An ARC trooper lieutenant in "official" armour only worn on coruscant when appearing in front of the Jedi Council or the republic senate. Usually the ARC's armour is customised and camouflaged to their needs and surroundings rather than left white which isn't very practical on most battlefields. It is only worn on coruscant or in public appearances as it is similar to modern dress uniform. This is done to make the the public believe that the war is a civilised affair rather than the brutal, messy hell it really is.
This ARC trooper is responding to an "altercation" in the senate. Six of the terrorists were killed in the ensuing brawl while two were taken away for interrogation.