Circular Electron Positron Collider

1. Circular Electron Positron Collider Enze Zhang 2019/3/27

2. Outline • Overviewofthecollider • SuperconductingRFcavities • Vertexdetector • PFAcalorimetrysystem

3. Motivation • “Higgs factory” After the discovery of the Higgs particle, it is natural to measure its properties as precise as possible, including mass, spin, CP nature, couplings, and etc. • Why electron positron collider? Cleaner events. • CircularcolliderHigherluminosityandmorethan1interactionpoint • Future upgradetoSuper Proton Proton Collider (SPPC)

4. Layout • The CEPC accelerator complex consists of a linear accelerator (Linac), a damping ring (DR), the Booster, the Collider and several transport lines.  • CEPC is a double ring collider with two interaction points (IPs). The CEPC collider ring, the CEPC booster ring and the future SPPC collider ring are all housed in the same 100 km circumference tunnel. 

5. Layout

6. Performance goal and running plan • Three different modes of operation

7. Superconducting RF cavities • Superconducting radio frequency (SRF) science and technology involves the application of electrical superconductors to radio frequency devices. • The ultra-low electrical resistivity of a superconducting material allows an RF resonator to obtain an extremely high quality factor Q. Such a very high Q resonator stores energy with very low loss and narrow bandwidth. • The most common application of superconducting RF is in particle accelerators.

8. How SRF cavities work • Accelerators typically use resonant RF cavities formed from or coated with superconducting materials. Electromagnetic fields are excited in the cavity by coupling in an RF source with an antenna. When the RF fed by the antenna is the same as that of a cavity mode, the resonant fields build to high amplitudes. • Charged particles passing through apertures in the cavity are then accelerated by the electric fields and deflected by the magnetic fields.

9. How SRF cavities work • Cavity immersed in a saturated liquid helium bath. • Pumping removes helium vapor boil-off and controls the bath temperature. The helium vessel is often pumped to a pressure below helium's superfluid lambda point to take advantage of the superfluid's thermal properties. High thermal conductivity makes superfluid an excellent coolant. • Superfluidsboil only at free surfaces, preventing the formation of bubbles on the surface of the cavity, which would cause mechanical perturbations. • The cold portions of the setup need to be extremely well insulated, which is best accomplished by a vacuum vessel surrounding the helium vessel and all ancillary cold components.

10. Advantages • The motivation for using superconductors in RF cavities is not to achieve a net power savings, but rather to increase the quality of the particle beam being accelerated. • High duty cycle or continues wave (cw) operation. SRF cavities allow the excitation of high electromagnetic fields at high duty cycle, or even cw, in such regimes that a copper cavity's electrical loss could melt the copper, even with robust water cooling.

11. Advantages • Low beam impedance. The low electrical loss in an SRF cavity allows their geometry to have large beampipe apertures while still maintaining a high accelerating field along the beam axis. Normal-conducting cavities need small beam apertures to concentrate the electric field as compensation for power losses in wall currents. • Nearly all RF power goes to the beam. The RF power dissipated in the SRF cavity walls is negligible, in contrast to normal-conducting cavities where the wall power loss can easily equal or exceed the beam power consumption.

12. CEPC RF parameters • P95

13. Cavities • The 650 MHz 2-cell cavity is made of bulk niobium and operates at 2 K with > at 22 MV/m for the vertical acceptance test, > at 20 MV/m for the horizontal test. The normal operation gradient is below 20 MV/m, and the lower limit of is for long term operation. 

14. CEPC tracking system

15. Vertex detector • The identification of heavy-flavor (b- and c-) quarks and tau leptons requires precise determination of the track parameters of charged particles in the vicinity of the Interaction Point (IP), permitting reconstruction of the displaced decay vertices of short-lived particles. • This drives the need for a vertex detector with low material budget and high spatial resolution.

16. Performance requirements • Impact parameter resolution in plane: • The first term describes the intrinsic resolution of the vertex detector in the absence of multiple scattering and is independent of the track parameters, while the second term reflects the effects of multiple scattering. • The parameters a = 5 μm and b = 10 μm · GeV are taken as the design values for the CEPC vertex detector.

17. Performance requirements The main physics performance goals can be achieved with a three concentric cylinders of double- layer pixelated vertex detector with the following characteristics:  • Single-point resolution of the first layer better than 3μm • Material budget below 0.15% per layer  • First layer located close to the beampipe at a radius of 16mm, with a material budget of 0.15% for the beampipe;  • Detector occupancy not exceeding 1%.  baseline design of CEPC vertex detector.

18. Vertex detector

19. PFA calorimetry system • Requirements that final states from decays of W and Z and Higgs boson be separately identified and reconstructed with high sensitivity, in particular, to clearly discriminate the and final states. • Therefore, a 3–4% invariant mass resolution for two-jet systems is required, which needs a jet energy resolution of  at energies below 100GeV. • This would be about a factor of two better than that of the LEP detectors and the currently operating calorimeters at the LHC.

21. PFA calorimetry system • Two different technology approaches are pursued for the CEPC calorimetry system. • Measure individual particles in a jet using a calorimetry system with very high granularity based on the particle flow concept • Ahomogeneous and integrated solution based on the dual-readout concept. • The first approach is the current baseline for the design of the CEPC calorimetry system in that it is integrated in the full CEPC detector simulation. 

22. Basic idea of PFA • The basic idea of the PFA is to make use of the optimal detector subsystem to determine the energy/momentum of each particle in a jet. An essential prerequisite for realization of this idea is to distinguish among energy deposits of individual particles from a jet in the calorimetry system. • PFA calorimeters feature finely segmented, three-dimensional granularity and compact, spatially separated, particle showers to facilitate the reconstruction and identification of every single particle shower in a jet. • A PFA calorimetry system generally consists of an electromagnetic calorimeter (ECAL), optimized for measurements of photons and electrons, and a hadronic calorimeter (HCAL) to measure hadronic showers. 

24. Extensive studies have been carried out within the CALICE collaboration and in the world-wide detector R&D efforts for the ILC to develop compact PFA calorimeters. Various detector technology options have been explored to address challenges from stringent performance requirements as shown in Figure 5.2. 

25. CEPC PFA calorimeter design • ECAL and HCAL are installed inside the bore of the solenoid to minimize the amount of inactive material in front of the calorimetry system and to enable the reliable association of tracks with energy deposits. • Both ECAL and HCAL are organized into one cylindrical barrel and two disk-like endcap sections 

26. CEPC ECAL baseline design

27. Reference • IHEP-CEPC-DR-2018-01 • IHEP-CEPC-DR-2018-02 • http://cepc.ihep.ac.cn/ • https://en.wikipedia.org/wiki/Superconducting_radio_frequency • https://indico.cern.ch/event/102998/contributions/17016/attachments/10442/15292/imaging_calorimeter_xia.pdf