First performance study on luminosity monitor and active mask by JIM simulator

weekly JLC meeting, 12/10/1999, T.Tauchi

This talk was presented at the JLC weekly meeting on 12/10/1999. The transparencies are contained in a pdf file (16 pages, 542kB). The content can be summarized as follows.

The purpose of this study was to show feasibilities of luminosity measurement inside a conical mask and activating the mask for a veto system with backgrounds caused by e+e- pairs. The luminosity monitor and the active mask were not optimized yet.

Luminosity monitor is located at 163cm from the interaction point(IP), which is made of tungsten(W) of 15cm thickness (42.86Xo) and the (polar) angular coverage ranges from 0.05 to 0.15 radian. It is segmented into 32, 16 and 128 divisions in radius (r), azimuthal angle(phi) and longitudinal coordinate(z). The physical dimensions are 5mm, 3.2-9.7cm and 1.17mm in r, phi and z, respectively. At present, all the tungsten material is a sensitive detector, that is, energy deposits in all segments are calculated in simulations. A real detector must have sensitive layers made of such as scintillators and silicon pads, or it may be crystal (BGO) instead of tungsten.

A front part of the conical mask is instrumented to measure energy deposits in electromagnetic showers. The mask is made of tungsten. The longitudinal position is 30cm from IP and the angular coverage is ranges from 0.15 to 0.2 radian. The active mask is consisted of 8 layers of silicon pads( Si(200 micron m) + G10(300 micron m) ), where the first W layer is 5mm thick and the other W layers are 1cm thick. In simulations, energy deposits in silicon pads are calculated. The active mask is segmented into 8-10 and 32 divisions in r and phi, respectively. The physical dimensions are 2mm and 0.9-1.2cm in r and phi, respectively.

The e+e- pairs were generated with a statistics corresponding to 100 bunch crossings ( ~ 1 train crossing) by cain21d, where machine parameters were nominal JLC-A at Ecm=500GeV. They should always overlap on any physics event at JLC. For the signals, an electron of 50 or 250 GeV was generated into the luminosity monitor and the active mask. Their energy deposits were calculated by the JIM simulation, and they are listed in the following table;

----------------------------------------------------
B=2T  e+e-pairs   50GeV e  250GeV e    250GeV muon
am     264MeV   120.4MeV   541.0MeV    0.48MeV
lm     152GeV    49.9GeV   249.5GeV    1.67GeV
B=3T  e+e-pairs
am     29.3MeV
lm     46.7GeV
-----------------------------------------------------
,where am=active mask, lm=luminosity monitor, and energy deposits due to a 250 GeV muon are also listed. As clearly seen, energy deposits of e+e- pairs decreases at higher magnetic field (B). The luminosity monitor contains all electromagnetic shower while the active mask has shower-linkages of about 10% in radial direction for a 250GeV electron compared with a 50 GeV electron. The longitudinal linkage in the active mask was estimated to be ~1% even for a 250GeV electron.

Although the total energy deposits of e+e- pairs are very large, the spatial distributions are very different from those of electrons as seen in figures (appended in pdf file), The simulations show that most of the pairs are absorbed near the surfaces of ~1Xo depth because of their low energies while the 50GeV / 250GeV electrons have shower maxima at 5 / 8Xo.

Conclusions

(1) Luminosity monitor

Among the total energy deposit of 152 (46.7) GeV/train due to e+e- pairs, only 54*(14*)GeV comes from the front at B=2(3)T, while most comes from the inner-back. (* sum of incoming energies). Phi-segmentation (16 div.) is very important, and r-segmentation is desired to determine polar angle with accuracy of ~a few m radian. So, a fine-segmented W/Silicon calorimeter seems to be ideal. Thickness of tungsten must be optimized in terms of energy-resolution.

(2) Active Mask

First layer (5mm thick W) has ~ 50% energy deposit for e+e- pairs. Phi segmentation (32 div.) is also very important. 8 layers of W/Si-pad calorimeter (21.4Xo) works very well for vetoing high energy electrons.

These results are only first order estimation. Apparently, we need more works for realistic detectors (detector type, radiation thickness, radiation damage, energy and angular resolutions, physics impacts.....).