Martin Breidenbach’s research while affiliated with SLAC National Accelerator Laboratory, Stanford University and other places

Higgs physics goals with detectors at future colliders demand unprecedented precision. Linear colliders, with energy reach to the TeV scale and low duty cycles and backgrounds, enable this high precision performance. The SiD Collaboration is developing Monolithic Active Pixel Sensor (MAPS) technology for tracking and electromagnetic calorimetry (ECal). This technology offers high granularity, thin sensors, good time resolution (<nsec), and small dead areas, enabled by gaseous cooling for tracking and passive thermal conduction for calorimetry. The first MAPS prototype (NAPA-p1), designed by SLAC in CMOS imaging 65 nm technology, is under test. The long-term objective is a wafer-scale sensor of area 5 × 20 cm². Detailed simulation of ECal performance confirms previous results, indicating electromagnetic energy resolution based on digital hit cluster counting provides better performance than the 13 mm² pixels SiD TDR analog design, and two particle separation in the ECal is excellent down to the millimeter scale. Recent heat management analysis indicates passive cooling for the low duty cycle linear colliders should work.

For C³, some 2000 accelerator modules must be 5D positioned, within 10 µm transversal, on a 2.3 km long straight line, for both linacs. In the Rasnik alignment system, light from a point-like monochromatic source falls on a zone lens, forming a Fraunhofer diffraction pattern on an image pixel sensor. The alignment of three objects can be obtained by analyzing the position of the diffraction pattern on the sensor. The alignment of a large number of objects can be realized by fixing a stick on each object, carrying all three Rasnik components. With this leap frog geometry, all sticks are mutually coupled, forming a multipoint alignment system. The system should operate in ambient air, in vacuum, and in liquid nitrogen. Due to the heat dissipation of these components, bubbles are formed, causing an error in the measured alignment when crossing the optical path. Various methods of beam shielding are presented. With the Quarter Cryo Module (QCM), essential studies will be carried out, enabling the realization of C³. The QCM will be equipped with four Rasnik chains, measuring alignment parameters with redundancy. In addition, the bubble-induced vibrations of the accelerator components can be registered accurately.

The particle physics community has agreed that an electron-positron collider is the next step for continued progress in this field, giving a unique opportunity for a detailed study of the Higgs boson. Several proposals are currently under evaluation by the international community. Any large particle accelerator will be an energy consumer and so, today, we must be concerned about its impact on the environment. This paper evaluates the carbon impact of the construction and operation of one of these Higgs factory proposals, the Cool Copper Collider. It introduces several strategies to lower the carbon impact of the accelerator. It proposes a metric to compare the carbon costs of Higgs factories, balancing physics reach, energy needs, and carbon footprint for both construction and operation, and compares the various Higgs factory proposals within this framework. For the Cool Copper Collider, the compact 8 km footprint and the possibility for cut-and-cover construction greatly reduce the dominant contribution from embodied carbon.

C ³ is an opportunity to realize an e ⁺ e ⁻ collider for the study of the Higgs boson at √ s = 250 GeV, with a well defined upgrade path to 550 GeV while staying on the same short facility footprint [2,3]. C ³ is based on a fundamentally new approach to normal conducting linear accelerators that achieves both high gradient and high efficiency at relatively low cost. Given the advanced state of linear collider designs, the key system that requires technical maturation for C ³ is the main linac. This paper presents the staged approach towards a facility to demonstrate C ³ technology with both Direct (source and main linac) and Parallel (beam delivery, damping ring, ancillary component) R&D. The primary goal of the C ³ Demonstration R&D Plan is to reduce technical and cost risk by building and operating the key components of C ³ at an adequate scale. This R&D plan starts with the engineering design, and demonstration of one cryomodule and will culminate in the construction of a 3 cryomodule linac with pre-production prototypes. This R&D program would also demonstrate the linac rf fundamentals including achievable gradient and gradient stability over a full electron bunch train and breakdown rates. It will also investigate beam dynamics including energy spread, wakefields, and emittance growth. This work will be critical to confirm the suitability of the C ³ beam parameters for the physics reach and detector performance in preparation for a Conceptual Design Report (CDR), as well as for follow-on technology development and industrialization. The C ³ Demonstration R&D Plan will open up significant new scientific and technical opportunities based on development of high-gradient and high-efficiency accelerator technology. It will push this technology to operate both at the GeV scale and mature the technology to be reliable and provide high-brightness electron beams. The timeline for progressing with C ³ technology development will be governed by practical limitations on both the technical progress and resource availability. It consists of four stages: Stage 0) Ongoing fundamental R&D on structure prototypes, damping and vibrations. Stage 1) Advancing the engineering maturity of the design and developing start-to-end simulations including space-charge and wakefield effects. This stage will include testing of strucutres operating at cryogenic temperatures. Beam tests would be performed with high beam current to test full beam loading. Stage 2) Production and testing of the first cryomodule at cryogenic temperatures. This would provide sufficient experimental data to compile a CDR and it is anticipated for Stage 2 to last 3 years and to culminate with the transport of photo-electrons through the first cryomodule. Stage 3) Updates to the engineering design of the cryomodules, production of the second and third cryomodule and their installation. Lower charge and lower emittance beams will be used to investigate emittance growth. The successful full demonstration of the 3 cryomodules to deliver up to a 3 GeV beam and achieve the C ³ five gradient will allow a comprehensive and robust evaluation of the technical design of C ³ as well as mitigate technical, schedule, and cost risks required to proceed with a Technical Design Report (TDR).

