The top quark, also known as the t quark (symbol: t) or truth quark, is an elementary particle and a fundamental constituent of matter. Like all quarks, the top quark is an elementary fermion with spin-¹⁄₂, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions. It has an electric charge of +²⁄₃ e, and is the most massive of all observed elementary particles (the Higgs boson, which may be nearly as massive, has not yet been experimentally observed). It has a mass of 172.9±1.5 GeV/c², which is about the same mass as an atom of tungsten. The antiparticle of the top quark is the top antiquark (symbol: t, sometimes called antitop quark or simply antitop), which differs from it only in that some of its properties have equal magnitude but opposite sign.

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, but it can also sometimes decay into a strange quark, and on the rarest of occasions, into a down quark. The Standard Model predicts its mean lifetime to be roughly 5×10⁻²⁵ s. This is about 20 times shorter than the timescale for strong interactions, and therefore it does not form hadrons, giving physicists a unique opportunity to study a "bare" quark. (All other quarks hadronize, meaning they can only be found in hadrons.) Because it is so massive, the properties of the top quark allow predictions to be made of the mass of the Higgs boson under certain extensions of the Standard Model (see Mass and coupling to the Higgs boson below). As such, it is extensively studied as a means to discriminate between competing theories.

William Henry "Hank" Green II (born May 5, 1980, in Birmingham, Alabama) is a professional entertainer and entrepreneur. He is best known for his online video blogs with his brother John Green. As of February 2012, their channel, Vlogbrothers, has more than 965 videos which, together, have been watched more than 225 million times.

Green was born in Birmingham, Alabama, but his family soon moved to Orlando, Florida, where he was raised. He graduated from Winter Park High School in 1998 and then earned a bachelor's degree in Biochemistry from Eckerd College and a master's degree in Environmental Studies from the University of Montana. Green currently resides in Missoula, Montana, with his wife Katherine, their cat Cameo, and their dog Lemon.

Green and his brother John ran a video blog project called Brotherhood 2.0. The original project ran from January 1 to December 31, 2007, with the premise that the brothers would cease all text-based ("textual") communication for the year and instead converse by video blogs, made available to the public via their YouTube channel Vlogbrothers and on their website. The initial idea for the project was John's when, during an instant messaging conversation between the two, he pointed out they had not gone past the realms of communication via phone conversations, e-mails, and instant messaging for nearly a year.