Eur. Phys. J. C (2016) 76:585
DOI 10.1140/epjc/s10052-016-4400-6
Regular Article - Experimental Physics
Search for Minimal Supersymmetric Standard Model Higgs
bosons H/ A and for a Z
′
boson in the ττ final state produced
in pp collisions at
√
s = 13 TeV with the ATLAS Detector
ATLAS Collaboration
⋆
CERN, 1211 Geneva 23, Switzerland
Received: 8 August 2016 / Accepted: 26 September 2016 / Published online: 27 October 2016
© CERN for the benefit of the ATLAS collaboration 2016. This article is published with open access at Springerlink.com
Abstract A search for neutral Higgs bosons of the minimal
supersymmetric standard model (MSSM) and for a heavneu-
tral Z
′
boson is performed using a data sample corresponding
to an integrated luminosity of 3.2 fb
−1
from proton–proton
collisions at
√
s = 13 TeV recorded by the ATLAS detector
at the LHC. The heavy resonance is assumed to decay to a
τ
+
τ
−
pair with at least one τ lepton decaying to final states
with hadrons and a neutrino. The search is performed in the
mass range of 0.2–1.2 TeV for the MSSM neutral Higgs
bosons and 0.5–2.5 TeV for the heavy neutral Z
′
boson. The
data are in good agreement with the background predicted by
the Standard Model. The results are interpreted in MSSM and
Z
′
benchmark scenarios. The most stringent constraints on
the MSSM m
A
–tan β space exclude at 95% confidence level
(CL) tan β>7.6form
A
= 200 GeV in the m
mod+
h
MSSM
scenario. For the Sequential Standard Model, a Z
′
SSM
mass
up to 1.90 TeV is excluded at 95% CL and masses up to
1.82–2.17 TeV are excluded for a Z
′
SFM
of the strong flavour
model.
Contents
1 Introduction ..................... 1
2 Data sample and Monte Carlo simulation ...... 2
3 Object reconstruction and identification ....... 3
4 Search channels ................... 4
4.1 τ
lep
τ
had
channel
................. 4
4.2 τ
had
τ
had
channel ................. 5
4.3 Event categories ................. 5
4.4 Di-tau mass reconstruction ........... 6
5 Background estimation ................ 6
5.1 τ
lep
τ
had
background estimate
.......... 6
5.2 τ
had
τ
had
background estimate
.......... 7
6 Systematic uncertainties ............... 9
7 Results ........................ 10
⋆
e-mail:
atlas.publications@cern.ch
8 Conclusions ..................... 13
References ........................ 14
1 Introduction
The discovery of a scalar particle at the LHC [
1,2] has pro-
vided important insight into the mechanism of electroweak
symmetry breaking. Experimental studies of the new parti-
cle [
3–7] demonstrate consistency with the standard model
(SM) Higgs boson [
8–13]. However, it remains possible that
the discovered particle is part of an extended scalar sector,
a scenario that is favoured by a number of theoretical argu-
ments [
14,15].
The minimal supersymmetric standard model (MSSM)
[
16–20] is the simplest extension of the SM that includes
supersymmetry. The MSSM requires two Higgs doublets of
opposite hypercharge. Assuming that CP symmetry is con-
served,this results in one CP-odd (A) and two CP-even (h, H )
neutral Higgs bosons and two charged Higgs bosons (H
±
).
At tree level, the properties of the Higgs sector in the MSSM
depend on only two non-SM parameters, which can be cho-
sen to be the mass of the CP-odd Higgs boson, m
A
, and
the ratio of the vacuum expectation values of the two dou-
blets, tan β. Beyond tree level, additional parameters affect
the Higgs sector, the choice of which defines various MSSM
benchmark scenarios. In some scenarios, such as m
mod+
h
[
21],
the top-squark mixing parameter is chosen such that the mass
of the lightest CP-even Higgs boson, m
h
, is close to the mea-
sured mass of the Higgs boson that was discovered at the
LHC. A different approach is employed in the hMSSM sce-
nario [
22,23] in which the value of m
h
can be used, with
certain assumptions, to predict the remaining masses and
couplings of the MSSM Higgs bosons without explicit ref-
erence to the soft supersymmetry-breaking parameters. The
couplings of the MSSM heavy Higgs bosons to down-type
fermions are enhanced with respect to the SM for large tan β
123
585 Page 2 of 30 Eur. Phys. J. C (2016) 76 :585
values, resulting in increased branching fractions to τ lep-
tons and b-quarks,
1
as well as a higher cross section for
Higgs boson production in association with b-quarks. This
has motivated a variety of searches for a scalar boson in ττ
and bb final states at LEP [
24], the Tevatron [25–27] and the
LHC [
28–32].
