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History of subatomic physics
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<h2 id="early-development">Early development</h2> <p class="note">Main articles: <a href="/facts/Atomism/8RMrHd5T">Atomism</a> and <a href="/facts/Atomic_theory/B2WvuLf6">Atomic theory</a></p> <p>The idea that all <a href="/facts/Matter/osyc04UN">matter</a> is composed of <a href="/facts/Elementary_particle/CTFDM97U">elementary particles</a> dates to as far as the 6th century BCE.<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a> The <a href="/facts/Jains/h0glxhjH">Jains</a> in ancient India were the earliest to advocate the particular nature of material objects between 9th and 5th century BCE. According to Jain leaders like <a href="/facts/Parshvanatha/8dVsg0Cf">Parshvanatha</a> and <a href="/facts/Mahavira/46A5AGc3">Mahavira</a>, the <a href="/facts/Ajiva/juDngnyS">ajiva</a> (non living part of universe) consists of matter or <i>pudgala</i>, of definite or indefinite shape which is made up tiny uncountable and invisible particles called <i>permanu</i>. <i>Permanu</i> occupies space-point and each <i>permanu</i> has definite colour, smell, taste and texture. Infinite varieties of <i>permanu</i> unite and form <i>pudgala</i>.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a> The philosophical doctrine of <a href="/facts/Atomism/8RMrHd5T">atomism</a> and the nature of elementary particles were also studied by <a href="/facts/Ancient_Greek_philosophers/LWEkp8JX">ancient Greek philosophers</a> such as <a href="/facts/Leucippus/T3qkY5Pj">Leucippus</a>, <a href="/facts/Democritus/yTzsod3v">Democritus</a>, and <a href="/facts/Epicurus/ww2teSaQ">Epicurus</a>; ancient <a href="/facts/Indian_philosophers/ddnT7Vec">Indian philosophers</a> such as <a href="/facts/Kanada_(philosopher)/DBk9j8R0">Kanada</a>, <a href="/facts/Dign%25C4%2581ga/RyYCgEfR">Dignāga</a>, and <a href="/facts/Dharmakirti/yhzVdcUK">Dharmakirti</a>; Muslim scientists such as <a href="/facts/Alhazen/Q5H9hm4J">Ibn al-Haytham</a>, <a href="/facts/Avicenna/EYKG4Vez">Ibn Sina</a>, and <a href="/facts/Al-Ghazali/haPCCftt">Mohammad al-Ghazali</a>; and in <a href="/facts/Early_modern_Europe/aeYvfAqx">early modern Europe</a> by physicists such as <a href="/facts/Pierre_Gassendi/2RFS2cpp">Pierre Gassendi</a>, <a href="/facts/Robert_Boyle/6tUs448C">Robert Boyle</a>, and <a href="/facts/Isaac_Newton/4L7gTncN">Isaac Newton</a>. The particle theory of <a href="/facts/Light/jeofpMn4">light</a> was also proposed by <a href="/facts/Alhazen/Q5H9hm4J">Ibn al-Haytham</a>, <a href="/facts/Avicenna/EYKG4Vez">Ibn Sina</a>, Gassendi, and Newton. </p><p>Those early ideas were founded through <a href="/facts/Abstraction/p7l9eHGd">abstract</a>, <a href="/facts/Philosophical/zAH5PVD6">philosophical</a> reasoning rather than <a href="/facts/Experimentation/esltM54f">experimentation</a> and <a href="/facts/Empirical_observation/A6cy8Rcy">empirical observation</a> and represented only one line of thought among many. In contrast, certain ideas of <a href="/facts/Gottfried_Wilhelm_Leibniz/cTc2er83">Gottfried Wilhelm Leibniz</a> (see <i><a href="/facts/Monadology/QJPjaptQ">Monadology</a></i>) contradict to almost everything known in modern physics. </p><p>In the 19th century, <a href="/facts/John_Dalton/oS8grQR4">John Dalton</a>, through his work on <a href="/facts/Stoichiometry/I5p9cJzV">stoichiometry</a>, concluded that each chemical element was composed of a single, unique type of particle. Dalton and his contemporaries believed those were the fundamental particles of nature and thus named them atoms, after the Greek word <i>atomos</i>, meaning "indivisible"<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> or "uncut". However, near the end of 19th century, physicists discovered that Dalton's atoms are not, in fact, the fundamental particles of nature, but conglomerates of even smaller particles. </p> <h2 id="from-atoms-to-nucleons">From atoms to nucleons</h2> <h3>The state of electromagnetic theory</h3> <p class="note">Main article: <a href="/facts/History_of_electromagnetism/D5SAJLe1">History of electromagnetism</a></p> <p>Throughout the 1800s scientists explored many phenomena of electricity and magnetism, culminating in an accurate theory by <a href="/facts/James_Clerk_Maxwell/jtYyze61">James Clerk Maxwell</a>.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> This theory was a continuous field model developed around the ideas of <a href="/facts/Luminiferous_aether/UzRf78a2">luminiferous aether</a>. When no experiment could produce evidence of such an ether, and in view of the growing evidence supporting the atomic model, <a href="/facts/Hendrik_Antoon_Lorentz/JbBpKFyr">Hendrik Antoon Lorentz</a> developed a theory of electromagnetism based on "ions" that reproduced Maxwell's model.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a>: 77 </p> <h3>The Electron</h3> <p>The <a href="/facts/Electron/L0KBo5cW">electron</a> was discovered between 1879 and 1897 in works of <a href="/facts/William_Crookes/q8U8VVwR">William Crookes</a>, <a href="/facts/Arthur_Schuster/d1Uzvlp9">Arthur Schuster</a>, <a href="/facts/J._J._Thomson/Y4jyndJF">J. J. Thomson</a>, and other physicists; its charge was carefully measured by <a href="/facts/Robert_Andrews_Millikan/rrWMCh2y">Robert Andrews Millikan</a> and <a href="/facts/Harvey_Fletcher/d6sdAFCs">Harvey Fletcher</a> in their <a href="/facts/Oil_drop_experiment/C2BBYkKh">oil drop experiment</a> of 1909. Physicists theorized that <a href="/facts/Electric_charge/xxsBfmlB">negatively charged</a> electrons are constituent part of "<a href="/facts/Atom/69syB8dH">atoms</a>", along with some (yet unknown) positively charged substance, and it was