arXiv subjects

Z. Was

Publications and source records attributed to Z. Was.

At least 19 recordsLinked to original sources

arXiv2026

Spin correlations in $τ$-lepton pair production due to anomalous magnetic and electric dipole moments

We present a simple algorithm for the calculation of event weights embedding the effects of anomalous electric and magnetic dipole moments in simulation of $e^-e^+\to τ^-τ^+ (nγ)$ events, and the subsequent decay of the $τ$ leptons produced. The impact of these weights on the spin-correlation matrix and the total cross section is taken into account. The algorithm is prepared to work in situ the Monte Carlo KKMC program without the need for introducing any external change to the generator libraries. As an example, $e^-e^+ \to τ^-τ^+ (nγ), \; τ^- \to ρ^- ν_τ\to π^- π^0 ν_τ, \ τ^+ \to ρ^+ \barν_τ\to π^+ π^0 \barν_τ$ events were simulated at a center-of-mass energy of 10.58 GeV. The distributions of the acoplanarity angle between the planes spanned by the $π^- π^0$ and the $π^+ π^0$ momenta of, respectively, $ρ^-$ and $ρ^+$ decays and in the rest frame of the entirely visible $ρ^-ρ^+$ system are presented for different values of the coupling constants incorporating anomalous electric and magnetic dipole moments in the $τ^- τ^+ γ$ vertex.

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arXiv2026

FCC precision requests: challenges for Monte Carlos and phenomenology tools

One can define three levels of ambiguities for the accelerator experiments: worse than 0.3%, around 0.3% and better than 0.3%. I will address issues important, for the last case, when Monte Carlo programs that simultaneously account for theoretical and experimental effects are necessary. To complete efforts, simultaneous effort in many specialities over years was necessary. The monumental projects of Bryan Lynn, Robin Stuart, Dima Bardin, Wolfgang Hollik and contributions of S. Jadach in the domain of precision physics need to be mentioned, they provide theoretical foundations. Incomplete lists of methodology domains and projects, usually left into references, appendices, and private notes include: (i) Phase space--> symmetries (ii) matrix element preparation --> factorizations (iii) program and development process design (iv) testing strategies (v) user interaction (vi) software tools (vii) partners and competitors. Usually we were publishing our own projects, let me mention incomplete lists of methodology domains and projects, which were usually left aside into references, appendices, and private notes: The work started on the basis of previous efforts which can be listed following names of the programs: (i) FOWL, (ii) GENRAP, (iii) Mustraal, (iv) Koralb, (v) Lesko, (vi) Tauola, (vii) KoralZ, (viii) Lumlog, (ix) Oldbab, (x) Bhlumi, (xi) Bhwide and (xii) KKMC. I will focus on some of these points. Others, hopefully, are covered in other talks. In particular I do not need to cover exponentiation and some issues of factorization; see contributions to the proceedings by B.F.L. Ward and A. Tapadar. Developments took years and did not follow a straight line; that is why there are simplifications, biases. Also a review of literature could not be completed.

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arXiv2026

Recent Developments in IR-Improved Amplitude-Based Resummation in Precision High Energy Collider Physics

We present recent developments in precision high energy collider physics based on the IR-improvement of unintegrable singularities in the infrared regime via amplitude-based resummation in $QED\times QCD \subset SU(2)_L \times U_1 \times SU(3)^c$. We focus on specific applications relevant to precision observables in LHC/FCC, LC, CLIC, CEPC, and CPPC physics, for which we present new results and some new issues.

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arXiv2026

TauSpinner algorithms for including spin and New Physics effects in $\bar q q \rightarrow Z/γ^* \to ττ$ process

