SNSN-323-63

Measurements of asymmetries in top-quark production and tests of lepton
universality in ATLAS

Nello Bruscino, on behalf of the ATLAS Collaboration 111Copyright 2020 CERN
for the benefit of the ATLAS Collaboration. CC-BY-4.0 license.

INFN Sezione di Roma 1 & Dipartimento di Fisica,
Sapienza Università di Roma, Roma; Italy

> The top quark is the heaviest known fundamental particle. As it is the only
> quark that decays without hadronisation, it provides the unique opportunity
> to probe the properties of bare quarks at the Large Hadron Collider (LHC).
> This article will present two recent measurements of the top quark using
> $13\,\textnormal{TeV}$ collision data with the ATLAS experiment: the top and
> anti-top quark pair ($t\overline{t}$) charge asymmetry measurement alongside
> the first test of lepton flavour universality (LFU) of leptons to $W$ bosons
> from $t\overline{t}$ events.

> PRESENTED AT
>
>
>
>
> $13^{\mathrm{th}}$ International Workshop on Top Quark Physics
> Durham, UK (videoconference), 14–18 September, 2020

##  1 Introduction

The large mass of the top quark, which is close to the electroweak symmetry
breaking scale, 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 top quark has a very short lifetime ($\tau=0.5\times
10^{-25}\,\textnormal{s}$) and decays before hadronisation
($\tau_{\textnormal{\scriptsize had}}\sim 10^{-23}\,\textnormal{s}$) or spin
de-correlation take place ($\tau_{\textnormal{\scriptsize spin dec.}}\sim
10^{-22}\,\textnormal{s}$). Therefore several properties of the top quark may
be measured precisely from its decay products.

Due to the large top-pair production ($t\overline{t}$) cross section for
$13\,\textnormal{TeV}$ proton–proton ($pp$) collisions, the Large Hadron
Collider (LHC) experiments collect an unprecedented number of top-quark
events. The copious amount of detected events allows for high precision
measurements in order to probe predictions of quantum chromodynamics (QCD),
which provides the largest contribution to $t\overline{t}$ production. This
process may also be employed to produce a large, unbiased sample of $W$-bosons
and study its properties.

This article focuses on two recent results in the top-quark sector by the
ATLAS [1] Collaboration, using proton-proton ($pp$) collisions at the Large
Hadron Collider (LHC):

  * •

the inclusive and differential measurements of the charge asymmetry ($A_{C}$)
in $t\overline{t}$ events at $13\,\textnormal{TeV}$;

  * •

the first test of lepton-flavour universality using di-leptonic
$t\overline{t}$ events at $13\,\textnormal{TeV}$.

##  2 Measurements of the charge asymmetry in $t\overline{t}$ events at
$13\,\textnormal{TeV}$ with the ATLAS detector

Production of top quark pairs is symmetric at leading-order (LO) under charge
conjugation. The asymmetry between the $t$ and $\overline{t}$ originates from
interference of the higher-order amplitudes in the $q\overline{q}$ and $qg$
initial states, with the $q\overline{q}$ annihilation contribution dominating.
The contribution from electro-weak corrections is about 13% for the inclusive
asymmetry. The $gq\to t\overline{t}q$ production process is also asymmetric,
but its cross section is much smaller than $q\overline{q}$. Gluon fusion
production is symmetric to all orders. As a consequence of these asymmetries,
the top quark is preferentially produced in the direction of the incoming
quark.

At a $p\overline{p}$ collider, where the preferential direction of the
incoming quark (antiquark) always almost coincides with that of the proton
(anti-proton), a forward-backward asymmetry $A_{\textnormal{\scriptsize FB}}$
can be measured directly. At the LHC $pp$ collider, since the colliding beams
are symmetric, it is not possible to measure $A_{\textnormal{\scriptsize FB}}$
as there is no preferential direction of either the top quark or the top
antiquark. However, due to the difference in the proton parton distribution
functions, on average the valence quarks carry a larger fraction of the proton
momentum than the sea antiquarks. This results in more forward top quarks and
more central top antiquarks.

A central-forward charge asymmetry for the $t\overline{t}$ production,
referred to as the charge asymmetry ($A_{\textnormal{\scriptsize C}}$), is
defined as $A_{\textnormal{\scriptsize
C}}^{t\overline{t}}=\frac{N(\Delta|y|>0)-N(\Delta|y|<0)}{N(\Delta|y|>0)+N(\Delta|y|<0)}$,
where $\Delta|y|=|y(t)|-|y(\overline{t})|$ is the difference between the
absolute value of the top-quark rapidity $|y_{t}|$ and the absolute value of
the top-antiquark rapidity $|y_{\overline{t}}|$.

The measurement of the $t\overline{t}$ charge asymmetry is performed using
data corresponding to an integrated luminosity of $139\,\textnormal{fb}^{-1}$
from the ATLAS experiment [2]. It is performed in the single-lepton channel
combining both the resolved and boosted topologies of top quark decays. A
Bayesian unfolding procedure is used to infer the asymmetry at parton level,
correcting for detector resolution and acceptance effects.

The inclusive $t\overline{t}$ charge asymmetry is measured as
$A_{\textnormal{\scriptsize C}}=0.0060\pm 0.0015\textnormal{(stat+syst.)}$,
which differs from zero by 4 standard deviations. It corresponds to the first
evidence for charge asymmetry in $pp$ collisions. Differential measurements
are performed as a function of the invariant mass and longitudinal boost of
the $t\overline{t}$ system. Both inclusive and differential measurements are
found to be compatible with the SM predictions, at NNLO in perturbation theory
with NLO electroweak corrections, and are shown in Figure 1.

