arXiv subjects

E. Richter-Was

Publications and source records attributed to E. Richter-Was.

At least 19 recordsLinked to original sources

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

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

$τ$-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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arXiv2021

Adequacy of Effective Born for electroweak effects and TauSpinner algorithms for LEP, Tevatron, HL-LHC and FCC simulated samples

Matching and comparing measurements of past and future experiments calls for consistency checks of calculations used for their interpretation. New schemes of the field theory may optimize precision, but obscure comparisons with earlier results. Over years concepts of Improved Born, Effective Born, effective couplings, and sin^2theta_W^eff mixing angle for electroweak interactions, have evolved. We use four DIZET electroweak library versions; of today and of the last 30 years, wich were used for phenomenology of practically all HEP accelerator experiments. Versions differ by incremented with time updates of corrections. We rely on the codes archived with the KKMC Monte Carlo program for e^+e^- to f bar f n(gamma). Versions became recently available for the TauSpinner reweighting of simulated and stored events. Documentation of TauSpinner upgrade, to version 2.1.0, and of its arrangement for semi-automated electroweak (EW) effects benchmark plots are provided. Details which can be used to complete simulated sample with EW effects, are given. Focus is placed on numerical results and on approximations introduced in Improved Born to obtain Effective Born, to match with QCD corrections. The tau lepton polarization P_tau, forward backward asymmetry A_FB and parton level cross section sigma^tot are used to monitor electroweak effects and effective sin^2theta_W^eff limitations. Results include: (i) feasibility of Effective Born approximation and sin^2theta_W^eff, (ii) differences between versions of electroweak libraries and (iii) parametric ambiguities due to e.g. m_t or Delta alpha_h^(5)(s). Example results allow to evaluate adequacy of Effective Born with respect to Improved Born.

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arXiv2020

Deep Neural Network application: Higgs boson CP state mixing angle in H to tau tau decay and at LHC

The consecutive steps of cascade decay initiated by H to tau tau can be useful for the measurement of Higgs couplings and in particular of the Higgs boson parity. In the previous papers we have found, that multi-dimensional signatures of the tau^pm to pi^pm pi^0 nu and tau^pm to 3pi^pm nu decays can be used to distinguish between scalar and pseudoscalar Higgs state. The Machine Learning techniques (ML) of binary classification, offered break-through opportunities to manage such complex multidimensional signatures. The classification between two possible CP states: scalar and pseudoscalar, is now extended to the measurement of the hypothetical mixing angle of Higgs boson parity states. The functional dependence of H to tau tau matrix element on the mixing angle is predicted by theory. The potential to determine preferred mixing angle of the Higgs boson events sample including $τ$-decays is studied using Deep Neural Network. The problem is adressed as classification or regression with the aim to determine the per-event: a) probability distribution (spin weight) of the mixing angle; b) parameters of the functional form of the spin weight; c) the most preferred mixing angle. Performance of methods are evaluated and compared. Numerical results are collected.

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arXiv2019

Machine learning classification: case of Higgs boson CP state in H to tau tau decay at LHC

Machine Learning (ML) techniques are rapidly finding a place among the methods of High Energy Physics data analysis. Different approaches are explored concerning how much effort should be put into building high-level variables based on physics insight into the problem, and when it is enough to rely on low-level ones, allowing ML methods to find patterns without explicit physics model. In this paper we continue the discussion of previous publications on the CP state of the Higgs boson measurement of the H to tau tau decay channel with the consecutive tau^pm to rho^pm nu; rho^pm to pi^pm pi^0 and tau^pm to a_1^pm nu; a_1^pm to rho^0 pi^pm to 3 pi^pm cascade decays. The discrimination of the Higgs boson CP state is studied as a binary classification problem between CP-even (scalar) and CP-odd (pseudoscalar), using Deep Neural Network (DNN). Improvements on the classification from the constraints on directly non-measurable outgoing neutrinos are discussed. We find, that once added, they enhance the sensitivity sizably, even if only imperfect information is provided. In addition to DNN we also evaluate and compare other ML methods: Boosted Trees (BT), Random Forest (RF) and Support Vector Machine (SVN).

