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JHEP08(2018)152

Published for SISSA by Springer

Received: June 9, 2018

Revised: July 21, 2018

Accepted: August 16, 2018

Published: August 23, 2018

Search for resonant pair production of Higgs bosons

decaying to bottom quark-antiquark pairs in

proton-proton collisions at 13 TeV

The CMS collaboration

E-mail: cms-publication-committee-chair@cern.ch

Abstract: A search for a narrow-width resonance decaying into two Higgs bosons, each

decaying into a bottom quark-antiquark pair, is presented. The search is performed us-

ing proton-proton collision data corresponding to an integrated luminosity of 35.9 fb

−1

√

s = 13 TeV recorded by the CMS detector at the LHC. No evidence for such a signal

is observed. Upper limits are set on the product of the production cross section for the

resonance and the branching fraction for the selected decay mode in the resonance mass

range from 260 to 1200 GeV.

Keywords: Beyond Standard Model, Hadron-Hadron scattering (experiments), Higgs

physics

ArXiv ePrint: 1806.03548

Open Access, Copyright CERN,

for the beneﬁt of the CMS Collaboration.

Article funded by SCOAP

https://doi.org/10.1007/JHEP08(2018)152

JHEP08(2018)152

with the CMS detector at the CERN LHC, corresponding to an integrated luminosity of

35.9 fb

−1

. The main challenge of this search is to discriminate the ﬁnal-state signature of

four bottom quark jets from the overwhelming multijet quantum chromodynamics (QCD)

background. This is addressed by dedicated online selection criteria that include b jet

identiﬁcation and by a model of the multijet background that is tested in control regions

of data. The analysis closely follows the approach adopted for the 8 TeV data [15] but the

sensitivity for high resonance mass values is enhanced because of the signiﬁcant increase in

production cross section at 13 TeV, a new trigger strategy and a more eﬃcient algorithm

for identifying jets originating from bottom quarks.

2 Detector and simulated samples

The central feature of the CMS apparatus is a superconducting solenoid of 6 m internal

diameter, providing a magnetic ﬁeld of 3.8 T. A silicon pixel and strip tracker, a lead

tungstate crystal electromagnetic calorimeter, and a brass and scintillator hadron calor-

imeter, each composed of a barrel and two endcap sections, reside within the solenoid.

Forward calorimeters extend the pseudorapidity (η) [23] coverage provided by the barrel

and endcap detectors. Muons are detected in gas-ionization chambers embedded in the

steel ﬂux-return yoke outside the solenoid. A more detailed description of the CMS detec-

tor, together with a deﬁnition of the coordinate system used and the relevant kinematic

variables, can be found in ref. [23]

Simulated samples of signal events are produced using various Monte Carlo (MC) event

generators, with the CMS detector response modeled using the Geant4 [24] program. To

model the production of a generic narrow-width spin-0 resonance, we use an MC simulation

of the bulk radion produced through gluon fusion. The momenta and angular distributions

for decay products of a spin-2 resonance are distinct from those for a spin-0 resonance, and

result in diﬀerent kinematic distributions. Therefore, we evaluate the signal eﬃciencies

for a narrow-width spin-2 resonance from a separate simulation of the ﬁrst excitation of a

bulk KK graviton produced through gluon fusion and forced to decay to a pair of Higgs

bosons with the parameters reported in ref. [25]. Bulk graviton and radion signal events

are simulated with masses in the range 260–1200 GeV and widths of 1 MeV (narrow-width

approximation), using the MadGraph5 amc@nlo 2.3.3 [26] event generator at leading

order (LO). The resonance is forced to decay into a pair of Higgs bosons which in turn decay

into bb. The parton distribution function (PDF) set NNPDF3.0 [27] with LO accuracy is

used. The showering and hadronization of partons are simulated with pythia 8.212 [28].

During the 2016 data-taking period the average number of pp interactions per bunch

crossing was approximately 23. The simulated samples include these additional pp interac-

tions, referred to as pileup interactions (or pileup), that overlap with the event of interest

in the same bunch crossing. Simulated events are weighted to match the number of pp

interactions per event in data.

– 2 –

JHEP08(2018)152

3 Event reconstruction

The particle-ﬂow (PF) algorithm [29] is used to reconstruct and identify individual particle

in an event with an optimized combination of information from the various elements of the

CMS detector. The algorithm identiﬁes each reconstructed particle (PF candidate) as an

electron, a muon, a photon, or a charged or neutral hadron.

The reconstructed vertex with the largest value of summed physics-object transverse

momentum squared (p

) is taken to be the primary pp interaction vertex. The physics

objects are the jets, clustered using the jet ﬁnding algorithm [30, 31] with the tracks

assigned to the vertex as inputs, and the associated missing transverse momentum, taken

as the negative vector sum of the p

of those jets. This vertex is used for all the objects

in the event reconstructed with the PF algorithm.

