Future Lepton Colliders Research Articles

Probing the symmetric Higgs portal with di-Higgs boson production

A coupling of a scalar, charged under an unbroken global U(1) symmetry, to the Standard Model via the Higgs portal is one of the simplest gateways to a dark sector. Yet, for masses $m_{S}\geq m_{H}/2$ there are few probes of such an interaction. In this note we evaluate the sensitivity to the Higgs portal coupling of di-Higgs boson production at the LHC as well as at a future high energy hadron collider, FCC-hh, taking into account the full momentum dependence of the process. This significantly impacts the sensitivity compared to estimates of changes in the Higgs-coupling based on the effective potential. We also compare our findings to precision single Higgs boson probes such as the cross section for vector boson associated Higgs production at a future lepton collider, e.g. FCC-ee, as well as searches for missing energy based signatures.

Pseudoscalar quarkonium+\u03b3 production at NLL+NLO accuracy

We consider the exclusive pseudoscalar heavy-quarkonium (ηb, c) production in association with a photon at future lepton colliders where the collider energies of O(102) GeV are far greater than the quarkonium mass. At these energies, the logarithm of mass to collision energy becomes increasingly large hence its resummation becomes particularly important. By making use of the light-cone-distribution factorization formula, we resum the logarithms up to next-to-leading-logarithmic accuracy (NLL) that corresponds to order-αs accuracy. We combine the resummed result with a known fixed-order result at next-to-leading order (NLO) such that both resummed-logarithmic terms and non-logarithmic terms are included at the same order in αs. This allowed us to provide reliable predictions at accuracies of order αs ranging from relatively low energies near quarkonium mass to the collider energies of O(102) GeV. We also include the leading relativistic corrections resummed at leading-logarithmic accuracy. Our prediction at the Belle energy is comparable with fixed-order predictions in literatures while it shows a large deviation from a recent Belle’s upper limit by about 4 σ. Finally, we make predictions for the energies of future Z - and Higgs factories.

Dark Matter Searches as New Physics at the Future Circular Collider (FCC)

We highlight the importance of the Future Circular Collider (FCC) in the search for possible Dark Matter (DM) signatures. The focus will be on well motivated Beyond the Standard Model (BSM) extensions. We consider a class of radiative neutrino mass models which extend the SM by adding heavy right handed neutrinos (RHNs), where the neutrino masses are generated at loop level. Within this class of models, the lightest RHN is considered to be stable and serves as DM candidate. We perform preliminary studies of final states which involve missing energy contributions from DM candidates to learn about the feasibility of observing these final states at the expected FCC-hh, FCC-eh and FCC-ee beam colliders. Some of our results are discussed where the topology of the the final states are analysed depending on the obtained sensitivity of the signal. The study also yields the most common promising discovery channels in exploring potential indirect DM signatures at future lepton and hadron colliders.

Enhanced Z boson decays as a new probe of first-order electroweak phase transition at future lepton colliders

We study phenomenological consequences of the strong first-order electroweak phase transition in an extension of the standard model with an inert doublet and vector-like leptons motivated by the muon $g-2$ anomaly and dark matter. We find that a condition for the strong first-order electroweak phase transition inevitably induces a large logarithmic enhancement in $Z$ boson decays, which relegates the explanation of the anomalous muon $g-2$ at below 2$\sigma$ level. Our analysis shows that future lepton collider experiments, especially the Giga-$Z$ at the International Linear Collider and Tera-$Z$ at the Circular Electron Positron Collider as well as Future Circular Collider have great capability to explore the nature of the electroweak phase transition, which is complementary to conventional approaches via measurements of the triple Higgs boson coupling and gravitational waves.

The full lepton flavor of the littlest Higgs model with T-parity

We re-examine lepton flavor violation (LFV) in the Littlest Higgs model with T-parity (LHT) including the full T-odd (non-singlet) lepton and Goldstone sectors. The heavy leptons induce two independent sources of LFV associated with the couplings necessary to give masses to the T-odd mirror fermions and to their partners in right-handed SO(5) multiplets, respectively. The latter, which have been neglected in the past, can be decoupled from gauge mediated processes but not from Higgs mediated ones and must therefore also be included in a general analysis of LFV in the LHT. We also further extend previous analyses by considering on-shell Z and Higgs LFV decays together with the LFV processes at low momentum transfer. We show that current experimental limits can probe the LHT parameter space up to global symmetry breaking scales f ∼ 10 TeV. For lower f values ≳ 1 TeV, μ − e transitions require the misalignment between the heavy and the Standard Model charged leptons to be ≲ 1%. Future LFV experiments using intense muon beams should be sensitive to misalignments below the per mille level. For τ LFV transitions, which could potentially be observed at Belle II and the LHC as well as future lepton colliders, we find that generically they can not discriminate between the LHT and supersymmetric models though in some regions of parameter space this may be possible.

