Police have released CCTV images of a woman who may have witnessed a serious assault in Leeds city centre.
The woman was seen having an argument with the suspect, who then assaulted a 37-year-old man when he intervened.
He suffered a broken jaw and needed surgery.
Detective Constable Andy Smurthwaite, of Leeds District CID, said: "This was a completely unprovoked attack that left the victim with very serious injuries and we urgently need to identify his attacker.
"The CCTV images of this woman are very clear and we are hoping she will recognise herself and contact us or that someone will know who she is.
"We believe she will have witnessed the incident and will have information that could assist the investigation.
"We would also still like to hear from anyone who recognises the suspect shown in the images."
The incident happened in Blayds Yard at about 1.30am on Christmas Eve.
Anyone with information is asked to contact DC 209 Smurthwaite at Elland Road via 101, quoting crime number 13160753763.
Alternatively, call the independent charity Crimestoppers anonymously on 0800 555 111.