Are you sure it's Pampas Grass (Cortaderia selloana)? I ask because it is generally not hardy to your zone. It's only barely hardy to mine.

Real Pampas Grass is evergreen, and generally does not require an annual haircut. On the fringes of it's hardiness zones (like in my yard) winter treats it roughly, so I cut it down to 4-6" in late winter (later this month). So far, it's come back fine.

If it's another tall grass just being called Pampas Grass, it definately needs to be cut back every year.