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Summary Report from FCAL at the DESY CMS Week.
B. Cox (Virginia) and R. Ruchti (Notre Dame)
For the FCAL Group
I. Background: At the CMS Week, the foci of the FCAL group were (1)
concentration on the various programmatic options for forward calorimetry and (2)
to assess the capabilities of these options. While a costing exercise has been
ongoing in parallel with these assessments, cost analyses were not a specific goal of
the FCAL sessions; However the discussions at the meeting were important to
informing the costing exercises to be undertaken in the weeks to follow.
DESY Meeting Schedule: The benefit of the organization of the DESY Workshop
was the ample accessibility across a number of groups of parallel sessions of
relevance. Here, FCAL benefitted by the availability and presence of ECAL, FCAL
and HCAL communities.
Meetings specific to FCAL: The FCAL had a plenary talk on Wednesday, June 5,
presented by Ruchti, a set of parallel sessions on Thursday morning, June 6,
coordinated by Cox and Ruchti, a report and summary of the parallel sessions to the
Forward Working Group on Thursday afternoon, June 6 presented by Cox.
Meetings and presentations of direct relevance to FCAL: Additionally, the FCAL
had strong interests in a variety of the other sessions including: the opening day
plenary session talks on June 3 on the HCAL Upgrades (Rumerio) and Status
(Abdoulline) and on the ECAL Status (Cavalleri); the HCAL parallel session, June 4,
coordinated by de Barbaro and Ruchti and Plenary Summary, June 5, by de Barbaro,
and the ECAL parallel session, June 4, coordinated by Cavallari and Jessop and
Plenary Summary, June 5, by Jessop; and the plenary talk on June 6 by Rusack on HE
options. The discussions of the final day plenary session, June 7, were very
important overall in summarizing outcomes, including the plenary talk from the
Forward Working Group by Manelli and the Scenarios/Next Steps Plenary Talk by
Spalding.
II. Synopsis of the FCAL Parallel Sessions of June 6: This note summarizes the
discussion and presentations from the FCAL parallel sessions of June 6th. The URL
for the meeting is: https://indico.cern.ch/conferenceDisplay.py?confId=254274
Choices will need to be made for EE, HE and HF subsystem upgrades, on the time
scale of a month. Given the tight schedule, one cannot cost all options, but we will
need to be selective; baseline and then perhaps an option or two at most. It should
be noted that these choices need to be considered in the context of rapidly changing
developments understanding, coming in part from analyses of the performance
degradation of existing detectors during the past running prior to LS1.
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III. New Developments: Several new developments have emerged in the relatively
near term leading up to this meeting. These are:
1. Radiation damage to the HE is more severe than previously thought, raising
new concerns about the longevity of that subdetector system in the time up
to LS3 as well as beyond.
2. The concept of digital calorimetry is being considered as a option for HE.
3. The ECAL community (through the action of their IB) is now established the
“working hypothesis” that the existing EE will need to be replaced with a new
one.
4. A very long period will be needed to accomplish the detector replacements
during LS3. Among these is new information about structural issues for
mounting of the endcap absorber as well as material activation issues for the
absorber material.
For purposes of the presentations and discussion, the topics focused on upgrades
for EE, HE and HF. Given physics considerations, material activation and expected
availability of resources for the upgrades, the HF detector in its current form, will
not be upgraded, but will continue to be operated in its current form, but with ever
decreasing capability in the region 4.5 < eta < 5 due to radiation degradation.
IV. FCAL Approach at DESY: Below is a synopsis of several the major options
under consideration for FCAL Upgrades under Scenarios #1 and #2. At the meeting,
we attempted to identify the key options, their approaches, strengths, and merits,
and to also identify key questions that should be answered in a timely way.
V. Electromagnetic Calorimetry - EE Overview: For the EE replacement, the
current strawman configuration is a Shashlik Detector with lead coversion plates
interleaved with LYSO scintillation tiles. The scintillation light is read out via Quartz
Capillaries with luminous waveshifting cores and GaInP photosensors. A costing
exercise for this option has been underway, and while the basic material costs (for
example LYSO) were considered the financially challenging aspect, in fact the
readout costs are the actual drivers, including the number of readout channels and
the adc range needed to record the information.
There are variants to the above materials, including other scintillators: plastics and
crystals (For example CeF3) and waveshifters, some of which might find use at
smaller eta values when the radiation environment is less severe. As an example,
plastic scintillator might find application for eta < 2. Figure 1 below illustrates the
range of options:
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Figure 1. Range of component options under consideration for the Shashlik EE.