Jets are reconstructed from PF candidates using the anti-k

clustering algorithm [30],

with a distance parameter of 0.4, as implemented in the FastJet package [31, 32]. Jet

identiﬁcation criteria are also applied to reject jets originating from detector noise. The

average neutral energy density from pileup interactions is evaluated from PF objects and

subtracted from the reconstructed jets [33]. Jet energy corrections are derived from the

simulation, and are conﬁrmed with in situ measurements of the energy balance in dijet and

photon+jet events [34].

Jets are identiﬁed as originating from b quarks (“b jets”) using the DeepCSV [35]

discriminator, a new b tagging algorithm based on a deep neural network with four hidden

layers [36]. The DeepCSV discriminator employs the same set of observables as those

used by the combined secondary vertex (CSV) algorithm [35, 37], except that the track

selection is expanded to include up to six tracks, further improving the b jet identiﬁcation.

The operating point chosen corresponds to a 1 (12)% rate for misidentifying a light-ﬂavor

(c-ﬂavor) jet as a b jet. The b tagging eﬃciency for jets with p

in the 30–150 GeV range

is approximately 69% and gradually decreases for lower and higher jet p

[35].

4 Event selection

The search for a narrow-width X → H(bb)H(bb) resonance is performed for mass values

in the 260–1200 GeV range. The angular distributions for the decay products of such a

resonance vary substantially over this range. In order to increase the sensitivity of this

search, diﬀerent criteria are used for events in two distinct mass regions: the low-mass

region (LMR), for resonance masses from 260 to 620 GeV, and the medium-mass region

(MMR), for masses from 550 to 1200 GeV. The boundary between the LMR and the MMR

is at 580 GeV. It has been chosen by optimizing for the expected sensitivity and takes

into account the uncertainties associated with the background modeling. The mass range

above 1200 GeV (high-mass region) is not covered by this search. Above 900 GeV, the Higgs

bosons have a momentum considerably higher than their mass and the Higgs to bb decays

are reconstructed more eﬃciently as single hadronic jets with a larger anti-k

distance

parameter (0.8) [38].

– 3 –

JHEP08(2018)152

Events are selected online by combining two diﬀerent trigger selections to identify b

jets, both using the CSV algorithm. For the ﬁrst trigger selection, four jets with p

30 GeV and |η| < 2.4 are required. The latter requirement ensures that the jet lies within

the tracker acceptance. Of those four jets, two are required to have p

> 90 GeV and at

least three jets are required to be tagged as b jets. The second trigger selection requires

four jets with p

> 45 GeV and at least three of those jets identiﬁed as b jets.

Events are selected oﬄine by requiring at least four b tagged jets with p

> 30 GeV

and |η| < 2.4. The selected jets are combined randomly into pairs to form two Higgs boson

candidates with masses m

and m

. For the LMR, HH candidates are chosen from the

four selected jets such that |m

−120 GeV| < 40 GeV for each candidate Higgs boson. For

the MMR the H candidates are selected using ∆R =

√

(∆η)

+ (∆φ)

less than 1.5, where

∆η and ∆φ are the diﬀerences in the pseudorapidities and azimuthal angles (in radians)

of the two jets.

In the two-dimensional space deﬁned by the reconstructed masses of the two Higgs

boson candidates, H

and H

, a circular signal region (SR) is deﬁned with R < 1, where

R is deﬁned as:

R =



− M





− M



(4.1)

The central mass value (M) is the average of the means of the m

and m

distributions

for simulated signal events and the parameter r is set to 20 GeV. The centers of these

circular regions have been determined separately for the LMR and MMR and found to

be 120 and 125 GeV, respectively. If there are multiple HH candidates in an event, the

combination that minimizes R

is used.

After these event selection criteria are applied, the dijet invariant mass resolution

for m

and m

is approximately 10–13%, depending on the p

of the reconstructed

Higgs boson, with a few percent shift in the value of the mass peak, relative to 125 GeV.

The Higgs boson mass resolution is further improved by applying multivariate regression

techniques similar to those used in the searches for SM Higgs bosons decaying to bb in

CMS [39, 40]. The regression estimates a correction that is applied after the standard

CMS jet energy corrections [34, 41], and it is computed for individual b jets to improve

the accuracy of the measured energy with respect to the b quark energy. To this end, a

specialized boosted decision tree [42] is trained on simulated b jets from tt events, with

inputs that include observables related to the jet structure and b tagging information. The

average improvement in the Higgs boson mass resolution, measured with simulated signal

samples, is 6–12%, depending on the p

of the reconstructed Higgs boson. The use of the

regression technique increases the sensitivity of the analysis by 5–20% depending on the

mass hypothesis. The regression technique is validated with data samples of Z → (ee, µµ)

events with two b tagged jets and in tt-enriched samples [39]. The cumulative selection

eﬃciencies of the selection criteria described above for the graviton and radion signal

benchmarks are reported in ﬁgure 1.

The reconstructed resonance mass (m

), computed as the invariant mass of H

and

, is displayed for simulated signal events with diﬀerent mass hypotheses in ﬁgure 2. In

– 4 –

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