EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)
CERN-EP-2017-304 LHCb-PAPER-2017-046 December 8, 2017
Updated determination of D
0
–D
0
mixing and CP violation parameters
with D
0
→ K
+
π
−
decays
LHCb collaboration†
Abstract
We report measurements of charm-mixing parameters based on the decay-time-dependent ratio of D0 → K+π−to D0→ K−π+rates. The analysis uses a data
sam-ple of proton-proton collisions corresponding to an integrated luminosity of 5.0 fb−1 recorded by the LHCb experiment from 2011 through 2016. Assuming charge-parity (CP ) symmetry, the mixing parameters are determined to be x02 = (3.9±2.7)×10−5, y0 = (5.28 ± 0.52) × 10−3, and RD = (3.454 ± 0.031) × 10−3. Without this
as-sumption, the measurement is performed separately for D0 and D0 mesons, yielding a direct CP -violating asymmetry AD = (−0.1 ± 9.1) × 10−3, and magnitude of the
ratio of mixing parameters 1.00 < |q/p| < 1.35 at the 68.3% confidence level. All results include statistical and systematic uncertainties and improve significantly upon previous single-measurement determinations. No evidence for CP violation in charm mixing is observed.
Published in Phys. Rev. D 97 (2018) 031101.
c
CERN on behalf of the LHCb collaboration, licence CC-BY-4.0.
†Authors are listed at the end of this paper.
1
Introduction
The mass eigenstates of neutral charm mesons are linear combinations of the flavor eigenstates, |D1,2i = p|D0i ± q|D0i, where p and q are complex-valued coefficients. This results in D0–D0 oscillations. In the limit of charge-parity (CP ) symmetry, oscillations are characterized by the dimensionless differences in mass, x ≡ ∆m/Γ ≡ (m2− m1)/Γ, and decay width, y ≡ ∆Γ/2Γ ≡ (Γ2 − Γ1)/2Γ, between the CP -even (D2) and CP -odd (D1) mass eigenstates, where Γ is the average decay width of neutral D mesons. If CP symmetry does not hold, the oscillation probabilities for mesons produced as D0 and D0 can differ, further enriching the phenomenology. Long- and short-distance amplitudes govern the oscillations of neutral D mesons [1–3]. Long-distance amplitudes depend on the exchange of low-energy gluons and are challenging to calculate. Short-distance amplitudes may include contributions from a broad class of particles not described in the standard model, which might affect the oscillation rate or introduce a difference between the D0 and D0 meson decay rates. The study of CP violation in D0 oscillations therefore offers sensitivity to non-standard-model phenomena [4–7].
The first evidence for D0–D0 oscillations was reported in 2007 [8, 9]. More recently, precise results from the LHCb collaboration [10–15] improved the knowledge of the mixing parameters, x = (4.6+1.4−1.5) × 10−3 and y = (6.2 ± 0.8) × 10−3 [16], although neither a nonzero value for the mass difference nor a departure from CP symmetry have been established. This paper reports measurements of CP -averaged and CP -violating mixing parameters in D0–D0 oscillations based on the comparison of the decay-time-dependent ratio of D0 → K+π− to D0 → K−π+ rates with the corresponding ratio for the charge-conjugate processes. The analysis uses data corresponding to an integrated luminosity of 5.0 fb−1 from proton-proton (pp) collisions at 7, 8, and 13 TeV center-of-mass energies, recorded with the LHCb experiment from 2011 through 2016. This analysis improves upon a previous measurement [12], owing to the tripling of the sample size and an improved treatment of systematic uncertainties. The inclusion of charge-conjugate processes is implicitly assumed unless stated otherwise.