Heavy Z
′
gauge bosons appear in several models [
33–
37] and are a common extension of the SM [38]. Such Z
′
bosons can appear in theories extending the electroweak
gauge group, where lepton universality is typically con-
served. A frequently used benchmark is the sequential stan-
dard model (SSM) [
39], which contains a single additional
Z
′
boson with the same couplings as the SM Z boson. Some
models offering an explanation for the high mass of the top-
quark, predict instead that such bosons couple preferentially
to third-generation fermions [
40–43]. A model predicting
additional weak gauge bosons Z
′
and W
′
coupling preferen-
tially to third-generation fermions is the strong flavour model
(SFM) [
41,43].
Direct searches for high-mass resonances decaying to ττ
have been performed by the ATLAS [
44] and CMS [45] col-
laborations using 5 fb
−1
of integrated luminosity at
√
s =
7 TeV. ATLAS [
46] updated the search with 20 fb
−1
of
integrated luminosity at
√
s = 8 TeV. Indirect limits on Z
′
bosons with non-universal flavour couplings have been set
based on measurements from LEP [
47].
This paper presents the results of a search for neutral
MSSM Higgs bosons as well as high-mass Z
′
resonances
in the ττ decay mode using 3.2 fb
−1
of proton–proton col-
lision data collected with the ATLAS detector [
48] in 2015
at a centre-of-mass energy of 13 TeV. The search is per-
formed for the τ
lep
τ
had
and τ
had
τ
had
decay modes, where τ
lep
represents the decay of a τ lepton to an electron or a muon
and neutrinos and τ
had
represents the decay to one or more
hadrons and a neutrino. The search considers narrow reso-
nances in the mass range of 0.2–1.2 TeV and tan β range of
1–60 for the MSSM Higgs bosons. For the Z
′
boson search,
the mass range of 0.5–2.5 TeV is considered. Higgs boson
production through gluon–gluon fusion and in association
with b-quarks is considered (Fig.
1a–c), with the latter mode
dominating for high tan β values. Hence both the τ
lep
τ
had
and
τ
had
τ
had
channels are split into b-tag and b-veto categories,
based on the presence or absence of jets originating from b-
quarks in the final state. Since a Z
′
boson is expected to be
predominantly produced via a Drell–Yan process (Fig.
1d),
there is little gain in splitting the data into b-tag and b-veto
categories. Hence, the Z
′
analysis uses an inclusive selection
instead.
1
Throughout this paper the inclusion of charge-conjugate decay modes
is implied.
(a) (b)
(c) (d)
Fig. 1 Lowest-order Feynman diagrams for a gluon–gluon fusion and
b-associated production in the b four-flavour and c five-flavour schemes
of a neutral MSSM Higgs boson. Feynman diagram for Drell–Yan pro-
duction of a Z
′
boson at lowest order (d)
2 Data sample and Monte Carlo simulation
The ATLAS detector [
48] at the LHC consists of an inner
tracking detector with a coverage in pseudorapidity
2
up to
|η|=2.5 surrounded by a thin superconducting solenoid
providing a 2 T axial magnetic field, electromagnetic and
hadronic calorimeters extending up to |η |=4.9 and a muon
spectrometer covering |η| < 2.7. A new innermost layer
was added to the pixel tracking detector after the end Run-
1 at a radial distance of 3.3 cm from the beam line [
49,50].
The ATLAS trigger system consists of a hardware-based first
level trigger, followed by a software-based high-level trigger
(HLT). The integrated luminosity used in this search, con-
sidering the data-taking periods of 2015 in which all relevant
detector subsystems were operational, is 3.2 fb
−1
.Thelumi-
nosity measurement and its uncertainty are derived following
a methodology similar to that detailed in Ref. [
51], from a cal-
ibration of the luminosity scale using x–y beam-separation
scans performed in August 2015.
