# Circular Electron Positron Collider

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The Circular Electron Positron Collider is an electron positron collider first proposed by the Chinese high-energy physics community in 2012. This machine could later be upgraded to a high-energy proton-proton collider, with potential far beyond the current production of the Higgs boson.[1][2] The low Higgs mass of ~125 GeV makes possible a Circular Electron Positron Collider (CEPC) as a Higgs Factory, which has the advantage of higher luminosity to cost ratio and the potential to be upgraded to a proton-proton collider to reach unprecedented high energy and discover new physics. The underground particle-smashing ring aims to be at least twice the size of the globe's current leading collider - the Large Hadron Collider (CERN) outside Geneva. With a circumference of 100 kilometres (60 miles), the Chinese accelerator complex would encircle the entire island of Manhattan.[3]

## Machine parameters

### Accelerator Parameters

The following parameters reflect the "baseline" configuration of a collider with 53.6 km circumference and 240 GeV center-of-mass energy, and the luminosities are 3×1034, 32×1034 and 10 ×1034 cm-2s-1, respectively. The primary physics goal is to use the CEPC as a Higgs factory. Therefore, a tentative “7-2-1” operation plan is to run first as a Higgs factory for 7 years and create one million Higgs particles or more, followed by 2 years of operation as a Super Z factory to create one trillion Z bosons and then 1 year as a W factory to create about 100 million W bosons. The vast amount of bottom quarks, charm quarks and τ-leptons produced in the decays of the Z bosons also makes the CEPC an effective B-factory and τ-charm factory. [4] Since the project is in the early stages of planning, they are subject to change.

Property Unit Value
Beam energy (E) GeV 120
Circumference (C) km 100[5]
Luminosity (L) cm−2s−1 1.80·1034
SR power/beam (P) MW 50
Bending radius (ρ) m 10,700 [6]
NIP 2
nB 50
Filling factor (κ) 0.71
Lorentz factor (γ) 234834.66
Revolution period (T0) s 1.79·10−4
Revolution frequency (f0) Hz 5591.66
Magnetic rigidity (Bp) T·m 400.27
Momentum compaction factor (αp) 4.15·10−5
Energy acceptance Ring (η) 0.02
Cross-section for radiative Bhabha scattering (σee) cm2 1.53·10−25
Lifetime due to radiative Bhabha scattering (τL) min 56.03
Build-up time of polarisation (τp) min 21

## Physics program

The CEPC enables a wide physics program. As an electron-positron collider, it is suited to precision measurements, but also has strong discovery potential for new physics. Some possible physics goals include:

• Higgs measurements: Running slightly above the production threshold for ZH, the CEPC is a Higgs factory. Over the course of a ten-year run, it is planned to collect 5 ab−1 with two detectors, which corresponds to approximately one million produced Higgs Bosons.[7] One target is to be able to measure the ZH production cross-section ${\displaystyle \sigma (ZH)}$ to 0.5% accuracy. Other goals include the measurement of the Higgs Boson self coupling, and its coupling to other particles.
• When running at the Z peak, a precision measurement of the Z Boson mass and other properties, e.g. the Zbb̅ coupling, can be made.[8]
• Physics beyond the Standard Model:[9] Despite the lower center-of-mass energy compared to the LHC, the CEPC will be able to make discoveries or exclusions in certain scenarios where the LHC cannot. A prominent situation is when there is supersymmetry, but the masses of the superpartners are very close to each other (near-degenerate). In this case, when one SUSY particle decays into another plus a Standard Model particle, the SM particle will likely escape detection in a Hadron collider. In an e+e- collider, since the initial state is completely known, it is possible to detect such events by their missing energy (the energy carried away by SUSY particles and neutrinos).

## Possible timeline

• Pre-study, R&D and preparation work
• Pre-study: 2013-15
• Pre-CDR (Conceptual Design Report) by the end of 2014 for R&D funding request
• R&D: 2016-2020
• Engineering Design: 2015-2020
• Construction: 2021-2029
• Data taking and experiments: 2030-2040
• Installation of superconducting magnets upgrades for the SPPC project: 2040 and beyond. [10]

## Possible Construction Locations

CEPC is conducting country wide site visits and study. Local government agencies are very receptive and supportive to CEPC. CDR study is based on site 1 (Qing Huang Dao). The site selection is still in processing, and CEPC team keep looking for more site candidates in China.[11] Below are a list of possible locations:

• Qinhuangdao, Hebei Province.
• Huangling, Shanxi Province.
• Shenshan, Guangdong Province.
• Baoding, Hebei Province.
• Huzhou, Zhejiang Province.
• Chuangchun, Jilin Province.
• Changsha, Hunan Province.

## References

1. ^ http://cepc.ihep.ac.cn/ Circular Electron Positron Collider - CEPC
2. ^ http://www.aljazeera.com/indepth/features/2014/09/china-unveils-world-largest-supercollider-science-physi-2014919131524321817.html China plans world's largest supercollider
3. ^ http://cepc.ihep.ac.cn/intro.html
4. ^ http://cepc.ihep.ac.cn/intro.html The CEPC Project
5. ^ http://cepc.ihep.ac.cn/intro.html
6. ^ http://cepc.ihep.ac.cn/intro.html
7. ^ LOU, Xinchou. "Overview of the CEPC project" (PDF). Workshop on Physics at the CEPC, August 10–12, 2015
8. ^ Gu, Jiayin. "Probing Zbb̅ couplings at the CEPC" (PDF). Workshop on Physics at the CEPC, August 10–12, 2015
9. ^ Craig, Nathaniel. "Naturalness and Higgs Measurements" (PDF). Workshop on Physics at the CEPC, August 10–12, 2015
10. ^ http://cepc.ihep.ac.cn/intro.html
11. ^ http://cepc.ihep.ac.cn/intro.html