Observation of Λb0→ ψ(2S)pK−and Λb0→ J/ψπ+π−pK−decays and a measurement of the Λb0baryon mass

JHEP05(2016)132

Published for SISSA by Springer

Received: March 24, 2016 Accepted: May 9, 2016 Published: May 23, 2016

Observation of Λ

0_b

→ ψ(2S)pK

−

and

Λ

0_b

→ J/ψπ

+

_π

−

_pK

−

_{decays and a measurement of}

the Λ

0_b

baryon mass

The LHCb collaboration

E-mail: Ivan.Belyaev@cern.ch

Abstract: The decays Λ0_b → ψ(2S)pK− and Λ0_b→ J/ψπ+_π−_pK−

are observed in a data

sample corresponding to an integrated luminosity of 3 fb−1, collected in proton-proton

collisions at 7 and 8 TeV centre-of-mass energies by the LHCb detector. The ψ(2S) mesons

are reconstructed through the decay modes ψ(2S) → µ+µ− and ψ(2S) → J/ψπ+π−.

The branching fractions relative to that of Λ0_b → J/ψ pK− are measured to be

B(Λ0 b→ ψ(2S)pK −₎ B(Λ0 b→ J/ψ pK −₎ = (20.70 ± 0.76 ± 0.46 ± 0.37) × 10 −2_, B(Λ0 b→ J/ψπ+π −_pK−₎ B(Λ0 b → J/ψ pK −₎ = (20.86 ± 0.96 ± 1.34) × 10 −2_,

where the first uncertainties are statistical, the second are systematic and the third is

related to the knowledge of J/ψ and ψ(2S) branching fractions. The mass of the Λ0_bbaryon

is measured to be

M (Λ0_b) = 5619.65 ± 0.17 ± 0.17 MeV/c2,

where the uncertainties are statistical and systematic.

Keywords: B physics, Flavor physics, Hadron-Hadron scattering (experiments), Particle and resonance production, Spectroscopy

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Contents

1 Introduction 1

2 Detector and simulation 2

3 Event selection 3

4 Measurement of branching fractions 4

4.1 Signal yields and efficiencies 4

4.2 Systematic uncertainties 6

4.3 Results 8

5 Measurement of Λ0_b baryon mass 9

6 Results and summary 12

The LHCb collaboration 17

1 Introduction

The Λ0_b baryon is the isospin singlet ground state of a bottom quark and two light quarks.

The rich phenomenology associated with decays of bottom baryons allows many mea-surements of masses, lifetimes and branching fractions, which test the theoretical un-derstanding of weak decays of heavy hadrons in the framework of heavy quark effective

theory (HQET) and the underlying QCD physics [1, 2]. At the Tevatron, properties of

the Λ0_b baryon, such as mass and lifetime, have been measured using two-body modes,

specifically Λ0_b → J/ψ Λ0 _{and Λ}0

b → Λ+cπ− decays [3–5].1 The high production rate of

b quarks at the Large Hadron Collider (LHC), along with the excellent momentum and mass resolution and the hadron identification capabilities of the LHCb detector, open up

a host of multibody and Cabibbo-suppressed decay channels of Λ0_b baryons, e.g. the

de-cays Λ0

b→ D0pK−, Λ0b→ Λc+K− [6], Λ0b→ Λ+cD−, Λ0b → Λ+cD−s [7] and Λ0b → J/ψ pπ− [8].

The high signal yield of the Λ0_b→ J/ψ pK− decay [9] allowed the precise measurement of

the Λ0_b lifetime [10,11]. The recent analysis of this decay mode uncovered a double

reso-nant structure in the J/ψ p system consistent with two pentaquark states [12]. LHCb has

also measured several B meson decays into final states with charmonia [13–18]. The first

observation of Λ0_b decays to excited charmonium, the Λ0_b→ ψ(2S)Λ0 _{decay, has been}

pre-sented by the ATLAS collaboration [19]. An experimental investigation of other similar

multibody decays of the Λ0_b baryon should lead to deeper insights into QCD.

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In this paper, the first observations of the decays Λ0_b→ ψ(2S)pK− _and

Λ0_b→ J/ψ π+_π−_pK−_{are reported, where ψ(2S) mesons are reconstructed in the final states}

µ+µ− and J/ψ π+π−. The ratios of the branching fractions of these decays to that of the

normalization decay Λ0 b→ J/ψ pK−, Rψ(2S) ≡ B(Λ 0 b→ ψ(2S)pK −₎ B(Λ0 b→ J/ψ pK−) , (1.1) RJ/ψ π+π− ≡ B(Λ 0 b→ J/ψ π+π−pK−) B(Λ0 b→ J/ψ pK−) , (1.2)

are measured. In measuring the branching fraction of Λ0_b→ J/ψ π+_π−_pK− _decays,

con-tributions via intermediate resonances, such as Λ0_b→ ψ(2S)pK−_{, are implicitly included.}

The low energy release in these decays allows a precise determination of the Λ0_b mass with

a small systematic uncertainty.

This study is based on a data sample corresponding to an integrated luminosity

of 3 fb−1, collected with the LHCb detector in pp collisions at centre-of-mass energies

√

s = 7 and 8TeV.

2 Detector and simulation

The LHCb detector [20, 21] 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 surrounding the pp interaction region, a large-area silicon-strip detector lo-cated upstream of a dipole magnet with a bending power of about 4 Tm, and three stations of silicon-strip detectors and straw drift tubes placed downstream of the magnet. The po-larity of the dipole magnet is reversed periodically throughout data-taking. 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 minimum distance of a track to a primary vertex (PV), the impact parameter, 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 [22]. Large samples of B+ → J/ψ K+ _{and J/ψ → µ}+_µ− _decays,

collected concurrently with the data set, were used to calibrate the momentum scale of the

spectrometer to a precision of 0.03 % [23].

Different types of charged hadrons are distinguished using information from two ring-imaging Cherenkov detectors (RICH). 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.

The trigger [24] comprises two stages. Events are first required to pass the hardware

trigger, which selects muon candidates with pT> 1.48 (1.76) GeV/c or pairs of opposite-sign

muon candidates with a requirement that the product of the muon transverse momenta is

larger than 1.7 (2.6) GeV2/c2for data collected at√s = 7 (8)TeV. The subsequent software

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while full event reconstruction is done at the second stage. At the first stage of the software trigger the invariant mass of well-reconstructed pairs of oppositely charged muons forming

a good-quality two-track vertex is required to exceed 2.7 GeV/c2, and the two-track vertex

is required to be significantly displaced from all PVs.

The analysis technique reported below has been validated using simulated events.

