The ATLAS Collaboration / Physics Letters B 800 (2020) 135069 5
Fig. 1. Distributions of representative kinematic quantities for different searches, channels and categories, before the fit as described in Section 7 is applied. Top row:
transverse mass m
T
(
1
, E
miss
T
) (eτ
μ
non-VBF), collinear mass m
coll
(eτ
had
non-VBF) and m
MMC
(eτ
μ
VBF). Bottom row: m
MMC
(μτ
e
non-VBF), muon p
T
(μτ
had
non-VBF) and
m
coll
(μτ
had
VBF). Entries with values that would exceed the x-axis range are included in the last bin of each distribution. The size of the combined statistical, experimental
and theoretical uncertainties in the background is indicated by the hatched bands. The H → eτ (H → μτ ) signal overlaid in top (bottom) plots assumes B(H → τ ) = 1%
and
is enhanced by a factor 10. In the data/background prediction ratio plots, points outside the displayed y-axis range are shown by arrows.
Another source of background comes from W + jets, top-quark
and multi-jet events, where jets are misidentified as leptons. This
background is estimated directly from OS data events where an
inverted isolation requirement is imposed on the subleading lep-
ton [17].
Normalization factors are applied to correct for the in-
verted
isolation requirement. The normalization factors are derived
in a dedicated region where the leptons are required to have same-
sign
(SS) charges. Additional corrections are made by reweight-
ing
the MC distributions of φ(
1
, E
miss
T
) and φ(
2
, E
miss
T
) to
data in the SS region, which improves the modelling of azimuthal
angles between leptons and the E
miss
T
direction as well as the
modelling of m
T
(
2
, E
miss
T
). A similar improvement is observed
in the nominal OS region. In most of the cases, the misidenti-
fied
jet mimics the lepton of lower p
T
,
2
, while the fraction of
events where both leptons are misidentified varies between 2%
to 8% across categories. The systematic uncertainties of the es-
timation
of the misidentified lepton background include contri-
butions
from closure tests in SS and OS regions enriched with
misidentified leptons, from the corrections made to the φ dis-
tributions,
and from the composition of the misidentified lepton
background.
5.2. τ
had
channel
The main background contributions come from the Z → ττ
process and events where either a jet or an electron is misiden-
tified
as τ
had-vis
. The shape of the Z → ττ background distribution
is modelled by simulation, and the corresponding normalization
factors are determined from the simultaneous fit of the event
yields in all signal and control regions. The Z → ττ normalization
factors are fully correlated with those of the τ
channel, in each
VBF and non-VBF category. Top-quark production represents less
than 1% of the total background in the τ
had
channel and is de-
termined
by simulation, including its normalization, which is kept
fixed in the fit.
The
main contributions to jets misidentified as τ
had-vis
come
from multi-jet events and W -boson production in association with
jets, and a fake-factor method is used to estimate the contribu-
tion
of each component separately. A fake factor is defined as the
ratio of the number of events where the highest-p
T
jet is identi-
fied
as a tight τ
had-vis
candidate to the number of events where
the highest-p
T
jet fails to satisfy this τ -ID criterion but satisfies
a looser criterion. The procedure, including systematic uncertain-
ties,
is described in Ref. [17]. Since a different τ -ID working point