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Register a new userPrecision tests of the Standard Model from Z physics
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Frederic Teubert
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The measurements performed at LEP and SLC have substantially improved the precision of the test of the Minimal Standard Model. The precision is such that there is sensitivity to pure weak radiative corrections. This allows to indirectly determine the top mass (mt=161$pm$8 GeV), the W-boson mass (MW=80.37$pm$0.03 GeV), and to set an upper limit on the the Higgs boson mass of 262 GeV at 95% confidence level.
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This write-up of lectures given at TASI 2020 provides an introduction into precision tests of the electroweak Standard Model. The lecture notes begin with a hands-on review of the (on-shell) renormalization procedure, and subsequently highlight a few subtleties that occur in the renormalization of a theory with electroweak symmetry breaking and massive gauge bosons. After that a set of typical electroweak precision observables is introduced, as well as a range of input parameter measurements that are needed for making predictions within the Standard Model. Finally, it is discussed how comparisons of the electroweak precision observables between experiment and theory can be used to stress-test the Standard Model and probe new physics.
We present a global analysis of leptonic and semileptonic kaon decays data, including all recent results by BNL-E865, KLOE, KTeV, ISTRA+, and NA48. Experimental results are critically reviewed and combined, taking into account theoretical (both analytical and numerical) constraints on the semileptonic kaon form factors. This analysis leads to a very accurate determination of Vus and allows us to perform several stringent tests of the Standard Model.
In the Standard Model there are several canonical examples of pure leptonic processes involving the muon, the electron and the corresponding neutrinos which are connected by the crossing symmetry: i) the decay of muon, ii) the inverse muon decay, and iii) the annihilation of a muon and an electron into two neutrinos. Although the first two reactions have been observed and measured since long ago, the third process, resulting in the invisible final state, has never been experimentally tested. It may go either directly, or, at low energies, via the annihilation of a muon and an electron from an atomic bound state, called muonium (M=mu^+e^-). The Mto u_mu u_e decay is expected to be a very rare process, with the branching fraction predicted to be Br(Mto u_mu u_e) = 6.6 10^{-12} with respect to the ordinary muon decay rate. Using the reported experimental results on precision measurements of the positive muon lifetime by the MuLan Collaboration, we set the first limit Br(M to invisible) < 5.7 10^{-6}, while still leaving a big gap of about six orders of magnitude between this bound and the predictions. To improve substantially the limit, we proposed to perform an experiment dedicated to the sensitive search for the Mto invisible decay. A feasibility study of the experimental setup shows that the sensitivity of the search for this decay mode in branching fraction Br(Mto invisible) at the level of 10^{-12} could be achieved. If the proposed search results in a substantially higher branching fraction than predicted, say Br(M to invisible) < 10^{-10}, this would unambiguously indicate the presence of new physics. We point out that such a possibility may occur due the muonium-mirror muonium conversion in the mirror matter model. A result in agreement with the Standard Model prediction would be a clean check of the pure leptonic bound state annihilation.
We review our expectations in the last year before the LHC commissioning.
I review recent theoretical work on electroweak symmetry breaking.
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