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    • 2018-05-08 16:30:15
    • Article ID: 694237

    The Weak Side of the Proton

    Precision measurement of the proton's weak charge narrows the search for new physics.

    • Credit: DOE's Jefferson Lab

      The Q-weak experiment was conducted in Jefferson Lab's Experimental Hall C, and its goal was to very precisely measure the proton's weak charge, a term that quantifies the influence that the weak force can exert on protons. The Q-weak apparatus, shown here, was installed in the hall for the experimental run, which concluded in 2012.

    A new result from the Q-weak experiment at the Department of Energy’s Thomas Jefferson National Accelerator Facility provides a precision test of the weak force, one of four fundamental forces in nature. This result, published recently in Nature, also constrains possibilities for new particles and forces beyond our present knowledge.

    “Precision measurements like this one can act as windows into a world of potential new particles that otherwise might only be observable using extremely high-energy accelerators that are currently beyond the reach of our technical capabilities,” said Roger Carlini, a Jefferson Lab scientist and a co-spokesperson for the Q-weak Collaboration.

    While the weak force is difficult to observe directly, its influence can be felt in our everyday world. For example, it initiates the chain of reactions that power the sun and it provides a mechanism for radioactive decays that partially heat the Earth’s core and that also enable doctors to detect disease inside the body without surgery.

    Now, the Q-weak Collaboration has revealed one of the weak force’s secrets: the precise strength of its grip on the proton. They did this by measuring the proton’s weak charge to high precision, which they probed using the high-quality beams available at the Continuous Electron Beam Accelerator Facility, a DOE Office of Science User Facility.

    The proton’s weak charge is analogous to its more familiar electric charge, a measure of the influence the proton experiences from the electromagnetic force. These two interactions are closely related in the Standard Model, a highly successful theory that describes the electromagnetic and weak forces as two different aspects of a single force that interacts with subatomic particles.

    To measure the proton’s weak charge, an intense beam of electrons was directed onto a target containing cold liquid hydrogen, and the electrons scattered from this target were detected in a precise, custom-built measuring apparatus. The key to the Q-weak experiment is that the electrons in the beam were highly polarized – prepared prior to acceleration to be mostly “spinning” in one direction, parallel or anti-parallel to the beam direction. With the direction of polarization rapidly reversed in a controlled manner, the experimenters were able to latch onto the weak interaction’s unique property of parity (akin to mirror symmetry) violation, in order to isolate its tiny effects to high precision: a different scattering rate by about 2 parts in 10 million was measured for the two beam polarization states.

    The proton’s weak charge was found to be QWp=0.0719±0.0045, which turns out to be in excellent agreement with predictions of the Standard Model, which takes into account all known subatomic particles and the forces that act on them. Because the proton’s weak charge is so precisely predicted in this model, the new Q-weak result provides insight into predictions of hitherto unobserved heavy particles, such as those that may be produced by the Large Hadron Collider (LHC) at CERN in Europe or future high energy particle accelerators.  

    “This very challenging experimental result is yet another clue in the world-wide search for new physics beyond our current understanding. There is ample evidence the Standard Model of Particle physics provides only an incomplete description of nature’s phenomena, but where the breakthrough will come remains elusive,” said Timothy J. Hallman, Associate Director for Nuclear Physics of the Department of Energy Office of Science. “Experiments like Q-weak are pressing ever closer to finding the answer.”

    For example, the Q-weak result has set limits on the possible existence of leptoquarks, which are hypothetical particles that can reverse the identities of two broad classes of very different fundamental particles – turning quarks (the building blocks of nuclear matter) into leptons (electrons and their heavier counterparts) and vice versa.

    "After more than a decade of careful work, Q-weak not only informed the Standard Model, it showed that extreme precision can enable moderate-energy experiments to achieve results on par with the largest accelerators available to science," said Anne Kinney, Assistant Director for the Mathematical and Physical Sciences Directorate at the National Science Foundation. "Such precision will be important in the hunt for physics beyond the Standard Model, where new particle effects would likely appear as extremely tiny deviations. 

