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

In view of the recent measurement of the muon g − 2 and finalization of the LEP electroweak data, we re-examine all the new particle contributions to those observables in the MSSM. The SM fits the latest electroweak data excellently with the observed top-quark mass and the Higgs-boson mass of around 100 GeV, and so does the MSSM in the decoupling limit. The MSSM gives a slightly better fit to the data than the SM (∆χmin ∼ −2) when relatively light left-handed sleptons of mass ∼ 200 GeV and a light chargino of mixed higgsinowino character (μ/M2 ∼ 1) with mass ∼ 100 GeV co-exist. The improvement in the fit diminishes quickly for winoor higgsino-dominant charginos, for heavier charginos, and for lighter sleptons. We find that the MSSM contributions to the muon g − 2 is most efficient when the light chargino has a mixed character. If tanβ < 10, the set of a light mixed chargino and light sleptons that is favored by the electroweak data is also favored by the g − 2 data for μ > 0. Models with light gauginos (μ/M2 > 10) or light higgsinos (μ/M2 < 0.1) give significant contributions to muon g − 2 only for large tanβ(∼> 15). Looking for signatures of supersymmetry (SUSY) at high energy experiments is one of the most important tasks of particle physics. Since the LEP experiments have been completed without finding any evidences of new physics beyond the Standard Model (SM), the chance to discover the superparticles at the energy frontier has been postponed to the next stage, at the Tevatron Run-II or at the LHC. However, we may find important constraints on, or an indication of, the supersymmetric models from precise measurements of the SM particles which are sensitive to the interactions of superparticles. In this letter, we would like to examine the superparticle contributions to the muon g − 2 in the light of its recent measurement [1] and the finalization of the LEP electroweak data [2]. The precise measurement of the muon g − 2 has been achieved at BNL [1] where the experimental uncertainty has been reduced by about factor 3 from the previous measurement [3]. The current world average of the muon g−2 is then given by [1] aμ(exp.) = 11 659 203(15)× 10. (1) The comparison of the data with the SM prediction is [1] aμ(exp.)− aμ(SM) = 43(16)× 10, (2) where the experimental and theoretical errors are added in quadrature. The theoretical uncertainty is dominated by the hadronic vacuum polarization effect. The SM prediction in eq. (2) has been obtained by using the estimate of ref. [4]. Although consensus among experts should yet to emerge on the magnitude and the error of the SM prediction [5], we adopt the estimate of eq. (2) as a distinct possibility. The purpose of this letter is to examine if this possible inconsistency of the data and the SM can be understood naturally in the context of minimal supersymmetric SM (MSSM), when taking account of the electroweak precision data. The enormous data of the electroweak measurements at LEP1 have been analyzed after its completion in 1995. The final combination of the results from four collaborations – ALEPH, DELPHI, L3 and OPAL– has been available on the Z-line shape and the leptonic asymmetry data [6]. The electroweak precision measurements consist of 17 Z-pole observables from LEP1 and SLC, and the W -boson mass from Tevatron and LEP2. Let us first summarize the constraints on the MSSM parameters [7] from the electroweak data. The supersymmetric particles affect these observables radiatively through the oblique corrections which are parametrized by SZ , TZ , mW , and