The previous two chapters emphasized high energy colliders and their detectors. In order that the reader does not get the impression that the only frontier is the high energy one, here we briefly mention other directions from which major discoveries might come. It turns out that our basic subject in this book, the Standard Model and its tests, is indeed more naturally the domain of high energy, high luminosity machines. That is because the natural scale of the Standard Model is of the order of MW or MZ, that is, of the order of 100 GeV. Testing the high energy predictions of the Standard Model requires TeV energies in the quark, lepton, or gluon collisions.
Nevertheless, our purpose is also to prepare the reader to understand any future developments in particle physics, both by providing an explanation of the Standard Model as the foundation on which anything new will stand, and by knowing whether the Standard Model is conceptually incomplete. Several kinds of experiments are in progress or planned which could extend the Standard Model in new directions. Some use low energy secondary beams at accelerators, and others are truly non-accelerator experiments. They include:
(1) searches for neutrino mass effects from neutrino oscillations, from detection of solar neuutrinos or atmospheric neutrinos, or from nuclear β decays;
(2) searches for neutrinoless double beta decay, which is sensitive to neutrino masses, right-handed currents, and any new light particles which might couple to neutrinos;
(3) searches for rare or forbidden decays of mesons or leptons or quarks;
(4) searches for dark matter;
(5) searches for nucleon decay;
(6) searches for magnetic moments. In particular, the reported magnetic moment of the muon currently deviates from the Standard Model by about three standard deviations, based on an important measurement at Brookhaven National Laboratory. The experiment has been rebuilt at Fermilab, and should take data in 2017. The experiment is based on a muon storage ring and measuring the correlation of the muon momenta and spins. In addition, the theory has subtleties that are being better understood.
(7) Electric dipole moments violate CP invariance. The CP invariance described in Chapter 21 implies a very tiny effect for electrons and neutrons, but some physics beyond the Standard Model could give larger observable effects, and searching for them is an active and important field since nearly all extensions of the Standard Model give effects, sometimes significant ones.
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