LL, 6/8/2010
Short summary of the meeting on CLIC HCAL geometry, held on 4/8/2010
Input provided orally at the meeting by Alain Herve on mechanical engineering aspects (to
be checked with Hubert Gerwig):
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Stretching the coil half-length by some ~60 cm is not a major cost driver. Alain advises
not to hesitate at this phase of the study to implement it in order to match the HCAL end
cap depth.
Plate thickness in the barrel and end-cap yokes:
o Typical plate thicknesses of 10 cm seem a good choice
o The first end-cap layer shall be 20 cm minimum
o The barrel yoke needs two thick layers, one at small radius and one at large
radius, in order to absorb the compressing forces of the end-cap yoke on the
barrel. Tentatively a thickness of 20 cm is given.
Radial space needs to be reserved outside the coil cryostat for cabling (5-10 cm?). This
space becomes available in the central barrel ring for additional tail catcher (CMS
example).
Discussion on choice of HCAL passive material in the end cap region:
Material
Steel
Advantage
 Lower cost
Tungsten
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Compact, therefore allowing
for a shorter L* and allowing
for a shorter coil
Disadvantage
 Deeper in Z, therefore inducing a
longer L*. It also requires stretching
the coil.
 Hadronic shower signal
development in Tungsten is slower
(see WG2 meeting of 3/8). It will
therefore be more difficult to use
timing for the separation of physics
and background.
 Higher cost
Discussion on the tail catcher:
Pandora uses tail catcher layers to improve the calorimetric performance. Pandora currently
uses 10 layers of tail catcher. Ten layers of 10 cm steel correspond to some 6 I. This seems
quite a lot. Previous studies (e.g. Peter and Christian) showed that one gains calorimetric
resolution using a 1 I tail catcher. This gain in resolution does not improve significantly for a
5 I deep tail catcher. This is compatible with Erik’s studies, where he sees only very few pion
hits after 40 cm of steel. One can test the usefulness of the tail catcher and the optimal choice
of the number of layers by using Pandora and successively turning off the hits in some tail
catcher layers. We expect the optimal number of active tail catcher layers to be larger in the
end cap than in the barrel, because of the existence of the coil.
Conclusions drawn at the meeting:
HCAL end-cap:
While there are compelling reasons for a tungsten-based HCAL in the barrel region, steel
currently seems the preferred CDR choice for the end-cap region. We may come back
on this choice much later, after the CDR, when we will have data from the test beam and
when we may have developed more sophisticated time-resolving algorithms to separate
physics from background.
Composition of the passive layers of the yoke:
Barrel:
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Start with an active layer
First steel plate is 20 cm thick
All successive layers are 10 cm think
End cap:
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Start with passive layer
First steel plate is 20 cm thick
All successive layers are 10 cm think
To do: simulation study to define HCAL depth and number of tail catcher layers
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Use Pandora
Simulate HCAL depth of 6, 7, 8, 9, 10 I
Turn hits on/off in a variable number of tail catcher layers
Use jets of 250, 500, 1000 GeV
Make plots of resolution as a function of energy, of I and of number of tail catcher layers
(for this simulation study it is OK to use tungsten-based HCAL, like in the current detector
model)