The Alphas cast.
Alphas is a sci-fi television series about a team with extraordinary abilities solving crimes linked to others like them. It aired on Syfy for two seasons and was canceled leaving it on an unresolved cliffhanger.
Alphas is mentioned in "The Closure Alternative" (S06E21) as being one of Sheldon's favorite TV shows. Naturally, Sheldon is devastated that the show had been canceled before it could end properly, so he calls Syfy to try to save "Alphas", but they hang up on him because they think he's crazy even though his mother had him tested. This behavior leads Amy to try and cure Sheldon's compulsive need for closure, but as soon as she leaves, her task proves to be all for naught. Afterwards, Sheldon calls Bruce Miller, the writer of the last "Alphas" episode, on how the cliffhanger would be resolved and hangs up, apparently disappointed with his response.
Ryan Cartwright, who portrayed Gary Bell on "Alphas", made a guest appearance as Cole, a British student in Penny's history course, in "The 43 Peculiarity" (S06E08).