Lepton Colliders Research Articles

Detector design for a multi-TeV muon collider

The design of a feasible multi-TeV muon collider facility is the mandate of the International Muon Collider Collaboration based at CERN and is considered with great interest along the presently on-going US Snowmass 2021 process. The physics potential of such a novel future collider is overwhelming, ranging from precision measurements to direct searches for new Physics. Despite the machine-design challenges, it gives access to the uncharted territory of leptonic collisions at a center-of-mass energy of 10 TeV or higher with an instantaneous luminosity up to a few 1035 cm−2 s−1. The experiment design, the detector technology choices, and the event reconstruction tools are strongly affected by the presence of the beam-induced background, generated by the interactions of the muon decay products with the machine elements. Full simulation studies at s=1.5 TeV and 3 TeV, adopting the CLIC experiment technologies, represent the starting point to optimize the detector design and propose future dedicated R&D.

Multi-photon Monte Carlo event generator KKMCee for lepton and quark pair production in lepton colliders

We present the KKMCee 5.00.2 Monte Carlo event generator for lepton and quark pair production for the high energy electron-positron annihilation process. It is still the most sophisticated event generator for such processes. Its entire source code is re-written in the modern C++ language. It reproduces all features of the older KKMC code in Fortran 77. However, a number of improvements in the Monte Carlo algorithm are also implemented. Most importantly, it is intended to be a starting point for the future improvements, which will be mandatory for the future high precision lepton collider projects. As in the older version, in addition to higher order QED corrections, it includes so-called O(α1.5) genuine weak corrections using a version of the classic DIZET library and polarized τ decays using TAUOLA program. Both DIZET and TAUOLA external libraries are still in Fortran 77. In addition, a HEPMC3 interface to other MC programs, like parton showers and detector simulation, replaces the older HepEvt interface. The HEPMC3 interface is also exploited in the implementation of the additional photon final state emissions in τ decays using an external PHOTOS library rewritten in C. Program summaryProgram title:KKMCee 5.00.2CPC Library link to program files:https://doi.org/10.17632/7drvvhbw92.1Licensing provisions: GPL-3.0Programming languages:C++, FORTRAN77External routines:CERN ROOT library, PHOTOS, HepMC v.3.0, TAUOLA, PHOTOS, FOAM, HEPMC3Nature of the problem: Fermion pair production is and will be used as an important data source for precise tests of the standard electroweak theory at a high luminosity future circular collider near the Z resonance and above and/or at the future linear lepton colliders of higher energies than those at LEP. The QED corrections to fermion pair production (especially τ leptons) have to be known to at least second order, including spin polarization effects, with 4-5 digit precision. The Standard Model predictions at the sub-permille precision level, taking into account multiple emission of photons for realistic experimental acceptances, can only be obtained using a Monte Carlo event generator. The realistic and precise simulation of τ lepton decays taking into account spin effects is an indispensable ingredient in the Monte Carlo event generator for the fermion pair production process.Solution method: Monte Carlo methods are used to simulate most of the two-fermion final-state processes in e+e− collisions in the presence of multiphoton initial and final state radiation. The multiphoton effects are described in the framework of coherent exclusive exponentiation (CEEX) extending/upgrading the older Yennie–Frautschi–Suura exclusive exponentiation (EEX) scheme. CEEX treats correctly to infinite order not only infrared cancellations but also QED interferences, including suppression of initial-final state interferences for narrow resonances. The matrix element according to the older YFS exponentiation is also implemented for the testing purpose. For τ leptons, the appropriate simulation of a very rich spectrum of the decays is included. Beam polarization and spin effects, both longitudinal and transverse, in tau decays are properly taken into account. Gaussian beam spread and an arbitrary spectrum of the beamstrahlung are also optionally simulated. The present version of the program is rewritten to C++ but in many respects corresponds to its FORTRAN predecessor KKMC v.4.13 [1] with later minor modifications in v. 4.32 [2].Additional comments including restrictions and unusual features: In the present version, electron (Bhabha) and t-quark final states are not included. (It is planned for a future version.) Third-order QED corrections in the leading-logarithmic approximation are included only in the auxiliary older YFS/EEX matrix element. The electroweak corrections should not be trusted above the t-quark threshold. The total cross section for light quarks for s<10 GeV (including narrow resonances) requires an improvement using experimental data. The program does not provide any handles for the beyond the Standard Model (BSM) physics, for instance in the Born Z boson couplings or in the electroweak (EW) formfactors.Running time depends on the CMS energy, final fermion type, upper phase space limit of the photon energy, and whether variable weight events or WT=1 events are generated. On a PC/Linux with a 2.2 GHz processor, producing 100k variable weight events at s=MZ takes 25 sec. of CPU time for μ-pairs and 30 sec. for τ-pairs including decays. At s=189 GeV 100k events with WT=1 costs 1200 sec. for μ-pairs and 830 sec. for τ-pairs (less hard photons).