2
ATLAS uses a right-handed coordinate system with its origin at the
nominal interaction point (IP) in the centre of the detector and the z-
axis along the beam pipe. The x-axis points from the IP to the centre of
the LHC ring, and the y-axis points upwards. Cylindrical coordinates
(r,φ) are used in the transverse plane, φ being the azimuthal angle
around the beam pipe. The pseudorapidity is defined in terms of the
polar angle θ as η =−ln tan(θ/2). Angular distance is measured in
units of R ≡
(η)
2
+ (φ)
2
.
123
Eur. Phys. J. C (2016) 76 :585 Page 3 of 30 585
Simulated events with a heavyneutral MSSM Higgs boson
produced via gluon–gluon fusion and in association with b-
quarks are generated with the POWHEG-BOX v2 [
52–54]
and MADGRAPH5_aMC@NLO 2.1.2 [
55,56] programs,
respectively. The CT10 [
57] and CT10nlo_nf4 [58]setsof
parton distribution functions (PDFs) are used, respectively.
PYTHIA 8.210 [
59] with the AZNLO [60](A14[61])
set of tuned parameters, or “tune”, is used for the parton
shower, underlying event and hadronization in the gluon–
gluon fusion (b-associated) production. The production cross
sections for the various MSSM scenarios are calculated using
SusHi [
62] for gluon fusion production [63–75] and b-
associated production in the five-flavour scheme [
76]; b-
associated production in the four-flavour scheme is calcu-
lated according to Refs. [
77,78]. The final b-associated pro-
duction cross section is obtained by using the method in
Ref. [
79] to match the four-flavour and five-flavour scheme
cross sections. The masses and the couplings of the Higgs
bosons are computed with FeynHiggs [
80–84], whereas
the branching fraction calculation follows the procedure
described in Ref. [
85]. In the case of the hMSSM scenario, the
procedure described in Ref. [
23] is followed for the produc-
tion cross sections and HDECAY [
86] is used for the branching
fraction calculation.
The Z
′
signals are simulated by reweighting a leading-
order (LO) Z/γ
∗
→ ττ sample using the TauSpinner
algorithm [
87–89] to account for spin effects in the τ decays.
The Z/γ
∗
→ ττ sample, enriched with high invariant mass
events, is generated with PYTHIA 8.165 [
90]usingthe
NNPDF2.3LO PDF set [
91] and the A14 tune for the under-
lying event. Interference between the Z
′
signals and the SM
Z/γ
∗
production is not included.
The simulated backgrounds consist of the production of
Z+jets, W+jets, t
¯
t pairs, single top quarks and electroweak
dibosons (WW/WZ/ZZ). These are modelled with sev-
eral event generators as described below, while contribu-
tions from multi-jet production are estimated with data as
described in Sect.
5.
Simulated samples of Z +jets events for the τ
lep
τ
had
and
τ
had
τ
had
channels and W +jets events for the τ
lep
τ
had
chan-
nel are produced using POWHEG-BOX v2 interfaced to
PYTHIA 8.186 with the AZNLO tune. In this sample,
PHOTOS++ v3.52 [
92,93] is used for final-state QED
radiation. A dedicated W +jets sample binned in p
W
T
,pro-
duced using the SHERPA 2.1.1 generator [
94], is used in
the τ
had
τ
had
channel in order to enhance the number of events
with high invariant mass. For this sample, matrix elements
are calculated for up to two partons at next-to-leading order
(NLO) and four partons at LO, merged with the SHERPA par-
ton shower model using the ME+PS@NLO prescription [
95].
Spin correlation effects between the W boson and its decay
products are simulated with the TauSpinner program. All
W /Z +jets samples use the CT10 PDF set and are normalized
to the next-to-next-to-leading-order (NNLO) cross sections
calculated using FEWZ [
96–98].
