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Physics Letters B 776 (2018) 355–378

Contents lists available at ScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Measurements of t

tcross sections in association with b jets and

inclusive jets and their ratio using dilepton ﬁnal states in pp collisions

√

s =13 TeV

.The CMS Collaboration



CERN, Switzerland

a r t i c l e i n f o a b s t r a c t

Article history:

Received

29 May 2017

Received

in revised form 9 November 2017

Accepted

21 November 2017

Available

online 23 November 2017

Editor:

M. Doser

Keywords:

CMS

Physics

Top

quark

The cross sections for the production of t

band t

tjj events and their ratio σ

/σ

tjj

are measured

using data corresponding to an integrated luminosity of 2.3 fb

−1

collected in pp collisions at

√

s =13 TeV

with

the CMS detector at the LHC. Events with two leptons (e or μ) and at least four reconstructed

jets, including at least two identiﬁed as b quark jets, in the ﬁnal state are selected. In the full phase

space, the measured ratio is 0.022 ±0.003 (stat) ±0.006 (syst), the cross section σ

is 4.0 ±0.6 (stat) ±

1.3 (syst) pb and σ

tjj

is 184 ±6 (stat) ± 33 (syst) pb. The measurements are compared with the standard

model expectations obtained from a powheg simulation at next-to-leading-order interfaced with pythia.

(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP

1. Introduction

Since the discovery of the Higgs boson [1–3], its properties have

been measured and compared to the standard model (SM) predic-

tion

[4–9]. However, the coupling of the top quark to the Higgs

boson remains to be determined. Although it appears indirectly

through loops in the gluon–gluon fusion production process and

in the H → γγ decay channel, a direct measurement has yet to

be completed. One of the most promising channels for a direct

measurement of the top quark Yukawa coupling in the SM is the

production of the Higgs boson in association with a t

tpair (t

tH),

where the Higgs boson decays to b

b, thus leading to a t

b ﬁnal

state. This ﬁnal state, which has not been observed yet [10], has an

irreducible nonresonant background from the production of a top

quark pair in association with a b quark pair produced via gluon

splitting (g → b

b).

Calculations

of the inclusive production cross section for t

events

with additional jets have been performed to next-to-

leading-order

(NLO) precision for proton–proton centre-of-mass

energies of 7, 8, and 13 TeV [11]. The dominant uncertainties in

these calculations are from the choice of the factorization (μ

) and

renormalization (μ

) scales [12,13], and are complicated by the

presence of two very different scales in this process: the top quark

mass and the jet transverse momentum (p

). Therefore, experi-



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

mental measurements of production cross sections pp → t

tjj (σ

tjj

)

and pp → t

b (σ

) can provide an important test of NLO quan-

tum

chromodynamics (QCD) theory calculations and important

input for describing the main background in the search for the t

process.

Previous cross section and ratio measurements at

√

s = 7

and

8TeVhave been reported by the CMS [14,15] and ATLAS Col-

laborations [16].

In this Letter, the measurements of the cross sections σ

and

tjj

and their ratio are presented using a data sample of pp colli-

sions

collected at a centre-of-mass energy of 13 TeV at the CERN

LHC by the CMS experiment, and corresponding to an integrated

luminosity of 2.3 fb

−1

[17]. Events are selected with the ﬁnal state

consisting of two leptons (e or μ) and at least four reconstructed

jets, of which at least two are identiﬁed as b quark jets. The cross

section ratio is measured with a smaller systematic uncertainty ex-

ploiting

the partial cancellation of uncertainties.

