Lepton Colliders Research Articles

Axion baryogenesis puts a new spin on the Hubble tension

We show that a rotating axion field that makes a transition from a matterlike equation of state to a kinationlike equation of state around the epoch of recombination can significantly ameliorate the Hubble tension, i.e., the discrepancy between the determinations of the present-day expansion rate H0 from observations of the cosmic microwave background on one hand and type Ia supernovae on the other. We consider a specific, UV-complete model of such a rotating axion and find that it can relax the Hubble tension without exacerbating tensions in determinations of other cosmological parameters, in particular the amplitude of matter fluctuations S8. We subsequently demonstrate how this rotating axion model can also generate the baryon asymmetry of our Universe, by introducing a coupling of the axion field to right-handed neutrinos. This baryogenesis model predicts heavy neutral leptons that are most naturally within reach of future lepton colliders, but in finely tuned regions of parameter space may also be accessible at the high-luminosity LHC and the beam dump experiment SHiP. Published by the American Physical Society 2024

Quantum tomography with τ leptons at the FCC-ee: Entanglement, Bell inequality violation, sinθW , and anomalous couplings

The Future Circular Collider (FCC)—in its first incarnation as a lepton collider—will produce, according to the proposed design, more than 100 billion pairs of τ leptons after working for four years at the energy of the Z-boson resonance. The τ lepton is special because its relatively long lifetime makes it possible to reconstruct the momenta of neutrinos emitted in the single pion decay mode. The resulting large number of events is an ideal source for a full quantum tomography of the process that will test quantum entanglement and the violation of Bell inequality with unprecedented precision. In addition, the study of polarizations and spin correlations can provide a competitive determination of the Weinberg angle θW and constrain possible anomalous couplings in the neutral electroweak current. We utilize analytic results and Monte Carlo simulations to explore to what extent these goals might be accomplished. Published by the American Physical Society 2024

Simulation of a capillary tube, fibre dual-readout calorimeter in DD4hep

Over the past years the dual-readout method for calorimetry, which exploits complementary information from Scintillation and Cherenkov channels, has emerged as candidate to fulfil the requirements for precision physics at future circular lepton colliders. While the dual-readout approach has been tested experimentally quite extensively, this type of calorimeter has never been used in an experiment at a collider. In recent years dedicated studies in simulation have investigated various detector geometries based on a fibre dual-readout calorimeter. One variation of the geometry, relying on capillary tubes, promises easy assembly with excellent geometrical accuracy at a moderate cost. In these proceedings, we present the status and latest results from a full 4π detector geometry simulation in DD4hep. The simulation is used to estimate the performance of a dual-readout calorimeter for single electromagnetic and hadronic particles. The results for the dual-readout technique show an improvement in energy resolution for both electromagnetic and hadronic showers, with respect to single channel measurements.

Charged Higgs decay to W±H\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$W^{\\pm }H$$\\end{document} at a high energy lepton collider

In this work, we present a search strategy for heavy charged Higgs boson at compact linear collider (CLIC) as a future e+e-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$e^+e^-$$\\end{document} collider. The signal is charged Higgs boson pair production in two Higgs doublet model (2HDM) followed by H±→W±H\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$H^{\\pm }\\rightarrow W^{\\pm }H$$\\end{document} and H→bb¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$H\\rightarrow b\\bar{b}$$\\end{document}. Here, H denotes the heavy CP-even neutral Higgs boson of the model. The collider center of mass energy is chosen to be s=1400\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sqrt{s}=1400$$\\end{document} GeV as the second stage of CLIC operation. In this case, mH+<s/2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$m_{H^+}<\\sqrt{s}/2$$\\end{document} can be explored due to the pair production. It is shown that the signal of charged Higgs in the mass range 250GeV<mH+<650GeV\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$250~ \ ext {GeV}<m_{H^+}< 650~ \ ext {GeV}$$\\end{document} in fully hadronic final state, containing four b-jets from neutral Higgs and four jets from W bosons, can well be observed on top of the standard model background. Finally 5σ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$5\\sigma $$\\end{document} contours are presented in (mH±,mH)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(m_{H^{\\pm }},m_{H})$$\\end{document} space for different tanβ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ an \\beta $$\\end{document} values.

Dark matter phenomenology in 2HDMS in light of the 95 GeV excess

The Two Higgs Doublet model extended with a complex scalar singlet (2HDMS) is a well-motivated Beyond Standard Model candidate addressing several open problems of nature. In this work, we focus on the dark matter (DM) phenomenology of the complex scalar singlet where the real part of the complex scalar obtains a vacuum expectation value. The model is characterized by an enlarged Higgs spectrum comprising six physical Higgs bosons and a pseudoscalar DM candidate. We address the impact of accommodating the 95 GeV excess on the 2HDMS parameter space and DM observables after including all theoretical and experimental constraints. Finally, we look into the prospects of this scenario at HL-LHC and future lepton colliders for a representative benchmark.

