| This analysis is to measure the branching ratio for the D0 → μ +μ− flavor-changing-neutral-current decay. Though this decay is allowed in high order diagrams in the Standard Model, the branching ratio is highly suppressed by both the GIM mechanism and helicity. The predicted branching ratio, within the context of the Standard Model, is of order 10−13 including the contribution from long distance interactions. However, in some extensions of the Standard Model, the branching ratio is predicted between order 10−6 and 10−11 which is well above the predicted Standard Model contributions. This makes this decay particularly interesting in testing these new models.; The HERA-B experiment observes the collision of a 920 GeV/c proton beam with fixed targets ( = 41.6 GeV). During the year 2000 run we collected 3800 J/Ψ → μ +μ− from 610k dimuon trigger events. With this data, I have determined an upper limit BR(D0 → μ +μ−) ≤ 1.5 × 10−5 at 90% confidence level. The dependence of the result on the analysis cuts is carefully studied. The result, mainly limited by the statistics, is about 4 times larger than the current best limit. With more data coming in the next runs, HERA-B is capable of lowering the best upper limit of this decay mode. |
