Next generation tracking and vertexing detectors

Abstract: Precise and efficient track and vertex reconstruction is essential to exploit the physics potential of collider experiments. The requirements of future tracking systems are determined by the collider type and its collision environment, required measurement precision, and beam structure. This contribution reviews the requirements and challenges of vertexing and tracking detectors at future $e<sup>+e<sup>-$, $e<sup>-$-hadron, $μ<sup>+μ<sup>-$, and hadron colliders. Several common trends emerge across the different collider types. Tracking systems require increasingly precise spatial measurements, low-material designs, and, in some cases, integrated particle identification capabilities. Monolithic active pixel sensors are currently the leading option for vertex detectors at $e<sup>+e<sup>-$ and $e<sup>-$-hadron colliders. Tracker concepts range from gaseous detectors, which provide many measurements per track and can achieve very low material budgets, to silicon and scintillating-fibre trackers, which provide fewer but more precise measurements and can tolerate higher hit rates. At 10 TeV parton centre-of-mass colliders, precision timing throughout the tracking system is needed for beam-background rejection or pile-up mitigation. For future hadron colliders specifically, radiation tolerance beyond that demonstrated by existing sensor technologies is required. Ultimately, meeting the vertexing and tracking requirements of future colliders will require substantial R&D and the integrated optimisation of sensors, front-end electronics, readout, cooling, powering, and mechanics.

• The paper compares tracking and vertexing requirements for future collider experiments, highlighting the need for integrated, collider-specific detector solutions.
• Future “lepton colliders” require precision tracking systems with 3 µm spatial precision and ultra-low material budget e.g., The FCC-ee program.
• “Muon and future hadron colliders” need highly radiation-tolerant, fully time-resolving detectors.”

The paper presents a comparative review of vertexing and tracking requirements for future collider experiments, organized around three governing variables: collision environment, measurement precision, and beam structure. Its central argument is that no single detector technology is optimal across all collider classes. Instead, future systems must combine sensor, front-end, readout, cooling, powering, mechanics, and DAQ design from the outset. The resulting design space extends from ultra-low-material precision trackers at lepton colliders to highly radiation-tolerant, fully time-resolving systems at future hadron and muon colliders (2608.28089).

Detector requirements across collider environments

Vertex detectors provide the precise impact-parameter measurements needed to reconstruct primary, secondary, and tertiary vertices, whereas outer trackers determine momentum from the curvature of charged-particle trajectories in a magnetic field. Across both subsystems, the basic performance requirements are high hit efficiency, low noise, precise position measurement, adequate bunch-crossing association, radiation tolerance, and broad angular coverage. The paper additionally emphasizes the need to design for long-lived-particle signatures and, where appropriate, integrate particle identification (PID) into the tracking system rather than treating it as an independent downstream function.

The relative importance of these requirements varies substantially. Lepton colliders are primarily precision-limited, with stringent demands on spatial resolution and material budget. Muon colliders and future hadron colliders are instead dominated by beam-induced backgrounds, pile-up, radiation damage, and timing. The bunch structure determines whether power pulsing is practical and whether a hardware trigger is required. This framework provides a useful basis for comparing technologies that otherwise differ substantially in operating principle.

Vertexing at future electron–positron colliders

Future $e^+e^-$ colliders combine comparatively moderate occupancies with the most demanding vertexing precision. The vertex detector is constrained by the machine–detector interface, which includes the beam pipe, luminosity calorimeter, and final-focus magnets. Its innermost radius and longitudinal extent cannot therefore be optimized independently of accelerator design. Incoherent pair production and synchrotron radiation are identified as the dominant sources of beam-induced background near the interaction point.

Linear colliders offer long intervals between bunch trains, enabling power pulsing. Turning off detector components between trains reduces average power consumption and makes air cooling feasible for the innermost layers. Circular colliders have a different beam structure, but their vertex-detector concepts similarly prioritize air cooling and minimal services in the active volume. These choices are not merely engineering optimizations: reducing cooling infrastructure directly limits the material contribution that degrades low-momentum tracking through multiple Coulomb scattering.

The relevant vertexing metric is the transverse impact-parameter resolution,

$$\sigma_{d_0} = a \oplus \frac{b}{p\sin^{3/2}\theta},$$

where $a$ captures intrinsic measurement precision and geometry, while $b$ describes multiple-scattering effects. The FCC-ee targets approximately $a = 3~\mu\mathrm{m}$ and $b = 15~\mu\mathrm{m}\,\mathrm{GeV}$ (2608.28089). The first term requires small-pitch pixel sensors placed close to the interaction point; the second requires an exceptionally low material budget in both the vertex detector and the beam pipe. The paper notes that improving these values would directly benefit rare-flavour measurements such as $B^0\rightarrow K^{*0}\tau^+\tau^-$, establishing a concrete physics consequence of detector-level gains.

