Measurement of the b-Quark Production Cross Section in 7 and 13 TeV pp Collisions
R. Aaijet al.*(LHCb Collaboration)
(Received 15 December 2016; revised manuscript received 9 January 2017; published 3 February 2017) Measurements of the cross section for producingb quarks in the reaction pp → bbX are reported in 7 and 13 TeV collisions at the LHC as a function of the pseudorapidityη in the range 2 < η < 5 covered by the acceptance of the LHCb experiment. The measurements are done using semileptonic decays of b-flavored hadrons decaying into a ground-state charmed hadron in association with a muon. The cross sections in the coveredη range are 72.0 0.3 6.8 and 154.3 1.5 14.3 μb for 7 and 13 TeV. The ratio is 2.14 0.02 0.13, where the quoted uncertainties are statistical and systematic, respectively. The agreement with theoretical expectation is good at 7 TeV, but differs somewhat at 13 TeV. The measured ratio of cross sections is larger at lowerη than the model prediction.
DOI:10.1103/PhysRevLett.118.052002
Production ofb quarks in high energy pp collisions at the LHC provides a sensitive test of models based on quantum chromodynamics[1]. Searches for physics beyond the stan-dard model (SM) often rely on the ability to accurately predict the production rates ofb quarks that can form backgrounds in combination with other high energy processes[2]. In addition, knowledge of theb-quark yield is essential for calculating the sensitivity of experiments testing the SM by measuring CP-violating and rare decay processes[3].
We present here measurements of production cross sec-tions for the average ofb-flavored and b-flavored hadrons, denotedpp → HbX, where X indicates additional particles, inpp collisions recorded by LHCb at both 7 and 13 TeV center-of-mass energies, and their ratio. These measurements are made as a function of the Hb pseudorapidity η in the interval2 < η < 5, where η ¼ − ln ½tanðθ=2Þ, and θ is the angle of the weakly decayingb or b hadron with respect to the proton direction. We report results over the full range ofb-hadron transverse momentum, pT. TheHbcross section has been previously measured at LHCb in 7 TeV collisions using semileptonic decays to D0μ−X [4] and b → J=ψX decays [5]. Previous determinations were made at the Tevatron collider inpp collisions near 2 TeV center-of-mass energy [6]. Other LHC experiments have also measured b-quark production characteristics at 7[7], and 13 TeV[8]. The method presented in this Letter is more accurate because the normalization is based on well-measured semileptonicB0 andB−branching fractions, and the equality of semileptonic widths for all b hadrons, in contrast to inclusive J=ψ production which relies on the assumption that theb-hadron
particle species are produced in the same proportions as at LEP[9], or those that just use one specificb hadron, which needs theb-hadron fractions to extrapolate to the total.
The production cross section for a hadron Hb that contains either ab or b quark, but not both, is given by
σðpp → HbXÞ ¼12½σðB0Þ þ σðB0Þ þ12½σðBþÞ þ σðB−Þ
þ12½σðB0
sÞ þ σðB0sÞ
þ1 þ δ
2 ½σðΛ0bÞ þ σðΛ0bÞ; ð1Þ
where δ is a correction that accounts for Ξb and Ω−b baryons; we ignoreBcmesons since their production level is estimated to be only 0.1% ofb hadrons[10].
Our estimate ofδ is based on a paper by Voloshin[11], in which two useful relations are given:
ΓðΞ− b → Ξ−Xμ−νÞ ¼ ΓðΛ0b→ ΛXμ−νÞ; and σðΞ − bÞ σðΛ0 bÞ¼ 0.11 0.03 0.03; ð2Þ
where the latter is determined from Tevatron data, and the second uncertainty is assigned from the allowable SU(3) symmetry breaking. The b-hadron fractions determined there[9]agree with the ones measured by LHCb for other b-flavored hadrons[12]. Since the lifetimes of theΛ0band Ξ−
b are equal within their uncertainties[9], assuming that
the two branching fractions are equal gives us an estimate of 0.11 for theΞ−b=Λ0b semileptonic decay ratio. However, this must be doubled, using isospin invariance, to account for theΞ0b. To this we must add theΩ−b contribution, taken as 15% of the Ξb, thus arriving at an estimate of δ of 0.25 0.10, where the uncertainty is the one in Eq. (2)
*Full author list given at end of the article.