The Strawman design uses the elements in red (Pb/LYSO/Capillaries/GaInP).
EE - General Questions: The general questions for a “pure” EE upgrade include:
 Transverse segmentation. What is the minimum cross sectional size for EE
elements to provide e/gamma identification, gamma/pizero separation, and
sufficient granularity for PF/GDE application?
 Longitudinal segmentation. Does this make sense for energy resolution and
perhaps timing information? What do we gain for PF?GDE from such
segmentation?
 Readout. How many readouts to we need per module and should it be
front/back. Key points here include optimal treatments for radiation
damage, calibration and uniformity; multiplexing to reduce the channel count
per module; and will the GaInP photosensors work as expected.
 What is the optimal mechanical construction for servicing or for module
removal or replacement?
 There are issues to be studied clarifying the performance of LYSO+Capillaries
for light yield and phosphorescence.
 The EE should be compatible (if not optimized) with respect to the HE
behind it.
EE – Shashlik Design Parameters: The key design parameters for the Shashlik EE
in two configurations, Pb/LYSO and W/LYSO are shown in Figure 2. Note the
compactness of the two options.
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Figure 2. Shashik EE design parameters for Pb/LYSO and W/LYSO options
(Borheim)
EE - Simulations: Many configurations of the sampling ECAL (see Fig. 1 for
examples) are under study with Standalone simulation tools. These are at the
position for providing information on the level of energy resolution under radiation
damage. Some important lessons have been learned:
 If a resolution is claimed, the noise term and photo-statistics (stochastic)
term should be in a safe zone.
 Longitudinal non-uniformity (the constant term) can be a major factor,
unless careful calibration (and the ability to make modest corrections) is
applied.
 Stay away from scintillators whose transparency depends upon dose.
 Experimental data on radiation damage is central.
Figure 3 below, shows Standalone simulations for the Strawman EE Shashlik
detector for integrated luminosities of 0 fb-1 and 3000 fb-1. From these, and
contingent upon successful developments in progress, such a detect will work for
the HL-LHC in the domain 1.4 < eta < 4. The next immediate simulation steps are
implementations in DELPHES and FastSim.
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Figure 3. Inergy resolution in the Pb/LYSO Shashlik Option (Ledovskoy, et al)
EE: Questions for the Pb/LYSO Option: There are a number of questions to be
addressed for the Shashlik Strawman option.
 Neutron issues, including backsplash from Pb material (an additionally for
the W variant) as well as contributions to the overall observed pulse length –
particularly important for W absorbers.
 Phosphorescence of the LYSO and its impact
 The number of needed readout channels and the associated dynamic range
for the readout.
 Calibration issues.
 Coverage from 1.4 < eta < 4. Can this be achieved with one set of
technologies?
 Compatibility with the HE, either current or new?
EE – Merit: The Pb/LYSO Shashlik Strawman has the best EM energy resolution of
the calorimeter options.
EE - Scintillator Variants: Several variants on the basic Shashlik design are
indicated in Figure 1. One or these is the possible use of CeF3 scintillator rather
than the LYSO. The use of this material has potential benefits and also its own
challenges. Among the latter are commercial availability and cost. But further study
of this material and others are underway. A table of comparative information of
LYSO and CeF3 is presented in Figure 4.
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Figure 4. Comparsion of properties of LYSO and CeF3 (Nessi-Tedaldi).
VI. Hadron Calorimetry - HE Technology Options when used in conjunction
with an upgraded EE. The Hadron Calorimetry situation became remarkably more
interesting and challenging with the realization that the existing HE is suffering
radiation damage at a rate significantly higher than originally expected. Originally it
was expected that an HE replacement might be restricted to a small eta region (eta >
2.5) and in HE layers closest to the IP. It would now appear that a much more
extensive reconfiguration is needed and an associated costing for such a
replacement must be made. Additionally, in April 2013 CMS Week, the RDMS
groups (talk in the HCAL meeting by Golutvin) had indicated that there would be
activation challenges with the Brass HE absorber during shutdown ahead (for
example in LS2 and LS3), making material handling difficult.
Impact in two scenarios: In Scenario #1, the HE absorber may or may not be
replaced, but the HE megatiles, as existing, would be replaced with a more rad hard
approach to the scintillating material. This could accomplished by using tiles of
smaller transverse dimensions, hence reducing the optical path length between
scintillation and waveshifter, the use of longer wave scintillators (such as 3HF) and
red-emission WLS fibers and/or capillaries. The use of the Shashlik EE would free
up considerable longitudinal space, perhaps allowing for the insertion of a transition
HE between the EE and HE subsystems, with the eye to providing an improved,
overall e/h response. At the DESY meeting, no proposal for such a “transition” HE
was presented or discussed, and the benefit (or use) of such a device has yet to be
fully explored.
