Measurements of asymmetries in top-quark production and tests of lepton universality in ATLAS
SSNSN-323-63February 26, 2021
Measurements of asymmetries in top-quark production andtests of lepton universality in ATLAS
Nello Bruscino, on behalf of the ATLAS Collaboration INFN Sezione di Roma 1 & Dipartimento di Fisica,Sapienza Universit`a di Roma, Roma; Italy
The top quark is the heaviest known fundamental particle. As it isthe only quark that decays without hadronisation, it provides the uniqueopportunity to probe the properties of bare quarks at the Large HadronCollider (LHC). This article will present two recent measurements of thetop quark using 13 TeV collision data with the ATLAS experiment: thetop and anti-top quark pair ( tt ) charge asymmetry measurement alongsidethe first test of lepton flavour universality (LFU) of leptons to W bosonsfrom tt events. PRESENTED AT th International Workshop on Top Quark PhysicsDurham, UK (videoconference), 14–18 September, 2020 Copyright 2020 CERN for the benefit of the ATLAS Collaboration. CC-BY-4.0 license. a r X i v : . [ h e p - e x ] F e b Introduction
The large mass of the top quark, which is close to the electroweak symmetry breakingscale, indicates that this particle could play a special role in the Standard Model(SM) as well as in beyond the Standard Model (BSM) theories. Moreover, the topquark has a very short lifetime ( τ = 0 . × − s) and decays before hadronisation( τ had ∼ − s) or spin de-correlation take place ( τ spin dec. ∼ − s). Thereforeseveral properties of the top quark may be measured precisely from its decay products.Due to the large top-pair production ( tt ) cross section for 13 TeV proton–proton( pp ) collisions, the Large Hadron Collider (LHC) experiments collect an unprece-dented number of top-quark events. The copious amount of detected events allowsfor high precision measurements in order to probe predictions of quantum chromody-namics (QCD), which provides the largest contribution to tt production. This processmay also be employed to produce a large, unbiased sample of W -bosons and studyits properties.This article focuses on two recent results in the top-quark sector by the AT-LAS [1] Collaboration, using proton-proton ( pp ) collisions at the Large Hadron Col-lider (LHC): • the inclusive and differential measurements of the charge asymmetry ( A C ) in tt events at 13 TeV; • the first test of lepton-flavour universality using di-leptonic tt events at 13 TeV. tt events at
13 TeV with the ATLAS detector
Production of top quark pairs is symmetric at leading-order (LO) under charge con-jugation. The asymmetry between the t and t originates from interference of thehigher-order amplitudes in the qq and qg initial states, with the qq annihilation con-tribution dominating. The contribution from electro-weak corrections is about 13%for the inclusive asymmetry. The gq → ttq production process is also asymmetric,but its cross section is much smaller than qq . Gluon fusion production is symmetricto all orders. As a consequence of these asymmetries, the top quark is preferentiallyproduced in the direction of the incoming quark.At a pp collider, where the preferential direction of the incoming quark (antiquark)always almost coincides with that of the proton (anti-proton), a forward-backwardasymmetry A FB can be measured directly. At the LHC pp collider, since the collidingbeams are symmetric, it is not possible to measure A FB as there is no preferentialdirection of either the top quark or the top antiquark. However, due to the differencein the proton parton distribution functions, on average the valence quarks carry a1arger fraction of the proton momentum than the sea antiquarks. This results inmore forward top quarks and more central top antiquarks.A central-forward charge asymmetry for the tt production, referred to as the chargeasymmetry ( A C ), is defined as A tt C = N (∆ | y | > − N (∆ | y | < N (∆ | y | > N (∆ | y | < , where ∆ | y | = | y ( t ) | − | y ( t ) | is the difference between the absolute value of the top-quark rapidity | y t | and theabsolute value of the top-antiquark rapidity | y t | .The measurement of the tt charge asymmetry is performed using data corre-sponding to an integrated luminosity of 139 fb − from the ATLAS experiment [2].It is performed in the single-lepton channel combining both the resolved and boostedtopologies of top quark decays. A Bayesian unfolding procedure is used to infer theasymmetry at parton level, correcting for detector resolution and acceptance effects.The inclusive tt charge asymmetry is measured as A C = 0 . ± . pp collisions. Differential measurements are performed as afunction of the invariant mass and longitudinal boost of the tt system. Both inclusiveand differential measurements are found to be compatible with the SM predictions,at NNLO in perturbation theory with NLO electroweak corrections, and are shownin Figure 1. Inclusive C A NNLO QCD + NLO EWPowheg+Pythia8Data (stat./total)
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Preliminary -1 = 13 TeV, 139 fbs (a) [0,0.3] [0.3,0.6] [0.6,0.8] [0.8,1] tz,t b - - C A NNLO QCD + NLO EWPowheg+Pythia8Data (stat./total)
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Preliminary -1 = 13 TeV, 139 fbs (b) <500 [500,750] [750,1000] [1000,1500] > 1500 [GeV] tt m0.03 - - - C A NNLO QCD + NLO EWPowheg+Pythia8Data (stat./total)
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Preliminary -1 = 13 TeV, 139 fbs (c) Figure 1: The unfolded inclusive (a) and differential charge asymmetries as a functionof the invariant mass (b) and the longitudinal boost (c) of the top pair system in data(resolved and boosted topologies are combined). Green hatched regions show SMtheory predictions calculated at NNLO in QCD and NLO in electroweak theory. Redhatched regions show parton-level truth asymmetry with its uncertainty extractedfrom the full phase space using the nominal tt signal sample. Vertical bars correspondto the total uncertainties. [2] 2 Test of the universality of τ and µ lepton cou-plings in W -boson decays from tt events with theATLAS detector The Standard Model of particle physics encapsulates our current best understandingof physics at the smallest scales. A fundamental axiom of this theory is the univer-sality of the couplings of the different generations of leptons to the electroweak gaugebosons. The measurement of the ratio of the rate of decay of W bosons to τ -leptonsand muons, R ( τ /µ ) = B ( W → τ ν τ ) /B ( W → µν µ ), constitutes an important test ofthis axiom. Previously, R ( τ /µ ) has been measured by the four experiments at theLarge Electron–Positron Collider (LEP), yielding a combined value of 1 . ± .
