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SiD Calorimeter R&D Report
I. Overview of SiD Calorimeter R&D.
The ILC physics program places strong requirements on the performance of calorimeter
systems. Many production channels to be studied require efficient identification and
separation of jets in multi-jet final states (e.g. in e+e- → t tbar → 6 jets), and the
measurement of jet energies and jet-jet invariant masses with unprecedented precision.
Our goal is to measure jet energies at the level of σ/E ~3-4%. Event-by-event separation
of hadronic decays of W- and Z-bosons and Higgs particles will be necessary to measure
cross-sections and branching ratios at the few percent level of error. The calorimeter
system is a critical element of the SiD detector concept design for achieving these goals.
The calorimeter will also provide excellent electromagnetic energy resolution and
efficient π0 reconstruction, charged hadron/photon shower separation, and tau lepton
reconstruction.
The SiD detector design is based on an all-silicon tracking system which allows for a
limited inner radius for the central calorimeter system. This, in turn, constrains the cost of
the silicon-tungsten electromagnetic calorimeter, and limits the outer radius of the hadron
calorimeter, thereby also constraining the size and cost of the superconducting coil. It is
taken as a basic design parameter that both the electromagnetic and hadron calorimeters
should lie inside the coil so as not to degrade their abilities to make particle-energy
cluster associations. A schematic view of part of the SiD concept is shown in Fig.1,
which indicates the dimensions and location of central and endcap components of the
calorimeter system.
Fig. 1 Quadrant view of the SiD detector design concept.
The forward calorimeter systems for SiD will be covered in a separate report by the FCal
collaboration, and so are not discussed here.
A promising approach for achieving the unprecendented jet energy resolution lies in the
use of a Particle Flow Algorithm (PFA). We have invested considerable time and effort
in the development of such algorithm(s), and in understanding the implications of a PFA
for the SiD detector design. This approach uses the tracking system to measure the
energy of the charged hadron component of a jet and the electromagnetic calorimeter to
measure photon energies. Accordingly, the neutral hadron energy is the only component
of jets to be measured directly in the hadron calorimeter. Leakage from the rear of the
hadron calorimeter could be estimated by the use of a “tail-catcher”, which would also be
the first part of the muon system.
The measurement of jet energies using data from several subsystems leads to a very
integrated view of the design of the SiD detector. The tracker must find the charged
particles with high efficiency and ensure accurate projection of the tracks into the
calorimeter to make the correct track/energy cluster associations. The electromagnetic
calorimeter must be sufficiently fine grained to provide efficient charged particle/photon
shower separation. The hadron calorimeter must be sufficiently fine grained to allow
efficient track following for charged hadrons and their hadronic shower products, and to
provide an approximately linear hits vs. energy for the digital hadron calorimeter case.
Simulations of PFA performance for light jet production at the Z-pole are beginning to
show excellent performance. Intense work is underway to optimize the algorithms for the
higher jet energies expected for higher center of mass energies. Prudence suggests we
also remain open to the investigation of more traditional calorimetry or more radical,
alternative ideas.
In the PFA approach, the electromagnetic energy resolution does not impose limits on jet
energy resolution: the current Si-W design is expected to have an electromagnetic shower
resolution of 17%/√E. However, the segmentation, both longitudinal and transverse, is a
critical consideration for the use of PFAs for hadronic jets. Transversely, not only should
the effective Moliere radius be kept small, but also the transverse cell size should be
about half of this radius. Longitudinally, the active gap between absorber layers should
be kept small to achieve a small effective Moliere radius, limiting transverse shower
spread. A sufficient number of longitudinal layers need to be provided to contain
electromagnetic showers and allow efficient recognition and reconstruction of these
showers. The resulting highly segmented electromagnetic calorimeter, in addition to
having the correct characteristics for use with PFAs, will then have outstanding
performance for the reconstruction of electrons, photons, and tau leptons and for tracking
charged hadrons and muons.
The choice of the silicon-tungsten combination for the electromagnetic calorimeter is a
centerpiece of the original SiD design. The Moliere radius for tungsten is 9mm, and our
present design has a pixel area of 12mm2. Longitudinally, there are 30 alternating layers
of tungsten and silicon giving a total thickness at normal incidence of 29X0. The goal for
the active gaps is 1mm width or less. Each group of approximately 1000 silicon pixels is
read out via one KPiX-ASIC mounted on the wafer. The first version of the silicon
wafers have been delivered and evaluated. The medium-term plan is to assemble a
prototype calorimeter section based on the second generation silicon wafers in
conjunction with the 1024-channel KPiX readout. The electromagnetic calorimeter is
discussed in detail in Section II.
The KPiX ASIC is being developed for the readout of the electromagnetic calorimeter,
and the central silicon tracker for SiD, and has applicability to several technology options
for the hadron calorimeter. The high energy density for electromagnetic showers
requires a large dynamic range for the readout. The KPiX chip uses a novel method by
which the feedback path on the front end amplifier can be switched between two
capacitors, switching in the large (10 pF) capacitor only when it is required. This allows
the amplified charge for smaller input signals to be well above the noise. There is an
event threshold, which can hold off bunch crossing resets in order to allow a fairly long
integration time of 1 s. The calculated noise level is about 1000 electrons, to be
compared with a MIP signal charge of 25 times this. Charge digitization uses two
overlapping 12-bit scales. The chip also allows up to four hits per bunch train to be stored
for each pixel. Several prototype versions of KPiX have been fabricated and tested, with
encouraging results for the present 64-channel version. A version of a 64-channel KPiX
has also been developed to read out the gas electron multiplier based version of the
hadron calorimeter active layers. Details of the KPiX development are given in section
III.
