Introduction to E787
E787 was approved in 198x, the first engineering run was in 1988, with long data taking runs in 1989-91. The detector and beam line were upgraded from 1992-94. The upgraded detector had a short engineering run in 1994, with long data taking runs in 1995-98.
The decay K+ -> pi nu nu is very interesting because it is possible to calculate the branching ratio from fundamental Standard Model (or more) parameters (|Vtd|) very precisely. Therefore a measurement of the branching ratio gives a very clean measurement of the important and elusive |Vtd|.
The first event of this elusive decay (BR ~ 10^-10) was observed during the 1995 run of E787. E787 has continued to take data through 1998.