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  • richardmitnick 8:57 am on October 18, 2017 Permalink | Reply
    Tags: , , , DAMA LIBRA Dark Matter Experiment, , DAYA BAY, , , NIST PROSPECT detector, U Washington ADMX, , ,   

    From COSMOS: “Closing in on dark matter” 

    Cosmos Magazine bloc

    COSMOS Magazine

    18 October 2017
    Cathal O’Connell

    Dark matter can’t be detected but it glues galaxies together. It outweighs ordinary matter by five to one. Maltaguy1/Getty Images

    One Saturday I hired a metal detector and drove four hours to the historic gold-rush town of Bright in Victoria, Australia, where my wedding ring lies lost, somewhere on the bed of the Ovens River. I spent the evening wading through the icy waters in gumboots, uncovering such treasures as a bottle cap, a fisher’s lead weight and a bracelet caked in rust. I did not uncover the ring. But that doesn’t mean the ring is not there.

    Like me, physicists around the world are in the midst of an important search that has so far proven fruitless. Their quarry is nothing less than most of the matter in the universe, so-called “dark matter”.

    So far their most sensitive detectors have found – to be pithy – nada. Despite the lack of results, scientists aren’t giving up. “The frequency with which articles show up in the popular press saying ‘maybe dark matter isn’t real’ massively exceeds the frequency with which physicists or astronomers find any reason to re-examine that question,” says Katie Mack, a theoretical astrophysicist at the University of Melbourne.

    In many respects, the quest for dark matter has only just begun. We can expect quite a few more null results before the real treasure turns up. So here is where we stand, and what we can expect from the next few years.

    Imagine a toddler sitting on one end of a seesaw and launching her father, at the other end, high into the air. It’s a weird and unsettling image, yet we regularly observe this kind of ‘impossible’ behaviour in the universe at large. Like the little girl on the seesaw, galaxies behave as if they have four or five times the mass we can see.

    Our first inkling of this discrepancy came in the 1930s, when the Swiss astronomer Fritz Zwicky noticed odd movements among the Coma cluster of galaxies.

    Fritz Zwicky: The Father of Dark Matter. https://www.youtube.com/watch?v=TV0c1EFIKy4

    Zwicky’s anomaly was largely ignored until the 1970s, when astrophysicist Vera Rubin, based at the Carnegie Institute in Washington, noticed that the way galaxies spin did not tally with the laws of physics.

    Astronomer Vera Rubin in 1974, with her “measuring engine” used to examine photographic plates. Credit: Courtesy of Carnegie Institution of Washington

    The meticulous observations by Rubin (who passed away in December 2016) convinced most of the astronomical community something was amiss. There were two possible answers to the problem: either galaxies were a lot heavier than they appeared, or our theory of gravity was kaput when it came to galaxy-scale movements.

    From the outset, astronomers preferred the first explanation. At first they thought the missing matter was probably nothing too weird – just regular astronomical objects (like planets, black holes and stars) too dim for us to see. But as we surveyed the sky with ever bigger telescopes, these so-called ‘massive compact halo objects’ (or MACHOs) never turned up in the numbers needed to explain all the extra mass.

    Other astrophysicists, such as the Mordehai Milgrom at Israel’s Weizmann Institute, explored models where gravity behaved differently at cosmic scales. [See https://sciencesprings.wordpress.com/2017/05/18/from-nautilus-the-physicist-who-denies-dark-matter/%5D

    Mordehai Milgrom. Cosmos on Nautilus

    They were not successful.

    Slowly astronomers realised they had something radically different on their hands – a new kind of stuff they called ‘dark matter’, which must outweigh the universe’s regular matter by about five to one. “Certainly, when all the evidence is taken together,” Mack says, “there’s no competing idea right now that comes anywhere close to explaining it as well.”

    We know four main facts about dark matter. First, it has gravity. Second, it doesn’t emit, absorb or reflect light. Third, it moves slowly. Fourth, it doesn’t seem to interact with anything, even itself.

    Like detectives in a TV murder mystery, physicists have compiled a list of suspects. Topping the list are three hypothetical particles already wanted on other charges: axions, sterile neutrinos and WIMPs. Besides nailing dark matter, each would help explain a grand mystery of their own.

    The axion is a particle proposed by Roberto Peccei and Helen Quinn back in 1977 to explain a quirk of the strong force (namely, why it can’t distinguish left from right, the way the weak force does). Thirty years on, axions are still our best explanation for that puzzle.

    Axions could have any mass, but if – and it is a big ‘if’ – they have a mass about 100 billion times lighter than an electron, theorists have calculated they would have been created in the Big Bang in such vast numbers that they could account for the universe’s dark matter. Like detectives with a dragnet, physicists are searching through different possible masses in an attempt to close in from both ends and corner the axion.

    The Axion Dark Matter eXperiment (ADMX), based at the University of Washington, is dragging the lightest end of the range.

    U Washington ADMX

    U Washington ADMX Axion Dark Matter Experiment

    Since 2010 the project has been trying to catch axions by turning them into photons using strong magnetic fields. So far ADMX has ruled out the featherweight mass range between 150 to 270 billion times lighter than the electron.

    The CERN Axion Solar Telescope (CAST) is dragging the heavyweight end of the range looking for axions that are a few tens of millions to about a million times lighter than the electron.

    CERN CAST Axion Solar Telescope

    The theorised source of these hefty axions is the Sun, where they might be created by X-rays in the presence of strong electric fields. In an example of recycling at its big-science best, CAST was assembled from a piece of the Large Hadron Collider -– a giant test magnet. It aims to detect solar axions by turning them back into X-rays. It has been running since 2003. The search goes on.

    Hypothetical particles known as axions could explain dark matter. Physicists at CERN have taken a giant magnet from the Large Hadron Collider and turned it into an axion detector, the CERN Axion Solar Telescope. Howard Cunningham/Getty Images

    Sterile neutrinos are the hypothetical heavier, lazier brothers of neutrinos – the ghostly, fast-moving particles created in nuclear reactions and in the centre of the Sun. They are called ‘sterile’ or ‘inactive’ because they only interact via gravity.