2. The CMS detector and event simulation

The central feature of the CMS apparatus is a superconduct-

ing

solenoid of 6m internal diameter, providing a magnetic ﬁeld

of 3.8 T. Within the solenoid volume are a silicon pixel and strip

tracker, a lead tungstate crystal electromagnetic calorimeter (ECAL),

and a brass and scintillator hadron calorimeter (HCAL), each com-

posed

of a barrel and two endcap sections. Forward calorimeters

extend the pseudorapidity (η) coverage provided by the barrel and

https://doi.org/10.1016/j.physletb.2017.11.043

0370-2693/

SCOAP

356 The CMS Collaboration / Physics Letters B 776 (2018) 355–378

endcap detectors. Muons are reconstructed in gas-ionization detec-

tors

embedded in the steel ﬂux-return yoke outside the solenoid.

Amore detailed description of the CMS detector, together with a

deﬁnition of the coordinate system used and the relevant kine-

matic

variables, can be found in Ref. [18].

The

Monte Carlo (MC) simulated samples for the t

tsig-

nal

are generated by the powheg (v2) event generator [19–21]

NLO, interfaced with pythia (v8.205) [22,23] using the tune

CUETP8M1 [24] to provide the showering of the partons and

to match soft radiation with the contributions from the matrix

elements (MEs). The NNPDF3.0 [25] set of the parton distribu-

tion

functions (PDFs) is used. The MadGraph (v5.1.5.11) event

generator [26] with MEs at leading order (LO), allowing up

to three additional partons, including b quarks, and the Mad-

Graph5_amc@nlo (v2.2.2)

event generator [27] are both used for

cross-checks and studies of systematic uncertainties. The t

tsam-

ples

are normalized to the next-to-next-to-leading-order (NNLO)

cross section calculation [28]. The W+jets and Z/γ

∗

+jets pro-

cesses

are simulated in MadGraph5_amc@nlo and are normalized

to their NNLO cross sections [29]. The single top quark associated

production with a W boson (pp → tW and pp →

tW) is simulated

in the ﬁve-ﬂavour scheme in powheg (v1) at NLO and normal-

ized

to an approximate NNLO cross section calculation [30], while

the t-channel single top quark events are simulated in the four-

ﬂavour

scheme in MadGraph5_amc@nlo. The multijet production

is modelled in pythia with LO MEs. The CMS detector response is

simulated using Geant4 (v9.4) [31]. The events in simulation in-

clude

the effects of additional interactions in the same or nearby

bunch crossings (pileup) and are weighted according to the vertex

distribution observed in data. The number of pileup interactions

in data is estimated from the measured bunch-to-bunch instanta-

neous

luminosity and the total inelastic cross section [32].

3. Deﬁnition of signal events

Measurements are reported for two different regions of the

phase space: the visible and the full phase space. The result in

the visible phase space is measured at the particle level, using

the stable particles after the hadronization, to reduce the possible

theoretical and modelling uncertainties, while the purpose of per-

forming

the result in the full phase space is to facilitate compar-

isons

to NLO calculations or measurements in other decay modes.

To deﬁne the visible phase space, all t

b ﬁnal-state particles

except the neutrinos, i.e. the charged leptons and jets originating

from the decays of the top quarks, as well as the two additional b

quark jets (“b jets”), are required to be within the experimentally

accessible kinematic region. The leptons must have p

> 20 GeV,

and |η| < 2.4. Electrons or muons originating from the leptonic

decays of τ leptons produced in W → τν decays are included. The

particle-level jets are obtained by combining all ﬁnal-state parti-

cles,

excluding neutrinos, at the generator level with an anti-k

clustering algorithm [33] with a distance parameter of 0.4 and are

required to satisfy |η| < 2.5 and p

> 20 GeV, which is lower than

the reconstructed minimum jet p

due to jet resolution – to have

all events that pass the reconstructed jet p

in the visible phase

space. Jets that are within R =



(φ)

+(η)

= 0.5units of

an identiﬁed electron or muon are removed, where φ and η

are the differences in azimuthal angle and pseudorapidity between

the directions of the jet and the lepton. To identify the b and c

quark jets (“c jets”) unambiguously, the b and c hadron momenta

are scaled down to a negligible value and included in the jet clus-

tering

(so called “ghost matching”) [34]. The b and c jets are then

identiﬁed by the presence of the corresponding “ghost” hadrons

among the jet constituents.

