Beam-induced backgrounds create several detector challenges: high occupancies, high rates, and large radiation doses. To deal with this environment, we need detectors with precision timing and on-chip intelligence.
Decay products from the beam pose unique challenges for particle detectors at a multi-TeV muon collider. To enable both energy reach and precision physics, the detector must also enable excellent reconstruction efficiency and performance for ~TeV final state particles produced via s-channel processes as well as more moderate momentum particles produced via vector boson fusion, respectively. As part of the Snowmass process, a 3 TeV detector concept was adopted from CLIC to the muon collider environment, and demonstrated to deliver high quality physics with full simulation. The most important modification to the design includes tungsten nozzles, with coverage up to ~10 degrees. High energy electrons produced from decays of the beam interact with the collider lattice and shower inside the nozzle material, resulting in a diffuse cloud of 108 particles per event entering into the detector region. These beam induced background (BIB) particles are mostly neutral, low momentum, and non-pointing, with an out of time component. The radiation environment is similar to that of the High Luminosity LHC, with doses up to 100 Mrad and fluences up to 1015 MeV neq/cm2 at the smallest tracker radii. Detector occupancies are somewhat higher than the HL-LHC (up to a factor of 10), while the event rate is much lower (~1/1000).
The current focus of detector development is the design of a 10 TeV concept. The properties of the BIB at 10 TeV are similar to that of 3 TeV. A challenge at 10 TeV is that the detector needs to grow with energy in order to maintain momentum resolution and fully contain high energy calorimeter showers. To accommodate these needs, the current plan is to move the solenoid inside of the electromagnetic calorimeter and increase the field from 3.5 to 5 T. The fully silicon tracker includes a vertex detector with 25x25 μm2 pixels and 30 ps timing resolution. The calorimeter is highly segmented and assumed to have ~100 ps timing resolution. The ECAL is assumed to be silicon tungsten with 5x5 mm2 cells while the HCAL is assumed to be steel and scintillator with 30x30 mm2 cells. The steel from the HCAL acts as the return flux for the solenoid. Occupancies in the ECAL are reduced by a factor of ~3 with respect to 3 TeV due to the presence of upstream material from the solenoid. The impact of this additional material on the energy resolution will be studied. The muon spectrometer design has not been optimized, but micro-patterned gaseous detectors should be sufficient to handle occupancies in the forward region. Preliminary studies suggest a streaming trigger architecture is feasible. However, bandwidth and power constraints pose challenges for future on-detector ASICs. Now that baseline performance has been established, it is essential to map sub-detector designs onto technology needs in order to ensure necessary technologies converge in time for construction.
Most open questions related to the detector design are related to the Machine Detector Interface (MDI), in particular the tungsten nozzles. Studying modifications of the nozzles is challenging because BIB needs to be simulated for each configuration. There is a great deal of interest in reducing the size of the nozzle to extend detector coverage beyond |η|=2.5. There’s also interest in tagging forward muons up to |η|=6 in order to separate WW and ZZ fusion processes by instrumenting the nozzle. A momentum measurement with ~20% resolution would additionally open up inclusive higgs measurements and higgs to invisible searches. High occupancies and the material from the nozzle itself pose significant challenges for such a measurement. Finally, dedicated detectors for a luminosity measurement need to be developed. Studies using muons from Bhabba scattering in the central region suggest we can achieve a luminosity measurement with ~1% uncertainty, but the limited statistics are insufficient for in time monitoring. Developing expertise simulating BIB in FLUKA or MARS in the US is crucial to enable these studies.
Tracker Technologies
Vertex and Tracking detectors at a muon collider face the unique challenge of BIB that drastically increases the multiplicity of hits in them. In order to successfully and with high accuracy reconstruct tracks, precision timing information O(30 ps) is a necessary tool to at least perform a basic rejection of BIB and achieve a realistic readout rate. As particularly in the vertex detector the hit multiplicity is extreme, further measures like stub identification and cluster shape analysis are necessary in addition to 4D tracking. Generally speaking, the tracking detector at a muon collider shares many requirements with the Phase-2 upgrades of the ATLAS and CMS detectors for the HL-LHC, but adds high precision timing to all of its tracking detectors. This is a non-trivial challenge and active R&D in this direction is being pursued:
- As 4D tracking is seen as the future by the community for any discovery machine, there are many examples of R&D that a muon collider tracking detectors could profit from: Monolithic Active Pixels Sensors (MAPS), Low Gain Avalanche Detectors (LGADs), 3D sensors, readout ASICs, and data transmission development. It is important for the muon collider community to highlight this R&D to the national and international organizational bodies using the RDC and DRD groups set up by CPAD and CERN.
