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Despite Chen-Ning Yang's objections, the Circular Electron Positron Collider (CEPC) project is progressing. The CEPC research team officially released its Technical Design Report (TDR) for the accelerator complex on December 25, as detailed in the CEPC Technical Design Report, with the first operation anticipated around 2035.

Isn't circular acceleration bad for electrons due to the synchrotron radiation? Why don't they build a linear collider instead? Is it because they are also planning to use it as a hadron collider in the future?

Furthermore, the CEPC's scientific objective of achieving a maximum of 240 GeV center-of-mass energy seems modest compared to the Compact Linear Collider (CLIC)'s ambitious 3 TeV goal. What justifies the continued promotion of the CEPC project despite this apparent discrepancy in scientific potential?

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  • $\begingroup$ because it wants to study the Higgs boson creation and decays? en.wikipedia.org/wiki/Circular_Electron_Positron_Collider $\endgroup$
    – anna v
    Jan 12 at 16:00
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    $\begingroup$ I suggest not using words like "stupid" in a question, let alone a question title. $\endgroup$
    – Javier
    Jan 12 at 16:07
  • $\begingroup$ @JavierJ Thanks for reminding $\endgroup$
    – Bababeluma
    Jan 12 at 16:21
  • $\begingroup$ You might find these videos helpful - Videos by Don Lincoln. See numbers 48-50, 52-53, and 55 $\endgroup$
    – mmesser314
    Jan 12 at 20:08
  • $\begingroup$ @mmesser314 Thanks for sharing, great videos! $\endgroup$
    – Bababeluma
    Jan 12 at 20:24

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This answer may be a glorified comment, but it looks like the CEPC is designed to study the Higgs. I do not know what the background looks like for $e^+e^-$, but it has to be much cleaner than LHC, where the vast majority of events in the "Higgs bump" are background. Meanwhile, going to 3 TeV may find new physics, most likely SUSY, though I do not know the nuances of where it's supposed to kick in...1 TeV iirc?

We're all aware of the synchrotron losses ($P\propto 1/m^4$) plaguing circular storage rings, but they do provide a great advantage: storage.

I'm not familiar with the CLIC's numbers, but (for example), the LHC stores protons with 300 MJ of energy (iirc, pls comment if it's wrong) which would need to be replenished every $27\,{\rm km}/c \approx 100\,\mu{\rm s}$ to achieve the same luminosity were it a LINAC... which is 300 GW. That's a lot of power.

Since luminosity is a combination of beam current and (inverse) beam cross sectional area, I presume the latter is going to be really small by 2035.

The boost in cross section by sitting on a resonance is also significant, but I don't have numbers. In the LHC, you just have to wait for two rando-partons to have the right energy (not necessarily in the lab frame), to make a Higgs...while a electron-positron machine can be tuned to make them "at-rest" in the lab frame...it's just so much cleaner.

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  • $\begingroup$ Thanks. I wasn't aware of the importance of luminosity. Seems like CEPC is aiming at precise measurement with the help of high luminosity while CLIC aims to find new physics? For new physics, isn't LHC already at 13TeV? Maybe it's CLIC which is in an awkward place. $\endgroup$
    – Bababeluma
    Jan 12 at 16:39
  • $\begingroup$ @Bababeluma "The physics aims of CLIC include high-precision measurements of the Higgs boson’s interactions with other particles and with itself. Unlike protons, electrons and positrons are truly point-like elementary particles. Therefore, compared to proton–proton collisions at the LHC, electron–positron collisions at CLIC could provide complementary and more accurate information about the Higgs boson." home.cern/science/accelerators/compact-linear-collider $\endgroup$
    – anna v
    Jan 12 at 16:54
  • $\begingroup$ @Bababeluma The bare basic approximation of $pp$ collision is two bags of partons: each with 3 valence quark carrying a large fraction of the proton momentum, and then a sea-quarks carrying the remaining-up-to 50%, and then gluons carrying the remaining 50%, so getting the full beam energy in a collision has almost zero probability. Most collisions are garbage. With $e^+e^-$ you know the exact energy, and you can tune to resonance (See: Breit-Wigner) and crank out real events. Having done 10 years of DIS, followed by much more space borne remote sensing, I think of $pp$ collision as... $\endgroup$
    – JEB
    Jan 13 at 3:48
  • $\begingroup$ ...remote sensing the SM without knowing what frequency you are transmitting. It makes the analysis much more difficult. It requires models based on decades nucleon parton distribution data, while $e^+e^-$ is a monochromatic probe. $\endgroup$
    – JEB
    Jan 13 at 3:51

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