The pp collisions are generated using Pythia [25, 26] with a specific LHCb

configura-tion [27]. Decays of hadronic particles are described by EvtGen [28], in which final-state

radiation is generated using Photos [29]. The interaction of the generated particles with

the detector, and its response, are implemented using the Geant4 toolkit [30, 31] as

de-scribed in ref. [32].

3 Event selection

The decays Λ0_b→ ψ(2S)pK−_{, Λ}0

b→ J/ψ π+π−pK− and Λ0b→ J/ψ pK− are reconstructed

using decay modes ψ(2S) → µ+µ−, ψ(2S) → J/ψ π+π− and J/ψ → µ+µ−. Common

se-lection criteria, based on those used in refs. [17, 33], are used for all channels, except

for those related to the selection of two additional pions in the Λ0

b→ J/ψ π+π−pK− and

Λ0_b → ψ(2S)[→ J/ψ π+_π−_]pK− _channels.

Muon, proton, kaon and pion candidates are selected from well-reconstructed tracks within the acceptance of the spectrometer that are identified using information from

the RICH, calorimeter and muon detectors [34, 35]. Muons, protons, kaons and pions

are required to have a transverse momentum larger than 550, 800, 500 and 200 MeV/c, respectively. To allow good particle identification, kaons and pions are required to have a momentum between 3.2 GeV/c and 150 GeV/c whilst protons must have a momentum be-tween 10 GeV/c and 150 GeV/c. To reduce combinatorial background involving tracks from the primary pp interaction vertices, only tracks that exceed a minimum impact parameter

χ2 with respect to every PV are used. The impact parameter χ2 is defined as the difference

between the χ2 of the PV reconstructed with and without the considered particle.

Pairs of oppositely-charged muons originating from a common vertex are combined to

form J/ψ → µ+_µ− _{or ψ(2S) → µ}+_µ− _{candidates. The resulting dimuon candidates are}

required to have an invariant mass between −5σ and +3σ around the known J/ψ or

ψ(2S) masses [36], where σ is the mass resolution. An asymmetric mass interval is chosen

to include part of the low-mass tail due to final-state radiation.

Candidate Λ0_b baryons are formed from J/ψ pK−, ψ(2S)pK− and J/ψ π+π−pK−

combi-nations. Each candidate is associated with the PV with respect to which it has the smallest

impact parameter significance. The Λ0_b mass resolution is improved by employing a

kine-matic fit [37] that constrains the mass of the J/ψ candidate to its known value and

re-quires the momentum of the Λ0_b candidate to point back to the PV. A requirement on

the quality of this fit is applied to further suppress combinatorial background. Finally,

the measured decay time of the Λ0_b candidate, calculated with respect to the associated

primary vertex, is required to be between 0.5 and 6.7 ps. The lower limit is used to sup-press background from particles coming from the PV while the upper limit removes poorly reconstructed candidates.

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5.6 5.65 0 50 100 150 5.6 5.65 0 1000 2000 3000 Candidates/(2 Me V /c 2 ) Candidates/(1 Me V /c 2 ) m(J/ψ pK−) m(ψ(2S)[→ µ+_µ−_]pK−_{)  GeV/c}2  GeV/c2 LHCb LHCb

Figure 1. Mass distributions of selected (left ) Λ0

b→ ψ(2S)[→ µ

+_µ−_]pK−_{and (right ) Λ}0

b→ J/ψ pK−

candidates. The total fit function (solid red), the Λ0

b signal contribution (dotted magenta) and

the combinatorial background (dashed blue) are shown. The error bars show 68% Poissonian con-fidence intervals.

To suppress cross-feed from decays of the B0_s meson into J/ψ K−K+, ψ(2S)K−K+ and

J/ψ π+π−K−K+ final states, with the positively-charged kaon misidentified as a proton,

a veto on the Λ0_b candidate mass, recalculated with a kaon mass hypothesis for the proton,

is applied. Any candidate with a recalculated mass consistent with the nominal B0_s mass

is rejected. A similar veto is applied to suppress cross-feed from decays of B0 _mesons

into J/ψ K−π+, ψ(2S)K−π+ and J/ψ π−π+π+K− decays with the positively-charged pion

misidentified as a proton.

4 Measurement of branching fractions

4.1 Signal yields and efficiencies

The mass distributions for selected Λ0_b→ ψ(2S)[→ µ+_µ−_]pK− _{candidates and candidates}

for the normalization channel Λ0_b→ J/ψ pK−_{are shown in figure1. Signal yields are}

deter-mined using unbinned extended maximum likelihood fits to these distributions. The signal

is modelled with a modified Gaussian function with power-law tails on both sides [38,39],

where the tail parameters are fixed from simulation and the mass resolution parameter is allowed to vary. The background is modelled with an exponential function multiplied by a first-order polynomial. The resolution parameters obtained from the fits are found to be

3.82 ± 0.17 MeV/c2 for the channel Λ0_b→ ψ(2S)[→ µ+_µ−_]pK− _{and 6.12 ± 0.05 MeV/c}2 _for

Λ0_b→ J/ψ pK−, in good agreement with expectations from simulation.

The mass distribution for selected Λ0_b→ J/ψ π+_π−_pK− _{candidates is shown in}

figure 2(left), along with the result of an unbinned extended maximum likelihood

fit using the model described above. The mass resolution parameter obtained

from the fit is 4.72 ± 0.23 MeV/c2. The mass distribution of the J/ψ π+π− system

from signal Λ0_b→ J/ψ π+_π−_pK− _{decays is presented in figure 2(right) in the region}

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5.6 5.65 0 50 100 150 Candidates/(2 Me V /c 2 ) m(J/ψ π+_π−_pK−₎ 3.67 3.68 3.69 3.7 0 20 40 Candidates/(1 Me V /c 2 ) m(J/ψ π+_π−₎  GeV/c2  GeV/c2 LHCb LHCb

Figure 2. Left: mass distribution of selected Λ0

b→ J/ψ π

+_π−_pK− _{candidates. Right:}

background-subtracted J/ψ π+_π−_{mass distribution for that mode. The total fit function and the signal}

contribu-tions are shown by solid red and dotted magenta lines, respectively. The combinatorial background in the left plot and nonresonant contribution in the right plot are shown by dashed blue lines.

Channel N (Λ0_b) Λ0_b→ J/ψ pK− 28 834 ± 204 Λ0 b→ ψ(2S)[→ µ+µ−]pK− 665 ± 28 Λ0_b→ ψ(2S)[→ J/ψ π+_π−_]pK− _{231 ± 17} Λ0_b→ J/ψ π+_π−_pK− _{793 ± 36}

Table 1. Signal yields of Λ0

bdecay channels. Uncertainties are statistical only.