Simulated events are categorized as coming from the t

tjj pro-

cess

if they contain at least four particle-level jets, including at

least two jets originating from b quarks, and two leptons (t

tjj →

−

jj → b

b

−

νjj). The t

tjj sample contains four compo-

nents

according to the number of b and c jets in addition to the

two b jets required from the top quark decays. The four compo-

nents

are the t

b ﬁnal state with two b jets, the t

tbj ﬁnal state

with one b jet and one lighter-ﬂavour jet, the t

c ﬁnal state with

two c jets, and the t

tLF ﬁnal state with two light-ﬂavour jets (from

a gluon or u, d, or s quark) or one light-ﬂavour jet and one c jet.

The t

tbj ﬁnal state mainly originates from the merging of two b

jets or the loss of one of the b jets caused by the acceptance re-

quirements.

4. Event selection

The events are recorded at

√

s = 13 TeV using a dilepton trig-

ger [35] that

requires the presence of two isolated leptons (e or μ)

both with p

larger than 17 GeV.

The particle-ﬂow (PF) event algorithm [36,37] reconstructs and

identiﬁes each individual particle with an optimized combination

of information from the various elements of the CMS detector. The

energy of photons is directly obtained from the ECAL measure-

ment.

The energy of electrons is determined from a combination

of the electron momentum at the primary interaction vertex as

determined by the tracker, the energy of the corresponding ECAL

cluster, and the energy sum of all bremsstrahlung photons spatially

compatible with originating from the electron track. The energy of

muons is obtained from the curvature of the corresponding track

reconstructed by combining information from the silicon tracker

and the muon system [38]. The energy of charged hadrons is de-

termined

from a combination of their momenta measured in the

tracker and the matching ECAL and HCAL energy deposits, cor-

rected

for zero-suppression effects and for the response function of

the calorimeters to hadronic showers. Finally, the energy of neutral

hadrons is obtained from the corresponding corrected ECAL and

HCAL energy.

The

leptons and all charged hadrons that are associated with

jets are required to originate from the primary vertex, deﬁned as

the vertex with the highest



of its associated tracks. Muon

candidates are further required to have a high-quality ﬁt includ-

ing

a minimum number of hits in both systems. Requirements on

electron identiﬁcation variables based on shower shape and track-

cluster

matching are further applied to the reconstructed electron

candidates [39–41]. Muons and electrons must have p

> 20 GeV

and

|η| < 2.4.

reduce the background contributions of muons or electrons

from semileptonic heavy-ﬂavour decays, relative isolation criteria

are applied. The relative isolation parameter, I

rel

, is deﬁned as

the ratio of the summed p

of all objects in a cone of R = 0.3

(

R = 0.4) units around the electron (muon) direction to the lep-

ton

. Different cone sizes for electron and muon are used to

maximize the sensitivity. The objects considered are the charged

hadrons associated with the primary vertex as well as the neutral

hadrons and photons, whose energies are corrected to take into

account pileup effects. Thus,

rel



charged hadron



neutral hadron



photon

lepton

. (1)

The muon candidates are required to have I

rel

< 0.15. For the

electron candidates, different I

rel

thresholds (0.077 or 0.068) are

applied depending on the pseudorapidity of the candidate (|η| <

1.48 or 1.48 ≤|η| < 2.40). These thresholds are obtained from a

multivariate analysis technique and result from the considerable

The CMS Collaboration / Physics Letters B 776 (2018) 355–378 357

differences in both the ECAL and the tracker in the two pseudo-

rapidity

regions. The eﬃciencies for the above lepton identiﬁca-

tion

requirements are measured using Z boson candidates in data

with a dilepton invariant mass between 70 and 130 GeV, and are

compared with the values from the simulation. The differences

between the two evaluations are applied as a correction to the

simulation.

