Below is an attempt to collect a few questions, and I apologize in advance if they are trivial and / or completely stupid (it is better to be stupid than to have stupid mistakes later in the detector).

- what should be the aspect ratio, why not ==1? One point is to move away problematic barrel/endcap overlap by increasing the aspect ratio. Another is to have sufficient level-arm for pt measurement even for small cos(theta) tracks. Probably, this is the question to tracker experts.

- What drives pz precision: do we need a big level-arm in Z (this also affects an aspect ratio) Some extra points: if track passes through a TPC readout endplate, the position there should be known very precisely (no distortions), so this point may probably be used to get a sufficient precision. If the track passes through the opposite plate (another TPC "endcap") - too bad because of long drifts and distortions, then, what to do?

Silicon just after TPC endcaps will help. The question, how to align it? One idea: align with a laser before TPC installation and hope that after it may move only as a rigid body as a whole, so only with 6 parameters (3 displacements and 3 rotations). They may be determined with physical tracks (ee->mumu, for example). Is this true? If alignment without TPC is done without magnetic field and at room temperature: with the field the detector may move a little due to magn. forces (how about thermal extension/shrinking of cool magnet?) Cooling down-up the detector to switch on-off magnetic field may be a long process. Probably, whether silicon acts as one solid body, depends on how it hangs, in how many points, whether they can move.

- whether one can calibrate the TPC distortions with the laser, by either ionizing the gas along laser ray or shooting to the opposite plate with some "photocathodes" emitting photoelectrons, which then drift through the whole TPC volume. What precision may be expected?

ECAL: problematic overlap regions:

- barrel-endcap

- between barrel modules with (phi modulo pi/4) close to +pi/8 (about 7% drop)

- endcap-endcap ring at 40 cm from the beam, it is not studied, but it is very "pointing" from IP

Other pointing cracks:

- first-second barrel alveola gap at z=9cm in DBD geometry

- first-second module barrel gap

- barrel-endcap

- gap between endcap modules at x or y = 40 cm

-- VladislavBalagura - 2014-11-21

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