later confirmed. Electron became the first elementary, truly fundamental particle discovered. </p> <h3>Radioactivity</h3> <p>Studies of the "radioactivity", that soon revealed the phenomenon of <a href="/facts/Radioactive_decay/Ya6FVjt4">radioactive decay</a>, provided another argument against considering <a href="/facts/Chemical_element/0jducgWD">chemical elements</a> as fundamental nature's elements. Despite these discoveries, the term <i>atom</i> stuck to Dalton's (chemical) atoms and now denotes the smallest particle of a chemical element, not something really indivisible. </p> <h3>Researching particles' interaction</h3> <p class="note">Further information: <a href="/facts/Rutherford_scattering/pDrXNfmK">Rutherford scattering</a></p> <p>Early 20th-century physicists knew only two <a href="/facts/Fundamental_force/WrFEJaFl">fundamental forces</a>: <a href="/facts/Electromagnetism/oAkmALHT">electromagnetism</a> and <a href="/facts/Gravitation/bqAN7DdQ">gravitation</a>, where the latter could not explain the structure of atoms. So, it was obvious to assume that unknown positively charged substance attracts electrons by <a href="/facts/Coulomb_force/V0lR5NKv">Coulomb force</a>. </p> <p>In 1909 <a href="/facts/Ernest_Rutherford/hjjMEskl">Ernest Rutherford</a> and <a href="/facts/Thomas_Royds/hzPVpsqW">Thomas Royds</a> demonstrated that an <a href="/facts/Alpha_particle/dzNMs9WP">alpha particle</a> combines with two electrons and forms a <a href="/facts/Helium/yRul93TG">helium</a> atom. In modern terms, alpha particles are doubly <a href="/facts/Ion/wrbwhF3z">ionized</a> helium (more precisely, <a href="/facts/Helium-4/vNsL5qhC">4He</a>) atoms. Speculation about the structure of atoms was severely constrained by Rutherford's 1907 <a href="/facts/Geiger%25E2%2580%2593Marsden_experiment/cmgI1Ha9">gold foil experiment</a>, showing that the atom is mainly empty space, with almost all its mass concentrated in a tiny <a href="/facts/Atomic_nucleus/orpIm5Xl">atomic nucleus</a>. </p> <h3>Inside the atom</h3> <p>By 1914, experiments by Ernest Rutherford, <a href="/facts/Henry_Moseley/W6Kd359o">Henry Moseley</a>, <a href="/facts/James_Franck/SooVoCUz">James Franck</a> and <a href="/facts/Gustav_Ludwig_Hertz/uudggEgb">Gustav Hertz</a> had largely established the structure of an atom as a dense nucleus of positive charge surrounded by lower-mass electrons.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> These discoveries shed a light to the nature of <a href="/facts/Radioactive_decay/Ya6FVjt4">radioactive decay</a> and other forms of <a href="/facts/Nuclear_transmutation/MjlvjsV6">transmutation</a> of elements, as well as of elements themselves. It appeared that <a href="/facts/Atomic_number/bgAr581S">atomic number</a> is nothing else than (positive) <a href="/facts/Electric_charge/xxsBfmlB">electric charge</a> of the atomic nucleus of a particular atom. Chemical transformations, governed by <a href="/facts/Electromagnetic_interaction/oAkmALHT">electromagnetic interactions</a>, do not change nuclei – that's why elements are chemically indestructible. But when the nucleus change its charge and/or mass (by emitting or capturing a <a href="/facts/Particle/JHKr2w6h">particle</a>), the atom can become the one of another element. <a href="/facts/Special_relativity/vsFFc63E">Special relativity</a> explained how the <i><a href="/facts/Mass_defect/2Q3poXgg">mass defect</a></i> is related to the <a href="/facts/Energy/wd8PrQM7">energy</a> produced or consumed in reactions. The branch of physics that studies transformations and the structure of nuclei is now called <a href="/facts/Nuclear_physics/fzQbt1aF">nuclear physics</a>, contrasted to <a href="/facts/Atomic_physics/7Jlm4I7k">atomic physics</a> that studies the structure and properties of atoms ignoring most nuclear aspects. The development in the nascent <a href="/facts/Quantum_physics/BRc3Mzgr">quantum physics</a>, such as <a href="/facts/Bohr_model/FmTHjaxy">Bohr model</a>, led to the understanding of <a href="/facts/Chemistry/SrNqkGVm">chemistry</a> in terms of the arrangement of electrons in the mostly empty volume of atoms. </p> <p>In 1918, Rutherford confirmed that the <a href="/facts/Hydrogen/BoYHjl0J">hydrogen</a> nucleus was a particle with a positive charge, which he named the <a href="/facts/Proton/clCjepXP">proton</a>. By then, <a href="/facts/Frederick_Soddy/gR9Qk6tc">Frederick Soddy</a>'s researches of radioactive elements, and experiments of J. J. Thomson and <a href="/facts/F.W._Aston/DVoj9ebB">F.W. Aston</a> conclusively demonstrated existence of <a href="/facts/Isotope/KD9B1y6o">isotopes</a>, whose nuclei have different masses in spite of identical atomic numbers. It prompted Rutherford to conjecture that all nuclei other than hydrogen contain chargeless particles, which he named the <a href="/facts/Neutron/oRCM1geb">neutron</a>. Evidences that atomic nuclei consist of some smaller particles (now called <i><a href="/facts/Nucleon/6fguU9Xk">nucleons</a></i>) grew; it became obvious that, while protons repulse each other <a href="/facts/Electrostatics/bxkuW6s4">electrostatically</a>, nucleons attract each other by some new force (<a href="/facts/Nuclear_force/A2MyJoWE">nuclear force</a>). It culminated in proofs of <a href="/facts/Nuclear_fission/LfkIylvm">nuclear fission</a> in 1939 by <a href="/facts/Lise_Meitner/BH51ypJ2">Lise Meitner</a> (based on experiments by <a href="/facts/Otto_Hahn/7alx1Db7">Otto Hahn</a>), and <a href="/facts/Nuclear_fusion/hP0gLDqj">nuclear fusion</a> by <a href="/facts/Hans_Bethe/X6bkxRRI">Hans Bethe</a> in that same year. Those discoveries gave rise to an active industry of generating one atom from another, even rendering possible (although it will probably never be