matter of this sort. There are, however, several attempts in the works to try and detect it via weak nuclear interactions on Earth and pin down its nature, such as the recently approved LUX-ZEPLIN experiment, which should be built and collecting data by 2020.

Direct detection, however, isn’t the only possible way physicists can get a handle on dark matter. In February 2014, an X-Ray signal at 3.5 keV was detected by the XMM-Newton, an X-ray spectroscopy project by the European Space Agency, in orbit around Earth. Ever since, there’s been buzz amongst particle cosmologists that the signal may be from some kind of dark matter annihilation process. One of the strongest candidates to explain the signal has been sterile neutrino, a hypothetical cousin of the Standard Model neutrino. Neutrinos are ghostly particles that also interact incredibly rarely with ordinary matter* but, thanks to the remarkable work of experimentalists, were detected in the late 1950s. Their exact nature was later probed by two famous experiments, SNO and Super-Kamiokande, that demonstrated that neutrinos do in fact have mass, by observing a phenomenon known as Neutrino Oscillations. As reported on this blog in October, the respective heads of each collaboration were awarded the 2015 Nobel Prize in Physics for their efforts in this field.

“Handedness” refers to how a particle spins about the axis it travels along. Standard Model neutrinos (first observed in 1956) are all observed as left handed. Sterile neutrinos, a hypothetical dark matter candidate, would be right-handed, causing them to spin the opposite way along their axes. Image source: ysfine.com

The hope amongst some physicists is that as well as the neutrinos that have been studied in detectors for the last half a century, there exists a sort of heavier “mirror image” to these particles that could act as a suitable dark matter candidate. Neutrinos are only found to “spin” in a certain way relative to the axis of their propagation, while the hypothesised sterile neutrinos would spin the opposite way round (in more technical terms, they have opposite chirality). This difference might seem trivial, but in the mathematical structure underpinning the Standard Model, it would fundamentally change how often these new particles interact with known particles. Although predicted to react incredibly rarely with ordinary matter, there are potentially processes that would allow these sterile neutrinos to emit an X-Ray signal, with half the mass-energy of the original particle. Due to the sheer number of them found in dense places such as the centres of galaxies, where XMM-Newton was collecting data from, in principle such a signal would be measurable from regions with a high density of sterile neutrinos.

This all seems well and good, but how well does the evidence measure up? Since the announcement of the signal, the literature has gone back and forth on the issue, with the viability of sterile neutrinos as a dark matter candidate being brought into question. It is thought that the gravitational presence of dark matter played a crucial role in the formation of galaxies in the early universe, and the best description we have relies on dark matter being “cold”, i.e. with a velocity dispersion such that the particles don’t whizz around at speeds too close to the speed of light, at which point their kinematic properties are difficult to reconcile with cosmological models. However, neutrinos are notorious for having masses so small they have yet to be directly measured and to explain the signal at 3.5 keV, the relevant sterile neutrino would have to have a relatively small mass of ~7 keV/c2, about 15,000 times lighter than the usual prediction for dark matter at ~100 GeV/c2. This means that under the energy predicted by cosmological models for dark matter production, our sterile neutrinos would have a sort of “luke-warm” characteristic, in which they move around at speeds comparable to but not approaching the speed of light.

A further setback has been that the nature of the signal has been called into question, since the resolution of the initial measurements from XMM-Newton (and accompanying X-ray satellite experiments such as Chandra) was not sharp enough to definitively determine the signal’s origin. XMM-Newton built up a profile of X-ray spectra by averaging across measurements from just 73 galaxy clusters, though it will take further measurements to fully rule out the possibility that the signal isn’t from the atomic spectra of potassium and sulpher ions found in hot cosmic plasmas.

But there remains hope.

A recent pre-print to the Monthly Notices of the Royal Astronomical Society (MNRAS) by several leading cosmologists has outlined the compatibility of a 7 keV/c2 sterile neutrino’s involvement with the development of galactic structure. To slow down the sterile neutrinos enough to bring them in line with cosmological observations, “lepton asymmetry” (a breaking of the symmetry between particles and antiparticles) has to be introduced in the model. While this may initially seem like extra theoretical baggage, since lepton asymmetry has yet to be observed, there are theoretical frameworks than can introduce such an asymmetry with the introduction of two much heavier sterile neutrinos at the GeV scale.

A Black Brant XII sounding rocket, similar to the type that could be used to carry microcalorimeters, capable of recording X-ray signals of the type XMM-Newton and Chandra have been observing in galactic nuclei. These rockets are used to conduct scientific experiments in sub-orbital flight, including attempts at dark matter detection. Source: NASA/Wallops

Under such a model, not only could our dark matter candidate be reconciled, but neutrino oscillations could also be explained. Finally, baryogenesis, the description of why there was slightly more matter than antimatter in the early universe, could also find an explanation in such a theory. This would resolve one of the largest puzzles in Physics; the Standard Model predicts nearly equivalent amounts of particles and antiparticles in the early universe which should have annihilated to leave nothing but radiation, rather than the rich and exciting universe we inhabit today. On the experimental side, there are a few proposed experiments to try and measure the X-ray signal more carefully to determine its shape and compare it with the prediction of such models, such as flying rockets around with calorimeters inside to try and pick up the signal by observing a broader section of the sky than XMM or Chandra did.

With the experts’ opinions divided and further research yet to be done, it would be facetious to end this article with any sort of comment on whether the signal can or will gather the support of the community and become verified as a full blown dark matter signal. At time of writing, a paper has been released claiming signal is better explained as an emission from the plasmas found in galactic nuclei. A further preprint to MNRAS, put on arXiv just days ago, claims the sterile neutrino hypothesis is incompatible with the signal but that axions (a dark matter model that supposes a totally different type of particle outside of the Standard Model) remain as a candidate to explain the signal. Perhaps sterile neutrinos, are not the particles we’re looking for.