    Besides being a dark-matter candidate, sterile neutrinos would plug a number of holes in the Standard Model,

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    which, like a subatomic version of the periodic table, has had great success in predicting the properties of the fundamental building blocks of the universe. For instance, sterile neutrinos could explain why neutrinos are so light, and why every neutrino we’ve ever seen has a ‘left-handed’ spin; sterile neutrinos would be the missing ‘right-handed’ partners.

    Physicists are trying to detect sterile neutrinos in different ways, including searching deep space for the X-rays emitted when they decay. NASA’s Chandra X-ray telescope has picked up an excess of X-rays from the Perseus cluster of galaxies, which is so far unexplained.

    NASA/Chandra Telescope

    Perseus cluster. NASA

    Meanwhile, regular neutrino detectors based at nuclear reactors, such as Daya Bay in China, have noticed anomalies that might be explained by sterile neutrinos.

    Daya Bay, approximately 52 kilometers northeast of Hong Kong and 45 kilometers east of Shenzhen, China

    “Like Elvis, people see hints of the sterile neutrino everywhere,” quipped Francis Halzen in August 2016, when he and his colleagues at the IceCube Neutrino Observatory announced the disappointing results of their own search.

    U Wisconsin ICECUBE neutrino detector at the South Pole

    Their detector, buried up to 2.5 km deep in ice near the South Pole, found no evidence of the elusive sterile neutrino – a result that seems to rule out the Daya Bay reactor sightings. For a conclusive answer, we’ll need to wait for the next neutrino searches, such as the Precision Reactor Antineutrino Oscillation and Spectrum Measurement (PROSPECT) under construction at the US National Institute of Standards and Technology (NIST) in Maryland.

    The PROSPECT detector will consist of an 11 x 14 array of long skinny cells filled with liquid scintillator, which is designed to sense antineutrinos emanating from the reactor core. If a sterile neutrino flavor exists, then PROSPECT will see waves of antineutrinos that appear and disappear with a period determined by their energy. Composition not drawn to scale. NIST.

    The third and most popular suspect is WIMPs – weakly interacting massive particles. The name covers a broad range of hypothetical particles that would interact via the weak force. They pop naturally out of the ideas of supersymmetry, an extension proposed to tidy up the loose ends of the Standard Model.

    Physicists calculate that the simplest possible WIMP, with a mass of about 100 billion electron volts, would have been created in the Big Bang at just the right numbers to explain dark matter: the so-called ‘WIMP miracle’.

    WIMP detectors are typically deep underground, watching for a telltale flash given out when a particle of dark matter bumps into an atomic nucleus.

    The most sensitive WIMP experiment yet is LUX, a bathtub-sized vat holding 370 kg of liquid xenon at the Sanford Underground Research Facility [SURF] in South Dakota. In 2016, the LUX team announced it had discovered no dark matter signals during its first 20-month-long search. Undeterred, the LUX team plan to upgrade to a 7,000-kg vat, LUX-ZEPLIN, by 2020.

    LBNL Lux Zeplin project at SURF

    The most intriguing dark matter result so far comes from the DAMA/LIBRA experiment in Italy. Using a detector made of highly purified sodium-iodide crystals, 1.5 km beneath Italy’s Gran Sasso mountain, scientists believe they have seen evidence of dark matter every year for the past 14 years (see Cosmos 65, p60). Their evidence comes in an annual rise and fall in background detections. Such a pattern might reflect the Earth’s relative speed through the dark-matter cloud that surrounds the Milky Way; while our planet moves around the Sun at 30 km/s, the Solar System as a whole is travelling at 230 km/s around the centre of the Milky Way.

    DAMA LIBRA Dark Matter Experiment, 1.5 km beneath Italy’s Gran Sasso mountain

    Gran Sasso LABORATORI NAZIONALI del GRAN SASSO, located in L’Aquila, Italy

    For half of the year the Earth’s orbital speed would add to the speed of the Solar System, increasing the rate of dark-matter interactions. For the other half, the speeds would subtract and the rate of interactions decrease. The problem is that lots of other things change with the seasons too, such as the thickness of the atmosphere. To rule out terrestrial effects, astronomers are setting up two identical detectors, called SABRE, in opposite hemispheres – so that one is collecting data in winter and the other in summer.

    One detector will be based at Gran Sasso, the other in Australia, in an abandoned gold mine near Stawell, Victoria. Each detector will be made of 50 kg of sodium iodide, and have noise levels 10 times lower than DAMA/LIBRA. Construction on each is under way, and could be finished this year.

    Rather than detecting dark matter, others are trying to make it. The closest we can get to the conditions of the Big Bang – where dark matter was presumably created – is in the collision chambers of the Large Hadron Collider, CERN’s 27-km long particle smasher. These chambers are ringed by sensors that can pick up the energies of millions of particles generated in each smash-up, and tally this against the known collision energy. If some energy is missing, it might indicate the creation of a particle that could not be detected by any sensors: dark matter.

    So far, notwithstanding a brief, hallucinatory blip in late 2015, the LHC has not discovered anything that might constitute a dark matter particle such as a WIMP. But the LHC has only collected about 1% of the data it is due to produce before it is retired in 2025. So it is too early to throw in the towel on producing dark matter yet. Plans are afoot for the LHC’s successor, which will be able to probe far higher energies.

    Snowmelt from the Alpine ranges had swelled the Ovens River. I had to hug the shore with my metal detector, where the water was shallow and easy to sweep. I searched those parts that I could search as thoroughly as possible. If I did not find my prize, I wanted to at least be able to point to the map and say with confidence where the ring was not.

    The map that physicists search has coordinates of energy levels and interaction strengths. Each new search sweeps out a new territory, so even a null result is valuable information. So far, in our search for the three primary candidates – axions, sterile neutrinos and WIMPs – we have only probed the most shallow, accessible waters. “There’s nothing really that says they have to be easy to detect,” Mack says. “It may just be that their interactions with our detectors are smaller than expected.”

    It took almost 50 years for the Higgs boson to be discovered. Gravitational waves took almost a century. Let’s not give up on dark matter just yet.