profitable) the <a href="/facts/Chrysopoeia/bNAzlDqf">transmutation of lead into gold</a>; and, those same discoveries also led to the development of <a href="/facts/Nuclear_weapon/QVdBcMdm">nuclear weapons</a>. </p> <h2 id="revelations-of-quantum-mechanics">Revelations of quantum mechanics</h2> <p class="note">Main article: <a href="/facts/History_of_quantum_mechanics/Xv77KBJM">History of quantum mechanics</a></p> <p>Further understanding of atomic and nuclear structures became impossible without improving the knowledge about the essence of particles. Experiments and improved theories (such as <a href="/facts/Erwin_Schr%25C3%25B6dinger/wCprepF2">Erwin Schrödinger</a>'s "electron waves") gradually revealed that there is <a href="/facts/Wave%25E2%2580%2593particle_duality/Au3UlcwQ">no fundamental difference</a> between particles and <a href="/facts/Wave/3ANn7WXS">waves</a>. For example, electromagnetic waves were reformulated in terms of particles called <i><a href="/facts/Photon/mFNhAEzA">photons</a></i>. It also revealed that physical objects do not change their parameters, such as <a href="/facts/Total_energy/wd8PrQM7">total energy</a>, <a href="/facts/Position_(vector)/Tpk5d9f2">position</a> and <a href="/facts/Momentum/TTkGv5eY">momentum</a>, as <a href="/facts/Continuous_function/sKbl02pB">continuous functions</a> of <a href="/facts/Time/3ShdpqLx">time</a>, as it was thought of in classical physics: see <a href="/facts/Atomic_electron_transition/HeaF4JJk">atomic electron transition</a> for example. </p><p>Another crucial discovery was <a href="/facts/Identical_particles/qg0mHeVR">identical particles</a> or, more generally, quantum <a href="/facts/Particle_statistics/hJaSBe1l">particle statistics</a>. It was established that all electrons are identical: although two or more electrons can exist simultaneously that have different parameters, but they do not keep separate, distinguishable histories. This also applies to protons, neutrons, and (with certain differences) to photons as well. It suggested that there is a limited number of sorts of smallest particles in the <a href="/facts/Universe/lfoqc5m6">universe</a>. </p><p>The <a href="/facts/Spin%25E2%2580%2593statistics_theorem/j2qqVpez">spin–statistics theorem</a> established that any particle in our <a href="/facts/Spacetime/uRIN1x4b">spacetime</a> may be either a <a href="/facts/Boson/XtPFcjb5">boson</a> (that means its statistics is <a href="/facts/Bose%25E2%2580%2593Einstein_statistics/Utb2jN3n">Bose–Einstein</a>) or a <a href="/facts/Fermion/LEckdDsR">fermion</a> (that means its statistics is <a href="/facts/Fermi%25E2%2580%2593Dirac_statistics/aLo4bton">Fermi–Dirac</a>). It was later found that all fundamental bosons transmit forces, like the photon that transmits light. Some of non-fundamental bosons (namely, <a href="/facts/Meson/9R0kIc1q">mesons</a>) also may transmit forces (see below), although non-fundamental ones. Fermions are particles "like electrons and nucleons" and generally comprise the matter. Note that any subatomic or atomic particle composed of <a href="/facts/Even_number/7zq2UM4V">even</a> <i>total</i> number of fermions (such as protons, neutrons, and electrons) is a boson, so a boson is not necessarily a force transmitter and perfectly can be an ordinary material particle. </p><p>The <a href="/facts/Spin_(physics)/4Jvp7e8P">spin</a> is the quantity that distinguishes bosons and fermions. Practically it appears as an intrinsic <a href="/facts/Angular_momentum/7TuSVFxq">angular momentum</a> of a particle, that is unrelated to its <a href="/facts/Motion_(physics)/dCnP7MlB">motion</a> but is linked with some other features like a <a href="/facts/Magnetic_dipole/Ny0TVUSX">magnetic dipole</a>. Theoretically it is explained from different types <a href="/facts/Representation_theory_of_the_Lorentz_group/XJRuPVkL">representations of symmetry groups</a>, namely <a href="/facts/Tensor/pNjFo5tV">tensor</a> representations (including vectors and scalars) for bosons with their integer (in <a href="/facts/Reduced_Planck_constant/qGHAe3qq">ħ</a>) spins, and <a href="/facts/Spinor/sKFNnEwP">spinor</a> representations for fermions with their <a href="/facts/Half-integer/oJ29mf8G">half-integer</a> spins. </p><p>Improved understanding of the world of particles prompted physicists to make bold predictions, such as <a href="/facts/Paul_Dirac/6bt5yRdk">Dirac</a>'s <a href="/facts/Positron/HgV4zBzA">positron</a> in 1928 (founded on the <a href="/facts/Dirac_Sea/x40KMcu3">Dirac Sea</a> model) and <a href="/facts/Wolfgang_Pauli/Ugk4ubXk">Pauli</a>'s <a href="/facts/Neutrino/yGWVfSg2">neutrino</a> in 1930 (founded on conservation of energy and angular momentum in <a href="/facts/Beta_decay/vxTBlckp">beta decay</a>). Both were later confirmed. </p><p>This culminated in the formulation of ideas of a <a href="/facts/Quantum_field_theory/VoewsNJV">quantum field theory</a>. The first (and the only mathematically complete) of these theories, <a href="/facts/Quantum_electrodynamics/zPteAr6G">quantum electrodynamics</a>, allowed to explain thoroughly the structure of atoms, including the <a href="/facts/Periodic_Table/umdGIfUb">Periodic Table</a> and <a href="/facts/Atomic_spectra/fxumR717">atomic spectra</a>. Ideas of <a href="/facts/Quantum_mechanics/BRc3Mzgr">quantum mechanics</a> and quantum field theory were applied to nuclear physics too. For example, <a href="/facts/Alpha_decay/rCsHNIUE">α decay</a> was explained as a <a href="/facts/Quantum_tunneling/WU4NMrKx">quantum tunneling</a> through nuclear potential, nucleons' fermionic statistics explained the <a href="/facts/Nucleon_pairing/x1A0CaB9">nucleon pairing</a>, and <a href="/facts/Hideki_Yukawa/Rz9vGmu3">Hideki Yukawa</a> proposed certain <a