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PARTICLE PHYSICS A brief intro to the quantum world of particles WHAT IS PARTICLE PHYSICS? Ordinary matter in our world is constructed of just 3 types of particles: Proton Neutron Electron There are HUNDREDS of other types of particles, most of which are unstable, that exist in our universe Particle Physics is the study of all of these particles—stable and unstable, ordinary and notso-ordinary. (It’s definitely a journey into a LOT of new and interesting names of things!) BRIEF HISTORY… Protons, Neutrons, Electrons—historically believed to be smallest particles of matter 1950’s and 60’s, during nuclear reaction tests and experiments, hundreds of other particles discovered! Pions (p+, p-, p0) Kaons (K+, K-, K0) Etas (h, h’) Hyperons (S+, S-, S0) Each was determined to have a very short lifetime (ranging from 10-10 s to 10-24 s) ELEMENTARY PARTICLES Any particle which is not made of any smaller component particles Three Classes of Elementary Particles: Leptons Responsible for ordinary matter Quarks Exchange Particles Involved with Fundamental Interactions LEPTONS Lepton Name Symbol Electric Charge/e Rest mass/ Spin/(h/2p) MeV·c-2 Electron e- -1 0.511 ½ Electron neutrino ne 0 - ½ Muon m- -1 106 ½ Muon Neutrino nm 0 - ½ Tau t- -1 1780 ½ Tau Neutrino nt 0 - ½ Electron and electron neutrino are seen in beta decay Neutrinos once believed to be massless, now known to have a very, very small mass Existence of all 6 has been supported with solid experimental evidence QUARKS Quark Flavor Symbol Electric Charge /e Rest mass/ Mev·c-2 Spin/ (h/2p) Up u + 2/3 330 ½ Down d - 1/3 333 ½ Strange s - 1/3 486 ½ Charm c + 2/3 1500 ½ Bottom b - 1/3 4700 ½ Top t + 2/3 175500 ½ Existence of all 6 has been supported with solid experimental evidence Quarks can never exist independently Quarks combine to form larger particles (i.e. protons and neutrons) QUANTUM NUMBERS Numbers (usually) used to characterize the properties of particles Electric Charge: units = e, where e = 1.6 x 10-19 C For example, the quantum number for electric charge of an electron is -1 and for a proton, +1 Flavor:--not specified by a number, and only really used for quarks Color Strangeness Baryon number Generation lepton number Some quantum numbers are conserved in particle reactions, others aren’t… SPIN The quantum number spin is a property that is analagous to rotation and angular momentum, but not the same It’s based on principles related to Einstein’s theory of relativity For elementary and composite particles: the unit of spin = h/2p All known particles have some quantity of spin that is a multiple of that unit Bosons have an integral spin Fermions have a half-integral spin MORE ON SPIN Depict a particle with spin using circle with arrow through it: B Spin up Spin down All particles with spin will align parallel (or antiparallel) to the magnetic field’s direction BOSONS AND FERMIONS Bosons All bosons have an integral spin (i.e. 1 or 2) Examples: Photon, W+ boson,… Fermions All Fermions have a ½-integral spin (i.e. ½ ) Examples: quarks, leptons, protons, and neutrons PAULI EXCLUSION PRINCIPLE It is impossible for two identical fermions to occupy the same quantum state if they have the same quantum numbers Example: electron distribution in energy levels… FUNDAMENTAL INTERACTION Range Relative Strength (to 2 protons touching) Exchange particle(s) Any particle with mass infinite 10-38 Graviton Weak interaction Any particle 10-18 m 10-5 W bosons, Z boson Electromagnetic Any charged particle Infinite 1 Photon Strong interaction Only quarks 10-15 m 100 gluons Fundamental Interaction Felt By… Gravitation EXCHANGE PARTICLES Exchange Particle Symbol electric Charge /e Spin/ (h/2p) Photon g 0 1 0 Electromagnetic W bosons W+ W- +1 -1 1 1 80.4 80.4 weak weak Z boson Z0 0 1 91.2 weak Gluons Gij 0 1 0 Strong Graviton g 0 2 0 Gravitational Rest Mass/ MeV·c-2 Associated Interaction There is solid experimental evidence for the existence of all of these, EXCEPT for the graviton VIRTUAL PARTICLES An interaction between particles will often take place while violating the law of conservation of energy (sorry, but it’s true!) This can happen IF the time in which the energy violation is happening is small enough to be undetectable, according to Heisenberg’s uncertainty principle The exchange particle involved in the interaction, therefore, is a virtual particle The exchanged particle is NOT observed in any way. RANGE OF AN INTERACTION We can estimate the range through which an interaction can occur by estimating: the mass (m) the time of the interaction, assuming maximum velocity of c (Range/c) We also use the Uncertainty Principle and Einstein’s mass-energy equivalence to derive a mathematical expression: h E t 4p h 2 R mc c 4p h R 4p mc HADRONS Baryons Mesons Made by combining 3 quarks Made by combining 1 quark and 1 anti-quark Anti-baryons are made by combining 3 antiquarks Anti-mesons are created by combining the antiparticles of each Examples: Proton = uud Neutron = udd Anti-proton = uu d Examples: Pion (p+) = Anti-pion = ud ud p - BARYON NUMBER protons and neutrons have a baryon number = +1 All anti-protons and anti-neutrons have a baryon number = -1 Quarks have baryon number = +1/3 Anti-quarks have baryon number = -1/3 Baryon number is always conserved STRANGENESS Particles were discovered during cosmic ray experiments in the 50’s and 60’s that had unexpected or unusual properties Strange particles Example of an unusual property: Decayed much, much more slowly than expected, based on observations of similar particles - = 10-10 s Decay of S Decay of S 0 = 10-20 s These properties were assigned a new quantum number, called “strangeness” Now known to be associated with the presence of a strange quark or anti-strange quark STRANGENESS Presence of a strange quark = -1 unit of strangeness Presence of an anti-strange quark = +1 unit of strangeness (yes, I know it seems backwards…think of it as a “strange” assigning of quantum numbers…) COLOR What’s wrong with having a baryon that has a total spin of 3/2? Color is a new quantum number that was created to explain the existence of particles that otherwise would violate the Pauli Exclusion Principle. This has nothing to do with visible light! Quarks carry one of 3 possible colors: Red, Blue, or Green Antiquarks carry anti-colors COLOR Color combinations that result in a white, or colorless, particle: Red + Blue + Green Red + anti-red Blue + Anti-blue Green + Anti-green Example: Energy is supplied to a meson, and two new mesons are created. Refer to the diagram on the board. What are the colors of the quarks indicated by X, Y, and Z?