Observation of the decay B¯s0→χc2K+K− in the ϕ mass region

JHEP08(2018)191

Published for SISSA by Springer

Received: June 28, 2018 Accepted: August 22, 2018 Published: August 29, 2018

Observation of the decay B

0_s

→ χ

_c2

K

+

K

−

in the φ

mass region

The LHCb collaboration

E-mail: Matthew.Needham@cern.ch

Abstract: The B0s → χc2K+K− decay mode is observed and its branching fraction

relative to the corresponding χc1 decay mode, in a ±15 MeV/c2 window around the φ

mass, is found to be B(B0 s → χc2K+K−) B(B0 s → χc1K+K−) = (17.1 ± 3.1 ± 0.4 ± 0.9)%,

where the first uncertainty is statistical, the second systematic and the third due to

the knowledge of the branching fractions of radiative χc decays. The decay mode

B0

s → χc1K+K− allows the Bs0 mass to be measured as

m(B_s0) = 5366.83 ± 0.25 ± 0.27 MeV/c2,

where the first uncertainty is statistical and the second systematic. A combination of this

result with other LHCb determinations of the B0

s mass is made.

Keywords: Flavor physics, B physics, Hadron-Hadron scattering (experiments)

JHEP08(2018)191

Contents

1 Introduction 1

2 Detector and simulation 2

3 Selection 3

4 Mass fit 5

5 Determination of the B0_s → χc2K+K− branching fraction 8

6 Measurement of the B0

s mass 10

7 Summary 11

The LHCb collaboration 15

1 Introduction

Studies of two-body b-hadron decays to final states containing a hidden charm meson

such as a χcJ state (J = 0, 1, 2) provide powerful probes of the strong interaction. These

decays proceed predominantly via a colour-suppressed b → c¯cs transition. Theoretically,

such decays are often studied in the factorization approach [1,2]. It is predicted, in the

absence of final-state interactions, that decays to spin-0 and 2 charmonium states are

highly suppressed compared to decays to spin-1 states [1]. Experimentally, factorization

has been observed to hold for B+_{→ χ}

c1,c2K+ decays,1 for which the Belle collaboration

reported B(B+_{→ χ}

c2K+)/B(B+→ χc1K+) = (2.25+0.73−0.69(stat)±0.17 (syst))×10−2 [3].

In other modes, less suppression is observed. For example, the LHCb

col-laboration has measured B(B0 _{→ χ}

c2K∗(892)0)/B(B0 → χc1K∗(892)0) =

(17.1 ± 5.0 (stat) ± 1.7 (syst) ± 1.1 (B)) × 10−2 [4], where the third uncertainty is due

to the knowledge of external branching fractions, and the Belle collaboration has

mea-sured B(B+_{→ χ}

c2K+π+π−)/B(B+→ χc1K+π+π−) = 0.36 ± 0.05 [5], where the total

uncertainty is quoted. Even more strikingly, the LHCb collaboration reported [6]

B(Λ0

b → χc2pK−)/B(Λ0b → χc1pK−) = 1.02 ± 0.10 (stat) ± 0.02 (syst) ± 0.05 (B). These

observations are difficult to reconcile with the factorization hypothesis. It is thus

interesting to probe this ratio with other exclusive decay modes.

In this paper, the decay B0

s → χc2K+K− (with charge conjugation implied) with

χc2→ J/ψ γ and J/ψ → µ+µ−is observed using the LHCb data set collected in pp collisions

up to the end of 2016. The data corresponds to an integrated luminosity of 3.0 fb−1

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W− B0_s 8 > > > > < > > > > : b s c c % χcJ s s % φ

Figure 1. Tree-level Feynman diagram for the B0

s→ χcJφ decay mode.

collected at centre-of-mass energies of 7 and 8 TeV during 2011 and 2012, together with

1.9 fb−1collected at a centre-of-mass energy of 13 TeV during 2015 and 2016. This analysis

focuses on the low K+_K− _{mass region, where the B}0

s → χcJK+K− decay is expected to

be dominated by the decay of an intermediate φ meson, as shown in figure 1. The same

data set allows a measurement of the B0

s mass with high precision due to the relatively

small energy release. These studies build on the previous observation of the B0

s → χc1φ

mode [4].

2 Detector and simulation

The LHCb detector [7,8] is a single-arm forward spectrometer covering the pseudorapidity

range 2 < η < 5, designed for the study of particles containing b or c quarks. The detector includes a high-precision tracking system consisting of a silicon-strip vertex detector

sur-rounding the pp interaction region [9], a large-area silicon-strip detector located upstream

of a dipole magnet with a bending power of about 4 Tm, and three stations of

silicon-strip detectors and straw drift tubes [10] placed downstream of the magnet. The tracking

system provides a measurement of the momentum, p, of charged particles with a relative uncertainty that varies from 0.5% at low momentum to 1.0% at 200 GeV/c. The

momen-tum scale is calibrated using samples of J/ψ → µ+_µ− _{and B}+ _{→ J/ψ K}+ _{decays collected}

concurrently with the data sample used for this analysis [11, 12]. The relative accuracy

of this procedure is estimated to be 3 × 10−4 using samples of other fully reconstructed

b-hadron, narrow-Υ , and K0

S decays. The minimum distance of a track to a primary vertex

(PV), the impact parameter (IP), is measured with a resolution of (15 + 29/pT) µm, where

pT is the component of the momentum transverse to the beam, in GeV/c.

Different types of charged hadrons are distinguished using information from two ring-imaging Cherenkov (RICH) detectors. Photons, electrons and hadrons are identified by a calorimeter system consisting of scintillating-pad and preshower detectors, an electromag-netic calorimeter and a hadronic calorimeter. Muons are identified by a system composed

of alternating layers of iron and multiwire proportional chambers [13].

The online event selection is performed by a trigger [14], which consists of a hardware

stage, based on information from the calorimeter and muon systems, followed by a software stage, where a full event reconstruction is made. Candidate events are required to pass

the hardware trigger, which selects muon and dimuon candidates with high pT based upon

JHEP08(2018)191

first performs a partial event reconstruction and requires events to have two well-identified

oppositely charged muons with an invariant mass larger than 2.7 GeV/c2_{. The second stage}

performs a full event reconstruction. Events are retained for further processing if they

contain a J/ψ → µ+_µ− _{candidate. The distance between the decay vertex of the J/ψ and}

each PV, divided by its uncertainty, is required to be larger than three.

To study the properties of the signal and the most important backgrounds, simulated

pp collisions are generated using Pythia [15,16] with a specific LHCb configuration [17].