    “It’s complementary information. So, if they find evidence for new physics in the future at the LHC, we can help identify what it might be, from the limits that we’re setting already in this paper,” said Greg Smith, Jefferson Lab scientist and Q-weak project manager.

    The Q-weak Collaboration consists of about 100 scientists and more than 20 institutions. The experiment was funded by the U.S. Department of Energy Office of Science, the National Science Foundation, the Natural Sciences and Engineering Research Council of Canada, and the Canadian Foundation for Innovation, with matching and in-kind contributions from a number of the collaborating institutions.

    Contact: Kandice Carter, Jefferson Lab Communications Office, 757-269-7263, kcarter@jlab.org

                                                                -end-

    Jefferson Science Associates, LLC, a joint venture of the Southeastern Universities Research Association, Inc. and PAE, manages and operates the Thomas Jefferson National Accelerator Facility, or Jefferson Lab, for the U.S. Department of Energy's Office of Science.

    Jefferson Lab is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

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    Science Up-Close: Developing a Cookbook for Efficient Fusion Energy

    To develop a future fusion reactor, scientists need to understand how and why plasma in fusion experiments moves into a "high-confinement mode" where particles and heat can't escape. Scientists at the Department of Energy's Princeton Plasma Physics Laboratory simulated the transition into that mode starting from the most basic physics principles.

    Peering into the Mist: How Water Vapor Changes Metal at the Atomic Level

    New insights into molecular-level processes could help prevent corrosion and improve catalytic conversion.

    Neutron science publications reach new highs at ORNL's flagship facilities

    The High Flux Isotope Reactor and the Spallation Neutron Source at the Department of Energy's Oak Ridge National Laboratory have reached new levels of increased science productivity. In 2018, a record high of more than 500 scientific instrument publications were produced between HFIR and SNS--based on neutron beamline experiments conducted by more than 1,200 US and international researchers who used the world-leading facilities.

    Fiery sighting: A new physics of eruptions that damage fusion experiments

    Feature describes first direct sighting of a trigger for bursts of heat that can disrupt fusion reactions.

    Microbial Types May Prove Key to Gas Releases from Thawing Permafrost

    Scientists discover key types of microbes that degrade organic matter and release carbon dioxide and methane into the atmosphere.

    An effect that Einstein helped discover 100 years ago offers new insight into a puzzling magnetic phenomenon

    Experiments at the Department of Energy's SLAC National Accelerator Laboratory have seen for the first time what happens when magnetic materials are demagnetized at ultrafast speeds of millionths of a billionth of a second: The atoms on the surface of the material move, much like the iron bar did. The work, done at SLAC's Linac Coherent Light Source (LCLS) X-ray laser, was published in Nature earlier this month.

    Found: A precise method for determining how waves and particles affect fusion reactions

    Like surfers catching ocean waves, particles within plasma can ride waves oscillating through the plasma during fusion energy experiments. Now a team of physicists led by PPPL has devised a faster method to determine how much this interaction contributes to efficiency loss in tokamaks.

    Discovery adapts natural membrane to make hydrogen fuel from water

    In a recent study from the U.S. Department of Energy's (DOE) Argonne National Laboratory, scientists have combined two membrane-bound protein complexes to perform a complete conversion of water molecules to hydrogen and oxygen.

    How Plants Regulate Sugar Deposition in Cell Walls

    Identified genes involved in plant cell wall polysaccharide production and restructuring could aid in engineering bioenergy crops.

    Scientists Identify Gene Cluster in Budding Yeasts with Major Implications for Renewable Energy

    How yeast partition carbon into a metabolite may offer insights into boosting production for biofuels.


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    Top 10 Discoveries of 2018

    Every year, the U.S. Department of Energy's (DOE) Brookhaven National Laboratory compiles a list of the biggest advances made by the Lab's staff scientists, engineers, and visiting researchers. From uncovering mysteries of the universe to building better batteries, here, in no particular order, are our picks for the top 10 discoveries of 2018.

