Search for bottom-type, vectorlike quark pair production in a fully hadronic
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
PHYSICAL REVIEW D 102, 112004 (2020)
Search for bottom-type, vectorlike quark pair production
pffiffi in a fully hadronic
final state in proton-proton collisions at s = 13 TeV
A. M. Sirunyan et al.*
(CMS Collaboration)
(Received 22 August 2020; accepted 9 October 2020; published 7 December 2020)
A search is described for the production of a pair of bottom-type vectorlike quarks (VLQs), each
decaying into a b or b̄ quark and either a Higgs or a Z boson, with a mass greater than 1000 GeV. The
analysis is based on data from proton-proton collisions at a 13 TeV center-of-mass energy recorded at the
CERN LHC, corresponding to a total integrated luminosity of 137 fb−1 . As the predominant decay modes
of the Higgs and Z bosons are to a pair of quarks, the analysis focuses on final states consisting of jets
resulting from the six quarks produced in the events. Since the two jets produced in the decay of a highly
Lorentz-boosted Higgs or Z boson can merge to form a single jet, nine independent analyses are performed,
categorized by the number of observed jets and the reconstructed event mode. No signal in excess of the
expected background is observed. Lower limits are set on the VLQ mass at 95% confidence level equal to
1570 GeV in the case where the VLQ decays exclusively to a b quark and a Higgs boson, 1390 GeV for
when it decays exclusively to a b quark and a Z boson, and 1450 GeV for when it decays equally in these
two modes. These limits represent significant improvements over the previously published VLQ limits.
DOI: 10.1103/PhysRevD.102.112004
I. INTRODUCTION Several alternative theories have been proposed for solving
this fine tuning problem. These theories include composite
One of the biggest puzzles in elementary particle physics
Higgs models [4–6], in which the Higgs boson is not a
concerns the large difference between the electroweak scale
fundamental particle, but rather contains constituents
and the Planck scale, and the related problem of the
bound by a new type of gauge interaction, and little
unexpectedly low value of the Higgs boson mass [1]. In
Higgs models [7,8], in which the Higgs boson is a
the standard model (SM), the Higgs boson H is assumed to
pseudo-Nambu–Goldstone boson that arises from sponta-
be a fundamental scalar (spin-0) particle. Unlike the
neous breaking of a global symmetry at the TeV energy
fundamental fermions (leptons and quarks) and the vector
scale. Both of these types of models predict a new class of
gauge bosons, the corrections to the Higgs boson mass due
vectorlike fermions [9] with the same charges as the SM
to vacuum energy fluctuations are quadratic, driving the
fermions, but with purely vector current couplings to the
Higgs boson mass to the cutoff value of the vacuum energy
weak gauge bosons. In composite Higgs models, the
fluctuations. In the absence of any new physics below the
vectorlike quarks (VLQs) are excited bound-state resonan-
Planck scale, this cutoff is about 1019 GeV. In that case, the ces, while in little Higgs models they are fundamental
Higgs boson mass would naturally be expected to be particles that cancel loop divergences.
seventeen orders of magnitude greater than its measured Since the VLQs are nonchiral, Lagrangian mass terms
mass of 125 GeV. not arising from Yukawa couplings to the Higgs field are
Although supersymmetry provides an elegant solution to allowed, thereby avoiding the constraints on heavy, sequen-
this problem [2,3], the lack of evidence for the production tial fourth-generation quarks set by the measured cross
of supersymmetric particles at the CERN LHC indicates section for Higgs boson production at the LHC [10,11].
that, if supersymmetry is realized in nature, it is broken at Requiring VLQs to have renormalizable couplings to
an energy scale greater than a few TeV and, therefore, does the SM quarks permits only four types of VLQs, defined
not solve the fine tuning of the 125 GeV Higgs boson mass. by their charge q: q ¼ −1=3 ðBÞ, q ¼ þ2=3 ðTÞ, q ¼
−4=3 ðXÞ, and q ¼ þ5=3 ðYÞ [12]. These are arranged
*
Full author list given at the end of the article. into seven multiplets: two singlets (T and B), three doublets
(TB, XT, and YB), and two triplets (XTB and TBY) [13].
Published by the American Physical Society under the terms of This analysis focuses on the q ¼ −1=3 ðBÞ type of VLQ.
the Creative Commons Attribution 4.0 International license.
Further distribution of this work must maintain attribution to The branching fractions B of the T and B are model
the author(s) and the published article’s title, journal citation, specific and depend upon the VLQ multiplet configuration,
and DOI. Funded by SCOAP3. the mass of the VLQ, and the coupling of the VLQ to chiral
2470-0010=2020=102(11)=112004(30) 112004-1 © 2020 CERN, for the CMS CollaborationA. M. SIRUNYAN et al. PHYS. REV. D 102, 112004 (2020)
pffiffi
quarks [14]. In general, up-type quark mass eigenstates will from proton-proton (pp) collisions at s ¼ 13 TeV with
be mixtures of the chiral up-type quarks with the T VLQ, an integrated luminosity of 36 fb−1 , have excluded a pair-
while down-type quark mass eigenstates will be mixtures of produced B with mass up to approximately 1100 GeV. The
the chiral down-type quarks and the B VLQ. Precision ATLAS analysis [17] was based on a classification of event
measurements of the couplings of the first and second signatures using neural networks, while the CMS analysis
generation SM quarks constrain their mixings with VLQs [18] used event shapes to identify Lorentz-boosted objects.
and indicate that the only sizable couplings of the T and B The ATLAS (CMS) results exclude masses, at 95% con-
VLQs allowed are to SM quarks of the third generation, fidence level (C.L.), up to 1010 (980), 710 (1070), and
although couplings to other quarks are not excluded 950 (1025) GeV for the 100% B → bH, 100% B → bZ,
[13,15,16]. In this analysis, we assume the B VLQ has and the BY doublet cases, respectively. In addition, an
three decay modes: B → bZ, B → bH, and B → tW. In ATLAS analysis [19] combining both fully hadronic and
most models, for a B VLQ mass greater than the current leptonic channels excludes values of the B mass up to
limit of approximately 1000 GeV, there is a small difference 1140 GeV for the BY doublet case. The analysis presented
between BðB → bZÞ and BðB → bHÞ, depending on the here improves on these results by using the full 137 fb−1
VLQ mass, but the difference is essentially zero for masses dataset collected by CMS in 2016–2018, and by fully
greater than 2000 GeV. The expected values of BðB → bZÞ reconstructing the event kinematics, thereby allowing the
and BðB → bHÞ also depend upon the multiplet configu- mass of the B to be reconstructed.
ration. They are 50% for both the XTB triplet and the BY
doublet, and 25% for the TBY triplet and the B singlet. The II. ANALYSIS OVERVIEW
branching fractions for the TB doublet depend upon the
mixing of the T and B VLQs with chiral quarks. If the Tt This analysis involves a search for the production
mixing is zero, the B → bH and B → bZ branching of a pair of bottom-type VLQs with masspgreaterffiffi than
fractions are 50%. If the Tt and Bb mixing are equal, 1000 GeV, using data from pp collisions at s ¼ 13 TeV
these branching fractions are 25%. If the Bb mixing is zero, at the LHC collected by the CMS detector during
these branching fractions are zero [12]. 2016–2018, corresponding to an integrated luminosity of
Results from both the ATLAS and CMS Collaborations 137 fb−1 . The analysis is focused on events in which each
using events with fully hadronic final states, based on data of the VLQs decays to a b or b̄ quark and to either a Higgs
FIG. 1. Dominant diagrams of the pair production of bottom-type VLQs (B) that subsequently decay to a b or b̄ quark and either a
Higgs or Z boson. In events targeted by this analysis, the Z boson then decays to a pair of quarks, where q denotes any quark other than a
top quark, while the Higgs boson decays to b quarks. Upper left: bHbH mode, upper right: bHbZ mode, lower: bZbZ mode.