In Scenario #2, the HE absorber is replaced by an entirely new absorber structure.
In the new structure, the sensing elements could include WCAL technology,
Extruded Crystals, Liquids, and GEMs. There was considerable discussion of these
possibilities at DESY.
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Questions for the HE (+EE): Among the fundamental questions here is, will the
existing HE survive up to Phase-II. An important feature of the Phase-I HE detector
(as well as HB and HO subsystems) is the use of SiPM readout. These allow the
possibility for more depth and transverse segmentation than would ever be possible
with the current HPDs is use. The SiPMs can provide for the opportunity to read
out the HE much more finely, and to reweight layers where light levels are being
reduced due to radiation damage. The additional benefit of SiPM is there very high
QE, particularly at longer wavelengths, making the use of red emitting waveshifters
a practical reality.
A listing of key questions of HE is the following:
 Will the existing HE, or with more radiation resistant component elements,
survive up to LS-3, and will any of it work afterward without replacement?
 Can HE (and EE in front) provide extended coverage to eta < 4?
 Can the HE performance with new replacement technologies match or
improved upon the current HE performance?
 For digital calorimetry, there are a number of open questions: Will it work
at high pileup? At what eta does it cease to be useful? How many readout
channels are needed and what is the segmentation? Is it triggerable? What
are the implications for HE construction? What are the issues of a gas-based
system for calorimetry?
 What are the costs of these systems?
HE Options for Scenario 2: The replacement of the HE absorber opens up a
number of possibilities for replacement technologies what would drive the actual
form and type of absorber material used and the sampling methodologies. A
number of these were presented at DESY, and several are indicated in this write up.
The interested reader is referred to the URL for the FCAL parallel sessions for a
complete set of the presentations.
GEM Detectors: GEMs are in strong favor for extending muon tracking toward the
IP, to take advantage of the higher magnetic field available and to exploit the
granularity of the GEM. These devices have been under active study by several
groups. Shown in Figure 5 is a schematic of the HE region in which the HE system is
replaced by a large number of triple GEM detectors (a total of 3000 chambers, with
1500 at each end of the detector). A R&D and contruction project is foreseen with
these devices ready by the time of LS-3.
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Figure 5. The HE region instrumented with Triple GEM detectors interleaved in
absorber material. The technology is currently being considered for
implementation in the ME1/1 region, but closer to the IP.
Questions for a GEM base HE:
 What is the manufacturing capability needed (and availability) for several
thousand GEM chambers?
 What is the effective number of readout channels?
 What are the edge effects due to chamber framing?
 What is the GEM lifetime?
 How does one deal with pattern recognition in such a system?
 What is the trigger capability and how is the energy measurement achieved?
Merits of the GEM technique:
 The granularity maps on to PF/GED
 It has common technological links to planning in the muon system
 It may be relatively “frugal” in system costing.
Glass RPC Option for PF/GED/Digital Calorimetry for HE: This is a different
variant for digital calorimetry based upon low resistance glass RPCs. The emphasis
is directed toward an imaging hadron calorimeter using 1-bit (or few bit) readout,
and readout pads of 1 x 1 cm2. The RPCs would be interleaved between absorber
plates of material such as tungsten and/or steel. Figure 6 shows an example of a
detected shower of a 120 GeV proton in such a calorimeter (the CALICE DHCAL).
Figures 7 and 8 show the energy resolution for pions and electrons respectively in
the DHCAL.
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Figure 6. The shower of a 120 GeV proton as detected in a glass RPC digital
calorimeter (Bilki).
Figure 7. The resolution for pions in the DHCAL, a glass RPC detector (Bilki).
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Figure 8. The energy resolution for positrons measured for the DHCAL RPC (Bilki).
Questions for the glass RPC for HE. As for the GEM option, the granularity of the
glass RPC is dense and provides a three dimensional image of the shower evolution.
Among the several questions facing this technology are:
 The effect of pileup on the image reconstruction
 Rate dependence of the RPC technology – including surface charge buildup.
 Pattern recognition techniques – these must be fast. What are the
approaches.
 What is the long term stability of the RPC?
 Is the system triggerable?
 How does the energy resolution and missing Et resolution evolve with time?
Merits of the RPC technique:
 The RPCs may afford a reasonable cost solution
 The imaging technique maps into PF/GED.