href="/facts/Virtual_particle/CfHFkVL3">virtual particles</a> (now knows as <a href="/facts/Pion/x5HamzG3">π-mesons</a>) as an explanation of the nuclear force. </p> <h2 id="from-nuclides-to-nuclear-engineering">From nuclides to nuclear engineering</h2> <p class="note">Further information: <a href="/facts/History_of_nuclear_power/WzqESEwd">History of nuclear power</a>, <a href="/facts/Synthetic_element/azU9Nr1m">Synthetic element</a>, and <a href="/facts/History_of_nuclear_weapons/KClpn4Pp">History of nuclear weapons</a></p> <p>Development of <a href="/facts/Nuclear_model/orpIm5Xl">nuclear models</a> (such as the <a href="/facts/Liquid-drop_model/XQhTYz7e">liquid-drop model</a> and <a href="/facts/Nuclear_shell_model/1Qgep495">nuclear shell model</a>) made prediction of properties of <a href="/facts/Nuclide/97Ykvzr3">nuclides</a> possible. No existing model of nucleon–nucleon interaction can <i>analytically</i> compute something more complex than 4He based on principles of quantum mechanics, though (note that complete computation of <a href="/facts/Electron_shell/QxI30eh9">electron shells</a> in atoms is also impossible as yet). </p><p>The most developed branch of nuclear physics in 1940s was studies related to <a href="/facts/Nuclear_fission/LfkIylvm">nuclear fission</a> due to its military significance. The main focus of fission-related problems is interaction of atomic nuclei with neutrons: a process that occurs in a <a href="/facts/Fission_bomb/QVdBcMdm">fission bomb</a> and a <a href="/facts/Nuclear_fission_reactor/8xqjIbge">nuclear fission reactor</a>. It gradually drifted away from the rest of subatomic physics and virtually became the <a href="/facts/Nuclear_engineering/wu26D25C">nuclear engineering</a>. The first synthesised <a href="/facts/Transuranium_element/a12PwdAw">transuranium elements</a> were also obtained in this context, through <a href="/facts/Neutron_capture/elsFpM12">neutron capture</a> and subsequent <a href="/facts/Beta_decay/vxTBlckp">β− decay</a>. </p><p>The <a href="/facts/Transfermium_elements/a12PwdAw">elements beyond fermium</a> cannot be produced in this way. To make a nuclide with more than 100 protons per nucleus one has to use an inventory and methods of particle physics (see details below), namely to accelerate and collide atomic nuclei. Production of progressively heavier synthetic elements continued into 21st century as a branch of nuclear physics, but only for scientific purposes. </p><p>The third important stream in nuclear physics are researches related to <a href="/facts/Nuclear_fusion/hP0gLDqj">nuclear fusion</a>. This is related to <a href="/facts/Thermonuclear_weapons/T9zFe3H0">thermonuclear weapons</a> (and conceived peaceful <a href="/facts/Thermonuclear_energy/jyETSzm4">thermonuclear energy</a>), as well as to <a href="/facts/Astrophysical/krFPRWLA">astrophysical</a> researches, such as <a href="/facts/Stellar_nucleosynthesis/qEQJdJe6">stellar nucleosynthesis</a> and <a href="/facts/Big_Bang_nucleosynthesis/IvM4gbKH">Big Bang nucleosynthesis</a>. </p> <h2 id="physics-goes-to-high-energies">Physics goes to high energies</h2> <h3>Strange particles and mysteries of the weak interaction</h3> <p class="note">Further information: <a href="/facts/Resonance_(particle_physics)/aBEXCbzT">Resonance (particle physics)</a>, <a href="/facts/Deep_inelastic_scattering/0kB9Leaf">Deep inelastic scattering</a>, and <a href="/facts/Strange_particle/VDWd3ky0">Strange particle</a></p> <p>In the 1950s, with development of <a href="/facts/Particle_accelerator/Pkuc08xC">particle accelerators</a> and studies of <a href="/facts/Cosmic_ray/uxCVdTKd">cosmic rays</a>, <a href="/facts/Inelastic_scattering/nEBTw75h">inelastic scattering</a> experiments on protons (and other atomic nuclei) with energies about hundreds of <a href="/facts/MeV/YMT1fqX9">MeVs</a> became affordable. They created some short-lived <a href="/facts/Resonance_(particle_physics)/aBEXCbzT">resonance "particles"</a>, but also <i><a href="/facts/Hyperon/9B37rDYq">hyperons</a></i> and <i><a href="/facts/Kaon/Yn2w4QlQ">K-mesons</a></i> with unusually long lifetime. The cause of the latter was found in a new quasi-<a href="/facts/Conservation_law_(physics)/6g1Ay2FR">conserved</a> quantity, named <i><a href="/facts/Strangeness/T1kLPfQU">strangeness</a></i>, that is conserved in all circumstances except for the <a href="/facts/Weak_interaction/f6upmKMs">weak interaction</a>. The strangeness of heavy particles and the <a href="/facts/Muon/BP2fLVIR">μ-lepton</a> were first two signs of what is now known as the <a href="/facts/Generation_(particle_physics)/hAbgvUfV">second generation</a> of fundamental particles. </p><p>The weak interaction revealed soon yet another mystery. In 1957 <a href="/facts/Chien-Shiung_Wu/ULvYEPgU">Chien-Shiung Wu</a> proved that it does not <a href="/facts/Parity_(physics)/emSbJ4hD">conserve parity</a>. In other words, the mirror symmetry was disproved as a fundamental <a href="/facts/Symmetry_(physics)/gJmgsooI">symmetry law</a>. </p><p>Throughout the 1950s and 1960s, improvements in particle accelerators and <a href="/facts/Particle_detector/rYj0qKtA">particle detectors</a> led to a bewildering variety of particles found in high-energy experiments. The term <i>elementary particle</i> came to refer to dozens of particles, most of them <a href="/facts/Particle_decay/3koAz34P">unstable</a>. It prompted Wolfgang Pauli's remark: "Had I foreseen this, I would have gone into botany". The entire collection was nicknamed the "<a href="/facts/Particle_zoo/Xxl2PFen">particle zoo</a>". It became evident that some smaller constituents, yet invisible, form <a href="/facts/Meson/9R0kIc1q">mesons</a> and <a href="/facts/Baryon/QFwAUWK2">baryons</a> that counted most of then-known