This kind of endeavour is just one of the hundreds of ways particle physicists and our colleagues in Astrophysics are looking to find evidence of new, fundamental physics. The appeal for me, as someone whose work will probably only have relevance to huge, Earth-bound experiments like the Large Hadron Collider, is the crossover between modelling the birth of colossal objects like galaxies and theories of subatomic particle production, using comparison between the two for consistency. Regardless of whether future rocket-based experiments can gather enough data to fully validate the signal in terms of theories produced by physicists here on Earth, it is a perfect example of breadth of activity physicists are engaged in, attempting to answer the big questions such as the nature of dark matter, through our research.

Kind regards to Piotr Oleśkiewicz (Durham University) for bringing this topic to my attention and for his insights on cosmology, and to Luke Batten (University College London) for a few corrections.

*The oft-quoted fact about neutrinos is that 65 billion solar neutrinos pass through just one square centimetre of area on earth every single second. The vast majority of these neutrinos will whizz straight through you without ever having noticed you were there, but by chance, in your entire lifetime, it’s likely that at least one or two will have the courtesy to notice you and bump off one of your atoms. The other interesting fact is that due to the decay of potassium in your bones, you actually emit about three hundred neutrinos a second.

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A measurement to watch

Monday, October 12th, 2015

This article appeared in symmetry on Oct. 7, 2015.

Finding a small discrepancy in measurements of the properties of neutrinos could show us how they fit into the bigger picture.

Finding a small discrepancy in measurements of the properties of neutrinos could show us how they fit into the bigger picture.

Physics, perhaps more so than any other science, relies on measuring the same thing in multiple ways. Different experiments let scientists narrow in on right answers that satisfy all parties—a scientific system of checks and balances.

That’s why it’s exciting when a difference, even a minute one, appears. It can teach physicists something about their current model – or physics that extends beyond it. It’s possible that just such a discrepancy exists between a certain measurement of neutrinos coming out of accelerator experiments and reactor-based experiments.

Neutrinos are minuscule, neutral particles that don’t interact with much of anything. They can happily pass through a light-year of lead without a peep. Trillions pass through you every second. In fact, they are the most abundant massive particle in the universe—and something scientists are, naturally, quite keen to understand.

The ghostly particles come in three flavors: electron, muon and tau. They transition between these three flavors as they travel. This means that a muon neutrino leaving an accelerator at Fermi National Accelerator Laboratory in Illinois can show up as an electron neutrino in an underground detector in South Dakota.

Not complicated enough for you? These neutrino flavors are made of mixtures of three different “mass states” of neutrinos, masses 1, 2 and 3.

At the end of the day, neutrinos are weird. They hang out in the quantum realm, a land of probabilities and mixing matrices and other shenanigans. But here’s what you should know. There are lots of different things we can measure about neutrinos—and one of them is a parameter called theta13 (pronounced theta one three). Theta13 relates deeply to how neutrinos mix together, and it’s here that scientists have seen the faintest hint of disagreement from different experiments.

Accelerators vs. reactors

There are lots of different ways to learn about neutrinos and things like theta13. Two of the most popular involve particle accelerators and nuclear reactors.

The best measurements of theta13 come from nuclear reactor experiments such as Double Chooz, RENO and Daya Bay Reactor Neutrino Experiment based in China (which released the best measurement to date a few weeks ago).

Detectors located near nuclear reactors provide such wonderful readings of theta13 because reactors produce an extremely pure fountain of electron antineutrinos, and theta13 is closely tied to how electron neutrinos mix. Researchers can calculate theta13 based on the number of electron antineutrinos that disappear as they travel from a near detector to the far detector, transforming into other types.

Accelerators, on the other hand, typically start with a beam of muon neutrinos. And while that beam is fairly pure, it can have a bit of contamination in the form of electron neutrinos. Far detectors can look for both muon neutrinos that have disappeared and electron neutrinos that have appeared, but that variety comes with a price.

“Both the power and the curse of long-baseline neutrino oscillation is that it’s sensitive to all of neutrino oscillation, not just theta13,” says Dan Dwyer, a scientist at Lawrence Berkeley National Laboratory and researcher on Daya Bay.

With that in mind, we come to the source of the disagreement. The results coming out of accelerator-based experiments, such as the United States-based NOvA and Japan-based T2K, see just a few more electron neutrinos than researchers would predict based on what the reactor experiments are saying.

“The theta13 value that fits the beam experiments, that really describes how much electron neutrino you get, is somewhat larger than what Daya Bay, RENO and Double Chooz measure,” says Kate Scholberg, professor of physics at Duke University and researcher on T2K. “So there’s a little bit of tension.”

Many grains of salt

Data coming out of the accelerator experiments is still very young compared to the strong readings from reactor experiments, and it is complicated by the nature of the beam. No one is jumping on the discrepancy yet because it can be explained in different ways. Most importantly, the accelerator experiments just don’t have enough information.

“We have to wait for T2K and NOvA to get sufficient statistics, and that’s going to take a while,” says Stephen Parke, head of the Theoretical Physics Department at Fermilab. Parke, Scholberg and Dwyer all estimated that about five more years of data collection will be required before researchers are able to start saying anything substantial.

“There’s been a lot of pressure on Daya Bay to try to eke out as precise a measurement as we possibly can,” Dwyer says. “Every bit of increased precision we provide further improves the ability of NOvA and T2K and eventually [proposed neutrino experiment] DUNE to measure the other parameters.”

Finding meaning in neutrinos

If the accelerator experiments gather more data and if a clear discrepancy emerges—a big if—what does it mean?

Turns out there are lots of reasons to love theta13. It’s one of the fundamental parameters that can define our universe. From a practical standpoint, it helps design future experiments to better understand neutrinos. And it could help physicists learn something new.