The possible anomalous New Physics contributions to dipole and weak dipole moments of the $τ$ lepton bring renewed interest in development and revisiting charge-parity violating signatures in $τ$-pair production in $Z$-boson decay at energies of the LHC. In this paper, we discuss effects of anomalous contributions to polarisation and spin correlations in the $\bar q q \to τ^+ τ^-$ production processes, with $τ$ decays included. Because of the complex nature of the resulting distributions, Monte Carlo techniques are useful, in particular of event reweighing with studied New Physics phenomena. Extensions of the Standard Model spin amplitudes, within Improved Born Approximation used for matrix element, are implemented in the TauSpinner program. This is mainly with $τ$ dipole and weak dipole moments in mind, but is applicable to arbitrary New Physics interactions, provided they can be encapsulated into the Standard Model $2 \to 2$ structure of matrix element extensions. Implementation allows one also to introduce arbitrary phase-shift between vector and axial-vector couplings of $Z$ boson to $τ$ leptons, which would have impact on observed transverse spin correlations. Basic formulas and algorithm principles are presented, together with distributions for spin correlation matrix. Numerical examples of impact on experimental signatures are shown in case of $τ^\pm \to ρ^\pm ν_τ\to π^\pm π^0 ν_τ$ decays. Information on how to use and configure the TauSpinner program is given in Appendix.

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arXiv2025

TauSpinner algorithms for including spin and New Physics effects in $γγ\rightarrow ττ$ process

The possible anomalous New Physics contributions to electric and magnetic dipole moments of the $τ$ lepton has brought renewed interest in development of new charge-parity violating signatures in the $τ$-pair production at Belle II energies, and also at higher energies of the LHC and the FCC. In this paper, we discuss effects of anomalous contributions to cross-section and spin correlations in the $γγ\to τ^-τ^+$ production processes, with $τ$ decays included. Such processes have been observed in the $pp$ and PbPb collisions at CERN LHC experiments. Because of complex nature of the resulting distributions, Monte Carlo techniques are useful, in particular of event reweighting with studied New Physics phenomena. For the $γγ$ processes, extensions of the Standard Model amplitudes are implemented in the TauSpinner program. This is mainly with electric and magnetic dipole moments in mind, but algorithm can be easily extended to other New Physics interactions, provided they can be encapsulated into similar form-factors in the Standard Model structure of matrix elements. Basic formulas and algorithm principles are presented, numerical examples are provided as illustration. Information on how to use the program is given in Appendix of the paper.

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arXiv2024

Monte Carlos for tau lepton -- Standard Model and New Physics signatures

One of purposes for High Energy accelerator experiments is confrontation of theory and measurements in ever new realms. Any new agreement extends theory applicability domain, any discrepancy hints to unexplained. That calls for better calculations or new, deeper theory. Often one has to search for small contributions over large Standard Model background. Multi-dimensional signatures and complex background subtraction cuts imply that Monte Carlo techniques are indispensable. My talk included description of: KKMC Monte Carlo for e+e- to tau+tau- (n gamma) (with tau decays), generation of additional lepton pairs of SM and New Physics, and of anomalous magnetic and electric dipole moments effects.

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arXiv2024

Monte Carlo Event Generator updates, for tau pair events at Belle II energies

The Monte Carlo for lepton pair production and tau decays consist of KKMC for lepton pair production, tauola for tau lepton decays and photos for radiative corrections in decays. An effort for adaptation of the system for precision data to be collected at Belle II experiment lead to extension of phase space generation modules both in photos and tauola to enable decays and/or radiative corrections of additional light lepton pairs. The phase-space and matrix element parts of the programs are separated, that is why extension to processes where lepton pair is produced through narrow resonances, like dark photon, was straight forward. In the present version of tauola, list of tau decay channels is enriched with multitude of exotic ones, useful for searches of new physics. The hadronic currents parametrizations of main decay channels is prepared for basic simulation in the experiment. The basis for future work on precise fits of hadronic currents including Machine Learning is retained, but development of necessary software solutions is left for the forthcoming years. Presented programs versions are available in stand-alone format from gitlab or through the basf2 system of Belle II software. Official distribution web pages, documenting programs tests, are retained but not necessarily up to the date.

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arXiv2024

Pseudo-observables and Deep Neural Network for mixed CP -- H to tau tau decays at LHC

The consecutive steps of H to tau tau cascade can be useful for the measurement of Higgs couplings and parity. The analysis methos of ATLAS and CMS Collaborations was to fit a one-dimensional distribution of the phi* angle, phi*, which is sensitive to transverse spin correlations and, hence, to the CP mixing angle, phi^CP. Machine Learning techniques (ML) offer opportunities to manage complex multidimensional signatures. The 4-momenta of the tau decay products can be used as input to the machine learning and to predict the CP-sensitive pseudo-observables and/or provide discrimination between different CP hypotheses. We show that the classification or regression methods can be used to train an ML model to predict the spin weight sensitive to the CP state of the decaying Higgs boson, parameters of the functional form of the spin weight, or the most preferred CP mixing angle of the analysed sample. The one-dimensional distribution of the predicted spin weight or the most preferred CP mixing angle of the experimental data can be examined further, with the statistical methods, to derive the measurement of the CP mixing state of the Higgs signal events. This paper extends our previous studies, with more details for the features and how proposed pseudo-observables can be used in the measurement.

