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WC5 - Recap Matter at the Atomic Level Recap Matter at the...

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Unformatted text preview: Recap: Matter at the Atomic Level Recap: Matter at the Atomic Level What was the meaning of atoms and valence numbers in the 19th century ? Which experiments showed that atoms are not indivisible? Describe Bohr’s model of the atom. Explain the difference between emission and absorption spectra. Define the Uncertainty Principle. How do you explain the atomic valence numbers ? What is a virtual particle in the Feynman diagram ? Recap: Matter at the Nuclear Level Recap: Matter at the Nuclear Level What is a positron ? Why were the first positrons seen in cosmic rays? How can nuclear energy be freed ? Explain energy production in the Sun/nuclear reactor/atomic bomb/hydrogen bomb. What are the 4 fundamental forces ? What is the meaning of the universality of forces ? The Particles “Jungle” - Classification by Weight (from the Appendix) Experiments with cosmic rays or with particles from accelerators produced a large number of new heavy particles, most of them with very short lives. A first attempt to categorize particles was based on their weight. Baryons: proton, neutron, etc Hadrons Mesons: pion,etc Leptons weight electron, electronic neutrino muon, muonic neutrino Note: for all particles there are anti-particles discovered. The Particles Jungle - Classification by Spin One way to think of spin is to imagine the particles like little tops spinning about an axis. Better interpret spin as a measure on how the particles look from different directions. It appears that the Universe contains two types of particles: fermions with spin ½ and bosons with integer spin (0,1,2). Fermions make up the matter in the Universe. Bosons are associated with the mechanism of forces. Fermions: baryons, leptons Bosons: photon, weakon True Jungle After 1950 hundreds of new particles (baryons, mesons, leptons) were discovered. Their lives were short by comparison to events in macro but very long by comparison to the collision time. Their numbers and their Greek names made E.Fermi declare: “if I could remember all these particles I would have been a botanist”. To make things worse various experiments were suggesting conflicting pictures. For instance, a pion can disintegrate into a proton and an anti­neutron, suggesting that a proton is a constituent of a pion. In a different experiment a proton can be manufactured from a pion and a neutron suggesting that the pion is a constituent of the proton. Quarks Model (I) Quarks Model (I) 1964 M.Gell­Mann at Caltech proposes the quarks model (also Sakata, Ne’eman, Zweig had similar models). In his model Gell­Mann used one of his older discoveries: strangeness, a property of some particles to live longer than expected. Like the electric charge strangeness can be quantified. Quarks Model (II) Quarks Model (II) Qem +2/3 d (338MeV) s (540MeV) -1/3 u (333MeV) Quarks Barions in Gell-Mann’s quark model: Qem is the electromagnetic charge Qsl is the strangeness charge. Searching for Quarks Searching for Quarks The observation of the Omega particle lead to a Noble price for Gell­Mann Looking for Quarks The real proof would have been the observation of a quark, a particle with an electric charge 2/3 or –1/3. Huge energy needed to obtain a pilot wave of a wavelength smaller than the size of a proton (10­15 cm). An electron for instance must be accelerated to an energy of 20 GeV. Note: at 4 MeV the speed of the electron is equal to the speed of light. Accelerated to higher energies it is its mass that will increase. Results at SLAC, Fermi­SPS and at CERN­ISR showed that the incident particles collided with particles inside the proton. No free quarks were observed. The Colour Force The Colour Force Colour force binds quarks together Quantum chromodynamics explains the mechanism of the colour force, in which the carriers are called gluons. Why use the word “colour ” ? Because quarks and the force which binds them behave according to a model which resembles the combination of colours. According to this theory all particles seen free in experiments are “white”. Quarks in a proton are red, blue, green (the 3 primary colours), while in an antiproton the anti­quarks are turquoise, yellow, mauve (the 3 anticolours of the primary colours). A meson contains a quark of a colour and an anti­quark of the corresponding anti­colour. The Strong Nuclear Force The Strong Nuclear Force Revisited How could the inter­nucleon force be related to the colour force, when the nucleons are not coloured ? The answer is related to a similar situation at a larger scale. In the 19th century the Dutch chemist Van der Waals demonstrated that molecules, which are electrically neutral, can attract each other through electromagnetic force (see next slide) The same way, colour­neutral nucleons can attract