Decays of hadronic particles are described by EvtGen [18], in which final-state radiation

is generated using Photos [19]. The interaction of the generated particles with the

detec-tor, and its response, are implemented using the Geant4 toolkit [20, 21] as described in

ref. [22]. Other sources of background, such as those from b → ψ(2S) transitions, where the

ψ(2S) decays radiatively to a χcJ meson, are studied using the RapidSim fast simulation

package [23].

3 Selection

A two-step procedure is used to optimize the selection of B0

s → χc1,c2K+K− candidates.

These studies use simulation samples together with the high-mass sideband of the data,

5550 < m(χc2K+K−) < 6150 MeV/c2, which is not used for subsequent analysis. In a

first step, loose selection criteria are applied to reduce the background significantly whilst retaining high signal efficiency. Subsequently, a multivariate selection is used to reduce further the combinatorial background.

The selection starts from a pair of oppositely charged particles, identified as muons, that form a common decay vertex. Combinatorial background is suppressed by requiring

that the χ2

IP of the muon candidates, defined as the difference between the χ2 of the

PV reconstructed with and without the considered particle, be larger than four for all

reconstructed PVs. The invariant mass of the dimuon candidate must be within 50 MeV/c2

of the known J/ψ mass [24].

Photons are selected from well-identified neutral clusters, reconstructed in the

electro-magnetic calorimeter [8], that have a transverse energy in excess of 700 MeV/c. Selected

J/ψ and photon candidates are combined to form χc1,c2 candidates. The invariant mass

of the combination, obtained from a kinematic fit [25] with a J/ψ mass constraint [24], is

required to be within the range 3400–3700 MeV/c2_.

Pairs of oppositely charged kaons with pT > 200 MeV/c and displaced from all PVs

(χ2

IP > 4) are selected. Good kaon identification is achieved by using information from

the RICH detectors. This is combined with kinematic and track quality information using neural networks which provide a response that varies between 0 and 1 for each of the

different mass hypotheses: kaon (PK_{), pion (P}π_{), and proton (P}p_{). The closer to one}

this value is, the higher the likelihood that the particular mass hypothesis is correct. The chosen requirements on these variables have an efficiency of (86.8 ± 0.2)% and (86.4 ± 0.2)%

for the B0

s → χc1K+K− and B0s → χc2K+K− modes, respectively, where the uncertainty

is due the size of the available simulation samples. The invariant mass of the selected kaon

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3400 3500 3600 3700 ] 2 c ) [MeV/ γ ψ J/ ( m 0 100 200 300 400 500 ) 2 c Candidates/ (12 MeV/ LHCb 5200 5300 5400 5500 ] 2 c ) [MeV/ − K + K γ ψ J/ ( m 0 100 200 300 400 500 ) 2 c Candidates/ (12 MeV/ LHCb

Figure 2. Invariant-mass distributions of (left) J/ψ γ and (right) J/ψ γK+_K− _{after the loose}

selection criteria.

substantially reduce background from K∗₍₈₉₂₎0 _{decays where a pion is misidentified as a}

kaon. To reduce background from Λ0

b decays to excited Λ states a loose proton veto is

applied to both kaon candidates.

The χc1,c2candidate is combined with the pair of kaons to make a candidate B0s meson,

which is associated to the PV giving the minimum χ2

IP. A kinematic fit is performed in

which the candidate is constrained to point to this PV and the dimuon mass is constrained

to the known value of the J/ψ mass [24]. The reduced χ2 _{of this fit is required to be less}

than five. Combinatorial background is further reduced by requiring the decay time of the

B0

s candidate to be larger than 0.3 ps and its χ2IP to be less than 20.

Several vetoes are applied to remove background from fully reconstructed b-hadron de-cay modes. By combining kinematic and particle-identification information it is possible to impose requirements that are almost fully efficient for signal decays. The upper-mass side-band is found to be polluted by fully reconstructed b-hadron decays where a random photon

is added. The most important of these is the B0

s→ J/ψ φ decay mode. This is removed by

rejecting candidates in which the reconstructed J/ψ K+_K− _{invariant mass, calculated with}

a J/ψ mass constraint, is within 18 MeV/c2 _{(±3σ) of the known B}0

s mass [24]. A similar

background is possible from the B0 _{→ J/ψ K}+_π− _{decay mode where the pion is}

misidenti-fied as a kaon. The candidate is rejected if either of the two possible J/ψ K+_π− _{masses is}

within 18 MeV/c2_{of the known B}0_{mass. These two requirements reject a negligible number}

of signal decays. Finally, candidates in which either of the kaons is consistent with being a

proton (Pp _{> P}K_{) are rejected if the reconstructed J/ψ pK}− _{mass is within 18 MeV/c}2 _of

the known Λ0

b mass. The efficiency of this veto is 99.3% for signal decays. Background from

the Λ0

b → χc1,c2pK

− _{decay mode peaks in the signal regions. Therefore, a veto is applied}

to each kaon candidate in turn. The candidate is rejected if the χc1,c2pK− mass is within

10 MeV/c2 _{of the Λ}0

b mass (a ±2σ window) and the proton well identified. After these

requirements a broad signal is seen in the χc1,c2 mass region and the B0s → J/ψ γK+K−

decay mode is observed (figure2) above a large combinatorial background.

The second step of the selection process is based on a multilayer perceptron (MLP)

classifier [26,27], trained using the B0

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0 0.2 0.4 0.6 0.8 1 MLP response 0 100 200 300 400 500 600 Candidates/ 0.05 LHCb

Figure 3. MLP response for (solid yellow) the B0

s→ χc1K+K− simulation sample, (hashed blue)

high-mass sideband and (black points) the background subtracted B0

s→ χc1K+K−signal in data.

The histogram areas are normalized to the number of B0

s→ χc1K+K−candidates observed in data

after the loose selection. The arrow indicates the selected threshold.

samples and the high-mass sideband of the data. As input, the classifier uses ten variables, related to the displacement of the candidate from the associated PV and kinematics, that

show good agreement between data and simulation. Figure3shows the output of the MLP

for the training samples and the B0

s → χc1K+K− signal in data where the background

is subtracted using the sPlot technique [28]. The MLP gives excellent separation between

signal and background and shows good agreement between data and simulation.

The requirement on the MLP output is chosen to maximize the figure of merit

/(a/2 +√NB) [29], where  is the signal efficiency for the χc2 mode obtained from the

simulation, a = 5 is the target signal significance, and NB is the background yield in a

±25 MeV/c2 _{window centred on the known B}0

s mass [24] estimated from the sideband. The

chosen threshold of 0.85 has an efficiency of (65.1 ± 0.3)% for the B0

s → χc1K+K− decay

mode and (66.1 ± 0.3)% for the B0

s → χc2K+K−decay mode whilst rejecting (96.0 ± 0.3)%

of the combinatorial background.

