Centre d'études et d'expertise sur les risques, l'environnement, la mobilité et l'aménagement

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The discovery of the Higgs boson with its mass around 125 GeV by the ATLAS and CMS Collaborations marked the beginning of a new era in high energy physics. The Higgs boson will be the subject of extensive studies of the ongoing LHC program. At the same time, lepton collider based Higgs factories have been proposed as a possible next step beyond the LHC, with its main goal to precisely measure the properties of the Higgs boson and probe potential new physics associated with the Higgs boson. The Circular Electron Positron Collider (CEPC) is one of such proposed Higgs factories. The CEPC is an e + e − circular collider proposed by and to be hosted in China. Located in a tunnel of approximately 100 km in circumference, it will operate at a center-of-mass energy of 240 GeV as the Higgs factory. In this paper, we present the first estimates on the precision of the Higgs boson property measurements achievable at the CEPC and discuss implications of these measurements.

We study the sensitivity of future medium baseline reactor antineutrino experiments on the neutrino mass hierarchy. By using the standard χ 2 analysis, we find that the sensitivity depends strongly on the baseline length L and the energy resolution (δE/E) 2 = a/ E/MeV 2 + b 2 , where a and b parameterize the statistical and systematic uncertainties, respectively. The optimal length is found to be L ∼ 40 − 55 km, where a slightly shorter L in the range is preferred for poorer energy resolution. The running time needed to determine the mass hierarchy also depends strongly on the energy resolution; for a 5 kton detector (with 12% weight fraction of free proton) placed at L ∼ 50 km away from a 20 GW th reactor, 3σ determination needs 14 years of running with a = 3% and b = 0.5%, which can be reduced to 5 years if a = 2% and b = 0.5%. On the other hand, the experiment can measure the mixing parameters accurately, achieving δ sin 2 2θ 12 ∼ 4 × 10 −3 , δ(m 2 2 − m 2 1 ) ∼ 0.03 × 10 −5 eV 2 , and δ|m 2 3 − m 2 1 | ∼ 0.007 × 10 −3 eV 2 , in 5 years, almost independently of the energy resolution for a < 3% and b < 1%. In order to compare our simple (∆χ 2 ) min results with those obtained by simulating many experiments, we develop an efficient method to estimate the uncertainty of (∆χ 2 ) min , and the probability for determining the right mass hierarchy by an experiment is presented as a function of the mean (∆χ 2 )

Combining ν oscillations at T2K or T2HK with ν oscillations from µ + decay at rest (DAR) allows a determination of the leptonic CP-violating phase δ. The degeneracies of this phase with θ 13 and θ 23 are broken and δ can be reliably distinguished from 180 • − δ. We present the sensitivity to δ of T2(H)K together with a µ + DAR experiment using Super-K as a near detector and Hyper-K at the Tochibora site as a far detector.

Neutrino oscillation data strongly support µ−τ symmetry as a good approximate flavor symmetry of the neutrino sector, which has to appear in any viable theory for neutrino mass-generation. The µ−τ breaking is not only small, but also the source of Dirac CP-violation. We conjecture that both discrete µ−τ and CP symmetries are fundamental symmetries of the seesaw Lagrangian (respected by interaction terms), and they are only softly broken, arising from a common origin via a unique dimension-3 Majorana mass-term of the heavy right-handed neutrinos. From this conceptually attractive and simple construction, we can predict the soft µ−τ breaking at low energies, leading to quantitative correlations between the apparently two small deviations θ 23 − 45 • and θ 13 − 0 • . This nontrivially connects the on-going measurements of mixing angle θ 23 with the upcoming experimental probes of θ 13 . We find that any deviation of θ 23 − 45 • must put a lower limit on θ 13 . Furthermore, we deduce the low energy Dirac and Majorana CP violations from a common soft-breaking phase associated with µ−τ breaking in the neutrino seesaw. Finally, from the soft CP breaking in neutrino seesaw we derive the cosmological CP violation for the baryon asymmetry via leptogenesis. We fully reconstruct the leptogenesis CP-asymmetry from the low energy Dirac CP phase and establish a direct link between the cosmological CP-violation and the low energy Jarlskog invariant. We predict new lower and upper bounds on the θ 13 mixing angle,• . In addition, we reveal a new hidden symmetry that dictates the solar mixing angle θ 12 by its group-parameter, and includes the conventional tri-bimaximal mixing as a special case, allowing deviations from it.