The neutral D-meson flavor at production is determined from the charge of the low-momentum pion (soft pion), πs+, produced in the flavor-conserving strong-interaction decay D∗(2010)+→ D0π+
s . The shorthand notation D
∗+ is used to indicate the D∗(2010)+ meson throughout. We denote as right-sign (RS) the D∗+ → D0(→ K−π+)π+
s process, which is dominated by a Cabibbo-favored amplitude. Wrong-sign (WS) decays, D∗+ → D0(→ K+π−)π+
s , arise from the doubly Cabibbo-suppressed D0 → K+π
− decay and the Cabibbo-favored D0 → K+π− decay that follows D0–D0 oscillation. Since the mixing parameters are small, |x|, |y| 1, the CP -averaged decay-time-dependent ratio of WS-to-RS rates is approximated as [1–4] R(t) ≈ RD + p RD y0 t τ + x02+ y02 4 t τ 2 , (1)
where t is the proper decay time, τ is the average D0 lifetime, and R
D is the ratio of suppressed-to-favored decay rates. The parameters x0 and y0 depend on the mixing parameters, x0 ≡ x cos δ + y sin δ and y0 ≡ y cos δ − x sin δ, through the strong-phase difference δ between the suppressed and favored amplitudes, A(D0 → K+π−)/A(D0 → K+π−) = −√RDe−iδ, which was measured at the CLEO-c and BESIII experiments [17,18]. If CP violation occurs, the decay-rate ratios R+(t) and R−(t) of mesons produced as D0
and D0, respectively, are functions of independent sets of mixing parameters, R± D, (x
0±)2, and y0±. The parameters R+D and R−D differ if the ratio between the suppressed and favored decay amplitudes is not CP symmetric, indicating direct CP violation. Violation of CP symmetry either in mixing, |q/p| 6= 1, or in the interference between mixing and decay amplitudes, φ ≡ argqA(D0 → K+π−)/pA(D0 → K+π−) 6= δ, are referred to as manifestations of indirect CP violation and generate differences between ((x0+)2, y0+) and ((x0−)2, y0−).
Experimental effects such as differing efficiencies for reconstructing WS and RS decays may bias the observed ratios of signal decays and, therefore, the mixing-parameter results. We assume that the efficiency for reconstructing and selecting the K∓π±π+s final state approximates as the product of the efficiency for the K∓π± pair from the D0 decay and the efficiency for the soft pion. The observed WS-to-RS yield ratio then equals R(t) multiplied by the ratio of the efficiencies for reconstructing K+π− and K−π+ pairs, which is the only relevant instrumental nuisance. The asymmetry in production rates between D∗+ and D∗− mesons in the LHCb acceptance and asymmetries in detecting soft pions of different charges cancel in the WS-to-RS ratio.
Candidate D∗+ mesons produced directly in the collision (primary D∗+) are recon-structed while suppressing background contributions from charm mesons produced in the decay of bottom hadrons (secondary D∗+) and misreconstructed decays. Residual con-taminations from such backgrounds are measured using control regions. The asymmetry in K±π∓ reconstruction efficiency is estimated using control samples of charged D-meson decays. The yields of RS and WS primary D∗+ candidates are determined, separately for each flavor, in intervals (bins) of decay time by fitting the D∗+ mass distribution of candidates consistent with being D0 decays. We fit the resulting WS-to-RS yield ratios as a function of decay time to measure the mixing and CP -violation parameters, including the effects of instrumental asymmetries, residual background contamination, and all considered systematic contributions. To ensure unbiased results, the differences in the decay-time dependence of the WS D0 and D0 samples are not examined until the analysis procedure is finalized.
2
The LHCb detector
The LHCb detector [19] 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 achieves high precision charged-particle tracking using 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 layers of silicon-strip detectors and straw drift tubes placed downstream of the magnet. The tracking system provides a measurement of charged-particle momentum p with a relative uncertainty varying from 0.5% at low momentum to 1.0% at 200 GeV/c. The typical decay-time resolution for D0 → K+π− decays is 10% of the D0 lifetime. The polarity of the dipole magnet is reversed periodically throughout data-taking. The minimum distance of a charged-particle trajectory (track) to a proton-proton interaction space-point (primary vertex), the impact parameter, is measured with (15 + 29/pT) µm resolution, where pT is the component of the momentum transverse to the beam, in GeV/c. Charged hadrons are identified using two ring-imaging Cherenkov detectors. Photons, electrons and hadrons
are identified by scintillating-pad and preshower detectors, and an electromagnetic and a hadronic calorimeter. Muons are identified by alternating layers of iron and multiwire proportional chambers. The online event selection is performed by a hardware trigger, based on information from the calorimeter and muon detectors, followed by a software trigger, based on information on displaced charged particles reconstructed in the event. Offline-like quality detector alignment and calibrations, performed between the hardware and software stages, are available to the software trigger for the 2015 and 2016 data [20,21]. Hence, for these data the analysis uses candidates reconstructed in the software trigger to reduce event size.
