arXiv subjects

A. A. Pankov

Publications and source records attributed to A. A. Pankov.

At least 19 recordsLinked to original sources

arXiv2022

Bounds on the mass and mixing of $Z^\prime$ and $W^\prime$ bosons decaying into different pairings of $W$, $Z$, or Higgs bosons using CMS data at the LHC

The full CMS Run 2 datasets with time-integrated luminosity of 137 fb$^{-1}$ in the diboson channels are used to probe benchmark models with extended gauge sectors such as $E_6$, left-right symmetric (LR) and the sequential standard model (extended gauge model, EGM), that predict the existence of neutral $Z'$- and charged $W'$-bosons decaying to a pair of bosons $WW$, $ZH$, $WZ$ and $WH$ in the semileptonic final state. These benchmark models are used to interpret the results. Exclusion limits at the 95% C.L. on the $Z'$ and $W'$ resonance production cross section times branching ratio to electroweak gauge boson pairs in the resonance mass range between 1.0 and 4.5 TeV are here converted to constraints on $Z$-$Z'$ and $W$-$W'$ mixing parameters and masses. We present exclusion regions on the parameter spaces of the $Z'$ and $W'$ and show that the obtained exclusion regions are significantly extended compared to those derived from the previous analysis performed with Tevatron data as well as with the CMS data collected at 7 and 8 TeV in Run 1. The reported limits are the most restrictive to date.

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arXiv2021

Updated constraints on $Z'$ and $W'$ bosons decaying into bosonic and leptonic final states using the Run 2 ATLAS data

The full ATLAS Run 2 data set with time-integrated luminosity of 139 fb$^{-1}$ in the diboson and dilepton channels is used to probe benchmark models with extended gauge sectors: the $E_6$-motivated Grand Unification models, the left-right symmetric $LR$ and the sequential standard model (EGM). These all predict neutral $Z'$ vector bosons, decaying into lepton pairs, $\ell\ell$, or into electroweak gauge boson pairs $WW$, where one $W$ in turn decays semileptonically. 95% C.L. exclusion limits on the $Z'$ resonance production cross section times branching ratio to electroweak gauge boson pairs and to lepton pairs in the mass range of $\sim$ 1 - 6 TeV are converted to constraints on the $Z$-$Z'$ mixing parameter and the heavy resonance mass. We present exclusion regions on the parameter space of the $Z'$ which are significantly extended compared to those obtained from the previous analyses performed with LHC data collected at 7 and 8 TeV in Run 1 as well as at 13 TeV in Run 2 at time-integrated luminosity of 36.1 fb$^{-1}$ and are the most stringent bounds to date. Also presented, from a similar analysis of electrically charged $W'$ bosons arising in the EGM, which can decay through $W'\to WZ$ and $W'\to \ellν$, are limits on the $W$-$W'$ mixing parameter and the charged $W'$ vector boson mass.

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arXiv2020

High-precision limits on $W$-$W'$ and $Z$-$Z'$ mixing from diboson production using the full LHC Run 2 ATLAS data set

The full ATLAS Run 2 data set with time-integrated luminosity of 139 fb$^{-1}$ in the diboson channels in hadronic final states is used to probe a simple model with an extended gauge sector (EGM), proposed by Altarelli et al., and often taken as a convenient benchmark by experimentalists. This model accommodates new charged $W'$ and neutral $Z'$ vector bosons with modified trilinear Standard Model gauge couplings, decaying into electroweak gauge boson pairs $WZ$ or $WW$, where $W$/$Z$ decay hadronically. Exclusion limits at the 95% C.L. on the $Z'$ and $W'$ resonance production cross section times branching ratio to electroweak gauge boson pairs in the mass range of $\sim$ 1 - 5 TeV are here converted to constraints on $W$-$W'$ and $Z$-$Z'$ mixing parameters and masses for the EGM. We present exclusion regions on the parameter space of the $W'$ and $Z'$ by using the full Run 2 data set comprised of $pp$ collisions at $\sqrt{s}=13$ TeV and recorded by the ATLAS detector at the CERN LHC. The obtained exclusion regions are significantly extended compared to those obtained from the previous analysis performed with Tevatron data as well as with LHC data collected at 7 and 8 TeV in Run 1 and are the most stringent bounds to date.