Monolithic active pixel sensors (MAPS) are presented as the leading, and currently sole, technology under consideration for future $e^+e^-$ vertex detectors. By integrating sensing, amplification, and readout within one silicon die, MAPS enable thin detector structures with pixel pitches of approximately $20\times20~\mu\mathrm{m}^2$. The reviewed R&D programmes illustrate different points in the performance space:

Programme	Process	Representative pitch	Spatial resolution	Timing target or performance
ARCADIA	110 nm LFoundry	$25\times25~\mu\mathrm{m}^2$	Down to $$\sigma_{d_0} = a \oplus \frac{b}{p\sin^{3/2}\theta},$$0	From nanoseconds to tens of picoseconds
OCTOPUS	65 nm TPSCo	Approximately $$\sigma_{d_0} = a \oplus \frac{b}{p\sin^{3/2}\theta},$$1	Goal of $$\sigma_{d_0} = a \oplus \frac{b}{p\sin^{3/2}\theta},$$2	$$\sigma_{d_0} = a \oplus \frac{b}{p\sin^{3/2}\theta},$$3 ns time tag with ToT
TaichuPix	180 nm TowerJazz	$$\sigma_{d_0} = a \oplus \frac{b}{p\sin^{3/2}\theta},$$4	Down to $$\sigma_{d_0} = a \oplus \frac{b}{p\sin^{3/2}\theta},$$5	Less than $$\sigma_{d_0} = a \oplus \frac{b}{p\sin^{3/2}\theta},$$6 ns time walk

The comparison also exposes the principal trade-offs. OCTOPUS targets a thickness of at most $$\sigma_{d_0} = a \oplus \frac{b}{p\sin^{3/2}\theta},$$7 and a power density below $$\sigma_{d_0} = a \oplus \frac{b}{p\sin^{3/2}\theta},$$8, whereas TaichuPix is thicker and has a higher power specification. ARCADIA offers a broader timing range and side-abuttable modules, but its thickness depends strongly on implementation. These are prototype and development targets rather than demonstrated full-detector performance, and the paper does not establish that any one MAPS programme satisfies the complete FCC-ee system requirement.

Proposed geometries include stave-based layouts and wafer-scale bent MAPS. The FCC-SEED concept, with overlapping curved layers and signal routing through flexible circuits, illustrates a broader design trend: mechanical integration and service routing are being treated as part of the vertex-detector architecture rather than as post hoc constraints. The remaining question is whether such geometries can simultaneously deliver the required alignment stability, manufacturability, power distribution, thermal performance, and radiation lifetime.

Tracking at electron–positron colliders

Momentum resolution is particularly important for precision Higgs and electroweak measurements. At FCC-ee, the stated requirement for tracks with momenta of order $$\sigma_{d_0} = a \oplus \frac{b}{p\sin^{3/2}\theta},$$9 GeV is $a$0 under a conservative design and below $a$1 for a more aggressive target (2608.28089). At low centre-of-mass energy, multiple scattering makes material minimization especially important; at higher energy, intrinsic hit resolution and lever arm become increasingly decisive.

The paper distinguishes two broad tracker strategies. Silicon and scintillating-fibre (SciFi) systems provide approximately five to ten highly precise measurements per track, while gaseous systems—drift chambers, time-projection chambers (TPCs), and straw tubes—provide of order one hundred lower-resolution measurements. Gaseous detectors offer continuous tracking and very low material budgets, and their ionization measurements provide useful PID. Silicon systems tolerate higher hit rates and can offer few-micrometre spatial precision, but require more demanding cooling and service integration. SciFi trackers provide low-to-medium material budgets and time-of-flight information, although light yield over long fibres is a design concern.

Tracker technology	Typical hit count	Hit resolution	Material budget	Principal challenge
Drift chamber	$a$2	Approximately $a$3	Ultra-low	Wire tension and failure
TPC	$a$4	Approximately $a$5	Low	Ion-backflow field distortions
Straw tubes	$a$6	Approximately $a$7	Low	Stereo geometry and mechanics
Silicon tracker	$a$8–$a$9	Few $b$0	Medium	Cooling and services
SciFi tracker	$b$1–$b$2	Tens of $b$3	Low–medium	Light yield along fibres

Magnetic-field and detector dimensions are coupled to collider operation. Circular machines are assumed to limit the solenoidal field to approximately $b$4 T to avoid adverse effects on the beams and luminosity. Their trackers compensate with outer radii around $b$5 m. Linear colliders can employ substantially higher fields; the SiD concept, for example, assumes $b$6 T (Breidenbach et al., 2021), allowing a smaller tracker for a comparable momentum-measurement performance. The paper therefore treats tracker radius, field strength, material, and spatial resolution as a coupled optimization problem rather than independent technology choices.

