J. Wotschack, 11 March 2012

Some comments on the 'Two alternative proposals for the New Small Wheel Upgrade'
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I am not convinced that proposing a multiple-technology solution is the right choice for the new Small Wheel given its overall complexity and cost. The only scientific argument (apart 'political' considerations) in favour of such an approach  that I can see is REDUNDANCY.

Therefore, given the two proposed options, I am favouring largely the 'homogeneous solution'. To go for a multiple-technology solution with all the complications and costs involved makes, to my mind, only sense, if it guarantees redundancy over the full area.

On the content of the proposal I have a general and a few detailed comments concerning the advantages/disadvantages listed for the two solutions. 

A. The general comment bears on what I consider a wrong understanding of the proposed trigger concepts:  

The idea that the sTGC trigger scheme is more simple than the MM trigger scheme is demonstrably not correct. If one looks at the TGC trigger scheme in detail one realizes that, in addition to the common functionality required by both sTGC and MM trigger, i.e. the track-angle determination (or track matching) in USA15, the sTGC trigger requires a number of additional hardware and decision steps, none of which is required by the MM trigger. These are:
- pad coincidences on-chamber
- RoI sélection on-chamber
- centroid finding in USA15.
All the rest is very comparable.
The first two of those require in addition electronics and a non-negligible cabling effort on the Small Wheel, and space. 

Furthermore: the sTGC trigger scheme, as described, is limited in the number of trigger track candidates that it can handle. Within one of the three regions of a sector only a single RoI can be accommodated, other RoI coincidences would be lost. In comparison, the MM trigger foresees in its minimal scheme four regions per sector and in each region four hits are allowed to be transported out via a single 4.8 Gbit link. 

It may, therefore, in the end turn out that the MM trigger is the safe-bet while the TGC trigger would be the back-up solution. Nevertheless, having a redundant trigger option over the full area is certainly an asset.

B. A few words about the various advantages/disadvantages listed:

Homogeneous solution: Disadvantages

- There is essentially no difference in the R&D efforts for large or medium-size chambers.

- No experience with large-scale production: This is true but applicable to all technologies. Any production of something new (also the sMDT chambers are new!, e.g., a completely new and complicated gas system) needs some learning.

- The number of channels is certainly larger in the homogeneous solution, however, in modern readout systems it is no longer the number of channels but the complexity of the overall system that is important. Here the MMs are proposing a very elegant solution with very low data bandwidth, thanks to on-chamber zero suppression and peak finding. Furthermore, for the trigger, the number of channels is low (<30k). In general, the channel count is far inferior to what is usually used in Si detectors (and extends over a much larger area).

- Pattern recognition algorithms will not depend on the track angle, precision tracking may (to be seen). However, I doubt that these algorithms will be more complicated than the MDT algorithms with rt-calibration, wire sag correction, etc… Note that MMs do not require any calibration constants.

Split solution: Advantages:

- the argument that the accuracy of the readout planes is probably easier to be kept on small MM planes is true, but all depends on what the differences in accuracy between small and large boards will be. If both accuracies are better than what we need, there is no argument.
In this context, there is another point that one should not underestimate: in a MM, the two sides with the strip ends are correlated by construction. For the tubes, the relative positions of the two tube ends rely on the alignment of the jigs during assembly and their transfer through the alignment system to the in-situ position. Even if this can be done with good precision, it will be certainly not more precise than what one can achieve with strips on a common board. 

- Tube based trigger: I do not think it makes sense to list this as an advantage. One should not install tubes in the SW to have a trigger test bed. This can and should be done elsewhere, if one thinks that the whole idea is promising enough.