Tungsten Wall (WCAL ) Option for HE + EE: With the replacement of the HE
absorber, entirely new sampling techniques (analogous to the HF and Shashlik EE
structures) could be utilized. One of these options is called the Tungsten Wall,
which is considered to be located in the special region vacated by the removal of the
existing HE and potentially extending to eta of 4. See Figure 9. The absorber and
sampling structure for the Tungsten Wall is shown in Figure 10, and contains fused
silica fibers for Cerenkov detection (and hence principally EM sampling) and
scintillation fibers for ionization (both EM and hadronic energy) sampling.
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Sampling fraction and directionality of the fibers (relative to the IP) are under
consideration for optimal performance.
Figure 9. Location of the proposed Tunsten Wall Calorimeter, which a potential
replacement for HE, and (potentially) also EE (Akchurin and Kunori).
Figure 10. A possible configuration of Quartz and Scintillation fibers used for dual
readout of the Tungsten Wall Calorimeter (Akchurin and Kunori).
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The use of the Dual Readout technique exploits the simultaneous measurement of
EM and ionization components to linearize the hadronic energy response. See
Figure 11. While the dual readout is under consideration in other detector schemes,
it is an intrinsic design choice for this proposed system. The use of Cerenkov signals
might also find application in fast timing, if the response of the photosensors is
correspondingly fast.
Figure 11. Use of the Cerenkov (EM signal) can be used to calibrate and linearize
the hadronic energy response in a dual readout calorimeter such as the Tungsten
Wall (Akchurin and Kunori).
Questions for the Tungsten Wall HE (or HE+EE) Calorimeter: Given the nature
of the proposed absorber material, a number of issues arise as with other proposals
for the use of Tungsten (or lead):
 The effect of the neutron backsplash on inner detectors (tracking) and
photosensors?
 As presented, this system lacks longitudinal segmentation. What are the
implications of this?
 What is the effect on the pulse shapes due to late arrival of neutrons
(principally the ionization measurement)?
 What are the numbers of readout channels, if an HE replacement or if an HE
and EE replacement?
 What are the fiber materials, their structure and lifetime?
 What is the system cost for such a system?
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Merits of the Tungsten Wall HE (or HE+EE) Calorimeter: This system builds
upon the opportunity afforded by a new absorber structure in the HE region.
 The system takes advantage of dual readout to provide equilibration of EM
and Hadronic response.
 It will provide excellent jet resolution.
Crystal Fiber CFCAL Option for HE + EE: For this option, a longitudinal
arrangement of densely packed crystal fibers is utilized, some scintillating for
ionization measurement, some non-scintillating for Cerenkov measurement. The
crystal fibers are made of LuAG, either doped with Ce3 for scintillation, or un-doped
for Cerenkov light detection. The crystal fibers can be incorporated (or not) into
Quartz capillaries to provide improved surface (and hence transmission)
characteristics. Readout would use GaInP photosensors. Figure 12 shows a sample
geometry in which crystal fibers are stacked. Figure 13 shows the results of a test
arrangement where the crystal fibers are incorporated as sensing elements in a
copper absorber in a packing arrangement not dissimilar to that of the Tungsten
Wall geometry. The preliminary measurements indicate that the technique is
functional, but performance optimization is needed for sampling fraction, and fiber
orientation [issues also of concern in the Tungsten Wall].
Figure 12. LuAG givers for calorimetry application (Auffray-Hillemans).
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Figure 13. Initial measurement of energy resolution for the CFCAL in test beam.
Currently the constant term is dominant in the initially tested configuration
(Auffray-Hillemans).
Questions for the Crystal Fiber CFCAL HE: The technology issues are refining the
crystal fibers for surface quality and how this configuration would fit into a full HE
subsystem, depending upon effective operable length of the crystal fibers.
 What is the optimal fiber structure and cladding?
 How the technique extrapolates to a full HE detector?
 How the scintillation and Cerenkov elements are read out?
 What number of readout channels is required?
 What are the costs of such a system for materials and readout?
Merits of the Crystal Fiber CFCAL HE option:
 The system is intrinsically dual readout
 The potential granularity maps onto PF/GED
 It could provide one technique for EM and hadronic measurement
VII. HF – Forward Calorimetry: At the meeting, it was the consensus that there
would be no changes, nor Phase-II upgrade of the HF detector, if physics does not
justify the need for coverage at eta > 4.5. In the absence of just justification, it is felt
that the extension of EE and HE to eta ~4 obviates the need for HF upgrade.
However it was generally acknowledged that the ongoing studies of HF radiation
damage – in situ – and of studies of Quartz with various amounts of OHconcentration – are extraordinarily important to inform choices for Quartz fiber and
Capillary options for EE and HE.