    U.S. Department of Energy Announces $33 Million for Small Business Research and Development Grants

    The U.S. Department of Energy announced it will award 189 grants totaling $33 million to 149 small businesses in 32 states.

    DOE to Provide $16 Million for New Research into Atmospheric and Terrestrial Processes

    The U.S. Department of Energy (DOE) announced a plan to provide $16 million for new observational research aimed at improving the accuracy of today's climate and earth system models.

    Machine learning award powers Argonne leadership in engine design

    When attempting to design engines to be more fuel-efficient and emissions-free, automotive manufacturers have to take into account all the complexity inherent in the combustion process.

    ORNL partners with industry to address multiple nuclear technology challenges

    The Department of Energy's Oak Ridge National Laboratory is collaborating with industry on six new projects focused on advancing commercial nuclear energy technologies that offer potential improvements to current nuclear reactors and move new reactor designs closer to deployment.

    Lithium earns honors for three physicists working to bring the energy that powers the sun to Earth

    Feature describes research of three PPPL physicists who have won the laboratory's 2018 outstanding research awards

    DOE approves technical plan and cost estimate to upgrade Argonne facility; Project will create X-rays that illuminate the atomic scale, in 3D

    The U.S. Department of Energy has approved the technical scope, cost estimate and plan of work for an upgrade of the Advanced Photon Source, a major storage-ring X-ray source at Argonne.

    Costas Soukoulis elected to National Academy of Inventors

    Costas Soukoulis, Ames Laboratory senior scientist and Iowa State University Frances M. Craig Endowed Chair and Distinguished Professor, has been named as a 2018 National Academy of Inventors (NAI) Fellow.

    Biophysicist F. William Studier Elected Fellow of the National Academy of Inventors

    F. William Studier, a Senior Biophysicist Emeritus at the U.S. Department of Energy's Brookhaven National Laboratory and Adjunct Professor of Biochemistry at Stony Brook University, has been elected as a Fellow of the National Academy of Inventors (NAI). He is among 148 renowned academic inventors being recognized by NAI for 2018.

    Blast to the future

    A grant from DOE's Technology Commercialization Fund will help researchers at Argonne and industry partners seek improvements to U.S. manufacturing by making discovery and design of new materials more efficient.


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    Rapid Lake Draining on Ice Sheets Changes How Water Moves in Unexpected Ways

    Widespread fracturing during lake drainage triggers vertical shafts to form that affect the Greenland Ice Sheet.

    New Historical Emissions Trends Estimated with the Community Emissions Data System

    The data system will allow for more detailed, consistent, and up-to-date global emissions trends that will aid in understanding aerosol effects.

    Peering into the Mist: How Water Vapor Changes Metal at the Atomic Level

    New insights into molecular-level processes could help prevent corrosion and improve catalytic conversion.

    Microbial Types May Prove Key to Gas Releases from Thawing Permafrost

    Scientists discover key types of microbes that degrade organic matter and release carbon dioxide and methane into the atmosphere.

    New Method Knocks Out Yeast Genes with Single-Point Precision

    Researchers can precisely study how different genes affect key properties in a yeast used industrially to produce fuel and chemicals.

    How Plants Regulate Sugar Deposition in Cell Walls

    Identified genes involved in plant cell wall polysaccharide production and restructuring could aid in engineering bioenergy crops.

    Scientists Identify Gene Cluster in Budding Yeasts with Major Implications for Renewable Energy

    How yeast partition carbon into a metabolite may offer insights into boosting production for biofuels.

    More Designer Peptides, More Possibilities

    A combined experimental and modeling approach contributes to understanding small proteins with potential use in industrial, therapeutic applications.

    Deep Learning for Electron Microscopy

    Artificial intelligence on Summit to discover atomic-scale structures.

    Clarifying Rates of Methylmercury Production

    New model provides more accurate estimates of how fast microbes produce a mercury-based neurotoxin.


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