Hockey referee question?

During a play in Team A's attacking zone, a helmet is on the ice near the blue line. The defenseman from Team A has control of the puck and is getting ready to p the puck to one of his teammates. A Team B player shoots the helmet at the puck and knocks it from the defenseman's stick. The action is not seen by either of the Referees, however it is seen by the Linesman. What does the Linesman do in this situation