Construction of an e ⁺ e ⁻ Higgs factory has been identified as a major goal for particle physics. Such a collider will offer precise measurements of the Higgs bosons couplings to other particles. A Higgs factory extendable in energy can also establish the Higgs self-coupling, measure the Higgs coupling to the top quark, and expand the reach to probe new phenomena. We propose a strategy for an energy-extendable Higgs factory based on a new linear accelerator technology. This strategy offers a compact and cost-effective design that could be realized as an accelerator project in the US. The core technologies to be developed have broad applications to accelerators for medicine and for X-ray science. The challenge of realizing these technologies will offer unique and exciting opportunities to young scientists.

The Rasnik 3-point alignment system, now widely applied in particle physics experiments and in the instrumentation of gravitational wave experiments, can be used as N-point alignment system by daisy chain N individual 3-point systems. The conceptual implementation of Rasnik chains in C3 is presented. The proper operation of a laser diode and a CMOS image sensor in liquid nitrogen has been verified. Next plans for testing a small but complete system, immersed in liquid nitrogen, are presented.

The particle physics community has agreed that an electron-positron collider is the next step for continued progress in this field, giving a unique opportunity for a detailed study of the Higgs boson. Several proposals are current under evaluation of the international community. Any large particle accelerator will be an energy consumer and so, today, we must be concerned about its impact on the environment. This paper evaluates the carbon impact of the construction and operations of one of these Higgs factory proposals, the Cool Copper Collider (C$^{3}$). It introduces several strategies to lower the carbon impact of the accelerator. It proposes a metric to compare the carbon costs of Higgs factories, balancing physics reach, energy needs, and carbon footprint for both construction and operations, and compares the various Higgs factory proposals within this framework. For C$^{3}$, the compact 8 km footprint and the possibility for cut-and-cover construction greatly reduce the dominant contribution from embodied carbon.

The SiD detector concept capitalizes on high granularity in its tracker and calorimeter to achieve the momentum resolution and particle flow calorimetry physics goals in a compact design. The collaboration has had a long interest in the potential for improved granularity in both the tracker and ECal with an application of monolithic active pixel sensors (MAPS) and a study of MAPS in the SiD ECal was described in the ILC TDR. Work is progressing on the MAPS application in an upgraded SiD design with a prototyping design effort for a common SiD tracker/ECal design based on stitched reticules to achieve 10 × 10 cm2 sensors with 25 × 100 micron2 pixels. Application of large area MAPS in these systems would limit delicate and expensive bump-bonding, provide possibilities for better timing, and should be significantly cheaper than the TDR concept due to being a more conventional CMOS foundry process. The small pixels significantly improve shower separation. Recent simulation studies confirm previous performance projections, indicating electromagnetic energy resolution based on digital hit cluster counting provides better performance than the SiD TDR analog design based on 13 mm2 pixels. Furthermore, the two shower separation is excellent down to the millimeter scale. Geant4 simulation results demonstrate these expectations.

C$^3$ is an opportunity to realize an e$^+$e$^-$ collider for the study of the Higgs boson at $\sqrt{s} = 250$ GeV, with a well defined upgrade path to 550 GeV while staying on the same short facility footprint. C$^3$ is based on a fundamentally new approach to normal conducting linear accelerators that achieves both high gradient and high efficiency at relatively low cost. Given the advanced state of linear collider designs, the key system that requires technical maturation for C$^3$ is the main linac. This white paper presents the staged approach towards a facility to demonstrate C$^3$ technology with both Direct (source and main linac) and Parallel (beam delivery, damping ring, ancillary component) R&D. The white paper also includes discussion on the approach for technology industrialization, related HEP R&D activities that are enabled by C$^3$ R&D, infrastructure requirements and siting options.