rpp2010-rev-higgs-boson

rpp2010-rev-higgs-boson - 1 HIGGS BOSONS THEORY AND...

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–1– HIGGS BOSONS: THEORY AND SEARCHES Updated May 2010 by G. Bernardi (LPNHE, CNRS/IN2P3, U. of Paris VI & VII), M. Carena (Fermi National Accelerator Laboratory and the University of Chicago), and T. Junk (Fermi National Accelerator Laboratory). I. Introduction Understanding the mechanism that breaks electroweak sym- metry and generates the masses of all known elementary par- ticles is one of the most fundamental problems in particle physics. The Higgs mechanism [1] provides a general framework to explain the observed masses of the W ± and Z gauge bosons by means of charged and neutral Goldstone bosons that end up as the longitudinal components of the gauge bosons. These Goldstone bosons are generated by the underlying dynamics of electroweak symmetry breaking (EWSB). However, the fun- damental dynamics of the electroweak symmetry breaking are unknown. There are two main classes of theories proposed in the literature, those with weakly coupled dynamics - such as in the Standard Model (SM) [2] - and those with strongly coupled dynamics. In the SM, the electroweak interactions are described by a gauge ±eld theory based on the SU(2) L × U(1) Y symmetry group. The Higgs mechanism posits a self-interacting complex doublet of scalar ±elds, and renormalizable interactions are arranged such that the neutral component of the scalar doublet acquires a vacuum expectation value v 246 GeV, which sets the scale of EWSB. Three massless Goldstone bosons are generated, which are absorbed to give masses to the W ± and Z gauge bosons. The remaining component of the complex doublet becomes the Higgs boson - a new fundamental scalar particle. The masses of all fermions are also a consequence of EWSB since the Higgs doublet is postulated to couple to the fermions through Yukawa interactions. If the Higgs boson mass m H is below 180 GeV, all ±elds remain weakly interacting up to the Planck scale, M Pl . The validity of the SM as an e²ective theory describing physics up to the Planck scale is questionable, however, because CITATION: K. Nakamura et al. (Particle Data Group), JPG 37 , 075021 (2010) (URL: http://pdg.lbl.gov) July 30, 2010 14:34
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–2– of the following “naturalness” argument. All fermion masses and dimensionless couplings are logarithmically sensitive to the scale Λ at which new physics becomes relevant. In contrast, scalar squared masses are quadratically sensitive to Λ. Thus, the observable SM Higgs mass has the following form: m 2 H =( m 2 H ) 0 + kg 2 Λ 2 16 π 2 , where ( m H ) 0 is a fundamental parameter of the theory. The second term is a one-loop correction in which g is an elec- troweak coupling and k is a constant, presumably of O (1), that is calculable within the low-energy eFective theory. The two contributions arise from independent sources and one would not expect that the observable Higgs boson mass is signi±cantly smaller than either of the two terms. Hence, if the scale of new physics Λ is much larger than the electroweak scale, unnatural cancellations must occur to remove the quadratic dependence
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rpp2010-rev-higgs-boson - 1 HIGGS BOSONS THEORY AND...

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