particles. </p> <h3>Deeper constituents of matter</h3> <p class="note">Further information: <a href="/facts/Hadron/C0sd1zty">Hadron</a></p> <p>The interaction of these particles by <a href="/facts/Scattering/nMou7lEH">scattering</a> and <a href="/facts/Particle_decay/3koAz34P">decay</a> provided a key to new fundamental quantum theories. <a href="/facts/Murray_Gell-Mann/7yg75mjR">Murray Gell-Mann</a> and <a href="/facts/Yuval_Ne%2527eman/W5VRPLnp">Yuval Ne'eman</a> brought some order to mesons and baryons, the most numerous classes of particles, by classifying them according to certain qualities. It began with what Gell-Mann referred to as the "<a href="/facts/Eightfold_way_(physics)/xDtTrHK2">Eightfold Way</a>", but proceeding into several different "octets" and "decuplets" which could predict new particles, most famously the <a href="/facts/Omega_baryon/u7lW74Yb">Ω−</a>, which was detected at <a href="/facts/Brookhaven_National_Laboratory/L2JrG0K1">Brookhaven National Laboratory</a> in 1964, and which gave rise to the <i><a href="/facts/Quark/hwuuVkV5">quark</a></i> model of hadron composition. While the <a href="/facts/Quark_model/mmaanHvp">quark model</a> at first seemed inadequate to describe <a href="/facts/Strong_interaction/JjRsKcLg">strong nuclear forces</a>, allowing the temporary rise of competing theories such as the <a href="/facts/S-matrix_theory/pkjVJ3sV">S-matrix theory</a>, the establishment of <a href="/facts/Quantum_chromodynamics/3cA2ctyb">quantum chromodynamics</a> in the 1970s finalized a set of fundamental and exchange particles (Kragh 1999). It postulated the fundamental <a href="/facts/Strong_interaction/JjRsKcLg">strong interaction</a>, experienced by quarks and mediated by <a href="/facts/Gluon/t6YH8I21">gluons</a>. These particles were proposed as a building material for hadrons (see <a href="/facts/Hadronization/MNVbDSPT">hadronization</a>). This theory is unusual because individual (free) quarks cannot be observed (see <a href="/facts/Color_confinement/W3XjqMAS">color confinement</a>), unlike the situation with composite atoms where electrons and nuclei can be isolated by transferring <a href="/facts/Ionization_energy/lezUajfA">ionization energy</a> to the atom. </p><p>Then, the old, broad <a href="/facts/Denotation_(semiotics)/Xz0csmxH">denotation</a> of the term <i>elementary particle</i> was deprecated and a replacement term <i><a href="/facts/Subatomic_particle/tMEzcTWQ">subatomic particle</a></i> covered all the "zoo", with its hyponym "<a href="/facts/Hadron/C0sd1zty">hadron</a>" referring to composite particles directly explained by the quark model. The designation of an "elementary" (or "fundamental") particle was reserved for <a href="/facts/Lepton/X4a6MGop">leptons</a>, quarks, their <a href="/facts/Antiparticle/7Ic43ThI">antiparticles</a>, and <a href="/facts/Quantum/4x5QTxGB">quanta</a> of fundamental interactions (see below) only. </p> <h3>Quarks, leptons, and four fundamental forces</h3> <p class="note">Main articles: <a href="/facts/Fundamental_interaction/WrFEJaFl">Fundamental interaction</a> and <a href="/facts/Standard_Model/LhVGf9ow">Standard Model</a></p> <p>Because the quantum field theory (see above) postulates no difference between particles and <a href="/facts/Fundamental_interaction/WrFEJaFl">interactions</a>, classification of elementary particles allowed also to classify interactions and <a href="/facts/Field_(physics)/FJdAKeRA">fields</a>. </p><p>Now a large number of particles and (non-fundamental) interactions is explained as combinations of a (relatively) small number of fundamental substances, thought to be <a href="/facts/Fundamental_interaction/WrFEJaFl">fundamental interactions</a> (incarnated in fundamental <a href="/facts/Boson/XtPFcjb5">bosons</a>), quarks (including antiparticles), and <a href="/facts/Lepton/X4a6MGop">leptons</a> (including antiparticles). As the theory distinguished <i>several</i> fundamental interactions, it became possible to see which elementary particles participate in which interaction. Namely: </p> <ul><li>All particles participate in gravitation.</li> <li>All charged elementary particles participate in electromagnetic interaction. <ul><li>As a consequence, neutron participates in it with its <a href="/facts/Magnetic_dipole/Ny0TVUSX">magnetic dipole</a> in spite of zero electric charge. This is because it is composed of <i>charged</i> quarks whose charges <a href="/facts/Summation/V2WUxoKn">sum</a> to zero.</li></ul></li> <li>All fermions participate in the weak interaction.</li> <li>Quarks participate in the strong interaction, along gluons (its own quanta), but not leptons nor any fundamental bosons other than gluons.</li></ul> <p>The next step was a reduction in number of fundamental interactions, envisaged by early 20th century physicists as the "<a href="/facts/United_field_theory/MMxqcM14">united field theory</a>". The first successful modern <a href="/facts/Unified_theory/MMxqcM14">unified theory</a> was the <a href="/facts/Electroweak_theory/QATz4dYF">electroweak theory</a>, developed by <a href="/facts/Abdus_Salam/M4GNh8dE">Abdus Salam</a>, <a href="/facts/Steven_Weinberg/G61D6kJw">Steven Weinberg</a> and, subsequently, <a href="/facts/Sheldon_Glashow/KNS7qjog">Sheldon Glashow</a>. This development culminated in the completion of the theory called the Standard Model in the 1970s, that included also the strong interaction, thus covering three fundamental forces. After the discovery, made at <a href="/facts/CERN/y1gwRchm">CERN</a>, of the existence of <a href="/facts/Neutral_current/eXCI2ldx">neutral weak currents</a>,<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a><a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a><a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a><a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> mediated by the <a href="/facts/Z_boson/gAncY5XV">Z boson</a> foreseen in the standard model, the physicists Salam, Glashow and Weinberg received the 1979 <a href="/facts/Nobel_Prize_in_Physics/o6w1nCbr">Nobel Prize in Physics</a> for their electroweak theory.