4 Mass fit

The energy resolution of the LHCb calorimeter results in an invariant-mass resolution for

the χc1and χc2states of about 50 MeV/c2. This makes it difficult to separate the two states

based on the J/ψ γ invariant mass alone. To improve the mass resolution, the approach used

in previous LHCb analyses [4, 6] is followed. Two kinematic fits are made to the dataset

in which constraints are applied to ensure the pointing of the candidate to the associated

primary vertex, on the J/ψ mass and either on the χc2 or χc1 mass. Owing to the small

radiative branching fraction any contribution from the B0

s → χc0K+K− decay mode can

be ignored. As can be seen in figure 4 the two components are then separable from the

B0

s invariant mass calculated from this fit. A mass model for the B0s → χc1,c2K+K−

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distribution into a component related to the constraints and a component related to the detector resolution.

The effect of applying the χc2 mass constraint can be seen as follows.2 To satisfy

the constraint, the kinematic fit adjusts the photon momentum, which is the most poorly measured quantity, by a factor, 1 − α, where

α = m 2 χc2 − m 2 J/ψ γ m2 J/ψ − m2J/ψ γ

and mχc2 and mJ/ψ are the known values of the χc2and J/ψ masses [24], respectively. For

each event in the simulation the value of α can be calculated using the generated four-momenta. Then the generated four-momentum of the photon is scaled by 1 − α and the

four-momentum of the B0

smeson recalculated. In this way the effect of the constraint is

em-ulated. For genuine B0

s → χc2K+K−decays, applying a χc2mass constraint transforms the

true B0

s invariant-mass distribution from a δ-function to a Breit-Wigner distribution whose

width is equal to the natural width of the χc2state. In the case of genuine B0s → χc1K+K−

decays the distribution is shifted upwards in mass by an amount equal to the mass splitting

between the χc2 and χc1 states and is broadened. The RMS of the resulting distribution

is 9.5 MeV/c2_{, which allows the separation of the χ}

c1 and χc2 components.

To obtain the mass models for the χc1and χc2components, the distributions described

above are convolved with a resolution function that accounts for the uncertainty in the measurement of the kaon four-momenta by the tracking system. Using the simulation, the resolution model is found to be well described by a Student’s t-distribution which has two resolution parameters: s which describes the core and n which controls the tail of the distribution. As part of the systematic studies, the following alternative resolution models

are also considered: Gaussian, sum of two Gaussians, double-sided Crystal Ball [30, 31]

and Bukin [32] functions. The advantage of factorizing the mass distribution in this way

is that it leads to a model where all parameters can be fixed from physics considerations

apart from an overall resolution scale factor, sf, that accounts for differences between data

and simulation. The simulation is tuned to match the mass resolution seen in data for the

B+_{→ J/ψ K}+_{, B}0_{→ J/ψ K}+_π−_{and B}0

s → J/ψ φ decay modes with a precision of 5%. The

validity of this tuning for B0

s → χc1,c2K+K−decays is cross-checked using Λ0b → χc1,2pK−

candidates, which have a similar topology, selected using the criteria described in ref. [6].

Similar agreement between data and simulation is found and consequently in this analysis

a Gaussian constraint is applied, sf = 1.00 ± 0.05.

After the selection described in section 3 three sources of background remain and are

included in the mass fit. By default, combinatorial background is modelled by a first-order polynomial. Both a power law and an exponential function are considered as systematic

variations. Partially reconstructed background from B0

s → ψ(2S)K+K− decays, with the

subsequent decay ψ(2S) → χcJγ, is studied using RapidSim and the resulting template

is added to the fit. The residual background from B0 _{→ χ}

c1,c2K∗(892)0 and partially

reconstructed B0 _{→ ψ(2S)K}∗₍₈₉₂₎0 _{decays is estimated to be 7 ± 2 candidates and is}

included as a fixed component in the fit with the shape modelled using the simulation.

2_{The same formalism applies for a χ}

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] 2 c ) [MeV/ − K + K c1 χ ( m 5200 5300 5400 ) 2 c Candidates/ (6 MeV/ 0 50 100 150 200 250 300 LHCb Pull 2 − 0 2 ] 2 c ) [MeV/ − K + K c2 χ ( m 5200 5300 5400 5500 ) 2 c Candidates/ (6 MeV/ 0 20 40 60 80 100 120 140 160 LHCb Pull 2 − 0 2

Figure 4. Invariant-mass distributions of selected candidates for (left) the χc1K+K− fit and

(right) the χc2K+K− fit. The total fitted function is superimposed (solid red line) together with

the (blue hashed area) χc1 component, (solid yellow) χc2 component and (dashed black line) the

background component. The pull, i.e. the difference between the observed and fitted value divided by the uncertainty, is shown below each of the plots.

Extended unbinned maximum likelihood fits are applied separately to the

invariant-mass distribution of selected candidates with either a χc1 or χc2 mass constraint applied.

The former fit (refered to as the χc1K+K− fit) is used to make further cross-checks of the

mass resolution and to determine the B0

s mass. The latter (refered to as the χc2K+K−fit)

is used to determine the yield of the B0

s → χc1,c2K+K− components. The χc1K+K− fit

has six free parameters: the B0

s → χc1K+K−decay yield, Nχc1, the B

0

s → χc2K+K−decay

yield relative to that of the χc1 mode, f , the Bs0 mass, m(Bs0), the yield of the partially

reconstructed background, Npart, the combinatorial background yield, Ncomb, and the slope

of the combinatorial background. In addition, sf is allowed to vary within the Gaussian

constraint of 1.00 ± 0.05. The χc2K+K− fit has the same free parameters apart from

m(B0

s), which is fixed to its known value [24]. The fit procedure is validated using both the

full simulation and pseudoexperiments which are fits to simulated distributions generated according to the density functions described above and using the yields from the fit to the data. No significant bias is found and the uncertainties estimated by the fit agree with the results of the pseudoexperiments.

The results of the fits to the data are shown in figure 4 and the relevant parameters

listed in table 1. The quality of the fit is judged to be good from the residuals and by a

binned χ2 _{test. The value of N}

part is consistent with the expectation based on the relevant

branching fractions [24]. The significance of the B0

s → χc2K+K− component, including

systematic uncertainties due to the choice of fit model and evaluated using the fit with

χc2γ mass constraint, is evaluated to be 6.7σ using Wilks’ theorem [33]. The values of f

determined from the two fits are consistent. That from the χc2K+K− fit is more precise

since, as can be seen from figure4, the width of the B0

s → χc2K+K−component is narrower

than in the B0

s → χc1K+K− case. Hence, this value is used in the determination of the

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Value Fit parameter χc1 fit χc2 fit Nχc1 745 ± 30 743 ± 30 f [%] 8.3 ± 2.2 10.5 ± 1.9 m(B0 s) [ MeV/c2] 5366.83 ± 0.25 – Npart 390 ± 47 343 ± 46 Ncomb 1024 ± 65 1013 ± 62 sf 1.01 ± 0.03 1.02 ± 0.05

Table 1. Results of the χc1K+K− and χc2K+K− fits to the invariant-mass distributions. A

Gaussian constraint is applied to the sf parameter.