The background subtraction is performed with the sPlot technique [40]

us-ing the J/ψ π+π−pK− mass as the discriminating variable. The signal yield of

Λ0_b→ ψ(2S)[→ J/ψ π+_π−_]pK− _{decays is determined using an unbinned extended maximum}

likelihood fit to the J/ψ π+π− invariant mass distribution. The ψ(2S) component is

mod-elled with a modified Gaussian function with power-law tails on both sides, where the tail parameters are fixed from simulation. The nonresonant component is taken to be constant.

The mass resolution parameter obtained from the fit is 2.29±0.17 MeV/c2. The signal yields

are summarized in table1.

The ratio of branching fractions Rψ(2S), defined in eq. (1.1), is measured in two different

decay modes, Rψ(2S)    ψ(2S)→µ+_µ− = N_ψ(2S)→µ+_µ− NJ/ψ × ε Λ0 b J/ψ εΛ 0 b ψ(2S)→µ+_µ− × B(J/ψ → µ +_µ−₎ B(ψ(2S) → µ+_µ−₎, Rψ(2S)    ψ(2S)→J/ψ π+_π− = Nψ(2S)→J/ψ π+_π− NJ/ψ × ε Λ0 b J/ψ εΛ0b ψ(2S)→J/ψ π+_π− × 1 B(ψ(2S) → J/ψ π+_π−₎, (4.1)

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Value εΛ 0 b J/ψ/ε Λ0 b ψ(2S)→µ+_µ− 1.188 ± 0.006 εΛ 0 b J/ψ/ε Λ0 b ψ(2S)→J/ψ π+_π− 8.84 ± 0.05 εΛ 0 b J/ψ/ε Λ0 b J/ψ π+_π− 7.59 ± 0.04

Table 2. Ratios of efficiencies. The uncertainties reflect the limited size of the simulation sample.

and the ratio RJ/ψ π+π−, defined in eq. (1.2), is measured as

RJ/ψ π+π− = NJ/ψ π+π− NJ/ψ × ε Λ0 b J/ψ εΛ 0 b J/ψ π+_π− , (4.2)

where NX represents the observed signal yield and ε

Λ0 b

X denotes the efficiency for the

de-cay Λ0

b→ XpK−. The ratio

B(J/ψ→µ+_µ−₎

B(ψ(2S)→µ+_µ−₎ is taken to be equal to the more precisely

measured ratio of dielectron branching fractions, _B(ψ(2S)→eB(J/ψ →e++e−_e−)₎ = 7.57 ± 0.17 [36].

For the ψ(2S) → J/ψ π+π− branching fraction the world average (34.46 ± 0.30)% [36]

is taken.

The efficiency is defined as the product of the geometric acceptance and the detection, reconstruction, selection and trigger efficiencies. The efficiencies for hadron identification as functions of kinematic parameters and event multiplicity are determined from data

using calibration samples of low-background decays: D∗+→ D0_π+_{followed by D}0 _{→ K}−_π−

for kaons and pions, and Λ0 _{→ pπ}− _{and Λ}+

c → pK−π+ for protons [34]. The remaining

efficiencies are determined using simulation.

In the simulation of Λ0_b→ J/ψ pK− _{decays, the model established in ref. [12] that}

in-cludes pentaquark contributions is used, while in the simulation of the other decay modes the events are generated uniformly in phase space. The simulation is corrected to reproduce

the transverse momentum and rapidity distributions of the Λ0_bbaryons observed in data [9]

and to account for small discrepancies between data and simulation in the reconstruction

of charged tracks [41]. The ratios of efficiencies to those in the Λ0_b→ J/ψ pK− channel are

presented in table2.

4.2 Systematic uncertainties

Most systematic uncertainties cancel in the measurements of the ratios of branching frac-tions, notably those related to the reconstruction, identification and trigger efficiencies of

the J/ψ → µ+µ−and ψ(2S) → µ+µ−candidates [13], due to the similarity of the muon and

dimuon spectra for these modes. The remaining systematic uncertainties are summarized

in table 3 and discussed below.

Alternative parametrizations for the signal and background are used to estimate

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Source Rψ(2S) ψ(2S)→µ+_µ− Rψ(2S)   ψ(2S)→J/ψ π+_π− RJ/ψ π +_π− Fit model 0.8 3.0 3.5 Cross-feed 0.8 0.9 0.9 Efficiency calculation: Λ0_b decay model 0.3 0.8 0.8

Reconstruction of additional pions:

Hadron interaction — 2 × 2.0 2 × 2.0

Track efficiency correction — 3.2 2.7

Hadron identification 0.1 0.1 0.2

Trigger 1.1 1.1 1.1

Selection criteria 0.6 0.9 0.2

Simulation sample size 1.0 1.6 1.7

Sum in quadrature 2.0 6.4 6.4

Table 3. Systematic uncertainties (in %) on the ratios of branching fractions Rψ(2S)and RJ/ψ π+π−.

an Apolonios function [43], an asymmetric variant of the Apolonios function and the

Stu-dent’s t-distribution are used for the Λ0_b signal shape, and an exponential function

multi-plied by a second-order polynomial is used for the background. The ratio of event yields is remeasured with the cross-check models, and the maximum deviation with respect to the nominal value is assigned as a systematic uncertainty.

The uncertainty associated with the B0_s and B0 cross-feed is estimated by varying

the widths of the rejected regions and recomputing the signal yields, taking into account the changes in efficiencies. As an additional cross-check, a veto is applied also on

possi-ble contributions from Λ0_b→ J/ψ pK+_{, Λ}0

b → ψ(2S)pK+ and Λ0b → J/ψ π+π−pK+ decays

where the positive kaon is misidentified as a proton and the antiproton is misidentified as a negative kaon. The maximum of the observed differences from the nominal values is assigned as the corresponding systematic uncertainty.

The remaining systematic uncertainties are associated with the efficiency

determina-tion. The systematic uncertainty related to the decay model for Λ0_b→ ψ(2S)pK− and

Λ0_b → J/ψ π+_π−_pK− _{decays is estimated using the simulated samples, corrected to}

repro-duce the invariant mass of the pK− and ψ(2S)p or J/ψ π+_π−_{p systems observed in data.}

The largest change in efficiency is taken as the corresponding systematic uncertainty.

The decay modes Λ0_b→ J/ψ π+_π−_pK− _{and Λ}0

b → ψ(2S)[→ J/ψ π+π

−_]pK− _{have two}

additional pions to reconstruct compared to the reference mode Λ0_b→ J/ψ pK− _{. The}

un-certainty associated with the reconstruction of these additional low-pT tracks has two

independent contributions. First, the uncertainties in the amount and distribution of ma-terial in the detector result in an uncertainty of 2.0% per additional final-state pion due to

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the modelling of hadron interactions [41]. Second, the small difference in the track

find-ing efficiency between data and simulation is corrected usfind-ing a data-driven technique [41].