Analytic NNLO QCD corrections to top quark pair production in electron-positron collisions

We present the analytic total cross section of top quark pair production in electron-positron annihilation at next-to-next-to-leading order (NNLO) in Quantum Chromodynamics (QCD). By utilizing the optical theorem, the NNLO corrections are related to the imaginary parts of three-loop self-energy Feynman diagrams, of which the master integrals are calculated with canonical differential equations. The analytic results for the NNLO corrections are expressed in terms of multiple polylogarithms as well as elliptic functions. We discuss the asymptotic expansions near the threshold and in the high energy limit in detail. Numerical results are provided for the total cross section of top quark pair production at future lepton colliders.

Quantum tops at circular lepton colliders

We study the quantum properties of top quark pairs in lepton colliders with unpolarised beams, including spin correlations, entanglement, and violation of Bell inequalities. We present analytical results in the SM and in the SMEFT and discuss several practical aspects, like the choice of quantisation axes and tt¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ t\\overline{t} $$\\end{document} threshold effects. We also note a correspondence between parity symmetry and entanglement. We find that quantum observables exhibit a rich phenomenology in the SM, and can also provide additional leverage in detecting new physics residing at higher scales.

ALLEGRO FCC-ee detector concept & Noble liquid calorimetry

ALLEGRO is a proposed FCC-ee general-purpose detector concept with a noble liquid (NL) electromagnetic calorimeter (ECAL) as a central feature. Calorimetry based on liquified noble gases is a well-proven technology that has been successfully applied in numerous high-energy physics (HEP) experiments. This technology provides excellent energy resolution, linearity, stability, uniformity and timing properties at a reasonable cost, making it a strong candidate for future HEP experiments. By using multi-layer printed circuit boards as readout electrodes, we can build a calorimeter with almost arbitrarily high granularity. This in turn allows for four-dimensional imaging, machine learning algorithms and particle-flow reconstruction to be fully exploited. In this proceedings contribution we give an overview to the ALLEGRO concept and present the ongoing R&D work for adapting NL calorimetry to an ECAL of a lepton collider experiment. In addition to simulation studies and expected performance, we show results on signal extraction and noise mitigation studies made with readout electrode prototypes and compare the measurements to simulations, as well as discuss test results of absorber prototypes. In addition we present progress of the mechanical design project and status of the barrel ECAL test-beam prototype module development.

CLFV leptophilic Z′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Z^\\prime $$\\end{document} as a dark matter portal: prospects for colliders

Extensions of the Standard Model featuring light vector bosons have been explored with the goal of resolving certain tensions between theory and experiment, among them the discrepancy in the anomalous magnetic moment of the muon, Δaμ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta a_{\\mu }$$\\end{document}. In particular, this is the case of a minimal construction including a leptophilic, strictly flavour violating, vector boson Z′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Z^\\prime $$\\end{document}. These new vector bosons are also well-motivated dark matter portals, with non-trivial couplings to stable, weakly interacting states which can account for the correct dark matter density. Here we study the prospects of a Standard Model extension (via a vector boson and a fermionic dark matter candidate) concerning signatures at the LHC, and at future lepton and hadron colliders. We discuss the cross-sections of several processes leading to same- and opposite-sign muon-tau lepton pairs in the final state, as well as final states with missing energy (in the form of neutrinos and/or dark matter). Our findings suggest that a future muon collider offers the best prospects to probe this model (together with searches for dilepton pairs and missing energy signatures at the FCC-ee running at the Z-pole); moreover, the complementarity of the different future high-energy colliders is also paramount to probing distinct Z′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Z^\\prime $$\\end{document} mass regimes.

A comparative study of flavour-sensitive observables in hadronic Higgs decays

Jet production from hadronic Higgs decays at future lepton colliders will have significantly different phenomenological implications than jet production via off-shell photon and Z-boson decays, owing to the fact that Higgs bosons decay to both pairs of quarks and gluons. We compute observables involving flavoured jets in hadronic Higgs decays to three partons at Born level including next-to-leading order corrections in QCD (i.e., up to O(αs2))\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathcal {O}(\\alpha _\ extrm{s}^2))$$\\end{document}. The calculation is performed in the framework of an effective theory in which the Higgs boson couples directly to gluons and massless b-quarks retaining a non-vanishing Yukawa coupling. For the energy of the leading and subleading flavoured jet, the angular separation and the invariant mass of the leading b–b¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\bar{b}$$\\end{document} pair, we contrast the results obtained in both Higgs-decay categories and using either of the infrared-safe flavoured jet algorithms flavour-kT\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$k_{\ extrm{T}}$$\\end{document} and flavour-dressing.

