Officers were called at 11am on Friday, April 9, to Peel Terrace, closed to the Kings Arms pub, following reports of an assault.
A good Samaritan female who was travelling in a red car stopped to help the female plain-clothed officer following the incident.
The offender is described as a white male in his early 30s travelling on a black and silver bike, wearing a black puffa jacket.
Anyone with information is asked to contact Staffordshire Police on 101, quoting incident number 229 of April 9.
Alternatively, contact Crimestoppers anonymously on 0800 555 111.