What will cops do to them?

First of all,if you are in the U.S.A., in an uncontrolled intersection the car on the right has the right of way.Now let's get down to the real question.The police car had slowed down or was stopped at the intersection and he was rear-ended by the other car.It was the fault of the driver of the other car.It is your fault if you rear-end another car.They can cite him for C&R driving.All the years as a police officer I never had an accident,but one hard fast rule was to never work your own accident.The bumper is not an issue,but he will have to have it replaced.His insurance will have to pay for the damages to the police car because he was at fault.There should be either signs or lights installed at the intersection.