The ePIC tracker at the Electron–Ion Collider

The ePIC tracking system at the EIC is used as an intermediate case between HL-LHC instrumentation and future lepton-collider systems. At a centre-of-mass energy of up to $b$7 GeV and luminosity of $b$8, its environment is less demanding than that of high-energy $b$9 machines, although higher-energy electron–hadron colliders such as the LHeC or FCC-eh would impose more severe rate and radiation constraints.

The design combines three subsystems optimized for different functions. A vertex detector with a $a = 3~\mu\mathrm{m}$0 integration time provides high-granularity spatial measurements. A moderately fast micro-pattern gas detector supplies timing of approximately $a = 3~\mu\mathrm{m}$1 ns, supporting pattern recognition and compensating for the slower vertex detector. A time-of-flight system based on AC-coupled low-gain avalanche detectors achieves approximately $a = 3~\mu\mathrm{m}$2 ps timing for PID.

The vertex detector uses wafer-scale bent MAPS derived from the ALICE ITS3 programme, implemented in the TPSCo 65 nm process. The inner barrel layers use bent MOSAIX sensors, while the outer barrel layers and forward disks use adapted flat versions. This architecture demonstrates that heterogeneous tracking can be advantageous: each subsystem is selected for a specific combination of spatial resolution, timing, integration time, and PID performance. The cost is substantial parallel development of sensors, ASICs, readout chains, and mechanical systems.

The paper identifies two alternatives to this multi-technology approach. Versatile sensors could be configured for different experiments and operating points, as pursued by the DRD3 MANTA programme. Alternatively, MAPS with internal gain could combine spatial precision and fast timing in a single sensor, as explored by CASSIA and ARCADIA MADPix. These approaches could simplify detector integration, but the paper does not demonstrate that multifunctional sensors can match the optimized performance of specialized subsystems across all relevant operating conditions.

Muon-collider tracking

Muon colliders present a qualitatively different background problem. Muon decays along the beam produce large fluxes of secondary particles, imposing stringent constraints on the machine–detector interface and requiring timing resolutions of tens of picoseconds throughout the tracking system. At collision energies up to $a = 3~\mu\mathrm{m}$3 TeV, the spatial-resolution requirements are also severe because of the large track momenta and the need to reject background hits without compromising pattern recognition.

The expected total ionizing dose and non-ionizing energy loss in the vertex detector and tracker may be comparable to HL-LHC levels (Accettura et al., 30 Apr 2025). The MAIA concept assumes vertex-detector pixels with $a = 3~\mu\mathrm{m}$4 spatial resolution and inner-tracker measurements of approximately $a = 3~\mu\mathrm{m}$5 (Bell et al., 31 Jan 2025). These specifications illustrate that muon-collider tracking is not reducible to a conventional high-energy lepton-collider design with an additional timing layer: timing must be distributed across the complete tracking volume because background rejection occurs at the hit level.

The low collision rate associated with single muon bunches may allow every collision to be read out and may enable power pulsing. This potentially reduces average sensor power and cooling requirements, but it does not remove the need for high instantaneous-rate capability, radiation tolerance, or precise time stamping. The compatibility between power-pulsed operation and continuous background exposure remains an important system-level constraint.

Future hadron-collider trackers

Future hadron colliders impose the most extreme combined requirements. At pile-up values approaching 1000 and centre-of-mass energies of order $a = 3~\mu\mathrm{m}$6 TeV, the tracker must separate hard-scatter products from a very large number of simultaneous interactions while operating in radiation fields one to two orders of magnitude above those expected at the HL-LHC. The cited FCC-hh estimates reach approximately $a = 3~\mu\mathrm{m}$7 neutron-equivalent particles per square centimetre and $a = 3~\mu\mathrm{m}$8 MGy (2608.28089).

The paper identifies precision timing as a fundamental requirement rather than an auxiliary capability: time resolutions of approximately $a = 3~\mu\mathrm{m}$9 ps or better throughout the tracking system are needed for 4D tracking and pile-up mitigation. The momentum range extends from approximately $b = 15~\mu\mathrm{m}\,\mathrm{GeV}$0 GeV to $b = 15~\mu\mathrm{m}\,\mathrm{GeV}$1 TeV. At $b = 15~\mu\mathrm{m}\,\mathrm{GeV}$2 TeV, maintaining a momentum resolution of $b = 15~\mu\mathrm{m}\,\mathrm{GeV}$3 while keeping occupancy below $b = 15~\mu\mathrm{m}\,\mathrm{GeV}$4 requires pixel granularities as small as $b = 15~\mu\mathrm{m}\,\mathrm{GeV}$5. The implication is direct: segmentation, timing, radiation hardness, bandwidth, and power density must be co-optimized, since improving one parameter can degrade the others.