The POWHEG-BOX v2 program with the CT10 PDF set
is used for the generation of t
¯
t pairs and single top quarks
in the Wt- and s-channels. Samples of t -channel single-
top-quark events are produced with the POWHEG-BOX v1
generator employing the four-flavour scheme for the NLO
matrix element calculations together with the fixed four-
flavour scheme PDF set CT10f4; the top-quark decay is simu-
lated with MadSpin [
99]. For all samples of top-quark pro-
duction, the spin correlations are preserved and the parton
shower, fragmentation and underlying event are simulated
using PYTHIA 6.428 [
100] with the CTQ6L1 PDF set and
the corresponding Perugia 2012 tune [
101]. Final-state QED
radiation is simulated using PHOTOS++ v3.52. The top-
quark mass is set to 172.5 GeV. The t
¯
t production sample is
normalized to the NNLO cross section, including soft-gluon
resummation to next-to-next-to-leading-logarithm accuracy
(Ref. [
102] and references therein). The normalization of the
single-top-quark event samples uses an approximate NNLO
calculation from Refs. [
103–105].
Finally, diboson processes are simulated using the
SHERPA 2.1.1 program with the CT10 PDF. They are
calculated for up to one additional parton at NLO, depend-
ing on the process, and up to three additional partons at LO.
The diboson samples use the NLO cross sections SHERPA
calculates.
The simulation of b- and c-hadron decays for all sam-
ples, excluding those generated with SHERPA,usesEvtGen
v1.2.0 [
106]. All simulated samples include the effect of
multiple proton-proton interactions in the same and neigh-
bouring bunch crossings (“pile-up”) by overlaying simu-
lated minimum-bias events on each generated signal or back-
ground event.These minimum-bias events are generated with
PYTHIA 8.186 [
90,100], using the A2 tune [107] and the
MSTW2008LO PDF [
108]. Each sample is simulated using
the full GEANT4 [
109,110] simulation of the ATLAS detec-
tor, with the exception of the b-associated MSSM Higgs
boson signal, for which the ATLFAST-II [
110,111]fast
simulation framework is used. Finally, the Monte Carlo (MC)
samples are processed through the same reconstruction soft-
ware as for the data.
3 Object reconstruction and identification
The primary vertex of each event is chosen as the proton–
proton vertex candidate with the highest sum of the squared
transverse momenta of all associated tracks. Electron can-
didates are reconstructed from energy deposits in the elec-
tromagnetic calorimeter associated with a charged-particle
track measured in the inner detector. The final electron candi-
dates are required to pass the “loose” likelihood-based iden-
123
585 Page 4 of 30 Eur. Phys. J. C (2016) 76 :585
tification selection [
112,113], to have a transverse energy
E
T
> 15 GeV and to be in the fiducial volume of the inner
detector, |η| < 2.47. The transition region between the barrel
and end-cap calorimeters (1.37 < |η| < 1.52) is excluded.
Muon candidates are reconstructed from track segments
in the muon spectrometer, matched with tracks found in the
inner detector within |η| < 2.5. The tracks of the final muon
candidates are refit using the complete track information from
both detector systems and are required to have a transverse
momentum p
T
> 15 GeV and to pass the “loose” muon
identification requirements [
114].
Both the electrons and muons are required to pass a p
T
-
dependent isolation selection, which utilizes both calorimet-
ric and tracking information, with an efficiency of 90 %
(99%) for transverse momentum of p
T
= 25 (60) GeV.
The isolation provides an efficiency that grows as a func-
tion of lepton p
T
, since the background from jets faking lep-
tons becomes less important as the lepton p
T
increases. The
contributions from pile-up and the underlying event activity
are corrected on an event-by-event basis using the ambient
energy density technique [
115].
Jets are reconstructed from topological clusters [
116]in
the calorimeter using the anti-k
t
algorithm [
117], with a
radius parameter value R = 0.4. To reduce the effect of
pile-up, a jet vertex tagger algorithm is used for jets with
p
T
< 50 GeV and | η| < 2.4. It employs a multivariate
technique based on jet energy, vertexing and tracking vari-
ables to determine the likelihood that the jet originates from
pile-up [
118]. In order to identify jets containing b-hadrons
(b-jets), a multivariate algorithm is used [
119,120]. A work-
ing point that corresponds to an average efficiency of 70%
for b-jets in t
¯
t simulated events is chosen. The misidentifi-
cation rates for c-jets, τ -jets and jets initiated by light quarks
or gluons for the same working point and in the same sam-
ple of simulated t
¯
t events are approximately 10, 4 and 0.2%
respectively.