The uncertainties in the correction factors are propagated to the efficiency ratios by means

of pseudoexperiments. This results in a systematic uncertainty of 3.2% for the ratio

Rψ(2S)

ψ(2S)→J/ψ π+_π− and 2.7% for the ratio RJ/ψ π +_π−

.

The systematic uncertainties related to the hadron identification efficiency, 0.1 (0.2)%

for Rψ(2S)(RJ/ψ π+π−) ratios, reflect the limited sizes of the calibration samples, and are

propagated to the ratios Rψ(2S) and RJ/ψ π+π− by means of pseudoexperiments.

The trigger efficiency for events with J/ψ → µ+µ− and ψ(2S) → µ+µ− produced in

beauty hadron decays is studied in data. A systematic uncertainty of 1.1% is assigned based on a comparison between data and simulation of the ratio of trigger efficiencies for

high-yield samples of B+→ J/ψ K+ _{and B}+_{→ ψ(2S)K}+ _{decays [13].}

Another source of uncertainty is the potential disagreement between data and simula-tion in the estimasimula-tion of efficiencies, due to effects not considered above. This is studied by varying the selection criteria in ranges that lead to as much as ±20% change in the mea-sured signal yields. The stability is tested by comparing the efficiency-corrected yields within these variations. The largest deviations range between 0.2% and 0.9% and are taken as systematic uncertainties.

Finally, a systematic uncertainty due to the limited size of the simulation sample is assigned. With all the systematic uncertainties added in quadrature, the total is 2.0% for

the ratio Rψ(2S)



ψ(2S)→µ+_µ−, 6.4% for the ratio Rψ(2S)

 

ψ(2S)→J/ψ π+_π− and 6.4% for the

ra-tio RJ/ψ π+π−.

4.3 Results

Using eq. (4.1) and the ratios of yields and efficiencies determined above, the ratio Rψ(2S)

is measured for each ψ(2S) decay mode separately: Rψ(2S)    ψ(2S)→µ+_µ− = (20.74 ± 0.88 ± 0.41 ± 0.47) × 10 −2 , Rψ(2S)    ψ(2S)→J/ψ π+_π− = (20.55 ± 1.52 ± 1.32 ± 0.18) × 10 −2 , (4.3) where the first uncertainty is statistical, the second is systematic and the third is re-lated to the uncertainties on the dielectron J/ψ and ψ(2S) branching fractions and

the ψ(2S) → J/ψ π+π− branching fraction. The average of the ratios in eq. (4.3) is

Rψ(2S) = (20.70 ± 0.76 ± 0.46 ± 0.37) × 10−2. (4.4) In this average the systematic uncertainties related to the normalization channel,

Λ0

b→ J/ψ pK−, and the trigger efficiency are considered to be 100% correlated while other

systematic uncertainties are treated as uncorrelated.

The ratio of the branching fractions of Λ0_b→ J/ψ π+_π−_pK− _{and Λ}0

b→ J/ψ pK− is

found to be

RJ/ψ π+π− = (20.86 ± 0.96 ± 1.34) × 10−2, (4.5) where contributions via intermediate resonances are included.

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The absolute branching fractions Λ0_b→ ψ(2S)pK− _{and Λ}0

b→ J/ψ π+π−pK−are derived

using the branching fraction B(Λ0_b→ J/ψ pK−) = (3.04 ± 0.04 ± 0.06 ± 0.33+0.43_−0.27) × 10−4,

measured in ref. [9], where the third uncertainty is due to the uncertainty on the branching

fraction of the decay B0 _{→ J/ψ K}∗₍₈₉₂₎0 _{and the fourth is due to the knowledge of the ratio}

of fragmentation fractions f_Λ0

b/fd. They are found to be

B(Λ0_b→ ψ(2S)pK−) = (6.29 ± 0.23 ± 0.14+1.14_−0.90) × 10−5,

B(Λ0_b→ J/ψ π+π−pK−) = (6.34 ± 0.29 ± 0.41+1.15_−0.91) × 10−5, (4.6)

where the third uncertainty comes from the uncertainties in the branching fractions of

Λ0_b→ J/ψ pK−_{, ψ(2S) → J/ψ π}+_π−_{, ψ(2S) → e}+_e− _{and J/ψ → e}+_e− _decays.

From the two separate measurements of the ratio Rψ(2S) via different decay modes

of the ψ(2S) meson (eq. (4.3)), the ratio of the ψ(2S) → µ+µ− and ψ(2S) → J/ψ π+π−

branching fractions is calculated as

B(ψ(2S) → µ+_µ−₎ B(ψ(2S) → J/ψ π+_π−₎ = Nψ(2S)→µ+_µ− Nψ(2S)→J/ψ π+_π− ×ε Λ0 b ψ(2S)→J/ψ π+_π− εΛ 0 b ψ(2S)→µ+_µ− × B(J/ψ → µ+µ−) = (2.30 ± 0.20 ± 0.12 ± 0.01) × 10−2, (4.7)

where the third uncertainty is related to the uncertainty of the known branching fraction

B(J/ψ → µ+_µ−_{) = (5.961 ± 0.033)% [36]. This result is in agreement with the world}

av-erage of (2.29 ± 0.25) × 10−2 [36] based on results of the E672/E706 [44] and BaBar [45]

collaborations, and has similar precision.

5 Measurement of Λ0

b baryon mass

The low energy release in Λ0_b→ ψ(2S)pK− and Λ0_b→ J/ψ π+_π−_pK− _decays

al-lows the Λ0_b mass to be determined with a small systematic uncertainty.

The mass is measured using four decay channels: Λ0_b→ ψ(2S)[→ µ+_µ−_]pK−_,

Λ0_b→ ψ(2S)[→ J/ψ π+_π−_]pK−_{, Λ}0

b→ J/ψ π+π

−_pK− _{and Λ}0

b→ J/ψ pK

−_{. The mass}

distri-butions for the Λ0_b→ ψ(2S)[→ µ+_µ−_]pK− _{and Λ}0

b→ J/ψ pK− channels are shown in

fig-ure 1. In the Λ0

b→ ψ(2S)[→ J/ψ π+π−]pK− channel, the J/ψ π+π− system is constrained

to the nominal ψ(2S) mass [36] to improve the precision. In the Λ0_b→ J/ψ π+_π−_pK−

chan-nel, to avoid overlap with the Λ0_b→ ψ(2S)[→ J/ψ π+_π−_]pK− _{channel the ψ(2S) region is}

vetoed, i.e. the mass of the J/ψ π+_π− _{combination is required to be outside the range}

3670 < m(J/ψ π+π−) < 3700 MeV/c2. The mass distributions for these two samples, along

with the result of an unbinned extended maximum likelihood fit using the model described

in section 4.1, are shown in figure3.