The hadron calorimeter in our PFA-oriented, baseline design consists of 35-40 layers of
alternating absorber plates and active sampling gaps. The critical feature of this
calorimeter is the small transverse cell size, O(1-3cm), required for the imaging
requirements imposed by a PFA. Depending on the approach taken, this device could be
analog, semi-digital, or fully digital. There is a premium on a small active gap size as
this has a large impact on the overall detector cost. The full depth of the baseline hadron
calorimeter is 4λI. The basic mechanical structure is foreseen to have three main
sections: a barrel and two endcaps. The barrel is subdivided along the beam direction into
three sections, and there are twelve modules in azimuth.
The baseline material choice for the absorber is steel, although we have also considered
tungsten and brass, with plates having thickness of 20mm or 1.1 X0. In contrast four
technologies, three gaseous and one plastic, are under consideration for the active layers
of the hadron calorimeter. These range from resistive plate chambers, gas electron
multipliers, and micromegas for digital implementations, to small scintillator tiles for
semi-digital and analog implementations. Prototypes of each technology have been built
and evaluated.
We have had extensive discussions on the process for selection of the best technology for
the SiD hadron calorimeter. We have enumerated a set of requirements for the
calorimeter and selection criteria for the technologies, and the steps that should be
followed to reach a decision. These requirements and criteria incorporate the demands of
PFAs. For instance, our PFA studies are now focusing on center-of-mass energies beyond
the Z-pole, and consequently, higher energy, more collimated, jets. This helps us
understand the requirement for the transverse cell size. Also our simulations have
highlighted the pros and cons of neutron detection. The issue is whether we gain in the
PFAs from the use of scintillator to see neutron clusters, or whether it is better to use a
gas active medium to suppress the neutron signal and thereby simplify the track-cluster
association in the PFA. Also at issue is the comparative ease of construction and cost
differences between the various technologies. A full discussion of the hadron calorimeter
is given in Section IV.
The option of installing a “tail-catcher” after the ~1.5 λI of the superconducting coil and
cryostat, to potentially identify and measure the last few percent of hadron shower
energies, is under discussion. Given that, for the PFA approach, we do not need to make a
high precision measurement of the neutral hadron energy, and the charged particle
energies are well measured in the tracking system, the tail-catcher may only be needed in
an analog, or hybrid, approach to SiD calorimetry. Recent prototype results from
CALICE data taking at CERN show improved energy resolution when the tail-catcher
data is included. The technology for implementation of the tail-catcher would follow that
of the muon system: currently scintillator strips and resistive plate chambers are being
considered. The tail-catcher is also discussed in Section IV.
Basic simulation studies of hadron calorimetry are being carried out in the SiD context.
The parameters studied include the absorber material, the active layer material, the
longitudinal and transverse segmentation, the total thickness of the hadron section, the
thickness (λI) per layer, the inner radius, and the magnetic field. The idea is to work from
a basic understanding of single particle energy resolutions, through jets of known
energy(s), to full dijet mass resolution in physics events. The single particle studies
already reveal interesting and significant differences in energy resolution between the
combinations of steel and tungsten absorber and scintillator and gas active media that will
be essential input to decisions on the hadron calorimeter. Studies of jet energy resolutions
and neutral energy measurement are ongoing. Section V discusses this simulation work.
Initially the study and development of PFAs for SiD was undertaken by a number of
independent physicists at several different locations. In order to bring some structure into
these studies, and provide a common basis for comparison of algorithms within the
various PFAs, a framework has been produced by a collaboration between SLAC and
ANL. Initial studies for SiD, as for the other concepts, have focused on the Z-pole region.
Based on physics processes requiring the separability of W and Z bosons, the initial goal
for the PFAs was set at achieving an energy resolution at the Z-pole of σ/E ~30%/√E.
However, more recent considerations particularly at higher center-of-mass energies have
revised this goal to σ/E ~3-4%. The current state-of-the-art for SiD PFAs is σ/E
~35%/√E. We are approaching the level of understanding of the PFA performance that
will provide the basis for meaningful comparisons of technology choices for the hadron
calorimeter,
DAQ, fiber cables, data rates?
Backgrounds in the electromagnetic calorimeter, mainly low energy photons from the
beamcal, and photons from gamma-gamma interactions, produce an occupancy rate of at
most 1 x 10-4/BX in the highly segmented silicon pixel detectors. This should be an
inconsequential effect in calorimeter pattern recognition, and the electronic buffer size is
more than adequate to ensure full efficiency.
We are forming a SiD Engineering Group, with initial participants from SLAC and
Fermilab, to carry out a preliminary engineering study of the SiD detector design. This
study will consider the basic design of the calorimeter modules, materials, support of the
barrel and endcap calorimeters and the superconducting coil, assembly procedures and
magnetic force effects .This generic study will assume a calorimeter inside the coil and
active gaps of order 1cm.
Stability, calibration, push-pull ?
In this report, we present the electromagnetic calorimeter and its readout in detail. For the
hadron, we summarize the R&D relevant to the SiD calorimeter design and technology
selection. The details of the hadron calorimeter and TCMT R&D will be discussed in the
CALICE report submitted to this review. We include a section on basic simulation
studies as these offer valuable insight into the relative importance of various materials
and size parameters for the calorimeter design. Finally, we summarize the results from
SiD-based PFA studies that also bear on the optimization of this design.