3
Event selection and candidate reconstruction
Events enriched in D∗+ candidates originating from the primary vertex are selected by the hardware trigger by imposing that either one or more D0 decay products are consistent with depositing a large transverse energy in the calorimeter or that an accept decision is taken independently of the D0 decay products and soft pion. In the software trigger, one or more D0 decay products are required to be inconsistent with charged particles originating from the primary vertex and, for 2015 and 2016 data, loose particle-identification criteria are imposed on these final-state particles. Each D0 candidate is then combined with a low-momentum positive-charge particle originating from the primary vertex to form a D∗+ candidate.
In the offline analysis, criteria on track and primary-vertex quality are imposed. To suppress the contamination from misidentified two-body D0 decays, the pion and kaon candidates from the D0 decay are subjected to stringent particle-identification criteria. An especially harmful background is generated by a 3% contribution of soft pions misreconstructed by combining their track segments in the vertex detector with unrelated segments in the downstream tracking detectors. The track segments in the vertex detector are genuine, resulting in properly measured opening angles in the D∗+ → D0π+
s decay. Since the opening angle dominates over the π+
s momentum in the determination of the D∗+ mass, such spurious soft pions tend to produce a signal-like peak in the D∗+ mass spectrum. In addition, they bias the WS-to-RS ratio because the mistaken association with downstream track-segments is prone to charge mismeasurements. We suppress such candidates with stringent requirements on a dedicated discriminant based on many low-level variables associated with track reconstruction [22]. Candidates consistent with the D∗+ decay topology are reconstructed by computing the two-body mass M (D0π+
s ) using the known D0 and π+ masses [23] and the reconstructed momenta [24]. The mass resolution is improved by nearly a factor of two with a kinematic fit that constrains the D∗+ candidate to originate from a primary vertex [25]. If multiple primary vertices are reconstructed, the vertex resulting from the fit with the best χ2 probability is chosen. The sample is further enriched in primary charm decays by restricting the impact-parameter chi-squared, χ2
IP, of the D0 and πs+ candidates such that the candidates point to the primary vertex. The χ2IP variable is the difference between the χ2 of the primary-vertex fit reconstructed including or excluding the considered particle, and offers a measure of consistency with the hypothesis that the particle originates from the primary vertex. Only opposite-charge particle pairs with K∓π± mass within 24 MeV/c2 (equivalent to approximately three times the mass resolution) of the known D0 mass [23] and K+K−
]
2c
) [MeV/
+π
0D
(
M
2005 2010 2015 2020 2c
Candidates per 0.1 MeV/
0 5 10 15 20 25 6 10 × LHCb (a) Data Fit Background
]
2c
) [MeV/
+π
0D
(
M
2005 2010 2015 2020 2c
Candidates per 0.1 MeV/
0 20 40 60 80 100 120 140 160 180 200 220 240 3 10 × LHCb (b) Data Fit Background
Figure 1: Distribution of M (D0πs+) for selected (a) right-sign D0 → K−π+ and (b) wrong-sign D0 → K+π− candidates.
and π+π− masses more than 40 MeV/c2 away from the D0 mass are retained. Accidental combinations of a genuine D0 with a random soft pion are first suppressed by removing the 13% of events where more than one D∗+ candidate is reconstructed. We then use an artificial neural-network discriminant that exploits the πs+ pseudorapidity, transverse momentum, and particle-identification information, along with the track multiplicity of the event. The discriminant is trained on an independent RS sample to represent the WS signal features and on WS events containing multiple candidates to represent background. Finally, we remove from the WS sample events where the same D0 candidate is also used to reconstruct a RS decay, which reduces the background by 16% with no significant loss of signal.