The systematic uncertainties on the measurement of the Λ0_b baryon mass for all four

channels are listed in table4. The precision of the absolute momentum scale calibration of

0.03% is the dominant source of uncertainty [23, 46]. This uncertainty is proportional to

the energy release in the decay and is minimal for the processes with a ψ(2S) in the final state. A further uncertainty is related to the energy loss in the material of the tracking

JHEP05(2016)132

5.6 5.65 0 20 40 Candidates/(2 Me V /c 2 ) m(ψ(2S)pK−) 5.6 5.65 0 50 100 Candidates/(2 Me V /c 2 ) m(J/ψ π+_π−_pK−₎  GeV/c2  GeV/c2 LHCb LHCb

Figure 3. Left: mass distribution of selected Λ0

b→ ψ(2S)[→ J/ψ π

+_π−_]pK− _{candidates with}

an additional constraint for the ψ(2S) mass [36]. Right: mass distribution of selected Λ0

b→ J/ψ π+π−pK− candidates with a requirement of the J/ψ π+π− combination mass to be

out-side the range 3670 < m(J/ψ π+π−) < 3700 MeV/c2. The total fit function (solid red), the Λ0_bsignal contribution (dotted magenta) and the combinatorial background (dashed blue) are shown.

J/ψ ψ(2S) → µ+_µ− _{ψ(2S) → J/ψ π}+_π− _{J/ψ π}+_π−_,

ψ(2S)

Momentum scale 0.34 0.19 0.15 0.26

Energy loss correction 0.03 0.02 0.06 0.07

Fit model 0.04 0.03 0.08 0.05

Sum in quadrature 0.34 0.19 0.18 0.27

Table 4. Systematic uncertainties (in MeV/c2_{) on the Λ}0

b mass using the decay

modes Λ0

b→ J/ψ pK−, Λ0b→ ψ(2S)[→ µ+µ−]pK−, Λ0b→ ψ(2S)[→ J/ψ π+π−]pK− and

Λ0_b→ J/ψ π+_π−_pK− _{with the J/ψ π}+_π− _{mass outside the ψ(2S) region.}

system [47], which is known with an accuracy of 10% [48]. This effect is estimated by

varying the energy loss correction in the reconstruction by 10% and taking the observed mass shift as an uncertainty. The uncertainty due to the fit model is estimated using the same set of cross-check models for the signal and background parameterization as

considered in section4, with the maximum deviation in the mass assigned as a systematic

uncertainty. The uncertainties on the masses of the J/ψ and ψ(2S) mesons [36] are small

and are therefore neglected.

As a cross-check, the data sample is divided into four parts, for data collected at√s = 7

and 8TeV and with different magnet polarities. The measured masses are consistent among these subsamples, and therefore no systematic uncertainty is assigned. To check the effect

of the selection criteria (see section 3), the high-yield Λ0_b→ J/ψ pK− decay channel is used.

No sizeable dependence of the mass on the selection criteria is observed and no additional uncertainty is assigned.

The results from the four decay channels are presented in table 5. To combine them,

JHEP05(2016)132

Channel M (Λ0_b) MeV/c2 Λ0_b→ J/ψ pK− 5619.62 ± 0.04 ± 0.34 Λ0_b→ ψ(2S)[→ µ+_µ−_]pK− _{5619.84 ± 0.18 ± 0.19} Λ0 b→ ψ(2S)[→ J/ψ π+π−]pK− 5619.38 ± 0.33 ± 0.18 Λ0_b→ J/ψ π+_π−_pK− _{excluding ψ(2S) 5619.08 ± 0.30 ± 0.27} Table 5. Measured Λ0

bmass in different decay channels. The first uncertainty is statistical and the

second is systematic.

to the fit procedure are treated as uncorrelated while those due to the momentum scale and energy loss correction are considered to be fully correlated. The combined value of

the Λ0_b mass is

M (Λ0_b) = 5619.65 ± 0.17 ± 0.17 MeV/c2, (5.1)

where the first uncertainty is statistical and the second systematic. The χ2/ndf calculated

for the individual measurements with respect to the combined value is 3.0/3. This is the most precise measurement of any b-hadron mass reported to date.

Previous direct measurements of the Λ0_b mass by LHCb were made using the

de-cay Λ0_b→ J/ψ Λ0 _{[23, 47] and are statistically independent of the results of this study.}

The combination obtained here is consistent with, and more precise than, the results of these earlier studies. The LHCb results are combined, taking the statistical uncertainties and those related to the fit procedure to be uncorrelated and those due to the energy loss correction to be fully correlated. The uncertainty due to the momentum scale in

ref. [23] is also taken to be fully correlated, whereas in ref. [47] a different alignment and

calibration procedure was used and so the corresponding uncertainty is considered to be uncorrelated with the other measurements. The result of the combination is dominated by the measurements of this analysis and is

M (Λ0_b) = 5619.65 ± 0.16 ± 0.14 MeV/c2, (5.2)

where the uncertainties are statistical and systematic. The χ2/ndf calculated for the

indi-vidual measurements with respect to the combined value is 3.4/5. The measured mass is

in agreement with, but more precise than, the results of the ATLAS [49] and CDF [5]

col-laborations.

From the value of the Λ0_b mass in eq. (5.2) and a precise measurement of the mass

difference between the Λ0_b and B0 hadrons reported in ref. [7], the mass of the B0 meson is

calculated to be

M (B0) = 5279.93 ± 0.39 MeV/c2, (5.3)

where the correlation of 41% between the LHCb measurements of the Λ0_b mass and

the Λ0_b–B0mass splitting has been taken into account. This is in agreement with the current

JHEP05(2016)132

6 Results and summary

The Λ0_b→ ψ(2S)pK− _{and Λ}0

b→ J/ψ π+π−pK− decay modes are observed using a

sam-ple of pp collisions at centre-of-mass energies of 7 and 8TeV, corresponding to an

integrated luminosity of 3 fb−1. With results from the channels ψ(2S) → µ+µ− and

ψ(2S) → J/ψ π+π− combined, the ratio of branching fractions is measured:

Rψ(2S) = B(Λ 0 b→ ψ(2S)pK−) B(Λ0 b→ J/ψ pK−) = (20.70 ± 0.76 ± 0.46 ± 0.37) × 10−2,

where the first uncertainty is statistical, the second is systematic and the third is related to the uncertainties of the known dielectron J/ψ and ψ(2S) branching fractions and of

the branching fraction of the ψ(2S) → J/ψ π+π− decay. The ratio of branching fractions

for Λ0_b→ J/ψ π+_π−_pK− _{and Λ}0 b→ J/ψ pK− is RJ/ψ π+π−= B(Λ 0 b→ J/ψ π+π−pK−) B(Λ0 b→ J/ψ pK−) = (20.86 ± 0.96 ± 1.34) × 10−2,

where the first uncertainty is statistical, the second is systematic and contributions via intermediate resonances are included.