Electroweak phase transition in 2HDM under Higgs, Z-pole, and W precision measurements

In this work we revisit the existence of a strong first order electroweak phase transition (SFOEWPT) and recent mW precision measurement in the Type-I and Type-II 2HDMs. The mathcal{O} (100) GeV new scalars in 2HDMs are favored by SFOEWPT, which is necessary for electroweak baryogenesis, and observed mW shift as well. We find that under current constraints, both Type-I and Type-II 2HDM can explain the SFOEWPT, Z-pole, Higgs precision measurements and mW precision measurement of CDF-II at same time, and all these precision measurements are sensitive to heavy Higgs mass splitting in 2HDM. The allowed regions are ∆mA/C ∈ (−400, 400) GeV, tan β ∈ (1, 50), and ∆mA/C ∈ (−200, 300) GeV, tan β ∈ (1, 10) for Type-I and Type-II 2HDM respectively. Furthermore future lepton collider measurements on Higgs and Z boson properties can explore this scenario in more detail or even rule out it.

The physics case for a neutrino lepton collider in light of the CDF W mass measurement

We propose a neutrino lepton collider where the neutrino beam is generated from TeV scale muon decays. Such a device would allow for a precise measurement of the W mass based on single W production [Formula: see text]. Although it is challenging to achieve high instantaneous luminosity with such a collider, we find that a total luminosity of 0.1[Formula: see text][Formula: see text] can already yield competitive physics results. In addition to a W mass measurement, a rich variety of physics goals could be achieved with such a collider, including W boson precision measurements, heavy leptophilic gauge boson searches and anomalous [Formula: see text] coupling searches. A neutrino lepton collider is both a novel idea in itself, and may also be a useful intermediate step, with less muon cooling required, towards the muon–muon collider already being pursued by the energy frontier community. A neutrino neutrino or neutrino proton collider may also be interesting future options for the high energy frontier.

Probing top-quark operators with precision electroweak measurements* *Yiming Liu, Cen Zhang and Jiayin Gu are supported by National Natural Science Foundation of China (NSFC) (12035008), Yuhao Wang and Lei Zhang are supported by NSFC (12122507)

In the standard model effective field theory, operators involving the top quark are generally difficult to probe and can generate sizable loop contributions to electroweak precision observables measured by past and future lepton colliders. Could the high precision of electroweak measurements compensate for loop suppression and provide competitive reaches on these operators? Would the inclusion of these contributions introduce too many additional parameters for a meaningful global electroweak analysis to be performed? In this paper, we perform a detailed phenomenological study to address these two important questions. Focusing on eight dimension-6 operators that generate anomalous couplings between electroweak gauge bosons and third-generation quarks, we calculate their one loop contributions to processes, both on and off the Z-pole, and the process. A global analysis is performed with these eight operators and those that contribute to the above processes at tree level using measurements at the LEP, SLC, and several low energy experiments. We find that although current electroweak precision measurements are sensitive to the one-loop effects of top-quark operators, it is difficult to separate them from the operators that contribute at tree level, making a global analysis rather challenging. Under further assumptions (for instance, new physics contributes to only third generation quark operators and the S and T parameters), competitive reaches may be obtained in a global fit. Another important finding of our study is that the two operators that generate the dipole interactions of the bottom quark have a significant impact on the Z-pole measurements and should not be omitted. We also discuss the implications of the recently reported W-boson mass measurement at the CDF for our results. Finally, we estimate the reaches of future lepton colliders in probing top-quark operators with precision electroweak measurements.