“We don’t expect things not to agree, but we kind of hope that they won’t,” says André de Gouvêa, professor of physics at Northwestern University. “It means that we’re missing something.”

That something could be CP violation, evidence that neutrinos and antineutrinos behave differently. CP violation has never been seen in neutrinos before, but if researchers observed it with accelerator experiments, it could help explain why our universe is made of matter rather than equal parts of matter and antimatter.

Figuring out if CP violation is occurring means nailing down all of the different neutrino mixing parameters, which in turn means building more powerful, next-generation experiments such as Hyper-K in Japan, JUNO in China and the Deep Underground Neutrino Experiment in the United States. DUNE will build on oscillation experiments like NOvA but will be able to better separate background noise from neutrino events, see a broader energy spectrum of neutrinos and find other neutrino characteristics.

DUNE, which will be built in a repurposed gold mine in South Dakota and detect neutrinos passed 800 miles through the Earth from Fermilab in Illinois, will be one of the best ways to see CP violation and rely on expertise gained from smaller neutrino experiments.

“Developing these types of experiments is very complicated,” de Gouvêa says. One of the major challenges of physics experiments is making sure you are measuring what you think you are measuring. “That’s part of the reason why we have a significant number of neutrino oscillation experiments.”

Ultimately, the neutrino puzzle is still missing many pieces. A variety of experiments are ramping up to fill in the gaps, making it an exciting time to be a neutrino physicist.

“We have to untangle the mysteries of the neutrino, and it’s not easy,” Parke says. “The neutrino doesn’t give up her secrets very easily.”

Lauren Biron

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Neutrinos have mass but are they their own antimatter partner?

The fortunate thing about international flights in and out of the US is that, likely, it is long enough for me to slip in a quick post. Today’s article is about the search for Majorana neutrinos.

mex_airport

Mexico City Airport. Credit: R. Ruiz

Neutrinos are a class of elementary particles that do not carry a color charge or electric charge, meaning that they do not interact with the strong nuclear force or electromagnetism. Though they are known to possess mass, their masses are so small experimentalists have not yet measured them. We are certain that they have mass because of neutrino oscillation data.

Words. Credit: Particle Zoo

Neutrinos in their mass eigenstates, which are a combination of their flavor (orange, yellow, red) eigenstates. Credit: Particle Zoo

This history of neutrinos is rich. They were first proposed as a solution to the mystery of nuclear beta (β)-decay, a type of radioactive decay. Radioactive decay is the spontaneous and random disintegration of an unstable nucleus in an atom into two or more longer-lived, or more stable, nuclei. A free neutron (which is made up of two down-type quarks, one up-type quark, and lots of gluons holding everything together) is unstable and will eventually undergo radioactive decay. Its half-life is about 15 minutes, meaning that given a pile of free neutrons, roughly half will decay by the end of those 15 minutes. A neutron in a bound system, for example in a nucleus, is much more stable. When a neutron decays, a down quark will become an up-type quark by radiating a (virtual) W- boson. Two up-type quarks and a down-type quark are what make a proton, so when a neutron decays, it turns into a proton and a (virtual) W- boson. Due to conservation of energy, the boson is very restricted into what it can decay; the only choice is an electron and an antineutrino (the antiparticle partner of a neutrino). The image below represents how neutrons decay.

Since neutrinos are so light, and interact very weakly with other matter, when neutron decay was first observed, only the outgoing electron and proton (trapped inside of a nucleus) were ever observed. As electrons were historically called β-rays (β as in the Greek letter beta), this type of process is known as nuclear beta-decay (or β-decay). Observing only the outgoing electron and transmuted atom but not the neutrino caused much confusion at first. The process

Nucleus A → Nucleus B + electron

predicts, by conservation of energy and linear momentum, that the electron carries the same fixed amount of energy in each and every decay. However, outgoing electrons in β-decay do not always have the same energy: very often they come out with little energy, but other times they come out with a lot of energy. The plot below is an example distribution of how often (vertical axis) an electron in β-decay will be emitted carrying away a particular amount of energy (horizontal axis).

Electron spectrum in beta decay: Number of electrons/beta-particles (vertical axis) versus energy/kinetic energy (KE) or electrons (horizontal axis). Credit: R. Church

Scientists at the time, including Wolfgang Pauli, noted that the distribution was similar to the decay process where a nucleus decays into three particles instead of two:

Nucleus A → Nucleus B + electron + a third particle.

Furthermore, if the third particle had no mass, or at least an immeasurably small mass, then the energy spectrum of nuclear β-decay could be explained. This mysterious third particle is what we now call the neutrino.

One reason for neutrinos being so interesting is that they are chargeless. This is partially why neutrinos interact very weakly with other matter. However, since they carry no charge, they are actually nearly indistinguishable from their antiparticle partners. Antiparticles carry equal but opposite charges of their partners. For example: Antielectrons (or positrons) carry a +1 electric charge whereas the electron carries a -1 electric charge. Antiprotons carry a -1 electric charge were as protons carry a +1 electric charge. Etc. Neutrinos carry zero charge, so the charges of antineutrinos are still zero. Neutrinos and antineutrinos may in fact differ thanks to some charge that they both possess, but this has not been verified experimentally. Hence, it is possible that neutrinos and antineutrinos are actually the same particle. Such particles are called Majorana particles, named after the physicist Ettore Majorana, who first studied the possibility of neutrinos being their own antiparticles.

The Majorana nature of neutrinos is an open question in particle physics. We do not yet know the answer, but this possibility is actively being studied. One consequence of light Majorana neutrinos is the phenomenon called neutrinoless double β-decay (or 0νββ-decay). In the same spirit as nuclear β-decay (discussed above), double β-decay is when two β-decays occur simultaneously, releasing two electrons and two antineutrinos. Double β-decay proceeds through the following diagram (left):

Double beta decay (L) and neutrinoless double beta decay (R). Credit: CANDLES experiment

Neutrinoless double β-decay is a special process that can only occur if neutrinos are Majorana. In this case, neutrinos and antineutrinos are the same and we can connect the two outgoing neutrino lines in the double β-decay diagram, as shown above. In 0νββ-decay, a neutrino/antineutrino is exchanged between the two decaying neutrons instead of escaping like the electrons.

