Linear Collider Detector R&D Issues

The planning for the linear collider detector research and development program will first require a careful analysis of the most important and relevant R&D goals. In order to present an initial point for discussion, we offer the following comments on some of the issues. Your comments and criticism are welcome.

-- Jim Brau

General Goals

Our goal is to be prepared to submit a detailed technical proposal for an experiment in a few years (when the accelerator proposal is ready). We must evaluate subsystem by subsystem what needs to be developed or demonstrated. Of course the overall detector function is the most important issue, and subsystem choices constrain other subsystems. So integration into a full detector, respecting the overall constraints, on the one hand, while optimizing the performance, can be a challenging effort.

In previous studies of the Linear Collider physics, the most important constraints on the detector have come from the following considerations:

In addition, special needs for the Linear Collider Detector include:

Special constraint: Final focus quads (2 meters from I.P.) that must be anchored to bedrock

Reminder of the LC Beam Parameters (see below for TESLA parameters ):

A central issue is the overall size of the detector. While many different options for the geometry could be considered, many considerations drive the design to one of two limits:

It is important to understand the strengths and weakness of each of these strategies, as well as alternative strategies which might be proposed.

CCD Vertex Detector Development

The physics of LC demands the best possible vertex detector performance, enabling clean separation of b, c, and udsg jets, and tau's

Vertexing provides:

The route to optimized flavor tagging follows from:

The current state-of-the-art on CCDs for linear collider vertex detectors is:

Linear Collider physics requirements justify further improvements, which can be divided in the following areas of R&D:

The Linear Collider Vertex Detector design goals are:

Directed R&D efforts will be aimed at

Simulation studies are a very important element of the R&D program. Some of the initial simulation studies will concentrate on the following issues:

Tracking Issues

Some of the issues that must be understood:

Particle ID?

Calorimetry

The goals for the calorimetry are precise electron and gamma measurements, jet reconstruction and measurements, and missing energy measurements.

For the electromagnetic calorimeter, one important benchmark process is the measurement of the Higgs decay to two photons. Naively, this process calls for the best possible electromagnetic resolution. However, the precise impact of lessened EM resolution (as a trade-off for improved jet resolution) needs to be more fully undersood.

The traditional jet measurement strategy is to measure the jet energy through total absorption in a calorimeter. It is thought that at the Linear Collider a different strategy will be more effective. This involves reconstructing the jet energy by measuring the deposition in the tracker and the calorimeter detectors, and then reconstructing the jet. The procedure, sometimes referred to as an "energy flow analysis," appears in at least two manifestations:

In each of these approaches, and particularly in the first, the granularity of the electromagnetic calorimeter is critical in separating the electromagnetic energy deposition from incoming charged particles. Very detailed longitudinal granularity could help here.

The calorimeter subsystem will focus on the key issues of

The calorimeter group must work through many options with different advantages and determine the relative importance of the feature each brings to the application.

The calorimeter working group must also ensure that the masking design to control the fluence of synchrotron radiation, and pairs, does not unacceptably impact the calorimeter performance (see Backgrounds below).

Muon Detection

The muon system must reside outside of the rest of the detector volume, and therefore is highly constrained and driven by inner detector choices. The physics requirements need to be more precisely defined. Presumably, the precision of the inner tracker is adequate for momentum measurements. This would require the muon system to tag and trigger on muons. What performance is needed to achieve these goals?

Trigger and DAQ

A system with flexibilty to adjust for the backgrounds must be developed. The strategy for triggering on each of the physics signatures needs to be enunciated.

Luminosity Measurement

The colliding electron and positron bunches disrupt one another and induce radiation, and the actual luminosity and energy spectrum of e+e- annihilation reactions depends on these effects. Therefore, it is important to measure the differential luminosity spectrum directly. This requires an acollinearity angular measurement for the small-amgle Bhabha scattering to better than 1 mrad at theta ~ 200-500 mrad. An electromagnetic calorimeter with such precision must reside within the synchrotron radiation mask.

Polarization Measurement

It will be necessary to measure the polarization of the beam (or beams) at the Linear Collider. Presumably this will be accomplished with the Compton scattering technique which has been so successfully exploited at the SLC. However, thinking and plans need to begin.

Backgrounds

Experience with SLC and simulations of the Linear Collider have led to the following estimates of the expected backgrounds. Here we see tolerable levels, but it will be wise to prepare for the unexpected. The shielding of the detector to backgrounds requires masking at small angles, as mentioned above. The design of this masking system must be studied in conjunction with the performance of the subsystems (i.e. calorimetry) to ensure acceptable impact on the hermiticity of the detector.

Simulation

see Plans for the US/Canada Linear Collider Detector Simulation Study

Comment

An important general issue for all subsystems is timing. Does an individual subdetector try to keep track of signal times well enough to make its own bunch assignment or does it rely on global pattern recognition to sort things out later?

Conclusion

There are many issues that need to be resolved in order confidently propose an experiment for the next Linear Collider. Now is the time to get on with planning and executing the detector R&D. Next we need to develop detailed plans covering all subsystems and issues.

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