5 Determination of the B0

s → χc2K

+_K− _{branching fraction}

The ratio of branching fractions is calculated as

B(B0 s → χc2K+K−) B(B0 s → χc1K+K−) = f · r· B(χc1→ J/ψ γ) B(χc2→ J/ψ γ) , where f = (10.5 ± 1.9)% and B(χc1→ J/ψ γ) B(χc2→ J/ψ γ) = 1.77 ± 0.09,

using the values given in ref. [24]. The ratio of reconstruction and selection efficiencies

between the two modes, r, is not one due to differences in the photon kinematics between

the two decay modes. It is evaluated using the simulation to be (92.0 ± 1.6)% where the uncertainty is statistical. Thus, the ratio of branching fractions is

B(B0

s → χc2K+K−)

B(B0

s → χc1K+K−)

= (17.1 ± 3.1)%, where the uncertainty is statistical.

Since the signal and normalization modes are identical in topology, systematic un-certainties largely cancel in the ratio of branching fractions. The assigned systematic

uncertainties are listed in table 2. The limited size of the available simulation samples

leads to a relative uncertainty of 1.8%. The uncertainty from the choice of the fit model

is evaluated to be 1.5% using the discrete profiling method described in ref. [34].

Prop-agating the uncertainty on the yield of the K∗₍₈₉₂₎0 _{background leads to an additional}

0.3% uncertainty. The effect of possible differences in the B0

s kinematics between the data

and simulation is studied by weighting the simulation such that pT spectra in data and

simulation agree for the B0

s→ χc1K+K− decay mode. Based on this study, a 0.4%

uncer-tainty is assigned. Summing in quadrature, the total systematic unceruncer-tainty amounts to

2.4%. No systematic uncertainty is included for the admixture of CP -odd and CP -even B0

s

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Source of systematic uncertainty Relative uncertainty (%)

Simulation sample size 1.8

Fit model 1.5

K∗(892)0 _{background 0.3}

Data/simulation agreement 0.4

Sum in quadrature of above 2.4

B(χc1→ J/ψ γ) 3.5

B(χc2→ J/ψ γ) 3.6

Sum in quadrature of external uncertainties 5.0

Table 2. Systematic uncertainties for the measurement of the ratio B(B0

s→ χc2K+K−)/B(B0s→ χc1K+K−). 1000 1010 1020 1030 1040 1050 ] 2 c ) [MeV/ − K + K ( m 0 20 40 60 80 100 120 140 160 180 200 ) 2 c Entries/ (2 MeV/ − K + K c1 χ → 0 s B LHCb 1000 1010 1020 1030 1040 1050 ] 2 c ) [MeV/ − K + K ( m 5 − 0 5 10 15 20 25 30 ) 2 c Entries/ (2 MeV/ − K + K c2 χ → 0 s B LHCb

Figure 5. Invariant-mass distribution of the K+_K− _{pair for the (left) B}0

s → χc1K+K− decay

obtained with the χc1 mass constraint applied to the Bs0 candidate invariant mass and (right)

B0

s→ χc2K+K−decay obtained with the χc2mass constraint applied to the Bs0candidate invariant

mass. The (red solid line) total fitted function is superimposed together with (blue hashed area) the S-wave component.

extreme case that one decay is only from the short-lifetime eigenstate and the other only from the long-lifetime eigenstate, the ratio would change by 2.8%.

External systematic uncertainties of 3.5% and 3.6% arise from the knowledge of the

radiative χc1→ J/ψ γ and χc2→ J/ψ γ branching fractions [24]. Adding these in quadrature

gives an additional uncertainty of 5.0%.

Both decay modes are expected to be dominated by contributions from an intermediate

φ resonance that decays to a K+_K− _{pair. Additional S-wave contributions may also be}

present. To check if this is the case, the resonance structure of the m(K+_K−₎

invariant-mass distribution is studied using the sPlot technique [28], with weights determined from

the χc1K+K− and χc2K+K− mass fits described in section 4. To increase the sensitivity

to an S-wave contribution, the K+_K− _{mass window 1000–1050 MeV/c}2 _{is considered. The}

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Source of uncertainty Value [ MeV/c2_]

Momentum scale 0.26 Material budget 0.02 Fit model 0.01 χc1 mass 0.08 K+ _{mass 0.02} Sum in quadrature 0.27

Table 3. Systematic uncertainties on the B0

s mass measurement.

The observed K+_K− _{invariant-mass distribution is modelled with two components.}

The first is a relativistic P-wave Breit-Wigner function with Blatt-Weisskopf form

fac-tors [36]. The natural width is fixed to the known value of the φ meson [24] and a meson

radius parameter of 3~c GeV−1 is used. The detector resolution of 0.9 MeV/c2 _{is accounted}

for by convolving the resonance lineshape with a Gaussian distribution. The second

con-tribution to the K+_K− _{invariant-mass distribution is the S-wave. This is assumed to}

be nonresonant in nature and is modelled by a phase-space function. The fit model has

two free parameters: the φ mass and the nonresonant S-wave fraction, fs. Applying an

unbinned maximum likelihood fit of this model to the B0

s → χc1K+K− sample gives

fs= (13.9 ± 2.3)%, where the statistical uncertainty is evaluated using pseudoexperiments.

This value is consistent at the 2σ-level with that found in the previous LHCb study [4] of

this mode, fs= (3.3 ± 5.1)%. This corresponds to an S-wave fraction of (9.2 ± 1.5)% in a

±15 MeV/c2 _{window around the φ mass.}

The same procedure is used for the B0

s → χc2K+K− sample. In this case the central

value of fs returned by the fit is zero, that is at the physical boundary. Pseudoexperiments

are used to set a limit fs < 0.30 at 90% confidence level in the 50 MeV/c2 wide K+K−

mass window. This corresponds to an S-wave fraction of less than 21% in a ±15 MeV/c2

window around the φ mass.

6 Measurement of the B_s0 mass

The fit to the χc1K+K− invariant-mass distribution in figure 4 (left) gives

m(B0

s) = 5366.83 ± 0.25 MeV/c2, where the uncertainty is statistical. The dominant source

of systematic uncertainty on the B0

s mass comes from the knowledge of the momentum scale

for charged-particles. This is found to be 0.26 MeV/c2 _{by adjusting the momentum scale by}

the 3×10−4 uncertainty on the calibration procedure and rerunning the mass fit. A further

uncertainty arises from the knowledge of the amount of material in the spectrometer. This

is known to 10% accuracy [8] and results in a 0.02 MeV/c2 _{uncertainty on the B}0

s mass.