- While there are some common R&D goals, the muon collider community could benefit from a dedicated R&D path to overcome the specific challenges a muon collider imposes on the tracking detectors. Due to the importance of 4D tracking in particular, it seems paramount to understand the requirements of such a system as a whole early in the process to drive the specific component development.
- Due to the unique nature of the BIB, detector development will heavily rely on simulation efforts and a close contact should be established between the two efforts.
- At a certain point in the R&D cycle it will also be necessary to establish test facilities that are suitable to testing such tracking detectors under realistic conditions.
Calorimeter Technologies
The past three decades of development of calorimeter technologies has resulted in maximal resolution within constraints, higher granularity and lower backgrounds for more sensitivity to new physics. In the Muon Collider Era, we expect to have sub-nanosecond tags for each individual particle region, multi-signal/multi-dimensional discrimination to further particle identification for all particles, tens to hundreds of millions of channels, tens of Mrad tolerances, tens of GBytes of per event data, triggers based on all real-time reco particles in the event and do all of this with the first AI/ML fully optimized detector design. Above all, the great leap for the muon collider detector calorimeter is to deliver new order of magnitude advances across the board while cleaning each event of beam-induced-background.
There is a complex landscape of potential technologies from Si-W based on-detector/MAPS readout, Cryo/Noble Liquid/LAr high granularity with cold readout, optical calorimetry with dual-readout/hybrid crystals to emerging timing-centric approaches/fast glass/Crilin. The time-domain is an important and pervasive dimension and includes (AC-)LGAD/silicon, fast glass/Crilin, LYSO/fast scint/SiPM technologies for dedicated sub-20 ps timing layers to several 10’s to 100’s of picosecond leading-edge discrimination for many of the full-detector calorimeter technologies. The main points for the muon collider calorimetry are as follows:
- We need to measure hit times along trajectories and design layer geometries to allow us to track from the interaction point outward and from beam collimators inward. Timing layers/walls should be arranged to efficiently catch beam backgrounds and maintain high event quality, providing multiple time measurements along trajectories leading up to the calorimeter.
- At the reconstruction level, the goal is to dig deep with high quality local data, but to save frugally and intelligently when it comes to pushing data off-detector. A strong guiding paradigm is Particle Flow (PF) in a graph theory approach taking advantage of improved spatial, temporal and energy resolutions.
- This level of design must be driven by AI/ML at its core, as nothing less than the most optimized interplay between analysis and intrinsic detector measurements will make new discoveries possible, forging a stronger foundation to understand the early Universe through particle physics.
- Simulation is absolutely central to optimizing the calorimeter in concert with PFA/PID performance. The software needs to be able to cycle through many available options and make quantitative comparisons.
- A physical self-cleaning demonstrator is an essential near-term goal as a focus of Detector R&D. This can be a relatively compact but structured tracking/timing/calorimeter detector slice capable of operating on a CASTOR-table (space between IP and forward calorimeters at CMS) setup in the HL-LHC environment. The demonstrator should prove in realistic high background conditions that out-of-time non-pointing particle backgrounds can be efficiently suppressed while maintaining high efficiency for IP signal particles.
Calorimeter R&D should continue to be impressive, pushing on ASICs, PID, and novel detector signals. It was even proposed to use a SQUID sampling array and polarized scintillation materials to estimate electron longitudinal polarization from the statistical sampling of the EM shower. One can look further to integrating embedded spatially distributed arrays of entangled coherent states for calorimetry-correlated decoherence signals from long-wavelength field transitions. There is much to discover and imagine.
Other systems
The remaining systems, including forward taggers, luminometers, and DAQ, are all crucial to the success of a muon collider, but so far much less explored.