Having only four particles in the final state for 0νββ-decay (two protons and two electrons) instead of six in double β-decay (two protons, two electrons, and two neutrinos) has an important effect on the kinematics, or motion, of the electrons, i.e., the energy and momentum distributions. In double β-decay:

Nucleus A → Nucleus B + electron + electrons + neutrino + neutrino

the two protons are so heavy compared to the energy released by the decaying neutrons that there is hardly any energy to give them a kick. So for the most part, the protons remain at rest. The neutrinos and electrons then shoot off in various directions and various energies. In neutrinoless double β-decay:

Nucleus A → Nucleus B + electron + electrons

since the remnant nucleus are still roughly at rest, the electron pair take away all the remaining energy allowed by energy conservation. There are no neutrinos to take energy away from the electrons and broaden their distribution. This difference between ββ-decay and 0νββ-decay is stark, particularly in the likelihood of how often (vertical axis) the electrons in β-decay will be emitted carrying away a particular amount of energy (horizontal axis). As seen below, the electron energy distribution in double β-decay is very wide and is centered around smaller energies, whereas the 0νββ-decay is very narrow and is peaked at the maximum of the 2νββ-decay curve.

For double beta decay (blue) and neutrinoless double beta decay (red peak), the electron spectrum in beta decay: Number of electrons/beta-particles (vertical axis) versus energy/kinetic energy (KE) or electrons (horizontal axis). Credit: COBRA experiment

Unfortunately, searches for 0νββ-decay have not yielded any evidence for Majorana neutrinos. This could be because neutrinos are not their own antiparticle, in which case we will never observe the decay. Alternatively, it could be the case that current experiments are simply not yet sensitive to how rarely 0νββ-decay occurs. The rate at which the decay occurs is proportional to the mass of the intermediate neutrino: a zero neutrino mass implies a zero 0νββ-decay rate.

Experiments such as KATRIN hope to measure the mass of neutrinos in the next coming years. If a mass measurement is obtained, it would be a very impressive and impacting result. Furthermore, definitive predictions for 0νββ-decay can be made, at which point the current generation of experiments, such as MAJORANA, COURE, and EXO will be in a mad dash for testing whether or not neutrinos are indeed their own antiparticle.

cuore_cryostat_01

Lower view of CUORE Cryostat. Credit: CUORE Experiment

Credit:

Inside view of CUORE Cryostat. Credit: CUORE Experiment

Happy Hunting and Happy Colliding,

Richard Ruiz (@BraveLittleMuon)

PS Much gratitude to Yury Malyshkin,  Susanne Mertens, Gastón Moreno, and Martti Nirkko for discussions and inspiration for this post. Cheers!

Update 2015 September 25: Photos of the Cryogenic Underground Observatory for Rare Events (CUORE) experiment have been added. Much appreciate to QD-er Laura Gladstone.

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MicroBooNE sees first cosmic muons

Wednesday, August 12th, 2015

This article appeared in Fermilab Today on Aug. 12, 2015.

This image shows the first cosmic ray event recorded in the MicroBooNE TPC on Aug. 6. Image: MicroBooNE

This image shows the first cosmic ray event recorded in the MicroBooNE TPC on Aug. 6. Image: MicroBooNE

A school bus-sized detector packed with 170 tons of liquid argon has seen its first particle footprints.

On Aug. 6, MicroBooNE, a liquid-argon time projection chamber, or LArTPC, recorded images of the tracks of cosmic muons, particles that shower down on Earth when cosmic rays collide with nuclei in our atmosphere.

“This is the first detector of this size and scale we’ve ever launched in the U.S. for use in a neutrino beam, so it’s a very important milestone for the future of neutrino physics,” said Sam Zeller, co-spokesperson for the MicroBooNE collaboration.

Picking up cosmic muons is just one brief stop during MicroBooNE’s expedition into particle physics. The centerpiece of the three detectors planned for Fermilab’s Short-Baseline Neutrino program, or SBN, MicroBooNE will pursue the much more elusive neutrino, taking data about this weakly interacting particle for about three years. When beam starts up in October, it will travel 470 meters and then traverse the liquid argon in MicroBooNE, where neutrino interactions will result in tracks that the detector can convert into precise three-dimensional images. Scientists will use these images to investigate anomalies seen in an earlier experiment called MiniBooNE, with the aim to determine whether the excess of low-energy events that MiniBooNE saw was due to a new source of background photons or if there could be additional types of neutrinos beyond the three established flavors.

One of MicroBooNE’s goals is to measure how often a neutrino that interacts with an argon atom will produce certain types of particles. A second goal is to conduct R&D for future large-scale LArTPCs. MicroBooNE will carry signals up to two and a half meters across the detector, the longest drift ever for a LArTPC in a neutrino beam. This requires a very high voltage and very pure liquid argon. It is also the first time a detector will operate with its electronics submerged in liquid argon on such a large scale. All of these characteristics will be important for future experiments such as the Deep Underground Neutrino Experiment, or DUNE, which plans to use similar technology to probe neutrinos.

“The entire particle physics community worldwide has identified neutrino physics as one of the key lines of research that could help us understand better how to go beyond what we know now,” said Matt Toups, run coordinator and co-commissioner for MicroBooNE with Fermilab Scientist Bruce Baller. “Those questions that are driving the field, we hope to answer with a very large LArTPC detector.”