The uncertainty from the choice of the fit model is evaluated to be 0.01 MeV/c2 _using

the discrete profiling method described in ref. [34]. Finally, uncertainties of 0.08 MeV/c2_and

0.02 MeV/c2 _{arise from the current knowledge [24] of the χ}

c1 and K+ masses, respectively.

These uncertainties are summarized in table 3. Adding them in quadrature results in

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7 Summary

The B0

s → χc2K+K−decay mode is observed for the first time with a significance of 6.7σ.

The branching fraction of this decay relative to that of the B0

s→ χc1K+K− mode within

a ±15 MeV/c2 _{window around the φ mass is measured to be}

B(B0

s→ χc2K+K−)

B(B0

s→ χc1K+K−)

= (17.1 ± 3.1 (stat) ± 0.4 (syst) ± 0.9 (B))%.

This ratio agrees with the value measured for the corresponding B0 _{decay by LHCb [4]}

B(B0 _{→ χ}

c2K∗(892)0)

B(B0 _{→ χ}

c1K∗(892)0)

= (17.1 ± 5.0 (stat) ± 1.7 (syst) ± 1.1 (B))%.

In the ±15 MeV/c2 _{window around the φ mass, the nonresonant S-wave fraction for the}

B0

s → χc1K+K− mode is measured to be (9.2 ± 1.5)% whilst for the B0s → χc2K+K−

mode it is limited to < 21% at 90% confidence level.

The B0

s → χc1K+K− signal is used to measure the Bs0 mass. The result is

m(B0_s) = 5366.83 ± 0.25 (stat) ± 0.27 (syst) MeV/c2.

This result is in good agreement with and has similar precision to previous LHCb

measure-ments of the B0

s mass made using the Bs0 → J/ψ φ [37] and Bs0→ J/ψ φφ [38] decay modes.

The LHCb results are combined, taking the statistical uncertainties and those related to

the fit procedure to be uncorrelated and those due to the detector material budget and K+

mass to be fully correlated. The uncertainty due to the momentum scale in ref. [38] is also

taken to be fully correlated, whereas in ref. [37] a different procedure was used and so the

corresponding uncertainty is considered to be uncorrelated with the other measurements. The result of this combination is

m(B0_s) = 5366.91 ± 0.18 (stat) ± 0.16 (syst) MeV/c2.

This value is in good agreement with the value published by the CDF collaboration,

m(B0

s) = 5366.01 ± 0.73 (stat) ± 0.33 (syst) MeV/c2 [39], and is the most precise value

to date.

Acknowledgments

We express our gratitude to our colleagues in the CERN accelerator departments for the excellent performance of the LHC. We thank the technical and administrative staff at the LHCb institutes. We acknowledge support from CERN and from the national agencies: CAPES, CNPq, FAPERJ and FINEP (Brazil); MOST and NSFC (China); CNRS/IN2P3 (France); BMBF, DFG and MPG (Germany); INFN (Italy); NWO (Netherlands); MNiSW and NCN (Poland); MEN/IFA (Romania); MinES and FASO (Russia); MinECo (Spain); SNSF and SER (Switzerland); NASU (Ukraine); STFC (United Kingdom); NSF (U.S.A.). We acknowledge the computing resources that are provided by CERN, IN2P3 (France), KIT

JHEP08(2018)191

and DESY (Germany), INFN (Italy), SURF (Netherlands), PIC (Spain), GridPP (United Kingdom), RRCKI and Yandex LLC (Russia), CSCS (Switzerland), IFIN-HH (Romania), CBPF (Brazil), PL-GRID (Poland) and OSC (U.S.A.). We are indebted to the communities behind the multiple open-source software packages on which we depend. Individual groups or members have received support from AvH Foundation (Germany), EPLANET, Marie Sk lodowska-Curie Actions and ERC (European Union), ANR, Labex P2IO and OCEVU, and R´egion Auvergne-Rhˆone-Alpes (France), Key Research Program of Frontier Sciences of CAS, CAS PIFI, and the Thousand Talents Program (China), RFBR, RSF and Yandex LLC (Russia), GVA, XuntaGal and GENCAT (Spain), Herchel Smith Fund, the Royal Society, the English-Speaking Union and the Leverhulme Trust (United Kingdom).

Open Access. This article is distributed under the terms of the Creative Commons

Attribution License (CC-BY 4.0), which permits any use, distribution and reproduction in

any medium, provided the original author(s) and source are credited.

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R.J. Barlow56_{, S. Barsuk}7_{, W. Barter}56_{, F. Baryshnikov}70_{, V. Batozskaya}31_{, B. Batsukh}61_,

V. Battista43_{, A. Bay}43_{, J. Beddow}53_{, F. Bedeschi}24_{, I. Bediaga}1_{, A. Beiter}61_{, L.J. Bel}27_,

N. Beliy63_{, V. Bellee}43_{, N. Belloli}20,i_{, K. Belous}39_{, I. Belyaev}34,42_{, E. Ben-Haim}8_{, G. Bencivenni}18_,

S. Benson27_{, S. Beranek}9_{, A. Berezhnoy}35_{, R. Bernet}44_{, D. Berninghoff}12_{, E. Bertholet}8_,

A. Bertolin23_{, C. Betancourt}44_{, F. Betti}15,42_{, M.O. Bettler}49_{, M. van Beuzekom}27_,

Ia. Bezshyiko44_{, S. Bhasin}48_{, J. Bhom}29_{, S. Bifani}47_{, P. Billoir}8_{, A. Birnkraut}10_{, A. Bizzeti}17,u_,

M. Bjørn57, M.P. Blago42, T. Blake50, F. Blanc43, S. Blusk61, D. Bobulska53, V. Bocci26, O. Boente Garcia41_{, T. Boettcher}58_{, A. Bondar}38,w_{, N. Bondar}33_{, S. Borghi}56,42_{, M. Borisyak}37_,

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M. Brodski42_{, J. Brodzicka}29_{, D. Brundu}22_{, E. Buchanan}48_{, A. Buonaura}44_{, C. Burr}56_,

A. Bursche22_{, J. Buytaert}42_{, W. Byczynski}42_{, S. Cadeddu}22_{, H. Cai}64_{, R. Calabrese}16,g_,

R. Calladine47_{, M. Calvi}20,i_{, M. Calvo Gomez}40,m_{, A. Camboni}40,m_{, P. Campana}18_,

D.H. Campora Perez42_{, L. Capriotti}56_{, A. Carbone}15,e_{, G. Carboni}25_{, R. Cardinale}19,h_,