112004-2SEARCH FOR BOTTOM-TYPE, VECTORLIKE QUARK PAIR … PHYS. REV. D 102, 112004 (2020)
or Z boson. Since the dominant decay modes of the Higgs III. THE CMS DETECTOR
and Z bosons are to a quark and antiquark pair, we select
The central feature of the CMS apparatus is a super-
final states consisting of jets resulting from the quarks and
conducting solenoid of 6 m internal diameter, providing a
antiquarks produced in the decays of the VLQs and
magnetic field of 3.8 T. Within the solenoid volume are a
subsequent decays of the two bosons. The events are
silicon pixel and strip tracker, a lead tungstate crystal
categorized into three modes, depending on the daughter
electromagnetic calorimeter (ECAL), and a brass and
bosons: bHbH, bHbZ, and bZbZ. Figure 1 shows the
scintillator hadron calorimeter (HCAL), each composed
dominant Feynman diagrams for these three modes.
of a barrel and two endcap sections. Forward calorimeters
Background from SM processes (predominantly “mul-
extend the pseudorapidity (η) coverage provided by the
tijet” events, consisting solely of jets produced through
barrel and endcap detectors. Muons are detected in gas-
the strong interaction) is reduced by requiring that the
ionization chambers embedded in the steel flux-return yoke
jets are consistent with the production of a pair of bosons
outside the solenoid.
(either Higgs or Z), that the reconstructed VLQs have
The electromagnetic calorimeter consists of 75 848 lead
equal masses, and that some of the jets are tagged as
tungstate crystals, which provide coverage in jηj < 1.48 in
originating from b quarks. For a highly boosted Higgs or
a barrel region and 1.48 < jηj < 3.0 in two endcap regions.
Z boson, the two jets resulting from its daughter quarks
In the region jηj < 1.74, the HCAL cells have widths of
might merge into a single reconstructed jet. In order to
0.087 in pseudorapidity and 0.087 in azimuth (ϕ). In the
include these events, three orthogonal, fully independent
η − ϕ plane, and for jηj < 1.48, the HCAL cells map on to
analyses are carried out using exclusive sets of events
5 × 5 arrays of ECAL crystals to form calorimeter towers
categorized by the observed jet multiplicity: 4, 5, or 6
projecting radially outwards from close to the nominal
jets. The final result is obtained by combining these three
interaction point. For jηj > 1.74, the coverage of the towers
independent analyses. In this paper, we use “jet tagging
increases progressively to a maximum of 0.174 in Δη and
requirements” to refer to both single jets tagged as being
Δϕ. Within each tower, the energy deposits in ECAL and
from a b quark, and merged jets tagged as containing a
HCAL cells are summed to define the calorimeter tower
bb̄ pair. energies, which are subsequently used to provide the
To select the correct assignment of reconstructed jets to energies and directions of hadronic jets. When combining
parent particles, a modified χ 2 metric, χ 2mod , is used. The information from the entire detector, the jet energy reso-
χ 2mod value is determined by the differences between the lution amounts typically to 15%–20% at 30 GeV, 10% at
masses of the two reconstructed bosons and the mass of 100 GeV, and 5% at 1 TeV [21].
the Higgs or Z boson, normalized by their resolutions, and Events of interest are selected using a two-tiered trigger
by the reconstructed fractional mass difference of the two system [22]. The first level, composed of custom hardware
VLQs. The event mode is assigned as bHbH, bHbZ, or processors, uses information from the calorimeters and
bZbZ, depending on which gives the smallest value of muon detectors to select events at a rate of around 100 kHz
χ 2mod . An upper cutoff on the value of χ 2mod is applied to within a fixed time interval of about 4 μs. The second level,
remove background. known as the high-level trigger (HLT), consists of a farm of
The expected background is first determined by fitting processors running a version of the full event reconstruction
the distribution of the number of events as a function of software optimized for fast processing, and reduces the
the reconstructed VLQ mass, before jet tagging require- event rate to around 1 kHz before data storage.
ments are applied, so this sample is overwhelmingly A more detailed description of the CMS detector,
background dominated. The fraction of background together with a definition of the coordinate system used
expected to remain after jet tagging requirements are and the relevant kinematic variables, can be found
applied, called the background jet-tagged fraction, is in Ref. [23].
measured using events with VLQ candidate masses in
the range 500–800 GeV, in which a VLQ signal has
IV. DATA AND SIMULATED EVENTS
already been excluded, and then corrected for a possible
dependence on the VLQ mass by using a control region The data used in this analysis were collected during the
with a higher χ 2mod value. Both the χ 2mod selection and jet 2016–2018 LHC running periods and correspond to an
tagging requirements are simultaneously optimized for integrated luminosity of 137 fb−1 [24–26].
maximal sensitivity to a potential signal. This optimiza- Signal events with pair production of VLQs
tion is done separately for each event mode and jet were simulated using the Monte Carlo generator
multiplicity. For the final result, all event mode and jet MadGraph5_aMC@NLO [27], version v2.3.3 (v2.4.2) for sam-
multiplicity analyses are combined using the procedure in ples corresponding to 2016 (2017–2018) data, at leading
Ref. [20] to obtain VLQ mass limits as a function of order with the NNPDF3.0 parton distribution functions
BðB → bHÞ and BðB → bZÞ, as described further (PDFs) [28]. The generated VLQ masses mB cover the
in Sec. X. range 1000–1800 GeV in steps of 100 GeV. Hadronization
112004-3A. M. SIRUNYAN et al. PHYS. REV. D 102, 112004 (2020)
of the underlying partons was simulated using PYTHIA v8.212 acceptance. Pileup can contribute additional tracks and
[29] with the CUETP8M1 tune [30] for samples corre- calorimetric energy depositions to the jet momentum. The
sponding to 2016 data, and with the CP5 tune [31] pileup per particle identification algorithm (PUPPI) [43] is
for samples corresponding to 2017 and 2018 data. used to mitigate the effect of pileup at the reconstructed
Corrections of the cross sections to next-to-next-to-leading particle level, making use of local shape information, event
order and next-to-next-to-leading logarithmic soft-gluon pileup properties, and tracking information. A local shape
resummation were obtained using TOP++ 2.0 [32] with the variable is defined, which distinguishes between collinear
MSTW2008NNLO68CL parton distribution set from and soft diffuse distributions of other particles surrounding
the LHAPDF 5.9.0 library [33,34]. To simulate the effect of the particle under consideration. The former is attributed to
additional pp interactions within the same or nearby bunch particles originating from the hard scatter and the latter to
crossings (“pileup”), PYTHIA v8.226 with a total inelastic pp particles originating from pileup interactions. Charged
cross section of 69.2 mb [35] was used. Following event particles identified as originating from pileup vertices are
generation, the GEANT4 package [36,37] was used to discarded. For each neutral particle, a local shape variable is
simulate the CMS detector response. Scale factors corre- computed using the surrounding charged particles com-
sponding to jet energy corrections, jet energy resolutions patible with the primary vertex within the tracker accep-
[21], pileup, and jet tagging [38,39] are applied to the tance (jηj < 2.5), and using both charged and neutral
simulated signal events so that the corresponding distribu- particles in the region outside of the tracker coverage.
tions agree with those in data. The momenta of the neutral particles are then rescaled
according to their probability to originate from the primary
V. JET RECONSTRUCTION AND TAGGING interaction vertex deduced from the local shape variable,
superseding the need for jet-based pileup corrections [44].