Light fermionic WIMP dark matter with light scalar mediator

A light fermionic weakly interacting massive particle (WIMP) dark matter is investigated by studying its minimal renormalizable model, where it requires a scalar mediator to have an interaction between the WIMP and standard model particles. We perform a comprehensive likelihood analysis of the model involving the latest but robust constraints and those will be obtained in the near future. In addition, we pay particular attention to properly take the kinematically equilibrium condition into account. It is shown that near-future experiments and observations such as low-mass direct dark matter detections, flavor experiments and CMB observations play important roles to test the model. Still, a wide parameter region will remain even if no WIMP and mediator signals are detected there. We also show that precise Higgs boson measurements at future lepton colliders will significantly test this remaining region.

Precision Description of Processes at Colliders in the SANC System

A review of the SANC computer system intended for theoretical support of experiments at colliders is presented. The system allows the precision predictions to be obtained semiautomatically for a wide class of particle interaction processes within the Standard Model. In this case, one considers total contributions of one-loop radiative corrections. Basic elements of the computer system are described, and examples of using the system for the high-precision description of certain processes of the Standard Model particle interactions are given. The MCSANC code developed using the Monte Carlo simulation is presented, which is used to calculate cross sections of a number of processes, investigated at the Large Hadron Collider. The prospects of research development in this field for theoretical support at future hadron and lepton colliders are discussed.

Dual-readout calorimetry

We summarize the fundamental aspects of dual-readout calorimetry, a calorimetric technique able to overcome the non-compensation problem by means of two independent scintillation and Cherenkov light signals detection. The expected ultimate energy resolution for single-hadron detection, together with the excellent particle identification capability, makes a dual-readout fiber calorimeter one of the most promising options for future leptonic colliders. In this paper, we include the main benefits of a new silicon photomultiplier-based readout system that allows to sample showers with an unprecedented spatial resolution.

\(\mathcal{O}(\alpha^3)\) ISR Corrections to Single Higgs Productions at Future Lepton Colliders

In this paper, we present \(\mathcal{O}(\alpha^3)\) QED corrections due to initial-state photon radiation (ISR) to processes \(e^{−} e^{+} \to f \bar{f} H\) with \(f = e, \mu_e, \nu_{\mu}, \nu_{\tau}\) at future lepton colliders. The impact of the ISR corrections on the total cross sections are studied. We find that the corrections vary from ∼ −30% to −10% in the range of center-of-mass energies from 200 GeV to 700 GeV. The ISR corrections to some realistic distributions of experimental interest for searching the properties of the Higgs boson are also evaluated and they are order of \(\sim −10\)% contributions at \(s = 250\) GeV. The corrections are massive contributions and they must be taken into account for matching high precision data at future lepton colliders.

First test-beam results obtained with IDEA, a detector concept designed for future lepton colliders

IDEA (Innovative Detector for Electron–positron Accelerators) is a detector concept designed for a future leptonic collider operating as a Higgs factory. It is based on innovative detector technologies developed over years of R&D. In September 2018, prototypes of the proposed sub-detectors have been tested for the first time on a beam line at CERN. The preliminary results from this test of a full slice of the IDEA detector and standalone measurements of dual read-out calorimeter prototypes are presented.

Linearity response measurement of a SiPM-based dual-readout calorimeter for future leptonic colliders

A dual-readout fibre sampling calorimeter module, based on Silicon Photomultiplier readout, was developed and tested on beam in 2016 and 2017. This test completed the proof-of-concept but also revealed a non-linear response to the scintillation light. In 2018, the signal linearity was studied after the interposition of a yellow filter between the scintillating fibres and the sensors. This paper reports the latest results on the module performance, highlighting as well the key and potentially critical parameters and the adopted solutions.