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arXiv2024

Amplitude-Based IR-Improvement in Precision LHC$/$FCC Physics

We present recent results based on the IR-improvement of unintegrable singularities in the infrared regime via amplitude-based resummation in $QED \times QCD \subset SU(2)_L \times U_1 \times SU(3)_c$. In the context of precision LHC$/$FCC physics, we focus on specific examples, such as the removal of QED contamination in PDFs evolved from data at ${Q_0}^2 \sim 2\; {\text{GeV}}^2$ and used in the evaluating precision observables in $pp \rightarrow Z + X \rightarrow \ell\bar{\ell} + X'$, in which we discuss new results and new issues.

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arXiv2024

Collinearly Enhanced YFS MC Approach to Precision High Energy Collider Physics

We improve the YFS IR resummation theory so that it includes all of the attendant collinear contributions which exponentiate. The attendant new resummed contributions are shown to agree with known results from the collinear factorization approach. We argue that they improve the corresponding precision tag for a given level of exactness in the respective YFS hard radiation residuals as the latter are realized in the YFS MC approach to precision high-energy collider physics.

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arXiv2024

$τ$-lepton pair spin in proton-proton LHC collisions for anomalous dipole moments

The spin correlations in the pair of $τ$ leptons produced in the high-energy proton-proton collisions are studied. The invariant mass of the $τ$-lepton pair is chosen close to $Z$-boson mass, in these conditions $Z$-boson exchange gives the dominant contribution to the Drell-Yan mechanism. The interaction of $Z$ boson with $τ$ lepton, in addition to the Standard Model couplings, can include weak anomalous magnetic and electric dipole moments (or form-factors) of the $τ$ lepton. Dependence of elements of the $pp$ spin-correlation matrix on the weak anomalous moments is determined for various regions of the rapidity of the $τ$-lepton pair. Based on this calculation, we construct a semi-realistic observable built on the momenta of pions in the $τ^\mp \to π^\mp ν_τ$ and $τ^\pm \to ρ^\pm ν_τ\to π^\pm π^0 ν_τ$ decay channels, and show that this observable can be sensitive to the weak anomalous dipole moments.

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arXiv2024

Electron-positron, parton-parton and photon-photon production of $τ$-lepton pairs: anomalous magnetic and electric dipole moments spin effects

Anomalous contributions to the electric and magnetic dipole moments of the $τ$ lepton from new physics scenarios have brought renewed interest in the development of new charge-parity violating signatures in $τ$-pair production at Belle II energies, and also at higher energies of the Large Hadron Collider and the Future Circular Collider. In this paper, we discuss the effects of spin correlations, including transverse degrees of freedom, in the $τ$-pair production and decay. These studies include calculating analytical formulas, obtaining numerical results, and building semi-realistic observables sensitive to the transverse spin correlations induced by the dipole moments of the $τ$ lepton. The effects of such anomalous contributions to the dipole moments are introduced on top of precision simulations of $e^-e^+ \to τ^-τ^+$, $q\bar{q} \to τ^-τ^+$ and $γγ\to τ^-τ^+$ processes, involving multi-body final states. The $τ$ decays are simulated along with radiative corrections, in particular electroweak box contributions of $WW$ and $ZZ$ exchanges are taken into account. Respective extensions of the Standard Model amplitudes and the reweighting algorithms are implemented into the KKMC Monte Carlo, which is used to simulate $τ$-pair production in $e^-e^+$ collisions, and the TauSpinner program, which is used to reweight events with $τ$ pair produced in $pp$ collisions.

