arXiv subjects

Z. Was

Publications and source records attributed to Z. Was.

At least 37 records · Page 2Linked to original sources

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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arXiv2020

Theory for the FCC-ee : Report on the 11th FCC-ee Workshop

The Future Circular Collider (FCC) at CERN, a proposed 100-km circular facility with several colliders in succession, culminates with a 100 TeV proton-proton collider. It offers a vast new domain of exploration in particle physics, with orders of magnitude advances in terms of Precision, Sensitivity and Energy. The implementation plan foresees, as a first step, an Electroweak Factory electron-positron collider. This high luminosity facility, operating between 90 and 365 GeV centre-of-mass energy, will study the heavy particles of the Standard Model, Z, W, Higgs, and top with unprecedented accuracy. The Electroweak Factory $e^+e^-$ collider constitutes a real challenge to the theory and to precision calculations, triggering the need for the development of new mathematical methods and software tools. A first workshop in 2018 had focused on the first FCC-ee stage, the Tera-Z, and confronted the theoretical status of precision Standard Model calculations on the Z-boson resonance to the experimental demands. The second workshop in January 2019, which is reported here, extended the scope to the next stages, with the production of W-bosons (FCC-ee-W), the Higgs boson (FCC-ee-H) and top quarks (FCC-ee-tt). In particular, the theoretical precision in the determination of the crucial input parameters, alpha_QED, alpha_QCD, M_W, m_t at the level of FCC-ee requirements is thoroughly discussed. The requirements on Standard Model theory calculations were spelled out, so as to meet the demanding accuracy of the FCC-ee experimental potential. The discussion of innovative methods and tools for multi-loop calculations was deepened. Furthermore, phenomenological analyses beyond the Standard Model were discussed, in particular the effective theory approaches. The reports of 2018 and 2019 serve as white papers of the workshop results and subsequent developments.

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arXiv2020

ISR and IFI in Precision AFB Studies with KKMC-hh

KKMC-hh is a hadronic event generator for Z boson production and decays, which includes exponentiated multi-photon radiation and first-order electroweak corrections. We have used KKMC-hh to investigate the role of initial sate radiation (ISR) and initial-final interference (IFI) in precision electroweak analyses at the LHC. We compare the effect of this radiation on angular distributions and forward-backward asymmetry, which are particularly important for the measurement of the weak mixing angle. We discuss the relation of the ISR implementation in KKMC-hh to ISR from parton distribution functions with QED corrections.

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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

The Belle II Physics Book

We present the physics program of the Belle II experiment, located on the intensity frontier SuperKEKB $e^+e^-$ collider. Belle II collected its first collisions in 2018, and is expected to operate for the next decade. It is anticipated to collect 50/ab of collision data over its lifetime. This book is the outcome of a joint effort of Belle II collaborators and theorists through the Belle II theory interface platform (B2TiP), an effort that commenced in 2014. The aim of B2TiP was to elucidate the potential impacts of the Belle II program, which includes a wide scope of physics topics: B physics, charm, tau, quarkonium, electroweak precision measurements and dark sector searches. It is composed of nine working groups (WGs), which are coordinated by teams of theorist and experimentalists conveners: Semileptonic and leptonic B decays, Radiative and Electroweak penguins, phi_1 and phi_2 (time-dependent CP violation) measurements, phi_3 measurements, Charmless hadronic B decay, Charm, Quarkonium(like), tau and low-multiplicity processes, new physics and global fit analyses. This book highlights "golden- and silver-channels", i.e. those that would have the highest potential impact in the field. Theorists scrutinised the role of those measurements and estimated the respective theoretical uncertainties, achievable now as well as prospects for the future. Experimentalists investigated the expected improvements with the large dataset expected from Belle II, taking into account improved performance from the upgraded detector.

hep-ex↗

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

A Precision Event Generator for EW Corrections in Hadron Scattering: ${\cal KK}$MC-hh

${\cal KK}$MC-hh is a precision event-generator for Z production and decay in hadronic collisions, which applies amplitude-level resummation to both initial and final state photon radiation, including perturbative residuals exact through ${\cal O}(α^2L)$, together with exact ${\cal O}(α)$ EW matrix element corrections. We present some comparisons to other programs and results showing the effect of multi-photon radiation for cuts motivated by a recent ATLAS W mass analysis. We also show preliminary untuned comparisons of the electroweak corrections of ${\cal KK}$MC-hh to those of HORACE, which includes exact ${\cal O}(α)$ corrections with resummed final-state photon radiation.