From measurements of the ratio Rψ(2S) via two different decay modes of the ψ(2S)

me-son it is determined that

B(ψ(2S) → µ+_µ−₎

B(ψ(2S) → J/ψ π+_π−₎ = (2.30 ± 0.20 ± 0.12 ± 0.01) × 10

−2

,

where the first uncertainty is statistical, the second is systematic and the third is related

to the uncertainty on B(J/ψ → µ+µ−). This is the most precise direct measurement of

this ratio to date.

Using Λ0_b→ ψ(2S)pK−, Λ0_b→ J/ψ π+_π−_pK− _{and Λ}0

b→ J/ψ pK

− _{decays, the mass of}

the Λ0_b baryon is measured to be

M (Λ0_b) = 5619.65 ± 0.17 ± 0.17 MeV/c2,

where the first uncertainty is statistical and the second is systematic. Combining this result

with previous LHCb measurements that used the channel Λ0_b → J/ψ Λ0 _{[23,47] gives}

M (Λ0_b) = 5619.65 ± 0.16 ± 0.14 MeV/c2, (6.1)

where the first uncertainty is statistical and the second is systematic. This is the most precise determination of the mass of any b hadron 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

JHEP05(2016)132

the national agencies: CAPES, CNPq, FAPERJ and FINEP (Brazil); NSFC (China); CNRS/IN2P3 (France); BMBF, DFG and MPG (Germany); INFN (Italy); FOM and NWO (The Netherlands); MNiSW and NCN (Poland); MEN/IFA (Romania); MinES and FANO (Russia); MinECo (Spain); SNSF and SER (Switzerland); NASU (Ukraine);

STFC (United Kingdom); NSF (U.S.A.). We acknowledge the computing

re-sources that are provided by CERN, IN2P3 (France), KIT and DESY (Germany), INFN (Italy), SURF (The 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

be-hind 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), Conseil G´en´eral de

Haute-Savoie, Labex ENIGMASS and OCEVU, R´egion Auvergne (France), RFBR and

Yandex LLC (Russia), GVA, XuntaGal and GENCAT (Spain), Herchel Smith Fund, The Royal Society, Royal Commission for the Exhibition of 1851 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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JHEP05(2016)132

J. He39_{, T. Head}40_{, V. Heijne}42_{, A. Heister}9_{, K. Hennessy}53_{, P. Henrard}5_{, L. Henry}8_,

J.A. Hernando Morata38_{, E. van Herwijnen}39_{, M. Heß}65_{, A. Hicheur}2_{, D. Hill}56_{, M. Hoballah}5_,

C. Hombach55, L. Hongming40, W. Hulsbergen42, T. Humair54, M. Hushchyn67, N. Hussain56, D. Hutchcroft53, M. Idzik28, P. Ilten57, R. Jacobsson39, A. Jaeger12, J. Jalocha56, E. Jans42, A. Jawahery59_{, M. John}56_{, D. Johnson}39_{, C.R. Jones}48_{, C. Joram}39_{, B. Jost}39_{, N. Jurik}60_,

S. Kandybei44_{, W. Kanso}6_{, M. Karacson}39_{, T.M. Karbach}39,†_{, S. Karodia}52_{, M. Kecke}12_,

M. Kelsey60, I.R. Kenyon46, M. Kenzie39, T. Ketel43, E. Khairullin67, B. Khanji21,39,k,

C. Khurewathanakul40_{, T. Kirn}9_{, S. Klaver}55_{, K. Klimaszewski}29_{, O. Kochebina}7_{, M. Kolpin}12_,

I. Komarov40_{, R.F. Koopman}43_{, P. Koppenburg}42,39_{, M. Kozeiha}5_{, L. Kravchuk}34_{, K. Kreplin}12_,

M. Kreps49_{, P. Krokovny}35_{, F. Kruse}10_{, W. Krzemien}29_{, W. Kucewicz}27,o_{, M. Kucharczyk}27_,

V. Kudryavtsev35, A. K. Kuonen40, K. Kurek29, T. Kvaratskheliya32, D. Lacarrere39, G. Lafferty55,39_{, A. Lai}16_{, D. Lambert}51_{, G. Lanfranchi}19_{, C. Langenbruch}49_{, B. Langhans}39_,

T. Latham49_{, C. Lazzeroni}46_{, R. Le Gac}6_{, J. van Leerdam}42_{, J.-P. Lees}4_{, R. Lef`evre}5_,

A. Leflat33,39, J. Lefran¸cois7, E. Lemos Cid38, O. Leroy6, T. Lesiak27, B. Leverington12, Y. Li7, T. Likhomanenko67,66, M. Liles53, R. Lindner39, C. Linn39, F. Lionetto41, B. Liu16, X. Liu3, D. Loh49_{, I. Longstaff}52_{, J.H. Lopes}2_{, D. Lucchesi}23,r_{, M. Lucio Martinez}38_{, H. Luo}51_,

A. Lupato23_{, E. Luppi}17,g_{, O. Lupton}56_{, N. Lusardi}22_{, A. Lusiani}24_{, F. Machefert}7_{, F. Maciuc}30_,

O. Maev31, K. Maguire55, S. Malde56, A. Malinin66, G. Manca7, G. Mancinelli6, P. Manning60, A. Mapelli39_{, J. Maratas}5_{, J.F. Marchand}4_{, U. Marconi}15_{, C. Marin Benito}37_{, P. Marino}24,39,t_,

J. Marks12_{, G. Martellotti}26_{, M. Martin}6_{, M. Martinelli}40_{, D. Martinez Santos}38_,

F. Martinez Vidal68_{, D. Martins Tostes}2_{, L.M. Massacrier}7_{, A. Massafferri}1_{, R. Matev}39_,

A. Mathad49, Z. Mathe39, C. Matteuzzi21, A. Mauri41, B. Maurin40, A. Mazurov46, M. McCann54, J. McCarthy46_{, A. McNab}55_{, R. McNulty}13_{, B. Meadows}58_{, F. Meier}10_{, M. Meissner}12_,

