International Journal of Modern Physics A

Opening Lecture at the Hong Kong University of Science and Technology Jockey Club Institute for Advanced Study Program on High Energy Physics Conference, January 18–21, 2016.

One item on the agenda of future colliders is certain to be the Higgs boson. What is it trying to tell us? The primary objective of any future collider must surely be to identify physics beyond the Standard Model, and supersymmetry is one of the most studied options. Is supersymmetry waiting for us and, if so, can LHC Run 2 find it? The big surprise from the initial 13 TeV LHC data has been the appearance of a possible signal for a new boson $X$ with a mass $\simeq$ 750 GeV. What are the prospects for future colliders if the X(750) exists? One of the most intriguing possibilities in electroweak physics would be the discovery of nonperturbative phenomena. What are the prospects for observing sphalerons at the LHC or a future collider?

I give an overview of the physics potential at possible future $e^{+}{e}^{-}$ colliders, including the ILC, FCC-ee, and CEPC. The goal is to explain some of the measurements that can be done in the context of electroweak precision tests and Higgs couplings, to compare some of the options under consideration, and to put the measurements in context by summarizing their implications for some new physics scenarios. This is a writeup of a plenary talk at the Hong Kong University of Science and Technology Jockey Club Institute for Advanced Study Program on High Energy Physics Conference, 18–21 January 2016. Some previously unpublished electroweak precision results for FCC-ee and CEPC are included.

The next-generation lepton colliders, such as CEPC, FCC-ee, and ILC will make precision measurement of the Higgs boson properties. We first extract the Higgs coupling precision from Higgs observables at CEPC to illustrate the potential of future lepton colliders. Depending on the related event rates, the precision can reach percentage level for most couplings. Then, we try to estimate the new physics scales that can be indirectly probed with Higgs and electroweak precision observables. The Higgs observables, together with the existing electroweak precision observables, can probe new physics up to 10 TeV (40 TeV for the gluon-related operator ${{𝒪}}_g$) at 95% C.L. Including the $Z/W$ mass measurements and $Z$-pole observables at CEPC further pushes the limit up to 35 TeV. Although $Z$-pole running is originally for the purpose of machine calibration, it can be as important as the Higgs observables for probing the new physics scales indirectly. The indirect probe of new physics scales at lepton colliders can mainly cover the energy range to be explored by the following hadron colliders of $pp$ (50–100 TeV), such as SPPC and FCC-hh.

I summarize our recent works on using differential observables to explore the physics potential of future $e^{+}{e}^{-}$ colliders in the framework of Higgs effective field theory. This proceeding is based upon Refs. 1 and 2. We study angular observables in the $e^{+}{e}^{-}\to ZH{\ell }^{+}{\ell }^{-}b\bar{b}$ channel at future circular $e^{+}{e}^{-}$ colliders such as CEPC and FCC-ee. Taking into account the impact of realistic cut acceptance and detector effects, we forecast the precision of six angular asymmetries at CEPC (FCC-ee) with center-of-mass energy $\sqrt{s}=240$ GeV and 5 (30) ab${}^{-1}$ integrated luminosity. We then determine the projected sensitivity to a range of operators relevant for the Higgsstrahlung process in the dimension-6 Higgs EFT. Our results show that angular observables provide complementary sensitivity to rate measurements when constraining various tensor structures arising from new physics. We further find that angular asymmetries provide a novel means of constraining the “blind spot” in indirect limits on supersymmetric scalar top partners. We also discuss the possibility of using $ZZ$-fusion at $e^{+}{e}^{-}$ machines at different energies to probe new operators.

The nonunitarity of the leptonic mixing matrix is a generic signal of new physics aiming at the generation of the observed neutrino masses. We discuss the Minimal Unitarity Violation (MUV) scheme, an effective field theory framework which represents the class of extensions of the Standard Model (SM) by heavy neutral leptons, and discuss the present bounds on the nonunitarity parameters as well as estimates for the sensitivity of the CEPC, based on the performance parameters from the preCDR.

In scenarios with sterile (right-handed) neutrinos with an approximate “lepton-number-like” symmetry, the heavy neutrinos (the mass eigenstates) can have masses around the electroweak scale and couple to the Higgs boson with, in principle, unsuppressed Yukawa couplings, while the smallness of the light neutrinos’ masses is guaranteed by the approximate symmetry. The on-shell production of the heavy neutrinos at lepton colliders, together with their subsequent decays into a light neutrino and a Higgs boson, constitutes a resonant contribution to the Higgs production mechanism. This resonant mono-Higgs production mechanism can contribute significantly to the mono-Higgs observables at future lepton colliders. A dedicated search for the heavy neutrinos in this channel exhibits sensitivities for the electron neutrino Yukawa coupling as small as $\sim 5\times 10^{-3}$. Furthermore, the sensitivity is enhanced for higher center-of-mass energies, when identical integrated luminosities are considered.