The global event reconstruction, also called the particle- Jet energy corrections are derived from simulation to bring
flow event reconstruction [40], aims to reconstruct and the measured response of jets to that of particle level jets on
identify each individual particle in an event, with an average. In situ measurements of the momentum balance in
optimized combination of all subdetector information. In dijet, photon þ jet, Z þ jet, and multijet events are used to
this process, the identification of the particle type (photon, account for any residual differences in the jet energy scale
electron, muon, charged hadron, or neutral hadron) plays an between data and simulation [21]. Additional selection
important role in the determination of the particle direction criteria are applied to each jet to remove jets potentially
and energy. First, photons, electrons, and muons are dominated by anomalous contributions from various sub-
identified using ECAL energy clusters, tracks in the tracker, detector components or reconstruction failures.
and hits in the muon system. Then, charged hadrons are This analysis only uses AK4 jets with pT > 50 GeV and
identified as charged particle tracks neither identified as AK8 jets with pT > 200 GeV, both within jηj < 2.4. For
electrons, nor as muons. Finally, neutral hadrons are AK8 jets, the constituents are reclustered using the
identified as HCAL energy clusters not linked to any Cambridge–Aachen algorithm [45,46]. The “modified
charged hadron trajectory, or as a combined ECAL and mass drop tagger” algorithm [47,48], also known as the
HCAL energy excess with respect to the expected charged “soft drop” algorithm, with angular exponent β ¼ 0, soft
hadron energy deposit. The energy of charged hadrons is cutoff threshold zcut < 0.1, and characteristic radius R0 ¼
determined from a combination of the track momentum and 0.8 [49], is applied to remove soft, wide-angle radiation
the corresponding ECAL and HCAL energies, corrected for from the jet. This results in a jet mass that, in the case of
the response function of the calorimeters to hadronic large mass, more accurately corresponds to the mass of the
showers, and the energy of neutral hadrons is obtained mother particle from which the jet originated. For events
from the corresponding corrected ECAL and HCAL with boosted Higgs and Z bosons, the AK8 soft-drop mass
energies. is used to obtain the mass of the merged jet.
For this analysis, two types of hadronic jets are The event jet multiplicity is determined by the number of
clustered from these reconstructed particles, using the AK4 jets passing the requirements above. In signal events,
infrared and collinear safe anti-kT algorithm [41,42]. The decays of the VLQ pair and subsequent decays of the Higgs
first type, “AK4 jets,” uses a distance parameter ΔR ¼ and Z boson daughters yield a total of six quarks. In all
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðΔηÞ2 þ ðΔϕÞ2 of 0.4. However, since merged jets from three event modes, at least two of these are b quarks.
a boosted Higgs or Z boson decay may be wider, a second Considering the predominant H → bb decay, for the bHbH
set, using a distance parameter of 0.8 (“AK8 jets”) is also event mode, all six are b quarks; for the bHbZ event mode,
reconstructed from the same set of input particles. Jet four or six are b quarks; while for the bZbZ event mode,
momentum is determined as the vectorial sum of all particle two, four or six are b quarks. We note that of the hadronic Z
momenta in the jet, and is found from simulation to be, on boson decays, 15% are to a bb̄ pair; however, Z → cc̄
average, within 5% to 10% of the true momentum over the decays also have a significant chance of passing the tagging
whole transverse momentum (pT ) spectrum and detector requirements, as discussed in Sec. VI, and are also included
112004-4SEARCH FOR BOTTOM-TYPE, VECTORLIKE QUARK PAIR … PHYS. REV. D 102, 112004 (2020)
(along with other possible Z boson decay modes) in the TABLE II. Summary of the minimum number of single and
signal efficiency. If all six quark jets are individually double b tags required for each jet multiplicity and event mode.
reconstructed, a 6-jet event is produced; if two jets merge
Jet multiplicity Tag bHbH bHbZ bZbZ
into a single reconstructed jet, this produces a 5-jet event;
and if two merged jets are produced, then a 4-jet event 4 jets Single b 2 2 2
results. Note that the VLQ reconstruction does not consider Double b 1 1 0
the possibility of additional jets produced by initial state or 5 jets Single b 3 3 3
Double b 0 0 0
final state radiation.
6 jets Single b 4 4 3
Because of the large number of b jets in signal events, b
tagging is a powerful tool to significantly reduce the
background from SM processes. Individual jets are tagged
using the DeepJet b discriminant [38] applied to AK4 jets, [38]. The double b tagger has an efficiency of 75% in
while merged jets from bb̄ pairs are double b tagged using simulated H → bb̄ events, and a mistag rate of 10% in
the algorithm in Ref. [39], developed in the context of H → simulated inclusive multijet events and 33% in simulated
bb̄ searches, applied to AK8 jets. H → cc̄ events [39], where a mistag in the double b tag
case means that at least one non-b quark subjet is present in
VI. EVENT SELECTION the tagged jet. The number of tags required depends on the
jet multiplicity as follows: in the 6-jet case, four AK4 jets
The events used in the analysis are first selected online are required to have a b tag, except in the bZbZ event
by the CMS trigger system. The HLT trigger used requires mode, for which three tags are required. In the 5-jet case,
the total pT measured in the calorimeters to be at least three of the AK4 jets not associated with the merged decay
900 (1050) GeV for the 2016 (2017–2018) dataset. Offline, products are required to be b tagged; no double b tag
events with HT > 1350 GeV are selected, where HT is requirement is applied to the AK8 jet associated with the
defined as the scalar sum of the jet pT for all AK4 jets with merged decay. In the 4-jet case, two of the nonmerged AK4
pT > 50 GeV and jηj < 2.4. The requirement is set higher jets are required to have a b tag, and one of the merged jets
than the trigger threshold to avoid effects due to trigger turn is required to have a double b tag, except in the bZbZ event
on. In order to minimize bias when measuring the effi- mode, for which no double b tag is required. These
ciency of the HLT triggers, the efficiencies are measured in requirements are summarized in Table II.
a dataset collected by an orthogonal trigger, which requires
the event to have a single muon. For all years, the measured
trigger efficiency for events with HT > 1350 GeV is at VII. EVENT RECONSTRUCTION
least 99.6%. Table I shows the efficiency for simulated In the case when the two jets produced from a H=Z
VLQ signal events after the H T requirement for each of the boson decay are individually resolved, the mass of the
three jet multiplicity channels and for each of three VLQ parent boson can be estimated from the invariant mass of
masses (1000, 1200, and 1400 GeV). the two jets. In the case where the two jets are merged, the
The number of tagged jets required to select an event, as parent boson mass is instead estimated using the soft-drop
well as the working points for the taggers used, are mass of the AK8 jet. Only those AK8 jets that are within
optimized separately for each of the three jet multiplicities ΔR < 0.3 of an AK4 jet are used. However, if a second
and event modes, in order to maximize the expected signal AK4 jet is within ΔR < 0.6 of the AK8 jet, this overlap
sensitivity. For the working points selected, the single b could cause the AK8 jet mass to be misreconstructed, so in
tagger has an efficiency of 82% for b jets in simulated tt̄ this case the AK8 jet is discarded and the two AK4 jets are
events with pT > 30 GeV and a mistag rate of 1% for light treated as a resolved dijet boson candidate.