    I certainly won’t be giving up my own search. Next summer, when the Ovens dries, I will return to Bright and sweep the next unprobed area of the riverbed. I’d say wish me luck, but the point is to be so rigorous that luck has nothing to do with it.

    See the full article here .

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  • richardmitnick 1:39 pm on May 4, 2017 Permalink | Reply
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    From Symmetry: “Sterile neutrino search hits roadblock at reactors” 

    Symmetry Mag


    Kathryn Jepsen


    A new result from the Daya Bay collaboration reveals both limitations and strengths of experiments studying antineutrinos at nuclear reactors.

    As nuclear reactors burn through fuel, they produce a steady flow of particles called neutrinos. Neutrinos interact so rarely with other matter that they can flow past the steel and concrete of a power plant’s containment structures and keep on moving through anything else that gets in their way.

    Physicists interested in studying these wandering particles have taken advantage of this fact by installing neutrino detectors nearby. A recent result using some of these detectors demonstrated both their limitations and strengths.

    The reactor antineutrino anomaly

    In 2011, a group of theorists noticed that several reactor-based neutrino experiments had been publishing the same, surprising result: They weren’t detecting as many neutrinos as they thought they would.

    Or rather, to be technically correct, they weren’t seeing as many antineutrinos as they thought they would; nuclear reactors actually produce the antimatter partners of the elusive particles. About 6 percent of the expected antineutrinos just weren’t showing up. They called it “the reactor antineutrino anomaly.”

    The case of the missing neutrinos was a familiar one. In the 1960s, the Davis experiment located in Homestake Mine in South Dakota reported a shortage of neutrinos coming from processes in the sun.

    Construction of the Homestake Mine tank. BNL.

    Other experiments confirmed the finding. In 2001, the Sudbury Neutrino Observatory in Ontario demonstrated that the missing neutrinos weren’t missing at all; they had only undergone a bit of a costume change.

    SNOLAB, Sudbury, Ontario, Canada.

    Neutrinos come in three types. Scientists discovered that neutrinos could transform from one type to another. The missing neutrinos had changed into a different type of neutrino that the Davis experiment couldn’t detect.

    Since 2011, scientists have wondered whether the reactor antineutrino anomaly was a sign of an undiscovered type of neutrino, one that was even harder to detect, called a sterile neutrino.

    A new result from the Daya Bay experiment in China not only casts doubt on that theory, it also casts doubt on the idea that scientists understand their model of reactor processes well enough at this time to use it to search for sterile neutrinos.

    The word from Daya Bay

    The Daya Bay experiment studies antineutrinos coming from six nuclear reactors on the southern coast of China, about 35 miles northeast of Hong Kong. The reactors are powered by the fission of uranium. Over time, the amount of uranium inside the reactor decreases while the amount of plutonium increases. The fuel is changed—or cycled—about every 18 months.

    The main goal of the Daya Bay experiment was to look for the rarest of the known neutrino oscillations. It did that, making a groundbreaking discovery after just nine weeks of data-taking.

    But that wasn’t the only goal of the experiment. “We realized right from the beginning that it is important for Daya Bay to address as many interesting physics problems as possible,” says Daya Bay co-spokesperson Kam-Biu Luk of the University of California, Berkeley and the US Department of Energy’s Lawrence Berkeley National Laboratory.

    For this result, Daya Bay scientists took advantage of their enormous collection of antineutrino data to expand their investigation to the reactor antineutrino anomaly.

    Using data from more than 2 million antineutrino interactions and information about when the power plants refreshed the uranium in each reactor, Daya Bay physicists compared the measurements of antineutrinos coming from different parts of the fuel cycle: early ones dominated by uranium through later ones dominated by both uranium and plutonium.

    n theory, the type of fuel producing the antineutrinos should not affect the rate at which they transform into sterile neutrinos. According to Bob Svoboda, chair of the Department of Physics at the University of California, Davis, “a neutrino wouldn’t care how it got made.” But Daya Bay scientists found that the shortage of antineutrinos existed only in processes dominated by uranium.

    Their conclusion is that, once again, the missing neutrinos aren’t actually missing. This time, the problem of the missing antineutrinos seems to stem from our understanding of how uranium burns in nuclear power plants. The predictions for how many antineutrinos the scientists should detect may have been overestimated.

    “Most of the problem appears to come from the uranium-235 model (uranium-235 is a fissile isotope of uranium), not from the neutrinos themselves,” Svoboda says. “We don’t fully understand uranium, so we have to take any anomaly we measured with a grain of salt.”

    This knock against the reactor antineutrino anomaly does not disprove the existence of sterile neutrinos. Other, non-reactor experiments have seen different possible signs of their influence. But it does put a damper on the only evidence of sterile neutrinos to have come from reactor experiments so far.

    Other reactor neutrino experiments, such as NEOS in South Korea and PROSPECT in the United States will fill in some missing details.


    Prospect. BNL

    NEOS scientists directly measured antineutrinos coming from reactors in the Hanbit nuclear power complex using a detector placed about 80 feet away, a distance some scientists believe is optimal for detecting sterile neutrinos should they exist. PROSPECT scientists will make the first precision measurement of antineutrinos coming from a highly enriched uranium core, one that does not produce plutonium as it burns.

    A silver lining

    The Daya Bay result offers the most detailed demonstration yet of scientists’ ability to use neutrino detectors to peer inside running nuclear reactors.

    “As a study of reactors, this is a tour de force,” says theorist Alexander Friedland of SLAC National Accelerator Laboratory. “This is an explicit demonstration that the composition of the reactor fuel has an impact on the neutrinos.”

    Some scientists are interested in monitoring nuclear power plants to find out if nuclear fuel is being diverted to build nuclear weapons.

    “Suppose I declare my reactor produces 100 kilograms of plutonium per year,” says Adam Bernstein of the University of Hawaii and Lawrence Livermore National Laboratory. “Then I operate it in a slightly different way, and at the end of the year I have 120 kilograms.” That 20-kilogram surplus, left unmeasured, could potentially be moved into a weapons program.

