Constraints on Light Top Squark from B-B̄ mixing

We discuss the constraints on the mass of the lighter top squark from BB̄ mixing in the minimal supersymmetric standard model. A light top squark whose mass is less than half of the Z-boson mass has not yet been excluded from direct search experiments at LEP. However, the existence of the light top squark may exceedingly enhance B-B̄ mixing, owing to the box diagrams exchanging the charginos and the up-type squarks. We show that for a sizable region of parameter space the light top squark contribution to B-B̄ mixing becomes the same order of magnitude as the standard W -boson contribution. Taking into account the experimental results for B-B̄ and K-K̄ mixings, the existence of the light top squark is excluded in an appreciable region of the parameter space which LEP experiments have not ruled out. Research Fellow of the Japan Society for the Promotion of Science. Supported by Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, contract number 6640409. Deceased. Supersymmetric particles have been searched for extensively in experiments, since supersymmetry (SUSY) is considered one of the most promising ideas for physics beyond the standard model (SM) from both aesthetical and phenomenological viewpoints. Up to now no direct experimental evidence for those new particles has been found, giving constraints on SUSY parameters [1]. In particular, abundant data of ee collisions at LEP almost exclude pair productions of charged SUSY particles at the Z resonance. However, there still remains a possibility that one of the top squarks (hereafter, we say “stop” for short) has a mass less than the half of the Z-boson mass. In the minimal supersymmetric standard model (MSSM) one stop could naturally be lighter than the other squarks [2]. Because, the large mass of the top quark induces a large mixing between the leftand the right-handed stops, while such a mixing is negligible in the first and second generations. One of the stops could be the lightest charged SUSY particle. Moreover, if the left-right mixing angle of the stops has a certain value, the lighter stop decouples with the Z-boson [3]. Then the cross section of the stop pair production is not large at the Z resonance, and the stop could escape detection at LEP. The experimental mass bounds on the lighter stop t̃1 have been given by CELLO at PETRA [4], AMY, TOPAZ, VENUS at TRISTAN [5], and OPAL at LEP [6], which depend on the assumed mass difference ∆m between the stop and the lightest neutralino. Assuming ∆m ≥ 5 GeV, the lower bound is about 45 GeV, if the left-right mixing angle has a value not close to the ‘decoupling’ value. On the other hand, if this mixing angle lies in the vicinity of the ‘decoupling’ value, the lower bound is about 40 GeV for ∆m ≥ 5 GeV and 25 GeV for ∆m ≥ 2 GeV. There also have appeared several analyses which constrain the light stop mass from other phenomena, such as the decay b → sγ [7] and the cosmological mass density [8]. However, the allowed region derived from the ee experiments is not much altered. In this letter we discuss the possibility of the existence of the light stop, whose mass is smaller than 1 2 MZ , by investigating its effects on B -B̄ and K-K̄ mixings. In the MSSM, these mixings receive contributions from the box diagrams mediated by the charginos and up-type squarks and those mediated by the charged Higgs bosons and up-type quarks, as well as the standard box diagrams. It was recently shown [9] that these new contributions are comparable to the SM contributions in sizable regions of the SUSY parameter space. In particular, the chargino