quarks (u, d, or s) and approximately 17% for charm quarks A central feature in the analysis is the selection of the
correct way of combining jets in order to reconstruct the
TABLE I. Signal efficiencies of the offline HT selection, in %, parent particles; this is a difficult task because of the large
for each of the jet multiplicity channels, for three VLQ masses number of jets in the event. In 6-jet events, there are two
(1000, 1200, and 1400 GeV). The efficiency is the fraction of pairs of jets originating from H=Z boson decay; and three
events in each jet multiplicity category satisfying the HT > jets (including two from the H=Z decay) associated with
1350 GeV selection. Statistical uncertainties are negligible and each VLQ decay. In 5-jet events, there is a pair of jets
therefore omitted.
associated with one H=Z boson and a merged jet associated
VLQ mass [GeV] 4 jets 5 jets 6 jets with the other H=Z boson; each of these is associated with
one of the remaining two jets to form a VLQ candidate. In
1000 95.1 89.4 81.4
1200 98.5 96.2 91.3
4-jet events, there is a merged jet associated with each H=Z
1400 99.5 98.4 95.0 boson, each of which is paired to one of the remaining two
jets to form a VLQ candidate. The final reconstructed VLQ
112004-5A. M. SIRUNYAN et al. PHYS. REV. D 102, 112004 (2020)
mass, mVLQ , is defined as the average mass of the two number of distinct 5-jet combinations and quadruples the
individual reconstructed VLQs in the event. number of distinct 4-jet combinations. The final number of
The number of possible ways to combine the jets to distinct combinations is then 10, 20, and 12 for the 6-, 5-,
reconstruct the two H=Z bosons and then to combine these and 4-jet multiplicities, respectively.
with the two remaining jets to form the VLQ candidates is For each jet combination, χ 2mod is determined using
720, 120, and 24 for the 6-, 5-, and 4-jet multiplicities, Eqs. (1)–(3) below, which depend on the measured mass
respectively. However, many of these different combina- of a dijet Higgs or Z boson candidate (mdijet ), the measured
tions simply involve different permutations among the jets soft-drop mass of a merged-jet Higgs or Z boson candidate
that constitute the individual VLQs, and these permutations (mmerged ), and the fractional mass difference of the two
do not affect the reconstructed VLQ mass. The numbers of VLQ candidates (ΔmVLQ ), where ΔmVLQ is the difference
combinations that give distinct VLQ masses are only 10, of the masses of the two VLQ candidates divided by the
10, and 3 for the 6-, 5-, and 4-jet multiplicities, respectively. average mass of the two. The only use of AK8 jets in this
For the 5- and 4-jet multiplicities, however, the jet calculation is to determine the mass of the merged H=Z
associated with the merged H=Z boson decay products candidates using the soft-drop mass of the matched
distinguishes different combinations, since this jet is treated AK8 jet. All other quantities are determined using
differently from the others by having a double b tag, rather AK4 jet kinematics.
than a single b tagging requirement. This doubles the
ðmdijet1 − mdijet Þ2 ðmdijet2 − mdijet Þ2 ðΔmVLQ − ΔmVLQ Þ2
6-jet events∶ χ 2mod ¼ þ þ ; ð1Þ
σ 2mdijet σ 2mdijet σ 2ΔmVLQ
ðmdijet − mdijet Þ2 ðmmerged − mmerged Þ2 ðΔmVLQ − ΔmVLQ Þ2
5-jet events∶ χ 2mod ¼ þ þ ; ð2Þ
σ 2mdijet σ 2mmerged σ 2ΔmVLQ
ðmmerged1 − mmerged Þ2 ðmmerged2 − mmerged Þ2 ðΔmVLQ − ΔmVLQ Þ2
4-jet events∶ χ 2mod ¼ þ þ : ð3Þ
σ 2mmerged σ 2mmerged σ 2ΔmVLQ
The means (m and ΔmVLQ ) and standard deviations (σ m value of the reconstructed VLQ mass for the jet combi-
and σ ΔmVLQ ) of the parameters used in these expressions nation with the lowest χ 2mod for simulated signal events
are determined from simulated signal events in which the with mB ¼ 1200 GeV. In most cases, the VLQ mass is
jets are matched to the generator-level quarks and H=Z correctly reconstructed. We observe that the reconstructed
bosons. These quantities are derived separately for each mass distribution is consistently peaked at a value about
jet multiplicity, but do not depend on the simulated signal 5% lower than the generated mass, for all generated
mass. For each parameter, the central core of the signal mass values. The low-side tail is due to the
distribution is fit with a Gaussian function, whose mean presence of incorrectly reconstructed events.
and standard deviation are then used as the parameters in Figure 3 shows the distributions of χ 2mod =ndf, where ndf
the expressions for χ 2mod. As the distribution of the is the number of degrees of freedom, for the best jet
merged Higgs boson mass is asymmetrical, two Gaussian combination (i.e., the combination with the lowest χ 2mod ),
functions are separately fit above and below the peak of from simulated 1200 GeV VLQ signal events, at each jet
the distribution. Since the underlying distributions used in multiplicity. Each χ 2mod expression has three degrees of
these expressions have non-Gaussian tails and are in freedom, one for each term. The distributions for the data
some cases asymmetric, the values of χ 2mod are not exactly are also shown for comparison. In these plots, the simulated
distributed as a χ 2 variable. However, the difference is signal and data distributions are normalized to the same
small, and χ 2mod is only used to select events, so these integral value within the displayed χ 2mod range. This figure
deviations do not affect the analysis. Choosing the jet demonstrates that requiring a small χ 2mod value for the best
combination that has the lowest value of χ 2mod gives a jet combination provides an effective method for removing
high probability of identifying the correct jet combina- background.
tion, and allows mVLQ to be reconstructed. In simulation, The χ 2mod value is also used to select the event mode.
this can then be compared with the generated B mass mB . There are three possible decays of the bottom-type VLQ:
This is indicated in Fig. 2, which shows the average B → bH, B → bZ, and B → tW. This results in six
112004-6SEARCH FOR BOTTOM-TYPE, VECTORLIKE QUARK PAIR … PHYS. REV. D 102, 112004 (2020)
137 fb-1 (13 TeV) 137 fb-1 (13 TeV) 137 fb-1 (13 TeV)
CMS CMS CMS
2.5 Simulation Simulation 8 Simulation
10
Events / 50 GeV
Events / 50 GeV
Events / 50 GeV
2 mB = 1200 GeV 8 mB = 1200 GeV mB = 1200 GeV
6
4-jet channel 5-jet channel 6-jet channel
bHbH mode bHbH mode bHbH mode
1.5 χ2 /ndf < 2 6 χ2 /ndf < 2 χ2 /ndf < 2
mod mod mod
4
1 4
2
0.5 2
0 0 0
0 500 1000 1500 2000 0 500 1000 1500 2000 0 500 1000 1500 2000
mVLQ [GeV] mVLQ [GeV] mVLQ [GeV]
137 fb-1 (13 TeV) 137 fb-1 (13 TeV) 137 fb-1 (13 TeV)
12
CMS 15 CMS CMS
8 Simulation Simulation Simulation
10
Events / 50 GeV
Events / 50 GeV
Events / 50 GeV
mB = 1200 GeV mB = 1200 GeV mB = 1200 GeV
6 4-jet channel 10 5-jet channel 8 6-jet channel
bHbZ mode bHbZ mode bHbZ mode
χ2 /ndf < 2 χ2 /ndf < 2 6 χ2 /ndf < 2
mod mod mod
4
5 4
2
2
0 0 0
0 500 1000 1500 2000 0 500 1000 1500 2000 0 500 1000 1500 2000
mVLQ [GeV] mVLQ [GeV] mVLQ [GeV]
137 fb-1 (13 TeV) 137 fb-1 (13 TeV) 137 fb-1 (13 TeV)
8
4
CMS 12 CMS CMS
Simulation Simulation Simulation
10
6 3
mB = 1200 GeV mB = 1200 GeV mB = 1200 GeV
Events / 50 GeV
Events / 50 GeV
Events / 50 GeV
4-jet channel 8 5-jet channel 6-jet channel
bZbZ mode bZbZ mode bZbZ mode
4 χ2 /ndf < 2 χ2 /ndf < 2 2 χ2 /ndf < 2
mod 6 mod mod
4
2 1
2
0 0 0
0 500 1000 1500 2000 0 500 1000 1500 2000 0 500 1000 1500 2000
mVLQ [GeV] mVLQ [GeV] mVLQ [GeV]
FIG. 2. Distributions of mVLQ for simulated signal events with a generated VLQ mass mB ¼ 1200 GeV. A requirement of χ 2mod =ndf <
2 is applied to the events. Mass distributions for 4-jet (left), 5-jet (center), and 6-jet (right) events are shown for the three event modes:
bHbH (upper row), bHbZ (middle row), and bZbZ (lower row).