Implications of the new CDF II W -boson mass on two-Higgs-doublet models

We present the implications of the recent measurement of $W$ boson at CDF II on the two-Higgs-doublet model (2HDM). In the analysis, we impose theoretical bounds such as vacuum stability and perturbative unitarity, and several experimental constraints. In addition, we take into account the measurement of $\sin^2\theta_W(m_Z)_{\rm \bar{MS}}$ on top of the CDF $W$-boson mass to investigate how the $S$ and $T$ parameters are determined. We explore two possible scenarios depending on whether the Higgs boson observed at the LHC is the lighter or heavier of $CP$-even neutral Higgs bosons for 2HDM type I and II. Using the results, we show how the parameter space is constrained, and compare it with the one based on the PDG average of $m_W$. Furthermore, we explore phenomenological consequences of electroweak precision observables that can be affected by $m_W$ within the predictions of the 2HDM, and the reduction in parameter space expected from future measurements at the Future Circular Lepton Collider.

Secondary Emission Calorimetry

Electromagnetic calorimetry in high-radiation environments, e.g., forward regions of lepton and hadron collider detectors, is quite challenging. Although total absorption crystal calorimeters have superior performance as electromagnetic calorimeters, the availability and the cost of the radiation-hard crystals are the limiting factors as radiation-tolerant implementations. Sampling calorimeters utilizing silicon sensors as the active media are also favorable in terms of performance but are challenged by high-radiation environments. In order to provide a solution for such implementations, we developed a radiation-hard, fast and cost-effective technique, secondary emission calorimetry, and tested prototype secondary emission sensors in test beams. In a secondary emission detector module, secondary emission electrons are generated from a cathode when charged hadron or electromagnetic shower particles penetrate the secondary emission sampling module placed between absorber materials. The generated secondary emission electrons are then multiplied in a similar way as the photoelectrons in photomultiplier tubes. Here, we report on the principles of secondary emission calorimetry and the results from the beam tests performed at Fermilab Test Beam Facility as well as the Monte Carlo simulations of projected, large-scale secondary emission electromagnetic calorimeters.

Energy Resolution Studies in Simulation for the IDEA Dual-Readout Calorimeter Prototype

Precision measurements of Z, W, and H decays at the next generation of circular lepton colliders will require excellent energy resolution for both electromagnetic and hadronic showers. The resolution is limited by event-to-event fluctuations in the shower development, especially in the hadronic system. Compensating for this effect can greatly improve the achievable energy resolution. Furthermore, the resolution can benefit greatly from the use of particle-flow algorithms, which requires the calorimeters to have a high granularity. The approach of dual-readout calorimetry has emerged as a candidate to fulfil both of these requirements by allowing to reconstruct the fluctuations in the shower development event-by-event and offering a high transverse granularity. An important benchmark of such a calorimeter is the electromagnetic energy resolution; a prototype of the IDEA calorimeter has been built for use in testbeams. In parallel, a simulation of this prototype has been developed in Geant4 for a testbeam environment. Here, we outline how this simulation was used to study the electromagnetic energy resolution and conclude that a resolution of 14%/E is achievable.

Including Calorimeter Test Beams in Geant-val—The Physics Validation Testing Suite of Geant4

The Geant4 simulation toolkit is currently adopted by many particle physics experiments, including those at the Large Hadron Collider and the ones proposed for future lepton and hadron colliders. In the present era of precision tests for the Standard Model and increasingly detailed detectors proposed for the future colliders scenario, Geant4 plays a key role. It is required to remain a reliable and stable toolkit for detector simulations and at the same time undergo major improvements in both physics accuracy and computational performance. Calorimeter beam tests involve various particles at different energy scales and represent ideal benchmarks for the physics modeling and assessment of Monte Carlo tools for radiation–matter simulation. We present the first results of a broad validation campaign on test beam data targeting data deployment and preservation with geant-val, the Geant4 validation and testing suite. We investigate the Geant4 capability to model the calorimeter response, energy fluctuations, and shower shapes using data from the ATLAS hadronic end-cap calorimeter and the CALICE silicon-tungsten calorimeter. The evolution over the recent years of the recommended set of physics processes for high-energy physics applications is outlined and compared to alternative models for hadronic interactions.