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added in quadrature to our estimate of the uncertainties from assuming isospin and lifetime equalities.
To measure these cross sections we determine the signal yields ofb decays into a charm hadron plus a muon for a given integrated luminosity L and correct for various efficiencies described below. Explicitly,
σðpp → HbXÞ ¼ 1 2L nðD0μÞ ϵD0×BD0þ nðDþμÞ ϵDþ×BDþ 1 BðB → DXμνÞ þ nðDþ sμÞ ϵDþ s ×BDþs 1 BðBs→ DsXμνÞ þ nðΛþ cμÞ ϵΛþ c ×BΛþc 1 þ δ BðΛ0 b→ ΛþcXμνÞ ; ð3Þ
wherenðXcμÞ means the number of detected charm hadron plus muon events and their charge conjugates, with corresponding efficiencies denoted by ϵXc. The charm branching fractions,BXc, used in this analysis, along with their sources, are listed in the Supplemental Material[13]. The PDG average is used for theD0andDþs modes[9]. For theDþ mode there is only one measurement by CLEO III, so that is used[14]. For theΛþc we average measurements by BES III [15] and Belle [16]. The expression BðB → DXμνÞ denotes the average branching fraction for B0 andB− semileptonic decays.
The B0 and B− semileptonic branching fractions are obtained with a somewhat different procedure than that adopted by the PDG, whose actual estimate is difficult to derive from the posted information. We take three
measurements that are mostly model independent and average them. The first one was made by CLEO using inclusive leptons at the ϒð4SÞ resonance without distin-guishing whether they are from B0 or B− meson decays
[17]. The ϒð4SÞ, however, does not have an equal branching fraction intoB0 B0 andB− Bþ mesons. In fact the fraction into neutralB pairs is α ¼ 0.486 0.006[9], with the remainder going into chargedB pairs. Therefore, to compute theB0andB−semileptonic branching fractions we need to use the following coupled equations
αB0
SLþ ð1 − αÞB−SL¼ ð10.91 0.09 0.24Þ%;
B0
SL=B−SL¼ τ0=τ−¼ 0.927 0.004; ð4Þ
whereτiare the lifetimes[9]. The numbers extracted from the solution are listed in Table I, along with direct measurements from CLEO [17], BABAR [18], and Belle
[19]. These latter two analyses measure the semileptonic decays ofB0andB− mesons separately. They do not cover the full momentum range so a correction has to be applied; this was done by the PDG[9]. SinceD0andDþmesons are produced in bothB0and B− decays, we sum their yields and use the average semileptonic branching fraction forB0 andB− decays,hB0þ B−i.
The semileptonicB branching fractions we use are listed in TableII. Since we are detecting onlyb → cμν modes, we have to correct later for the fact that there is a small 1% b → uμν component[9].
The semileptonic widthsΓSLare equal for allHbspecies used in this analysis except for a small correction for Λ0b decays (BSL¼ ΓSL=Γ ¼ ΓSL×τ). This has proven to be true in the case of charm hadron decays even though the lifetimes ofD0andDþdiffer by a factor of 2.5. The decays of theΛ0b are slightly different due to the absence of the chromomagnetic correction that affects B-meson decays but is absent inb baryons[20–22]. ThusΓSL, and alsoBSL, are increased for theΛ0b byð4 2Þ%[12].
The input for theB0s lifetime listed in TableIIuses only measurements in the flavor-specific decay B0s→ Dþsπ− from CDF [23] and LHCb [24]. Other measurements can in principle be used, e.g., in J=ψϕ or J=ψf0ð980Þ final states, but they then involve also determining ΔΓs. Older measurements involving semileptonic decays are
TABLE I. Measured semileptonic decay branching fractions for ¯B0andB−mesons. The correlation of the errors in the underlying
measurements in the average is taken into account. The CLEO numbers result from solving Eq.(4).
B0 SL(%) B−SL(%) Source 10.49 0.27 11.31 0.27 CLEO[17] 9.64 0.43 10.28 0.47 BABAR[18] 10.46 0.38 11.17 0.38 Belle[19] 10.31 0.19 11.09 0.20 Average
TABLE II. Measured semileptonic decay branching fractions forB mesons and derived branching fractions for ¯B0s andΛ0b based on the equality of semileptonic widths and the lifetime ratios.