026 [3].This deviates from the SM expectation of unity by 2 . σ , motivating a precise mea-surement of this ratio at the LHC.The measurement of this quantity is performed with a novel technique using thelarge number of tt events produced at LHC. It is based on 139 fb − of data recordedwith the ATLAS detector in proton–proton collisions at 13 TeV [4]. Given the large B ( t → W q ), close to 100%, a very large sample of W boson pairs can be exploited.These are used in a tag-and-probe technique to obtain a large sample of clean and un-biased W boson decays to muons and τ -leptons. The τ -leptons are identified throughtheir decay to muons. The displacement of the τ decay vertex and the different muontransverse momentum ( p T ) spectra are used to distinguish between muons from the W → τ ν τ and W → µν µ processes, to extract R ( τ /µ ). This is achieved by utilisingthe precise reconstruction of muon tracks obtainable by the ATLAS experiment.Muons originating from W bosons and those originating from an intermediate τ -lepton are distinguished using the lifetime of the τ -lepton, through the muon trans-verse impact parameter, and differences in the muon transverse momentum spectra.The two largest backgrounds are Z ( → µµ )+jets and events in which the probe muondoes not originate from a W boson decay. Three dedicated control regions are usedto extract the normalisation of these backgrounds.A profile likelihood fit is performed in three bins in p T µ (boundaries of: 5, 10, 20,250 GeV) and eight bins in the transverse impact parameter, | d µ | (boundaries of: 0,0.01, 0.02, 0.03, 0.04, 0.06, 0.09, 0.15, 0 . e – µ and µ − µ ), making 48 bins in total.Figure 2 shows the differential distributions of | d µ | in the six signal regions for thedata and the expectation after the fit to data. Good agreement is observed betweenthe corrected simulation samples and the data.The value of R ( τ /µ ) is found to be 0 . ± .
013 [ ± .
007 (stat) ± .
011 (syst)]and is in agreement with the hypothesis of universal lepton couplings as postulatedin the Standard Model. This is the most precise measurement of this ratio, and theonly such measurement from the Large Hadron Collider, to date (Figure 3).3 m |d0.90.9511.05 D a t a / P r ed . E v en t s / . mm ATLAS -1 = 13 TeV, 139 fbsSignal Region<10 GeV m T , 5
Figure 2: The | d µ | distributions for each channel ((a),(b),(c): e – µ channel, (d),(e),(f): µ − µ channel) and probe muon p T µ bin ((a)-(d): 5 < p T µ <
10 GeV, (b)-(e): 10
20 GeV, (c)-(f): 220 < p T µ <
250 GeV) used in the analysis. Plots areshown after the fit has been performed. The data are represented by points and astacked histogram represents the different simulated processes. The bottom panelshows the ratio of the data to the expectation. Blue bands indicate the systematicuncertainties with the constraints from the analysis fit applied. Different componentsare labelled according to the muon source and process. The contribution from ’otherSM processes’ is dominated by di-boson and tt + V production. [4] References [1] ATLAS Collaboration, 2008 JINST 3 S08003.[2] ATLAS Collaboration, ATLAS-CONF-2019-026.[3] LEP Electroweak Working Group, Phys. Rept. 532 (2013) 119[4] ATLAS Collaboration, CERN-EP-2020-139, Submitted to Nature Physics.4 .8 0.9 1 1.1 1.2 ’) ν l’ → W ( Β )/ ν l → W ( Β ) νµ→ W ( Β ) ντ→ W ( Β ) ν e → W ( Β ) ντ→ W ( Β ) ν e → W ( Β ) νµ→ W ( Β ATLAS
UA1UA2CDFD0LHCbLEPATLASPDG averages
Z.Phys. C44 (1989) 15 61PLB. 280 (1992) 137 145J.Phys.G 34 (2007) 2457 2544, PRL. 68 (1992) 3398 3402PRL. 75 (1995) 1456, PRL. 84 (2000) 5710JHEP 10 (2016) 030Phys.Rept. 532 (2013) 119EPJC. 77 (2017) 367PRD. 98 (2018) 030001
ATLAS this result = 13 TeV, 139 fbs Statistical Uncert.Systematic Uncert.Total Uncertainty Figure 3: The measured value of R ( τ /µ ) with previous measurements, as well asprevious measurements of R ( µ/e ) and R ( τ /eτ /e