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| 27 July 2021

3D simulation image of ILD detector Image: Akiya Miyamoto

Things are busy at the ILD detector concept at the ILC. ILD stands for “International Large Detector”, and this is one of two current detector concept groups at the ILC. The ILD team has just published its interim design report (IDR) in which the current state of the detector, the state of the technologies and the situation in the collaboration are discussed. 

The IDR is the result of about five years of studies, tests, calculations and simulations. It is meant to bring the status of the detector as recorded in the 2011 Technical Design Report up to scratch with the latest technical and physical standard and results. 

“We looked at many things,” explains ILD spokesperson Ties Behnke from DESY. “For example, we studied the dependence of the performance on the size of the detector with the goal to optimise the cost-performance benefits, or the impact of changes to the interaction region design on the detector performance. We looked at the basic technologies we propose to use at the ILD, and brought them up to date with latest technological developments. We also performed systematic demonstrations of key technologies for ILD in test-experiments in close collaboration with technology collaborations like CALICE or LCTPC.” In particular, though, the size of the detector is highly relevant, as it has a major impact on the cost of the system. To evaluate the impact different sizes and different technologies might have on the performance, ILD has created two different versions of the detector, a smaller one and a larger one, and developed novel techniques to compare different sub-detector technologies in one simulation model.

So does size matter? Yes, bigger is better, but the benefits are not always obvious. “Looking more closely”, Behnke says, “bigger in particular has significant advantages if the ILC would be upgraded to higher energies.” Scientists hope this will happen after an initial running period to increase the discovery potential of the collider. 

The design of the ILC accelerator is also constantly evolving, and the detector needs to track these developments. For example, the distance between the interaction point (where particles collide) to the last component of the accelerating system can have a huge impact on the way collisions go, and the detectors have to be able to adapt to whatever system accelerator designers deem most useful and cost effective. Close interaction between detector physicists and accelerator designers have led to an optimised design of the forward region of the detector, an area which is notoriously difficult to instrument. 

A suite of software tools to enable simulation of events and of the detector plays a central role in these studies. ILD has developed a robust, scalable and flexible software system, including simulation tools, access to reconstruction tools, and means for the user to easily access the simulated events. Based on these tools large sets of simulated data are available, which can be used for a wide variety of studies. To ease the access to these data sets, and to ILD in general, ILD has initiated a guest -membership programme which allows people to participate with minimal hurdles, and which will hopefully lead to a further increase in the participation in ILD studies. 

Claude Vallee, until recently ILD technical coordinator and now deputy-Chair of the IDT Physics and Detector working group, summarises: “The ILD Interim Design Report constitutes the most up-to-date comprehensive description of a detector for an e+e- Higgs factory. It provides a good overview of the motivation for most concepts currently on the market, as well as the main directions of potential improvements in the subdetector core technologies, their internal integration and the overall detector configuration. I strongly recommend it to all experimentalists willing to engage now on the ILC programme and, more generally, on any e+e- Higgs factory detector design.” The report also clearly points out areas where developments are needed and promising, and how the concept will be developed into the future.

The ILD team currently consists of some 360 scientists from 68 different labs. The group recently re-elected its management team. Ties Behnke and Kiyotomo Kawagoe (Kyushu University) got confirmed as spokespeople, Filip Zarnecki (Warsaw University) joined the physics coordination and will share the work with Keisuke Fujii (KEK). Mary-Cruz Fouz (CIEMAT) and Karsten Buesser (DESY) will lead the technical coordination. The software team remains unchanged with Frank Gaede (DESY) and Daniel Jeans (KEK) as coordinators.

“With this optimised ILD detector we see ourselves well positioned to participate in the process now initiated by the International Development Team that will eventually led to the submission of expressions of interest for an experiment at the ILC,” Behnke concludes. But the work continues: over the next years, the team plans to undergo a critical scrutiny of the key technologies which are currently used, folding in the experience gained in the construction of the upgraded LHC detectors, and exploring where new technologies could be used to further improve the performance – for example the use of timing in the detector, and the use of modern very thin pixel detectors.

About detector concepts at the ILC

Just to get you up to date with detectors at ILC: the current planning foresees two independent detectors, SiD and ILD. There will only be one place where electrons and positrons are made to collide, so the detectors will have a time-share on this popular spot. While one is in taking data, the other is in a parking position at the side and can undergo checks and repairs. The ingenious rotation scheme by which these two gigantic high-tech devices swap positions is called “push-pull” (while one is pushed in, the other is pulled out) and is the subject of a lot of study – but that’s another story.

ILD and SiD use different systems to study the collisions. That’s intentional, because one needs to be able to check and ideally verify the discoveries of the other to make sure it’s a true discovery and not just a bug in the system. One central difference is the tracking system, closest to the beampipe just like the forward region. It is based fully on silicon at SiD, while ILD uses a time projection chamber. Otherwise they are very similar in size and performance.

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