Particle τ (ps) measured BSL(%) measured ΓSL(ps−1) measured BSL (%) to be used
¯B0 1.519 0.005 10.31 0.19 0.0678 0.0013 10.31 0.19 B− 1.638 0.004 11.09 0.20 0.0680 0.0013 11.09 0.20 h ¯B0þ B−i 10.70 0.19 10.70 0.19 ¯B0 s 1.533 0.018 10.40 0.30 Λ0 b 1.467 0.010 10.35 0.28 PRL 118, 052002 (2017) P H Y S I C A L R E V I E W L E T T E R S 3 FEBRUARY 2017
suspected of having larger uncontrolled systematic uncer-tainties [25]. Finally, the Λ0b lifetime is taken from the HFAG average [26].
Corrections due to cross feeds among the modes, for example, from B0s → DKμ−X events or Λ0b → DNμ−X decays are well below our sensitivity, and thus we do not include them.
The data used here correspond to integrated luminosities of 284.10 4.86 pb−1 collected at 7 TeV and 4.60 0.18 pb−1 at 13 TeV [27], where special triggers were
implemented to minimize uncertainties. The LHCb detector
[28,29]is a single-arm forward spectrometer covering the pseudorapidity range 2 < η < 5. Components include a high-precision tracking system consisting of a silicon-strip vertex detector surrounding the pp interaction region, 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 placed downstream of the magnet. Different types of charged hadrons are distinguished using information from two ring-imaging Cherenkov detectors (RICH). Muons are identified by a system composed of alternating layers of iron and multiwire proportional chambers.
Events of potential interest are triggered by the identi-fication of a muon in real time with a minimum pT of 1.48 GeV in the 7 TeV data[30], and 0.9 GeV in the 13 TeV data (further restricted in the higher level trigger to pT > 1.3 GeV)[31]. In addition, to test for inconsistency
with production at the primary vertex (PV), theχ2IPfor the muon is computed as the difference between the vertex fit χ2of the PV reconstructed with and without the considered
track. We require thatχ2IPbe larger than 200 at 7 TeV (16 at 13 TeV), and in the 7 TeV data only, the impact parameter of the muon must be greater than 0.5 mm. There is a prescale by a factor of 2 for both energies and an additional prescale of a factor of 2 for the D0μ− channel in the 7 TeV data.
These events are subjected to further requirements in order to select those with a charmed hadron decay which forms a vertex with the identified muon that is detached from the PV. The charmed hadron must not be consistent with originating from the PV. We use the decays D0→ K−πþ, Dþ → K−πþπþ, Dþ
s → KþK−πþ, and
Λþ
c → pK−πþ. (The related branching fractions are given
in the Supplemental Material [13]). The RICH system is used to determine a likelihood for each particle hypothesis. We use selections on the differences of log-likelihoods (L) to separate protons from kaons and pions, LðpÞ − LðKÞ > 0 and LðpÞ − LðπÞ > 10, kaons from pions LðKÞ − LðπÞ > 4, and pions from kaons LðKÞ − LðπÞ < 4 for 7 and< 10 for 13 TeV. In addition, in order to suppress background, the averagepT of the charm hadron daughters must be larger than 700 MeV for three-body and 600 MeV for two-body decays, and the invariant mass of the charm hadron plus muon must range from approximately 3 to
5 GeV. Furthermore, the charm plus μ vertex must be within a radius less than 4.8 mm from the beam line to remove contributions of secondary interactions in the detector material due to long-lived particles, and the charm hadron must decay downstream of this vertex.
Since detection efficiencies vary over the available phase space, we divide the data into two-dimensional intervals in pT of the charm plusμ system, and η, where the latter is
determined from the relative positions of the charm plusμ vertex and the PV. We fit the data for each charm plusμ combination in each interval simultaneously in invariant mass of the charm hadron and ln(IP=mm) variables, where IP is the measured impact parameter of the charmed hadron with respect to the PV in units of mm.
