see if this answers your query:
</quote>
When an alarm is acknowledged, vCenter Server records the time the alarm was acknowledged and the user account that acknowledged the alarm. As long as the alarm condition persists, the alarm will remain in the Triggered Alarms view but is grayed out. When the alarm condition is resolved, the activated alarm disappears.
<quote\>
ref: http://searchsystemschannel.techtarget.com/feature/Monitoring-VMware-vSphere-performance-using-alarm...
I will try to test this out in my home lab as well..
