Nuclear and Hadronic Physics
Protons, neutrons and atomic nuclei, the strong force that binds them, and the dense matter inside neutron stars.
arXiv: nucl-th, nucl-ex, hep-lat
16 topics
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Nuclear forces and ab initio nuclei from QCD
6 problemsProtons and neutrons attract and repel each other through forces that are leftovers of the quark-gluon interaction. Physicists try to derive these forces from QCD and then compute whole nuclei without fitted shortcuts, called an ab initio (from first principles) calculation.
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If neutrinos are their own antiparticles, some nuclei can change two neutrons into two protons while emitting only two electrons. Converting a measured or bounded rate into a neutrino mass needs a nuclear factor, the matrix element, on which theorists disagree by a factor of two to three.
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EMC effect and short-range correlations
5 problemsQuarks inside a nucleus move differently from quarks inside a free proton or neutron, an effect found by the European Muon Collaboration (EMC) in 1983. Pairs of nucleons that briefly overlap at short distance appear to be connected to it.
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Most strongly interacting particles are three quarks or a quark-antiquark pair, but QCD also allows four- and five-quark states, particles made mostly of gluons (glueballs), and quark-antiquark pairs with an excited gluon field (hybrids). Dozens of candidates have been found since 2003, and their internal structure is disputed.
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Microscopic theory of nuclear fission
6 problemsIn fission a heavy nucleus stretches, forms a neck and snaps into two fragments, and physicists still cannot compute this process from the forces between protons and neutrons alone. The open questions concern how the fragments end up spinning, how the released energy is split between them, and how high the energy barrier to fission is for nuclei never made in the lab.
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Gluon saturation at small momentum fraction
5 problemsInside fast-moving protons and nuclei the number of gluons (the particles that bind quarks) grows rapidly as one looks at gluons carrying smaller fractions of the total momentum. Theory predicts that this growth must slow down when gluons become so dense that they overlap and recombine, a state called the color glass condensate.
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Hypernuclei and strange-baryon interactions
4 problemsA hypernucleus is a nucleus in which one proton or neutron is replaced by a hyperon, a heavier relative containing a strange quark. Their binding energies, together with scattering of antikaons on nucleons, are the main data on how strange particles interact with ordinary nucleons.
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Only some combinations of protons and neutrons form nuclei that hold together: add too many neutrons and the extras fall off, add too many nucleons overall and the nucleus splits apart. Where these edges lie is known only for the lightest elements and is uncertain for the heaviest.
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Experiments that measure how neutrinos change type detect them through collisions with nuclei, such as argon or the oxygen in water. To infer the neutrino energy and type from what comes out, they need a model of these collisions that is much better than current ones.
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A neutron star packs more than the Sun's mass into a ball the size of a city, squeezing matter to several times the density of an atomic nucleus. What this matter is made of, and its equation of state (how pressure grows with density), are unknown above nuclear density.
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A free neutron decays into a proton, an electron and an antineutrino in about 15 minutes, but two measuring methods disagree by about 9 seconds. Other low-energy puzzles include a reported $17\,\mathrm{MeV}$ particle seen in nuclear decays and a small deficit in quark-mixing measurements from nuclear $\beta$ decays.
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Proton spin, mass and charge radius
7 problemsThe proton is made of quarks and gluons, yet the quarks' own spins supply only about a third of its spin and the quark masses only about one percent of its mass. Where the rest comes from, and exactly how large the proton is, are still being measured.
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At zero net baryon density, heating turns ordinary nuclear matter into a quark-gluon plasma smoothly, near $1.8e12\,\mathrm{K}$. At higher densities the change may become abrupt, and mapping where this happens is a central goal of the field.
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Quark-gluon plasma in nuclear collisions
5 problemsCollisions of nuclei at nearly the speed of light briefly create droplets of quark-gluon plasma, a liquid of quarks and gluons that filled the early universe. How it forms so fast, why small collisions seem to make it too, and how it responds to rotation and magnetic fields are not fully understood.
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About half of the elements heavier than iron, including gold and uranium, were built by rapid capture of neutrons in violent events such as neutron-star mergers. Predicting how much of each element forms needs properties of thousands of very neutron-rich nuclei never made in a laboratory.
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Stars make elements through nuclear reactions at energies so low that the reactions are too rare to measure directly in the laboratory, so their rates must be extrapolated. Some elements, such as certain molybdenum and ruthenium isotopes or the products of a medium-speed neutron-capture process, still have no confirmed stellar origin.