It happened at about 4am on Saturday May 29, over the Bank Holiday weekend.
Police were called to reports of a fight outside the Samsung store on Briggate.
A group of youths - five males and a female - assaulted a 29-year-old man, who was alone at the time of the assault.
The victim was wearing a blue jacket at the time of the attack.
Officers believe a number of people may have witnessed the assault and they would like to speak to those with information.
Anyone with information is asked to contact 101 quoting crime reference 13210266522.
Information can also be given anonymously to the independent charity Crimestoppers on 0800 555 111.
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