The $\tau$ Magnetic Dipole Moment at Future Lepton Colliders

Author(s): Howard, Jessica N; Rajaraman, Arvind; Riley, Rebecca; Tait, Tim MP | Abstract: The magnetic moment of the $\tau$ lepton is an interesting quantity that is potentially sensitive to physics beyond the Standard Model. Electroweak gauge invariance implies that a heavy new physics contribution to it takes the form of an operator which involves the Higgs boson, implying that rare Higgs decays are able to probe the same physics as $a_\tau$. We examine the prospects for rare Higgs decays at future high energy lepton (electron or muon) colliders, and find that such a project collecting a few ab$^{-1}$ would be able to advance our understanding of this physics by roughly a factor of 10 compared to the expected reach of the high luminosity LHC.

Long live the Higgs factory: Higgs decays to long-lived particles at future lepton colliders * * Supported by the US Department of Energy under the Early Career Award DE-SC0014129 and the Cottrell Scholar Program through the Research Corporation for Science Advancement

We initiate the study of exotic Higgs decays to long-lived particles (LLPs) at proposed future lepton colliders, focusing on scenarios with displaced hadronic final states. Our analysis entails a realistic tracker-based search strategy involving the reconstruction of displaced secondary vertices and the imposition of selection cuts appropriate for eliminating the largest irreducible backgrounds. The projected sensitivity is broadly competitive with that of the LHC and potentially superior at lower LLP masses. In addition to forecasting branching ratio limits, which may be freely interpreted in a variety of model frameworks, we interpret our results in the parameter space of a Higgs portal Hidden Valley and various incarnations of neutral naturalness, illustrating the complementarity between direct searches for LLPs and precision Higgs coupling measurements at future lepton colliders.

Dark matter, neutrino mass, cutoff for cosmic-ray neutrino, and the Higgs boson invisible decay from a neutrino portal interaction

We study an effective theory beyond the standard model (SM) where either of the two additional gauge singlets, a Majorana fermion and a real scalar, constitutes all or some fraction of dark matter. In particular, we focus on the masses of the two singlets in the range of (10) MeV- (10) GeV with a neutrino portal interaction, which plays an important role not only in particle physics but also in cosmology and astronomy. We point out that the thermal dark matter abundance can be explained by (co-)annihilation, where the dark matter with a mass greater than 2 GeV can be tested in future lepton colliders, CEPC, ILC, FCC-ee and CLIC, in the light of the Higgs boson invisible decay. When the gauge singlets are lighter than (100) MeV, the interaction can affect the neutrino propagation in the universe due to its annihilation with cosmic background neutrino into the gauge singlets. Although in this case it can not be the dominant dark matter, the singlets are produced by the invisible decay of the Higgs boson at such a rate which is fully within reach of future lepton colliders. In particular, a high energy cutoff of cosmic-ray neutrino, which may account for the non-detection of Greisen-Zatsepin-Kuzmin (GZK) neutrino or the non-observation of the Glashow resonance, can be set. Interestingly, given the cutoff and the mass (range) of WIMPs, a neutrino mass can be " measured” kinematically.

Higgs inflation and cosmological electroweak phase transition with N scalars in the post-Higgs era

We study inflation and cosmological electroweak phase transitions utilizing the Standard model augmented by $N$ scalars respecting a global $O(N)$ symmetry. We observe that the representation of the global symmetry is restricted by the inflationary observables and the condition of a strongly first order electroweak phase transition. Theoretical constraints including the stability, perturbativity and unitarity are used to bound the model parameter space. The Electroweak precision observables and Higgs precisions limit the representation of the symmetry. We evaluate the possibility to simultaneously address the inflation and the dark matter after considering the experimental constraints from the future leptonic colliders. When the $O (N)$ symmetry respected by the N-scalar is spontaneously broken to the $O (N-1)$ symmetry, both the one-step and two-step SFOEWPT can occur within the inflation viable parameter regions, which will be tested by the future CEPC, ILC and FCC-ee. The relation between the number of Goldstones and the SFOEWPT condition depends on phase transition patterns. The situation of Goldstone faking neutrinos and contributing to the dark radiation are investigated.