Another benefit of the experiment, Zeller said, is training the next generation of LArTPC experts for future programs and experiments. MicroBooNE is a collaborative effort of 25 institutions, with 55 students and postdocs working tirelessly to perfect the technology. Collaborators are keeping their eyes on the road toward the future of neutrino physics and liquid-argon technology.

“It’s been a long haul,” said Bonnie Fleming, MicroBooNE co-spokesperson. “Eight and a half years ago liquid argon was a total underdog. I used to joke that no one would sit next to me at the lunch table. And it’s a world of difference now. The field has chosen liquid argon as its future technology, and all eyes are on us to see if our detector will work.”

Ali Sundermier

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This article appeared in Fermilab Today on Aug. 10, 2015.

The Fermilab Short-Baseline Neutrino program will use three detectors: SBND, MicroBooNE (shown here) and ICARUS. Photo: Reidar Hahn

The Fermilab Short-Baseline Neutrino program will use three detectors: SBND, MicroBooNE (shown here) and ICARUS. Photo: Reidar Hahn

In 1995, physicists working on the Liquid Scintillator Neutrino Detector, or LSND, at Los Alamos National Laboratory stumbled upon some curious results.

The experiment, whose goal was to investigate oscillations between the three different flavors of the elusive neutrino, saw evidence that there might be at least one additional flavor of neutrino lurking just out of reach. In 2002, an experiment at Fermilab called MiniBooNE started collecting data to explore this anomaly, but the results were inconclusive: some data seemed to refute the possibility of a fourth neutrino, but other data seemed to indicate particle interactions that couldn’t be explained with conventional three-neutrino models. The possibility of a mysterious, fourth neutrino remained alive.

“It’s a question that’s been first lingering with the anomalies from LSND and then MiniBooNE,” said Bonnie Fleming, co-spokesperson of a new neutrino experiment at Fermilab called MicroBooNE. “There’s now a worldwide campaign to address whether these short-baseline oscillations and hints from other experiments are indicating new physics.”

Scientists from Fermilab and more than 45 institutions around the world have teamed up to design a program to catch this hypothetical neutrino in the act. The program, called the Short-Baseline Neutrino (SBN) program, makes use of a trio of detectors positioned along one of Fermilab’s neutrino beams. Although there are other reactor and source-based experiments in the world that actively seek a fourth neutrino, also called a sterile neutrino, SBN is the only program that uses a particle accelerator to produce neutrinos and multiple neutrino detectors for this search.

“No one else is doing an experiment like this,” said Peter Wilson, coordinator for the SBN program. “There are no other experiments on this energy scale using the concept of a near detector and a far detector.”

Determining whether there are more than three neutrino flavors would affect how scientists interpret data from experiments like the planned Deep Underground Neutrino Experiment, which is expected to make transformative discoveries about neutrinos, and perhaps other aspects of the universe, in the future. Solving the mystery of the anomalies seen at LSND and MiniBooNE, however, will not be easy. Because the sterile neutrino would not interact through the weak nuclear force as the other three do (hence the name “sterile”), detecting this particle would be like chasing the shadow of a ghost.

It begins at the Fermilab Booster, where protons are accelerated to 8 GeV and smashed into a target, creating new particles. Charged particles are bent forward by a magnetic focusing device into a tunnel where most decay to produce muon neutrinos. The three detectors — named the Short-Baseline Near Detector, or SBND, MicroBooNE and ICARUS — will be spread out over a distance of 600 meters. SBND, 100 meters from the target, will take data close to the source to reduce systematic uncertainties by measuring the initial characteristics of the muon neutrino beam. Four hundred meters beyond the planned site for SBND is MicroBooNE, which is already installed. ICARUS will be located 110 meters past MicroBooNE. ICARUS is an existing detector from a previous experiment at the Italian INFN laboratory at Gran Sasso that is currently being refurbished at CERN. It will have a massive chamber holding 760 tons of liquid argon to beat down statistical uncertainties in the experiment.

All three of the detectors are time projection chambers, a type of detector that allows physicists to analyze particle collisions in three dimensions. For these particular TPCs, scientists use liquid argon because its relatively heavy mass ensures a higher rate of interactions.

MicroBooNE received its last fill of liquid argon in July and recently began taking data. Scientists are expecting to break ground on buildings for both ICARUS and SBND by this fall. In 2017, ICARUS will be fully refurbished and delivered to Fermilab. Scientists hope to complete building SBND that same year.

Since experimenters won’t be able to directly detect the sterile neutrino, they will search for clues in the trails of particles the three known neutrino flavors leave behind in the liquid argon after they interact. If the experiments, expected to begin running in 2018, see deviations in the expected neutrino oscillation pattern, scientists will know that they’re on the right track in their hunt for this fugitive particle. If not, they will be able to put the mystery of the sterile neutrino to rest.

“If we design a strong enough experiment, which I believe we have, then one of two things will happen when we start taking data,” said David Schmitz, co-spokesperson for SBND. “Either we will rule out the earlier hints, or we make, frankly, the most exciting discovery in particle physics in some time.”

Ali Sundermier

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This article appeared in Fermilab Today on May 5, 2015.

Technicians John Cornele, Pat Healey and Skyler Sherwin have been crucial in preparing the LArIAT detector for beam. The liquid-argon-filled detector saw first beam on Thursday. Photo: Jen Raaf

Technicians John Cornele, Pat Healey and Skyler Sherwin have been crucial in preparing the LArIAT detector for beam. The liquid-argon-filled detector saw first beam on Thursday. Photo: Jen Raaf

Fermilab’s Test Beam Facility (FTBF) now runs a second beamline to provide particles for R&D experiments. The MCenter beamline came back to life last year after an eight-year slumber to join the facility’s other beamline, MTest.

On Thursday, April 30, accelerator operators began using the revived beamline to send particles to its first major experiment, Liquid Argon TPC in a Test Beam (LArIAT), which will help advance particle detector technologies for neutrino experiments.