    Current monitoring techniques involve checking what goes into a nuclear power plant before the fuel cycle begins and then checking what comes out after it ends. In the meantime, what happens inside is a mystery.

    Neutrino detectors allow scientists to understand what’s going on in a nuclear reactor in real time.

    Scientists have known for decades that neutrino detectors could be useful for nuclear nonproliferation purposes. Scientists studying neutrinos at the Rovno Nuclear Power Plant in Ukraine first demonstrated that neutrino detectors could differentiate between uranium and plutonium fuel.

    Most of the experiments have done this by looking at changes in the aggregate number of antineutrinos coming from a detector. Daya Bay showed that neutrino detectors could track the plutonium inventory in nuclear fuel by studying the energy spectrum of antineutrinos produced.

    “The most likely use of neutrino detectors in the near future is in so-called ‘cooperative agreements,’ where a $20-million-scale neutrino detector is installed in the vicinity of a reactor site as part of a treaty,” Svoboda says. “The site can be monitored very reliably without having to make intrusive inspections that bring up issues of national sovereignty.”

    Luk says he is dubious that the idea will take off, but he agrees that Daya Bay has shown that neutrino detectors can give an incredibly precise report. “This result is the best demonstration so far of using a neutrino detector to probe the heartbeat of a nuclear reactor.”

    See the full article here .

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    Symmetry is a joint Fermilab/SLAC publication.

  • richardmitnick 7:38 am on April 5, 2017 Permalink | Reply
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    From LBNL: “New Measurements Suggest ‘Antineutrino Anomaly’ Fueled by Modeling Error” 

    Berkeley Logo

    Berkeley Lab

    April 5, 2017

    Antineutrinos produced by reactors at the Daya Bay Nuclear Power Plant complex in Shenzhen, China, are measured in a particle physics experiment that is conducted by an international collaboration involving Berkeley Lab researchers. (Credit: Roy Kaltschmidt/Berkeley Lab)

    Daya Bay, approximately 52 kilometers northeast of Hong Kong and 45 kilometers east of Shenzhen, China

    Results from a new scientific study may shed light on a mismatch between predictions and recent measurements of ghostly particles streaming from nuclear reactors—the so-called “reactor antineutrino anomaly,” which has puzzled physicists since 2011.

    The anomaly refers to the fact that scientists tracking the production of antineutrinos—emitted as a byproduct of the nuclear reactions that generate electric power—have routinely detected fewer antineutrinos than they expected. One theory is that some neutrinos are morphing into an undetectable form known as “sterile” neutrinos.

    But the latest results [submitted to Phys. Rev. Letters] from the Daya Bay reactor neutrino experiment, located at a nuclear power complex in China, suggest a simpler explanation—a miscalculation in the predicted rate of antineutrino production for one particular component of nuclear reactor fuel.

    Antineutrinos carry away about 5 percent of the energy released as the uranium and plutonium atoms that fuel the reactor split, or “fission.” The composition of the fuel changes as the reactor operates, with the decays of different forms of uranium and plutonium (called “isotopes”) producing different numbers of antineutrinos with different energy ranges over time, even as the reactor steadily produces electrical power.

    The new results from Daya Bay—where scientists have measured more than 2 million antineutrinos produced by six reactors during almost four years of operation—have led scientists to reconsider how the composition of the fuel changes over time and how many neutrinos come from each of the decay chains.

    The scientists found that antineutrinos produced by nuclear reactions that result from the fission of uranium-235, a fissile isotope of uranium common in nuclear fuel, were inconsistent with predictions.

    In this chart, the yields of reactor antineutrinos produced by plutonium-239 (vertical) and uranium-235 (horizontal) measured by the Daya Bay experiment (red triangle at center) are compared to the theoretical prediction (black dot at right), showing a discrepancy that could explain the so-called “antineutrino anomaly.” (Credit: Daya Bay Collaboration)

    “The model predicts almost 8 percent more antineutrinos coming from decays of uranium-235 than what we have measured,” said Kam-Biu Luk, a Daya Bay Collaboration co-spokesperson who is a faculty senior scientist at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and a physics professor at UC Berkeley.

    Patrick Tsang, who conceptualized a new data-analysis technique that was key in this study while working as a postdoctoral fellow in Berkeley Lab’s Physics Division, added, “The finding is surprising because it is the first time we are able to identify the disagreement with predictions for a particular fission isotope.” Tsang is now a project scientist working at SLAC National Accelerator Laboratory.

    Meanwhile, the number of antineutrinos from plutonium-239, the second most common fuel ingredient, was found to agree with predictions, although this measurement is less precise than that for uraninum-235.

    If sterile neutrinos—theoretical particles that are a possible source of the universe’s vast unseen or “dark” matter—were the source of the anomaly, then the experimenters would observe an equal depletion in the number of antineutrinos for each of the fuel ingredients, but the experimental results disfavor this hypothesis.

    The latest analysis suggests that a miscalculation of the rate of antineutrinos produced by the fission of uranium-235 over time, rather than the presence of sterile neutrinos, may be the explanation for the anomaly. These results can be confirmed by new experiments that will measure antineutrinos from reactors fueled almost entirely by uranium-235.

    The work could help scientists at Daya Bay and similar experiments understand the fluctuating rates and energies of those antineutrinos produced by specific ingredients in the nuclear fission process throughout the nuclear fuel cycle. An improved understanding of the fuel evolution inside a nuclear reactor may also be helpful for other nuclear science applications.

    A view inside a particle detector tank at Daya Bay, where photomultiplier tubes measure signals from antineutrinos. (Credit: Roy Kaltschmidt/Berkeley Lab)

    Situated about 32 miles northeast of Hong Kong, the Daya Bay experiment uses an array of detectors to capture antineutrino signals from particle interactions occurring in a series of liquid tanks. The Daya Bay collaboration involves 243 researchers at 41 institutions in the U.S., China, Chile, Russia and the Czech Republic.

    Daya Bay physics research is supported by the U.S. Department of Energy’s Office of Science and the National Science Foundation.

    See the full article here .