Indirect probe of electroweak-interacting particles at the μTRISTAN μ+μ+ collider

A novel collider, called μTRISTAN, was recently proposed, offering the capability to achieve high-energy collisions of antimuons. This high-energy collider presents an exceptional opportunity for the discovery of electroweak-interacting massive particles (EWIMPs), which are predicted by various new physics models. In a lepton collider like μTRISTAN, the potential for discovering EWIMPs extends beyond their direct production. Quantum corrections arising from EWIMP loops can significantly enhance our prospects for discovery by precise measurement of Standard Model processes. This study focuses on the indirect detection method within the μTRISTAN experiment, with a specific emphasis on TeV-scale EWIMP dark matter scenarios that yield the correct thermal relic density. At collision energies for s=O(1–10) TeV, these EWIMPs introduce noticeable effects, typically in the range of O(0.1–1)%. Our findings indicate that at s=2(10) TeV, with an integrated luminosity of 10 ab−1, μTRISTAN can detect Higgsinos at a mass of 1.3 (3.0) TeV and winos at a mass of 1.9 (4.4) TeV, assuming an optimistic level of systematic uncertainty in the observation of the Standard Model processes. Published by the American Physical Society 2024

Composite dark matter and neutrino masses from a light hidden sector

We study a class of models in which the particle that constitutes dark matter arises as a composite state of a strongly coupled hidden sector. The hidden sector interacts with the Standard Model through the neutrino portal, allowing the relic abundance of dark matter to be set by annihilation into final states containing neutrinos. The coupling to the hidden sector also leads to the generation of neutrino masses through the inverse seesaw mechanism, with composite hidden sector states playing the role of the singlet neutrinos. We focus on the scenario in which the hidden sector is conformal in the ultraviolet, and the compositeness scale lies at or below the weak scale. We construct a holographic realization of this framework based on the Randall-Sundrum setup and explore the implications for experiments. We determine the current constraints on this scenario from direct and indirect detection, lepton flavor violation and collider experiments and explore the reach of future searches. We show that in the near future, direct detection experiments and searches for μ → e conversion will be able to probe new parameter space. At colliders, dark matter can be produced in association with composite singlet neutrinos via Drell Yan processes or in weak decays of hadrons. We show that current searches at the Large Hadron Collider have only limited sensitivity to this new production channel and we comment on how the reconstruction of the singlet neutrinos can potentially expand the reach.

Investigation of vertex a1&gt;&gt;VP for hadronic tau decays

In the framework of U(3) symmetric quark model, triangular diagrams are calculated that describe the strongly interacting vertices a1&gt;&gt;rhopi and a1&gt;&gt;K*K. The divergences arising when considering quark loops are regularized by covariant cutoff parameter. Based on four-particle hadronic τ decays, the quark structure of the resonance with quantum numbers JPC=1++ was studied. Theoretical estimates of intermediate channels with the meson for the partial decay widths of tau&gt;&gt;pipipinu, tau&gt;&gt;KKpinu and tau&gt;&gt;KKetanu are obtained. Contributions from contact channels describing the direct production of 3pi, KKpi and KKeta mesons by τ lepton current are taken into account. The interference of contact and intermediate axial-vector channels in determining the integral decay widths is studied. The matrix elements of the processes are obtained in the leading approximation of 1/Nc expansion. It is shown that calculations on τ lepton mesonic decays confirm the quark-antiquark structure of the a1 meson and clarify the role of the intermediate non-strange axial-vector channel in determining the integral partial decay widths. The obtained results are compared with experimental data by BaBar and Belle collaborations at the BEPC II and KEK lepton colliders.