4
Yield determination
The RS and WS signal yields are determined by fitting the M (D0π+
s ) distribution of D0 signal candidates. The decay-time-integrated M (D0πs+) distributions of the selected RS and WS candidates are shown in Fig. 1. The smooth background is dominated by favored D0 → K−π+ and D0 → K+π− decays associated with random soft-pion candidates. The sample contains approximately 1.77×108 RS and 7.22×105 WS signal decays. Each sample is divided into 13 subsamples according to the decay time, and signal yields are determined for each subsample using an empirical shape [11]. We assume that the signal shapes are common to WS and RS decays for a given D∗ meson flavor whereas the descriptions of the backgrounds are independent. The decay-time-dependent WS-to-RS rate ratios R+ and R− observed in the D0 and D0 samples, respectively, and their difference, are shown in Fig. 2. The ratios and difference include corrections for the relative efficiencies for reconstructing K−π+ and K+π− final states.
0
2
4
6
20
τ
/
t
0.2
−
0
0.2
]
3 −[10
−R
−
+R
(c)
0
2
4
6
8
10
4
5
6
]
3 −[10
−R
CPV allowed
No direct CPV
No CPV
(b)
0
2
4
6
8
10
4
5
6
]
3 −[10
+R
LHCb
(a)
Figure 2: Efficiency-corrected ratios of WS-to-RS yields for (a) D∗+ decays, (b) D∗− decays, and (c) their differences as functions of decay time in units of D0 lifetime. Projections of fits allowing for (dashed line) no CP violation, (dotted line) no direct CP violation, and (solid line) direct and indirect CP violation are overlaid. The last two curves overlap. The abscissa of each data point corresponds to the average decay time over the bin. The error bars indicate the statistical uncertainties.
5
Determination of oscillation parameters
The mixing parameters are determined by minimizing a χ2 function that includes terms for the difference between the observed and predicted ratios and for systematic effects,
χ2 =X i ri+− + rRe+i σi+ !2 + r − i − − rRe−i σ−i !2 + χ 2 corr. (2)
The observed WS-to-RS yield ratio and its statistical uncertainty in the decay-time bin i are denoted by r±i and σ±i , respectively. The associated predicted value eR±i corresponds to the decay-time integral over bin i of Eq. (1), including bin-specific corrections. The parameters associated with these corrections are determined separately for data collected in different LHC and detector configurations and vary independently in the fit within their constraint χ2corr in Eq. (2). Such corrections account for small biases due to (i) the decay-time evolution of the 1%–10% fraction of signal candidates originating from b-hadron decays, (ii) the approximately 0.3% component of the background from misreconstructed charm decays that peak in the signal region, and (iii) the effect of instrumental asymmetries in the K±π∓ reconstruction efficiencies. The secondary-D∗+ fraction is determined by fitting, in
each decay-time bin, the χ2
IP distribution of RS D0 signal decays. The peaking background, dominated by D0 → K−π+ decays in which both final-state particles are misidentified, is determined by extrapolating into the D0 signal mass region the contributions from misreconstructed charm decays identified by reconstructing the two-body mass under various mass hypotheses for the decay products. The relative efficiency ±r accounts for the effects of instrumental asymmetries in the K±π∓ reconstruction efficiencies, mainly caused by K− mesons having a larger nuclear interaction cross-section with matter than K+ mesons. These asymmetries are measured in data to be typically 0.01 with 0.001 precision, independent of decay time. They are derived from the efficiency ratio +r = 1/−r = (K+π−)/(K−π+), obtained by comparing the ratio of D− → K+π−
π− and D− → K0
S(→ π
+π−)π− yields with the ratio of the corresponding charge-conjugate decay yields. The asymmetry between D+ and D− production rates [26] cancels in this ratio, provided that the kinematic distributions are consistent across samples. We therefore weight the D− → K+π−π− candidates so that their kinematic distributions match those in the D− → K0
Sπ
− sample. We then determine ±