The

event selection requires the presence of two isolated

opposite-sign leptons of invariant mass M



> 12 GeV. Lepton pairs

of the same ﬂavour (e

−

, μ

−

) are rejected if their invariant

mass is within 15 GeV of the Z boson mass. The missing trans-

verse

momentum vector



miss

is deﬁned as the projection on the

plane perpendicular to the beams of the negative vector sum of

the momenta of all reconstructed PF candidates in the event. Its

magnitude is referred to as p

miss

. In the same-ﬂavour channels,

remaining backgrounds from Z+jets processes are suppressed by

demanding p

miss

> 30 GeV. For the e

∓

channel, no p

miss

re-

quirement

is applied.

Jets

are reconstructed using the same anti-k

clustering algo-

rithm

as particle-level jets in the simulations, with the PF candi-

dates

as input particles. The jet momentum is determined as the

vectorial sum of all PF candidate momenta in the jet and is found

from simulation to be within 5 to 10% of the true momentum over

the whole p

spectrum and detector acceptance. An offset correc-

tion

is applied to jet energies to take into account the contribution

from pileup interactions. Jet energy corrections are derived from

simulation and conﬁrmed with in situ measurements of the en-

ergy

balance in dijet and photon+jet events [42]. Additional selec-

tion

criteria are applied to each event to remove spurious jet-like

features originating from isolated noise patterns in certain HCAL

regions. The event must contain at least four reconstructed jets

with p

> 30 GeV and |η| < 2.4, of which at least two jets must

be identiﬁed as b jets, using the combined secondary vertex (CSV)

algorithm (v2), which combines secondary vertex information with

lifetime information of single tracks to produce a b tagging dis-

criminator [43].

A b tagging requirement on this discriminator is

applied, which has an eﬃciency of about 60–70% for b jets and a

misidentiﬁcation probability of 1% for light-ﬂavour jets and 15–20%

for c-ﬂavour jets [44].

Differences

in the b tagging eﬃciencies between data and simu-

lation [43] are

accounted for by reweighting the shape of the CSV b

tagging discriminator distribution in the simulation to match that

in the data. Data/simulation p

- and η-dependent correction fac-

tors

are derived from the control samples separately for light- and

heavy-ﬂavour jets, that are described in Section 6.

The

diboson, W+jets and multijet contributions are found to

be negligible after the full event selection. The Z+jets background

is estimated from data using control samples enriched in Z boson

events.

Table 1 gi

ves the predicted number of events for each physics

process and for each lepton category, as well as a comparison of

the total number of events expected from the simulation and ob-

served

in data. Since the full event selection requires at least two

b-tagged jets, a condition which is usually satisﬁed by t

tevents,

only 5% of the events are from non-t

t processes. The t

tbj ﬁnal

state is predominantly composed of t

bevents where there is one

lost b jet due to acceptance requirements (73% of t

tbj events). The

background contribution from t

tevents that fail the visible phase

space requirements is labelled “t

tothers”. The number of observed

events with four or more reconstructed jets is lower than the pre-

diction

from the simulation, a condition that is also observed in

the lepton+jets decay mode [45].

Table 1

Predicted

number of events for each physics process and for each dilepton category,

their total, and the observed number of events. Results are shown after the ﬁnal

event selection. The Z+jets normalization and uncertainty are calculated from data,

while all other predictions and statistical uncertainties come from the simulated

data samples. The t

tsample for event categorization is from the powheg (v2) event

generator interfaced with pythia (v8.205).