As an example of the fitting technique consider Dþsμ− candidates integrated over pT and η for the 7 TeV data. Figure1(a)shows the KþK−πþ invariant mass spectrum, while (b) shows the lnðIP=mmÞ distribution. The invariant mass signal is fit for theDþs yield with a double-Gaussian function where the means of the two Gaussians are con-strained to be the same. The common mean and the widths are determined in the fit. (A second double-Gaussian shape is used to fit the higher mass decay of Dþ→ πþD0, D0→ KþK−, an additional consideration only in this
mode.) The lnðIP=mmÞ shape of the signal component, determined by simulation, is a bifurcated Gaussian where the peak position and width parameters are determined by the fit. The combinatorial background is modeled with a linear shape. (The other modes at both energies are shown in the Supplemental Material[13].) The signal yields for charm hadron plus muon candidates integrated overη are also given in the Supplemental Material[13].
The major components of the total efficiency are the off-line and trigger efficiencies. The latter is measured with respect to the off-line, which has several components from tracking, particle identification, event selection, and overall event size cuts. These have been evaluated in a data-driven manner whenever possible. Only the event selection effi-ciencies have been simulated. Samples of simulated events, produced with the software described in Refs.[32–34], are used to characterize signal and background contributions. The particle identification efficiencies are determined from calibration samples ofDþ→ πþD0,D0→ K−πþ decays for kaons and pions, andΛ → pπ−for protons. The trigger efficiencies including the muon identification efficiency are determined using samples of b → J=ψX, J=ψ → μþμ− decays, where one muon is identified and the other used to measure the efficiencies. For the overall sample they are typically 20% for the 7 TeV data and 70% for the 13 TeV data, only weakly dependent onη. The difference is caused primarily by the impact parameter cut on the muon of 0.5 mm in the 7 TeV data. The efficiency for the overall event size requirement is determined usingB− → J=ψK− decays where much looser criteria were applied. These efficiencies are all above 95% and are determined with
negligible uncertainties. The total efficiencies given as a function of η and pT for both energies are shown in the Supplemental Material [13].
There is dwindling efficiency toward smallpT values of the charmed hadron plus muon. Data in the regions with negligible efficiency are excluded, and a correction is made using simulation to calculate the fraction of events that fall within inefficient regions. These numbers are calculated for each bin of η for 7 and 13 TeV data separately, and the averages are 38% at 7 TeV and 46% at 13 TeV. The pT distributions from simulation in each η bin have been checked and found to agree within error with those observed in the data in bins with sufficient statistics.
The signal yields are obtained from fits that subtract the uncorrelated backgrounds. There are, however, two back-ground sources that must be dealt with separately. One results from real charm hadron decays that form a vertex with a charged track that is misidentified as a muon and the other is fromb decays into two charmed hadrons where one decays either leptonically or semileptonically into a muon. In most cases the requirement that the muon forms a vertex with the charmed hadron eliminates this background, but some remains. The background from fake muons combined with a real charmed hadron, and a real muon combined with a charm hadron from another b decay as estimated from wrong-sign muon and hadron combinations is 0.7% at 7 TeV and 2.0% at 13 TeV. The fake rates caused by b decays to two charmed hadrons where one decays semi-leptonically have been evaluated from simulation and are about 2% when averaged over all charmed species.
The inclusiveb-hadron cross sections as functions of η are given in Fig. 2, along with a theoretical prediction called FONLL[35]. These results are consistent with and super-sede our previous results at 7 TeV[4]. The ratio of cross sections is predicted with less uncertainty, and indeed most of the experimental uncertainties (discussed below) also cancel, with the largest exception being the luminosity error. In Fig.2(c), we compare theη-dependent cross-section ratio for 13 TeV divided by 7 TeV with the FONLL prediction. We see higher ratios at lower values ofη than given by the prediction, which indicates that the cross section atη values near 2 is growing faster than at larger values.
The results as a function of η are listed in Table III. The total cross sections at 7 and 13 TeV integrated over 2 < η < 5 are 72.0 0.3 6.8 and 154.3 1.5 14.3 μb for 7 and 13 TeV. The ratio is 2.14 0.02 0.13. This agrees with the theoretical prediction at 7 TeV of62þ28−22 μb, and is a bit larger than the 13 TeV prediction of111þ51−44 μb. While the measured ratio is consistent with the prediction of1.79þ0.21−0.15, it disagrees with the combination of shape and normalization.