Hadronic decays of τ leptons are predominantly character-
ized by the presence of one or three charged particles, accom-
panied by a neutrino and possibly neutral pions. The recon-
struction of the visible decay products, hereafter referred to
as τ
had−vis
, starts with jets with p
T
> 10 GeV. The τ
had−vis
candidate must have energy deposits in the calorimeters in
the range |η| < 2.5, with the transition region between the
barrel and end-cap calorimeters excluded. Additionally, they
must have p
T
> 20 GeV, one or three associated tracks and
an electric charge of ±1. A multivariate boosted decision tree
(BDT) identification, based on calorimetric shower shape and
track multiplicity of the τ
had−vis
candidates, is used to reject
backgrounds from jets. In this analysis, two τ
had−vis
identifi-
cation criteria are used: “loose” and “medium” with efficien-
cies measured in Z → ττ decays of about 60 % (50%) and
55% (40 %) for one-track (three-track) τ
had−vis
candidates,
respectively [
121]. An additional dedicated likelihood-based
veto is used to reduce the number of electrons misidentified
as τ
had−vis
.
Signals in the detector can be used in more than one recon-
structed object. Objects that have a geometric overlap are
removed according to the following priorities:
• Jets within a R = 0.2 cone around a selected τ
had−vis
are excluded.
• Jets within a R = 0.4 cone around an electron or muon
are excluded.
• Any τ
had−vis
within a R = 0.2 cone around an electron
or muon is excluded.
• Electrons within a R = 0.2 cone around a muon are
excluded.
The missing transverse momentum (E
miss
T
) is calculated
as the modulus of the negative vectorial sum of the p
T
of
all fully reconstructed and calibrated jets and leptons [
122].
This procedure includes a “soft term”, which is calculated
based on the inner-detector tracks originating from the pri-
mary vertex that are not associated to reconstructed objects.
4 Search channels
4.1 τ
lep
τ
had
channel
Eventsin the τ
lep
τ
had
channel are recorded using single-muon
triggers and a logical-OR combination of single-electron trig-
gers. Single-electron triggers with p
T
thresholds of 24 GeV,
60 GeV and 120 GeV are used for the τ
e
τ
had
channel. For the
τ
μ
τ
had
channel, a single-muon trigger with a p
T
threshold of
50 GeV is used if the muon p
T
is larger than 55 GeV and a
single-muon trigger with a p
T
threshold of 20 GeV is used
otherwise. The triggers impose electron and muon quality
requirements which are tighter for the triggers with lower p
T
thresholds.
Events must have at least one identified τ
had−vis
candidate
and either one electron or one muon candidate which is geo-
metrically matched to the HLT object that triggered the event.
Events with more than one electron or muon fulfilling the cri-
teria described in Sect.
3 are rejected in order to reduce the
backgrounds from Z/γ
∗
→ ℓℓ production, where ℓ = e, μ.
The selected lepton must have a transverse momentum p
T
>
30 GeV and pass the “medium” identification requirement.
The τ
had−vis
candidate is required to have p
T
> 25 GeV,
pass the “medium” BDT-based identification requirement
and lie in the range |η| < 2.3. The latter requirement is moti-
vated by a larger rate of electrons misidentified as τ
had−vis
candidates at higher |η| values: the rate is above 10 % for
|η| > 2.3, while it ranges from 0.5 to 3 % for lower |η| val-
ues. If there is more than one τ
had−vis
candidate, the candidate
with the highest p
T
is selected and the others are treated as
123
Eur. Phys. J. C (2016) 76 :585 Page 5 of 30 585
Events / GeV
1−
10
1
10
2
10
3
10
4
10
5
10
6
10
ATLAS
-1
= 13 TeV, 3.2 fbs
channel
had
τ
lep
τ
Data
ττ→H/A
= 20β = 600 GeV, tan
A
m
fakesτl,→Jet
ττ→Z
, single toptt
Diboson
µµ ee/→Z
Uncertainty
) [GeV]
miss
T
(lepton, E
T
m
0 50 100 150 200
Data/Pred
0.8
1
1.2
Events / 0.14
1
10
2
10
3
10
4
10
ATLAS
-1
= 13 TeV, 3.2 fbs
channel
had
τ
had
τ
Data
ττ→H/A
= 20β = 500 GeV, tan
A
m
Multijet
ττ→Z
τν→W
, single toptt
Others
Uncertainty
ττ
φ∆
1 1.5 2 2.5 3
Data/Pred
0.8
1
1.2
(a) (b)
Fig. 2 The distributions of a m
T
(ℓ, E
miss
T
) in the τ
lep
τ
had
channel and
b φ(τ
had−vis,1
,τ
had−vis,2
) for the τ
had
τ
had
channel for the inclusive
selection with the criterion for the variable displayed removed. The label
“Others” in b refers to contributions due to diboson, Z (→ ℓℓ)+jets and
W (→ ℓν)+jets production. Bins have a varying size and overflows are
included in the last bin of the distribution on the left
jets. Finally, the identified lepton and the τ
had−vis
are required
to have opposite electric charge.