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> The discovery of the <a href="/facts/W_and_Z_bosons/gAncY5XV">weak gauge bosons</a> (quanta of the <a href="/facts/Weak_interaction/f6upmKMs">weak interaction</a>) through the 1980s, and the verification of their properties through the 1990s is considered to be an age of consolidation in particle physics. </p><p>While accelerators have confirmed most aspects of the Standard Model by detecting expected particle interactions at various collision energies, no theory reconciling general relativity with the Standard Model has yet been found, although <a href="/facts/Supersymmetry/hkWGrMet">supersymmetry</a> and <a href="/facts/String_theory/HUJFIPYH">string theory</a> were believed by many theorists to be a promising avenue forward. The <a href="/facts/Large_Hadron_Collider/XWJCFSqT">Large Hadron Collider</a>, however, which began operating in 2008, has failed to find any evidence whatsoever that is supportive of supersymmetry and string theory,<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> and appears unlikely to do so, meaning "the current situation in fundamental theory is one of a serious lack of any new ideas at all."<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> This state of affairs should not be viewed as a crisis in physics, but rather, as <a href="/facts/David_Gross/W76ShB3u">David Gross</a> has said, "the kind of acceptable scientific confusion that discovery eventually transcends."<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> </p><p><a href="/facts/Gravitation/bqAN7DdQ">Gravitation</a>, the fourth fundamental interaction, is not yet integrated into particle physics in a consistent way. </p> <h3>Higgs boson</h3> <p class="note">Further information: <a href="/facts/Higgs_boson/3LAYXfft">Higgs boson</a></p> <p>As of 2011, the <a href="/facts/Higgs_boson/3LAYXfft">Higgs boson</a>, the quantum of a field that is thought to provide particles with <a href="/facts/Rest_mass/aRloR0nW">rest masses</a>, remained the only particle of the Standard Model to be verified. On July 4, 2012, physicists working at CERN's <a href="/facts/Large_Hadron_Collider/XWJCFSqT">Large Hadron Collider</a> announced that they had discovered a new subatomic particle greatly resembling the Higgs boson, a potential key to an understanding of why elementary particles have masses and indeed to the existence of diversity and life in the universe.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> <a href="/facts/Rolf-Dieter_Heuer/ovYCAX1M">Rolf-Dieter Heuer</a>, the director general of CERN, said that it was too soon to know for sure whether it is an entirely new massive particle – one of the heaviest subatomic particles yet – or, indeed, the elusive particle predicted by the <a href="/facts/Standard_Model/LhVGf9ow">Standard Model</a>, the theory that has ruled physics for the last half-century.<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> It is unknown if this particle is an impostor, a single particle or even the first of many particles yet to be discovered. The latter possibilities are particularly exciting to physicists since they could point the way to new deeper ideas, <a href="/facts/Physics_beyond_the_Standard_Model/UX68py44">beyond the Standard Model</a>, about the nature of reality. For now, some physicists are calling it a "Higgslike" particle.<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> <a href="/facts/Joe_Incandela/pBheRWeD">Joe Incandela</a>, of the <a href="/facts/University_of_California%2c_Santa_Barbara/G7UvBWlK">University of California, Santa Barbara</a>, said, "It's something that may, in the end, be one of the biggest observations of any new phenomena in our field in the last 30 or 40 years, going way back to the discovery of quarks, for example."<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> The groups operating the large detectors in the collider said that the likelihood that their signal was a result of a chance fluctuation was less than one chance in 3.5 million, so-called "five sigma," which is the gold standard in physics for a discovery. <a href="/facts/Michael_Turner_(cosmologist)/vAxc6x6q">Michael Turner</a>, a cosmologist at the University of Chicago and the chairman of the physics center board, said </p> <blockquote><p><i>This is a big moment for particle physics and a crossroads — will this be the high water mark or will it be the first of many discoveries that point us toward solving the really big questions that we have posed?</i></p>— <a href="/facts/Michael_Turner_(cosmologist)/vAxc6x6q">Michael Turner</a>, University of Chicago<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a></blockquote> <p>Confirmation of the Higgs boson or something very much like it would constitute a rendezvous with destiny for a generation of physicists who have believed the boson existed for half a century without ever seeing it. Further, it affirms a grand view of a universe ruled by simple and elegant and symmetrical laws, but in which everything interesting in it being a result of flaws or breaks in that symmetry.<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a> According to the Standard Model, the Higgs boson is the only visible and particular manifestation of an <a href="/facts/Higgs_field/3LAYXfft">invisible force field</a> that permeates space and imbues elementary particles that would otherwise be massless with mass. Without this Higgs field, or something like it, physicists say all the elementary forms of matter would zoom around at the speed of light; there would be neither <a href="/facts/Atoms/FoQ4kGN5">atoms</a> nor life. The Higgs boson achieved a notoriety rare for abstract physics.