Review of top charge asymmetries for LHC run II
aa r X i v : . [ h e p - ph ] M a r Review of top charge asymmetries for LHC run II
Susanne Westhoff
Institute for Theoretical Physics, Heidelberg University, 69120 Heidelberg, GermanyE-mail: [email protected]
This is a brief review of charge asymmetries in top-antitop quark production at the LHC. Proposedasymmetry observables in processes of associated top-pair production t ¯ t + jet, t ¯ t + W , and t ¯ t + γ ,are analyzed and compared. While the focus is on standard-model predictions, we also commenton the sensitivity to possible new physics and observation prospects during the second runtime ofthe LHC. c (cid:13) Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ op charge asymmetries for LHC
1. Introduction
The precise predictions and measurements of abundantly produced top-quarks at the LHC allow usto probe features beyond cross sections, such as the charge asymmetry in top-antitop production. Inthe standard model (SM), the top charge asymmetry provides us with an important test of QCD. Inthis respect, the charge asymmetry is complementary to charge-symmetric observables, as it probesdifferent structures of QCD contributing to the hard production process. Beyond the standardmodel, the charge asymmetry can reveal potential new particles with top-quark interactions with ahigh sensitivity, provided that a good experimental accuracy can be achieved.In inclusive top-pair production from proton-proton collisions at the LHC, the observation of acharge asymmetry is hindered by the huge background from symmetric gg → t ¯ t parton interactions.Furthermore, in the forward-backward symmetric experiment, the charge asymmetry at the partonlevel is not translated into an angular forward-backward asymmetry at the hadron level. Therefore,rapidity distributions of top and antitop quarks can only give limited access to the original chargeasymmetry in the hard scattering process. To overcome these complications, two strategies havebeen pursued to find asymmetry observables that are tailored to the LHC environment: to designobservables with a suppressed gluon-gluon background ; and to exploit final-state kinematics thatgive the most direct access to the charge asymmetry in the partonic process. This approach has ledto considering processes of associated top-antitop production, namely t ¯ t + jet, t ¯ t + W , and t ¯ t + γ ,where the additional particle in the final state is used to define new charge asymmetry observables.In this write-up, we review three proposed observables of the charge asymmetry in associatedtop-pair production. Our goal is to provide the reader with a concise reference of available asym-metry predictions for the LHC. We compare these observables with respect to their sensitivity tophysics in and beyond the standard model. Finally, we briefly comment on proposals to observe thecharge asymmetry in inclusive top-antitop production in the boosted regime at ATLAS and CMS,and in the forward region with the LHCb detector.
2. Top energy asymmetry in t ¯ t + jet production In top-antitop production with an associated hard jet, the charge asymmetry can be probed throughthe difference between top and antitop energies, ∆ E = E t − E ¯ t , in the partonic center-of-mass sys-tem. The energy asymmetry is defined by [1] A E = σ t ¯ t j ( ∆ E > ) − σ t ¯ t j ( ∆ E < ) σ t ¯ t j ( ∆ E > ) + σ t ¯ t j ( ∆ E < ) . (2.1)This observable probes the charge asymmetry in partonic quark-gluon interactions. Due to thehigher abundance of quark-gluon versus quark-antiquark states in proton-proton interactions, thegluon-gluon background is significantly reduced. This is especially true if the jet is energetic andemitted in the direction of the incoming quark. A strong kinematic cut on the transverse momentumof the hardest jet, p T ( j ) , can thus be used to efficiently suppress the gluon-gluon background.In QCD, the energy asymmetry in t ¯ t + jet production is induced at the leading order (LO),where it corresponds with an angular asymmetry of the (quark-)jet in the top-antitop center-of-mass frame. Measuring the energy distributions of the top and antitop quarks thus gives direct1 op charge asymmetries for LHC access to the charge asymmetry at parton level. Beyond LO, the energy asymmetry can be affectedby additional jet radiation in the production process and from top decays. In Ref. [2], it has beenshown that the observable is stable under next-to-leading order (NLO) QCD corrections to topproduction and that effects of radiation in top decay are expected to be small, if the jet is sufficientlyhard and emitted perpendicular to the beam axis in the parton center-of-mass frame. In this regime,with suitable phase-space cuts, the energy asymmetry is predicted to reach A E = − . + . − . % at √ s =
13 TeV, corresponding with a cross section of σ t ¯ t j =
3. Top rapidity asymmetry in t ¯ t + W production In t ¯ t + W production, the presence of the additional W boson strongly affects the production processof top-quarks and thereby the top charge asymmetry [4]. As in inclusive top-pair production, arapidity asymmetry can be defined as A W η = σ t ¯ tW ( ∆ | η | > ) − σ t ¯ tW ( ∆ | η | < ) σ t ¯ tW ( ∆ | η | > ) + σ t ¯ tW ( ∆ | η | < ) , (3.1)with ∆ | η | = | η t | − | η ¯ t | , where η t and η ¯ t denote the pseudo-rapidities of the top and antitop quarks.Since the W boson in t ¯ t + W production can be emitted only from a light quark, gluon-gluoncontributions are absent up to the NNLO in QCD. In the SM, the top-antitop charge asymmetryis induced first at the NLO in quark-antiquark interactions. At the LHC with √ s =
