1. Layout Tool results In a first exercise https://mersi.web.cern.ch/mersi/layouts/.LongBarrel/LongBarrel_4_15_PS/index.html we have reproduced something similar to the "traditional" long barrel layout. The phi sector is 15 degrees, and consists of 7 rods, 1+2+4 in the central region, becoming 6, 1+2+3, when the fourth super-layer ends and the short super-layer 3 takes over. There is only one short layer, otherwise you cannot have self-contained 15 degrees phi sectors (... I never understood how/if this aspect was handled in the LB layouts presented so far). pT_cut is 2.5 GeV in this geometry, the layer average radii are 330, 655, 875, 1060, and the phi multiplicities are 24, 48, 72, 96. You see some performance estimates in the various tabs. The number of modules is quite large: 44064! In a second exercise https://mersi.web.cern.ch/mersi/layouts/.LongBarrel/LongBarrel_4_15/index.html we have have used the same geometry but we have used PS modules in super-layers 1 and 2 and 2S modules in 3 and 4. PS modules are heavier, power hungry, more difficult to fabricate and certainly more expensive, we cannot propose to use them at large radii unless we have an absolutely compelling argument. 2S modules also offer a small advantage in momentum resolution, thanks to the pitch of 90 microns instead of 100 microns - which we can't see very well now because of the wrong pitches. The drawbacks of using 2S modules in the outer part could be (1) The z0 resolution for L1 tracks is degraded, because of the reduced lever arm between precision z coordinates: from the potential performance the degradation seems to be rather moderate: e.g. from ~400 microns to ~550 microns for 10 GeV tracks in the central and intermediate regions. (2) Tracklets from layer 3 would have a poor polar angle measurement, and therefore would be probably unsuitable to extrapolate down to layers 1 and 2. This implies that the track finding would have a double-redundant logic, where the tracklets from Layer 1 and 2 can be used as seed for the extrapolation, instead of the triple- redundant logic that you have today. I think point (1) is acceptable, and can be a bit improved with a more optimized geometry (see below). Point (2) should also be acceptable, if we build a detector of good quality. Considering also the effort that several groups are putting in the development of 2S modules (which looks quite promising!) I believe it adds a lot of credibility to the LB concept if 2S modules are used in the outer part. The total n of modules is 17376 + 11904 (PS + 2S), which is already much better.

2. Layout Tool results In a third exercise https://mersi.web.cern.ch/mersi/layouts/.LongBarrel/LongBarrel_4_18/index.html we have reduced the radii of the layers keeping the same geometry. As there is no interposer, in PS modules the sensor spacing can be easily increased, and therefore they work pretty well down to ~20 cm, in barrel configuration, with a spacing of ~2.5 mm. In this geometry the phi sector is 18 degrees, and still consists of 7 rods, 1+2+4 in the central region, becoming 6, 1+2+3, when the fourth super-layer ends and the short super-layer 3 takes over. The layer average radii are now 260, 545, 730, 911, and the phi multiplicities are 20, 40, 60, 80. With this geometry, the pT_cut is 2.0 GeV! The n of modules is now 13880 + 9920 (PS + 2S). This option has less modules, better pT acceptance for L1 tracks, the z0 resolution @ L1 is slightly improved because of the smaller radius of the 1st layer (~ 500 microns for high momentum tracks), but there is a relevant drawback in pT resolution (up to ~30% worse at high momentum) due to the lower overall lever arm of the Tracker. In a fourth exercise https://mersi.web.cern.ch/mersi/layouts/.LongBarrel/LongBarrel_5_20_a/index.html we have changed the geometry trying to fully exploit the available radial range and the capability of the modules. The phi sector is now 20 degrees, there are two short super-layers, the average radii are 243, 500, 670, 843 and 993, and phi multiplicities 18, 36, 54, 72, 90. The phi sector consists now of 8 rods in the central region, 1+2+5, becoming 7, 1+2+4, when the fifth super-layer ends and the short super-layer 4 takes over, and then 6, 1+2+3 when the super-layer 4 ends and the super-layer 3 takes over. The pT_cut is still 2.0 GeV (with ample margin). The n of modules is now 12492 + 11592 (PS + 2S). The z0 resolution @ L1 is about the same and the pT resolution is essentially restored. This layout is both good and cheap (... so to speak...), as it stretches the use of the modules in the whole possible range. In a fifth and last exercise https://mersi.web.cern.ch/mersi/layouts/.LongBarrel/LongBarrel_5_20_b/index.html https://mersi.web.cern.ch/mersi/layouts/.LongBarrel/LongBarrel_5_20_b/index.html we have tried to move outward the intermediate full-length super-layer, and evaluate advantages and disadvantages. The phi sector is still 20 degrees and the phi multiplicities are the same as in the previous case. The advantage is an improvement in the L1 z0 resolution (from 500 to 400 microns at high momentum), due to the increased lever arm between precision z coordinates. On the other hand the n of modules is now 18360 + 11232 (PS + 2S), which is a large difference (plus the increase in power that goes with it). I don't think that such an increase in the n of modules can be proposed without a compelling justification.