The FTBF provides experiments with different types of particle beams with a range of energies. Its main purpose is the research and development of particle detectors. It is one of only two sites in the world that provides this service with high-energy hadrons, which are particles made of quarks. Since 2005, the FTBF, with its distinctive orange and blue corrugated-steel roof, has staged more than 50 experiments, conducted by scientists from more than 170 institutions in 30 countries.

“We’re very busy and fully subscribed,” said JJ Schmidt, deputy facility manager at FTBF. “The existence of two beams allows us to serve a broader class of experiments.”

Not only does the new beamline allow FTBF to serve a larger number of users, it also provides room for a greater diversity of experiments. While MTest is aimed at experiments with a turnover of about one to four weeks, MCenter caters to more long-term experiments like LArIAT that will last for months, or even years.

Beautiful tracks at first try
LArIAT is a liquid-argon time projection chamber. Charged particles traveling through the sea of liquid argon ionize the argon atoms, and an electric field causes liberated electrons to drift toward the detector readout. Different particles cause different amounts of ionization, allowing researchers to distinguish between particles such as pions, kaons and protons.

This plot shows LArIAT's first tracks: two views of a charged particle interacting inside the LArIAT detector, which is filled with liquid argon.

This plot shows LArIAT’s first tracks: two views of a charged particle interacting inside the LArIAT detector, which is filled with liquid argon.

The first spill of particles delivered to LArIAT led to immediate success. The detector recorded picture-perfect tracks of charged particles.

Like the test beam, LArIAT will act as a research and development vehicle for future projects. Because neutrinos can be studied only through the particles produced when they interact with material inside a particle detector, being able to reliably characterize these other particles is of great importance.

“This is going to be fantastic not only for LArIAT but all the neutrino experiments that will use its results,” said Jen Raaf, co-spokesperson for LArIAT.

LArIAT will run the test beam for 24 hours a day while experimenters take data. The first run will last about three months, after which the detector’s cryogenic system will undergo upgrades to prepare for longer follow-up runs.

“It’s great that we have a facility where a small experiment can take beam over a long term,” said Brian Rebel, a scientist involved in LArIAT.

About 75 people from 22 institutions from the United States, Europe and Japan work on this experiment.

“Most are young postdocs and Ph.D. students that are enthusiastically doing a great job,” said Flavio Cavanna, LArIAT co-spokesperson.

“It’s an exciting combination of many years of work by the Accelerator, Particle Physics, Neutrino and Scientific Computing divisions to have the capability to do research that is important for making this the premier neutrino laboratory in the world,” Schmidt said.

Diana Kwon

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This article appeared in symmetry on April 22, 2015.

The world’s largest liquid-argon neutrino detector will help with the search for sterile neutrinos at Fermilab. Photo: INFN

The world’s largest liquid-argon neutrino detector will help with the search for sterile neutrinos at Fermilab. Photo: INFN

Mysterious particles called neutrinos seem to come in three varieties. However, peculiar findings in experiments over the past two decades make scientists wonder if a fourth is lurking just out of sight.

To help solve this mystery, a group of scientists spearheaded by Nobel laureate Carlo Rubbia plans to bring ICARUS, the world’s largest liquid-argon neutrino detector, across the Atlantic Ocean to the United States. The detector is currently being refurbished at CERN, where it is the first beneficiary of a new test facility for neutrino detectors.

Neutrinos are some of the most abundant and yet also most mysterious particles in the universe. They have tiny masses, but no one is sure why—or where those masses come from. They interact so rarely that they can pass through the entire Earth as if it weren’t there. They oscillate from one type to another, so that even if you start out with one kind of neutrino, it might change to another kind by the time you detect it.

Many theories in particle physics predict the existence of a sterile neutrino, which would behave differently from the three known types of neutrino.

“Finding a fourth type of neutrinos would change the whole picture we’re trying to address with current and future experiments,” says Peter Wilson, a scientist at Fermi National Accelerator Laboratory.

The Program Advisory Committee at Fermilab recently endorsed a plan, managed by Wilson, to place a suite of three detectors in a neutrino beam at the laboratory to study neutrinos—and determine whether sterile neutrinos exist.

Over the last 20 years, experiments have seen clues pointing to the possible existence of sterile neutrinos. Their influence may have caused two different types of unexpected neutrino behavior seen at the Liquid Scintillator Neutrino Detector experiment at Los Alamos National Laboratory in New Mexico and the MiniBooNE experiment at Fermilab.

Both experiments saw indications that a surprisingly large number of neutrinos may be morphing from one kind to another a short distance from a neutrino source. The existence of a fourth type of neutrino could encourage this fast transition.

The new three-detector formation at Fermilab could provide the answer to this mystery.

In the suite of experiments, a 260-ton detector called Short Baseline Neutrino Detector will sit closest to the source of the beam, so close that it will be able to detect the neutrinos before they’ve had a chance to change from one type into another. This will give scientists a baseline to compare with results from the other two detectors. SBND is under construction by a team of scientists and engineers from universities in the United Kingdom, the United States and Switzerland, working with several national laboratories in Europe and the US.

The SBND detector will be filled with liquid argon, which gives off flashes of light when other particles pass through it.

“Liquid argon is an extremely exciting technology to make precision measurements with neutrinos,” says University of Manchester physicist Stefan Soldner-Rembold, who leads the UK project building a large section of the detector. “It’s the technology we’ll be using for the next 20 to 30 years of neutrino research.”

Farther from the beam will be the existing 170-ton MicroBooNE detector, which is complete and will begin operation at Fermilab this year. The MicroBooNE detector was designed to find out whether the excess of particles seen by MiniBooNE was caused by a new type of neutrino or a new type of background. Identifying either would have major implications for future neutrino experiments.