Subsequently, the following selection requirements are
applied:
• φ(τ
had−vis
,ℓ) >2.4.
• m
T
(ℓ, E
miss
T
) ≡
2p
T
(ℓ)E
miss
T
1 − cos φ(ℓ, E
miss
T
)
< 40 GeV.
• For the τ
e
τ
had
channel, events are vetoed if the invariant
mass of the electron and the visible τ lepton decay prod-
ucts is in the range 80 < m
vis
(e,τ
had−vis
)<110 GeV.
The requirement on φ(τ
had−vis
,ℓ) gives an overall
reduction of SM backgrounds with little signal loss. The
requirement on m
T
(ℓ, E
miss
T
), the distribution of which is
shown in Fig.
2a, serves to remove events that originate from
processes containing a W boson: in signal events, the missing
transverse momentum is usually in the same direction as the
τ
lep
, resulting in a low value of m
T
(ℓ, E
miss
T
). The require-
ment on m
vis
(e,τ
had−vis
) reduces the contribution of Z → ee
decays, where an electron is misidentified as a τ
had−vis
can-
didate. These selection criteria define the inclusive τ
lep
τ
had
selection.
4.2 τ
had
τ
had
channel
Events in the τ
had
τ
had
channel are selected by a trigger that
requires a single τ
had−vis
satisfying the “medium” τ
had−vis
identification criterion with p
T
> 80 GeV. The leading
τ
had−vis
candidate in p
T
must geometrically match the HLT
object. A p
T
requirement is applied to the leading τ
had−vis
candidate, p
T
> 110 GeV, and to the sub-leading τ
had−vis
candidate, p
T
> 55 GeV. Furthermore, the leading (sub-
leading) τ
had−vis
candidate has to satisfy the “medium”
(“loose”) τ
had−vis
identification criterion. Events with elec-
trons or muons fulfilling the loose selection criteria described
in Sect.
3 (with the exception of the isolation requirement)
are vetoed to reduce electroweak background processes and
guarantee orthogonality with the τ
lep
τ
had
channel.
The leading and sub-leading τ
had−vis
candidates must have
opposite electric charge and have a back-to-back topology in
the transverse plane, φ(τ
had−vis,1
,τ
had−vis,2
)>2.7. The
distribution of φ(τ
had−vis,1
,τ
had−vis,2
) before this require-
mentisshowninFig.
2b. This selection defines the inclusive
τ
had
τ
had
selection.
4.3 Event categories
In the search for Z
′
bosons, the event selections described
in Sects.
4.1 and 4.2 result in a signal selection efficiency
3
varying between 0.8% (2.0%) at m
Z
′
= 500 GeV and 3.4%
(3.8%) at m
Z
′
= 2.5 TeV for the τ
lep
τ
had
(τ
had
τ
had
) chan-
nel. In the search for the H/ A bosons, events satisfying the
inclusive selection are categorized to exploit the two different
signal production modes as follows:
• b-veto: no b-tag jets in the event,
• b-tag: at least one b-tag jet in the event.
3
The term “signal selection efficiency” refers to the fraction of signal
events decaying to τ
lep
τ
had
or τ
had
τ
had
that are subsequently recon-
structed within the detector acceptance and pass the selection require-
ments.
123