Systematic uncertainties are considerably larger than the statistical errors. The ones that are independent of η are listed in TableIV. The luminosity and muon trigger efficiency uncertainties in the ratio are each obtained by assuming a−50% correlated error[36]. The uncertainty in the tracking efficiency is given by taking 0.5% per muon track and 1.5% per hadron track [37]. The various final states used to simulate the efficiencies can contribute to an overall efficiency change. This is estimated by taking the [MeV] ) π m(KK 1900 1950 2000 Events / ( 1 MeV ) 0 500 1000 1500 2000 2500 3000 3500 LHCb 7 TeV (a) /mm) IP ln( -6 -4 -2 0 2 Events / ( 0.15 ) 0 2000 4000 6000 8000 10000 LHCb 7 TeV (b) [MeV] ) π m(KK 1900 1950 2000 Events / ( 1 MeV ) 1 10 2 10 3 10 4 10 LHCb 7 TeV (c) /mm) IP ln( -6 -4 -2 0 2 Events / ( 0.15 ) 1 10 2 10 3 10 4 10 LHCb 7 TeV (d)
FIG. 1. Fits to theKþK−πþinvariant mass (a) and lnðIP=mmÞ (b) distributions for data taken at 7 TeV data integrated over 2 < η < 5. The data are shown as solid circles (black), and the overall fits as solid lines (blue). The dot-dashed (green) curve shows theDþs signal fromb decay, while the dashed (purple) curve Dþs from prompt production. The dotted curve (orange) shows theDþcomponent. The dashed line (red) shows the combinatorial background. The same fits using a logarithmic scale are shown in (c) and (d).
difference between the efficiencies of the higher multi-plicityDμ−ν states and Dμ−ν states, where Drefers to excited states that decay into a charmed particle and pions, and taking into account the uncertainties on the measured branching fractions. These are then added in quadrature and referred to as the b decay cocktail in Table IV.
The fraction of higher massb-baryon states with respect to theΛ0b is given byδ ¼ 0.25 0.10, which represents a 40% relative uncertainty that affects only the baryon contribution to Eq. (3).
There are also η-dependent systematic uncertainties in the cross section that arise from the trigger efficiency, the event selection, the hadron identification, and the corrections for the low pT region with low efficiencies. When added in quadrature with the η-independent uncer-tainties, the total errors range from (8.5–11.0)% at 7 TeV to
(8.7–-9.7)% at 13 TeV. There is some cancellation in the ratio giving a range of (5.6–7.3)%.
In conclusion, new results for the bb production cross section at 7 TeV are in good agreement with the original η-dependent cross-section measurement previously reported
[4], and are in agreement with the theoretical prediction (FONLL)[35]. The 13 TeV results are somewhat higher in magnitude than the theory, and generally agree with the shape and magnitude measured using inclusiveb → J=ψX decays [36]. The cross-section ratio of 13 to 7 TeV as a function ofη differs from the FONLL model by 5 standard deviations, including the systematic uncertainties. This discrepancy is mainly the difference in the low η bins. To get an idea of the cross section in the fullη range we use η 2 3 4 5 b]μ [η d X)/b H → (ppσ d 0 5 10 15 20 25 30 35 40 45 50 (a) FONLL Data 2 3 LHCb 7 TeV η 2 3 4 5 b]μ [η d X)/b H → (ppσ d 0 10 20 30 40 50 60 70 80 90 (b) FONLL Data 2 3 LHCb 13 TeV η 2 3 4 )η d X)/b H → (ppσ d( 13/7 R 0 0.5 1 1.5 2 2.5 3 3.5 4 (c) FONLL Data 7 TeV 13 TeV LHCb 5
FIG. 2. The differential cross section as a function ofη for σðpp → HbXÞ, where Hbis a hadron that contains either ab or a ¯b quark, but not both, at center-of-mass energies of 7 TeV (a) and 13 TeV (b). The ratio is shown in (c). The smaller error bars (black) show the statistical uncertainties only, and the larger ones (blue) have the systematic uncertainties added in quadrature. The solid line (red) gives the theoretical prediction, while the solid shaded band gives the estimated uncertainty on the predictions at1σ, the cross-hatched at 2σ, and the dashes at 3σ.
