Supersymmetry versus precision experiments revisited

Abstract

We study constraints on the supersymmetric standard model from the updated electroweak precision measurements --- the Z-pole experiments and the $W$-boson mass measurements. The supersymmetric-particle contributions to the universal gauge-boson-propagator corrections are parametrized by the three oblique parameters Sz, Tz and mw. The oblique corrections, the Zqq and Zll vertex corrections, and the vertex and box corrections to the \mu-decay width are separately studied in detail. We first study individual contribution from the four sectors of the model, the squarks, the sleptons, the supersymmetric fermions (charginos and neutralinos), and the supersymmetric Higgs bosons, to the universal oblique parameters, where the sum of individual contributions gives the total correction. We find that the light squarks or sleptons, whose masses just above the present direct search limits, always make the fit worse than that of the Standard Model (SM), whereas the light charginos and neutralinos generally make the fit slightly better. The contribution from the supersymmetric Higgs sector is found small. We then study the vertex/box corrections carefully when both the supersymmetric fermions (-inos) and the supersymmetric scalars (squarks and sleptons) are light, and find that no significant improvement over the SM fit is achieved. The best overall fit to the precision measurements are found when charginos of mass $\sim 100\gev$ with a dominant wino-component are present and the doublet squarks and sleptons are all much heavier. The improvement over the SM is marginal, however, where the total \chi^2 of the fit to the 22 data points decreases by about one unit, due mainly to a slightly better fit to the $Z$-boson total width.Comment: 61 pages, 16 figures. References added. version to appear in Nuclear Physics

Supersymmetric contributions to muon g−2 and the electroweak precision measurements

“…(5), x α = 0.4, the SM best fit is found given at The supersymmetric contributions to the oblique parameters have been studied in ref. [7] in detail. In the MSSM, the oblique corrections are given as a sum of the individual contributions of (i) squarks, (ii) sleptons, (iii) Higgs bosons and the (iv) ino-particles (charginos and neutralinos).…”

“…The R-parameter, which accounts for the difference between T and T Z , represents the running effect of the Z-boson propagator corrections between q 2 = m 2 Z and q 2 = 0 [7]. The parameter x α ≡ (1/α(m 2 Z ) − 128.90) /0.09 allows us to take account of improvements in the hadronic uncertainty of the QED coupling α(m 2 Z ).…”

“…The parameter x α ≡ (1/α(m 2 Z ) − 128.90) /0.09 allows us to take account of improvements in the hadronic uncertainty of the QED coupling α(m 2 Z ). ∆δ G denotes new physics contribution to the muon lifetime which has to be included in the oblique parameters because the Fermi coupling G F is used as an input in our formalism [7,9]. The third oblique parameter ∆m W = m W − 80.402(GeV) is given as a function of ∆S, ∆T, ∆U, x α and ∆δ G [7].…”

“…We find no improvement of the fit through the oblique corrections in the Higgs sector. The ino-particles give ∆T Z < 0, owing to the large negative contribution to the R-parameter when there is a light chargino of mass ∼ 100 GeV [7]. They also make ∆S Z negative when the light chargino is either gaugino-like or higgsinolike [7].…”

“…The ino-particles give ∆T Z < 0, owing to the large negative contribution to the R-parameter when there is a light chargino of mass ∼ 100 GeV [7]. They also make ∆S Z negative when the light chargino is either gaugino-like or higgsinolike [7]. However, we find that both ∆S Z and ∆T Z can remain small in the presence of a light chargino, if the ratio of the higgsino mass µ and the SU(2) L gaugino mass M 2 is order unity [13].…”