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arXiv2019

Improved bounds on W-W' mixing with ATLAS resonant WZ production data at the LHC at $\sqrt{s}=13$ TeV

New charged vector bosons $W'$ decaying into gauge boson pairs $WZ$ are predicted in many scenarios of new physics, including models with an extended gauge sector (EGM). Due to the large variety of models (other unification groups, models with Supersymmetry, Little Higgs Models, Extra Dimensions) the more general EGM approach is here considered. For what concerns $W'$-production, these models are parametrised by two parameters, the $W^\prime$ mass $M_{W^\prime}$ and the $W$-$W^\prime$ mixing parameter $ξ$. The diboson $WZ$ production allows to place stringent constraints on this mixing angle and the $W'$ mass, which we determine and present for the first time by using data from $pp$ collisions at $\sqrt{s}=13$ TeV recorded by the ATLAS detector at the CERN LHC, with integrated luminosity of 36.1 fb$^{-1}$. By comparing the experimental limits to the theoretical predictions for the total cross section of $W'$ resonant production and its subsequent decay into $WZ$ pairs, we show that the derived constraints on the mixing angle for the benchmark model are rather small, between $10^{-4}$ and $10^{-3}$, i.e., greatly improved with respect to those derived from the global analysis of electroweak data which yield $ξ\sim 10^{-2}$. We combine the limits derived from $WZ$ production data with those obtained from the $W'\to eν$ process in order to significantly extend the exclusion region in the $M_{W'}$-$ξ$ parameter plane and obtain the most stringent exclusion limits to date. We present the combined allowed parameter space for the EGM $W'$ boson after incorporating indirect constraints from low energy electroweak data, direct search constraints from Tevatron and from the LHC Run I with 7 and 8 TeV as well as at Run II with 13 TeV data.

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arXiv2018

Improved constraints on the mixing and mass of $Z'$ bosons from resonant diboson searches at the LHC at $\sqrt{s}=13$ TeV and predictions for Run II

New neutral vector bosons $Z'$ decaying to charged gauge boson pairs $W^+W^-$ are predicted in many scenarios of new physics, including models with an extended gauge sector such as $E_6$, left-right symmetric $Z^\prime_{\rm LRS}$ and the sequential standard model $Z'_{\rm SSM}$. For these benchmark models we calculate and present theoretical expectations for different values of the $Z^\prime$ mass $M_2$ and mixing parameter $ξ$. Our results are based on the narrow width approximation which allows to make a convenient comparison of experiment to theoretical benchmark models. The diboson production allows to place stringent constraints on the $Z$-$Z'$ mixing angle and the $Z'$ mass, which we determine by using data from $pp$ collisions at $\sqrt{s}=13$ TeV recorded by the ATLAS detector at the CERN LHC, with integrated luminosity of $\sim$ 36 fb$^{-1}$. By comparing the experimental limits to the theoretical predictions for the total cross section of $Z'$ resonant production and its subsequent decay into $W^+W^-$ pairs, we show that the derived constraints on the mixing angle for the benchmark models are of the order of a few $\times 10^{-4}$, i.e., greatly improved with respect to those derived from the global analysis of electroweak data. We combine the limits derived from diboson production data with those obtained from the Drell--Yan process in order to significantly extend the exclusion region in the $M_{2}$-$ξ$ parameter plane. Also, we demonstrate that further improvement on the constraining of this mixing can be achieved through analysis of the full set of Run~II data.

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arXiv2017

Probing $Z$-$Z'$ mixing with ATLAS and CMS resonant diboson production data at the LHC at $\sqrt{s}=13$ TeV

The study of electroweak boson pair production provides a powerful test of the spontaneously broken gauge symmetry of the Standard Model (SM) and can be used to search for new phenomena beyond the SM. Extra neutral vector bosons $Z'$ decaying to charged gauge vector boson pairs $W^+W^-$ are predicted in many scenarios of new physics, including models with an extended gauge sector. The diboson production allows to place stringent constraints on the $Z$-$Z'$ mixing factor $ξ$ and $Z'$ mass, $M_{Z'}$. We present the $Z'$ exclusion region in the $ξ-M_{Z'}$ plane for the first time by using data comprised of $pp$ collisions at $\sqrt{s}=13$ TeV and recorded by the ATLAS and CMS detectors at the CERN LHC, with integrated luminosities of 13.2 and 35.9 fb$^{-1}$, respectively. The exclusion region has been significantly extended compared to that obtained from the previous analysis performed with Tevatron data as well as with LHC data collected at 7 and 8 TeV. Also, we found that these constraints on the $Z$-$Z'$ mixing factor are more severe than those derived from the global analysis of electroweak data. Further improvement on the constraining of this mixing can be achieved from the analysis of data to be collected at higher luminosity.

