Without any photos it's hard for us to figure out what exact bolt you need. The bolt screws into the bottom bracket (which likely has a slightly tapered spindle, hence the need for the bolt fastened to the correct torque).
Based on your Bikepedia link, it seems that your bike originally had an SRAM S150 crankset, which seems in the 2011 era to use a Truvativ Powerspline bottom bracket. The Powerspline in turn seems to use these fixing bolts: Truvativ Crank Arm Bolts M8 Capless for Square Taper and PowerSpline, 11.6115.115.000-2 Pieces
Caveat: Your bike's bottom bracket may have been changed to something else, even at the time of original purchase, in which case these bolts may not be correct. Don't forget to put a dab of grease on the threads before attaching the bolt, and use a torque wrench to get the correct torque.
Important edit: OP's photo shows his BB is NOT a powerspline cartridge, but rather an external-bearing BB, so the above bolt suggestion doesn't match his bike.