hep-ph↗

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.

hep-ph↗

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.

hep-ph↗

arXiv2018

KKMC-hh: A Precision Event Generator for EW Radiative Corrections in Hadron Scattering

KKMC-hh is a precision event-generator for Z production and decay in hadronic collisions, which applies amplitude-level exponentiation to both initial and final state photon radiation, including perturbative residuals through order $α^2 L$, together with electroweak matrix element corrections. We present results showing the effect of multi-photon radiation for cuts motivated by a recent ATLAS W-mass analysis. We also show preliminary untuned comparisons of the electroweak corrections of KKMC-hh to those of HORACE, which includes order $α$ corrections with exponentiated final-state photon radiation.

hep-ph↗

arXiv2017

Extra lepton pair emission corrections to Drell-Yan processes in PHOTOS and SANC

In the paper we present results for final state emissions of lepton pairs in decays of heavy intermediate states such as Z boson. Short presentations of PHOTOS and SANC algorithms and physics assumptions are given. Numerical distributions of relevance for LHC observables are shown. They are used in discussions of systematic errors in the predictions of pair emissions as implemented in the two programs. Suggestions for the future works are given. Present results confirm, that for the precision of 0.3% level, in simulation of final state the pair emissions can be avoided. For the precision of 0.1-0.2%, the results obtained with the presented programs should be enough. To cross precision tag of 0.1%, the further work is however required.

hep-ph↗

arXiv2017

TAUOLA of tau lepton decays-- framework for hadronic currens, matrix elements and anomalous decays

We present an update of the Monte Carlo event generator TAUOLA for tau lepton decays, with substantially increased list of decay channels and new initialization options. The core of the program remains written in FORTRAN but necessary arrangements have been made to allow handling of the user-provided hadronic currents and matrix elements at the execution time. Such solution may simplify preparation of new hadronic currents and may be useful for fitting to the experimental data as well. We have implemented as default for TAUOLA a set of hadronic currents, which is compatible with the default initialization used by BaBar collaboration. Options for currents available in previous releases are still stored in the code, sometimes left defunct or activated by internal flags only. The new version of the program, includes also implementation of Lepton Flavour Violating tau decays. Finally, we present, as an example, a set of C++ methods for handling user-provided currents, matrix elements or complete new decay channels initialization which can be performed at the program execution time.

hep-ph↗

arXiv2017

Automated calculation of matrix elements and physics motivated observables

The central aspect of my personal scientific activity, has focused on calculations useful for interpretation of High Energy accelerator experimental results, especially in a domain of precision tests of the Standard Model. My activities started in early 80's, when computer support for algebraic manipulations was in its infancy. But already then it was important for my work. It brought a multitude of benefits, but at the price of some inconvenience for physics intuition. Calculations became more complex, work had to be distributed over teams of researchers and due to automatization, some aspects of the intermediate results became more difficult to identify. In my talk I will not be very exhaustive, I will present examples from my personal research only: (i) calculations of spin effects for the process e+ e- to tau+tau- gamma at Petra/PEP energies, calculations (with the help of the {\tt Grace} system of Minami-tateya group) and phenomenology of spin amplitudes for (ii) e+ e- to 4f and for (iii) e+ e- to nu_e bar nu_e gamma gamma processes, (iv) phenomenology of CP-sensitive observables for Higgs boson parity in H to tau+ tau- , tau+- to nu 2(3)pi cascade decays

hep-ph↗

arXiv2016

Tau lepton production and decays: perspective of multi-dimensional distributions and Monte Carlo methods

Status of tau lepton decay Monte Carlo generator TAUOLA, its main applications and recent developments 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: lesson from comparison and fits to the BaBar and Belle data is recalled. It was found, that as in the past at a time of comparisons with CLEO and ALEPH data, proper fitting, to as detailed as possible representation of the experimental data, is essential for appropriate developments of models of tau decay dynamic. This multi-dimensional nature of distributions is also important for observables where tau leptons are used to constrain experimental data. In later part of the presentation,use of the TAUOLA program for phenomenology of W,Z,H decays at LHC is addressed, in particular in the context of the Higgs boson parity measurements. Some new results, relevant for QED lepton pair emission are mentioned as well.

hep-ph↗

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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