Muon g − 2 and precision electroweak physics in the MSSM

The minimal supersymmetric extension of standard model (MSSM) is examined by analyzing its quantum effects on the precision electroweak measurements and the muon g − 2. We examine carefully the effects of light charginos and neutralinos that are found to improve the fit to the electroweak data. We identify two distinct regions on the (μ,M2)-plane that fit well to the electroweak data and give significant contribution to muon g − 2. The minimal supersymmetric standard model (MSSM) has been one of the extensively investigated theories beyond the standard model since it was recognized [1] that it has the potential to unify strong and electroweak gauge interactions. On the other hand, despite enormous efforts to find its signature, there has been no direct evidence for the existence of the supersymmetric particles. In this letter we study quantitatively the MSSM predictions for the muon g − 2 in the light of the latest precision electroweak data [2, 3], and that of the present and future g − 2 experiments [4]. The importance of the muon g − 2, conventionally denoted as aμ = 1 2 (gμ − 2), has been widely discussed in the context of supersymmetric theories [5, 6, 7, 8]. The observation of the effect from the MSSM is found generally accessible with the target accuracy of the current experiment at Brookhaven National Laboratory (BNL) [4] ∆aμ(expt) = 4.0× 10 −10 . (1) This accuracy is about 1/20 of the error (numerals in the parenthesis) of the current measurement [4] aμ(expt) = 11659 235 (73)× 10 −10 . (2) The prediction of the standard model (SM) is aμ(SM) = 11659 168.75 (9.56)× 10 −10 , (3) where the details of specific contributions is found in Ref. [9]. We shall come back to the theoretical uncertainties of the SM prediction not included in the above quoted error of 9.56× 10 in the summary part. As noted above the experimental results on the precision measurements around the Z-pole have played an essential role in revealing the possibility of grand unification. This implies the existence of many supersymmetric particles below the TeV scale and hence we may expect to observe their quantum effects in the precision electroweak experiments as well as in the muon g − 2. Although no signal of supersymmetry has so far been identified, recent systematic study of the precision electroweak data found that the existence of the relatively light (∼ 100 GeV) charginos and neutralinos can make the MSSM fit slightly better than that of the standard model [10]. We therefore study carefully the MSSM effects on the muon g − 2 in the range |μ|, M2 < 500 GeV. It is found that analysis performed here could give rise to a systematic criterion to identify the preferred range of the MSSM parameters.