I’m not a boffin but the question intrigued me. You would have been quicker googling than posting

but here it is
Yes it is dead.
28.3 Protective helmets belonging to the fielding side
28.3.1 Protective helmets, when not in use by fielders, may not be placed on the ground, above the surface except behind the wicket-keeper and in line with both sets of stumps.
28.3.2 If the ball while in play strikes a helmet, placed as described in 28.3.1
28.3.2.1 the ball shall become dead
and, subject to 28.3.3,
28.3.2.2 an award of 5 Penalty runs shall be made to the batting side;
28.3.2.3 any runs completed by the batsmen before the ball strikes the protective helmet shall be scored, together with the run in progress if the batsmen had already crossed at the instant of the ball striking the protective helmet.
28.3.3 If the ball while in play strikes a helmet, placed as described in 28.3.1, unless the circumstances of Law 23.3 (Leg byes not to be awarded), 25.7 (Restriction on the striker’s runner) or Law 34 (Hit the ball twice) apply, the umpire shall
- permit the batsmen’s runs as in 28.3.2.3 to be scored
- signal No ball or Wide ball to the scorers if applicable
- award 5 Penalty runs as in 28.3.2.2
- award any other Penalty runs applicable.