r as functions of kaon momentum to account for the known momentum-dependence of the asymmetry between K+ and K− interaction rates with matter. In addition, a systematic uncertainty for possible residual contamination from spurious soft pions is included through a 1.05–1.35 scaling of the overall uncertainties. The scaling value is chosen such that a fit with a constant function of the time-integrated WS-to-RS ratio versus false-pion probability has unit reduced χ2. The observed WS-to-RS yield ratios for the D0 and D0 samples are studied first with bin-by-bin arbitrary offsets designed to mimic the effect of significantly different mixing parameters in the two samples. To search for residual systematic uncertainties, the analysis is repeated on statistically independent data subsets chosen according to criteria likely to reveal biases from specific instrumental effects. These criteria include the data-taking year (2011–2012 or 2015–2016), the magnet field orientation, the number of primary vertices in the event, the candidate multiplicity per event, the trigger category, the D0 momentum and χ2IP with respect to the primary vertex, and the per-candidate probability to reconstruct a spurious soft pion. The resulting variations of the measured CP -averaged and CP -violating parameters are consistent with statistical fluctuations, with p-values distributed uniformly in the 4%–85% range.
6
Results
The efficiency-corrected WS-to-RS yield ratios are subjected to three fits. The first fit allows for direct and indirect CP violation; the second allows only for indirect CP violation by imposing R+D = R−D; and the third is a fit under the CP -conservation hypothesis, in which all mixing parameters are common to the D0 and D0 samples. The fit results and their projections are presented in Table 1 and Fig. 2, respectively. Figure 3 shows the central values and confidence regions in the (x02, y0) plane. For each fit, 208 WS-to-RS ratio data points are used, corresponding to 13 ranges of decay time; distinguishing D∗+ from D∗− decays; two magnetic-field orientations; and 2011, 2012, 2015, and 2016 data sets. The consistency of the data with the hypothesis of CP symmetry is determined from the change in χ2 probability between the fit that assumes CP conservation and the fit in which CP violation is allowed. The resulting p-value is 0.57 (0.37) for the fit in which both direct and indirect (indirect only) CP violation is allowed, showing that the data are
compatible with CP symmetry.
The fit uncertainties incorporate both statistical and systematic contributions. The statistical uncertainty, determined in a separate fit by fixing all nuisance parameters to their central values, dominates the total uncertainty. The systematic component is obtained by subtraction in quadrature. The leading systematic uncertainty is due to residual secondary-D∗+ contamination and does not exceed half of the statistical uncertainty. The second largest contribution is due to spurious soft pions. Smaller effects are due to peaking backgrounds for the CP -averaged results, and uncertainties in detector asymmetries for the CP -violating results. All reported results, p-values, and the contours shown in Fig. 3, include total uncertainties.
Direct CP violation would produce a nonzero intercept at t = 0 in the efficiency-corrected difference of WS-to-RS yield ratios between D0and D0mesons shown in Fig. 2 (c). We parametrize this effect with the asymmetry measured in the fit that allows for direct CP violation, AD ≡ (R+D− R − D)/(R + D + R − D) = (−0.1 ± 8.1 ± 4.2) × 10
−3, where the first uncertainty is statistical and the second systematic. Indirect CP violation would result in a time dependence of the efficiency-corrected difference of yield ratios, which is not observed in Fig. 2 (c). From the results of the fit allowing for direct and indirect CP violation, a likelihood for |q/p| is constructed using the relations x0± = |q/p|±1(x0cos φ ± y0sin φ) and y0± = |q/p|±1(y0cos φ ∓ x0sin φ). Confidence intervals are derived with a likelihood-ratio ordering [27], assuming that the parameter correlations are independent of the true values
Table 1: Results of fits for different CP -violation hypotheses. The first contribution to the uncertainties is statistical and the second systematic. Correlations include both statistical and systematic contributions.