The $WW$ production is the primary channel to directly probe the triple gauge couplings (TGCs). We analyze the $e^{+}{e}^{-}\to W^{+}{W}^{-}$ process at the proposed circular electron–positron collider (CEPC), and find that the anomalous TGCs and relevant dimension six operators can be probed up to the order of $10^{-4}$. We also estimate constraints at the 14 TeV (LHC), with both 300 fb${}^{-1}$ and 3000 fb${}^{-1}$ integrated luminosity from the leading lepton $p_T$ and azimuthal angle difference $\mathrm{\Delta }{\varphi }_{ll}$ in the di-lepton channel. The constrain is somewhat weaker, up to the order of $10^{-3}$. The limits on the TGCs are complementary to those on the electroweak precision observables and Higgs couplings.

The two-Higgs-doublet model contains extra Higgs bosons with mass ranges spanning from several hundred GeV to about 1 TeV. We study the possible experimental searches for the neutral Higgs bosons of $A$ and $H$ at the future high-luminosity LHC runs. Besides of the conventional search modes that are inspired by the supersymmetric models, we discuss two search modes which were not quite addressed previously. They are the decay modes of $A\to hZ$ and $A/H\to t\bar{t}$. Thanks to the technique of tagging boosted objects of SM-like Higgs bosons and top quarks, we show the improved mass reaches for heavy neutral Higgs bosons with masses up to $\sim {𝒪}(1)\mathrm{\text{TeV}}$. The modes proposed here are complementary to the conventional experimental searches motivated by the MSSM.

I summarize some results related with the determination of the Higgs potential, including a hadron-level Monte Carlo study of the sensitivity of Higgs boson pair production via the $W{W}^{\ast }W{W}^{\ast }$ channel with the final state $3\ell 2j+E/$ and the sensitivity study on the triple Higgs boson final state via $4b2\gamma$ in a 100 TeV collider.

Grand Unified Theories (GUTs) are attractive candidates for more fundamental elementary particle theories. They cannot only unify the Standard Model (SM) interactions but also different types of SM fermions, in particular quarks and leptons, in joint representations of the GUT gauge group. We discuss how comparing predictive supersymmetric GUT models with the experimental results for quark and charged lepton masses leads to constraints on the SUSY spectrum. We show an example from a recent analysis where the resulting superpartner masses where found just beyond the reach of LHC Run 1, but fully within the reach of a 100 TeV $pp$ collider.

Similar to a super B-factory, a circular Higgs factory (CHF) will require strong focusing systems near the interaction points and a low-emittance lattice in the arcs to achieve a factory luminosity. At electron beam energy of 125 GeV, beamstrahlung effects during the collision pose an additional challenge to the collider design. In particular, a large momentum acceptance at the 2% level is necessary to retain an adequate beam lifetime. This turns out to be the most challenging aspect in the design of a CHF. In this paper, an example will be provided to illustrate the beam dynamics in a CHF, emphasizing the chromatic optics. Basic optical modules and advanced analysis will be presented. Most importantly, we will show that 2% momentum aperture is achievable.

CEPC was proposed as an electron and positron collider ring with a circumference of 50–100 km to study the Higgs boson. Since the proposal was made, the lattice design for CEPC has been carried out and a preliminary conceptual design report has been written at the end of 2014. In this paper, we will describe the principles of pretzel scheme design, which is one of most important issues in CEPC lattice design. Then, we will show the modification of the lattice based on the lattice design shown in the Pre-CDR. The latest pretzel orbit design result will also be shown. The issues remained to be solved in the present design will be discussed and a brief summary will be given at the end.

We discuss beam dynamics issues in CEPC/FCC-ee, especially focusing on the beam–beam and electron cloud effects. Beamstrahlung is strong in extreme high energy collision such as Higgs and top factory. Beam–beam simulations considering beamstrahlung are now ready. Several points of beam–beam effects for FCC-ee are presented. Electron cloud effects are serious for high current positron machine, especially in Z factory that many bunches are stored. Analytical estimate for threshold of electron density and electron build-up for CEPC are presented.