possible modes for the BB̄ pair production events: bHbH, in the signal simulation and are added according to
bHbZ, bZbZ, tWbH, tWbZ, and tWtW. The latter three their reconstruction efficiency. For events that satisfy the
modes involve one or two decays of a VLQ to a t quark and HT requirement and that are categorized as either 4-, 5-, or
a W boson. These events either have a jet multiplicity 6-jet multiplicity events, the χ 2mod described above is
greater than six, or contain leptons and missing transverse calculated for each of the three event modes: bHbH,
energy from the W decays. Although this analysis is not bHbZ, or bZbZ, and the mode of the event is selected
optimized for sensitivity to these events, events with as the one that has the best χ 2mod value. Events are
B → tW, if present, have some probability to be selected categorized by their jet multiplicity and their reconstructed
as one of the other three event modes and can affect mode, regardless of the underlying decay mode for simu-
the sensitivity of the analysis. These events are included lated signal events.
112004-7A. M. SIRUNYAN et al. PHYS. REV. D 102, 112004 (2020)
137 fb-1 (13 TeV) 137 fb-1 (13 TeV) 137 fb-1 (13 TeV)
0.2
0.25
CMS mB = 1200 GeV CMS mB = 1200 GeV
0.14 CMS mB = 1200 GeV
4-jet channel 5-jet channel 6-jet channel
Data Data Data
Normalized Events / 1.0 units
Normalized Events / 1.0 units
Normalized Events / 1.0 units
0.12
0.15
0.2
0.1
0.15 0.08
0.1
0.06
0.1
0.05 0.04
0.05
0.02
0 0 0
0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50
χ2 /ndf χ2 /ndf χ2 /ndf
mod mod mod
FIG. 3. Distribution of χ 2mod =ndf for the best jet combination for simulated 1200 GeV VLQ events (red histogram) and data (black
points), for 4-jet (left), 5-jet (center), and 6-jet events (right). The simulated signal events and data events are normalized to the same
integral value within the displayed χ 2mod range.
Before examining the data in the potential signal region,
TABLE III. Optimized values of the χ 2mod =ndf selection as a the event selection parameters (the jet tagging parameters
function of jet multiplicity and event mode. and χ 2mod ) are optimized. This optimization is performed by
Jet multiplicity varying the parameters and selecting the values that
maximize the sensitivity to a 1600 GeV VLQ signal.
Event mode 4 5 6
The mass of 1600 GeV is chosen because it is the point
bHbH 5.5 2 2.75 with maximum sensitivity of the analysis; however, the
bHbZ 2.75 2.5 2.5 optimized parameters are largely independent of the point
bZbZ 2 2 2.25 chosen. The optimized jet tagging parameters are described
in Section VI, and the optimized χ 2mod =ndf values are
shown below in Table III. With the optimized selection, the
TABLE IV. Values of the BJTF for data events with mVLQ in the overall signal efficiency measured in simulation for a
range 500–800 GeV for each of the three event modes and three generated VLQ mass of 1600 GeV is approximately 5%
jet multiplicities. in the BðB → bZÞ ¼ 100% scenario, increasing to 10%
for BðB → bHÞ ¼ 100%.
bHbH bHbZ bZbZ
4 jets 0.0042 % 0.0014 0.0019 % 0.0004 0.0025 % 0.0004
VIII. BACKGROUND ESTIMATION
5 jets 0.0041 % 0.0003 0.0036 % 0.0002 0.0048 % 0.0009
6 jets 0.0019 % 0.0002 0.0019 % 0.0002 0.0020 % 0.0005 The expected background is estimated in a low-mass
sideband region in data, using the ratio of the number of
137 fb-1 (13 TeV) 137 fb-1 (13 TeV) 137 fb-1 (13 TeV)
105 105 105
CMS Data CMS Data CMS Data
104 4-jet channel 104 5-jet channel 104 6-jet channel
Fit Fit Fit
Events / 40 GeV
Events / 40 GeV
Events / 40 GeV
103 103 103
102 102 102
10 10 10
1 1 1
1 1 1
(Data-Fit)/Fit
(Data-Fit)/Fit
(Data-Fit)/Fit
0.5 0.5 0.5
0 0 0
− 0.5 − 0.5 − 0.5
0 500 1000 1500 2000 0 500 1000 1500 2000 0 500 1000 1500 2000
mVLQ [GeV] mVLQ [GeV] mVLQ [GeV]
FIG. 4. Distributions of mVLQ for the jet combination with the lowest χ 2mod in 4-jet (left), 5-jet (center), and 6-jet (right) multiplicity events.
The red lines show the exponential fit in the range 1000–2000 GeV. The lower panels show the fractional difference, ðdata − fitÞ=fit.