TABLE III. pp → HbX differential cross sections as a function of η for 7 and 13 TeV collisions and their ratio. The first uncertainty is statistical and the second systematic. To get the cross section in each interval divide by a factor of 2.
η 7 TeV (μb) 13 TeV (μb) Ratio13=7
2.0–2.5 27.2 0.5 3.0 68.6 2.4 6.7 2.53 0.10 0.18 2.5–3.0 29.9 0.2 2.8 63.4 0.9 6.2 2.12 0.03 0.13 3.0–3.5 29.8 0.2 2.7 58.3 1.0 5.3 1.96 0.04 0.11 3.5–4.0 25.8 0.2 2.2 51.9 0.7 4.7 2.01 0.03 0.11 4.0–4.5 18.9 0.1 1.6 39.3 0.6 3.6 2.08 0.04 0.12 4.5–5.0 12.5 0.1 1.3 27.2 0.7 2.6 2.17 0.06 0.16
TABLE IV. Systematic uncertainties independent ofη on the pp → HbX cross sections at 7 and 13 TeV and their ratio.
Source 7 TeV 13 TeV Ratio13=7
Luminosity 1.7% 3.9% 3.8%
Tracking efficiency 3.8% 4.3% 2.5%
b semileptonic B 2.1% 2.1% 0
Charm hadronB 2.6% 2.6% 0
b decay cocktail 1.0% 1.0% 0
Ignoringb cross feeds 1.0% 1.0% 0
Background 0.2% 0.3% 0
b → u decays 0.3% 0.3% 0
δ 2.0% 2.0% 0.2%
Total 5.9% 7.1% 4.6%
multiplicative factors derived from Pythia 8 simulations of 4.1 at 7 TeV and 3.9 at 13 TeV[33,34]and extrapolate the totalbb cross sections as ≈ 295 μb at 7 TeV and ≈ 600 μb at 13 TeV.
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); NSFC (China); CNRS/IN2P3 (France); BMBF, DFG and MPG (Germany); INFN (Italy); FOM and 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 (USA). We acknowledge the computing resources that are provided by CERN, IN2P3 (France), KIT 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 (USA). 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 Skłodowska-Curie Actions and ERC (European Union), Conseil Général de Haute-Savoie, Labex ENIGMASS and OCEVU, Région 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).
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A. Zhokhov,32 X. Zhu,3 V. Zhukov,9and S. Zucchelli15 (LHCb Collaboration)
1
Centro Brasileiro de Pesquisas Físicas (CBPF), Rio de Janeiro, Brazil
2Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil 3
Center for High Energy Physics, Tsinghua University, Beijing, China
4LAPP, Université Savoie Mont-Blanc, CNRS/IN2P3, Annecy-Le-Vieux, France 5
Clermont Université, Université Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France
6CPPM, Aix-Marseille Université, CNRS/IN2P3, Marseille, France 7
LAL, Université Paris-Sud, CNRS/IN2P3, Orsay, France
8
LPNHE, Université Pierre et Marie Curie, Université Paris Diderot, CNRS/IN2P3, Paris, France
9
I. Physikalisches Institut, RWTH Aachen University, Aachen, Germany
10
Fakultät Physik, Technische Universität Dortmund, Dortmund, Germany
11Max-Planck-Institut für Kernphysik (MPIK), Heidelberg, Germany 12
Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany
13School of Physics, University College Dublin, Dublin, Ireland 14
Sezione INFN di Bari, Bari, Italy
15Sezione INFN di Bologna, Bologna, Italy 16
Sezione INFN di Cagliari, Cagliari, Italy
17Sezione INFN di Ferrara, Ferrara, Italy 18
Sezione INFN di Firenze, Firenze, Italy
19Laboratori Nazionali dell’INFN di Frascati, Frascati, Italy 20
Sezione INFN di Genova, Genova, Italy
21Sezione INFN di Milano Bicocca, Milano, Italy 22
Sezione INFN di Milano, Milano, Italy
23Sezione INFN di Padova, Padova, Italy 24
Sezione INFN di Pisa, Pisa, Italy
25Sezione INFN di Roma Tor Vergata, Roma, Italy 26
Sezione INFN di Roma La Sapienza, Roma, Italy
27Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Kraków, Poland 28
AGH - University of Science and Technology, Faculty of Physics and Applied Computer Science, Kraków, Poland
29National Center for Nuclear Research (NCBJ), Warsaw, Poland 30
Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania
31Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia 32
Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia
33Institute 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
35Yandex School of Data Analysis, Moscow, Russia 36
Budker Institute of Nuclear Physics (SB RAS), Novosibirsk, Russia