Three-point energy correlators in hadronic Higgs boson decays

We present the analytic calculation of the leading-order three-point energy correlator (EEEC) in hadronic Higgs decays, including both the gluon-initiated channel H→gg+X and quark-initiated channel H→qq¯+X. The phase-space integration is evaluated directly using Mandelstam variables sij=(pi+pj)2, and the appearing square roots can be rationalized by either conformal ratios or celestial coordinate variables. Throughout the calculation, we observe the same transcendental function space as in N=4 super–Yang-Mills theory and e+e−→hadrons. Different infrared limits are also explored using the full analytic result, offering the fixed-order data for EEEC factorization and resummation. Given its nontrivial shape dependence, the EEEC presents an excellent opportunity to explore the dynamics of gluon jets originating from the H→gg decay channel at future lepton colliders. Published by the American Physical Society 2024

Probing dipole operators and four-fermion operators at low-energy lepton colliders

In this study, we explore the potentials of dipole operators and four-fermion operators at low-energy lepton colliders such as Belle II and the Super Tau Charm Facility (STCF). We utilize high-dimension operators to characterize such anomalous interactions, focusing on those that do not interfere with the Standard Model (SM) contributions. With polarized beams, the four-fermion operators and dipole moment operators can be tested with high precision.

Search for R-Parity-Violation-Induced Charged Lepton Flavor Violation at Future Lepton Colliders

Search for R-Parity-Violation-Induced Charged Lepton Flavor Violation at Future Lepton Colliders

From optimal observables to machine learning: an effective-field-theory analysis of e+e−→ W+W− at future lepton colliders

We apply machine-learning techniques to the effective-field-theory analysis of the e+e− → W+W− processes at future lepton colliders, and demonstrate their advantages in comparison with conventional methods, such as optimal observables. In particular, we show that machine-learning methods are more robust to detector effects and backgrounds, and could in principle produce unbiased results with sufficient Monte Carlo simulation samples that accurately describe experiments. This is crucial for the analyses at future lepton colliders given the outstanding precision of the e+e− → W+W− measurement (~ 10−4 in terms of anomalous triple gauge couplings or even better) that can be reached. Our framework can be generalized to other effective-field-theory analyses, such as the one of e+e− → tt¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ t\\overline{t} $$\\end{document} or similar processes at muon colliders.

Probing positivity at the LHC with exclusive photon-fusion processes

By tagging one or two intact protons in the forward direction, it is possible to select and measure exclusive photon-fusion processes at the LHC. The same processes can also be measured in heavy ion collisions, and are often denoted as ultraperipheral collisions (UPC) processes. Such measurements open up the possibility of probing certain dimension-8 operators and their positivity bounds at the LHC. As a demonstration, we perform a phenomenological study on the γγ → ℓ+ℓ− processes, and find out that the measurements of this process at the HL-LHC provide reaches on a set of dimension-8 operator coefficients that are comparable to the ones at future lepton colliders. We also point out that the γq → γq process could potentially have better reaches on similar types of operators due to its larger cross section, but a more detailed experimental study is need to estimate the signal and background rates of this process. The validity of effective field theory (EFT) and the robustness of the positivity interpretation are also discussed.

Secluded scalar dark matter and the muon anomalous magnetic moment

Abstract We consider a dark matter model with a singlet scalar, $\chi$, as our dark matter (DM) candidate which is secluded from the Standard Model (SM) and annihilates to the singlet scalar, $\phi$, via a contact interaction. &amp;#xD;The singlet scalar, $\phi$, has a leptophilic interaction with the SM leptons &amp;#xD;and may decay leptonically at tree level, and decays into a pair of photons at loop level. &amp;#xD;The focus in this work is to consider DM masses below 10 GeV. &amp;#xD;It is found a viable secluded region in the parameter space after imposing the observed &amp;#xD;relic density. There is a one-loop interaction between scalar dark matter and the atomic electron in this model. We then apply the available direct detection bounds from Xenon10, Xenon1T, and DarkSide on the DM-electron elastic scattering cross section. While the model can explain the muon anomalous magnetic moment, we put bounds from current and future lepton collider experiments.

Probing the electromagnetic properties of the neutrinos at future lepton colliders

In this study, we explore the non-standard νν¯γγ couplings parametrized by dimension-seven operators via e+e−→νν¯γ process at the FCC-ee/CEPC and μ+μ−→νν¯γ process at the Muon Colliders. For the detailed Monte Carlo simulation, all signal and relevant background events are produced within the framework of Madgraph where non-standard νν¯γγ couplings are implemented. After passing through Pythia for parton showering and hadronization, detector effects are included via tuned corresponding detector cards for each collider in Delphes. Projected sensitivities on νν¯γγ couplings are obtained at a 5σ confidence level without and with 5% systematic uncertainties for the FCC-ee/CEPC and the Muon Colliders, showcasing the complementarity between lepton colliders. Our best limit on the anomalous νν¯γγ couplings even with 5% systematic uncertainties for muon collider with s=10 TeV and Lint=10 ab−1 are found to be fourteen orders of magnitude stronger than the upper bound obtained from rare decay Z→γγνν¯ analysis using LEP data.