A. Cardini22_{, P. Carniti}20,i_{, L. Carson}52_{, K. Carvalho Akiba}2_{, G. Casse}54_{, L. Cassina}20_,

M. Cattaneo42_{, G. Cavallero}19,h_{, R. Cenci}24,p_{, D. Chamont}7_{, M.G. Chapman}48_{, M. Charles}8_,

Ph. Charpentier42_{, G. Chatzikonstantinidis}47_{, M. Chefdeville}4_{, V. Chekalina}37_{, C. Chen}3_,

S. Chen22_{, S.-G. Chitic}42_{, V. Chobanova}41_{, M. Chrzaszcz}42_{, A. Chubykin}33_{, P. Ciambrone}18_,

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JHEP08(2018)191

O.G. Girard43, L. Giubega32, K. Gizdov52, V.V. Gligorov8, D. Golubkov34, A. Golutvin55,70,

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L.A. Granado Cardoso42_{, E. Graug´es}40_{, E. Graverini}44_{, G. Graziani}17_{, A. Grecu}32_{, R. Greim}27_,

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M. Heß67_{, A. Hicheur}2_{, R. Hidalgo Charman}56_{, D. Hill}57_{, M. Hilton}56_{, P.H. Hopchev}43_{, W. Hu}65_,

W. Huang63_{, Z.C. Huard}59_{, W. Hulsbergen}27_{, T. Humair}55_{, M. Hushchyn}37_{, D. Hutchcroft}54_,

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X. Liang61_{, T. Likhomanenko}69_{, R. Lindner}42_{, F. Lionetto}44_{, V. Lisovskyi}7_{, X. Liu}3_{, D. Loh}50_,

A. Loi22_{, I. Longstaff}53_{, J.H. Lopes}2_{, G.H. Lovell}49_{, D. Lucchesi}23,o_{, M. Lucio Martinez}41_,

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V. Macko43_{, P. Mackowiak}10_{, S. Maddrell-Mander}48_{, O. Maev}33,42_{, K. Maguire}56_,

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C. Marin Benito7_{, M. Marinangeli}43_{, P. Marino}43_{, J. Marks}12_{, P.J. Marshall}54_{, G. Martellotti}26_,

M. Martin6_{, M. Martinelli}42_{, D. Martinez Santos}41_{, F. Martinez Vidal}72_{, A. Massafferri}1_,

M. Materok9_{, R. Matev}42_{, A. Mathad}50_{, Z. Mathe}42_{, C. Matteuzzi}20_{, A. Mauri}44_{, E. Maurice}7,b_,

B. Maurin43_{, A. Mazurov}47_{, M. McCann}55,42_{, A. McNab}56_{, R. McNulty}13_{, J.V. Mead}54_,

B. Meadows59_{, C. Meaux}6_{, F. Meier}10_{, N. Meinert}67_{, D. Melnychuk}31_{, M. Merk}27_{, A. Merli}21,q_,

E. Michielin23_{, D.A. Milanes}66_{, E. Millard}50_{, M.-N. Minard}4_{, L. Minzoni}16,g_{, D.S. Mitzel}12_,

A. Mogini8_{, J. Molina Rodriguez}1,z_{, T. Momb¨acher}10_{, I.A. Monroy}66_{, S. Monteil}5_,

M. Morandin23_{, G. Morello}18_{, M.J. Morello}24,t_{, O. Morgunova}69_{, J. Moron}30_{, A.B. Morris}6_,

R. Mountain61_{, F. Muheim}52_{, M. Mulder}27_{, C.H. Murphy}57_{, D. Murray}56_{, A. M¨odden}10_,

D. M¨uller42_{, J. M¨uller}10_{, K. M¨uller}44_{, V. M¨uller}10_{, P. Naik}48_{, T. Nakada}43_{, R. Nandakumar}51_,

A. Nandi57_{, T. Nanut}43_{, I. Nasteva}2_{, M. Needham}52_{, N. Neri}21_{, S. Neubert}12_{, N. Neufeld}42_,

M. Neuner12_{, T.D. Nguyen}43_{, C. Nguyen-Mau}43,n_{, S. Nieswand}9_{, R. Niet}10_{, N. Nikitin}35_,

A. Nogay69_{, D.P. O’Hanlon}15_{, A. Oblakowska-Mucha}30_{, V. Obraztsov}39_{, S. Ogilvy}18_,

R. Oldeman22,f_{, C.J.G. Onderwater}68_{, A. Ossowska}29_{, J.M. Otalora Goicochea}2_{, P. Owen}44_,

A. Oyanguren72_{, P.R. Pais}43_{, A. Palano}14_{, M. Palutan}18,42_{, G. Panshin}71_{, A. Papanestis}51_,

M. Pappagallo52_{, L.L. Pappalardo}16,g_{, W. Parker}60_{, C. Parkes}56_{, G. Passaleva}17,42_{, A. Pastore}14_,

M. Patel55_{, C. Patrignani}15,e_{, A. Pearce}42_{, A. Pellegrino}27_{, G. Penso}26_{, M. Pepe Altarelli}42_,

S. Perazzini42_{, D. Pereima}34_{, P. Perret}5_{, L. Pescatore}43_{, K. Petridis}48_{, A. Petrolini}19,h_,

A. Petrov69_{, S. Petrucci}52_{, M. Petruzzo}21,q_{, B. Pietrzyk}4_{, G. Pietrzyk}43_{, M. Pikies}29_{, M. Pili}57_,

D. Pinci26_{, J. Pinzino}42_{, F. Pisani}42_{, A. Piucci}12_{, V. Placinta}32_{, S. Playfer}52_{, J. Plews}47_,

JHEP08(2018)191

E. Polycarpo2, G.J. Pomery48, S. Ponce42, A. Popov39, D. Popov47,11, S. Poslavskii39,

C. Potterat2_{, E. Price}48_{, J. Prisciandaro}41_{, C. Prouve}48_{, V. Pugatch}46_{, A. Puig Navarro}44_,

H. Pullen57_{, G. Punzi}24,p_{, W. Qian}63_{, J. Qin}63_{, R. Quagliani}8_{, B. Quintana}5_{, B. Rachwal}30_,

J.H. Rademacker48_{, M. Rama}24_{, M. Ramos Pernas}41_{, M.S. Rangel}2_{, F. Ratnikov}37,x_{, G. Raven}28_,

M. Ravonel Salzgeber42_{, M. Reboud}4_{, F. Redi}43_{, S. Reichert}10_{, A.C. dos Reis}1_{, F. Reiss}8_,

C. Remon Alepuz72_{, Z. Ren}3_{, V. Renaudin}7_{, S. Ricciardi}51_{, S. Richards}48_{, K. Rinnert}54_,

P. Robbe7_{, A. Robert}8_{, A.B. Rodrigues}43_{, E. Rodrigues}59_{, J.A. Rodriguez Lopez}66_,

M. Roehrken42_{, A. Rogozhnikov}37_{, S. Roiser}42_{, A. Rollings}57_{, V. Romanovskiy}39_,

A. Romero Vidal41_{, M. Rotondo}18_{, M.S. Rudolph}61_{, T. Ruf}42_{, J. Ruiz Vidal}72_,

J.J. Saborido Silva41_{, N. Sagidova}33_{, B. Saitta}22,f_{, V. Salustino Guimaraes}62_{, C. Sanchez Gras}27_,