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  • richardmitnick 1:11 pm on February 19, 2016 Permalink | Reply
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    From Brookhaven: “Most Precise Measurement of Reactor Antineutrino Spectrum Reveals Intriguing Surprise” 

    Brookhaven Lab

    February 12, 2016
    Karen McNulty Walsh, (631) 344-8350
    Peter Genzer, (631) 344-3174

    Daya Bay
    Antineutrino detectors installed in the far hall of the Daya Bay experiment. Credit: LBL Qiang

    Members of the International Daya Bay Collaboration, who track the production and flavor-shifting behavior of electron antineutrinos generated at a nuclear power complex in China, have obtained the most precise measurement of these subatomic particles’ energy spectrum ever recorded. The data generated from the world’s largest sample of reactor antineutrinos indicate two intriguing discrepancies with theoretical predictions and provide an important measurement that will shape future reactor neutrino experiments. The results have been published in the journal Physical Review Letters.

    Studying the behavior of elusive neutrinos holds the potential to unlock many secrets of physics, including details about the history, makeup, and fate of our universe. Neutrinos were among the most abundant particles at the time of the Big Bang, and are still generated abundantly today in the nuclear reactions that power stars and in collisions of cosmic rays with Earth’s atmosphere.

    They are also emitted as a by-product of power generation in man-made nuclear reactors, giving scientists a powerful way to study them on Earth in a controlled manner. In fact, the study of particles emitted by reactors led to the first detection of neutrinos in the 1950s, a finding once considered impossible due to the extreme inert nature of these particles, which were then only predicted. Since that time reactor experiments, including Daya Bay, have played a crucial role in uncovering the secrets of neutrino oscillation—their tendency to switch among three known flavors: electron, muon, and tau—and other important neutrino properties.

    A crucial factor for many of these experiments is knowing how many antineutrinos are emitted in total in these nuclear reactions (the flux), and how many are being produced at particular energies (the energy distribution, or spectrum). In early studies, scientists relied on calculations or other indirect means, such as electron spectrum measurements made on reactor fuels, to estimate these numbers, based on their understanding of the complex fission processes in the reactor core. These methods have rather strong dependence on theoretical models.

    The Daya Bay Collaboration now provides the most precise model-independent measurement of the energy spectrum of these elusive particles, and a new measurement of total antineutrino flux. The data were gathered by analyzing more than 300,000 reactor antineutrinos collected over the course of 217 days. The most challenging part of this work was to accurately calibrate the energy response of the detectors. Through dedicated calibration and analysis effort, Daya Bay was able to measure the neutrino energy to an unprecedented precision, better than 1 percent, over a broad energy range of the reactor antineutrinos.

    The measured reactor antineutrino spectrum shows a surprising feature: an excess of antineutrinos at an energy of around 5 million electron volts (MeV) compared with theoretical expectations. This represents a deviation of about 10 percent between the experimental measurement and calculations based on the theoretical models—well beyond the uncertainties—leading to a discrepancy of up to four standard deviations [σ]. “This unexpected disagreement between our observation and predictions strongly suggested that the current calculations would need some refinement,” commented Kam-Biu Luk of the University of California at Berkeley and DOE’s Lawrence Berkeley National Laboratory, a co-spokesperson of the Daya Bay Collaboration. Two other experiments have shown a similar excess at this energy, though with less precision than the new Daya Bay result.

    Such deviation shows the importance of the direct measurement of the reactor antineutrino spectrum, particularly for experiments that use the spectrum to measure neutrino oscillations, and may indicate the need to revisit the models underlying the calculations. “We expect that the spectrum measured by Daya Bay will improve with more data and better understanding of the detector response. These improved measurements will be essential for next-generation reactor neutrino experiments such as JUNO,” said Jun Cao of the Institute of High Energy Physics (IHEP) in China, a co-spokesperson of Daya Bay and the deputy spokesperson of JUNO, an experiment being built 200 kilometers away from Daya Bay.

    Daya Bay’s measurement of antineutrino flux—the total number of antineutrinos emitted across the entire energy range—indicates that the reactors are producing 6 percent fewer antineutrinos overall when compared to some of the model-based predictions. This result is consistent with past measurements. This observed deficit has been named the “Reactor Antineutrino Anomaly.” This disagreement could arise from the imperfection of the models. Or, more intriguingly, it could be the result of an oscillation involving a new kind of neutrino, the so-called sterile neutrino—postulated by some theories but yet to be detected. Whether the anomaly exists is still an open question.

    Background on Daya Bay

    The Daya Bay nuclear power complex is located on the southern coast of China, 55 kilometers northeast of Hong Kong. It consists of three nuclear power plants, each with two reactor cores. All six cores are pressurized water reactors with similar design, and each can generate up to 2.9 gigawatt thermal power. Every second, the six reactors emit 3,500 billion billon electron antineutrinos. For this measurement, the Daya Bay experiment used six detectors located at 360 meters to 1.9 kilometers from the reactors. Each detector contains 20 tons of gadolinium-doped liquid scintillator to catch the reactor antineutrinos.
    Contact Information

    Jun Cao, co-spokesperson, IHEP, +86-10-88235808, caoj@ihep.ac.cn
    Kam-Biu Luk, co-spokesperson, UC Berkeley and Lawrence Berkeley National Laboratory, 510-642-8162, 510-486-7054, k_luk@berkeley.edu

    For more information, visit http://dayabay.ihep.ac.cn/

    See the full article here .

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    One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world.Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.

  • richardmitnick 9:00 am on September 11, 2015 Permalink | Reply
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    From BNL: “Best Precision Yet for Neutrino Measurements at Daya Bay” 

    Brookhaven Lab

    Bird’s-eye view of the underground Daya Bay Far Hall during installation. The four antineutrino detectors are immersed in a large pool filled with ultra pure water as a cosmic muon veto system. (Photo by Roy Kaltschmidt, Berkeley Lab)

    In the Daya Bay region of China, about 55 kilometers northeast of Hong Kong, a a research project is underway to study ghostlike, elusive particles called neutrinos. Today, the international Daya Bay Collaboration announces new findings on the measurements of neutrinos, paving the way forward for further neutrino research, and confirming that the Daya Bay neutrino experiment continues to be one to watch.