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arXiv2019

Monte Carlo, fitting and Machine Learning for Tau leptons

Status of tau lepton decay Monte Carlo generator TAUOLA, and its main recent applications are reviewed. It is underlined, that in recent efforts on development of new hadronic currents, the multi-dimensional nature of distributions of the experimental data must be taken with a great care. Studies for H to tau tau; tau to hadrons indeed demonstrate that multi-dimensional nature of distributions is important and available for evaluation of observables where tau leptons are used to constrain experimental data. For that part of the presentation, use of the TAUOLA program for phenomenology of H and Z decays at LHC is discussed, in particular in the context of the Higgs boson parity measurements with the use of Machine Learning techniques. Some additions, relevant for QED lepton pair emission and electroweak corrections are mentioned as well.

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arXiv2019

The TauSpinner approach for electroweak corrections in LHC Z to ll observables

The LHC enters era of the Standard Model Z-boson couplings precise measurements, to match precision of LEP. The calculations of electroweak (EW) corrections in the Monte Carlo generators become of relevance. Precise predictions of Z-boson production and decay require classes of QED/EW/QCD corrections, preferably in the manner which allows for separation from the QCD dynamics of the production. At LEP, calculations, genuine weak and lineshape corrections were introduced into electroweak form-factors and Improved Born Approximation. This was well suited for so-called doubly-deconvoluted observables around the Z-pole; observables for which the initial- and final-state QED real and virtual emissions are treated separately or integrated over. This approach to EW corrections is followed for LHC pp collisions. We focus on the EW corrections to doubly-deconvoluted observables of Z to ll process, in a form of per-event weight and on numerical results. The reweighting technique of TauSpinner package is revisited and the program is enriched with the EW sector. The Dizet library, as interfaced to KKMC Monte Carlo of the LEP era, is used to calculate O(alpha) weak loop corrections, supplemented by some higher-order terms. They are used in the form of look-up tables by the TauSpinner package. The size of the corrections is evaluated for the following observables: the Z-boson resonance line-shape, the outgoing leptons forward-backward asymmetry, effective leptonic weak mixing angles and the lepton distribution spherical harmonic expansion coefficients. Evaluation of the EW corrections for observables with simplified calculations based on Effective Born of modified EW couplings, is also presented and compared with the predictions of Improved Born Approximation where complete set of EW form-factors is used.

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arXiv2019

LHC Higgs CP Sensitive Observables in H to tau^+ tau^-; tau^pm to (3pi)^pm nu and Machine Learning Benefits

In phenomenological preparation for new measurements one searches for the carriers of quality signatures. Often, the first approach quantities may be difficult to measure or to provide sufficiently precise predictions for comparisons. Complexity of necessary details grow with precision. To achieve the goal one can not break the theory principles, and take into account effects which could be ignored earlier. Mixed approach where dominant effects are taken into account with intuitive even simplistic approach was developed. Non dominant corrections were controlled with the help of Monte Carlo simulations. Concept of Optimal Variables was successfully applied for many measurements. New techniques, like Machine Learning, offer solutions to exploit multidimensional signatures. Complementarity of these new and old approaches is studied for the example of Higgs Boson CP-parity measurements in H to tau^+tau^-, tau^pm \to nu (3pi)^pm cascade decays.

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arXiv2018

Documentation of TauSpinner algorithms -- program for simulating spin effects in tau-lepton production at LHC

The tau-lepton plays an important role in the physics program at the Large Hadron Collider (LHC). It offers a probe in searches for New Physics and to measure parameters of the Standard Model. Spin of tau lepton is an interesting phenomenological quantity for separation of signal from background or in measuring properties of particles. A proper treatment of tau spin effects in the Monte Carlo simulations is important for understanding the detector acceptance as well as for the measurements/use of tau polarization and tau spin correlations. The TauSpinner is a tool to modify tau$spin effects in any sample. Also production matrix elements can be modified with its help. The general principle of TauSpinner algorithm rely on the kinematic of outgoing particles (or jets) only, and to average over all possible initial states accordingly to assumed parton distributions and cross-sections. Incoming on mass-shell parton four-momenta are reconstructed in approximation, but energy-momentum conservation is obeyed. Samples of events featuring tau leptons with their decay products, can be used as an input. The information on the polarization and spin correlations, and production/decay matrix elements is reconstructed from the kinematics of the tau lepton(s) (also nu_tau in case of W-mediated processes) and tau decay products (also jets). No other information is required. By calculating spin (or production/decay matrix element) weights, attributed to each event, it enables evaluation of modifications without regenerating events. Elements of program functionalities together with explanation of the use are given in Appendices. Present publication collect program documentation, previously scattered over several publiications.

