A novel approach to confront electroweak data and theory

Abstract

A novel approach to study electroweak physics at one-loop level in generic SU(2) L × U(1) Y theories is introduced. It separates the 1-loop corrections into two pieces: process specific ones from vertex and box contributions, and universal ones from contributions to the gauge boson propagators. The latter are parametrized in terms of four effective form factorsē 2 (q 2 ),s 2 (q 2 ),ḡ 2 Z (q 2 ) andḡ 2 W (q 2 ) corresponding to the γγ, γZ, ZZ and W W propagators. Under the assumption that only the Standard Model contributes to the process specific corrections, the magnitudes of the four form factors are determined at q 2 = 0 and at q 2 = m 2 Z by fitting to all available precision experiments. These values are then compared systematically with predictions of SU(2) L × U(1) Y theories. In all fits α s (m Z ) andᾱ(m 2 Z ) are treated as external parameters in order to keep the interpretation as flexible as possible. The treatment of the electroweak data is presented in detail together with the relevant theoretical formulae used to interpret the data. No deviation from the Standard Model has been identified. Ranges of the top quark and Higgs boson masses are derived as functions of α s (m Z ) andᾱ(m 2 Z ). Also discussed are consequences of the recent precision measurement of the left-right asymmetry at SLC as well as the impact of a top quark mass and an improved W mass measurement.To be published in Zeitschrift für Physik C into account by introducing appropriate external parameters. Up to this stage, our analysis is quite general, as the formulae are valid for any electroweak model respecting the flavor and chirality conservation laws of the SM, that is, for all new physics contributions which can interfere with the leading SM amplitudes.Although one may attempt to constrain these model-independent transition form factors directly by experiment, we find it impractical, since the number of independent 4 transition form factors exceeds by far the effective number of degrees of freedom provided by precision measurements. Hence, we perform the quantitative comparison of data with theory in a more restricted class of models which are minimal extensions from the SM, i.e. those models which respect the SU(2) L × U(1) Y gauge symmetry broken spontaneously down to U(1) EM .3: The transition form factors are expanded perturbatively in SU(2) L × U(1) Y gauge couplings, and radiative effects are classified either as the universal gauge boson propagator corrections or as the process specific vertex and box corrections. The universal propagator correction factors are then parameterized by four charge form factors,ē 2 (q 2 ),s 2 (q 2 ),ḡ 2 Z (q 2 ) andḡ 2 W (q 2 ), corresponding to the γγ, γZ, ZZ and W W propagator degrees of freedom.The restriction to the electroweak gauge group SU(2) L × U(1) Y implies at the tree level that all fermions, quarks and leptons, couple to the electroweak gauge bosons universally with the same coupling constant as long as they have common SU(2) L × U(1) Y quantum numbers. This universality of the g...

Constraints on the electroweak chiral Lagrangian from the precision data

“…[25], we observe that the m H dependence cancel accurately for m H > 300GeV. For definiteness, we set m H = 500GeV and find…”

“…The latest results from the LEP, SLC, and Tevatron [24] are shown in Table III. In the standard model, the relations among Γ ℓ , sin 2 θ eff W , m W and S, T , U can be expressed as [25] m W (GeV) = 80.377…”

“…where the central values are obtained by using the Zfitter 6.41 [26] and Higgs mass dependence of the SM predictions are incorporated in the shifts ∆S, ∆T , and ∆U [25]. In order to utilize the above formula designed for the theories with a Higgs boson to theories like the EWCL without a Higgs boson, we follow the prescription of Bagger, Falk and Swartz, [11];…”

Muon g-2 and LHC phenomenology in the L μ − L τ gauge symmetric model

“…We adopt formulas in, for example, refs. [39][40][41] for the calculation. We perform a χ 2 fit by varying input parameters m t , m h , ∆α (5) had and α s , in order to identify the parameter region consistent with the data.…”

How well do we need to measure Higgs boson couplings?

“…We have used the one loop expressions that appear in Ref. [14]. To compute this bound we have taken m h = 125 GeV and imposed the requirement that the contributions to the S and T parameters are within the 90% CL S − T contour in Ref.…”