each other through the colour force. The number of fundamental forces stays equal to four: gravitation, electromagnetism, weak nuclear and strong nuclear (or colour) forces. The Van der Waals Interaction The Van der Waals Interaction + + - Step 1: the electronic cloud in one of the molecules becomes polarized (probabilistically it is possible that at a certain moment the electrons are predominantly in one side of the molecule) Step 2: this induces the polarization of the other molecule (due to the electric repulsion of the electrons) Step 3: As a result, the molecules attract each other because they show each other charges of opposite sign. A New Quark Needed A New Quark Needed Qel -1 neutrino 0 electron Building Bricks of the Matter muon u (333MeV) ? muonic neutrino Qel +2/3 d (338MeV) s (540MeV) -1/3 Leptons Quarks 1974 Sheldon Glashow proposes a new quark c (with charm). Particles with Charm (I) Particles with Charm (I) 1976 ­ First experimental proof of the charm existence was the discovery of the J/psi particle: S.Ting at Brookhaven and B.Richter at Stanford find almost simultaneously a particle with a 3.1 GeV mass. •The Brookhaven experiment had fast protons hitting a metallic target. The mass­energy of the proton and target was changing into new forms of matter, which disintegrated into electron­positron pairs. •The Stanford SPEAR supercollider experiment had an electron beam colliding head­on with a positron beam. Their detectors observed the picture shown below. J/psi (c anti-c) Particles with Charm (II) Particles with Charm (II) How can we recognize a particle with charm ? The c quark would have to disintegrate into an s quark and the s quark into u or d quarks. All this is through the weak force and a lot of neutrinos should be produced. In a photo from Gargamelle a neutrino creates a charmed particle (Do) and becomes a muon; the charmed particle creates a strange particle which disintegrates into two normal particles (the V in the image) The second photo is from Brookhaven showing a neutrino interacting with a proton. The characteristic V corresponds to the disintegration of a strange particle. Particles with Charm (III) Particles with Charm (III) 1976anti­proton with charm at Fermi. High energy gamma rays were used to hit a beryllium target. Its mass was 2250 MeV, in agreement with the theory. At the end of 1976 even the visualization of charmed particle was possible by using a special type of photographic emulsion. With charm the symmetry of the two groups of baryons look like in the diagrams shown on the right. Super Heavy Particles (I) Super Heavy Particles (I) Four leptons, 4 quarks and their anti­particles seem to be a nice model for the “bricks” of matter. 1975 Stanford’s SPEAR discovered a super­heavy electron which was baptized tau and a new neutrino associated with the tau lepton. The mass of tau was 1.8 GeV, or about twice the mass of a proton (!) and its life only 2x10­12 seconds. A theoretical model with 6 leptons and 6 quarks was already proposed by H.Harari. He named the two extra quarks top and bottom, but those names were soon changed to the more poetic names truth and beauty. Super Heavy Particles (II) Super Heavy Particles (II) 1977 at Fermi lab L.Lederman and his team observed the first particle with beauty – a meson with b and anti­b having a total mass of 9.7 GeV (about 10 times the mass of a proton). It was named Upsilon. As in the case of the particles with charm a lot of experiments were performed designed to find new super­heavy particles which contain beauty quarks in combination with other quarks. August 2000 the first particle with the truth quark. Super Heavy Particles (III) Super Heavy Particles (III) Are these super­heavy particles important ? The only heavy particles which seem to be fairly stable are the proton and the neutrino, which contain up and down quarks. But all the super­heavy particles indirectly influence the nuclear force Super­heavy particles are very important: in places where matter is in a compressed form (such as inside stars). in the Big Bang: without them the temperature of the Universe in the first fractions of a second would have been much higher and a very different future. In fact the modeling of the Big Bang provides support for a model with 6 leptons and 6 quarks. The Standard Model The Standard Model Building Bricks of the Matter Qel -1 0 tau electron muon (0.5MeV) (105MeV) (1800MeV) neutrino muonic neutrino tau neutrino Leptons Qel +2/3 -1/3 u (333MeV) d (338MeV) c (1550MeV) t (12000MeV) Quarks s (540MeV) b (4700MeV) The Standard Clasification The Standard Clasification Leptons Anti-leptons Quarks Baryons “Bricks” Anti-quarks Mesons Anti-baryons Anti-mesons Hadrons and Anti-hadrons Force Carriers foton weakon gluon graviton Force weak colour gravitational Acts on all charged particles all particles quarks all particles electromagnetic ...
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