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arXiv2018

Production of tau tau jj final states at the LHC and the TauSpinner algorithm: the spin-2 case

The TauSpinner algorithm is a tool that allows to modify the physics model of the Monte Carlo generated samples due to the changed assumptions of event production dynamics, but without the need of re-generating events. With the help of weights $τ$-lepton production or decay processes can be modified accordingly to a new physics model. In a recent paper a new version TauSpinner ver.2.0.0 has been presented which includes a provision for introducing non-standard states and couplings and study their effects in the vector-boson-fusion processes by exploiting the spin correlations of $τ$-lepton pair decay products in processes where final states include also two hard jets. In the present paper we document how this can be achieved taking as an example the non-standard spin-2 state that couples to Standard Model particles and tree-level matrix elements with complete helicity information included for the parton-parton scattering amplitudes into a $τ$-lepton pair and two outgoing partons. This implementation is prepared as the external (user provided) routine for the TauSpinner algorithm. It exploits amplitudes generated by MadGraph5 and adopted to the TauSpinner algorithm format. Consistency tests of the implemented matrix elements, reweighting algorithm and numerical results for observables sensitive to $τ$ polarization are presented.

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arXiv2016

W production at LHC: lepton angular distributions and reference frames for probing hard QCD

Precision tests of the Standard Model in the Strong and Electroweak sectors play an important role, among the physics goals of LHC experiments. Because of the nature of proton-proton processes, observables based on the measurement of the direction and energy of leptons provide the most precise signatures. In the present paper, we concentrate on the angular distribution of leptons from W to l nu decays in the lepton-pair rest-frame. The vector nature of the intermediate state imposes that distributions are to a good precision described by spherical polynomials of at most second order. We argue, that contrary to general belief often expressed in the literature, the full set of angular coefficients can be measured experimentally, despite the presence in the final state of neutrino escaping detection. There is thus no principle difference with respect to the phenomenology of the Z/gamma to l^+ l^- Drell-Yan process. We show also, that with the proper choice of the coordinate frames, only one coefficient in this polynomial decomposition remains sizable, even in the presence of one or more high p_T jets. The necessary stochastic choice of the frames relies on probabilities independent from any coupling constants. In this way, electroweak effects (dominated by the V-A nature of W couplings to fermions) can be better separated from the ones of strong interactions. The separation is convenient for the measurements interpretation.

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arXiv2016

Multiplicity and transverse momentum fluctuations in inelastic proton-proton interactions at the CERN Super Proton Synchrotron

Measurements of multiplicity and transverse momentum fluctuations of charged particles were performed in inelastic p+p interactions at 20, 31, 40, 80 and 158 GeV/c beam momentum. Results for the scaled variance of the multiplicity distribution and for three strongly intensive measures of multiplicity and transverse momentum fluctuations \$Δ[P_{T},N]\$, \$Σ[P_{T},N]\$ and \$Φ_{p_T}\$ are presented. For the first time the results on fluctuations are fully corrected for experimental biases. The results on multiplicity and transverse momentum fluctuations significantly deviate from expectations for the independent particle production. They also depend on charges of selected hadrons. The string-resonance Monte Carlo models EPOS and UrQMD do not describe the data. The scaled variance of multiplicity fluctuations is significantly higher in inelastic p+p interactions than in central Pb+Pb collisions measured by NA49 at the same energy per nucleon. This is in qualitative disagreement with the predictions of the Wounded Nucleon Model. Within the statistical framework the enhanced multiplicity fluctuations in inelastic p+p interactions can be interpreted as due to event-by-event fluctuations of the fireball energy and/or volume.