37Institute for High Energy Physics (IHEP), Protvino, Russia 38
ICCUB, Universitat de Barcelona, Barcelona, Spain
39Universidad de Santiago de Compostela, Santiago de Compostela, Spain 40
European Organization for Nuclear Research (CERN), Geneva, Switzerland
41Institute of Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland 42
Physik-Institut, Universität Zürich, Zürich, Switzerland
43Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands 44
Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, The Netherlands
45NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine 46
Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine
47University of Birmingham, Birmingham, United Kingdom 48
H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom
49Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom 50
Department of Physics, University of Warwick, Coventry, United Kingdom
51STFC Rutherford Appleton Laboratory, Didcot, United Kingdom 52
School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom
53School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom 54
Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom
55Imperial College London, London, United Kingdom 56
School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom
57Department of Physics, University of Oxford, Oxford, United Kingdom 58
Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
59University of Cincinnati, Cincinnati, Ohio, USA 60
University of Maryland, College Park, Maryland, USA
61Syracuse University, Syracuse, New York, USA 62
Pontifícia Universidade Católica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil, associated to Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil
63
University of Chinese Academy of Sciences, Beijing, China,
associated to Center for High Energy Physics, Tsinghua University, Beijing, China
64
Institute of Particle Physics, Central China Normal University, Wuhan, Hubei, China, associated to Center for High Energy Physics, Tsinghua University, Beijing, China
65
Departamento de Fisica, Universidad Nacional de Colombia, Bogota, Colombia,
associated to LPNHE, Université Pierre et Marie Curie, Université Paris Diderot, CNRS/IN2P3, Paris, France
66Institut für Physik, Universität Rostock, Rostock, Germany,
associated to Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany
67National Research Centre Kurchatov Institute, Moscow, Russia,
associated to Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia
68Instituto de Fisica Corpuscular (IFIC), Universitat de Valencia-CSIC, Valencia, Spain,
associated to ICCUB, Universitat de Barcelona, Barcelona, Spain
69Van Swinderen Institute, University of Groningen, Groningen, The Netherlands,
associated to Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands †Deceased.
a
Universidade Federal do Triângulo Mineiro (UFTM), Uberaba-MG, Brazil. bLaboratoire Leprince-Ringuet, Palaiseau, France.
c
P.N. Lebedev Physical Institute, Russian Academy of Science (LPI RAS), Moscow, Russia. dUniversità di Bari, Bari, Italy.
e
Università di Bologna, Bologna, Italy. fUniversità di Cagliari, Cagliari, Italy. g
Università di Ferrara, Ferrara, Italy. hUniversità di Genova, Genova, Italy.
I
Università di Milano Bicocca, Milano, Italy. jUniversità di Roma Tor Vergata, Roma, Italy. k
Università di Roma La Sapienza, Roma, Italy.
lAGH - University of Science and Technology, Faculty of Computer Science, Electronics and Telecommunications, Kraków, Poland. m
LIFAELS, La Salle, Universitat Ramon Llull, Barcelona, Spain. nHanoi University of Science, Hanoi, Vietnam.
o
Università di Padova, Padova, Italy. pUniversità di Pisa, Pisa, Italy. q
Università degli Studi di Milano, Milano, Italy. rUniversità di Urbino, Urbino, Italy.
s
Università della Basilicata, Potenza, Italy. tScuola Normale Superiore, Pisa, Italy. u
Università di Modena e Reggio Emilia, Modena, Italy. vIligan Institute of Technology (IIT), Iligan, Philippines. w
Novosibirsk State University, Novosibirsk, Russia.