112004-8SEARCH FOR BOTTOM-TYPE, VECTORLIKE QUARK PAIR … PHYS. REV. D 102, 112004 (2020)
events passing the tag requirements to the number before 1350 GeV requirement. The distributions are then fit with
these requirements are applied. The estimation is done an exponential function for VLQ candidate masses greater
separately for each of the three event modes, jet multiplic- than 1000 GeV; in all three cases, the function (shown by
ities, and three data-taking years, for a total of 27 cases. the red line) agrees with the data. An F-test [50] shows that
Figure 4 shows the distribution of mVLQ for the jet a more complex model, namely an exponential plus
combination with the lowest χ 2mod for each of the three jet constant background, offers no significant improvement
multiplicities. All events shown in this plot are required to over the exponential distribution. The lower plots show the
pass a selection of χ 2mod =ndf < 4. The falloff in the fractional difference between the data and the fit. At this
distribution at lower masses is due to the HT > stage, since there is no requirement made on jet tagging, the
137 fb-1 (13 TeV) 137 fb-1 (13 TeV) 137 fb-1 (13 TeV)
CMS Data CMS Data CMS Data
4-jet channel 0.025 5-jet channel 0.004 6-jet channel
Background jet-tagged fraction
Background jet-tagged fraction
Background jet-tagged fraction
0.15
bHbH mode Fit bHbH mode Fit bHbH mode Fit
12 < χ2 /ndf < 48 12 < χ2 /ndf < 48 12 < χ2 /ndf < 48
0.02
0.003
0.1
0.015
0.002
0.01
0.05
0.001
0.005
0 0
500 1000 1500 2000 500 1000 1500 2000 500 1000 1500 2000
mVLQ [GeV] mVLQ [GeV] mVLQ [GeV]
137 fb-1 (13 TeV) 137 fb-1 (13 TeV) 137 fb-1 (13 TeV)
0.01
0.08 CMS Data CMS Data CMS Data
4-jet channel 5-jet channel 6-jet channel
Background jet-tagged fraction
Background jet-tagged fraction
Background jet-tagged fraction
0.015 bHbZ mode 0.008
bHbZ mode Fit Fit bHbZ mode Fit
12 < χ2 /ndf < 48 12 < χ2 /ndf < 48 12 < χ2 /ndf < 48
0.06
0.006
0.01
0.04
0.004
0.005
0.02
0.002
0 0
500 1000 1500 2000 500 1000 1500 2000 500 1000 1500 2000
mVLQ [GeV] mVLQ [GeV] mVLQ [GeV]
137 fb-1 (13 TeV) 137 fb-1 (13 TeV) 137 fb-1 (13 TeV)
0.14
CMS Data CMS Data 0.2
CMS Data
4-jet channel 5-jet channel 6-jet channel
Background jet-tagged fraction
Background jet-tagged fraction
Background jet-tagged fraction
0.06 0.12
bZbZ mode Fit bZbZ mode Fit bZbZ mode Fit
12 < χ2 /ndf < 48 0.1 12 < χ /ndf < 48
2 12 < χ2 /ndf < 48
0.15
0.04 0.08
0.1
0.06
0.02 0.04
0.05
0.02
0 0 0
500 1000 1500 2000 500 1000 1500 2000 500 1000 1500 2000
mVLQ [GeV] mVLQ [GeV] mVLQ [GeV]
FIG. 5. Dependence of the BJTF on mVLQ in the control region 12 < χ 2mod =ndf < 48, for 4-jet (left column), 5-jet (center column), and
6-jet (right column) multiplicities, and for the bHbH (upper row), bHbZ (middle row), and bZbZ (lower row) event modes. The data are
shown as black points with vertical error bars, and the linear fit and associated uncertainty are shown as a solid red line and the shaded
red band.
112004-9A. M. SIRUNYAN et al. PHYS. REV. D 102, 112004 (2020)
ratio of background to signal event acceptance is more than between 500 and 800 GeV, which is below the current
two orders of magnitude larger than after jet tagging, so the lower exclusion limit on the VLQ mass [17,18]. Table IV
fits are insensitive to any possible signal events in the data. shows the BJTF for data events with mVLQ in the range
The background jet-tagged fraction (BJTF) is the frac- 500–800 GeV for each of the three event modes and three
tion of background events that remain after the jet tagging jet multiplicities.
requirements, as described in Sec. V, are applied. Since the Because the jet tagging efficiency depends on the pT of
BJTF for events with mVLQ > 1000 GeV could be biased the jet, the BJTF might depend on the mass of the VLQ
due to signal events that might be in the data, the BJTF is candidate, since events with greater VLQ mass generally
initially determined only for events in which mVLQ is have higher pT jets. A control region is therefore used to
137 fb-1 (13 TeV) 137 fb-1 (13 TeV) 137 fb-1 (13 TeV)
0.01 CMS Data CMS Data CMS Data
Background jet-tagged fraction
Background jet-tagged fraction
Background jet-tagged fraction
4-jet channel 5-jet channel 0.003 6-jet channel
0.005
bHbH mode Fit bHbH mode Fit bHbH mode Fit
0.005 0.002
0.004
0.003 0.001
0
0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50
Upper bound of χ2 window Upper bound of χ2 window Upper bound of χ2 window
mod mod mod
137 fb-1 (13 TeV) 137 fb-1 (13 TeV) 137 fb-1 (13 TeV)
CMS Data
0.006 CMS Data
0.006 CMS Data
Background jet-tagged fraction
Background jet-tagged fraction
Background jet-tagged fraction
0.006 4-jet channel 5-jet channel 6-jet channel
bHbZ mode Fit bHbZ mode Fit bHbZ mode Fit
0.005
0.004 0.004
0.002 0.004
0.002
0
0.003
0
0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50
Upper bound of χ2 window Upper bound of χ2 window Upper bound of χ2 window
mod mod mod
137 fb-1 (13 TeV) 137 fb-1 (13 TeV) 137 fb-1 (13 TeV)
0.015 CMS Data CMS Data CMS Data
Background jet-tagged fraction
Background jet-tagged fraction
Background jet-tagged fraction
4-jet channel 5-jet channel 6-jet channel
bZbZ mode Fit bZbZ mode Fit 0.15 bZbZ mode Fit
0.01 0.08
0.005 0.1
0.06
0
0.05
0.04
−0.005
0
0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50
Upper bound of χ2 window Upper bound of χ2 window Upper bound of χ2 window
mod mod mod
FIG. 6. Dependence of the BJTF on χ 2mod =ndf in the low-mass (500–800 GeV) VLQ region, for 4-jet (left column), 5-jet (center
column), and 6-jet (right column) multiplicities, and for the bHbH (upper row), bHbZ (middle row), and bZbZ (lower row) event
modes. The data are shown as black points with vertical error bars, and the linear fit and associated uncertainty are shown as a solid red
line and the shaded red band.
112004-10SEARCH FOR BOTTOM-TYPE, VECTORLIKE QUARK PAIR … PHYS. REV. D 102, 112004 (2020)
determine the VLQ mass dependence of the BJTF by uncertainty. This covers the uncertainty due to the choice of
offsetting the window of the χ 2mod selection. The signal χ 2mod the BJTF shape.
region depends on the event mode and multiplicity, as In order to validate that the control region used has the
determined by the optimization procedure described in same BJTF behavior as the signal region, we perform a test
Sec. VII, but in all cases is at most χ 2mod =ndf < 5.5. A where the BJTF in the low VLQ mass range (500–
control region is defined by using a region of 800 GeV) is plotted as a function of χ 2mod =ndf, in twelve
12 < χ 2mod =ndf < 48. Figure 5 shows the mass dependence equally spaced regions for χ 2mod =ndf from 0 to 48. This is
of the BJTF for data events in the χ 2mod control region. A shown in Fig. 6. The slope of this plot is consistent with
first-order polynomial fit is used to determine the BJTF zero, indicating no statistically significant dependence
mass dependence. Another F-test shows that there is no on χ 2mod =ndf.
improvement for a second-order polynomial fit compared Figure 7 shows the two-dimensional dependence of the
to a first-order one, and also that the first-order polynomial BJTF in data on mVLQ and χ 2mod =ndf, and Fig. 8 shows the
fit performs better than a constant fit. A systematic corresponding distributions for simulated VLQ signal
uncertainty is assigned by comparing the first-order poly- events with a generated VLQ mass of 1200 GeV. The
nomial fit to an exponential fit, and using the average signal region is indicated in these plots by the red rectangle,
difference of these two fits over the mass range as the and is excluded from the data plots.