Development of a Novel Highly Granular Hadronic Calorimeter with Scintillating Glass Tiles

Based on the particle-flow paradigm, a new hadronic calorimeter (HCAL) with scintillating glass tiles is proposed to address major challenges from precision measurements of jets at the future lepton colliders, such as the Circular Electron Positron Collider (CEPC). Tiles of high-density scintillating glass, with a high-energy sampling fraction, can significantly improve the hadronic energy resolution in the low-energy region (typically below 10 GeV for major jet components at Higgs factories). The hadronic energy resolution of single hadrons and the effects of key parameters of scintillating glass have been evaluated in the Geant4 full simulation, followed by the physics benchmark studies on the Higgs boson with jets in the final state. R&amp;D efforts of scintillating glass materials are ongoing within a dedicated collaboration since 2021 with the aim to achieve a high light yield, a high density, and a low cost. Measurements have been performed for the first batches of scintillating glass samples including the light yield, emission and scintillation spectra, scintillation decay times, and cosmic responses. An optical simulation model of a single scintillating glass tile has been established to provide guidance in the development of scintillating glass. Highlights of the expected detector performance and the latest scintillating glass developments are presented in this contribution.

Flux Concentrator Optimization for Future Positron Sources

Future lepton colliders, such as CLIC, FCC-ee or ILC require high quality positron sources. Positrons are created after the collision of electrons on a tungsten target and then focused to match the aperture of the capture structures with a flux concentrator (FC) often called an Adiabatic Matching Device (AMD), a strong pulsed magnet. The FC is a tapered solenoid powered with fast pulses (microsecond) of high current (kA). The current pulse produces a strong magnetic field (3-8 T) at the magnet entrance that rapidly decays (over few cm) to zero. This paper describes the finite element model of the FC and the transient electromagnetic simulation capable of describing the experimental data in terms of current, field and voltage. The computed field map is transferred into particle tracking codes to compute the figure of merit, the positron yield. The coil configuration is optimized to minimize the voltage and Lorentz forces and to maximize the yield. This paper presents the numerical model and the result of the optimization process.

Characterisation of HV-MAPS ATLASPix3 and its applications for future lepton colliders

HV-MAPS are a novel type of CMOS depleted active pixel sensors for ionizing particles, implemented in standard CMOS processes, that have been proposed in several future particle physics experiments for particle tracking. In depleted monolithic sensors, the sensor element is the n-well/p-substrate diode. The sensor matrix and the readout are integrated in one single piece of silicon and the electronics is embedded in shallow wells inside deep n-wells, isolated from the substrate. High voltage biasing increases the depth of the depletion region, improving sensor properties as signal amplitude, charge collection speed and radiation tolerance. ATLASPix3 is the first full reticle size high voltage Monolithic Active Pixel CMOS sensor, designed to meet the specifications of the outer layers of the ATLAS inner tracker (ITk). Its thin design, the excellent position resolution, high readout rate and high radiation tolerance make ATLASPix3 an ideal candidate for large-area tracking detector R&D of future collider experiments such as the Circular Electron Positron Collider (CEPC) silicon tracker.