C. Sanchez Mayordomo72_{, B. Sanmartin Sedes}41_{, R. Santacesaria}26_{, C. Santamarina Rios}41_,

M. Santimaria18, E. Santovetti25,j, G. Sarpis56, A. Sarti18,k, C. Satriano26,s, A. Satta25, M. Saur63_{, D. Savrina}34,35_{, S. Schael}9_{, M. Schellenberg}10_{, M. Schiller}53_{, H. Schindler}42_,

M. Schmelling11_{, T. Schmelzer}10_{, B. Schmidt}42_{, O. Schneider}43_{, A. Schopper}42_{, H.F. Schreiner}59_,

M. Schubiger43_{, M.H. Schune}7_{, R. Schwemmer}42_{, B. Sciascia}18_{, A. Sciubba}26,k_{, A. Semennikov}34_,

E.S. Sepulveda8_{, A. Sergi}47,42_{, N. Serra}44_{, J. Serrano}6_{, L. Sestini}23_{, A. Seuthe}10_{, P. Seyfert}42_,

M. Shapkin39_{, Y. Shcheglov}33,†_{, T. Shears}54_{, L. Shekhtman}38,w_{, V. Shevchenko}69_{, E. Shmanin}70_,

B.G. Siddi16_{, R. Silva Coutinho}44_{, L. Silva de Oliveira}2_{, G. Simi}23,o_{, S. Simone}14,d_,

N. Skidmore12_{, T. Skwarnicki}61_{, J.G. Smeaton}49_{, E. Smith}9_{, I.T. Smith}52_{, M. Smith}55_,

M. Soares15_{, l. Soares Lavra}1_{, M.D. Sokoloff}59_{, F.J.P. Soler}53_{, B. Souza De Paula}2_{, B. Spaan}10_,

P. Spradlin53_{, F. Stagni}42_{, M. Stahl}12_{, S. Stahl}42_{, P. Stefko}43_{, S. Stefkova}55_{, O. Steinkamp}44_,

S. Stemmle12_{, O. Stenyakin}39_{, M. Stepanova}33_{, H. Stevens}10_{, S. Stone}61_{, B. Storaci}44_,

S. Stracka24,p_{, M.E. Stramaglia}43_{, M. Straticiuc}32_{, U. Straumann}44_{, S. Strokov}71_{, J. Sun}3_,

L. Sun64_{, K. Swientek}30_{, V. Syropoulos}28_{, T. Szumlak}30_{, M. Szymanski}63_{, S. T’Jampens}4_,

Z. Tang3_{, A. Tayduganov}6_{, T. Tekampe}10_{, G. Tellarini}16_{, F. Teubert}42_{, E. Thomas}42_,

J. van Tilburg27_{, M.J. Tilley}55_{, V. Tisserand}5_{, S. Tolk}42_{, L. Tomassetti}16,g_{, D. Tonelli}24_,

D.Y. Tou8_{, R. Tourinho Jadallah Aoude}1_{, E. Tournefier}4_{, M. Traill}53_{, M.T. Tran}43_{, A. Trisovic}49_,

A. Tsaregorodtsev6_{, A. Tully}49_{, N. Tuning}27,42_{, A. Ukleja}31_{, A. Usachov}7_{, A. Ustyuzhanin}37_,

U. Uwer12_{, C. Vacca}22,f_{, A. Vagner}71_{, V. Vagnoni}15_{, A. Valassi}42_{, S. Valat}42_{, G. Valenti}15_,

R. Vazquez Gomez42_{, P. Vazquez Regueiro}41_{, S. Vecchi}16_{, M. van Veghel}27_{, J.J. Velthuis}48_,

M. Veltri17,r_{, G. Veneziano}57_{, A. Venkateswaran}61_{, T.A. Verlage}9_{, M. Vernet}5_{, N.V. Veronika}13_,

M. Vesterinen57_{, J.V. Viana Barbosa}42_{, D. Vieira}63_{, M. Vieites Diaz}41_{, H. Viemann}67_,

X. Vilasis-Cardona40,m_{, A. Vitkovskiy}27_{, M. Vitti}49_{, V. Volkov}35_{, A. Vollhardt}44_{, B. Voneki}42_,

A. Vorobyev33_{, V. Vorobyev}38,w_{, J.A. de Vries}27_{, C. V´azquez Sierra}27_{, R. Waldi}67_{, J. Walsh}24_,

J. Wang61_{, M. Wang}3_{, Y. Wang}65_{, Z. Wang}44_{, D.R. Ward}49_{, H.M. Wark}54_{, N.K. Watson}47_,

D. Websdale55_{, A. Weiden}44_{, C. Weisser}58_{, M. Whitehead}9_{, J. Wicht}50_{, G. Wilkinson}57_,

M. Wilkinson61_{, I. Williams}49_{, M.R.J. Williams}56_{, M. Williams}58_{, T. Williams}47_,

F.F. Wilson51,42_{, J. Wimberley}60_{, M. Winn}7_{, J. Wishahi}10_{, W. Wislicki}31_{, M. Witek}29_,

G. Wormser7_{, S.A. Wotton}49_{, K. Wyllie}42_{, D. Xiao}65_{, Y. Xie}65_{, A. Xu}3_{, M. Xu}65_{, Q. Xu}63_,

Z. Xu3_{, Z. Xu}4_{, Z. Yang}3_{, Z. Yang}60_{, Y. Yao}61_{, L.E. Yeomans}54_{, H. Yin}65_{, J. Yu}65,ab_{, X. Yuan}61_,

O. Yushchenko39_{, K.A. Zarebski}47_{, M. Zavertyaev}11,c_{, D. Zhang}65_{, L. Zhang}3_{, W.C. Zhang}3,aa_,

Y. Zhang7_{, A. Zhelezov}12_{, Y. Zheng}63_{, X. Zhu}3_{, V. Zhukov}9,35_{, J.B. Zonneveld}52_{, S. Zucchelli}15

1

Centro Brasileiro de Pesquisas F´ısicas (CBPF), Rio de Janeiro, Brazil

2

Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil

3 _{Center for High Energy Physics, Tsinghua University, Beijing, China} 4

Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IN2P3-LAPP, Annecy, France

5

Clermont Universit´e, Universit´e Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France