Results [10−3] Correlations
Direct and indirect CP violation
Parameter Value R+D y0+ (x0+)2 R− D y 0− (x0−)2 R+D 3.454 ± 0.040 ± 0.020 1.000 −0.935 0.843 −0.012 −0.003 0.002 y0+ 5.01 ± 0.64 ± 0.38 1.000 −0.963 −0.003 0.004 −0.003 (x0+)2 0.061 ± 0.032 ± 0.019 1.000 0.002 −0.003 0.003 R−D 3.454 ± 0.040 ± 0.020 1.000 −0.935 0.846 y0− 5.54 ± 0.64 ± 0.38 1.000 −0.964 (x0−)2 0.016 ± 0.033 ± 0.020 1.000 No direct CP violation Parameter Value RD y0+ (x0+)2 y0− (x0−)2 RD 3.454 ± 0.028 ± 0.014 1.000 −0.883 0.745 −0.883 0.749 y0+ 5.01 ± 0.48 ± 0.29 1.000 −0.944 0.758 −0.644 (x0+)2 0.061 ± 0.026 ± 0.016 1.000 −0.642 0.545 y0− 5.54 ± 0.48 ± 0.29 1.000 −0.946 (x0−)2 0.016 ± 0.026 ± 0.016 1.000 No CP violation Parameter Value RD y0 x02 RD 3.454 ± 0.028 ± 0.014 1.000 −0.942 0.850 y0 5.28 ± 0.45 ± 0.27 1.000 −0.963 x02 0.039 ± 0.023 ± 0.014 1.000
0.1 − 0 0.1 3 4 5 6 7 8
]
3 −[10
y'
LHCb (a) CPV allowed 68.3% CL 0 D 68.3% CL 0 D 0.1 − 0 0.1]
3 −[10
2x'
3 4 5 6 7 8 No direct CPV (b) 68.3% CL 0 D 68.3% CL 0 D 0.1 − 0 0.1 3 4 5 6 7 8 No CPV (c) 99.7% CL 95.5% CL 68.3% CLFigure 3: Two-dimensional confidence regions in the (x02, y0) plane obtained (a) without any restriction on CP violation, (b) assuming no direct CP violation, and (c) assuming CP conserva-tion. The dashed (solid) curves in (a) and (b) indicate the contours of the mixing parameters associated with D0 (D0) decays. The best-fit value for D0 (D0) decays is shown with an open (filled) point. The solid, dashed, and dotted curves in (c) indicate the contours of CP -averaged mixing parameters at 68.3%, 95.5%, and 99.7% confidence levels (CL), respectively, and the point indicates the best-fit value.
of the mixing parameters. We determine 1.00 < |q/p| < 1.35 and 0.82 < |q/p| < 1.45 at the 68.3% and 95.5% confidence levels, respectively.
The RD result departs from the previous result based on a subset of the same data [12], which was biased by the then-undetected residual spurious-pion background. Since such background induces an apparent global shift toward higher WS-to-RS ratio values, the bias affects predominantly the RD measurement and less severely the mixing-parameter de-termination. The systematic uncertainties are significantly reduced because the dominant components are statistical in nature or sensitive to a generally improved understanding of the data quality.
7
Summary
We study D0–D0 oscillations using D∗+ → D0(→ K+π−)π+ decays reconstructed in a data sample of pp collisions collected by the LHCb experiment from 2011 through 2016, corresponding to an integrated luminosity of 5.0 fb−1. Assuming CP conservation, the mixing parameters are measured to be x02= (3.9 ± 2.7) × 10−5, y0 = (5.28 ± 0.52) × 10−3, and RD = (3.454 ± 0.031) × 10−3. The results are twice as precise as previous LHCb results [12] that were based on a subset of the present data, and supersede them. Studying D0 and D0 decays separately shows no evidence for CP violation and provides the current most stringent bounds on the parameters AD and |q/p| from a single measurement, AD = (−0.1 ± 9.1) × 10−3 and 1.00 < |q/p| < 1.35 at the 68.3% confidence level.
