Possible futures of electroweak precision: ILC, FCC-ee, and CEPC

Journal of High Energy Physics, Sep 2015

The future of high-precision electroweak physics lies in e + e − collider measurements of properties of the Z boson, the W boson, the Higgs boson, and the top quark. We estimate the expected performance of three possible future colliders: the ILC, FCC-ee (formerly known as TLEP), and CEPC. In particular, we present the first estimates of the possible reach of CEPC, China’s proposed Circular Electron-Positron Collider, for the oblique parameters S and T and for seven-parameter fits of Higgs couplings. These results allow the physics potential for CEPC to be compared with that of the ILC and FCC-ee. We also show how the constraints on S and T would evolve as the uncertainties on each of the most important input measurements change separately. This clarifies the basic physics goals for future colliders. To improve on the current precision, the highest priorities are improving the uncertainties on m W and sin2 θ eff . At the same time, improved measurements of the top mass, the Z mass, the running of α, and the Z width will offer further improvement which will determine the ultimate reach. Each of the possible future colliders we consider has strong prospects for probing TeV-scale electroweak physics.

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Possible futures of electroweak precision: ILC, FCC-ee, and CEPC

Received: June Possible futures of electroweak precision: ILC, FCC-ee, and CEPC JiJi Fan 0 1 2 5 Matthew Reece 0 1 2 3 Lian-Tao Wang 0 1 2 4 Open Access 0 1 2 c The Authors. 0 1 2 0 5640 S Ellis Ave , Chicago, IL 60637 , U.S.A 1 17 Oxford Street, Cambridge, MA 02138 , U.S.A 2 900 South Crouse Ave , Syracuse, NY 13210 , U.S.A 3 Department of Physics, Harvard University 4 Enrico Fermi Institute and Kavli Institute for Cosmological Physics, University of Chicago 5 Department of Physics, Syracuse University The future of high-precision electroweak physics lies in e+e− collider measurements of properties of the Z boson, the W boson, the Higgs boson, and the top quark. We estimate the expected performance of three possible future colliders: the ILC, FCC-ee (formerly known as TLEP), and CEPC. In particular, we present the first estimates of the possible reach of CEPC, China's proposed Circular Electron-Positron Collider, for the oblique parameters S and T and for seven-parameter fits of Higgs couplings. These results allow the physics potential for CEPC to be compared with that of the ILC and FCC-ee. We also show how the constraints on S and T would evolve as the uncertainties on each of the most important input measurements change separately. This clarifies the basic physics goals for future colliders. To improve on the current precision, the highest priorities are improving the uncertainties on mW and sin2 θeff . At the same time, improved measurements of the top mass, the Z mass, the running of α, and the Z width will offer further improvement which will determine the ultimate reach. Each of the possible future colliders we consider has strong prospects for probing TeV-scale electroweak physics. Higgs Physics; Beyond Standard Model; LEP HERA and SLC Physics 1 Introduction 4.1.1 4.1.2 4.1.3 Global fit of electroweak observables with oblique corrections Prospects for electroweak precision at the ILC and FCC-ee Prospects for CEPC electroweak precision Hypothetical improvements of CEPC EWPT Details of electroweak fit The top mass mt The Z and Higgs masses Non-nuisance parameters Experimental uncertainties Parametric uncertainties To do list for a successful electroweak program Higgs measurements at CEPC New physics reach and complementarity A Treatment of theory uncertainties The discovery of the Higgs boson has ushered in a new era of electroweak physics. The Standard Model has proved to be essentially correct, at least as a low-energy effective field theory, in its description of electroweak symmetry breaking as due to a light, weakly coupled scalar boson. However, the physics giving rise to the Higgs potential remains completely unclear. If there is a small amount of fine-tuning in the Higgs sector, we expect new physics at nearby scales. Perhaps the Higgs is composite (e.g. a pseudo-Nambu Goldstone boson), or perhaps supersymmetry cuts off the quadratic divergence in the Higgs mass. Although the Large Hadron Collider may yet discover new particles that offer clues to these possibilities, precision measurements of electroweak physics including the Higgs boson’s properties may also offer powerful probes of electroweak symmetry breaking. Several compelling possibilities for the next step forward in high-precision electroweak physics exist: the International Linear Collider [1], which may be built in Japan; FCC-ee, a future circular collider formerly known as TLEP [2]; and the CEPC, a new proposal for an electron-positron collider in China (see http://cepc.ihep.ac.cn). Our goal in this paper is to assess the physics potential of these different colliders, including a first look at CEPC’s potential accuracy in measurements of Higgs boson couplings and in fits of the oblique parameters S and T [3, 4] (see also [5–7]). These correspond, in an effective operator language (reviewed in ref. [8, 9]), to adding to the Lagrangian the following dimension-six operators from the minimal basis of operators [10]: Loblique = S troweak symmetry breaking — for instance, the left-handed doublet of stops and sbottoms in a supersymmetric theory — will produce a contribution to S. The masses must additionally be split by custodial symmetry-violating effects to contribute to T . For example, in the case of the stop and sbottom sector we have both, and T is numerically dominant [11]. In this paper we estimate the size of the region in the (S, T ) plane that will be allowed after several suites of high-precision measurements: a “GigaZ” program at the ILC, a “TeraZ” program at FCC-ee, extended runs of FCC-ee combining Z pole data with data present a self-contained discussion of many of the relative advantages and disadvantages of the different machines; for example, the Z mass measurement will be improved only at circular colliders, which can follow LEP in exploiting resonant spin depolarization. We also emphasize the basic physics of the fits and their potential bottlenecks, specifying the goals of the electroweak program (...truncated)


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JiJi Fan, Matthew Reece, Lian-Tao Wang. Possible futures of electroweak precision: ILC, FCC-ee, and CEPC, Journal of High Energy Physics, 2015, pp. 196, Volume 2015, Issue 9, DOI: 10.1007/JHEP09(2015)196