Physical assault at the workplace or any other place is illegal. Whatever the differences or whatever provocation may be, these need to be handled professionally.
At this stage, you may order domestic enquiry to investigate the incident. First, design the terms of references for the enquiry and decide who will conduct the enquiry. After that send an official letter to the delinquent employee to depose before the enquiry. In the letter, write clearly when an enquiry will be conducted, where it will be conducted, who will conduct it etc. Send the letter by RPAD.
If the delinquent employee fails to depose before the enquiry, order the enquiry once more after a one month period. If the employee fails to depose second time also, you may take action ex parte and terminate his services.
Larger issues: -
What I have outlined is only the administrative procedure. Nevertheless, there are larger issues to be looked at. As of now, the focus is on the individual but at the same time, we need to give our focus on the organisation.
Why things have come to such a pass where an employee had to resort to physical assault? He has not just physically assaulted but abandoned duties also. This shows his extreme frustration. Why was he frustrated?
For how long the tension was brewing? Why it went unnoticed by HOD and if noticed, what steps did he take to resolve it amicably? Overall, what is the interpersonal environment in the company? If not physical then is a verbal attack a common feature in the workplace? Has any training been conducted on fostering a healthy interpersonal environment in the company?
What about the employee in question who attacked his colleague? Was his behaviour abrasive? Was he irascible? Did anyone counsel him earlier? If yes, then was the counsellor professionally trained?
While it is easy to blame the delinquent employee for his unprofessional behaviour, such incidents erode the credibility of the organisation in the eyes of the employee. These incidents also vitiate the organisation's culture. Therefore, steps must be taken to nip such incidents in the bud.