    The latest findings involve measurements that track the way neutrinos change types or flavors as they move, a characteristic called neutrino oscillation. By measuring neutrino oscillation, the researchers can home in on two key neutrino properties: their “mixing angle” and “mass splitting.”

    Measurements of these properties by the Daya Bay Collaboration are the most precise to date, an improvement of about a factor of two over previous measurements published by the collaboration in early in 2014. The new results will be published in Physical Review Letters.

    “We are trying to measure a small effect to a very high precision. Our new result is an important milestone marking the start of the precision era of neutrino physics,” said physicist Xin Qian of the U.S. Department of Energy’s Brookhaven National Laboratory, which plays multiple roles in this international project, ranging from management to detector engineering and data analysis. The Collaboration includes more than 200 scientists from seven regions and countries.

    It’s important to measure the mixing angle and mass splitting parameters as precisely as possible, the scientists say, because neutrino behavior could hold the key to understanding the asymmetry between matter and antimatter in the universe. This asymmetry, known as the charge-parity or CP violation, explains why shortly after the Big Bang, when most matter and antimatter annihilated each other, some matter was left over to make up the universe we see today.

    Electron antineutrino survival probability versus the ratio of neutrino propagation distance divided by the energy. The points represent the ratio of the observed number of events divided by the expectation assuming the inverse-square law. A clear deficit is seen and is well described by neutrino oscillation theory (solid line).

    The Fluctuating Neutrino

    The behavior of neutrinos is unlike any other fundamental particle—they seem to disappear, reappear, and transform themselves as they travel, unimpeded, from sources like the sun and other stars, through space, planets, and even our own bodies.

    Neutrinos come in three flavors—electron, muon, and tau.

    Six flavours of leptons

    And as a neutrino travels, thanks to quantum mechanical fluctuations, it oscillates between flavors. That is, a particle that starts out as an electron neutrino might at some point turn into a tau neutrino. Then at another point it will present itself more like it did in the beginning. As time goes by, these transformations happen again and again, with the oscillation having a particular amplitude and frequency—similar to sound and light waves.

    The amplitude of neutrino oscillations gives scientists information about the rate at which neutrinos transform into different flavors, known as the mixing angle. The frequency of the oscillations gives information about the difference between the masses, a property known as mass splitting.

    The Neutrino Net

    To study neutrino oscillations, the Daya Bay Collaboration has immersed eight detectors in three large underground pools of water. These detectors sit at different distances from the six China General Nuclear Power Group reactors in Daya Bay. As a by-product of generating electricity, the reactors emit steady streams of electron antineutrinos, which for purposes of the experiment are essentially the same as electron neutrinos. The detectors pick up the transformations that occur as these millions of quadrillions of electron antineutrinos travel farther away from their origin in the reactors.

    Based on the data collected over 217 days with six of the Daya Bay detectors and 404 days using all eight of the Daya Bay detectors, the research team has determined the value for a specific mixing angle, called theta13 (pronounced theta-one-three), to a precision two times better than previous results. Similar improvement was made in the precision of measuring the mass splitting.

    “We’ve been able to collect so much data and achieved this level of precision thanks to the spectacular performance of our detectors,” said physicist Chao Zhang of Brookhaven Lab. The measurements support the three-neutrino model, which describes physicists’ current understanding of the nature of neutrinos, and will have far-reaching implications for future neutrino experiments, he added.

    The Daya Bay Collaboration continues to take data. At the end of 2017 it will have roughly four times more data to further improve precision for both the mixing angle of theta13 and the corresponding mass splitting. By then, all three mixing angles and two mass splittings may be determined to comparable precisions, better than three percent, which are essential for future neutrino experiments to measure the remaining unknown properties of the elusive neutrinos.

    The unprecedented precision of the data set allows for many other studies: For example, the team is looking for evidence of a possible “sterile” neutrino, a hypothetical type that may mix with the three known neutrino flavors. If this sterile neutrino shows itself in the data, scientists will need to rethink the three-neutrino model. The team is also looking for a variety of other possible deviations from expectations of the Standard Model, the theory physicists use to describe particle interactions.

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    “By advancing our knowledge about neutrinos, the Daya Bay experiment will expand our understanding of fundamental physics,” Zhang said.

    Brookhaven Lab’s role in the Daya Bay Collaboration is supported by the DOE Office of Science (HEP, NP).

    See the full article here .

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  • richardmitnick 2:43 pm on October 3, 2014 Permalink | Reply
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    From BNL: “Brookhaven and the Daya Bay Neutrino Experiment” 

    Brookhaven Lab

    October 1, 2014
    Karen McNulty Walsh

    The Daya Bay Collaboration, an international group of scientists studying the subtle transformations of subatomic particles called neutrinos, is publishing its first results on the search for a so-called sterile neutrino, a possible new type of neutrino beyond the three known neutrino “flavors,” or types. The existence of this elusive particle, if proven, would have a profound impact on our understanding of the universe, and could impact the design of future neutrino experiments. The new results, appearing in the journal Physical Review Letters, show no evidence for sterile neutrinos in a previously unexplored mass range. Read the collaboration press release.

    Daya Bay
    Daya Bay
    The U.S. Department of Energy’s Brookhaven National Laboratory plays multiple roles in the Daya Bay experiment, ranging from management to data analysis. In addition to coordinating detector engineering and design efforts and developing software and analysis techniques, Brookhaven scientists perfected the “recipe” for a very special, chemically stable liquid that fills Daya Bay’s detectors and interacts with antineutrinos. This work at Daya Bay builds on a legacy of breakthrough neutrino research by Brookhaven Lab that has resulted in two Nobel Prizes in Physics.

    Members of the BNL team on the Daya Bay Neutrino Project include: (seated, from left) Penka Novakova, Laurie Littenberg, Steve Kettell, Ralph Brown, and Bob Hackenburg; (standing, from left) Zhe Wang, Chao Zhang, Jiajie Ling, David Jaffe, Brett Viren, Wanda Beriguete, Ron Gill, Mary Bishai, Richard Rosero, Sunej Hans, and Milind Diwan. Missing from the picture are: Donna Barci, Wai-Ting Chan, Chellis Chasman, Debbie Kerr, Hide Tanaka, Wei Tang, Xin Qian, Minfang Yeh, and Elizabeth Worcester.