After the Higgs discovery, it is believed that a circular $e^{+}{e}^{-}$ collider could serve as a Higgs factory. The high energy physics community in China launched a study of a 50–100 km ring collider. A preliminary conceptual design report (Pre-CDR) has been published in early 2015. This report is based on a 54-km ring design. Some progress on beam–beam effect study after Pre-CDR is shown in the paper. We estimate the beamstrahlung lifetime using a pure strong–strong code as a comparison with the result obtained using a quasi-strong–strong method. The effect of parasitic crossing in the pretzel scheme is also estimated for the very first time. The feasibility of the main parameters for partial double ring scheme are evaluated from the point view of beam–beam interaction.

In order to avoid the pretzel orbit, CEPC is proposed to use partial double ring scheme in CDR. In this paper, a general method of how to make an consistent machine parameter design of CEPC with crab-waist by using analytical expression of maximum beam–beam tune shift and beamstrahlung beam lifetime started from given IP vertical beta, beam power and other technical limitations were developed. FFS with crab sextupoles will be developed and the arc lattice will be redesigned to acheive the lower emittance for crab-waist scheme.

In this paper, we introduce the layout and lattice design of Circular-Electron-Positron-Collider (CEPC) partial double ring scheme and the lattice design of Super-Proton-Proton-Collider (SPPC). The baseline design of CEPC is a single beam-pipe electron positron collider, which has to adopt pretzel orbit scheme and it is not suitable to serve as a high luminosity $Z$ factory. If we choose partial double ring scheme, we can get a higher luminosity with lower power and be suitable to serve as a high luminosity $Z$ factory. In this paper, we discuss the details of CEPC partial double ring lattice design and show the dynamic aperture study and optimization. We also show the first version of SPPC lattice although it needs lots of work to do and to be optimized.

IHEP (the Institute of High Energy Physics, Beijing, China) has started the R&D of high field accelerator magnet technology from 2014 for recently proposed CEPC-SppC (Circular Electron Positron Collider, Super proton–proton Collider) project. The conceptual design study of a 20-T dipole magnet is ongoing with the common coil configuration, and a 12-T model magnet will be fabricated in the next two years. A 3-step R&D process has been proposed to realize this 12-T common-coil model magnet: first, a 12-T subscale magnet will be fabricated with Nb₃Sn and NbTi superconductors to investigate the fabrication process and characteristics of Nb₃Sn coils, then a 12-T subscale magnet will be fabricated with only Nb₃Sn superconductors to test the stress management method and quench protection method of Nb₃Sn coils; the final step is fabricating the 12-T common-coil dipole magnet with HTS (YBCO) and Nb₃Sn superconductors to test the field optimization method of the HTS and Nb₃Sn coils. The characteristics of these R&D steps will be introduced in the paper.

We report an efficient dynamic aperture (DA) optimization approach using multi-objective genetic algorithm (MOGA), which is driven by nonlinear driving terms computation. It was found that having small low order driving terms is a necessary but insufficient condition of having a decent DA. Then direct DA tracking simulation is implemented among the last generation candidates to select the best solutions. The approach was demonstrated successfully in optimizing NSLS-II storage ring DA.

The talks in the Program and the Conference parallel sessions make clear that high quality pixel vertex chambers are presently well developed and with continuing improvements (M. Caccia,¹ X. Sun,² M. Stanitzki,³ J. Qian⁴); that there are at least two major tracking chambers that are well studied, a TPC and silicon-strip chambers (H. Qi,${}^{5,6}$ C. Young,${}^{7,8}$ A. de Roeck${}^{9,10}$); that the energy measurement of photons and electrons is generally very good (H. Yang,${}^{11}$ S. Franchino${}^{12}$); and, that the last remaining detector that has not yet achieved the high precision required for good $e^{+}{e}^{-}$ physics is the hadronic calorimeter for the measurement of jets, most importantly, jets from the decays of $W$ and $Z$ to quarks (S. Lee,${}^{13,14}$ M. Cascella,${}^{15}$ A. de Roeck${}^{16}$). The relationship of the detectors to physics and the overall design of detectors was addressed and questioned (Y. Gao,${}^{17}$ M. Ruan,${}^{18}$ G. Tonelli,${}^{19}$ H. Zhu,${}^{20}$ M. Mangano,${}^{21}$ C. Quigg${}^{22}$) in addition to precision time measurements in detectors (C. Tully${}^{23}$).

