Phy107Spr06Lect38

Phy107Spr06Lect38 - Course evaluations on Monday Question...

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1 Fri. Apr. 28, 2006 Phy107 Lecture 38 1 From Last Time… • Discussed the weak interaction • All quarks and leptons have a ‘weak charge’ – They interact through the weak interaction • Weak interaction often swamped by electromagnetic or strong interaction. • Most clearly manifested in particle decays, where the weak interaction can change one particle into another. Course evaluations on Monday Fri. Apr. 28, 2006 Phy107 Lecture 38 2 Question Which of the following particles is not a ‘matter particle’ A. Electron B. Quark C. Gluon D. Neutrino Fri. Apr. 28, 2006 Phy107 Lecture 38 3 Matter particles & force carriers • Matter Particles – Quarks and leptons – These are fermions (half-integer spin) • Force pariticles – These are all bosons – (integer spin) Fri. Apr. 28, 2006 Phy107 Lecture 38 4 Question Which of the following particles cannot interact with a paticle that carries only the weak charge? A. Quark B. Neutrino C. Electron D. All of them can Fri. Apr. 28, 2006 Phy107 Lecture 38 5 Fundamental Matter Particles Three generations of leptons and quarks All feel the weak force. Charged leptons feel weak & EM forces Quarks feel weak, EM, & strong Fri. Apr. 28, 2006 Phy107 Lecture 38 6 Quarks and the weak force Quarks have color charge, electric charge, and weak charge — other interactions swamp the weak interaction But similar to leptons, quarks can change their flavor (decay) via the weak force, by emitting a W particle. u d Generation I c s Generation II Emit W + 2x10 -12 seconds t b Generation III Emit W + 10 -23 seconds Charge +2/3 -1/3
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2 Fri. Apr. 28, 2006 Phy107 Lecture 38 7 Flavor change between generations But for quarks, not limited to within a generation Similar to leptons, quarks can change their flavor (decay) via the weak force, by emitting a W particle. u d Generation I c s Generation II Emit W - t b Generation III Emit W - 10 -12 seconds Charge +2/3 -1/3 Fri. Apr. 28, 2006 Phy107 Lecture 38 8 ‘Combining’ the quarks • In this sense, different quarks can be considered as different ‘manifestations’ of the same particle. • Just as we wouldn’t call spin-up and spin- down electrons different particles, the different flavor quarks don’t have to be considered as different particles. • Three generations, six quarks, can be considered different ‘facets’ of same field. • Why six? This is only an input to the theory. Fri. Apr. 28, 2006 Phy107 Lecture 38 9 Actual quark decays • Quarks are bound into hadrons – bayrons (3 bound quarks), mesons(2 bound quarks). • But the internal quarks can still emit W particles and change into something else. • Most of the time, hadrons containing heavy quarks (c, s, b, t) decay by ‘emitting’ a W and transforming into the next lightest quark 10 Decay of heavy quarks Top quark decays so fast (10 -23 s) , it doesn’t have time to form a meson.
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This note was uploaded on 01/05/2012 for the course PHYS 107 taught by Professor N/a during the Spring '08 term at Wisconsin.

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Phy107Spr06Lect38 - Course evaluations on Monday Question...

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