The global head of trading for UBS' central risk book - a maths prodigy who achieved the coveted rank of managing director at the unusually young age of 28 - is leaving to join the hedge fund Citadel.

Paul Jefferys, a quantitative trader who plays a key role in managing risk for the Swiss bank's equities business, will join Ken Griffin's $26 billion investment group in an as yet unknown role, according to people familiar with the matter.