Crystal-based pair production for a lepton collider positron source

An intense positron sources is a demanding element in the design of future lepton colliders. A crystal-based hybrid positron source could be an alternative to a more conventional scheme based on the electron conversion into positron in a thick amorphous target. The conceptual idea of the hybrid source is to have two separate objects, a photon radiator and a photon-to-positron converter target. In such a scheme an electron beam crosses a thin axially oriented crystal with the emission of a channeling radiation, characterized by a considerably larger amount of photons if compared to Bremsstrahlung. The net result is an increase in the number of produced positrons at the converter target. In this paper we present the results of a beam test conducted at the DESY TB 21 with 5.6 GeV electron beam and a crystalline tungsten radiator. Experimental data clearly highlight an increased production of photons and they are critically compared with the outcomes of novel method to simulate the number of radiated photons, showing a very good agreement. Strong of this, the developed simulation tool has been exploited to design a simple scheme for a positron source based on oriented crystal, demonstrating the advantages in terms of reduction of both deposited energy and the peak energy deposition density if compared to conventional sources. The presented work opens the way for a realistic and detailed design of a hybrid crystal-based positron source for future lepton colliders.

Deeply learned preselection of Higgs dijet decays at future lepton colliders

Future electron-positron colliders will play a leading role in the precision measurement of Higgs boson couplings which is one of the central interests in particle physics. Aiming at maximizing the performance to measure the Higgs couplings to the bottom, charm and strange quarks, we develop machine learning methods to improve the selection of events with a Higgs decaying to dijets. Our methods are based on the Boosted Decision Tree (BDT), Fully-Connected Neural Network (FCNN) and Convolutional Neural Network (CNN). We find that the BDT and FCNN algorithms outperform the conventional cut-based method. With our improved selection of Higgs decaying to dijet events using the FCNN, the charm quark signal strength is measured with a 16% error, which is roughly a factor of two better than the 34% precision obtained by the cut-based analysis. Also, the strange quark signal strength is constrained as μss≲35 at the 95% C.L. with the FCNN, which is to be compared with μss≲70 obtained by the cut-based method.

Probing Triple Higgs Self-Coupling and Effect of Beam Polarization in Lepton Colliders

One of the main objectives of almost all future (lepton) colliders is to measure the self-coupling of triple Higgs in the Standard Model. By elongating the Standard Model’s scalar sector, using incipient Higgs doublet along with a quadratic (Higgs) potential can reveal many incipient features of the model and the possibility of the emergence of additional Higgs self-couplings. The self-coupling of the Higgs boson helps in reconstructing the scalar potential. The main objective of this paper is to extract Higgs self-coupling by numerically analyzing several scattering processes governed by two Higgs doublet models (2HDM). These scattering processes include various possible combinations of final states in the triple Higgs sector. The determination of production cross-section of scattering processes is carried out using two different scenarios, one with and other without polarization of incoming beam, and is extended to a center of mass energy up to s = 3 TeV . The computation is carried out in type-1 2HDM. Here, we consider the case of exact alignment limit ( s β α =1) and masses of extra Higgs states are equal, that is, m H = m H 0 = m A 0 = m H ± . This choice minimizes the oblique parameters. The decays of the final state of each process are investigated to estimate the number of events at an integrated luminosity of 1 a b − 1 and 3 a b − 1 .

Meter-scale plasma waveguides for multi-GeV laser wakefield acceleration

We present results from two new techniques for the generation of meter-scale, low density (∼1017 cm−3 on axis) plasma waveguides, the “two-Bessel” technique, and the “self-waveguiding” technique. Plasma waveguides of this density and length range are needed for demonstration of a ∼10 GeV laser wakefield accelerator module, key for future staging for a ∼TeV lepton collider. Both techniques require the use of high quality ultrashort pulse Bessel beams to efficiently and uniformly ionize hydrogen gas in meter-scale supersonic gas jets via optical field ionization. We review these two techniques, describe our meter-scale gas jets, and present a new method for correction of optical aberrations in Bessel beams. Finally, we briefly present results from recent experiments employing one of our techniques, demonstrating quasi-monoenergetic acceleration of ∼5 GeV electron bunches in 20 cm long, low density plasma waveguides.