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arXiv2015

The Higgs-like boson spin from the center-edge asymmetry in the diphoton channel

We discuss the discrimination of the 125 GeV spin-parity 0^+ Higgs-like boson observed at the LHC, decaying into two photons, H\toγγ, against the hypothesis of a minimally coupled J^{P}=2^+ narrow diphoton resonance with the same mass and giving the same total number of signal events under the peak. We apply, as the basic observable of the analysis, the center-edge asymmetry A_{CE} of the cosine of the polar angle of the produced photons in the diphoton rest frame to distinguish between the tested spin hypotheses. We show that the center-edge asymmetry A_{CE} should provide strong discrimination between the possibilities of spin-0 and spin-2 with graviton-like couplings, depending on the fraction of q\bar{q} production of the spin-2 signal, reaching a {CL}_s<10^{-6} for f_{qq}=0. Indeed, the A_{CE} has the potential to do better than existing analyses for f_{qq}<0.4.

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arXiv2014

Precise determination of Z-Z' mixing at the CERN LHC

We discuss the expected sensitivity to Z' boson effects in the W^\pm boson pair production process at the Large Hadron Collider (LHC). The results of a model-dependent analysis of Z' boson effects are presented as constraints on the Z-Z' mixing angle phi and Z' boson mass. The process pp\to W^+W^- + X allows to place stringent constraints on the Z-Z' mixing angle. Specifically, we find that the present LHC bounds on the mixing angle are of the order a few times 10^{-3}, what is of the same order as those derived from the electroweak data. These results were derived from analysis of W-pair production at sqrt{s}=8 TeV and integrated luminosity of 20 fb^{-1}. Further improvement on the constraining of this mixing can be achieved from the analysis of data on WW \to l nu l'nu' (l,l'=e or mu) and WW \to l nu jj final states collected at the LHC with nominal energy and luminosity, 14 TeV and 100 fb^{-1}, and should be phi \sim 10^{-4}-10^{-3}.

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arXiv2013

Unique heavy lepton signature at $e^+e^-$ linear collider with polarized beams

We explore the effects of neutrino and electron mixing with exotic heavy leptons in the process e^+e^-\to W^+W^- within E_6 models. We examine the possibility of uniquely distinguishing and identifying such effects of heavy neutral lepton exchange from Z-Z' mixing within the same class of models and also from analogous ones due to competitor models with anomalous trilinear gauge couplings (AGC) that can lead to very similar experimental signatures at the e^+e^- International Linear Collider (ILC) for \sqrt{s}=350, 500 GeV and 1 TeV. Such clear identification of the model is possible by using a certain double polarization asymmetry. The availability of both beams being polarized plays a crucial role in identifying such exotic-lepton admixture. In addition, the sensitivity of the ILC for probing exotic-lepton admixture is substantially enhanced when the polarization of the produced W^\pm bosons is considered.

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arXiv2012

Discriminating Z' from anomalous trilinear gauge coupling signatures in e+e- \to W+W- at ILC with polarized beams

New heavy neutral gauge bosons Z' are predicted by many models of physics beyond the Standard Model. It is quite possible that Z's are heavy enough to lie beyond the discovery reach of the CERN Large Hadron Collider LHC, in which case only indirect signatures of Z' exchanges may emerge at future colliders, through deviations of the measured cross sections from the Standard Model predictions. We discuss in this context the foreseeable sensitivity to Z's of W^\pm-pair production cross sections at the e^+e^- International Linear Collider (ILC), especially as regards the potential of distinguishing observable effects of the Z' from analogous ones due to competitor models with anomalous trilinear gauge couplings (AGC) that can lead to the same or similar new physics experimental signatures at the ILC. The sensitivity of the ILC for probing the Z-Z' mixing and its capability to distinguish these two new physics scenarios is substantially enhanced when the polarization of the initial beams and the produced W^\pm bosons are considered. A model independent analysis of the Z' effects in the process e^+e^- \to W^+W^- allows to differentiate the full class of vector Z' models from those with anomalous trilinear gauge couplings, with one notable exception: the sequential SM (SSM)-like models can in this process not be distinguished from anomalous gauge couplings. Results of model dependent analysis of a specific Z' are expressed in terms of discovery and identification reaches on the Z-Z' mixing angle and the Z' mass.