13 TeV, thepredicted rapidity asymmetry at NLO is A W η = − . + . − . %. The result includes a parton shower,and uncertainties are due to scale variations.The emission of the W boson from the initial-state quark implies the production of polarizedtop-quarks. This polarization imprints itself on the angular distributions of the top decay productsfrom t → b ℓ + and thus generates different rapidity distributions for bottom-quarks and leptons fromtop and antitop decays at the LO. The rapidity asymmetry A W η with ∆ | η | = η b ,ℓ + − η ¯ b ,ℓ − can there-fore be used to probe the top-quark polarization in the production process. While a measurementof A W η for reconstructed top-quarks at the LHC requires a large data set, comparably better statis-tical precision can be obtained for the lepton and bottom-quark asymmetries, which are larger inmagnitude.Beyond the SM, the rapidity asymmetries in t ¯ t + W production can help to observe and dis-entangle potential contributions from new physics, especially if they affect the polarization of thetop-quarks. As for the energy asymmetry, modifications of the charge asymmetry are generally ex-pected to be larger than in inclusive top-pair production, mainly because A W η does not suffer fromgluon-gluon background. 2 op charge asymmetries for LHC
4. Top rapidity asymmetry in t ¯ t + γ production A third option to observe the top charge asymmetry is to consider the rapidity asymmetry in t ¯ t + γ production [5], A γ y = σ t ¯ t γ ( ∆ | y | > ) − σ t ¯ t γ ( ∆ | y | < ) σ t ¯ t γ ( ∆ | y | > ) + σ t ¯ t γ ( ∆ | y | < ) , (4.1)with ∆ | y | = | y t | − | y ¯ t | , where y t and y ¯ t are the rapidities of the top and antitop quarks. While thesource of the QCD charge asymmetry is the same as in inclusive top-pair production, the presenceof the photon enhances the fraction of quark-antiquark versus gluon-gluon interactions. This effectlead to a larger net rapidity asymmetry, due to a reduced gluon-gluon background.The main challenge in measuring A γ y consists in suppressing photon radiation from top decays,which makes up the dominant part of the cross section, but does not contribute to the charge asym-metry. This can be achieved by applying selection cuts on the photon kinematics and a veto onradiative top and W boson decays. After cuts, the rapidity asymmetry in t ¯ t + γ production at theLO in QCD is predicted to be A γ y = − . ± .
2% at √ s =
14 TeV. Quoted uncertainties are dueto limited statistics in Monte-Carlo simulation. A crucial limiting factor for an observation of therapidity asymmetry in t ¯ t + γ production will be the experimental statistical sensitivity, due to thesmall production cross section.The fact that the photon re-weights the contributions to the asymmetry from initial up- versusdown-quarks and from initial quarks versus gluons can be used to search for new physics. Specifi-cally, charge-asymmetric contributions that cancel in the rapidity asymmetry in inclusive top-pairproduction can be visible in A γ y .
5. Boosted top asymmetry and lepton charge asymmetry at LHCb in inclusive t ¯ t production An alternative to associated top-antitop production is to consider the rapidity asymmetry in inclu-sive top-pair production in a kinematic regime with enhanced quark-antiquark contributions [6]. Ithas been shown that the asymmetry can be enhanced up to a few percent in regions of high invariantmass M t ¯ t or high boost β t ¯ t of the top-antitop pair [7]. The main challenge to observe the rapidityasymmetry in these regions consists in suppressing QCD background close to the beam axis and/ordealing with a strong reduction of data statistics.In this spirit, a measurement of the top charge asymmetry in the forward region at LHCbhas been proposed [8]. While one of the top-quarks escapes the detector, the top-antitop chargeasymmetry can still be observed through a charge asymmetry of the leptons from top or antitopdecays, where the lepton charge reflects the top charge. A severe complication for a measurementof this lepton charge asymmetry is the control of large background from mis-tagged W + jet, Z +jet, or single-top events [9].
6. Summary
The main features of the discussed charge asymmetry observables in associated top-antitop pro-duction are summarized in Table 1. All three observables offer access to the charge asymmetry in3 op charge asymmetries for LHC A E in t ¯ t + jet A W η in t ¯ t + W A γ y in t ¯ t + γ production cross section O ( ) pb O ( ) fb O ( ) fbcharge asymmetry in QCD LO NLO LOavailable prediction in QCD NLO NLO LOasymmetry in parton channel qg q ¯ q q ¯ q gluon-gluon background reduced absent up to NNLO reduced Table 1:
Comparison of charge asymmetry observables in associated top-antitop production at the LHCwith √ s =
13 TeV. a way that is complementary to inclusive top-antitop production and to the respective other asso-ciated channels. The rapidity asymmetry in t ¯ t + W production offers the best suppression of thegluon-gluon background. In turn, at the parton level, the energy asymmetry in t ¯ t + jet and therapidity asymmetry in t ¯ t + γ production are relatively larger, since they occur at the LO in QCD.Predictions beyond the leading contribution in QCD are currently only available for the energyasymmetry, which has been calculated at the NLO.The observation prospects of the three asymmetries at the LHC strongly depend on experimen-tal aspects, which makes their comparison difficult. The energy asymmetry in t ¯ t + jet is certainlybased on the largest data set, such that kinematic cuts are a viable option to enhance the asymmetryby focusing on specific phase-space regions. Due to the smaller cross section for t ¯ t + W and t ¯ t + γ production, kinematic cuts are less suitable here. In turn, the W boson or photon might provide acleaner signature than an additional hard jet. From the theory point of view, all three observableshave interesting and complementary features that are worthwhile exploring. We therefore stronglysuggest to pursue their measurement in the increasingly large data set obtained from the secondruntime of the LHC.
7. Acknowledgements
A warm acknowledgement goes to the organizers of CKM 2017 for an interesting conference andfor introducing us visitors to the character of Mumbai. SW acknowledges funding by the Carl Zeissfoundation through a
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