Energy reconstruction of hadronic showers at the CERN PS and SPS using the Semi-Digital Hadronic Calorimeter

The CALICE Semi-Digital Hadronic CALorimeter (SDHCAL) is the first technological prototype in a family of high-granularity calorimeters developed by the CALICE Collaboration to equip the experiments of future lepton colliders. The SDHCAL is a sampling calorimeter using stainless steel for absorber and Glass Resistive Plate Chambers (GRPC) as a sensitive medium. The GRPC are read out by 1 cm× 1 cm pickup pads combined to a multi-electronics. The prototype was exposed to hadron beams in both the CERN PS and the SPS beamlines in 2015 allowing the test of the SDHCAL in a large energy range from 3 GeV to 80 GeV. After introducing the method used to select the hadrons of our data and reject the muon and electron contamination, we present the energy reconstruction approach that we apply to the data collected from both beamlines and we discuss the response linearity and the energy resolution of the SDHCAL. The results obtained in the two beamlines confirm the excellent SDHCAL performance observed with the data collected with the same prototype in the SPS beamline in 2012. They also show the stability of the SDHCAL in different beam conditions and different time periods.

Unambiguously Testing Positivity at Lepton Colliders.

The diphoton channel at lepton colliders, e^{+}e^{-}(μ^{+}μ^{-})→γγ, has a remarkable feature that the leading new physics contribution comes only from dimension-eight operators. This contribution is subject to a set of positivity bounds, derived from the fundamental principles of quantum field theory, such as unitarity, locality, analyticity and Lorentz invariance. These positivity bounds are thus applicable to the most direct observable: the diphoton cross section. This unique feature provides a clear, robust, and unambiguous test of these principles. We estimate the capability of various future lepton colliders in probing the dimension-eight operators and testing the positivity bounds in this channel. We show that positivity bounds can lift certain flat directions among the effective operators and significantly change the perspectives of a global analysis. We also discuss the positivity bounds of the Zγ/ZZ processes which are related to the γγ ones, but are more complicated due to the massive Z boson.

Noble liquid calorimetry for a future FCC-ee experiment

Noble liquid calorimetry is a well proven technology that successfully operated in numerous particle physics detectors (D0, H1, NA48, NA62, ATLAS, …). Its excellent linearity, stability, uniformity and radiation hardness as well as very good energy and time resolution make it a strong candidate for future hadron and lepton colliders. Recently, a highly granular noble liquid sampling calorimeter was proposed for a possible FCC-hh experiment. It has been shown that, on top of its intrinsic excellent conventional calorimetry performance, this technology can be optimized in terms of granularity to allow for 4D imaging, machine learning and – in combination with the tracker measurements – particle-flow reconstruction. This paper discusses how such a detector can be adapted to an electron–positron collider (FCC-ee) experiment, shows that a signal over noise ratio above five can be reached while keeping the cross-talk at the percent level, presents a possible signal extraction scheme and reports on the first performance studies derived with the Key4hep full simulation framework.

Triple Higgs couplings in the 2HDM: the complete picture

The measurement of the triple Higgs coupling is one of the main tasks of the (HL-)LHC and future lepton colliders. Similarly, triple Higgs couplings involving BSM Higgs bosons are of high interest. Within the framework of Two Higgs Doublet Models (2HDM) we investigate the allowed ranges for all triple Higgs couplings involving at least one light, SM-like Higgs boson. We present newly the allowed ranges for 2HDM type III and IV and update the results within the type I and II. We take into account theoretical constraints from unitarity and stability, experimental constraints from direct BSM Higgs-boson searches, measurements of the rates of the SM-like Higgs-boson at the LHC, as well as flavor observables and electroweak precision data. For the SM-type triple Higgs coupling w.r.t. its SM value, lambda _{hhh}/lambda _{mathrm{SM}}, we find allowed intervals of sim [{-0.5}, {1.3}] in type I and sim [{0.5}, {1.0}] in the other Yukawa types. These allowed ranges have important implications for the experimental determination of this coupling at future collider experiments. We find the coupling lambda _{hhH} between sim -1.5 and sim +1.5 in the four Yukawa types. For the triple Higgs couplings involving two heavy neutral Higgs bosons, lambda _{hHH} and lambda _{hAA} we find values between sim -0.5 and sim 16, and between sim -1 and sim 32 for lambda _{hH^+H^-}. These potentially large values could lead to strongly enhanced production of two Higgs-bosons at the HL-LHC or high-energy lepton colliders.