# Circular Electron Positron Collider

The Circular Electron Positron Collider is a proposed electron positron collider by the Chinese high energy physics community in 2012. This machine could be later upgraded to a high energy proton-proton collider with physics potential far beyond the Higgs factory.[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 80 kilometres (49.70 miles), the Chinese accelerator complex would encircle the entire island of Manhattan.

## Machine parameters

### Accelerator Parameters

The following parameters reflect the "baseline" configuration of a collider with 53.6 km circumference and 2×120 GeV center-of-mass energy.[3] Since the project is in the early stages of planning, they are subject to change.

Property Unit Value Property Unit Value
Beam energy (E) GeV 120 Lorentz factor (γ) 234834.66
Circumference (C) km 53.6 Revolution period (T0) s 1.79·10−4
Luminosity (L) cm−2s−1 1.80·1034 Revolution frequency (f0) Hz 5591.66
SR power/beam (P) MW 50 Magnetic rigidity (Bp) T·m 400.27
Bending radius (ρ) m 6094 Momentum compaction factor (αp) 4.15·10−5
NIP 2 Energy acceptance Ring (η) 0.02
nB 50 Cross-section for radiative Bhabha scattering (σee) cm2 1.53·10−25
Filling factor (κ) 0.71 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 5000 fb−1 with two detectors, which corresponds to approximately one million produced Higgs Bosons.[4] 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.[5]
• Physics beyond the Standard Model:[6] 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 a 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: 2021-2027
• Data taking: 2028-2035

## 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 The CEPC Project
4. ^ LOU, Xinchou. "Overview of the CEPC project" (PDF). Workshop on Physics at the CEPC, August 10–12, 2015
5. ^ Gu, Jiayin. "Probing Zbb̅ couplings at the CEPC" (PDF). Workshop on Physics at the CEPC, August 10–12, 2015
6. ^ Craig, Nathaniel. "Naturalness and Higgs Measurements" (PDF). Workshop on Physics at the CEPC, August 10–12, 2015