D. Melnychuk29_{, M. Merk}42_{, A Merli}22,u_{, E Michielin}23_{, D.A. Milanes}64_{, M.-N. Minard}4_,

D.S. Mitzel12, J. Molina Rodriguez61, I.A. Monroy64, S. Monteil5, M. Morandin23, P. Morawski28, A. Mord`a6, M.J. Morello24,t, J. Moron28, A.B. Morris51, R. Mountain60, F. Muheim51,

D. M¨uller55_{, J. M¨uller}10_{, K. M¨uller}41_{, V. M¨uller}10_{, M. Mussini}15_{, B. Muster}40_{, P. Naik}47_,

T. Nakada40_{, R. Nandakumar}50_{, A. Nandi}56_{, I. Nasteva}2_{, M. Needham}51_{, N. Neri}22_{, S. Neubert}12_,

N. Neufeld39, M. Neuner12, A.D. Nguyen40, C. Nguyen-Mau40,q, V. Niess5, S. Nieswand9,

R. Niet10_{, N. Nikitin}33_{, T. Nikodem}12_{, A. Novoselov}36_{, D.P. O’Hanlon}49_{, A. Oblakowska-Mucha}28_,

V. Obraztsov36_{, S. Ogilvy}52_{, O. Okhrimenko}45_{, R. Oldeman}16,48,f_{, C.J.G. Onderwater}69_,

B. Osorio Rodrigues1_{, J.M. Otalora Goicochea}2_{, A. Otto}39_{, P. Owen}54_{, A. Oyanguren}68_,

A. Palano14,d, F. Palombo22,u, M. Palutan19, J. Panman39, A. Papanestis50, M. Pappagallo52, L.L. Pappalardo17,g_{, C. Pappenheimer}58_{, W. Parker}59_{, C. Parkes}55_{, G. Passaleva}18_{, G.D. Patel}53_,

M. Patel54_{, C. Patrignani}20,j_{, A. Pearce}55,50_{, A. Pellegrino}42_{, G. Penso}26,m_{, M. Pepe Altarelli}39_,

S. Perazzini15,e, P. Perret5, L. Pescatore46, K. Petridis47, A. Petrolini20,j, M. Petruzzo22, E. Picatoste Olloqui37, B. Pietrzyk4, M. Pikies27, D. Pinci26, A. Pistone20, A. Piucci12, S. Playfer51_{, M. Plo Casasus}38_{, T. Poikela}39_{, F. Polci}8_{, A. Poluektov}49,35_{, I. Polyakov}32_,

E. Polycarpo2_{, A. Popov}36_{, D. Popov}11,39_{, B. Popovici}30_{, C. Potterat}2_{, E. Price}47_{, J.D. Price}53_,

J. Prisciandaro38, A. Pritchard53, C. Prouve47, V. Pugatch45, A. Puig Navarro40, G. Punzi24,s, W. Qian56_{, R. Quagliani}7,47_{, B. Rachwal}27_{, J.H. Rademacker}47_{, M. Rama}24_{, M. Ramos Pernas}38_,

M.S. Rangel2_{, I. Raniuk}44_{, G. Raven}43_{, F. Redi}54_{, S. Reichert}55_{, A.C. dos Reis}1_{, V. Renaudin}7_,

S. Ricciardi50_{, S. Richards}47_{, M. Rihl}39_{, K. Rinnert}53,39_{, V. Rives Molina}37_{, P. Robbe}7,39_,

A.B. Rodrigues1, E. Rodrigues55, J.A. Rodriguez Lopez64, P. Rodriguez Perez55, A. Rogozhnikov67_{, S. Roiser}39_{, V. Romanovsky}36_{, A. Romero Vidal}38_{, J. W. Ronayne}13_,

M. Rotondo23_{, T. Ruf}39_{, P. Ruiz Valls}68_{, J.J. Saborido Silva}38_{, N. Sagidova}31_{, B. Saitta}16,f_,

V. Salustino Guimaraes2, C. Sanchez Mayordomo68, B. Sanmartin Sedes38, R. Santacesaria26, C. Santamarina Rios38_{, M. Santimaria}19_{, E. Santovetti}25,l_{, A. Sarti}19,m_{, C. Satriano}26,n_,

JHEP05(2016)132

A. Satta25_{, D.M. Saunders}47_{, D. Savrina}32,33_{, S. Schael}9_{, M. Schiller}39_{, H. Schindler}39_,

M. Schlupp10_{, M. Schmelling}11_{, T. Schmelzer}10_{, B. Schmidt}39_{, O. Schneider}40_{, A. Schopper}39_,

M. Schubiger40, M.-H. Schune7, R. Schwemmer39, B. Sciascia19, A. Sciubba26,m, A. Semennikov32, A. Sergi46, N. Serra41, J. Serrano6, L. Sestini23, P. Seyfert21, M. Shapkin36, I. Shapoval17,44,g, Y. Shcheglov31_{, T. Shears}53_{, L. Shekhtman}35_{, V. Shevchenko}66_{, A. Shires}10_{, B.G. Siddi}17_,

R. Silva Coutinho41_{, L. Silva de Oliveira}2_{, G. Simi}23,s_{, M. Sirendi}48_{, N. Skidmore}47_,

T. Skwarnicki60, E. Smith54, I.T. Smith51, J. Smith48, M. Smith55, H. Snoek42, M.D. Sokoloff58,39, F.J.P. Soler52_{, F. Soomro}40_{, D. Souza}47_{, B. Souza De Paula}2_{, B. Spaan}10_{, P. Spradlin}52_,

S. Sridharan39_{, F. Stagni}39_{, M. Stahl}12_{, S. Stahl}39_{, S. Stefkova}54_{, O. Steinkamp}41_{, O. Stenyakin}36_,

S. Stevenson56_{, S. Stoica}30_{, S. Stone}60_{, B. Storaci}41_{, S. Stracka}24,t_{, M. Straticiuc}30_,

U. Straumann41, L. Sun58, W. Sutcliffe54, K. Swientek28, S. Swientek10, V. Syropoulos43,

M. Szczekowski29_{, T. Szumlak}28_{, S. T’Jampens}4_{, A. Tayduganov}6_{, T. Tekampe}10_{, G. Tellarini}17,g_,

F. Teubert39_{, C. Thomas}56_{, E. Thomas}39_{, J. van Tilburg}42_{, V. Tisserand}4_{, M. Tobin}40_{, J. Todd}58_,

S. Tolk43, L. Tomassetti17,g, D. Tonelli39, S. Topp-Joergensen56, E. Tournefier4, S. Tourneur40, K. Trabelsi40, M. Traill52, M.T. Tran40, M. Tresch41, A. Trisovic39, A. Tsaregorodtsev6, P. Tsopelas42_{, N. Tuning}42,39_{, A. Ukleja}29_{, A. Ustyuzhanin}67,66_{, U. Uwer}12_{, C. Vacca}16,39,f_,

