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Elementarteilchenphysik Antonio Ereditato University of Bern historical development: discoveries of new particles A.Ereditato SS 2017 1 The atom (around 1905) Before ~1905, nobody really knew: “ What does the inside of an atom look like ? ” Thomson atom Early “plum-pudding” model Positive charge (uniformly distributed) A.Ereditato SS 2017 corpuscles (electrons) The positive charge is spread out like a “plum-pudding” 2 1906-1911 Electromagnetism predicted that the heavy α particles should be only slightly deflected by the “plum-pudding” atom…but, contrary to expectations, large scattering angles were detected: Probability < 10-10 ! α The atom must have a solid core capable of applying large electric forces onto an incoming (charged) particle. The atom is not an ‘elementary particle’ Ernest Rutherford 1871-1937 α α A.Ereditato SS 2017 3 Interpretation of the Rutherford experiment (discovery of the atomic nucleus) soon followed by the Bohr atom α α α α α Electrons ~10-13 cm Nucleus Volume Nucleus = 4/3π (10-13)3 ~ 4 x 10-39 cm3 Volume Atom = 4/3π (10-8)3 ~ 4 x 10-24 cm3 ~10-8 cm A.Ereditato SS 2017 Fraction ~ 10-15 4 The atomic nucleus In addition to the “discovery” of the nucleus and of the proton, Rutherford also noted the need of a “neutral” particle in the atomic nucleus, due to the disagreement between the atomic number of an atom (number of positive charges) and its mass computed in atomic mass units. In 1920 Rutherford proposed that the difference could be explained by the existence of a neutrally charged particle (the neutron) within the atomic nucleus. This was conceived as a “couple” of an electron orbiting around a proton. As an example, the nitrogen nucleus (14N) would be composed of 14 protons and 7 (nuclear) electrons. 7 more “external” electrons would have built up the nitrogen atom. This hypothesis worked for a while, but was eventually questioned with the advent of quantum mechanics (~1930). The Heisenberg principle: Δx⋅Δp ≥ ħ led to an energy of ~100 MeV for nuclear electrons, larger than the nuclear binding energy and larger than the β electron energy. More quantitative objections came from detailed QM calculations and the Pauli principle. 1932: Chadwick discovered the “neutron” and, soon after, Heisenberg worked out a nuclear model with protons and neutrons. A.Ereditato SS 2017 5 History: discovery of particles coming somewhere from the Universe and hitting the Earth ~1900: charged electroscopes discharge with time, even if accurately shielded from radioactive sources (ambient radioactivity) One would expect a strong reduction of this radioactivity with increasing altitude (gamma absorption: 50% of the radiation to survive after 80 m, 0% at the 330 m of the Eiffel Tower) 1910: Jesuit T. Wulf measured the radiation on top of the Eiffel Tower still measuring a 60% factor 1912: Victor Hess discovered that there are “particles” coming from the outer space. The ionization even increases with height! Hess balloon flights: up to 5300 m A.Ereditato SS 2017 6 Cosmic-ray spectrum Direct measurements A.Ereditato SS 2017 Indirect (ground based detectors) 7 1912-1925: development of the first particle detectors discovery of new particles with cosmic-rays A.Ereditato SS 2017 8 High energy cosmic-rays: discovery of new particles 1932: Discovery of the antiparticle of the electron, the positron (Anderson). Confirmed the existence and prediction that anti-matter does exist (Dirac). 1936: Discovery of the muon (Anderson and Neddermeyer, confirmed by Street and Stevenson). It’s very much like a “heavy electron”. 1947: Discovery of the pion (Powell). Hadron lighter than the proton. A.Ereditato SS 2017 9 Evidence for antimatter Anderson’s experiment: cloud chamber (discovery of the positron) Carl Anderson 1936 Nobel Prize Direction of deflection inside a magnetic field: it is a positively charged particle Momentum measured before and after the 6 mm thick lead plate: pb = 63 MeV, pa = 23 MeV Assuming a proton, the kinetic energy after the plate would be ~200 keV, implying a range in the cloud chamber gas of about 5 mm, not compatible with the observed 50 mm: positron track The particle should have the electron mass A.Ereditato SS 2017 10 Evidence for antimatter in this early bubble chamber photo. The magnetic field in this chamber makes negative particles curl left and positive particles curl right. Many electronpositron pairs appear as if from nowhere, but are in fact from photons, which don't leave a track. Positrons (anti-electrons) behave just like the electrons but curl in the opposite way because they have the opposite charge. (One such electron-positron pair is highlighted.) A.Ereditato SS 2017 11 The muon and its role in the early times The discovery of the muon was published in Seth H. Neddermeyer and Carl. D. Anderson, Note on the Nature of Cosmic-Ray Particles, Phys. Rev., Vol. 51, 884 1937. J. C. Street and E. C. Stevenson, New Evidence for the