Experimental setup

The experiment is performed at the IHEP 70 GeV proton synchrotron U-70. The experimental apparatus "ISTRA+" is the result of the modification of "ISTRA-M" [5], which, in turn, evolved from "ISTRA" that yielded important results on $\pi^{-}$ and $K^{-}$ decays in the late 1980's [6]. The setup is located in the 4A negative unseparated secondary beam. The beam momentum in the measurements is $\sim 25$ GeV with $\Delta p/p \sim 2 \%$. The admixture of $K^{-}$ in the beam is $\sim 3 \%$. The beam intensity is $\sim 3 \cdot 10^{6}$ per 1.9 sec. U-70 spill. A schematic view of the detector is shown in Fig.1.

Figure 1: The layout of the "ISTRA+" setup.

The momentum of the beam particle deflected by M$_{1}$ is measured by $BPC_{1}\div BPC_{4}$ PC's with 1mm wire step, the kaon identification is done by $\check{C_{1}} \div \check{C_{3}}$ threshold $\check{C}$-counters. The 9 meter long vacuumated decay volume is surrounded by 8 lead glass rings $LG_{1} \div LG_{8}$ used to veto low energy photons. The same role is played by $SP_{2}$- a 72-cell lead glass calorimeter. The decay products deflected in M2 with 1Tm field integral are measured with $PC_{1} \div PC_{3}$- 2mm step proportional chambers; $DC_{1} \div DC_{3}$- 1cm cell drift chambers and finally with 2cm diameter drift tubes $DT_{1} \div DT_{4}$. A wide aperture threshold Cerenkov counter $\check{C_{4}}$ is filled with He and used to trigger the electrons. $SP_{1}$ is a 576-cell lead glass calorimeter, followed by HC- a scintillator-iron sampling hadron calorimeter, subdivided into 7 longitudinal sections 7$\times$7 cells each. MH is a 11$\times$11 cell scintillating hodoscope, used to solve the ambiguity for multitrack events and improve the time resolution of the tracking system, MuH is a 7$\times$7 cell muon hodoscope.
The trigger is provided by $S_{1} \div S_{5}$ scintillation counters, $\check{C_{1}} \div \check{C_{3}}$ Cerenkov counters, analog sum of amplitudes from the last dinodes of the $SP1$ and is very loose: $T=S_{1} \cdot S_{2} \cdot S_{3} \cdot \bar{S_{4}} \cdot \check{C_{1}} \cdot \... ...ck{C_{2}}} \cdot \bar{\check{C_{3}}} \cdot \bar{S_{5}} \cdot \Sigma(SP_{1})$, here S4 is a scintillator counter with a hole to supress beam halo ; $S5$ is a counter downstream of the setup at the beam focus; $\Sigma(SP_{1})$- a requirement for the analog sum of amplitudes from $SP1$ to be larger than $\sim$700 MeV - a MIP signal. The last requirement surves to suppress the $K \rightarrow \mu \nu$ decay. Some complementary triggers: $T_{e}=S_{1} \cdot S_{2}\cdot S_{3} \cdot \bar S_{4} \cdot \bar S_{5} \cdot \check{C_{4}}$ - the electron trigger and prescaled "decay" trigger $T_{d}=S_{1} \cdot S_{2}\cdot S_{3} \cdot \bar S_{4} \cdot \bar S_{5}$ were used to crosscheck the efficiency of the main one.

The main difference between "ISTRA-M" and "ISTRA+" is in the electronics and DAQ: all the CAMAC based electronics was changed by IHEP developed MICC [7] ECL-based electronics. "ISTRA+" has now 12 MICC crates with ADC's, TDC's and latches. The DAQ, described in some detailes in [8] is based on IHEP-developed VME master V-08 [9], which writes the MICC stream into standard VME memory. Between the spills, the information is written into PC through BIT-3 VME-PCI interface. The saturated event rate is $\sim 6500$ of 1 Kb events per 1.9 sec. spill.