(a)  (b)  (c)

Figure 1: The unfolded inclusive (a) and differential charge asymmetries as a
function of 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 SM theory predictions calculated at NNLO in QCD and NLO
in electroweak theory. Red hatched regions show parton-level truth asymmetry
with its uncertainty extracted from the full phase space using the nominal
$t\overline{t}$ signal sample. Vertical bars correspond to the total
uncertainties. [2]

##  3 Test of the universality of $\tau$ and $\mu$ lepton couplings in
$W$-boson decays from $t\overline{t}$ events with the ATLAS detector

The Standard Model of particle physics encapsulates our current best
understanding of physics at the smallest scales. A fundamental axiom of this
theory is the universality of the couplings of the different generations of
leptons to the electroweak gauge bosons. The measurement of the ratio of the
rate of decay of $W$ bosons to $\tau$-leptons and muons,
$R(\tau/\mu)=B(W\to\tau\nu_{\tau})/B(W\to\mu\nu_{\mu})$, constitutes an
important test of this axiom. Previously, $R(\tau/\mu)$ has been measured by
the four experiments at the Large Electron–Positron Collider (LEP), yielding a
combined value of $1.070\pm 0.026$ [3]. This deviates from the SM expectation
of unity by $2.7\sigma$, motivating a precise measurement of this ratio at the
LHC.

The measurement of this quantity is performed with a novel technique using the
large number of $t\overline{t}$ events produced at LHC. It is based on
$139\,\textnormal{fb}^{-1}$ of data recorded with the ATLAS detector in
proton–proton collisions at $13\,\textnormal{TeV}$ [4]. Given the large
$B(t\to Wq)$, 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 unbiased W boson decays to muons and $\tau$-leptons. The
$\tau$-leptons are identified through their decay to muons. The displacement
of the $\tau$ decay vertex and the different muon transverse momentum
($p_{\textnormal{\scriptsize T}}$) spectra are used to distinguish between
muons from the $W\to\tau\nu_{\tau}$ and $W\to\mu\nu_{\mu}$ processes, to
extract $R(\tau/\mu)$. This is achieved by utilising the precise
reconstruction of muon tracks obtainable by the ATLAS experiment.

Muons originating from $W$ bosons and those originating from an intermediate
$\tau$-lepton are distinguished using the lifetime of the $\tau$-lepton,
through the muon transverse impact parameter, and differences in the muon
transverse momentum spectra. The two largest backgrounds are
$Z(\to\mu\mu)$+jets and events in which the probe muon does not originate from
a $W$ boson decay. Three dedicated control regions are used to extract the
normalisation of these backgrounds.

A profile likelihood fit is performed in three bins in
$p_{\textnormal{\scriptsize T}\,\mu}$ (boundaries of: 5, 10, 20,
$250\,\textnormal{GeV}$) and eight bins in the transverse impact parameter,
$|d_{0\,\mu}|$ (boundaries of: 0, 0.01, 0.02, 0.03, 0.04, 0.06, 0.09, 0.15,
$0.5\,\textnormal{mm}$), of the probe muon for each channel ($e–\mu$ and
$\mu-\mu$), making 48 bins in total.

Figure 2 shows the differential distributions of $|d_{0\,\mu}|$ in the six
signal regions for the data and the expectation after the fit to data. Good
agreement is observed between the corrected simulation samples and the data.

(a)  (b)  (c)

(d)  (e)  (f)

Figure 2: The $|d_{0\,\mu}|$ distributions for each channel ((a),(b),(c):
$e–\mu$ channel, (d),(e),(f): $\mu-\mu$ channel) and probe muon
$p_{\textnormal{\scriptsize T}\,\mu}$ bin ((a)-(d):
$5<p_{\textnormal{\scriptsize T}\,\mu}<10\,\textnormal{GeV}$, (b)-(e):
$10<p_{\textnormal{\scriptsize T}\,\mu}<20\,\textnormal{GeV}$, (c)-(f):
2$20<p_{\textnormal{\scriptsize T}\,\mu}<250\,\textnormal{GeV}$) used in the
analysis. Plots are shown after the fit has been performed. The data are
represented by points and a stacked histogram represents the different
simulated processes. The bottom panel shows the ratio of the data to the
expectation. Blue bands indicate the systematic uncertainties with the
constraints from the analysis fit applied. Different components are labelled
according to the muon source and process. The contribution from ’other SM
processes’ is dominated by di-boson and $t\overline{t}+V$ production. [4]

The value of $R(\tau/\mu)$ is found to be $0.992\pm 0.013$ [$\pm 0.007$ (stat)
$\pm 0.011$ (syst)] and is in agreement with the hypothesis of universal
lepton couplings as postulated in the Standard Model. This is the most precise
measurement of this ratio, and the only such measurement from the Large Hadron
Collider, to date (Figure 3).

Figure 3: The measured value of $R(\tau/\mu)$ with previous measurements, as
well as previous measurements of $R(\mu/e)$ and $R(\tau/e)$. The statistical
(yellow box) and systematic (purple box) errors are shown separately as well
as the total error on the measurement (black circular marker). A vertical
dashed line indicates the Standard Model prediction of equal branching ratios
to different lepton flavours. [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.