Lecture13-16

U red shired to 273 degrees due to expansion of

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Unformatted text preview: u༇  Spin 1 par;cles (bosons) mediate the EM, Weak, and Strong interac;ons. u༇  Bosons can be created and destroyed singly and can occupy the same quantum state. u༇  Weak and strong interac;ons are very short range (due to high mass and confinement respec;vely). u༇  Photon can transmit energy long distances. u༇  Spin 2 graviton is very weakly interac;ng and has not yet been detected although we infer its existence from energy loss in binary systems. 72 Ques;on: Fermions and Bosons u༇  The elementary par;cles that we have detected are either spin 1/2 fermions and spin 1 bosons. Which statement is false? A) Bosons can be created singly. B) Fermions must be created in pairs (with antiparticle). C) No two identical Bosons can be in the same state. D) Fermions take up space. E) None of the above. 73 The Weak Interac;on Problem There was one big problem! (not as simple as 1 2 3) u༇  Unlike the EM, strong, and gravita;onal interac;ons, the bosons which mediate the weak interac;on were experimentally found to be very massive. u༇  o  W mass is about 90 ;mes the proton mass. o  That’s why force seems weak. Theory with massive vector bosons is not renormalizable (cannot remove infini;es). u༇  Solu;on to this problem was found about 1970 using spontaneous symmetry breaking and the Higgs mechanism. u༇  74 Spontaneous Symmetry Breaking u༇  The SU(2) symmetry is broken. o  Electron and neutrino have different charges and masses… o  W mass is far from zero. o  SM uses Spontaneous Symmetry Breaking. u༇  Standard example: Ferromagnet o  Space symmetric Hamiltonian o  Spins (in a domain) align producing a lower energy state. Field strength set to minimize energy. u༇  Direc;on is chosen spontaneously even though system is symmetric. u༇  75 Curie Point u༇  If we raise the temperature, the domains of magne;za;on are destroyed and each atom has a random spin direc;on. u༇  Object no longer behaves as a strong magnet (ferromagnet). u༇  Canadian quarters have a small amount of iron allowing us to do the experiment. u༇  At high temperature the rota;onal symmetry is restored. u༇  As system cools, symmetry is spontaneously broken again. 76 The Higgs Poten;al !0" # $ % v+H & + "φ # $ 0% &φ ' u༇  u༇  u༇  v Higgs boson is an SU(2) (complex) doublet. (Mexican Hat) Poten;al is symmetric (µ²<0). Neutral Higgs field takes on a...
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