Process e

−

∓

All

b6.3± 0.4 8.6 ± 0.4 24 ± 139± 1

tbj 16 ± 121±157± 295± 2

c7.7± 0.4 11 ± 127± 146± 1

tLF 157 ± 2 220 ± 2 596 ± 3 972 ± 4

tothers 18± 119± 161± 199± 1

tV 2.5± 0.1 3.2 ± 0.2 7.3 ± 0.2 14 ± 1

Single t 6.6

± 0.8 8.4 ± 0.8 23 ± 239± 2

+jets 0.8

+1.0

−0.8

5.4 ± 1.5 0.6 ± 0.5 6.8 ± 1.9

Total 215

± 2 297 ± 3 796 ± 4 1311 ± 6

Data 186 288 682 1156

5. Cross section measurements

The ﬁrst and the second jets in decreasing order of the b tag-

ging

discriminator usually (in 85% of t

tjj events) correspond to the

b jets from the decays of top quarks, and hence these jets pro-

vide

no discriminating power between t

b and t

tjj events. The

third and the fourth jets from t

tjj events are mostly light-ﬂavour

jets, while these are heavy-ﬂavour jets for t

bevents. The nor-

malized

2D distributions of the discriminators from simulation for

the third and the fourth jets are shown in Fig. 1. These 2D dis-

tributions

are used to separate t

bevents from other processes.

To extract the ratio of the number of t

bevents to t

tjj events,

a binned maximum-likelihood ﬁt is performed on the 2D distribu-

tion

of the CSV b tagging discriminators of the third and the fourth

jets, where the three event categories e

∓

, e

∓

, and μ

∓

are

merged.

The

number of t

tjj events and the ratio of the numbers of t

events

to t

tjj events are free parameters in the ﬁt. The t

c and

tLF processes have similar 2D distributions so their contributions

are combined based on the MC simulation.

The

likelihood function is constructed as the product over all

bins of a Poisson probability with a mean deﬁned in each bin by

M(N

tjj

, R) = N

tjj



R F

norm



norm

tbj

+(1 − R − R



) F

norm

tLF+t



tjj

tothers

+ N

bkg

, (2)

where F

norm

, F

norm

tbj

, and F

norm

tLF+t

are the normalized expecta-

tions

for each bin of t

b, t

tbj, and the combination of t

tLF and

c, respectively. The parameter N

tjj

denotes the number of the

tjj events from the ﬁt. The quantity f

t others

reﬂects the fraction

of other t

t processes in the t

tjj sample as calculated in simulation

tothersdivided by the sum of the t

tjj components in Table 1).

The other backgrounds, such as t

tV (V = W or Z) and single top

quark processes are ﬁxed to the simulation expectations, while the

Z+jets background is ﬁxed to its estimation from control samples

in data. This remaining background not from the t

t process is la-

belled

bkg

. The parameter R is the ratio of the number of t

events

with respect to the number of t

tjj events, and R



is the

fraction of t

tbj events at the reconstruction level and constrained

to the ratio of the number of the t

tbj events to t

bevents. It is

ﬁxed to 2.43 as calculated from the MC simulation (powheg inter-

faced

with pythia). The effect of this assumption is estimated as

a systematic uncertainty in Section 6. Values for N

tjj

of 950 ± 30

358 The CMS Collaboration / Physics Letters B 776 (2018) 355–378

Fig. 1. Normalized 2D distributions of the b jet discriminators of the third (x-axis) and the fourth (y-axis) jets sorted in decreasing order of b tagging discriminator value,

after the full event selection for t

b (upper left), t

tbj (upper right), t

c(lower left), and t

tLF (lower right) processes.

events and R of 0.056 ±0.008 are obtained from the ﬁt. The cor-

relation

coeﬃcient between the two parameters is 0.002.

The

result obtained for R is corrected to account for the differ-

ent

selection eﬃciencies for the two processes. The event selection

eﬃciencies, deﬁned as the number of t

b and t

tjj events after the

full event selection divided by the number of events in the cor-

responding

visible phase space, are 27% and 12%, respectively. For

the t

b process, there are at least 4 b jets in the events, therefore,

it is easier to fulﬁll the requirement of at least two b-tagged jets

than the t

tjj process.

