The top quark is the third-generation up-type quark with a charge of +(2/3)e. It was discovered in 1995 by the CDF and D0 experiments at Fermilab, and is by far the most massive of known elementary particles. Its mass is measured at 172.6±1.4 GeV/c2 , about the same weight as the nuclei of tantalum or tungsten atoms. The top quark interacts primarily by the strong interaction but can only decay through the weak force. It almost exclusively decays to a W boson and a bottom quark. The Standard Model predicts its lifetime to be roughly 1×10−25 s; this is about 20 times shorter than the timescale for strong interactions, and therefore it does not hadronize, giving physicists a unique opportunity to study a "bare" quark.