    Comments from U.S. Daya Bay Chief Scientist Steve Kettell

    Steve Kettell

    This body of research is helping to unlock the secrets of the least understood constituents of matter—an important quest considering that neutrinos outnumber all other particle types with a billion neutrinos for every quark or electron.

    The fairly recent discovery that neutrinos have mass changes how we must think about the Standard Model of particle physics because it cannot be explained by that well-accepted description of all known particles and their interactions.

    The Standard Model of elementary particles, with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Understanding the details of neutrino mass could have huge implications for our understanding of how the universe evolved. And those details—including how neutrinos oscillate, or switch from one flavor to another, are the essence of the research at Daya Bay and a key to unlocking these mysteries.

    The unusual properties of the known neutrinos, particularly their unique mass properties compared to other particles in the Standard Model, give us good reason to suspect that the universe may be full of such neutral particles of other flavors, such as the sterile neutrino. These particles could potentially help account for a large portion of matter in the universe that we cannot detect directly, so called dark matter.

    Daya Bay has been an exciting experiment to work on. It has been exquisitely designed and built, enabling us to make several important discoveries (first result and new result) and to search for these particles. And while the latest study from Daya Bay did not detect evidence of sterile neutrinos, it did greatly narrow the range in which we need to search. We will continue to exploit this beautiful experiment to further explore and understand the properties of the mysterious neutrino.

    The existence of neutrino mass and mixing leads to further deep questions, in particular whether neutrinos are responsible for the dominance of matter over antimatter in the universe. With the first results from Daya Bay this question now seems answerable with the long-baseline neutrino project planned at DOE’s Fermi National Accelerator Laboratory. Brookhaven scientists identified this scientific opportunity and continue to lead the development of this project, which has now been endorsed by recent national advisory panels as the highest priority domestic project in fundamental particle physics.
    See the full article here.

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  • richardmitnick 4:10 pm on October 1, 2014 Permalink | Reply
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    From LBL: “News Center Hide & Seek: Sterile Neutrinos Remain Elusive” 

    Berkeley Logo

    Berkeley Lab

    October 1, 2014
    Kate Greene

    The Daya Bay Collaboration, an international group of scientists studying the subtle transformations of subatomic particles called neutrinos, is publishing its first results on the search for a so-called sterile neutrino, a possible new type of neutrino beyond the three known neutrino “flavors,” or types. The existence of this elusive particle, if proven, would have a profound impact on our understanding of the universe, and could impact the design of future neutrino experiments. The new results, appearing in the journal Physical Review Letters, show no evidence for sterile neutrinos in a previously unexplored mass range.

    There is strong theoretical motivation for sterile neutrinos. Yet, the experimental landscape is unsettled—several experiments have hinted that sterile neutrinos may exist, but the others yielded null results. Having amassed one of the largest samples of neutrinos in the world, the Daya Bay Experiment is poised to shed light on the existence of sterile neutrinos.

    Daya Bay
    Daya Bay

    The reactors at Daya Bay in southeast China. Credit: Kam-Biu Luk

    The Daya Bay Experiment is situated close to the Daya Bay and Ling Ao nuclear power plants in China, 55 kilometers northeast of Hong Kong. These reactors produce a steady flux of antineutrinos that the Daya Bay Collaboration scientists use for research at detectors located at varying distances from the reactors. The collaboration includes more than 200 scientists from six regions and countries.

    The Daya Bay experiment began its operation on December 24, 2011. Soon after, in March 2012, the collaboration announced its first results: the observation of a new type of neutrino oscillation—evidence that these particles mix and change flavors from one type to others—and a precise determination of a neutrino “mixing angle,” called θ13, which is a definitive measure of the mixing of at least three mass states of neutrinos.

    The fact that neutrinos have mass at all is a relatively new discovery, as is the observation at Daya Bay that the electron neutrino is a mixture of at least three mass states. While scientists don’t know the exact values of the neutrino masses, they are able to measure the differences between them, or “mass splittings.” They also know that these particles are dramatically less massive than the well-known electron, though both are members of the family of particles called “leptons.”

    These unexpected observations have led to the possibility that the electrically neutral, almost undetectable neutrino could be a special type of matter and a very important component of the mass of the universe. Given that the nature of matter and in particular the property of mass is one of the fundamental questions in science, these new revelations about the neutrino make it clear that it is important to search for other light neutral particles that might be partners of the active neutrinos, and may contribute to the dark matter of the universe.

    Search for a light sterile neutrino

    The new Daya Bay paper describes the search for such a light neutral particle, the “sterile neutrino,” by looking for evidence that it mixes with the three known neutrino types—electron, muon, and tau. If, like the known flavors, the sterile neutrino also exists as a mixture of different masses, it would lead to mixing of neutrinos from known flavors to the sterile flavor, thus giving scientists proof of its existence. That proof would show up as a disappearance of neutrinos of known flavors.

    “The signal of sterile neutrinos, if exists, can be very subtle and easily confused by fluctuations,” says Yasuhiro Nakajima, Chamberlain Fellow in the Physics Division at the U.S. Department of Energy’s Lawrence Berkeley National Lab (Berkeley Lab) and one of the corresponding authors on the paper. “This investigation required very careful examination of the data. We developed multiple analysis methods and cross checked the analyses in many aspects.”

    Measuring disappearing neutrinos isn’t as strange as it seems. In fact that’s how Daya Bay scientists detect neutrino oscillations. The scientists count how many of the millions of quadrillions of electron antineutrinos produced every second by the six China General Nuclear Power Group reactors are captured by the detectors located in three experimental halls built at varying distances from the reactors. The detectors are only sensitive to electron antineutrinos. Calculations based on the number that disappear along the way to the farthest reactor give them information about how many have changed flavors.

