The earliest known cultural tribes of the area were members of the Hallstatt and La Tène cultures, named after the archaeological site of La Tène on the north side of Lake Neuchâtel. La Tène culture developed and flourished during the late Iron Age from around 450 BC, possibly under some influence from the Greek and Etruscan civilisations. One of the most important tribal groups in the Swiss region was the Helvetii. Steadily harassed by the Germans, in 58 BC the Helvetii decided to abandon the Swiss plateau and migrate to western Gallia, but Julius Caesar's armies pursued and defeated them at the Battle of Bibracte, in today's western France, forcing the tribe to move back to its original homeland. In 15 BC, Tiberius, who was destined to be the second Roman emperor and his brother, Drusus, conquered the Alps, integrating them into the Roman Empire. The area occupied by the Helvetii—the namesakes of the later Confoederatio Helvetica—first became part of Rome's Gallia Belgica province and then of its Germania Superior province, while the eastern portion of modern Switzerland was integrated into the Roman province of Raetia. Sometime around the start of the Common Era, the Romans maintained a large legionary camp called Vindonissa, now a ruin at the confluence of the Aare and Reuss rivers, near the town of Windisch, an outskirt of Brugg.

Answer this question, if possible (if impossible, reply "unanswerable"): Which Swiss tribal group was defeated by Julius Caesar at the Battle of Bribacti as they tried to escape the Swiss plateau?
the Helvetii