As already mentioned in the introduction, work since theLoIhas shown that the physics program is significantly enhanced with an increased pseudo-rapidity coverage with respect to theLoIlayout. The Reference scenario therefore assumes anη-coverage to|η|=4.0as being the best compromise between the requirements of physics and constraints of space, material and services. The details of the best way to realise the extension are still an active area of investigation within theITkcommunity. The exact pseudo-rapidity cut-off must take into account the combined effects of: the decreased magnetic field at large z and small radius, the decreased lever-arm for tracks that originate in that region and the effects of increased material from the services that would be located in front of the calorimeter. In particular, additional material in this location must be carefully monitored, since it not only degrades the resolution of the forward calorimetry, but it also increases the flux of secondary particles into the forward regions of theITk, which in turn increases the neutron fluence throughout the tracker cavity.
In preparation for the Strip and PixelTDRs, a tracker layout task-force has been created, and has been reviewing the general requirements for the ITk layout and design, as well as looking at further optimisation of the Pixel and Strip layouts beyond the currentLoIdesigns. Several variations are being examined, including detailed engineering considerations for the local and global supports, as well as the services. A wide range of performance and physics benchmarks are being used to evaluate these layouts, and the task-force is expected to report in early 2016. As a result of this work, the baselineITklayout which will be described in detail in theITk TDRsis expected to evolve from the present LoI designs discussed in this document. However, the overall cost for theITkis not expected to change substantially in this process, although both cost and performance will be further optimised within the constraints imposed by theITkrequirements.
IV.2.1
Reference scenario layout
For the Reference tracker layout presented in this document the angular coverage provides full tracking capability to|η|=4.0. This layout of the ITksystem is shown in Fig.7and it is referred to here and elsewhere in the document as theLoI-VFlayout. It has twelve pixel discs on each side of the detector to keep the number of space points on a track approximately constant up to |η|=4.0. This detector layout can be considered as idealised, in the sense that it has not yet been optimised in terms of mechanical construction and maximum performance for a given silicon area, and the routing of services is very challenging, but the performance should depend mainly on the number
of space points and the segmentation, and so we have confidence that this layout provides a useful assessment of the relative performance variation between scenarios.
Disc s Strips x 7 Pixel Discs x 12 Long (47.8mm) Strips x 2 (r=762mm, 1000mm) Stub Layer x 1 (r=862mm) (z=1415mm,1582mm,1800mm, 2040mm, 2320mm,2620mm,3000mm) Pixel Barrel x 4 (r=39mm,78mm,155mm, 250mm) Short (23.8mm) Strips x 3 (r=405mm,519mm, 631mm) 877mm,1059mm, 1209mm, 1358mm, 1509mm,1675mm,1875mm, 2075mm, 2275mm,2500mm,2750mm,3000mm
Figure 7. A cross-section of theLoI-VFlayout showing the coverage of the pixel detector in red and the strip detector in blue. The pseudo-rapidity coverage extends up to |η|=4.0. Blue and red lines represent strip and pixel layers, respectively. Horizontal and vertical lines represent barrel and end-cap layers, respectively. Lines of constant pseudo-rapidity are indicated. The blue line outside theITkvolume represents the coil of the solenoid magnet. This layout is used in the Reference scenario.
IV.2.2
Middle scenario layout
The layout of theITkin the Middle scenario (shown in Fig. 8) introduces notable reductions com- pared to the Reference scenario. A pair of strip discs (i.e. the next to the last disc in z) and the stub layer are removed from the strip detector; and the η-range covered by the pixel discs is limited to 3.2. Due to the limitations of time for the preparation of this scoping document, no re-optimisation of the relative positions of the different detector elements has been attempted for this layout. The hit information (digitisation) from the regions which have been removed are not provided to the recon- struction software and are not used in the performance analysis. The material of the elements which have been removed, however, remains in the detector simulations. This layout explores a modest
reduction in the area of the Strip system at largerη-values to evaluate whether such reductions pro- duce a non-negligible loss of performance. Similarly, the reduction inη-coverage provides sensitivity to the importance of the larger very forward tracking coverage provided in the Reference scenario.
Figure 8. A cross-section of the Middle scenario layout, which is based on theLoI-VFlayout. The pseudo- rapidity coverage extends up to |η|=3.2. Blue and red lines represent strip and pixel layers, respectively. Horizontal and vertical lines represent barrel and end-cap layers, respectively. Lines of constant pseudo- rapidity are indicated. The blue line outside theITkvolume represents the coil of the solenoid magnet.
IV.2.3
Low scenario layout
In the Low scenario further reductions have to be made with respect to the Middle scenario. The tracker (pixel disc) η-coverage is further reduced to|η|=2.7. The middle barrel of the strip system and one side of each of the double sided silicon strip modules on the two barrel layers which are adjacent to the middle layer, are also removed (see Fig.9). The innermost and outermost strip barrel layers are left unchanged. Again, no attempt is made to re-optimise the module plane positions and the hit information (digitisation) from these regions is masked from the reconstruction software. The material for the elements which have been removed is still in the detector simulations. No z
information from the stereo layers is available from the two barrel layers where one side of the strip modules has been removed. For these modules the only measurement available is from the r-φ coordinate and the coarse segmentation of the strips in z. The changes (parts removed) from the LoI-VF(Reference) layout are shown in Table12.
1 Pixel Barrel x 4 (r=39mm,78mm,155mm,250mm) Pixel Discs x 6 (z=877mm,1059mm,1209mm,1359mm,1509mm,1675mm) Disc Strips x 6 m Long (47.8mm) Strips x 2 (r=762mm, 1000mm) (z=1415mm,1582mm,1800mm,2040mm, 2320mm, 3000mm) Short (23.8mm) Strips x 2 (r=405mm,519mm)
Figure 9. The Low scenario layout, which is based on the LoIlayout. Dramatic cuts are made to the strip system removing measurements along the length of the track and theη-coverage range is limited to|η|=2.7 by important reductions in the pixel system. The strip barrels shown in green are those where one side of the strip module has been removed. The blue line outside theITkvolume represents the coil of the solenoid magnet.
Table 12.The differences between the layouts for the Reference, Middle, and Low scenarios.
Reference Middle Low ITkstrips - changes w.r.t.LoIlayout
Remove Barrel layer 3 7
Remove 1 Disc set 7 7
Remove 2 stereo layers 7
Remove stub 7 7
ITkη-coverage 4.0 3.2 2.7
This layout explores the performance impact of a very substantial reduction in the area of the Strip system in the barrel region, to evaluate the minimal number of large-radius measurements required for a successfulITklayout, as well as the performance degradation that results from such a reduction.