Fig. 2 sho

ws the comparisons of the b tagging discriminator dis-

tributions

of the third and the fourth jets in the events from data

and simulation, where the simulated histograms have been scaled

to the ﬁt result.

The

b-tagged jet multiplicity distribution in Fig. 3 shows the

comparison between data and the simulation after the requirement

of at least four jets, together with the ratio of the number of data

events to the expectation in the lower panel, where the simulated

histograms have been scaled to the ﬁt result.

The

b and t

tjj cross sections in the visible phase space are

calculated using the relationship σ

visible

= N/(L), where L is the

integrated luminosity, N is the number of events from the ﬁt re-

sult,

and  is the eﬃciency for each process. For the purpose of

comparing with the theoretical prediction and the measurements

in the other decay modes, the cross sections in the full phase space

are extrapolated from the cross sections in the visible phase space

using the relation σ

full

=σ

visible

/A, where A is the acceptance, de-

ﬁned

as the number of events in the corresponding visible phase

space divided by the number of events in the full phase space. The

acceptances are calculated based on the powheg simulation and

are 2.2% and 2.0% for t

b and t

tjj, respectively, including the lep-

tonic

branching fraction of both W bosons [46].

6. Estimation of systematic uncertainties

The systematic uncertainties are determined separately for the

b and t

tjj cross sections, and their ratio. In the ratio, many

systematic effects cancel, speciﬁcally normalization uncertainties,

such as the ones related to the measurement of the integrated

luminosity and the lepton identiﬁcation, including trigger eﬃcien-

cies,

since they are common to both processes. The various sys-

tematic

uncertainties in the measured values are shown in Table 2

for

the visible phase space.

The

systematic uncertainties associated with the b tagging ef-

ﬁciency

for heavy- and light-ﬂavour jets are studied separately,

varying their values within the corresponding uncertainties. The b-

ﬂavour

correction factors are obtained using t

t enriched events by

tagging one b jet and probing the other b jet. Their dominant un-

certainty

comes from the contamination when one of the b jets is

not reconstructed [47] (indicated as “b quark ﬂavour” in Table 2).

The light-ﬂavour jet correction factors are determined from Z+jets

enriched events with at least two jets (indicated as “light ﬂavour”

in Table 2). The uncertainty arises because in this control sample

of Z+jets, the contamination from the Z+b

b process is not well

modelled. The correction factor for c jets is not measured, owing

to the limited amount of data, and is assumed to be unity with an

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用ATLAS测量在$$ \ sqrt {s} = 13〜\ text {TeV} $$ s = 13TeV pp碰撞中pp碰撞中的$$ W ^ {\ pm} Z $$ W±Z生产横截面并测量玻色子极化 探测器

在s = 13TeV的pp碰撞中t通道单顶夸克生成的截面测量

在s = 13 $$ \ sqrt {s} = 13 $$ TeV的pp碰撞中测量正J /ψ产生横截面

搜索新的共振，在事件中有一个轻子和缺少横向动量的pp碰撞中的s = 13 TeV的pp碰撞与ATLAS探测器

Z玻色子在s = 8 TeV pp碰撞中产生的Z玻色子的角系数，并随横向动量和速度的变化而衰减到¼+¼

搜索希格斯玻色子衰变成s = 8 TeV的pp碰撞中低Dilepton质量的ÂÎÎÎ¡

W玻色子的生产截面的测量与s = 8TeV的pp碰撞中的两个b射流相关

最新资源

“使用ATLAS探测器在s = 13 TeV的pp碰撞中使用带有b标记喷头的eμ事件和b标记喷头测量tt生产横截面”的更正。来吧 B 761（2016）136]

用ATLAS测量在$$ \ sqrt {s} = 13〜\ text {TeV} $$ s = 13TeV pp碰撞中pp碰撞中的$$ W ^ {\ pm} Z $$ W±Z生产横截面并测量玻色子极化探测器