Abstract

Abstract The discovery by the ATLAS and CMS experiments of a new boson with mass around 125 GeV and with measured properties compatible with those of a Standard-Model Higgs boson, coupled with the absence of discoveries of phenomena beyond the Standard Model at the TeV scale, has triggered interest in ideas for future Higgs factories. A new circular e+e− collider hosted in a 80 to 100 km tunnel, TLEP, is among the most attractive solutions proposed so far. It has a clean experimental environment, produces high luminosity for top-quark, Higgs boson, W and Z studies, accommodates multiple detectors, and can reach energies up to the $$ \mathrm{t}\overline{\mathrm{t}} $$ threshold and beyond. It will enable measurements of the Higgs boson properties and of Electroweak Symmetry-Breaking (EWSB) parameters with unequalled precision, offering exploration of physics beyond the Standard Model in the multi-TeV range. Moreover, being the natural precursor of the VHE-LHC, a 100 TeV hadron machine in the same tunnel, it builds up a long-term vision for particle physics. Altogether, the combination of TLEP and the VHE-LHC offers, for a great cost effectiveness, the best precision and the best search reach of all options presently on the market. This paper presents a first appraisal of the salient features of the TLEP physics potential, to serve as a baseline for a more extensive design study.

Highlights

  • The Higgs boson with mass around 125 GeV recently discovered by the ATLAS and CMS experiments [1, 2] at the LHC is found to have properties compatible with the Standard Model predictions [3,4,5], as shown for example in figure 1 [6]

  • Movable spin rotators as designed for HERA [30] would allow a program of longitudinally polarized beams at the Z peak. (The spin rotator design foreseen for LEP requires tilting the experiments and is unpractical for TLEP.) For the same level of polarization in collisions as that observed at LEP, and assuming that a fraction of the bunches can be selectively depolarized, a simultaneous measurement [31] of the beam polarization and of the left-right asymmetry ALR can be envisioned at TLEP

  • The LHC run at 13-14 TeV may well discover something else, and it would be premature to mortgage the future of high-energy physics before knowing what it reveals

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Summary

Introduction

The Higgs boson with mass around 125 GeV recently discovered by the ATLAS and CMS experiments [1, 2] at the LHC is found to have properties compatible with the Standard Model predictions [3,4,5], as shown for example in figure 1 [6]. The same tunnel will be designed to host a hadron collider (called the VHE-LHC), at a centre-of-mass energy of up to 100 TeV, which would give direct access to new physics up to scales of 30 TeV This vision was already put forward by the ICFA beam-dynamics workshop [11] where the design study of a circular Higgs factory was recommended. The Council calls upon the Organization to develop a proposal for an ambitious post-LHC accelerator project at the high-energy frontier, and recalls the strong scientific case for an e+e− collider that can study the properties of the Higgs boson and other particles with unprecedented precision This global vision is being implemented at CERN under the “Future Circular Colliders” (FCC) international design study. A five-year-long design study — responding to the recent European Strategy update and part of the CERN medium-term plan [13] for 2014–2018 — has been launched to refine this understanding, as well as to ascertain the feasibility of TLEP and the VHE-LHC, as input to the European Strategy update

Luminosity and energy
Beamstrahlung
Motivation
Transverse polarization
20 A Bunch 2 colliding
Beam energy measurement
Integrated luminosity measurement
Detectors
Possible timescale and physics programme
Elements of costing
Precise measurements of the Higgs boson properties
Global fit for Higgs boson couplings
Sensitivity to new physics and theory uncertainties
Precise measurements of the EWSB parameters
The Z mass and width
The Z hadronic and leptonic partial widths
The leptonic weak mixing angle
Rare decays
Measurements with OkuW
The W mass
The Z invisible width and the number of neutrinos
The strong coupling constant
Measurements with MegaTop
Reducing the theory uncertainties
Global fit of the EWSB parameters
High-energy upgrades
The Htt coupling
Findings
Conclusion

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