EL's+Masterclass+Summary



τ+τ-: The event is e+ e- --> Z0 --> tau+ tau- where the taus are observed by their decay products: a neutrino accompanied by an electron or a muon. The picture shows one decay to a positron (right track), the other to a muon (left track) with respectively 37 and 25 GeV/c momenta. Using the energy balance, there is approximately 30 GeV energy missing in the event and carried away by the neutrinos.

μ+μ-: Positive muon (//µ//+) decay showing the case where the neutrino and antineutrino go off in the same direction together, which gives the highest energy positron (//e//+), roughly 52 MeV. Since (in this case) the spins of the neutrino and antineutrino cancel each other, the positron spin must be in the same direction as that of the muon in order to balance angular momentum. In the weak interaction governing this decay, such an ultrarelativistic positron acts pretty much like an antineutrino and [|therefore can only be created with positive helicity] (right-handed, with its spin pointing along its momentum). Therefore the reaction has maximum probability when the positron exits along the direction of the //muon's// spin (and zero probability of being emitted in the opposite direction).

q+q-: The decay of hadrons by the weak interaction can be viewed as a process of decay of their constituent quarks. There is a pattern of these quark decays: a quark of charge +2/3 ( u,c,t) is always transformed to a quark of charge -1/3 (d,s,b) and vice versa. This is because the transformation proceeds by the exchange of charged W bosons, which must change the charge by one unit.

e+e-: The electron would decay into a photon and neutrino if the law of electric charge conservation is not respected. Such a decay would cause vacancy in closed shells of atoms giving rise to emission of x-rays and Auger electrons. Experimental searches for such very rare decay have given an estimate for the life time to be greater than $2.7 \times 10^{23}$ years. The simplest theoretical model which would give rise to such a decay is one where the electron is regarded as the first excited state and neutrino as the ground state of a fundamental spin 1/2 particle bound to a scalar particle by a super strong force and the photon is considered as a bound state of a fundamental spin 1/2 fermion-antifermion pair. The fine structure constant of the super strong coupling is found to be unity from the masslessness of the neutrino and the lower bound of the mass of the fundamental particles is estimated by using quantum mechanical formula for photon emission by atoms and found to be $10^{22}$ GeV from the bound for electron decay time indicating thereby that the composite nature of electron, neutrino and the photon would be revealed in the Planckian energy regime. A model based on extension of $SU(2)\otimes SU(2)$ symmetry of Dirac equation to $SU(3)\otimes SU(3)$ gives a lower bound for the mass of the gauge boson mediating the decay to be $10^9 GeV$ which is the geometric mean of the masses of the electron and the fundamental particles.

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· Conservation of energy? Momentum? Charge?