Hydrogen: The Key to Unlocking the Atom's Fundamental Building Blocks

Newswise — NEWPORT NEWS, VA – The very first element on the Periodic Table may be the key to solving mysteries hiding inside the atom’s nucleus. Nuclear physicists are comparing different hydrogen nuclei to learn more about its simplest building blocks. New results from such comparisons carried out at DOE’s Thomas Jefferson National Accelerator Facility are providing more detailed information for refining our understanding of how the building blocks of protons and neutrons build different nuclei. Protons and neutrons make up the center of atoms of ordinary matter, the building blocks of our existence. These two particles are both composed of smaller particles called quarks and gluons. While quark-gluon dynamics are still not fully understood, physicists are at least able to probe the proton directly: hydrogen’s nucleus contains one proton, offering an easy source of these positively charged particles. But neutrons are trickier. “A free neutron decays in about 10 minutes, so we don’t have any of those laying around the lab,” said William Henry, a staff scientist at Jefferson Lab. “If we want to study neutrons, we have to get creative with the elements and isotopes available to us.” One method to access the neutron involves using deuterium, an isotope—or variation—of hydrogen whose nucleus contains the typical proton as well as a neutron. Every element is defined by the number of protons it carries in its nucleus. Hydrogen comes first on the Periodic Table, because it carries just one proton. But it can also carry neutrons. Samples of the same element whose nuclei contain the same number of protons but a different number of neutrons are called isotopes. The nucleus of the simplest hydrogen isotope contains only a single proton and is called protium – originating from the Greek word prôtos, meaning first. Deuterium is the next-simplest nucleus. It is also a hydrogen isotope that contains a single proton and also a neutron. Its name also originates in ancient Greek, from the word for second: deúteros. Nuclear physicists often refer to the deuterium nucleus as a deuteron. An experiment at Jefferson Lab recently measured scattering probabilities from the nuclei of protium and deuterium – the proton and deuteron. The first results from this experiment, recently published in Physical Review Letters, will help theorists refine their models of quark distributions in the neutron and proton, as well as aid in other experiments pursuing the neutron. In particular, these measurements will help determine the probabilities of scattering from a down quark relative to an up quark as a function of the quark momentum. “These data really complement the world dataset through which we want to understand the nuclear structure as a whole,” said Henry, who led the analysis. A precise, revealing ratio The experiment was carried out in the Jefferson Lab’s Continuous Electron Beam Accelerator Facility (CEBAF). CEBAF is an Office of Science user facility that supports the research of more than 1,650 nuclear physicists worldwide. In the experiment, energetic electrons from CEBAF were aimed at proton and deuteron targets. A detector called the Super High Momentum Spectrometer (SHMS) measured the post-collision properties of the electrons. From these properties, the team extracted a ratio: the probability that the electron scattered from the deuteron at a certain angle and with a certain energy to the probability that the electron scattered from the proton at a certain angle and energy. These probabilities, known as cross sections, describe the likelihood that a certain interaction will take place between two particles. These cross sections can also inform nuclear physicists about what’s going on inside the building blocks of these nuclei. Differences in the probability that an electron will interact with each can be related back to the distribution of the building blocks of protons and neutrons, their quarks and gluons. Quantum Chromodynamics (QCD) is the theory that nuclear physicists use to describe how quarks and gluons interact. And certain QCD models have been developed to describe the distribution of quarks and gluons inside neutrons and protons. For instance, protons, which contain two up quarks and one down quark, behave and have different characteristics compared to neutrons, which have two down quarks and just one up quark. The deuteron to proton cross section ratio provided access to the neutron to proton ratio, and therefore the down quark to up quark ratio. This new measurement provides the most precise proton-deuteron cross section ratio to date in the kinematic region dominated by scattering from a single quark (the valence quark region). These first results will help theorists refine their models of quark distribution in the neutron and proton, as well as aid in other experiments pursuing the neutron. One immediate application of these results is that they will enable more refined QCD models of quark-gluon dynamics inside nuclei. “Uncertainties on previous measurements of this type were between 10 and 20 percent,” Henry said. “From these data, the uncertainty is less than 5 percent.” Further, the energetic electrons from CEBAF, which provides the world’s highest intensity electron beam, allowed the team to push the data to a higher kinematic region, which describes the energy carried by different particles in the system. The new data are already being added to existing knowledge and expanding the envelope for the entire field. “These data really complement the world dataset through which we want to understand the nuclear structure as a whole,” said Henry, who led the analysis. One dataset, many applications Further analyses with the new data may unlock discoveries about the structures of neutrons and other particles in the future. For instance, the new dataset is useful for other experiments exploring the building blocks of matter. “One of the important things about this experiment is that we took the data in an increased kinematic range, which means many people can use this data for different things,” said Debaditya Biswas, a postdoc at Virginia Tech. Biswas worked on this experiment for his thesis project as a Ph.D. student at Hampton University. Abel Sun, a student at Carnegie Mellon University, was also involved in the analysis. With others, Biswas plans to use the expanded range of the data to study quark-hadron duality, a phenomenon in which the behavior of particles like protons and neutrons can be described using either their fundamental constituents, quarks and gluons, or by the composite particles they form (known as hadrons). “It’s also important for things like calculations of backgrounds from QCD processes at the Large Hadron Collider, or LHC, and other studies seeking to understand this theory,” said Eric Christy, a staff scientist at Jefferson Lab in the Radiation Detector and Imaging Group and spokesperson for this experiment. Putting effects into perspective for neutrons Knowing the wide impact this new dataset could potentially have on other experiments, the researchers conducting the analysis intentionally worked closely with other groups studying similar mysteries, including the EMC Effect collaboration and the BONuS12 and nuclei">MARATHON experiments. Ultimately, it’s hoped that the synergy among all of these experiments will add to our understanding of the structure of the neutron and matter at the smallest scales. Further Reading MARATHON Measures Mirror Nuclei Postdoc Takes Multipronged Approach to Muon Detection What’s Going on in the Nucleon -end- Jefferson Science Associates, LLC, manages and operates the Thomas Jefferson National Accelerator Facility, or Jefferson Lab, for the U.S. Department of Energy's Office of Science. JSA is a wholly owned subsidiary of the Southeastern Universities Research Association, Inc. (SURA).