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A bstract Collider processes at the highest available partonic center-of-mass energies — 10 TeV and above — exhibit a new regime of electroweak interactions where electroweak gauge bosons mostly act as quasi-massless partons in vector boson fusion processes. We scrutinize these processes using the Equivalent Vector boson Approximation (EVA) based on its implementation in the Monte Carlo generator framework W hizard . Using a variety of important physics processes, including top pairs, Higgs pairs, neutrino pairs, and vector boson pairs, we study the behavior of processes initiated by transverse and longitudinal vector bosons, both W and Z induced. By considering several distributions for each process, we conclude that: there is no universal, process-independent prescription which minimizes the discrepancies between EVA- and matrix-element-based predictions; even by resorting to process-by-process prescriptions, we typically observe significant observable-dependent effects; the uncertainties associated with parameter dependencies in the EVA can be as large as , and can only possibly be reduced by careful process-dependent kinematical selections.

We present an update of the Universal FeynRules Output model format, commonly known as the UFO format, that is used by several automated matrix-element generators and high-energy physics software. We detail different features that have been proposed as extensions of the initial format during the last ten years, and collect them in the current second version of the model format that we coin the Universal Feynman Output format. Following the initial philosophy of the UFO, they consist of flexible and modular additions to address particle decays, custom propagators, form factors, the renormalisation group running of parameters and masses, and higher-order quantum corrections.

We describe a new parallel approach to the evaluation of phase space for Monte-Carlo event generation, implemented within the framework of the Whizard package. The program realizes a twofold self-adaptive multi-channel parameterization of phase space and makes use of the standard OpenMP and MPI protocols for parallelization. The modern MPI3 feature of asynchronous communication is an essential ingredient of the computing model. Parallel numerical evaluation applies both to phase-space integration and to event generation, thus covering computing-intensive parts of physics simulation for a realistic collider environment.

A bstract We explore the sensitivity of directly testing the muon-Higgs coupling at a high-energy muon collider. This is strongly motivated if there exists new physics that is not aligned with the Standard Model Yukawa interactions which are responsible for the fermion mass generation. We illustrate a few such examples for physics beyond the Standard Model. With the accidentally small value of the muon Yukawa coupling and its subtle role in the high-energy production of multiple (vector and Higgs) bosons, we show that it is possible to measure the muon-Higgs coupling to an accuracy of ten percent for a 10 TeV muon collider and a few percent for a 30 TeV machine by utilizing the three boson production, potentially sensitive to a new physics scale about Λ ∼ 30 − 100 TeV.

This contribution lists challenges of Monte Carlo event generators for future lepton, especially linear colliders. A lot of the recent development benefits from the achievements at the Large Hadron Collider (LHC), but several aspects are unique for lepton colliders like beam simulation, polarization, electroweak higher order corrections and resummed QED corrections. We will describe the status of multi-purpose event generators and specialized codes and outline the challenges for these tools until such a collider starts data taking.

A bstract We present results on NLO electroweak (EW) corrections to multiple massive boson production processes at a future muon collider. Inclusive cross sections with O ( α ) corrections for processes for up to four bosons in the final state as well as differential distributions for HZ production are computed for s = 3, 10 and 14 TeV by using FKS subtraction in the NLO EW automated Monte-Carlo framework WHIZARD+RECOLA. Large logarithmic effects due to collinear ISR and EW virtual correction factors as well as the impacts of an energy cut on hard photons are discussed with an emphasis on the properties of Higgsstrahlung. The potential of a proposed muon collider for studying physics of the EW sector is underlined by the EW corrections significantly affecting observables for processes at high energies and boson multiplicities.

A bstract Neutrinos are among the most mysterious particles in nature. Their mass hierarchy and oscillations, as well as their antiparticle properties, are being intensively studied in experiments around the world. Moreover, in many models of physics beyond the Standard Model, the baryon asymmetry or the dark matter density in the Universe are explained by introducing new species of neutrinos. Among others, heavy neutrinos of Dirac or Majorana nature were proposed to solve open questions in High Energy Physics. Such neutrinos with masses above the electroweak (EW) scale could be produced at future linear e + e − colliders, like the Compact LInear Collider (CLIC) or the International Linear Collider (ILC). We studied the possibility of observing decays of heavy Dirac and Majorana neutrinos in the qqℓ final state with ILC running at 500 GeV and 1 TeV, and CLIC at 3 TeV. The analysis is based on the W hizard event generation and fast simulation of detector response with Delphes. Neutrinos with masses from 200 GeV to 3.2 TeV were considered. We estimated the limits on the production cross sections, interpreted them in terms of the neutrino-lepton coupling parameter V ℓ N 2 (effectively the neutrino mixing angle) and compared them with current limits coming from the LHC running at 13 TeV, as well as the expected limits from future hadron colliders. The limits for the future lepton colliders, extending down to the coupling values of 10 − 7 − 10 − 6 , are stricter than any other limit estimates published so far.

A bstract In this paper we investigate how well the nature of heavy neutral leptons can be determined at a future lepton collider, after its potential discovery. Considered in a simplified model are prompt decays of the neutrino in the mass range from 100 GeV to 10 TeV. We study event selection and application of multivariate analyses to determine whether such a newly discovered particle is of the Dirac or Majorana nature. Combining lepton charge and kinematic event variables, we find that the nature of a heavy neutrino, whether it is a Dirac or a Majorana particle, can be determined at 95% C.L. almost in the whole discovery range. We will briefly speculate about other than the studied channels and the robustness of this statement in more general models of heavy neutral leptons, particularly on the complementarity of high-energy electron-positron vs. muon colliders on resolving the flavor structure of heavy neutrinos.

We study the discrimination power of future multi-TeV muon colliders for a large set of models with extended gauge symmetries and additional neutral gauge bosons (“ Z ′ -models”). Our study is carried out using a χ 2 -analysis of leptonic observables of s-channel scattering in effective Z ′ -models. We make use of angular and chiral asymmetries induced in such models to find the discovery reach of a given muon collider setup in terms of the Z ′ mass and to discriminate between the different scenarios. In this context, we discuss how polarized beams – should they become available at muon colliders – or polarization measurements can help in the discrimination. Our results show that typical muon collider setups which are currently under consideration can give a significantly higher reach compared to existing bounds and projections for high-luminosity LHC.

We present a new study of quasi-elastic W and Z scattering processes in high-energy e + e - collisions, based on and extrapolating the low-energy effective theory which extends the standard model with a 125 GeV Higgs boson. We parameterize deviations in the low-energy range in terms of the dimension-eight operators that arise in the effective theory. Smoothly extending this to higher energy, we study a set of simplified models of new physics in W  /  Z scattering, (1) a structureless extrapolation of the effective theory, and (2) scalar and tensor resonance multiplets. The high-energy asymptotics of all models is regulated by a universal unitarization procedure. This enables us to provide benchmark scenarios which can be meaningfully evaluated off shell and in exclusive event samples, and to determine the sensitivity of an e + e - collider to the model parameters. We analyze the longitudinal vector-boson scattering modes, where we optimize the cuts for the fiducial cross section for different collider scenarios. Here, we choose energy stages of 1.0, 1.4 and 3 TeV, as motivated by the extendability of the ILC project and the staging scenario of the CLIC project.