Existence of a Particle Intermediate Between the Proton and Electron, Phys. Rev. 52, 1003 (1937). Isidor Rabi: “who ordered this?” Before that the fundamental particles were presumed to be electrons, protons and the (then) newly discovered neutron. The discovery brought attention to the prediction by Yukawa in 1935 that an intermediate mass "meson" might be responsible for the nuclear strong force. Yukawa had predicted a mass of about 100 MeV and the muon had a mass very close to that. Moreover, “the mesons” decayed emitting electrons, and Yukawa’s nuclear quanta were expected to be responsible for β-radioactivity by disintegrating into electrons and undetectable neutrinos. Was the new intermediate-mass particle the responsible for the strong interaction? A.Ereditato SS 2017 12 Yukawa Theory (1935) Relativistic Klein-Gordon equation: For m = 0 reduces to the wave propagation equation, with Ψ either the wave function of the photon or the potential U(r). For a static potential we obtain: 1 ∂ ⎛ 2 ∂U ⎞ m 2 c 2 ∇ U(r) = 2 ⎜ r ⎟ = 2 U(r) ⎝ r ∂r ∂r ⎠ h 2 The solution is: The analogous equation for EM (Laplace): ∇ 2U(r) = 0 with gives This implies that g can be called the “nuclear charge” € For a typical R =10-13 cm one obtains mc2 ~ 100 MeV The Yukawa theory was compatible with the assumption that the “meson” was the quantum of the strong interaction A.Ereditato SS 2017 13 From 1941 and through the difficult years of World War II, three young Italian physicists, Conversi, Pancini and Piccioni, carried on a series of observations of mesons stopped in matter, which seemed at the beginning to support Yukawa’s predictions. At the end of 1946, they reported that the rates of absorption of mesons in light materials were in strong disagreement with the theory. The experiment was based on the magnetic separation of positive and negative muons. A positive particle would have been repelled by the nucleus and decay as in vacuum. A negative one would have been captured by a nucleus and, as the quanta of the strong interaction, quickly be absorbed. The results of the experiment showed instead that also negative muons decayed when at rest in Carbon, rather than being absorbed by the nucleus. This disproved the possibility for the muon to be the quantum of strong interaction. A.Ereditato SS 2017 14 Discovery of the pion Cecil Powell and colleagues (Bristol University) used nuclear emulsions to see charged tracks in the upper atmosphere. In 1947, they announced the discovery of a particle called the π-meson or pion (π) for short. Pion (π) comes to rest here, and then decays: π!µ+ν One neutrino is also produced but escapes undetected. Muon (µ) comes to rest here, and then decays: µ ! e +ν +ν µ Cecil Powell 1950 Nobel Prize Two more neutrinos are also produced but escape undetected. e π The π (140 MeV) was then assumed to be the mediator of the strong interaction at short distances (the Yukawa particle). Today we know that the actual mediator is the massless gluon. A.Ereditato SS 2017 15 Interaction of an antiproton in a bubble chamber: 8 pions are produced. One of them (positive) decays into a muon and a muon-neutrino A.Ereditato SS 2017 16 The pion • Pions are produced copiously in strong interactions • Charged pions decay by weak interaction • The neutral pion decays by electromagnetic interaction • All pions have spin 0. m = 135.0 MeV, τ = 8.4×10−17 s m = 139.6 MeV, τ = 26×10−9 s A.Ereditato SS 2017 17 “Strangeness” In 1947 Rochester and Butler observed the joint production of two unstable particles. Pais (1952) proposed the “associated production” for these particles. These particles were always observed together: one with a mass of ~500 MeV (the K meson), the other heavier than the proton (hyperons: Λ, Σ), producing the so-called V events. These particles had other “strange” properties: 1) Produced in strong interaction (fast) 2) Decaying by weak interaction (slow) The solution by Nishijima and Gell-Mann (1953): New additive quantum number S (strangeness), conserved in strong and EM interaction, violated in weak K! Λ, Σ ! p, n, π ! A.Ereditato SS 2017 S= +1 or -1 S= -1 S= 0 18 V “Strange” particles are produced in pair (ΔS = 0) via strong interaction π but they decay weakly (ΔS = ±1) 19 Examples of reactions involving “strange” particles strong EM weak weak A.Ereditato SS 2017 20 Discovery of many more particles (1950-1960) Because one has no control over cosmic rays (energy, types of particles, location, etc), scientists focused their efforts on accelerating particles in the lab and smashing them together. Generically people refer to them as “particle accelerators”. Circa 1950, these particle accelerators began to uncover many new particles. Most of these particles are unstable and decay very quickly, and hence had not been seen in cosmic rays. Notice the discovery of the proton’s antiparticle, the antiproton, in 1955: more antimatter. A.Ereditato SS 2017 21 CERN: European Laboratory for Particle Physics A.Ereditato SS 2017 22