Detector Development Group

Overview

The NIU team has been investigating a finely segmented scintillator-based hadron calorimeter for some time now. This option combines proven detection techniques with new photodetector devices. The absence of fluids/gases and very high voltages inside the detector increases longevity and operational stability.

Challenges

The main challenge for a scintillator-based hadron calorimeter is the architecture and cost of converting light from a large number of channels to an electrical signal. Our studies demonstrate that small cells (six-10) with embedded Silicon Photomultipliers (SiPMs)/Metal Resistive Semiconductor (MRS) photodetectors offer the most promise in tackling this issue. The use of these photodetectors opens the door to integration of the full readout chain to an extent that makes a multimillion channel scintillator calorimeter plausible. Also, in large quantities the devices are expected to cost a few dollars per channel, making the construction of a full-scale detector equipped with these photodiodes financially feasible.

The large number of readout channels can still pose a significant challenge in the form of complexity and cost of signal processing and data acquisition. Reducing the dynamic range of the readout is a potential solution. Monte Carlo studies have shown that this is a promising possibility. Scintillator cells with an area in the six-10 range are good candidates for one (digital) or two-bit (semi-digital) readout, where the lowest threshold is set to detect the passage of a minimum ionizing particle. Performance of PFAs on scintillator hadron calorimeter Monte Carlo's with a minimum of amplitude information in the form of thresholds also looks very competitive. The fabrication of cheap and compact electronics with just a few thresholds (three in the case of a two-bit readout) that will deliver the required performance is a realistic possibility for a scintillator hadron calorimeter.

Collaboration

We have coordinated our efforts with European groups pursuing similar interests. This interaction takes place under the umbrella of the CALICE collaboration, which bands together universities and labs from all over the world that share an interest in developing calorimeters for the linear collider. We are the only group in the United States actively investigating the promising option of a scintillator-based hadron calorimeter.

Muon ID and Reconstruction

Many key physics channels expected to appear at the linear collider have muons in their final states. Given the smallness of the expected cross sections, high efficiency in the tracking and identification of the muons will be very important. Since the precise measurement of the muon momentum will be done with the central tracker, a high-granularity muon system that can efficiently match hits in it with those in the tracker and calorimeter will be needed.

Energy Leakage

Hermeticity and resolution constraints require that the calorimeters be placed inside the superconducting coil to avoid serious degradation of calorimeter performance. On the other hand, cost considerations associated with the size of the coil imply that the total calorimetric system will be relatively thin (4.5-5.5 λ). Additional calorimetric sampling may be required behind the coil to estimate and correct for hadronic leakage.

Shower Validation

Current hadronic shower models differ significantly from each other. This puts conclusions on detector performances drawn from PFAs on rather shaky ground. One of the most important goals of the LC test beam program is the validation of hadronic simulation packages. A TCMT that can provide a reasonably detailed picture of the tail end of showers will be very helpful in this task.

The TCMT prototype will have fine and coarse sections distinguished by the thickness of the steel absorber plates. The fine section will sit directly behind the hadron calorimeter and have the same longitudinal segmentation as the HCAL. It will provide a detailed measurement of the tail end of the hadron showers. This is crucial to the validation of hadronic shower models, since the biggest deviations between models occurs in the tails. The following coarse section will serve as a prototype muon system for any design of a linear collider detector. It will facilitate studies of muon tracking and identification within the particle flow reconstruction framework. Additionally, the TCMT will provide valuable insights into hadronic leakage and punch-through from thin calorimeters and the impact of the coil in correcting for this leakage.

Overview

The detector development group is interested in calorimeter research and development for the proposed ILC. We propose to develop, in simulation and in prototype, designs for a hadron calorimeter (HCal) optimized for jet reconstruction using particle-flow algorithms (also called energy-flow algorithms). Simulation/algorithm development and hardware prototyping are envisioned as the two main components of our efforts. The text below addresses the first component.

High-precision Measurements

An e+e- linear collider is a precision instrument that can elucidate Standard Model (SM) physics near the electroweak energy scale and discover new physics processes in that regime, should they exist. In order to fully realize the potential anticipated from a machine of this type, the collection of standard high-energy physics detector components comprising an experiment must be optimized, sometimes in ways not yet realized at current experiments. One such example is the hadron calorimeter, which will play a key role in measuring jets from decays of vector bosons and other heavy particles, such as the top quark and the Higgs boson(s).

In particular, it will be important to be able to distinguish, in the final state of an e+e- interaction, the presence of a Z or a W boson by its hadronic decay into two jets. This means that the dijet mass must be measured within ~ 3 GeV, or, in terms of jet energy resolution, σ(E) ≈ 0.3 E^1/2 (E in GeV). Such high precision in jet energy measurement cannot be achieved by any existing calorimeter in the absence of a kinematically overconstrained event topology. Similar precision in measurements of jet and missing momentum will be crucial for discovery and characterization of several other new physics processes, as well as for precision tests of the Standard Model. Such ambitious objectives place strong demands on the performance of the calorimeters working in conjunction with the tracking system at the ILC, and require the development of new algorithms and technology.

Particle-flow Algorithms

The most promising means to achieving such unprecedented jet energy resolutions is through particle-flow algorithms (PFA). A PFA attempts to separately identify in a jet its charged, electromagnetic and neutral hadron components, in order to use the best means to measure each. On average, neutral hadrons carry only ~11 percent of a jet's total energy, which can only be measured with the relatively poor resolution of the HCal. The tracker is used to measure with much better precision the charged components (~64 percent of jet energy), and the electromagnetic calorimeter (ECal) to measure the photons with σ(E) ≈ 0.15 E^1/2 (~24 percent of jet energy).

On average, only a small fraction of a jet's energy is carried by particles with momenta greater than 20 GeV. Measurements from the tracker are at least two orders (one order) of magnitude more precise than those from the calorimeter for particles below 20 GeV (100 GeV). A net jet energy resolution of σ(E) ≈ 0.3 E^1/2 is thus deemed achievable by using the HCal only to measure the neutral hadrons with σ(E) ≈ 0.6 E^1/2. However, this will certainly require extensive and simultaneous optimization of detector design and tuning of algorithm parameters.

Calorimeter Design and Event Simulation

A calorimeter designed for PFAs must be finely segmented both transversely and longitudinally for 3D shower reconstruction, separation of neutral and charged clusters, and association of the charged clusters to corresponding tracks. This requires realistic simulation of parton shower evolution and of the detector's response to the particles passing through it. Accurate simulation relies heavily on analysis of data from beam test of prototype modules. The detector optimization requires the simulation, visualization and analysis packages to be highly flexible, which calls for careful design and implementation of the software itself.

Very large numbers of events will have to be simulated to evaluate competing detector designs in relation to ILC physics goals. Characterization of signatures arising from processes predicted by some extensions of the Standard Model will require simultaneous coverage of broad ranges of undetermined parameters. Parametrized fast simulation programs will thus have to be developed once the algorithms have stabilized. Parametrization of PFAs will require much work and is one of our key objectives.