Autist mathematician trader in disgrace.

Former bank trader Tom Hayes was sentenced to 14 years in jail on Monday after a London jury convicted him of fraudulently trying to rig the London interbank offered rate, or Libor.

14-year prison sentence, is one of the harshest penalties meted out against a banker since the financial crisis. While several big banks have pleaded guilty to manipulating Libor, it was the first criminal conviction of an individual for rigging the widely used benchmark.

Mr. Hayes, a mildly autistic mathematician whose quirky personality earned him the nickname “Rain Man” among colleagues, was accused by British prosecutors of conspiring with other bank traders and brokers to manipulate Libor to make more money for himself and his employers.

The 35-year-old Briton argued that his behavior while at UBS Group AG and Citigroup Inc. was in line with industry standards, that his bosses knew about and condoned what he was doing and that he never realized his behavior was improper.

Mr. Hayes, who moved to Tokyo in 2006, quickly became one of the market’s elite traders. He generated hundreds of millions of dollars in revenue for UBS by trading complex instruments known as interest-rate swaps. He joined Citigroup in late 2009.

In September 2010, less than two weeks before his wedding in England, Citigroup fired him for Libor manipulation.