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arXiv2012

Sneutrino Identification in Lepton Pair Production at ILC with Polarized Beams

Numerous non-standard dynamics are described by contact-like effective interactions that can manifest themselves in electron-positron collisions only through deviations of the observables (cross sections, asymmetries) from the Standard Model predictions. If such a deviation were observed, it would be important to identify the actual source among the possible non-standard interactions as many different new physics scenarios may lead to very similar experimental signatures. We study the possibility of uniquely identifying the indirect effects of s-channel sneutrino exchange, as predicted by supersymmetric theories with R-parity violation, against other new physics scenarios in high-energy e^+e^- annihilation into lepton pairs at the International Linear Collider. These competitive models are interactions based on gravity in large and in TeV-scale extra dimensions, anomalous gauge couplings, Z' vector bosons and compositeness-inspired four-fermion contact interactions. To evaluate the identification reach on sneutrino exchange, we use as basic observable a double polarization asymmetry, that is particularly suitable to directly test for such s-channel sneutrino exchange effects in the data analysis. The availability of both beams being polarized plays a crucial role in identifying the new physics scenario.

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arXiv2011

Spin-analysis of s-channel diphoton resonances at the LHC

The high mass neutral quantum states envisaged by theories of physics beyond the standard model can at the hadron colliders reveal themselves through their decay into a pair of photons. Once such a peak in the diphoton invariant mass distribution is discovered, the determination of its spin through the distinctive photon angular distributions is needed in order to identify the associated nonstandard dynamics. We here discuss the discrimination of the spin-2 Randall-Sundrum graviton excitation against the hypothesis of a spin-0 exchange giving the same number of events under the peak, by means of the angular analysis applied to resonant diphoton events expected to be observed at the LHC. The spin-0 hypothesis is modelled by an effective interaction of a high mass gauge singlet scalar particle interacting with the standard model fields. The basic observable of our analysis is the symmetrically integrated angular asymmetry $A_{CE}$, calculated for both graviton and scalar s-channel exchanges to next-to-leading order in QCD.

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arXiv2010

Spin identification of heavy nonstandard bosons in dilepton and diphoton events at the LHC

New Physics scenarios generally predict the existence of very heavy quantum states that can possibly manifest themselves as peaks in the cross sections at the LHC. For values of the parameters in certain domains, different nonstandard models can generate peaks with the same mass and same number of events. In this case, the spin determination of a peak, requiring the angular analysis of the events, becomes crucial in order to identify the relevant nonstandard source. We here discuss, using a particularly suitable symmetrically integrated angular asymmetry applied to Drell-Yan dilepton and diphoton events at LHC, the identification reach on the exchanges in these reactions of the following heavy bosons: spin-2 Randall-Sundrum graviton excitations; spin-1 heavy neutral gauge bosons Z^\prime; and spin-0 SUSY R-parity violating sneutrinos.

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arXiv2010

Sneutrino identification in dilepton events at the LHC

Heavy neutral resonances appearing in the clean Drell-Yan channel may be the first new physics to be observed at the proton-proton CERN LHC. If a new resonance is discovered at the LHC as a (narrow) peak in the dilepton invariant mass distribution, the characterization of its spin and couplings will proceed via the measurement of production rates and angular distributions of the decay products. We discuss the discrimination of a spin-0 resonance (sneutrino) predicted by supersymmetric theories with R-parity breaking against the spin-1 of Z^\prime bosons and the Randall-Sundrum graviton resonance (spin-2) with the same mass and producing the same number of events under the observed peak. To assess the region of sneutrino parameters (couplings and masses) where the spin determination can be performed to a given confidence level, we focus on the event rate and the angular distributions of the Drell-Yan leptons, in particular using the center-edge asymmetry, A_{\rm CE}. We find that although the measured event rate permits solving the above problem partially, the center-edge asymmetry, on the contrary allows to differentiate the various spins entirely with a minimal number of events around 200.