50 1 50 1 50 1
CMS CMS CMS
Background jet-tagged fraction
Background jet-tagged fraction
Background jet-tagged fraction
4-jet channel 5-jet channel 6-jet channel
40 40 40
bHbH mode bHbH mode bHbH mode
10−1 10−1 10−1
Best χ2 /ndf
Best χ2 /ndf
Best χ2 /ndf
30 30 30
mod
mod
mod
20 10−2 20 10−2 20 10−2
10 10 10
Signal region 10−3 Signal region 10−3 Signal region 10−3
0 0 0
0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500
mVLQ [GeV] mVLQ [GeV] mVLQ [GeV]
50 1 50 1 50 1
CMS CMS CMS
Background jet-tagged fraction
Background jet-tagged fraction
Background jet-tagged fraction
4-jet channel 5-jet channel 6-jet channel
40 40 40
bHbZ mode bHbZ mode bHbZ mode
−1 −1
10 10 10−1
Best χ2 /ndf
χ2 /ndf
χ2 /ndf
30 30 30
mod
mod
mod
Best
Best
20 10−2 20 10−2 20 10−2
10 10 10
Signal region 10−3 Signal region 10−3 Signal region 10−3
0 0 0
0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500
mVLQ [GeV] mVLQ [GeV] mVLQ [GeV]
50 1 50 1 50 1
CMS CMS CMS
Background jet-tagged fraction
Background jet-tagged fraction
Background jet-tagged fraction
4-jet channel 5-jet channel 6-jet channel
40 40 40
bZbZ mode bZbZ mode bZbZ mode
10−1 10−1 10−1
Best χ2 /ndf
χ2 /ndf
χ2 /ndf
30 30 30
mod
mod
mod
Best
Best
20 10−2 20 10−2 20 10−2
10 10 10
Signal region 10−3 Signal region 10−3 Signal region 10−3
0 0 0
0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500
mVLQ [GeV] mVLQ [GeV] mVLQ [GeV]
FIG. 7. Dependence of the BJTF on mVLQ and the best χ 2mod =ndf in data events, for 4-jet (left column), 5-jet (center column), and 6-jet
(right column) multiplicities, and for the bHbH (upper row), bHbZ (middle row), and bZbZ (lower row) event modes. The red box
indicates the signal region, which is excluded from these plots.
112004-11A. M. SIRUNYAN et al. PHYS. REV. D 102, 112004 (2020)
The final estimate of the number of background events IX. SYSTEMATIC UNCERTAINTIES
nb as a function of VLQ mass m is given by the following
We consider two types of systematic uncertainties, those
expression:
that are common to all event modes and jet multiplicities,
εðmÞ and those that depend on the particular channel. The
nb ðmÞ ¼ nðmÞε0 R 800 GeV ; ð4Þ
ð 500 GeV εðm0 Þdm0 Þ=ð300 GeVÞ uncertainties in the first category are listed in Table V;
these are the integrated luminosity, trigger efficiency, and
where nðmÞ is the number of candidates as a function of the choice of fit function for the dependence of the BJTF on
mVLQ before jet tagging for candidates passing the χ 2mod VLQ mass. The integrated luminosities of the 2016, 2017,
selection shown in Fig. 4, ε0 is the BJTF at low VLQ mass and 2018 data-taking periods are individually known with
as shown in Table IV, and the last factor accounts for the uncertainties in the range 2.3%–2.5% [24–26], while the
potential mass dependence of the BJTF, with εðmÞ the total 2016–2018 integrated luminosity has an uncertainty of
distribution of the BJTF as a function of mass, as shown in 1.8%, the improvement in precision reflecting the (uncor-
Fig. 5; the factor of 300 GeV is to normalize over the range related) time evolution of some systematic effects. The
considered. The final estimate is also validated by com- uncertainty associated with the choice of fit function for the
parison with the region 8 < χ 2mod =ndf < 12 and with mVLQ dependence of the BJTF is determined by finding
simulated events.
-1 -1 -1
137 fb (13 TeV) 137 fb (13 TeV) 137 fb (13 TeV)
50 1 50 1 50 1
CMS CMS CMS
Signal jet-tagged fraction
Simulation Simulation Simulation
Signal jet-tagged fraction
40 40 40
Signal jet-tagged fraction
−1 −1
4-jet channel 10 5-jet channel 10 6-jet channel 10−1
Best χ2 /ndf
Best χ2 /ndf
Best χ2 /ndf
30 bHbH mode 30 bHbH mode 30 bHbH mode
mod
mod
mod
20 10−2 20 10−2 20 10−2
10 10 10
Signal region 10−3 Signal region 10−3 Signal region 10−3
0 0 0
0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500
mVLQ [GeV] mVLQ [GeV] mVLQ [GeV]
137 fb-1 (13 TeV) 137 fb-1 (13 TeV) 137 fb-1 (13 TeV)
50 1 50 1 50 1
CMS CMS CMS
Simulation Simulation Simulation
40 40 40
Signal jet-tagged fraction
Signal jet-tagged fraction
Signal jet-tagged fraction
−1 −1
4-jet channel 10 5-jet channel 10 6-jet channel 10−1
Best χ2 /ndf
Best χ2 /ndf
Best χ2 /ndf
30 bHbZ mode 30 bHbZ mode 30 bHbZ mode
mod
mod
mod
20 10−2 20 10−2 20 10−2
10 10 10
Signal region 10−3 Signal region 10−3 Signal region 10−3
0 0 0
0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500
mVLQ [GeV] mVLQ [GeV] mVLQ [GeV]
-1 -1 -1
137 fb (13 TeV) 137 fb (13 TeV) 137 fb (13 TeV)
50 1 50 1 50 1
CMS CMS CMS
Simulation Simulation Simulation
40 40 40
Signal jet-tagged fraction
Signal jet-tagged fraction
Signal jet-tagged fraction
4-jet channel 10−1 5-jet channel 10−1 6-jet channel 10−1
Best χ2 /ndf
Best χ2 /ndf
Best χ2 /ndf
30 bZbZ mode 30 bZbZ mode 30 bZbZ mode
mod
mod
mod
20 10−2 20 10−2 20 10−2
10 10 10
Signal region 10−3 Signal region 10−3 Signal region 10−3
0 0 0
0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500
mVLQ [GeV] mVLQ [GeV] mVLQ [GeV]
FIG. 8. Dependence of the BJTF on mVLQ and the best χ 2mod =ndf in simulated VLQ signal events with mB ¼ 1200 GeV, for 4-jet (left
column), 5-jet (center column), and 6-jet (right column) multiplicities, and for the bHbH (upper row), bHbZ (middle row), and bZbZ
(lower row) event modes. The red box indicates the signal region.
112004-12SEARCH FOR BOTTOM-TYPE, VECTORLIKE QUARK PAIR … PHYS. REV. D 102, 112004 (2020)
TABLE V. Systematic uncertainties common to all three event There are several sources of uncertainty in the back-
modes and all three jet multiplicities. All uncertainties listed here ground estimation, corresponding to the three terms in
are rate uncertainties, meaning they affect only the normalization. Eq. (4). The first uncertainty arises from the exponential fit
Type Signal=Background Uncertainty
to the distribution nðmÞ, the number of events as a function
of mass before jet tagging is applied. The uncertainties in
Integrated luminosity Signal 1.8% the fit parameters p0 and p1 are used to determine the
Trigger efficiency Signal 0.02% uncertainty in the fit value for a given mass. The second is
Choice of fit function Background 4.9%
the uncertainty in the BJTF determined for low-mass VLQ
candidates, ε0 , as shown in Table IV. Finally, the third
uncertainty arises from the third term, to account for a
the average difference between the fit functions used for potential mass dependence of the BJTF, and is obtained
each mode and multiplicity combination, and using the from the uncertainties in the fit parameters, as in the
maximum one as the common uncertainty for all modes and first case.
multiplicities. Table V also indicates whether an uncertainty The efficiencies for jet tagging are measured in simulated
affects the signal efficiency or the background estimate. events and then corrected to data events using a data-to-
The uncertainties that depend on event mode and jet simulation scale factor. The uncertainty in this scale factor
multiplicity are those due to the background estimation, jet is propagated to the signal reconstruction efficiency by
tag scale factors, jet energy resolution and scale, choice of varying the scale factors within their uncertainties [39]. The
PDF, and pileup. uncertainties due to the scale factors for jet energy scale and
TABLE VI. Table of systematic uncertainties for each event mode and jet multiplicity. The reported values indicate the uncertainty in
the event yield in a %75 GeV window about the signal peak for a generated signal mass mB ¼ 1600 GeV.