Finally, farthest from the beam would be a liquid-argon detector more than four times the size of MicroBooNE. The 760-ton detector was used in the ICARUS experiment, which studied neutrino oscillations at Gran Sasso Laboratory in Italy using a beam of neutrinos produced at CERN from 2010 to 2014.

Its original beam at CERN is not optimized for the next stage of the sterile neutrino search. “The Fermilab beamline is the only game in town for this type of experiment,” says physicist Steve Brice, deputy head of Fermilab’s Neutrino Division.

And the ICARUS detector “is the best detector in the world to detect this kind of particle,” says Alberto Scaramelli, the former technical director of Gran Sasso National Laboratory. “We should use it.”

Rubbia, who initiated construction of ICARUS and leads the ICARUS collaboration, proposed bringing the detector to Fermilab in August 2013. Since then, the ICARUS, MicroBooNE and SBND groups have banded together to create the current proposal. The updated plan received approval from the Fermilab Program Advisory Committee in February.

“The end product was really great because it went through the full scrutiny of three different collaborations,” says MicroBooNE co-leader Sam Zeller. “The detectors all have complementary strengths.”

In December, scientists shipped the ICARUS detector from the Gran Sasso laboratory to CERN, where it is currently undergoing upgrades. The three-detector short-baseline neutrino program at Fermilab is scheduled to begin operation in 2018.

Kathryn Jepsen

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The dawn of DUNE

Wednesday, March 25th, 2015

This article appeared in symmetry on March 25, 2015.

A powerful planned neutrino experiment gains new members, new leaders and a new name. Image: Fermilab

A powerful planned neutrino experiment gains new members, new leaders and a new name. Image: Fermilab

The neutrino experiment formerly known as LBNE has transformed. Since January, its collaboration has gained about 50 new member institutions, elected two new spokespersons and chosen a new name: Deep Underground Neutrino Experiment, or DUNE.

The proposed experiment will be the most powerful tool in the world for studying hard-to-catch particles called neutrinos. It will span 800 miles. It will start with a near detector and an intense beam of neutrinos produced at Fermi National Accelerator Laboratory in Illinois. It will end with a 10-kiloton far detector located underground in a laboratory at the Sanford Underground Research Facility in South Dakota. The distance between the two detectors will allow scientists to study how neutrinos change as they zip at close to the speed of light straight through the Earth.

“This will be the flagship experiment for particle physics hosted in the US,” says Jim Siegrist, associate director of high-energy physics for the US Department of Energy’s Office of Science. “It’s an exciting time for neutrino science and particle physics generally.”

In 2014, the Particle Physics Project Prioritization Panel identified the experiment as a top priority for US particle physics. At the same time, it recommended the collaboration take a few steps back and invite more international participation in the planning process.

Physicist Sergio Bertolucci, director of research and scientific computing at CERN, took the helm of an executive board put together to expand the collaboration and organize the election of new spokespersons.

DUNE now includes scientists from 148 institutions in 23 countries. It will be the first large international project hosted by the US to be jointly overseen by outside agencies.

This month, the collaboration elected two new spokespersons: André Rubbia, a professor of physics at ETH Zurich, and Mark Thomson, a professor of physics at the University of Cambridge. One will serve as spokesperson for two years and the other for three to provide continuity in leadership.

Rubbia got started with neutrino research as a member of the NOMAD experiment at CERN in the ’90s. More recently he was a part of LAGUNA-LBNO, a collaboration that was working toward a long-baseline experiment in Europe. Thomson has a long-term involvement in US-based underground and neutrino physics. He is the DUNE principle investigator for the UK.

Scientists are coming together to study neutrinos, rarely interacting particles that constantly stream through the Earth but are not well understood. They come in three types and oscillate, or change from type to type, as they travel long distances. They have tiny, unexplained masses. Neutrinos could hold clues about how the universe began and why matter greatly outnumbers antimatter, allowing us to exist.

“The science is what drives us,” Rubbia says. “We’re at the point where the next generation of experiments is going to address the mystery of neutrino oscillations. It’s a unique moment.”

Scientists hope to begin installation of the DUNE far detector by 2021. “Everybody involved is pushing hard to see this project happen as soon as possible,” Thomson says.

Jennifer Huber and Kathryn Jepsen

Image: Fermilab

Image: Fermilab

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Expanding the cosmic search

Friday, March 20th, 2015

This article appeared in Fermilab Today on March 20, 2015.

The South Pole Telescope scans the skies during a South Pole winter. Photo: Jason Gallicchio, University of Chicago

The South Pole Telescope scans the skies during a South Pole winter. Photo: Jason Gallicchio, University of Chicago

Down at the South Pole, where temperatures drop below negative 100 degrees Fahrenheit and darkness blankets the land for six months at a time, the South Pole Telescope (SPT) searches the skies for answers to the mysteries of our universe.

This mighty scavenger is about to get a major upgrade — a new camera that will help scientists further understand neutrinos, the ghost-like particles without electric charge that rarely interact with matter.

The 10-meter SPT is the largest telescope ever to make its way to the South Pole. It stands atop a two-mile thick plateau of ice, mapping the cosmic microwave background (CMB), the light left over from the big bang. Astrophysicists use these observations to understand the composition and evolution of the universe, all the way back to the first fraction of a second after the big bang, when scientists believe the universe quickly expanded during a period called inflation.

One of the goals of the SPT is to determine the masses of the neutrinos, which were produced in great abundance soon after the big bang. Though nearly massless, because neutrinos exist in huge numbers, they contribute to the total mass of the universe and affect its expansion. By mapping out the mass density of the universe through measurements of CMB lensing, the bending of light caused by immense objects such as large galaxies, astrophysicists are trying to determine the masses of these elusive particles.

