The Z boson spin observables as messengers of new physics

Aguilar-Saavedra, J. A.; Bernabéu, J.; Mitsou, V. A.; Segarra, Alejandro

doi:10.1140/epjc/s10052-017-4795-8

Cited by 28 publications

(22 citation statements)

References 33 publications

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“…[12,61] for the W boson and in Ref. [14] for the Z boson. This is the reason why the angular coefficients are also called spin or polarization observables.…”

Section: Definition Of Fiducial Polarization Observablesmentioning

confidence: 99%

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Fiducial polarization observables in hadronic WZ production: a next-to-leading order QCD+EW study

Baglio

Ninh

2019

J. High Energ. Phys.

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We present a study at next-to-leading-order (NLO) of the process pp → W ± Z → ν l + − , where , = e, µ, at the Large Hadron Collider. We include the full NLO QCD corrections and the NLO electroweak (EW) corrections in the double-pole approximation. We define eight fiducial polarization coefficients directly constructed from the polar-azimuthal angular distribution of the decay leptons. These coefficients depend strongly on the kinematical cuts on the transverse momentum or rapidity of the individual leptons. Similarly, fiducial polarization fractions are also defined and they can be directly related to the fiducial coefficients. We perform a detailed analysis of the NLO QCD+EW fiducial polarization observables including theoretical uncertainties stemming from the scale variation and parton distribution function uncertainties, using the fiducial phase space defined by the ATLAS and CMS experiments. We provide results in the helicity coordinate system and in the Collins-Soper coordinate system, at a center-of-mass energy of 13 TeV. The EW corrections are found to be important in two of the angular coefficients related to the Z boson, irrespective of the kinematical cuts or the coordinate system. Meanwhile, those EW corrections are very small for the W ± bosons.

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“…[12,61] for the W boson and in Ref. [14] for the Z boson. This is the reason why the angular coefficients are also called spin or polarization observables.…”

Section: Definition Of Fiducial Polarization Observablesmentioning

confidence: 99%

“…Following Ref. [9] there has been a number of phenomenological studies of the spin-density matrix of the W boson [10][11][12] as well as of the Z boson [13,14], that relate to the corresponding angular coefficients. One-loop QCD effects have also been studied in Refs.…”

mentioning

confidence: 99%

Fiducial polarization observables in hadronic WZ production: a next-to-leading order QCD+EW study

Baglio

Ninh

2019

J. High Energ. Phys.

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“…A 0−4 are called P-even and A 5−7 P-odd according to the parity transformation where φ flips sign while θ remains unchanged [19,20]. We also note here that A 5−7 are proportional to the imaginary parts of the spin-density matrix of the W and Z bosons in the DPA at LO [5,21,22]. This is important to understand why the values of these coefficients are very small, as will be later shown.…”

Section: Polarization Observablesmentioning

confidence: 77%

Polarization Observables in WZ Production at the 13 TeV LHC: Inclusive Case

Baglio

Ninh²

2020

Comm. in Phys.

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We present a study of the polarization observables of the W and Z bosons in the process pp → W ± Z → e ± ν e µ + µ − at the 13 TeV Large Hadron Collider. The calculation is performed at next-to-leading order, including the full QCD corrections as well as the electroweak corrections, the latter being calculated in the double-pole approximation. The results are presented in the helicity coordinate system adopted by ATLAS and for different inclusive cuts on the di-muon invariant mass. We define left-right charge asymmetries related to the polarization fractions between the W + Z and W − Z channels and we find that these asymmetries are large and sensitive to higher-order effects. Similar findings are also presented for charge asymmetries related to a P-even angular coefficient.

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“…[5] are related to the ones defined in refs. [4,16] through Table 1. Asymmetries with their associated angular observables and their relation to the top-quark polarisation and W -boson spin observables.…”

Section: Jhep04(2017)124mentioning

confidence: 99%

Probing the W tb vertex structure in t-channel single-top-quark production and decay in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

Aaboud¹,

Aad²,

Alstaty³

et al. 2017

J. High Energ. Phys.

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To probe the W tb vertex structure, top-quark and W -boson polarisation observables are measured from t-channel single-top-quark events produced in proton-proton collisions at a centre-of-mass energy of 8 TeV. The dataset corresponds to an integrated luminosity of 20.2 fb −1 , recorded with the ATLAS detector at the LHC. Selected events contain one isolated electron or muon, large missing transverse momentum and exactly two jets, with one of them identified as likely to contain a b-hadron. Stringent selection requirements are applied to discriminate t-channel single-top-quark events from background. The polarisation observables are extracted from asymmetries in angular distributions measured with respect to spin quantisation axes appropriately chosen for the top quark and the W boson. The asymmetry measurements are performed at parton level by correcting the observed angular distributions for detector effects and hadronisation after subtracting the background contributions. The measured top-quark and W -boson polarisation values are in agreement with the Standard Model predictions. Limits on the imaginary part of the anomalous coupling g R are also set from model-independent measurements. The ATLAS collaboration 33 IntroductionAt hadron colliders, top quarks are predominantly produced in pairs (tt) via the flavourconserving strong interaction, but single top-quark production can occur via chargedcurrent electroweak processes involving a W tb vertex. At leading order in QCD perturbation theory, three sub-processes contribute to single top-quark production: an exchange of a virtual W boson either in the t-channel or in the s-channel, or the associated production of a top quark with an on-shell W boson (W t). The t-channel and s-channel processes do not interfere at next-to-leading-order in QCD and are thus well defined with that precision [1]. In proton-proton (pp) collisions, the t-channel exchange, depicted in figure 1, is the dominant production process of single top quarks. The exchange of a space-like W boson due to the interaction of a light quark with a b-quark produces a top quark and a forward light-quark (called the spectator quark) in the final state. Furthermore, as a consequence of the vector minus axial-vector (V-A) form of the W tb vertex in the Standard Model, the Figure 1. Leading-order Feynman diagram for t-channel production of single top quarks in pp collisions. In the depicted four-flavour scheme (2 → 3 process) the initial b-quark arises from a gluon splitting into a bb pair.produced top quarks are highly polarised, in particular along the direction of the spectatorquark momentum [2,3]. Within the Standard Model the top quark decays through the electroweak interaction into an on-shell W boson and a b-quark, with a lifetime much shorter than the time scale necessary to depolarise the spin. The information on the top-quark spin can thus be obtained from its decay products. The produced real W boson also possesses a polarisation (or helicity state), which can be extracted from angular ...

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The Z boson spin observables as messengers of new physics

Cited by 28 publications

References 33 publications

Fiducial polarization observables in hadronic WZ production: a next-to-leading order QCD+EW study

Fiducial polarization observables in hadronic WZ production: a next-to-leading order QCD+EW study

Polarization Observables in WZ Production at the 13 TeV LHC: Inclusive Case

Probing the W tb vertex structure in t-channel single-top-quark production and decay in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

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