CEPC Technical Design Report

The Circular Electron Positron Collider(CEPC)is a large scientific project initiated and hosted by China,fostered through extensive collaboration with international partners.The complex comprises four accelerators:a 30 GeV Linac,a 1.1 GeV Damping Ring,a Booster capable of achieving energies up to 180 GeV,and a Collider operating at varying energy modes(Z,W,H,and tt).The Linac and Damping Ring are situated on the surface,while the subterranean Booster and Collider are housed in a 100 km circumference underground tunnel,strategically accommodating future expansion with provisions for a potential Super Proton Proton Collider(SPPC).The CEPC primarily serves as a Higgs factory.In its baseline design with synchrotron radiation(SR)power of 30 MW per beam,it can achieve a luminosity of 5×10^(34)cm^(-2)s^(-1)per interaction point(IP),resulting in an integrated luminosity of 13 ab^(-1)for two IPs over a decade,producing 2.6 million Higgs bosons.Increasing the SR power to 50 MW per beam expands the CEPC's capability to generate 4.3 million Higgs bosons,facilitating precise measurements of Higgs coupling at sub-percent levels,exceeding the precision expected from the HL-LHC by an order of magnitude.This Technical Design Report(TDR)follows the Preliminary Conceptual Design Report(Pre-CDR,2015)and the Conceptual Design Report(CDR,2018),comprehensively detailing the machine's layout,performance metrics,physical design and analysis,technical systems design,R&D and prototyping efforts,and associated civil engineering aspects.Additionally,it includes a cost estimate and a preliminary construction timeline,establishing a framework for forthcoming engineering design phase and site selection procedures.Construction is anticipated to begin around 2027-2028,pending government approval,with an estimated duration of 8 years.The commencement of experiments and data collection could potentially be initiated in the mid-2030s.

New signature of low mass Z'in J/ψdecays

We explored a new approach to search for a low-mass Z′particle through J/ψdecays by identifying its existence through parity-violating phenomena in the isospin-violating final states ofΛΣ¯^(0)and the corresponding charge conjugated states ofΛΣ¯^(0).Our investigation centered on a generation-independent and leptophobic Z′with mass below 10 GeV.Given the present experimental conditions at Beijing Spectrometer III(BESIII)and the anticipated opportunities at the Super Tau Charm Factory(STCF),we conducted Monte-Carlo simulations to predict possible events at both facilities.Notably,we foresee a substantial enhancement in the precision of the lower limit estimation ofαNP as well as a reduction in statistical uncertainty with upcoming STCF experiments.Furthermore,it is essential to highlight that a null result in the measurement ofαNP would impose stringent constraints,requiring the Z′−q−q couplings to be in the order of 10^(−2).

Time-reversal asymmetries in Λ_(b)→Λ(→pπ^(−))ℓ^(+)ℓ^(−)

We study the decays ofΛb→Λ(→pπ^(−))ℓ^(+)ℓ^(−)withℓ=(e,μ,τ).We examine the full angular distributions with polarizedΛb,where the T-odd observables are identified.We discuss the possible effects of new physics(NP)and find that the T-odd observables are sensitive to them as they vanish in the standard model.Special attention is given to the interference of(pseudo)scalar operators with(axial)vector operators in polarized Λ_(b)→Λ(→pπ^(−))τ^(+)τ^(−),which are studied for the first time.Their effects are proportional to the lepton masses and therefore may evade the constraint from Λ_(b)→Λ(→pπ^(−))μ^(+)μ^(−) at the LHCb naturally.AsΛ_(b)→Λ(→pπ^(−))τ^(+)τ^(−) is uncontaminated by the charmonia resonance,it provides a clean background to probe NP.In addition,we show that the experimental central value of K10 in Λ_(b)→Λ(→pπ^(−))μ^(+)μ^(−) at the LHCb can be explained by the NP case,which couples to the right-handed quarks and leptons.The polarization fraction of Λ_(b) at the LHCb is found to be consistent with zero regardless of the NP scenarios.

The Buchdahl stability bound in Eddington-inspired Born-Infeld gravity

We give the Buchdahl stability bound in Eddington-inspired Born-Infeld(EiBI)gravity.We show that this bound depends on an energy condition controlled by the model parameter K.From this bound,we can constrain κ≤10~8 m^2 if a neutron star with a mass around 3 M is observed in the future.In addition,to avoid the potential pathologies in EiBI,a Hagedorn-like equation of state associated with κ at the center of a compact star is inevitable,which is similar to the Hagedorn temperature in string theory.

Observational constraints on running vacuum model

We investigate the power spectra of the CMB temperature and matter density in the running vacuum model(RVM) with the time-dependent cosmological constant of A=3 vH^2+ Λ_0, where H is the Hubble parameter. In this model, dark energy decreases in time and decays to both matter and radiation. By using the Markov chain Monte Carlo method, we constrain the model parameter v as well as the cosmological observables. Explicitly, we obtain v ≤1.54× 10^(-4)(68% confidence level) in the RVM with the best-fit χ_(RVM)~2 =13968.8, which is slightly smaller thanχ_(ΛCDM)~2= 13969.8 in the ΛCDM model of v = 0.

Running vacuum model in a non-flat universe

We investigate observational constraints on the running vacuum model(RVM)ofΛ=3v(H^2+K/a^2)+c0 in a spatially curved universe,where is the model parameter,K corresponds to the spatial curvature constant,represents the scalar factor,and co is a constant defined by the boundary conditions.We study the CMB power spectra with several sets of and K in the RVM.By fitting the cosmological data,we find that the best fitted x^2 value for RVM is slightly smaller than that of CDM in the non-flat universe,along with the constraints of v≤O(10^-4)(68%C.L.)and|ΩK=-K/(aH)^2|≤O(10^-2)(95%C.L.).In particular,our results favor the open universe in both CDM and RVM.In addition,we show that the cosmological constraints of∑mv=0.256^+0.224-0.234(RVM)and∑mv=0.257^+0.219-0.234(ΛCDM)at 95%C.L.for the neutrino mass sum are relaxed in both models in the spatially curved universe.