A wafer of detectors for the SPT-3G camera undergoes inspection at Fermilab. Photo: Bradford Benson, University of Chicago

A wafer of detectors for the SPT-3G camera undergoes inspection at Fermilab. Photo: Bradford Benson, University of Chicago

To conduct these extremely precise measurements, scientists are installing a bigger, more sensitive camera on the telescope. This new camera, SPT-3G, will be four times heavier and have a factor of about 10 more detectors than the current camera. Its higher level of sensitivity will allow researchers to make extremely precise measurements of the CMB that will hopefully make it possible to cosmologically detect neutrino mass.

This photo shows an up-close look at a single SPT-3G detector. Photo: Volodymyr Yefremenko, Argonne National Laboratory

This photo shows an up-close look at a single SPT-3G detector. Photo: Volodymyr Yefremenko, Argonne National Laboratory


“In the next several years, we should be able to get to the sensitivity level where we can measure the number of neutrinos and derive their mass, which will tell us how they contribute to the overall density of the universe,” explained Bradford Benson, the head of the CMB Group at Fermilab. “This measurement will also enable even more sensitive constraints on inflation and has the potential to measure the energy scale of the associated physics that caused it.”

SPT-3G is being completed by a collaboration of scientists spanning the DOE national laboratories, including Fermilab and Argonne, and universities including the University of Chicago and University of California, Berkeley. The national laboratories provide the resources needed for the bigger camera and larger detector array while the universities bring years of expertise in CMB research.

“The national labs are getting involved because we need to scale up our infrastructure to support the big experiments the field needs for the next generation of science goals,” Benson said. Fermilab’s main role is the initial construction and assembly of the camera, as well as its integration with the detectors. This upgrade is being supported mainly by the Department of Energy and the National Science Foundation, which also supports the operations of the experiment at the South Pole.

Once the camera is complete, scientists will bring it to the South Pole, where conditions are optimal for these experiments. The extreme cold prevents the air from holding much water vapor, which can absorb microwave signals, and the sun, another source of microwaves, does not rise between March and September.

The South Pole is accessible only for about three months during the year, starting in November. This fall, about 20 to 30 scientists will head down to the South Pole to assemble the camera on the telescope and make sure everything works before leaving in mid-February. Once installed, scientists will use it to observe the sky over four years.

“For every project I’ve worked on, it’s that beginning — when everyone is so excited not knowing what we’re going to find, then seeing things you’ve been dreaming about start to show up on the computer screen in front of you — that I find really exciting,” said University of Chicago’s John Carlstrom, the principal investigator for the SPT-3G project.

Diana Kwon

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This article appeared in Fermilab Today on Nov. 26, 2014

The plastic scintillator extrusion line, shown here, produces detector material for export to experiments around the world. Photo: Reidar Hahn

The plastic scintillator extrusion line, shown here, produces detector material for export to experiments around the world. Photo: Reidar Hahn

Small, clear pellets of polystyrene can do a lot. They can help measure cosmic muons at the Pierre Auger Observatory, search for CP violation at KEK in Japan or observe neutrino oscillation at Fermilab. But in order to do any of these they have to go through Lab 5, located in the Fermilab Village, where the Scintillation Detector Development Group, in collaboration with the Northern Illinois Center for Accelerator and Detector Design (NICADD), manufactures the exclusive source of extruded plastic scintillator.

Like vinyl siding on a house, long thin blocks of plastic scintillator cover the surfaces of certain particle detectors. The plastic absorbs energy from collisions and releases it as measurable flashes of light. Fermilab’s Alan Bross and Anna Pla-Dalmau first partnered with local vendors to develop the concept and produce cost-effective scintillator material for the MINOS neutrino oscillation experiment. Later, with NIU’s Gerald Blazey, they built the in-house facility that has now exported high-quality extruded scintillator to experiments worldwide.

“It was clear that extruded scintillator would have a big impact on large neutrino detectors,” Bross said, “but its widespread application was not foreseen.”

Industrially manufactured polystyrene scintillators can be costly — requiring a labor-intensive process of casting purified materials individually in molds that have to be cleaned constantly. Producing the number of pieces needed for large-scale projects such as MINOS through casting would have been prohibitively expensive.

Extrusion, in contrast, presses melted plastic pellets through a die to create a continuous noodle of scintillator (typically about four centimeters wide by two centimeters tall) at a much lower cost. The first step in the production line mixes into the melted plastic two additives that enhance polystyrene’s natural scintillating property. As the material reaches the die, it receives a white, highly reflective coating that holds in scintillation light. Two cold water tanks respectively bathe and shower the scintillator strip before it is cool enough to handle. A puller controls its speed, and a robotic saw finally cuts it to length. The final product contains either a groove or a hole meant for a wavelength-shifting fiber that captures the scintillation light and sends the signal to electronics in the most useful form possible.

Bross had been working on various aspects of the scintillator cost problem since 1989, and he and Pla-Dalmau successfully extruded experiment-quality plastic scintillator with their vendors just in time to make MINOS a reality. In 2003, NICADD purchased and located at Lab 5 many of the machines needed to form an in-house production line.

“The investment made by Blazey and NICADD opened extruded scintillators to numerous experiments,” Pla-Dalmau said. “Without this contribution from NIU, who knows if this equipment would have ever been available to Fermilab and the rest of the physics community?”

Blazey agreed that collaboration was an important part of the plastic scintillator development.

“Together the two institutions had the capacity to build the resources necessary to develop state-of-the-art scintillator detector elements for numerous experiments inside and outside high-energy physics,” Blazey said. “The two institutions remain strong collaborators.”

Between their other responsibilities at Fermilab, the SDD group continues to study ways to make their scintillator more efficient. One task ahead, according to Bross, is to work modern, glass wavelength-shifting fibers into their final product.

“Incorporation of the fibers into the extrusions has always been a tedious part of the process,” he said. “We would like to change that.”

Troy Rummler

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