Type Signal=Background Rate=Shape 4 jets 5 jets 6 jets
bHbH event mode
Background fit p0 Background Shape 59% 14% 13%
Background fit p1 Background Shape 78% 18% 16%
BJTF m dependence p0 Background Shape 1.3% 5.9% 4.5%
BJTF m dependence p1 Background Shape 19% 25% 17%
Low-mass BJTF Background Rate 34% 9.7% 11%
Jet tag scale factors Signal Shape 16% 15% 17%
Jet energy scale Signal Shape 4.0% 5.3% 6.4%
Jet energy resolution Signal Shape 2.4% 1.5% 1.6%
Pileup Signal Shape 28% 28% 27%
PDF Signal Rate 1.5% 1.5% 1.5%
bHbZ event mode
Background fit p0 Background Shape 21% 12% 10%
Background fit p1 Background Shape 21% 14% 12%
BJTF m dependence p0 Background Shape 2.1% 7.7% 3.5%
BJTF m dependence p1 Background Shape 21% 30% 27%
Low-mass BJTF Background Rate 22% 7.7% 11%
Jet tag scale factors Signal Shape 15% 13% 17%
Jet energy scale Signal Shape 4.9% 5.7% 5.1%
Jet energy resolution Signal Shape 1.8% 2.7% 3.2%
Pileup Signal Shape 33% 28% 21%
PDF Signal Rate 1.6% 1.5% 1.5%
bZbZ event mode
Background fit p0 Background Shape 26% 17% 24%
Background fit p1 Background Shape 28% 21% 32%
BJTF m dependence p0 Background Shape 3.7% 0.6% 11%
BJTF m dependence p1 Background Shape 15% 7.8% 21%
Low-mass BJTF Background Rate 16% 19% 25%
Jet tag scale factors Signal Shape 8.9% 8.0% 11%
Jet energy scale Signal Shape 4.0% 2.9% 1.6%
Jet energy resolution Signal Shape 2.5% 2.5% 3.2%
Pileup Signal Shape 28% 28% 10%
PDF Signal Rate 1.5% 1.5% 1.5%
112004-13A. M. SIRUNYAN et al. PHYS. REV. D 102, 112004 (2020)
resolution [21] are determined similarly. The uncertainty Table VI summarizes these uncertainties, and indicates
due to the choice of PDF weighting is calculated from a set whether they affect the signal efficiency or the background
of 100 weights selected from the NNPDF3.0 distribution, estimate, and whether the uncertainty affects the overall
following the prescription in Ref. [51]. The pileup uncer- rate or the shape of the mass distribution. For the PDF
tainties are due to a 4.6% systematic uncertainty in the pp systematic uncertainties, the values refer only to the event
inelastic cross section. acceptance rate. There is in addition an uncertainty in the
137 fb-1 (13 TeV) 137 fb-1 (13 TeV) 137 fb-1 (13 TeV)
60
CMS mB = 1000 GeV CMS mB = 1000 GeV 50 CMS mB = 1000 GeV
4-jet channel, bHbH mode mB = 1200 GeV 5-jet channel, bHbH mode mB = 1200 GeV 6-jet channel, bHbH mode mB = 1200 GeV
6
mB = 1400 GeV mB = 1400 GeV 40 mB = 1400 GeV
Events / 50 GeV
Events / 50 GeV
Events / 50 GeV
mB = 1600 GeV 40 mB = 1600 GeV mB = 1600 GeV
mB = 1800 GeV mB = 1800 GeV 30 mB = 1800 GeV
4
Background Background Background
Data Data 20 Data
20
2 Systematic Uncertainty Systematic Uncertainty Systematic Uncertainty
10
0 0 0
30
data - bkgd
data - bkgd
data - bkgd
2
20 2
bkgd
bkgd
bkgd
1
10 0
0
−1
0
500 1000 1500 2000 500 1000 1500 2000 500 1000 1500 2000
mVLQ [GeV] mVLQ [GeV] mVLQ [GeV]
-1 -1 -1
137 fb (13 TeV) 137 fb (13 TeV) 137 fb (13 TeV)
80
20
CMS mB = 1000 GeV CMS mB = 1000 GeV CMS mB = 1000 GeV
4-jet channel, bHbZ mode mB = 1200 GeV 5-jet channel, bHbZ mode mB = 1200 GeV 30 6-jet channel, bHbZ mode mB = 1200 GeV
60
15 mB = 1400 GeV mB = 1400 GeV mB = 1400 GeV
Events / 50 GeV
Events / 50 GeV
Events / 50 GeV
mB = 1600 GeV mB = 1600 GeV mB = 1600 GeV
mB = 1800 GeV mB = 1800 GeV 20 mB = 1800 GeV
10 40
Background Background Background
Data Data Data
5 Systematic Uncertainty 20 Systematic Uncertainty 10 Systematic Uncertainty
0 0 0
2 2 2
data - bkgd
data - bkgd
data - bkgd
1 1 1
bkgd
bkgd
bkgd
0 0 0
−1 −1 −1
500 1000 1500 2000 500 1000 1500 2000 500 1000 1500 2000
mVLQ [GeV] mVLQ [GeV] mVLQ [GeV]
137 fb-1 (13 TeV) 137 fb-1 (13 TeV) 137 fb-1 (13 TeV)
30 250 140
CMS mB = 1000 GeV CMS mB = 1000 GeV CMS mB = 1000 GeV
120
4-jet channel, bZbZ mode mB = 1200 GeV 200 5-jet channel, bZbZ mode mB = 1200 GeV 6-jet channel, bZbZ mode mB = 1200 GeV
mB = 1400 GeV mB = 1400 GeV 100 mB = 1400 GeV
Events / 50 GeV
Events / 50 GeV
Events / 50 GeV
20
mB = 1600 GeV mB = 1600 GeV mB = 1600 GeV
150
mB = 1800 GeV mB = 1800 GeV 80 mB = 1800 GeV
Background Background 60
Background
100
10 Data Data Data
Systematic Uncertainty Systematic Uncertainty 40 Systematic Uncertainty
50
20
0 0 0
2 2 2
data - bkgd
data - bkgd
data - bkgd
1 1 1
bkgd
bkgd
bkgd
0 0 0
−1 −1 −1
500 1000 1500 2000 500 1000 1500 2000 500 1000 1500 2000
mVLQ [GeV] mVLQ [GeV] mVLQ [GeV]
FIG. 9. Data (black points), expected background (solid blue histogram), and expected background plus a VLQ signal for different
VLQ masses (colored lines), for 4-jet (left column), 5-jet (center column), and 6-jet (right column) multiplicities and for bHbH (upper
row), bHbZ (middle row), and bZbZ (lower row) event modes. For the signal, BðB → bHÞ ¼ 100% is assumed. The hatched regions
for the background and background plus signal distributions indicate the systematic uncertainties. All three data-taking years are
combined.
112004-14You can also read