The physics potential of the Circular Electron Positron Collider (CEPC) can be significantly strengthened by two detectors with complementary designs. A promising detector approach based on the Silicon Detector (SiD) designed for the International Linear Collider (ILC) is presented. Several simplifications of this detector for the lower energies expected at the CEPC are proposed. A number of cost optimizations of this detector are illustrated using full detector simulations. We show that the proposed changes will enable one to reach the physics goals at the CEPC.

Advancements in fast timing particle detectors have opened up new possibilities to design $4\pi$ collider detectors that fully reconstruct and separate event vertices and individual particles in the time domain. The applications of these techniques are considered for the physics at CEPC.

The reconstruction and high-precision measurement of the four-vectors of $W$ and $Z$ decays to quarks, which constitute nearly 70% of their decay branching fractions, are critical to a high efficiency and high quality experiment. Furthermore, it is crucial that the energy resolution, and consequently the resolution on the invariant mass of the two fragmenting quarks, is comparable to the energy–momentum resolution on the other particles of the standard model, in particular, electrons, photons, and muons, at energies around 100 GeV. I show that this “unification of resolutions” on all particles of the standard model is now in sight, and will lead to excellent physics at an electron–positron collider.

The RD52 (DREAM) collaboration is performing R&D on dual readout calorimetry techniques with the aim of improving hadronic energy resolution for future high energy physics experiments. The simultaneous detection of Cherenkov and scintillation light enables us to measure the electromagnetic fraction of hadron shower event-by-event. As a result, we could eliminate the main fluctuation which prevented from achieving precision energy measurement for hadrons. We have tested the performance of the lead and copper fiber prototypes calorimeters with various energies of electromagnetic particles and hadrons. During the beam test, we investigated the energy resolutions for electrons and pions as well as the identification of those particles in a longitudinally unsegmented calorimeter. Measurements were also performed on pure and doped PbWO₄ crystals, as well as BGO and BSO, with the aim of realizing a crystal based dual readout detector. We will describe our results, focusing on the more promising properties of homogeneous media for the technique. Guidelines for additional developments on crystals will be also given. Finally we discuss the construction techniques that we have used to assemble our prototypes and give an overview of the ones that could be industrialized for the construction of a full hermetic calorimeter.

The luminosity ${ℒ}$ of colliding beams in a storage ring such as CEPC depends strongly on $l^{\ast }$, the half-length of the free space centered on the intersection point (IP). $l^{\ast }$ is also the length from the IP to the front edges of the two near-in quadrupoles that are focusing the counter-circulating beams to the IP spot. The detector length cannot, therefore, exceed $2l^{\ast }$. Since ${ℒ}$ increases strongly with decreasing $l^{\ast }$, there is incentive for reducing $l^{\ast }$; but this requires the detector to be shorter than desirable. This paper proposes a method for integrating these adjacent quadrupoles into the particle detector to retain (admittedly degraded) active particle detection of those forward going particles that would otherwise be obscured by the quadrupole. A gently conical quadrupole shape is more natural for merging the quadrupole into the particle detector than is the analytically exact cylindrical shape. This is true whether or not the calorimeter is integrated. It will be the task of accelerator physicists to determine the extent to which deviation from the pure quadrupole field compromises (or improves) accelerator performance. Superficially, both the presence of strongest gradient close to the IP and largest aperture farther from the IP seem to be advantageous. A tentative design for this merged, quadrupole-calorimeter is given.

The Circular Electron Positron Collider (CEPC) as a Higgs factory was proposed in September 2013. The preliminary conceptual design report was completed in 2015.¹ The CEPC detector design was using International Linear Collider Detector — ILD² as an initial baseline. The CEPC calorimeters, including the high granularity electromagnetic calorimeter (ECAL) and the hadron calorimeter (HCAL), are designed for precise energy measurements of electrons, photons, taus and hadronic jets. The basic resolution requirements for the ECAL and HCAL are about 16%$\sqrt{E}$ (GeV) and 50%$\sqrt{E}$ (GeV), respectively. To fully exploit the physics potential of the Higgs, $W$, $Z$ and related Standard Model processes, the jet energy resolution is required to reach 3%–4%, or 30%/$\sqrt{E}$ (GeV) at energies below about 100 GeV. To achieve the required performance, a Particle Flow Algorithm (PFA) — oriented calorimetry system is being considered as the baseline design. The CEPC ECAL detector options include silicon–tungsten or scintillator–tungsten structures with analog readout, while the HCAL detector options have scintillator or gaseous detector as the active sensor and iron as the absorber. Some latest R&D studies about ECAL and HCAL within the CEPC working group is also presented.