WW fusion and Higgsstrahlung interplay in Higgs precision tests with the e−e+→νν¯h process

Higgs precision program at future lepton collider aims at (sub) percent level precision measurement of the Higgs properties, shedding light to new physics through the Higgs lamppost. Amongst many exclusive Higgs channels that can be measured precisely, the $WW$-fusion to Higgs with subsequent decays into $b\bar b$ final state are of particular importance. This channel provide leading constrains on Higgs total width in the $\kappa$-framework and greatly improves the constraints in the EFT framework as a distinct production mode other than the Higgsstrahlung process. We argue in this paper that, there are two interference effects both affects the physical information one can extract from the precision measurements. One takes place at quantum level from the interference between the two amplitudes that amounts to $-10\%$ of the $WW$-fusion signal strength, failing to take into account which will result in a $4$-$5\sigma$ discrepancy between theory and measurement. The other takes place at the classical level from the global fitting procedure where the $ZH$ process is the dominant background with its cross section around six times larger than the $WW$-fusion signal. Despite that $ZH$ process can be measured to great precision at future lepton colliders, failing take this interplay in the coupling extraction will result in a 100\% too aggressive constraints on $\kappa_W$, the Higgs coupling to $W$-boson pairs. This effect will also be important for lepton colliders running at slightly higher energies where the phase space overlap are still sizable between the two processes.

A potential scenario for Majorana neutrino detection at future lepton colliders

The existence of Majorana neutrinos must lead to lepton-number violating processes, and the Majorana nature of neutrinos can only be experimentally verified via lepton-number violating processes. We study an approach to search for Majorana neutrinos at future electron-positron colliders by exploiting this feature. We investigate ΔL = 2 like-sign dilepton production and find that lepton colliders with different center-of-mass (c.m.) energies have comparative advantages in resonant production of a Majorana neutrino in both light and heavy neutrino mass range. At the future Circular Electron-Positron Collider (CEPC), with 250 GeV c.m. energy and 5 ab−1 integrated luminosity, we find that there could be more significant sensitivity for resonant production of a Majorana neutrino in the mass range of 30-50 GeV than previous results at LEP2 or LHC. At the 1 TeV ILC with 1 ab−1 integrated luminosity, the sensitivity can be improved for neutrino mass in the range of 450-600 GeV.

Searching for weak singlet charged scalars at lepton colliders

A weak singlet charged scalar exists in many new physics models beyond the Standard Model. The discovery potential of the singlet charged scalar is explored at future lepton colliders, e.g. the CEPC, ILC-350 and ILC-500. We demonstrate that one can discover the singlet charged scalar up to 118 GeV at the CEPC with an integrated luminosity of $5~\mathrm{ab}^{-1}$. At the ILC-350 and the ILC-500 with an integrated luminosity of $1~\mathrm{ab}^{-1}$ such a discovery limit can be further improved to 136 GeV and 160 GeV, respectively.

Axionic electroweak baryogenesis

An axion can make the electroweak phase transition strongly first-order as required for electroweak baryogenesis even if it is weakly coupled to the Higgs sector. This is essentially because the axion periodicity naturally allows the structure of phase transition to be insensitive to the axion decay constant that determines the strength of axion interactions. Furthermore, the axion can serve as a CP phase relevant to electroweak baryogenesis if one introduces an effective axion coupling to the top quark Yukawa operator. Then, for f between about TeV and order 10 TeV, the observed baryon asymmetry can be explained while avoiding current experimental constraints. It will be possible to probe the axion window for baryogenesis in future lepton colliders and beam-dump experiments.

High-quality positrons from a multi-proton bunch driven hollow plasma wakefield accelerator

By means of a hollow plasma, multiple proton bunches work well in driving nonlinear plasma wakefields and accelerate electrons to the energy frontier with preserved beam quality. However, the acceleration of positrons is different because the accelerating structure is strongly charge dependent. There is a trade-off between keeping a small normalized emittance and a small energy spread. This results from the conflict that the plasma electrons, used to provide focusing to the multiple proton bunches, dilute the positron bunch. By loading an extra electron bunch to repel the plasma electrons and meanwhile reducing the plasma density slightly to shift the accelerating phase with a conducive slope to the positron bunch, the positron bunch can be accelerated to 400 GeV (40% of the driver energy) with an energy spread as low as 1% and well preserved normalized emittance. The successful generation of high quality and high energy positrons paves the way to the future energy frontier lepton colliders.