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arXiv2016

Potential for optimizing Higgs boson CP measurement in H to tau tau decay at LHC and ML techniques

We investigate potential for measuring CP state of the Higgs boson in the H to tau tau$ decay with consecutive tau-lepton decays in channels: tau^+- to rho^+- nu_tau and tau^+- to a1^+- nu_tau combined. Subsequent decays rho^+- to pi^+- pi^0, a1^+- to rho^0 pi^+- and rho^0 to pi^+ pi^- are taken into account. We will explore extensions of the method, where acoplanarity angle for the planes build on the visible decay products, pi^+- pi^0 of tau^+- to pi^pm pi^0 nu_tau, was used. The angle is sensitive to transverse spin correlations, thus to parity. We show, that in the case of the cascade decays of tau to a1 nu, information on the CP state of Higgs can be extracted from the acoplanarity angles as well. Because in the cascade decay a1^+- to rho^0 pi^pm,rho^0 to pi^+ pi^- up to four planes can be defined, up to 16 distinct acoplanarity angles are available for H \to tau tau to a1^+ a1^- nu nu decays. These acoplanarities carry in part supplementary but also correlated information. It is thus cumbersome to evaluate an overall sensitivity. We investigate sensitivity potential of such analysis, by developing and implementing model in the Machine Learning (ML) techniques. We quantify possible improvements when multi-dimensional phase-space of outgoing decay products directions is used, instead of 1-dimensional projections i.e. the acoplanarity angles. We do not take into account ambiguities resulting from detector uncertainties or background contamination, we concentrate on usefulness of ML methods and tau to 3pi nu decays for Higgs boson parity measurement.

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arXiv2016

Separating Electroweak and Strong interactions in Drell-Yan processes at LHC: leptons angular distributions and reference frames

Among the physics goals of LHC experiments, precision tests of the Standard Model in the Strong and Electroweak sectors play an important role. Because of nature of the proton-proton processes, observables based on the measurement of the direction and energy of leptons provide the most precise signatures. In the present paper, we concentrate on the angular distribution of Drell-Yan process leptons, in the lepton-pair rest-frame. The vector nature of the intermediate state imposes that distributions are to a good precision described by spherical polynomials of at most second order. We show that with the proper choice of the coordinate frames, only one coefficient in this polynomial decomposition remains sizable, even in the presence of one or two high $p_T$ jets. The necessary stochastic choice of the frames relies on probabilities independent from any coupling constants. This remains true when one or two partons accompany the lepton pairs. In this way electroweak effects can be better separated from strong interaction ones for the benefit of the interpretation of the measurements. Our study exploits properties of single gluon emission matrix elements which are clearly visible if a conveniently chosen form of their representation is used. We rely also on distributions obtained from matrix element based Monte Carlo generated samples of events with two leptons and up to two additional partons in test samples. Incoming colliding protons' partons are distributed accordingly to PDFs and are strictly collinear to the corresponding beams.

hep-ph↗

arXiv2016

Production of tau lepton pairs with high pT jets at the LHC and the TauSpinner reweighting algorithm

The TauSpinner algorithm allows to modify the physics of the Monte Carlo generated samples due to the changed assumptions of event production dynamics, without re-generating events. To each event it attributes weights: the spin effects of tau-lepton production or decay, or the production mechanism are modified. There is no need to repeat the detector response simulation. We document the extension to 2 to 4 processes in which the matrix elements for the parton-parton scattering amplitudes into a tau-lepton pair and two outgoing partons are used. Tree-level matrix elements for the Standard Model processes, including the Higgs boson production are used. Automatically generated codes by MadGraph5 have been adapted. Tests of the matrix elements, reweighting algorithm and numerical results are presented. For averaged tau lepton polarisation, we perform comparison of 2 to 2 and 2 to 4 matrix elements used to calculate the spin weight in pp to tau tau j j events. We show, that for events with tau-lepton pair close to the Z-boson peak, the tau-lepton polarisation calculated using 2 to 4 matrix elements is very close to the one calculated using 2 to 2 Born process only. For the m_(tautau) masses above the Z-boson peak, the effect from including 2 to 4 matrix elements is also marginal, however when restricting into subprocesses qq,q bar q to tau tau j j only, it can lead to a 10% difference on the predicted tau-lepton polarisation. Choice of electroweak scheme can have significant impact. The modification of the electroweak or strong interaction can be performed with the re-weighting technique. TauSpinner v.2.0.0, allows to introduce non-standard couplings for the Higgs boson and study their effects in the vector-boson-fusion. The discussion is relegated to forthcoming publications.

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