abstract
 

1998 Km21 and Kp22 monitor study shows a 1.0  MeV/c systematic offsets on the muon and  pion momenta. This deviation stems from the actual magnetic field inside the Ultra Thin Chamber (UTC) with an average  9.954 kG, which is smaller than the expected constant 10 kG used in the previous charged track reconstruction.  A correction is thereby introduced and implemented in the codes for 1998 data processing.

1.   1998 Km21 and Kp22 monitor check

To monitor the experiment condition, during the standard physics data taking period, the E787 DAQ system also recorded down two major kaon decay modes: K->muon + neutrino (called Km2) and K-> pion + pion-zero (called Kp2). Different from their trigger condition on the delayed coincidence, namely requiring Kaon stops in the target and can only decays  2 ns  afterwards, these samples are classified as Km22 or Kp22, otherwise are treated as Km21 or Kp21. This trigger tends to eliminate the beam background and the case of kaon decay in flight, but has little impact on the study of the momentum peaks. In this study, only Km21 and Kp22 monitors are used.

Since both Km21 and Kp22 modes are two-body Kaon decays, the daughter muon and pion momenta should be of monochrome  and are expected to be 235.5 MeV/c  and 205.1 MeV/c  separately. A first look at the muon and the pion momentum spectrum in 98 data shows

                    Pmuon = 236.9 MeV/c,   Ppion = 206.4 MeV/c.

                              Pmuon = 236.3 MeV/c,   Ppion = 206.3 MeV/c.

Even by doing so, the upward offset is still significant. The momentum measurement comes mainly from the UTC. As the above cuts exclude any possibility of wrong transervse momentum measurement, only the UTC geometry inputs in offline reconstruction or the constant 10 kG magnetic field used in calculating the transverse momentum can produce the momentum offset. Since the same geometry inputs also apply for the previous data sets e.g.  1995 data, no such a big shift is observed (see Table 1), the magnetic field downward variation must account for this offset.  Simply from the scaling, one can estimate the average magnetic field in 1998 to be 9.954 kG.   In order to observe the trend of magnetic field in different years, the same check is also done for all the previous data, as given in Table 1.
 

Year	Pmuon (MeV/c)	Ppion (MeV/c)	B0 (kG)
1995	235.3	205.2	10.002
1996	235.3	205.2	10.002
1997	236.0	205.7	9.975
1998	236.3	206.3	9.954

Table 1. The measured muon and pion momenta from Km21 and Kp22 monitors in different years.  Since the cuts applied reduce the longitude momentum contribution, the momenta in this table are regarded as full transverse momentum and the magnetic field B0 for different years are obtained by comparing to the known mometum values.

A trend of decreasing magnetic field is clearly observed.

2. The variation of magnetic field in 1998

In E787, the magnetic field is continuously read out from two Hall probes installed inside the detector.  They are both mounted on the inside face of the magnet end plate on the downstream at a radial position close to the barrel photon veto system. The power supply to the magnet is regulated through a shunt in the output current path and is usually subject to temperature and aging effects. It has been noticed that the Hall  probes readout drifted downward slowly over the years to about 1% lower than the 10 kG designed value. In 1998, the variation of magnetic field shows a clear periodic behavior-- gently decreasing from 36030 to 38983 run and stability jumping up during the rest runs (see Fig. 1). The online logbook indicates that there was a power dip happening in run 38893 which corresponds to the date of October 6, 1998. All high power crates and all magnets tripped at that time. When the power supply was resumed, the magnetic field is monitored to have a 2% variation (also see Fig. 1). One may concerns whether or not this variation truly reflects the actual situation of magnetic field.  A further investigation is therefore performed by checking the correlation between the measured magnetic field and the track momentum from a constant 10 kG field used in the reconstruction (see Fig.2).

Fig. 1 The average magnetic field for every eight runs as a function of run number in 1998 data.	Fig. 2 The pion momentum using 10 kG constant magnetic field in the UTC reconstruction as a function of actual magnetic field in 1998 Kp22 data.

Besides the offset related to the conversion factor, a descent trend of momentum is seen associated with the increasing magnetic field. Since the momentum is derived from a 10 kG constant value. If the actual magnetic field is small, assigning a larger magnetic field results in a smaller momentum observed.  In conclusion, the Hall probes do detect the variation of magnetic field and there is no reason not to take full advantage of it,  though the reason for why magnetic field changes so frequently is still unknown. Despite of this, one also observes that there are some fake field measurements which may be due to something wrong in the DAQ readout system, since they stay quite far away from the neighboring ones (see Fig. 3 and Fig.4 ). It is noted that starting from run 39465 to 39555, there are a lot of spills recorded a low magnetic field. The online logbook also recorded the warning message for a possible field change. However, the check on the pion and muon momenta does not show any evidence of a low magnetic field. Looking into individual run, one finds a rather stable magnetic field (see Fig. 4) except for some fields with a flip-flopping problem.

Fig. 3 The magnetic field variation as a function of run number.	Fig. 4 The magnetic field variation as a function of spill record in a single problematic run.

3. Magnetic field calibration for 1998 data

Calibration for the magnetic field is performed in two steps. A global calibration constant for converting the scaler record to the magnetic field value in kG unit is obtained from the comparison between the expected field as given in Table 1. and the previous one, which is found to be smaller because of using a smaller conversion factor 10/1994. In 1998 data, a factor of  0.005071 is determined from the muon and pion momenta.  Using the magnetic field  from this factor gives a  better result when the field is above 9.925 kG and below this shows a problem that is completely due to the fake scaler record (see Fig. 5)

Fig.5 The pion momentum as a function of simple calibrated magnetic field in 1998 Kp22 data. The problem of momentum corresponding to smaller magnetic fields comes from the fake scaler record. A slight increase of momentum is also observed for the larger magnetic field, indicating a necessary fine calibration regarding the periodic variation of magnetic field.

For the case of fake magnetic field measurement, the updated "my_end_spill" will abandon any field below  9.925  kG and use the average value in the corresponding run. Consequently, a new CFM file "ut_bav.00001" is created and can be read by "my_end_spill".  In addition, to take into account the slight correlation between momentum and magnetic field, a linear correction term is applied. The final calibration result shows a stable momentum absorbing the actual magnetic field variation (see Fig. 6).

Fig.6 The pion momentum from using the final calibrated magnetic field in 1998 Kp22 data. No correlation between the momentum and the actual field change is observed since the field variation is absorbed in the UTC charged track reconstruction.

 

4. Conclusion

A study of magnetic field is done for the 1998 data. The investigation of muon and pion momenta in Km21 and Kp22 monitors shows  a necessary calibration for the magnetic field which has not been performed before. This calibration has been completed and is ready for the forthcoming 1998 data processing.