    Photomultiplier tubes in the Daya Bay detectors. Credit: Lawrence Berkeley Nat’l Lab – Roy Kaltschmidt

    The rate at which they transform is the basis for measuring the mixing angles (for example, θ13), and the mass splitting is determined by how the rate of transformation depends on the neutrino energy and the distance between the reactor and the detector.

    That distance is also referred to as the “baseline.” With six detectors strategically positioned at three separate locations to catch antineutrinos generated from the three pairs of reactors, Daya Bay provides a unique opportunity to search for a light sterile neutrino with baselines ranging from 360 meters to 1.8 kilometers.

    Daya Bay performed its first search for a light sterile neutrino using the energy dependence of detected electron antineutrinos from the reactors. Within the searched mass range for a fourth possible mass state, Daya Bay found no evidence for the existence of a sterile neutrino.

    This data represents the best world limit on sterile neutrinos over a wide range of masses and so far supports the standard three-flavor neutrino picture. Given the importance of clarifying the existence of the sterile neutrino, there are continuous quests by many scientists and experiments. The Daya Bay’s new result remarkably narrowed down the unexplored area.

    “We continue to collect a steady stream of data with all eight antineutrino detectors in place,” says Kam-Biu Luk, co-spokesperson for the Daya Bay experiment and senior scientist in Berkeley Lab’s Physics Division and physics professor at the University of California, Berkeley. “This will allow us to hunt for sterile neutrino in an even larger virgin land in the future.”

    See the full article here.

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  • richardmitnick 3:10 pm on August 21, 2013 Permalink | Reply
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    From Berkeley Lab: “New Results from Daya Bay – Tracking the Disappearance of Ghostlike Neutrinos” 

    Berkeley Lab

    Daya Bay neutrino experiment releases high-precision measurement of subatomic shape shifting and new result on differences among neutrino masses

    August 21, 2013
    Lynn Yarris (510) 486-5375 lcyarris@lbl.gov

    “The international Daya Bay Collaboration has announced new results about the transformations of neutrinos – elusive, ghostlike particles that carry invaluable clues about the makeup of the early universe. The latest findings include the collaboration’s first data on how neutrino oscillation – in which neutrinos mix and change into other “flavors,” or types, as they travel – varies with neutrino energy, allowing the measurement of a key difference in neutrino masses known as mass splitting.

    ‘Understanding the subtle details of neutrino oscillations and other properties of these shape-shifting particles may help resolve some of the deepest mysteries of our universe,’ said Jim Siegrist, Associate Director of Science for High Energy Physics at the U.S. Department of Energy (DOE), the primary funder of U.S. participation in Daya Bay.

    U.S. scientists have played essential roles in planning and running of the Daya Bay experiment, which is aimed at filling in the details of neutrino oscillations and mass hierarchy that will give scientists new ways to test for violations of fundamental symmetries. For example, if scientists detect differences in the way neutrinos and antineutrinos oscillate that are beyond expectations, it would be a sign of charge-parity (CP) violation, one of the necessary conditions that resulted in the predominance of matter over antimatter in the early universe. The new results from the Daya Bay experiment about mass-splitting represent an important step towards understanding how neutrinos relate to the structure of our universe today.

    ‘Mass splitting represents the frequency of neutrino oscillation,’ says Kam-Biu Luk of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), the Daya Bay Collaboration’s Co-spokesperson, who identified the ideal site for the experiment. ‘Mixing angles, another measure of oscillation, represent the amplitude. Both are crucial for understanding the nature of neutrinos.’ Luk is a senior scientist in Berkeley Lab’s Physics Division and a professor of physics at the University of California (UC) Berkeley.

    The Daya Bay Collaboration, which includes more than 200 scientists from six regions and countries, is led in the U.S. by DOE’s Berkeley Lab and Brookhaven National Laboratory (BNL). The Daya Bay Experiment is located close to the Daya Bay and Ling Ao nuclear power plants in China, 55 kilometers northeast of Hong Kong. The latest results from the Daya Bay Collaboration will be announced at the XVth International Workshop on Neutrino Factories, Super Beams and Beta Beams in Beijing, China.

    The Daya Bay Neutrino Experiment is designed to provide new understanding of neutrino oscillations that can help answer some of the most mysterious questions about the universe. Shown here are the photomultiplier tubes in the Daya Bay detectors. (Photo by Roy Kaltschmidt)

    ‘These new precision measurements are a great indication that our efforts will pay off with a deeper understanding of the structure of matter and the evolution of the universe – including why we have a universe made of matter at all,’ says Steve Kettell, a Senior Scientist at BNL and U.S. Daya Bay Chief Scientist.”

    See the full article here.

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  • richardmitnick 3:34 pm on March 21, 2012 Permalink | Reply
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    From isgtw: “A new kind of neutrino transformation” 


    Linda Vu
    March 21, 2012

    “Neutrinos, the wispy particles that flooded the universe in the earliest moments after the Big Bang, are continually produced in the hearts of stars and other nuclear reactions. Untouched by electromagnetism, they respond only to the weak nuclear force and even weaker gravity, passing mostly unhindered through everything from planets to people.

    Years ago scientists also discovered another hidden talent of neutrinos. Although they come in three basic “flavors”—electron, muon and tau—neutrinos and their corresponding antineutrinos can transform from one flavor to another while they are traveling close to the speed of light. How they do this has been a long standing mystery.

    But some new, and unprecedentedly precise, measurements from the multinational Daya Bay Neutrino Experiment are revealing how electron antineutrinos “oscillate” into different flavors as they travel. This new finding from Daya Bay opens a gateway to a new understanding of fundamental physics and may eventually solve the riddle of why there is far more ordinary matter than antimatter in the universe today.

    The international collaboration of researchers is made possible by advanced networking and computing facilities. In the U.S., the Department of Energy’s high-speed science network, ESnet, speeds data to the National Energy Research Scientific Computing Center (NERSC) where it is analyzed, stored and made available to researchers via the Web. Both facilities are located at the DOE’s Lawrence Berkeley National Laboratory (Berkeley Lab).”

    Daya Bay Neutrino Facility in China. Photo by: Roy Kaltschmidt, Lawrence Berkeley National Laboratory.

    See the full article here.

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