<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a> To the eternal dismay of his colleagues, Leon Lederman, the former director of <a href="/facts/Fermilab/8yyTx3oj">Fermilab</a>, called it the "God particle" in his book of the same name, later quipping that he had wanted to call it "the goddamn particle".<a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a> Professor Incandela also stated, </p> <blockquote><p><i>This boson is a very profound thing we have found. We're reaching into the fabric of the universe at a level we've never done before. We've kind of completed one particle's story [...] We're on the frontier now, on the edge of a new exploration. This could be the only part of the story that's left, or we could open a whole new realm of discovery.</i></p>— Joe Incandela, University of California<a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a></blockquote> <p>Dr. <a href="/facts/Peter_Higgs/zGvqlC99">Peter Higgs</a> was one of six physicists, working in three independent groups, who in 1964 invented the notion of the cosmic molasses, or Higgs field. The others were <a href="/facts/Tom_W._B._Kibble/3D1YbVel">Tom Kibble</a> of <a href="/facts/Imperial_College_London/SoXBRyax">Imperial College, London</a>; <a href="/facts/C._R._Hagen/6HRxK7gX">Carl Hagen</a> of the <a href="/facts/University_of_Rochester/RT7TWLzd">University of Rochester</a>; <a href="/facts/Gerald_Guralnik/4oeqlw4V">Gerald Guralnik</a> of <a href="/facts/Brown_University/o9MRu4jx">Brown University</a>; and <a href="/facts/Fran%25C3%25A7ois_Englert/nNbs7PxG">François Englert</a> and <a href="/facts/Robert_Brout/c7uArLqg">Robert Brout</a>, both of <a href="/facts/Universit%25C3%25A9_libre_de_Bruxelles/MlBw2Ijt">Université Libre de Bruxelles</a>.<a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a> One implication of their theory was that this Higgs field would produce its own quantum particle if hit hard enough by the right amount of energy. The particle would be fragile and fall apart within a millionth of a second in a dozen different ways depending upon its own mass. Unfortunately, the theory did not predict the particle mass making it difficult to find. The particle eluded researchers at a succession of particle accelerators.<a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a>[<i>better source needed</i>] </p><p>Further experiments continued and in March 2013 it was tentatively confirmed that the newly discovered particle was a Higgs Boson. </p><p>Although they have never been seen, Higgslike fields play an important role in theories of the universe and in string theory. Under certain conditions, according to the strange accounting of Einsteinian physics, they can become suffused with energy that exerts an antigravitational force. Such fields have been proposed as the source of an enormous burst of expansion, known as inflation, early in the universe and, possibly, as the secret of the dark energy that now seems to be speeding up the expansion of the universe.<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a> </p> <h2 id="notes">Notes</h2> <ul><li>Kragh, Helge (1999), <i>Quantum Generations: A History of Physics in the Twentieth Century</i>, Princeton: Princeton University Press.</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Salerno, Daniel (2019). The Higgs Boson Produced With Top Quarks in Fully Hadronic Signatures. Springer Theses. Springer, Cham. p. 7. Bibcode:2019hbpt.book.....S. doi:10.1007/978-3-030-31257-2. ISBN 978-3-030-31256-5. S2CID 209954340. <a href="978-3-030-31256-5" target="_blank">978-3-030-31256-5</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Glasenapp, Helmuth von (1999). Jainism: An Indian Religion of Salvation. Motilal Banarsidass Publ. ISBN 9788120813762. <a href="9788120813762" target="_blank">9788120813762</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>"Scientific Explorer: Quasiparticles". Sciexplorer.blogspot.com. 2012-05-22. Retrieved 2012-07-21. <a href="http://sciexplorer.blogspot.com/2012/05/quasiparticles.html" target="_blank">http://sciexplorer.blogspot.com/2012/05/quasiparticles.html</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Whittaker, E. T. (1910). A history of the theories of aether and electricity from the age of Descartes to the close of the 19th century. Dublin University Press series. London: Longmans, Green and Co.; [etc.]. <a href="/wiki/E._T._Whittaker" target="_blank">/wiki/E._T._Whittaker</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Pais, Abraham (2002). Inward bound: of matter and forces in the physical world (Reprint ed.). Oxford: Clarendon Press [u.a.] ISBN 978-0-19-851997-3. <a href="978-0-19-851997-3" target="_blank">978-0-19-851997-3</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Smirnov, B.M. (2003). Physics of Atoms and Ions. Springer. pp. 14–21. ISBN 0-387-95550-X. <a href="0-387-95550-X" target="_blank">0-387-95550-X</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Hasert, F.J.; Faissner, H.; Krenz, W.; Von Krogh, J.; Lanske, D.; et al. (1973). "Search for elastic muon-neutrino electron scattering". Physics Letters B. 46 (1). Elsevier BV: 121–124. Bibcode:1973PhLB...46..121H. doi:10.1016/0370-2693(73)90494-2. ISSN 0370-2693. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Hasert, F.J.; Kabe, S.; Krenz, W.; Von Krogh, J.; Lanske, D.; et al. (1973). "Observation of neutrino-like interactions without muon or electron in the gargamelle neutrino experiment". Physics Letters B. 46 (1). Elsevier BV: 138–140. Bibcode:1973PhLB...46..138H. doi:10.1016/0370-2693(73)90499-1. ISSN 0370-2693. