22 May 2014
Summary FCC-HC ECAL preliminary discussions
1st General Comment: While we are considering right now only the EM components, it is important
early on to consider the full calorimeter system aiming at an optimal configuration of both EM and
Hadronic
2nd General Comment: The overall experimental layout with the large radius solenoid has an impact
on both the requirements and possible technical implementations of all the subsystem and in
particular of the calorimeters. The initial considerations summarized below are based on this layout
but it is important to analyze an alternative layout, e.g.one based on a compact solenoid, with size
similar to the CMS one.
3rd General Comment: We have singled out a few approaches that have different and complementary
characteristics in terms of intrinsic energy resolution, and granularity (transverse and longitudinal).
4th General Comment: There is a need early on to establish the physics drivers that set the
requirements on the stochastic (eg. physics with Higgs) and constant term (eg searches at high mass,
Z’ etc) of the energy resolution. It is also necessary (though likely more challenging) to establish the
impact of granularity on event reconstruction and particle flow performance. In this sense, the
tracking and calorimeter detectors need to be considered together for particle/event reconstruction.
5th General Comment: EM calorimeter depth up to 30X0 to ensure full containment for highest energy
showers (may be less critical if sufficient longitudinal segmentation to allow for correction of energy
leakage and/or ability to use HCAL to measure and correct)
We consider two broad families of systems, with candidate technologies for each of these:
1. Photo-detection based systems
a. Homogeneous Crystals and/or Crystal based Shashlick
b. Crystal fiber based Dual read-out, with/without dense inert absorber
c. Quartz fiber based Dual read-out, with dense inert absorber
2. Ionization based systems
a. LAr “ATLAS” like sampling calorimeter
b. Silicon sampling calorimeter
In what follows we briefly summarize the pros and cons of each of these, in the different eta regions
of the experiment
EM divided in 4 eta regions (base=Large Solenoid, CMS-inspired drawing)
-Barrel
(0< <1.5)
- Ext-Barrel
(1.5<  <2.7)
-End-Cap
(2.7<  <4)
3m radius minus 1m radius for FCAL
~ 25 m2 each side
-FCAL
(4<  <6)
1m radius
~ 3m2 each side.
16 m long, 2.5 m radius (front face)
~ 250 m2
~ 20 m2 each side
Barrel
Radiation levels likely manageable for most/all technologies, size may be a challenge for some

Homogeneous Crystals
o E/E ~ 3%/sqrt(E) + 0.7 % (CMS Barrel)
o Radiation levels probably manageable; cost likely to be an issue
o Have potentially the lowest stochastic energy resolution term
 If cost drives towards Shashlik sampling then go to ~ 10%/sqrt(E)
o Drawbacks: no depth segmentation; signal decay times longer than bunch crossing
interval, even at 25ns
o Transverse granularity: the Moliere radius ranges from ~2cm to 2.6cm for plausible
candidate crystals, such as LYSO, PbWO4 and CeF3
 Due to the larger radius, Crystals of same size as CMS would result in cells of
 ~ 0.012 x 0.012 (at 900)
o Longitudinal segmentation: none

Crystal fiber based dual read-out
o Most of the above applies equally
o Have the advantage of potentially improved particle id and hadronic response
 nb hadronic response dependent on HCAL…
o Drawback: expect some degradation in the EM stochastic energy resolution, even in
the absence of passive absorber, but likely to remain below 10%

LAr sampling calorimeter
o E/E ~ 10%/sqrt(E) + 1% (ATLAS)
o Radiation levels and rates ok; cost likely to be ok
o Offer an intermediate stochastic energy resolution term, together with good
transverse granularity and some longitudinal segmentation
 nb Energy resolution sufficient for early Higgs discovery with ATLAS
experiment!
o
o
o

Drawbacks: charge collection time much longer than bunch crossing interval,
although this is mitigated by electronics shaping.
Transverse granularity: the Moliere radius is ~ 3.6cm, the read-out electrodes can be
designed to accommodate varying granularity (eg constant  segmentation,
should this be desirable)
 Due to the larger radius, cells of same size as ATLAS would result in cells of
 ~ 0.015 x 0.015 (at 900)
Longitudinal segmentation: 3 segments for the current ATLAS calorimeter
 Could be extended to 4, going beyond this would require substantial new
development of the read-out electrodes
Silicon sampling calorimeter
o E/E ~ 16%/sqrt(E) + 1.1% (CALICE Test-Beam Prototype)
o 30 to 40 layers*250m2 ~ 7’500m2 – 10’000 m2 => cost likely to be a problem
o Have potentially the highest segmentation, both transverse as well as longitudinal,
and a signal formation faster than the bunch crossing interval
o Drawback: will have the worst stochastic energy resolution term
o Transverse granularity: the Moliere radius is likely to be in the range of ~ 2cm to
2.5cm. However, EM showers are much narrower than this up to at least shower
max, so that a highly longitudinally segmented calorimeter one can profit from
substantially finer granularity than the Moliere radius. Representative transverse
granularities for a Silicon sampling calorimeter range from ~ 0.25cm2 to 1cm2.
o Longitudinal segmentation: in the range from 30 to 40 layers
Extended Barrel:
Radiation levels likely still manageable… Smaller surfaces relieve cost pressure. Energy and
granularity requirements remain similar to Barrel, but emphasis likely to be shifting from stochastic
energy resolution term to granularity.
Most of the comments above remain applicable. The services feed-troughs for the LAr cryostat could
be problematic.
End-Caps:
Radiation starts to be a problem, even if the large distance to vertex mitigates it. The stochastic
energy resolution term will likely no longer be a driving consideration (boost), while segmentation
and (VBF) jet energy response will be increasingly important.

Crystal based Shashlik
o High radiation levels, and the reduced importance of excellent low energy EM
resolution make this better suited than a homogeneous crystal calorimeter.

Quartz fiber based dual read-out
o High radiation levels, and the reduced importance of excellent low energy EM
resolution make this better suited than a crystal fiber based calorimeter.
o Potentially suitable for the radiation environment
o Could be well suited to the performance requirements

LAr sampling calorimeter
o Space for feed-troughs is not constrained
o Given the large distance from the vertex, a geometry similar to ATLAS barrel can be
envisaged (ie simpler than present ATLAS EC)
o Transverse granularity of .015x.015 may be hard to maintain up to  = 4
(corresponds to 1.5cm at ~20 m distance)

Silicon sampling calorimeter
o Silicon resistance to neutrons may be a challenge
 Need more detailed radiation fluence information
FCAL:
Radiation is a major challenge, and (VBF) jet energy response likely to be the driving performance
requirement. High density will be mandatory, so that a dense passive absorber material (Pb or W)
will be a must.

Quartz fiber based dual read-out
o This could be well suited to the performance requirements, provided the radiation
resistance can be demonstrated

LAr sampling calorimeter
o
o
LAr with rods like present ATLAS, but with narrower gaps (100microns) may be a
possibility
Granularity likely increasing as eta increases, because of finite size of elements (say
1cmx1cm is a minimum). For example, at 20m from the collision point, eta=6
corresponds to a radius of 10cm,and a 1cm size corresponds to delta_phi=0.1
(roughly)