Particle Physics
The elementary particles and forces, the Standard Model (the accepted theory of them), and what may lie beyond it.
arXiv: hep-ph, hep-ex
16 topics
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Dark sector searches at accelerators
3 problemsThere may be a hidden set of particles that barely interacts with ordinary matter, connected by a weak link such as a dark photon. Accelerators can make such particles and see them decay or disappear.
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Origin of the matter-antimatter asymmetry
3 problemsThe Big Bang should have made equal amounts of matter and antimatter, yet the universe contains almost only matter. The SM cannot produce the observed excess.
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DAMA/LIBRA annual modulation and NaI tests
5 problemsAn experiment in Italy called DAMA/LIBRA has seen its event rate rise and fall once a year for over two decades, as expected if Earth moves through a cloud of dark matter while orbiting the Sun. Other experiments with the same crystal material, sodium iodide, see no such yearly change, so the cause of the DAMA signal is unknown.
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Some very precise electroweak and Higgs-sector measurements have disagreed with Standard Model predictions, which could signal new particles or mistakes in theory or experiment. Each needs to be confirmed or explained.
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Flavor anomalies: CKM unitarity and B decays
4 problemsSeveral measurements of how quarks change type disagree with Standard Model predictions by about three standard deviations. Each could be a sign of new particles or an error in theory or experiment.
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Matter particles come in three copies (generations) with identical charges but masses spread over more than ten powers of ten. Nobody knows why there are three, or why the masses and mixings take their values.
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The strong, weak and electromagnetic forces may be parts of a single force at very high energy. If so, protons should decay, very slowly.
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The Higgs boson weighs about 130 times as much as a proton, yet quantum effects should push its mass up toward far heavier scales such as the Planck mass, about 1e19 proton masses. Something must keep it light, and nobody knows what.
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In the Standard Model every quark and charged lepton gets its mass from the Higgs field, with an interaction strength proportional to its mass. This is confirmed for the three heaviest fermions (top, bottom, tau) but not for the lighter ones that make up ordinary matter.
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The shape of the Higgs field's energy curve decides whether our vacuum is permanent or could one day decay. That shape is set by how strongly the Higgs interacts with itself, which has barely been measured.
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The most sensitive dark matter detectors, cooled crystals and special camera chips called CCDs (charge-coupled devices), all see far more tiny energy deposits than radioactivity or dark matter can explain. Until the cause is found, these events block searches for light dark matter.
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Dark matter searches inside the neutrino fog
4 problemsNeutrinos from the Sun and from cosmic rays hitting the atmosphere can bump atomic nuclei exactly as heavy dark matter particles would. Large xenon detectors have now started to see these neutrino bumps, and the question is how a search can still find dark matter beneath them.
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Neutrinos change type as they travel, which proves they have mass, but we do not know how heavy each is, which is heaviest, or whether a neutrino is its own antiparticle. Their mass needs physics beyond the SM.
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Several experiments saw more or fewer neutrinos than expected over short distances, hinting at a fourth, non-interacting (sterile) neutrino. Newer experiments do not confirm the simplest version, but some excesses remain unexplained.
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Strong CP problem and the QCD axion
4 problemsThe theory of the strong force allows an angle $\theta$ that would make the neutron a tiny electric dipole, but experiments find theta below one ten-billionth. A hypothetical light particle called the axion could explain this.
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Dark matter lighter than a proton would deposit far too little energy to detect by knocking atoms, so new detectors aim to sense single vibrations of a crystal (phonons), single spin waves (magnons) or single electrons. The questions are which of these detectors can work and whether favored models of light dark matter are already ruled out.