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arXiv2010

Signals of Warped Extra Dimensions at the LHC

We discuss the signatures of the spin-2 graviton excitations predicted by the Randall-Sundrum model with one warped extra dimension, in dilepton and diphoton production at LHC. By using a specific angular analysis, we assess the ranges in mass and coupling constant where such gravitons can be discriminated against competitor spin-1 and spin-0 objects, that potentially could manifest themselves in these processes with the same mass and rate of events. Depending on the value of the coupling constant to quarks and leptons, the numerical results indicate graviton identification mass ranges up to 1.1-2.4 TeV and 1.6-3.2 TeV for LHC nominal energy of 14 TeV and time-integrated luminosity of 10 and 100~${\rm fb}^{-1}$, respectively.

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arXiv2009

Identification of extra neutral gauge bosons at the International Linear Collider

Heavy neutral gauge bosons, Z's, are predicted by many theoretical schemes of physics beyond the Standard Model, and intensive searches for their signatures will be performed at present and future high energy colliders. It is quite possible that Z's are heavy enough to lie beyond the discovery reach expected at the CERN Large Hadron Collider LHC, in which case only indirect signatures of Z' exchanges may occur at future colliders, through deviations of the measured cross sections from the Standard Model predictions. We here discuss in this context the foreseeable sensitivity to Z's of fermion-pair production cross sections at an e^+e^- linear collider, especially as regards the potential of distinguishing different Z' models once such deviations are observed. Specifically, we assess the discovery and identification reaches on Z' gauge bosons pertinent to the E_6, LR, ALR and SSM classes of models, that should be attained at the planned International Linear Collider (ILC). With the high experimental accuracies expected at the ILC, the discovery and the identification reaches on the Z' models under consideration could be increased substantially. In particular, the identification among the different models could be achieved for values of Z' masses in the discovery (but beyond the identification) reach of the LHC. An important role in enhancing such reaches is played by the electron (and possibly the positron) longitudinally polarized beams. Also, although the purely leptonic processes are experimentally cleaner, the measurements of c- and b-quark pair production cross sections are found to carry important, and complementary, information on these searches.

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arXiv2009

Spin and model identification of Z' bosons at the LHC

Heavy resonances appearing in the clean Drell-Yan channel may be the first new physics to be observed at the proton-proton CERN LHC. If a new resonance is discovered at the LHC as a peak in the dilepton invariant mass distribution, the characterization of its spin and couplings will proceed via measuring production rates and angular distributions of the decay products. We discuss the discrimination of the spin-1 of Z' representative models (Z'_{SSM}, Z'_{psi}, Z'_{eta}, Z'_{chi}, Z'_{LR}, and Z'_{ALR}) against the Randall-Sundrum graviton resonance (spin-2) and a spin-0 resonance (sneutrino) with the same mass and producing the same number of events under the observed peak. To assess the range of the Z' mass where the spin determination can be performed to a given confidence level, we focus on the angular distributions of the Drell-Yan leptons, in particular we use as a basic observable an angular-integrated center-edge asymmetry, A_{CE}. The spin of a heavy Z' gauge boson can be established with A_{CE} up to M_{Z'} \simeq 3.0 TeV, for an integrated luminosity of 100 fb^{-1}, or minimal number of events around 110. We also examine the distinguishability of the considered Z' models from one another, once the spin-1 has been established, using the total dilepton production cross section. With some assumption, one might be able to distinguish among these Z' models at 95% C.L. up to M_{Z'} \simeq 2.1 TeV.

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arXiv2009

Spin Identification of the Randall-Sundrum Graviton at the LHC

Using as basic observable an angular-integrated asymmetry to be measured in Drell-Yan lepton-pair production at the LHC, we discuss the identification reach on the spin-2 of the lowest-lying Randall-Sundrum resonance predicted by gravity in one warped extra dimension, against the spin-1 and spin-0 hypotheses. Numerical results indicate that, depending on the graviton coupling strength to the standard model particles, such a spin-2 identification can extend up to mass scales of 1.0-1.6 TeV and 2.4-3.2 TeV for LHC integrated luminosities of 10 and 100 fb^{-1}, respectively.

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