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Hasert, F.J.; Kabe, S.; Krenz, W.; Von Krogh, J.; Lanske, D.; et al. (1974). "Observation of neutrino-like interactions without muon or electron in the Gargamelle neutrino experiment". Nuclear Physics B. 73 (1). Elsevier BV: 1–22. Bibcode:1974NuPhB..73....1H. doi:10.1016/0550-3213(74)90038-8. ISSN 0550-3213. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>The discovery of the weak neutral currents, CERN courier, 2004-10-04, retrieved 2008-05-08 <a href="http://cerncourier.com/cws/article/cern/29168" target="_blank">http://cerncourier.com/cws/article/cern/29168</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>The Nobel Prize in Physics 1979, Nobel Foundation, retrieved 2008-09-10 <a href="http://www.nobel.se/physics/laureates/1979" target="_blank">http://www.nobel.se/physics/laureates/1979</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Woit, Peter (20 October 2013). "Last Links For a While". Not Even Wrong. Retrieved 2 November 2013. <a href="/wiki/Peter_Woit" target="_blank">/wiki/Peter_Woit</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>Peter Woit (28 May 2013). "A Tale of Two Oxford Talks". Not Even Wrong. Retrieved 19 October 2013. <a href="/wiki/Peter_Woit" target="_blank">/wiki/Peter_Woit</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>Peter Byrne (24 May 2013). "Waiting for the Revolution". Quanta Magazine. simonsfoundation.org. Retrieved 19 October 2013. <a href="http://www.simonsfoundation.org/quanta/20130524-waiting-for-the-revolution/" target="_blank">http://www.simonsfoundation.org/quanta/20130524-waiting-for-the-revolution/</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>Overbye, Dennis (4 July 2012). "Physicists Find Elusive Particle Seen as Key to Universe". The New York Times. <a href="https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science" target="_blank">https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>Overbye, Dennis (4 July 2012). "Physicists Find Elusive Particle Seen as Key to Universe". The New York Times. <a href="https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science" target="_blank">https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li> <li id="fn:17"><p>Overbye, Dennis (4 July 2012). "Physicists Find Elusive Particle Seen as Key to Universe". The New York Times. <a href="https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science" target="_blank">https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></p></li> <li id="fn:18"><p>Overbye, Dennis (4 July 2012). "Physicists Find Elusive Particle Seen as Key to Universe". The New York Times. <a href="https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science" target="_blank">https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></p></li> <li id="fn:19"><p>Overbye, Dennis (4 July 2012). "Physicists Find Elusive Particle Seen as Key to Universe". The New York Times. <a href="https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science" target="_blank">https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science</a> <a href="#fnref:19" class="footnote-back-ref">↩</a></p></li> <li id="fn:20"><p>Overbye, Dennis (4 July 2012). "Physicists Find Elusive Particle Seen as Key to Universe". The New York Times. <a href="https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science" target="_blank">https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science</a> <a href="#fnref:20" class="footnote-back-ref">↩</a></p></li> <li id="fn:21"><p>Overbye, Dennis (4 July 2012). "Physicists Find Elusive Particle Seen as Key to Universe". The New York Times. <a href="https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science" target="_blank">https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science</a> <a href="#fnref:21" class="footnote-back-ref">↩</a></p></li> <li id="fn:22"><p>Overbye, Dennis (4 July 2012). "Physicists Find Elusive Particle Seen as Key to Universe". The New York Times. <a href="https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science" target="_blank">https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science</a> <a href="#fnref:22" class="footnote-back-ref">↩</a></p></li> <li id="fn:23"><p>Rincon, Paul (2012-07-04). "BBC News - Higgs boson-like particle discovery claimed at LHC". Bbc.co.uk. Retrieved 2013-04-20. <a href="https://www.bbc.co.uk/news/world-18702455" target="_blank">https://www.bbc.co.uk/news/world-18702455</a> <a href="#fnref:23" class="footnote-back-ref">↩</a></p></li> <li id="fn:24"><p>Overbye, Dennis (4 July 2012). "Physicists Find Elusive Particle Seen as Key to Universe". The New York Times. <a href="https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science" target="_blank">https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science</a> <a href="#fnref:24" class="footnote-back-ref">↩</a></p></li> <li id="fn:25"><p>Overbye, Dennis (4 July 2012). "Physicists Find Elusive Particle Seen as Key to Universe". The New York Times. <a href="https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science" target="_blank">https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science</a> <a href="#fnref:25" class="footnote-back-ref">↩</a></p></li> <li id="fn:26"><p>Overbye, Dennis (4 July 2012). "Physicists Find Elusive Particle Seen as Key to Universe". The New York Times. <a href="https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science" target="_blank">https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html?pagewanted=3&_r=1&ref=science</a> <a href="#fnref:26" class="footnote-back-ref">↩</a></p></li> </ol>