The reference FCC-hh tracker radius of $b = 15~\mu\mathrm{m}\,\mathrm{GeV}$6 m is substantially larger than the approximately $b = 15~\mu\mathrm{m}\,\mathrm{GeV}$7–$b = 15~\mu\mathrm{m}\,\mathrm{GeV}$8 m radii of the ATLAS and CMS trackers. This increases the instrumented volume and consequently the demands on mechanics, services, cooling, calibration, and material control. Forward coverage to $b = 15~\mu\mathrm{m}\,\mathrm{GeV}$9 additionally requires dedicated forward tracking systems, extending the design problem beyond a conventional central barrel-and-endcap architecture.

A particularly strong conclusion is that no currently available sensor technology satisfies the combined performance and radiation-tolerance requirements. The paper therefore points to 3D sensors, low-gain avalanche detectors (LGADs), and wide-band-gap semiconductors as R&D directions rather than established solutions. This distinguishes the hadron-collider case from the $B^0\rightarrow K^{*0}\tau^+\tau^-$0 case, where MAPS already constitute a credible baseline, even though substantial engineering development remains.

System integration and technology trade-offs

Across all collider types, the paper argues against evaluating sensors in isolation. The detector performance ultimately depends on the complete chain from charge deposition to reconstructed track: sensor geometry and depletion, front-end shaping and discrimination, timestamping, data sparsification, optical or electrical transmission, powering, cooling, mechanical stability, alignment, and reconstruction software. A sensor that meets a nominal spatial or timing specification may still be unsuitable if its power density produces excessive material, its readout cannot sustain the hit rate, or its radiation-induced parameter shifts cannot be calibrated.

The comparison also reveals a persistent trade-off between measurement multiplicity and per-hit precision. Gaseous detectors exploit many measurements and low material, while silicon and SciFi systems provide fewer but more accurate measurements and generally higher rate tolerance. Similarly, heterogeneous systems such as ePIC can assign timing, spatial measurement, and PID to specialized subsystems, whereas multifunctional sensors seek to reduce integration complexity at the possible cost of less optimized performance.

The paper’s synthesis is therefore methodological as much as technological: detector concepts should be optimized against physics-level observables, including impact-parameter resolution, momentum resolution, pile-up rejection, long-lived-particle acceptance, and PID performance. Component-level specifications are necessary but insufficient.

Limitations and open questions

The contribution is a review of requirements and R&D directions rather than a comparative performance study based on a common simulation or benchmark. Many quoted values are design targets, conceptual assumptions, or results from individual prototypes. They should not be interpreted as a uniform demonstration that the corresponding technologies satisfy complete detector requirements.

The discussion also leaves unresolved how the competing technologies should be selected once realistic constraints on cost, manufacturing yield, radiation-induced degradation, services, alignment, and long-term maintenance are included. For $B^0\rightarrow K^{*0}\tau^+\tau^-$1 colliders, the extent to which MAPS can simultaneously achieve sub-$B^0\rightarrow K^{*0}\tau^+\tau^-$2 spatial resolution, low power, low mass, adequate timing, and large-area manufacturability remains an engineering question. For muon colliders, the required background model and MDI configuration remain coupled to the detector timing specification. For future hadron colliders, the absence of a demonstrated sensor satisfying the simultaneous $B^0\rightarrow K^{*0}\tau^+\tau^-$3 ps, fine-granularity, high-rate, and $B^0\rightarrow K^{*0}\tau^+\tau^-$4 requirements is an explicit technology gap, not merely a missing optimization.

A further open question is whether integrated multifunctional sensors can deliver a net system advantage over heterogeneous architectures. Combining gain, spatial measurement, timing, and readout in one device may reduce subsystem count, but it can also increase circuit complexity, power consumption, radiation sensitivity, and calibration burden. The paper identifies this tension but does not resolve it.

Conclusion

The paper establishes a collider-dependent hierarchy of tracking challenges. Future $B^0\rightarrow K^{*0}\tau^+\tau^-$5 experiments are dominated by spatial precision and material minimization, the EIC illustrates the value of complementary detector technologies, muon colliders require tens-of-picoseconds timing to reject decay backgrounds, and future hadron colliders demand unprecedented combinations of timing, granularity, radiation tolerance, and instrumented volume. The principal conclusion is that progress will depend on integrated detector optimization rather than isolated sensor advances. In the most demanding hadron-collider regime, the required sensor technology has not yet been demonstrated; in all regimes, the decisive performance will emerge from the coupled design of the entire tracking system (2608.28089).