V. Vagnoni15_{, G. Valenti}15_{, A. Vallier}7_{, R. Vazquez Gomez}19_{, P. Vazquez Regueiro}38_,

C. V´azquez Sierra38, S. Vecchi17, M. van Veghel42, J.J. Velthuis47, M. Veltri18,h, G. Veneziano40, M. Vesterinen12_{, B. Viaud}7_{, D. Vieira}2_{, M. Vieites Diaz}38_{, X. Vilasis-Cardona}37,p_{, V. Volkov}33_,

A. Vollhardt41_{, D. Voong}47_{, A. Vorobyev}31_{, V. Vorobyev}35_{, C. Voß}65_{, J.A. de Vries}42_{, R. Waldi}65_,

C. Wallace49_{, R. Wallace}13_{, J. Walsh}24_{, J. Wang}60_{, D.R. Ward}48_{, N.K. Watson}46_{, D. Websdale}54_,

A. Weiden41, M. Whitehead39, J. Wicht49, G. Wilkinson56,39, M. Wilkinson60, M. Williams39, M.P. Williams46_{, M. Williams}57_{, T. Williams}46_{, F.F. Wilson}50_{, J. Wimberley}59_{, J. Wishahi}10_,

W. Wislicki29_{, M. Witek}27_{, G. Wormser}7_{, S.A. Wotton}48_{, K. Wraight}52_{, S. Wright}48_{, K. Wyllie}39_,

Y. Xie63, Z. Xu40, Z. Yang3, H. Yin63, J. Yu63, X. Yuan35, O. Yushchenko36, M. Zangoli15, M. Zavertyaev11,c, L. Zhang3, Y. Zhang3, A. Zhelezov12, Y. Zheng62, A. Zhokhov32, L. Zhong3, V. Zhukov9_{, 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

LAPP, Universit´e Savoie Mont-Blanc, CNRS/IN2P3, Annecy-Le-Vieux, France 5

Clermont Universit´e, Universit´e Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France 6 _{CPPM, Aix-Marseille Universit´e, CNRS/IN2P3, Marseille, France}

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

8 _{LPNHE, Universit´e Pierre et Marie Curie, Universit´e Paris Diderot, 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

Sezione INFN di Bari, Bari, Italy 15

Sezione INFN di Bologna, Bologna, Italy 16

Sezione INFN di Cagliari, Cagliari, Italy 17

Sezione INFN di Ferrara, Ferrara, Italy 18 _{Sezione INFN di Firenze, Firenze, Italy}

19 _{Laboratori Nazionali dell’INFN di Frascati, Frascati, Italy} 20 _{Sezione INFN di Genova, Genova, Italy}

JHEP05(2016)132

22 _{Sezione INFN di Milano, Milano, Italy}

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

Sezione INFN di Pisa, Pisa, Italy 25

Sezione INFN di Roma Tor Vergata, Roma, Italy 26

Sezione INFN di Roma La Sapienza, Roma, Italy 27

Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Krak´ow, Poland 28

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

29

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

30 _{Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele,} Romania

31 _{Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia}

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

33 _{Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow, Russia} 34

Institute for Nuclear Research of the Russian Academy of Sciences (INR RAN), Moscow, Russia 35

Budker Institute of Nuclear Physics (SB RAS) and Novosibirsk State University, Novosibirsk, Russia

36

Institute for High Energy Physics (IHEP), Protvino, Russia 37

Universitat de Barcelona, Barcelona, Spain 38

Universidad de Santiago de Compostela, Santiago de Compostela, Spain 39

European Organization for Nuclear Research (CERN), Geneva, Switzerland 40 _{Ecole Polytechnique F´ed´erale de Lausanne (EPFL), Lausanne, Switzerland} 41 _{Physik-Institut, Universit¨at Z¨urich, Z¨urich, Switzerland}

42 _{Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands}

43 _{Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, The} Netherlands

44

NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine 45

Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine 46

University of Birmingham, Birmingham, United Kingdom 47

H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom 48

Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom 49

Department of Physics, University of Warwick, Coventry, United Kingdom 50

STFC Rutherford Appleton Laboratory, Didcot, United Kingdom

51 _{School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom} 52 _{School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom} 53 _{Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom} 54 _{Imperial College London, London, United Kingdom}

55 _{School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom} 56

Department of Physics, University of Oxford, Oxford, United Kingdom 57

Massachusetts Institute of Technology, Cambridge, MA, United States 58

University of Cincinnati, Cincinnati, OH, United States 59

University of Maryland, College Park, MD, United States 60

Syracuse University, Syracuse, NY, United States 61

Pontif´ıcia Universidade Cat´olica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil, associated to2

62

University of Chinese Academy of Sciences, Beijing, China, associated to3

63 _{Institute of Particle Physics, Central China Normal University, Wuhan, Hubei, China,} associated to3

64 _{Departamento de Fisica , Universidad Nacional de Colombia, Bogota, Colombia, associated to}8 65 _{Institut f¨ur Physik, Universit¨at Rostock, Rostock, Germany, associated to}12

66

National Research Centre Kurchatov Institute, Moscow, Russia, associated to32 67

JHEP05(2016)132

68 _{Instituto de Fisica Corpuscular (IFIC), Universitat de Valencia-CSIC, Valencia, Spain,}

associated to37 69

Van Swinderen Institute, University of Groningen, Groningen, The Netherlands, associated to42 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 <sub>Universit`a di Cagliari, Cagliari, Italy</sub> g _{Universit`a di Ferrara, Ferrara, Italy</sub> h <sub>Universit`a di Urbino, Urbino, Italy}

i _{Universit`a di Modena e Reggio Emilia, Modena, Italy</sub> j <sub>Universit`a di Genova, Genova, Italy}

k

Universit`a di Milano Bicocca, Milano, Italy l

Universit`a di Roma Tor Vergata, Roma, Italy m

Universit`a di Roma La Sapienza, Roma, Italy n

Universit`a della Basilicata, Potenza, Italy o

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

p

LIFAELS, La Salle, Universitat Ramon Llull, Barcelona, Spain q _{Hanoi University of Science, Hanoi, Viet Nam}

r _{Universit`a di Padova, Padova, Italy</sub> s <sub>Universit`a di Pisa, Pisa, Italy} t _{Scuola Normale Superiore, Pisa, Italy}

u _{Universit`a degli Studi di Milano, Milano, Italy} †