JHEP08(2018)191

7 _{LAL, Univ. Paris-Sud, CNRS/IN2P3, Universit´e Paris-Saclay, Orsay, France}

8

LPNHE, Sorbonne Universit´e, Paris Diderot Sorbonne Paris Cit´e, CNRS/IN2P3, Paris, France

9 _{I. Physikalisches Institut, RWTH Aachen University, Aachen, Germany} 10

Fakult¨at Physik, Technische Universit¨at Dortmund, Dortmund, Germany

11 _{Max-Planck-Institut f¨ur Kernphysik (MPIK), Heidelberg, Germany} 12

Physikalisches Institut, Ruprecht-Karls-Universit¨at Heidelberg, Heidelberg, Germany

13

School of Physics, University College Dublin, Dublin, Ireland

14 _{INFN Sezione di Bari, Bari, Italy} 15

INFN Sezione di Bologna, Bologna, Italy

16 _{INFN Sezione di Ferrara, Ferrara, Italy} 17

INFN Sezione di Firenze, Firenze, Italy

18 _{INFN Laboratori Nazionali di Frascati, Frascati, Italy} 19

INFN Sezione di Genova, Genova, Italy

20 _{INFN Sezione di Milano-Bicocca, Milano, Italy} 21

INFN Sezione di Milano, Milano, Italy

22

INFN Sezione di Cagliari, Monserrato, Italy

23 _{INFN Sezione di Padova, Padova, Italy} 24

INFN Sezione di Pisa, Pisa, Italy

25 _{INFN Sezione di Roma Tor Vergata, Roma, Italy} 26

INFN Sezione di Roma La Sapienza, Roma, Italy

27 _{Nikhef National Institute for Subatomic Physics, Amsterdam, Netherlands} 28

Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, Netherlands

29 _{Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Krak´ow, Poland} 30

AGH - University of Science and Technology, Faculty of Physics and Applied Computer Science, Krak´ow, Poland

31

National Center for Nuclear Research (NCBJ), Warsaw, Poland

32

Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania

33

Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia

34 _{Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia} 35

Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow, Russia

36 _{Institute for Nuclear Research of the Russian Academy of Sciences (INR RAS), Moscow, Russia} 37

Yandex School of Data Analysis, Moscow, Russia

38 _{Budker Institute of Nuclear Physics (SB RAS), Novosibirsk, Russia} 39

Institute for High Energy Physics (IHEP), Protvino, Russia

40 _{ICCUB, Universitat de Barcelona, Barcelona, Spain} 41

Instituto Galego de F´ısica de Altas Enerx´ıas (IGFAE), Universidade de Santiago de Compostela, Santiago de Compostela, Spain

42

European Organization for Nuclear Research (CERN), Geneva, Switzerland

43

Institute of Physics, Ecole Polytechnique F´ed´erale de Lausanne (EPFL), Lausanne, Switzerland

44

Physik-Institut, Universit¨at Z¨urich, Z¨urich, Switzerland

45 _{NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine} 46

Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine

47 _{University of Birmingham, Birmingham, United Kingdom} 48

H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom

49 _{Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom} 50

Department of Physics, University of Warwick, Coventry, United Kingdom

51

STFC Rutherford Appleton Laboratory, Didcot, United Kingdom

52 _{School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom} 53

School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom

JHEP08(2018)191

55 _{Imperial College London, London, United Kingdom}

56

School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom

57 _{Department of Physics, University of Oxford, Oxford, United Kingdom} 58

Massachusetts Institute of Technology, Cambridge, MA, United States

59 _{University of Cincinnati, Cincinnati, OH, United States} 60

University of Maryland, College Park, MD, United States

61

Syracuse University, Syracuse, NY, United States

62 _{Pontif´ıcia Universidade Cat´olica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil,}

associated to2

63

University of Chinese Academy of Sciences, Beijing, China, associated to 3

64 _{School of Physics and Technology, Wuhan University, Wuhan, China, associated to}3 65

Institute of Particle Physics, Central China Normal University, Wuhan, Hubei, China, associated to3

66 _{Departamento de Fisica, Universidad Nacional de Colombia, Bogota, Colombia, associated to}8 67

Institut f¨ur Physik, Universit¨at Rostock, Rostock, Germany, associated to 12

68 _{Van Swinderen Institute, University of Groningen, Groningen, Netherlands, associated to}27 69

National Research Centre Kurchatov Institute, Moscow, Russia, associated to34

70

National University of Science and Technology “MISIS”, Moscow, Russia, associated to34

71

National Research Tomsk Polytechnic University, Tomsk, Russia, associated to34

72

Instituto de Fisica Corpuscular, Centro Mixto Universidad de Valencia - CSIC, Valencia, Spain, associated to40

73 _{University of Michigan, Ann Arbor, United States, associated to}61 74

Los Alamos National Laboratory (LANL), Los Alamos, United States, associated to 61

a

Universidade Federal do Triˆangulo Mineiro (UFTM), Uberaba-MG, Brazil

b _{Laboratoire Leprince-Ringuet, Palaiseau, France} c

P.N. Lebedev Physical Institute, Russian Academy of Science (LPI RAS), Moscow, Russia

d _{Universit`a di Bari, Bari, Italy} e

Universit`a di Bologna, Bologna, Italy

f

Universit`a di Cagliari, Cagliari, Italy

g _{Universit`a di Ferrara, Ferrara, Italy} h

Universit`a di Genova, Genova, Italy

i _{Universit`a di Milano Bicocca, Milano, Italy} j

Universit`a di Roma Tor Vergata, Roma, Italy

k _{Universit`a di Roma La Sapienza, Roma, Italy} l

AGH - University of Science and Technology, Faculty of Computer Science, Electronics and Telecommunications, Krak´ow, Poland

m _{LIFAELS, La Salle, Universitat Ramon Llull, Barcelona, Spain} n

Hanoi University of Science, Hanoi, Vietnam

o

Universit`a di Padova, Padova, Italy

p _{Universit`a di Pisa, Pisa, Italy} q

Universit`a degli Studi di Milano, Milano, Italy

r _{Universit`a di Urbino, Urbino, Italy} s

Universit`a della Basilicata, Potenza, Italy

t _{Scuola Normale Superiore, Pisa, Italy} u

Universit`a di Modena e Reggio Emilia, Modena, Italy

v

MSU - Iligan Institute of Technology (MSU-IIT), Iligan, Philippines

w _{Novosibirsk State University, Novosibirsk, Russia} x

National Research University Higher School of Economics, Moscow, Russia

y _{Sezione INFN di Trieste, Trieste, Italy} z

Escuela Agr´ıcola Panamericana, San Antonio de Oriente, Honduras

aa _{School of Physics and Information Technology, Shaanxi Normal University (SNNU), Xi’an, China} ab

Physics and Micro Electronic College, Hunan University, Changsha City, China

†