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arXiv2023

Evaluation of Photos Monte Carlo ambiguities in case of four fermion final states

With the increasing precision requirements and growing spectrum of applications of Monte Carlo simulations the evaluation of different components of such simulations and their systematic ambiguities become of utmost interest. In the following, we will address the question of systematic errors for Photos Monte Carlo for simulation of bremsstrahlung corrections in final states, which can not, in principle, be identified as a decay of resonances. It is possible, because the program features explicit and exact parametrization of phase space for multi-body plus multi-photon final states. The Photos emission kernel for some processes consist of complete matrix element, in the remaining cases appropriate approximation is used. Comparisons with results of simulations, from generators based on exact phase space and exact fixed order matrix elements, can be used. For the purpose of such validations Photos provides an option to restrict emissions to single photon only. In the current work we concentrate on final state bremsstrahlung in $q \bar{q}(e^+e^-) \to l^+l^- l^+l^- γ$ and $γγ\to l^+l^- γ$ processes. The reference distributions used as a cross-check are obtained from the fixed-order MadGraph Monte Carlo simulations. For the purpose of validation we concentrate on those phase space regions where Photos is not expected to work on the basis of its design alone. These phase space regions of hard, non-collinear photons, do not contribute to large logarithmic terms. We find that in these phase space regions the differences between Photos and MadGraph results do not surpass a few percent and these regions, in turn, contribute about 10% to the observed process rates. This is encouraging in view of the possible ambiguities for precise calculation of realistic observables.

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arXiv2022

TAUOLA update for decay channels with $e^+e^-$ pairs in the final state

With the arrival of high luminosity B-factories like the Belle II experiment, $τ$ decay measurements have become more precise than ever, allowing rarer processes to be explored, and finer details of $τ$ decays to be studied. These are important to understand the spectrum of intermediate particles produced in $τ$ decays. Therefore Monte Carlo generators, like the TAUOLA program, have to facilitate precision analysis as well as confront new models that constantly emerge with the availability of high statistics experimental data. New decay channels and models may lead to large variation of matrix elements size within the available phase space, as in the case when $e^+e^-$ pairs are present in final state. It requires appropriate presampler of the phase space generator and a proper documentation to help users introduce their own models. While releasing a new update, it is important for the TAUOLA Monte Carlo library to maintain the general structure of the previous versions to preserve backward compatibility. The aim is to minimize changes from the user perspective. This paper presents a demonstration of new models implementation facilitated by the current update.

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arXiv2022

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

We present the {\tt 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 \order{α^{1.5}} genuine weak corrections using a version of the classic {\tt DIZET} library and polarized $τ$ decays using {\tt TAUOLA} program. Both {\tt DIZET} and {\tt TAUOLA} external libraries are still in Fortran 77. In addition, a {\tt HEPMC3} interface to other MC programs, like parton showers and detector simulation, replaces the older {\tt HepEvt} interface. The {\tt HEPMC3} interface is also exploited in the implementation of the additional photon final state emissions in $τ$ decays using an external {\tt PHOTOS} library rewritten in C.

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arXiv2022

IR-Improved Amplitude-Based Resummation in Quantum Field Theory: New Results and New Issues

With the advancement of strategies for the precision physics programs for the HL-LHC, FCC-ee, FCC-hh, ILC, CLIC, CEPC, and CPPC, the need for proper control of the attendant theoretical precision tags is manifest. We discuss the role that amplitude-based resummation may play in this regard with examples from the LHC, the proposed new colliders and quantum gravity.

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arXiv2022

Mini-Proceedings of the STRONG2020 Virtual Workshop on "Space-like and Time-like determination of the Hadronic Leading Order contribution to the Muon $g-2$"

The mini-proceedings of the STRONG2020 Virtual Workshop "Space-like and Time-like determination of the Hadronic Leading Order contribution to the Muon $g-2$", November 24--26 2021, are presented. This is the first workshop of the STRONG2020 WP21: JRA3-PrecisionSM: Precision Tests of the Standard Model (http://www.strong-2020.eu/joint-research-activity/jra3-precisionsm.html). The workshop was devoted to review of the working group activitity on: $(\it i)$ Radiative Corrections and Monte Carlo tools for low-energy hadronic cross sections in $e^+ e^-$ collisions; $(\it ii)$ Annotated database for $e^+e^-$ into hadrons processes at low energy; $(\it iii)$ Radiative Corrections and Monte Carlo tools for $μ$-$e$ elastic scattering.

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arXiv2021

The tau lepton Monte Carlo Event Generation -- imprinting New Physics models with exotic scalar or vector states into simulation samples

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