1. P ARTICLE P HYSICS A brief intro to the quantum world of particles

2. W HAT IS P ARTICLE P HYSICS ? 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 not- so-ordinary. (It’s definitely a journey into a LOT of new and interesting names of things!)

3. B RIEF H ISTORY … 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 (  +,  -,  0 ) Kaons (  +,  -,  0 ) Etas (   ’) Hyperons (  +,  -,  0 ) Each was determined to have a very short lifetime (ranging from 10 -10 s to 10 -24 s)

4. E LEMENTARY P ARTICLES Any particle which is not made of any smaller component particles Three Classes of Elementary Particles: Leptons Quarks Exchange Particles Responsible for ordinary matter Involved with Fundamental Interactions

5. L EPTONS Lepton Name SymbolElectric Charge/e Rest mass/ MeV·c -2 Spin/(h/2  ) Electrone-e- 0.511½ Electron neutrino e 0-½ Muon -- 106½ Muon Neutrino  0-½ Tau  1780½ Tau Neutrino  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

6. Q UARKS Quark Flavor SymbolElectric Charge /e Rest mass/ Mev·c -2 Spin/ (h/2  ) Upu+ 2/3330½ Downd- 1/3333½ Stranges- 1/3486½ Charmc+ 2/31500½ Bottomb- 1/34700½ Topt+ 2/3175500½ 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)

7. Q UANTUM N UMBERS 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…

8. S PIN 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 / 2  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

9. M ORE ON S PIN Depict a particle with spin using circle with arrow through it: All particles with spin will align parallel (or antiparallel) to the magnetic field’s direction Spin up Spin down

10. B OSONS AND F ERMIONS 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

11. P AULI E XCLUSION P RINCIPLE 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…

12. F UNDAMENTAL I NTERACTION Fundamental Interaction Felt By…Range Relative Strength (to 2 protons touching) Exchange particle(s) Gravitation Any particle with mass infinite10 -38 Graviton Weak interactionAny particle10 -18 m10 -5 W bosons, Z boson Electromagnetic Any charged particle Infinite1Photon Strong interaction Only quarks10 -15 m100gluons

13. E XCHANGE P ARTICLES Exchange Particle Symbolelectric Charge /e Spin/ (h/2  ) Rest Mass/ MeV·c -2 Associated Interaction Photon  010Electromagnetic W bosonsW+W-W+W- +1 1111 80.4 weak Z bosonZ0Z0 0191.2weak GluonsG ij 010Strong Gravitong020Gravitational There is solid experimental evidence for the existence of all of these, EXCEPT for the graviton

14. V IRTUAL P ARTICLES 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.

15. R ANGE OF AN I NTERACTION 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:

16. HADRONS Baryons Made by combining 3 quarks Anti-baryons are made by combining 3 anti- quarks Examples: Proton = uud Neutron = udd Anti-proton = Mesons Made by combining 1 quark and 1 anti-quark Anti-mesons are created by combining the anti- particles of each Examples: Pion (  + ) = Anti-pion =

17. B ARYON N UMBER 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

18. S TRANGENESS 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 Decay of = 10 -10 s Decay of = 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

19. S TRANGENESS 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…)

20. C OLOR 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

21. C OLOR 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 ?