Acknowledgements
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 (The 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 (The 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 Sk lodowska-Curie Actions and ERC (European Union), ANR, Labex P2IO, ENIGMASS and OCEVU, and R´egion Auvergne-Rhˆone-Alpes (France), RFBR 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).
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LHCb collaboration
R. Aaij40, B. Adeva39, M. Adinolfi48, Z. Ajaltouni5, S. Akar59, J. Albrecht10, F. Alessio40, M. Alexander53, A. Alfonso Albero38, S. Ali43, G. Alkhazov31, P. Alvarez Cartelle55,
A.A. Alves Jr59, S. Amato2, S. Amerio23, Y. Amhis7, L. An3, L. Anderlini18, G. Andreassi41, M. Andreotti17,g, J.E. Andrews60, R.B. Appleby56, F. Archilli43, P. d’Argent12,
J. Arnau Romeu6, A. Artamonov37, M. Artuso61, E. Aslanides6, M. Atzeni42, G. Auriemma26, M. Baalouch5, I. Babuschkin56, S. Bachmann12, J.J. Back50, A. Badalov38,m, C. Baesso62, S. Baker55, V. Balagura7,b, W. Baldini17, A. Baranov35, R.J. Barlow56, C. Barschel40, S. Barsuk7, W. Barter56, F. Baryshnikov32, V. Batozskaya29, V. Battista41, A. Bay41, L. Beaucourt4, J. Beddow53, F. Bedeschi24, I. Bediaga1, A. Beiter61, L.J. Bel43, N. Beliy63, V. Bellee41, N. Belloli21,i, K. Belous37, I. Belyaev32,40, E. Ben-Haim8, G. Bencivenni19, S. Benson43, S. Beranek9, A. Berezhnoy33, R. Bernet42, D. Berninghoff12, E. Bertholet8, A. Bertolin23, C. Betancourt42, F. Betti15, M.O. Bettler40, M. van Beuzekom43, Ia. Bezshyiko42, S. Bifani47, P. Billoir8, A. Birnkraut10, A. Bizzeti18,u, M. Bjørn57, T. Blake50, F. Blanc41, S. Blusk61, V. Bocci26, T. Boettcher58, A. Bondar36,w, N. Bondar31, I. Bordyuzhin32, S. Borghi56,40, M. Borisyak35, M. Borsato39, F. Bossu7, M. Boubdir9, T.J.V. Bowcock54, E. Bowen42, C. Bozzi17,40, S. Braun12, J. Brodzicka27, D. Brundu16, E. Buchanan48, C. Burr56, A. Bursche16,f, J. Buytaert40, W. Byczynski40, S. Cadeddu16, H. Cai64, R. Calabrese17,g, R. Calladine47, M. Calvi21,i, M. Calvo Gomez38,m, A. Camboni38,m, P. Campana19, D.H. Campora Perez40, L. Capriotti56, A. Carbone15,e, G. Carboni25,j, R. Cardinale20,h, A. Cardini16, P. Carniti21,i, L. Carson52, K. Carvalho Akiba2, G. Casse54, L. Cassina21,
M. Cattaneo40, G. Cavallero20,40,h, R. Cenci24,t, D. Chamont7, M.G. Chapman48, M. Charles8, Ph. Charpentier40, G. Chatzikonstantinidis47, M. Chefdeville4, S. Chen16, S.F. Cheung57, S.-G. Chitic40, V. Chobanova39, M. Chrzaszcz42, A. Chubykin31, P. Ciambrone19,
X. Cid Vidal39, G. Ciezarek40, P.E.L. Clarke52, M. Clemencic40, H.V. Cliff49, J. Closier40, V. Coco40, J. Cogan6, E. Cogneras5, V. Cogoni16,f, L. Cojocariu30, P. Collins40, T. Colombo40, A. Comerma-Montells12, A. Contu16, G. Coombs40, S. Coquereau38, G. Corti40, M. Corvo17,g,
C.M. Costa Sobral50, B. Couturier40, G.A. Cowan52, D.C. Craik58, A. Crocombe50, M. Cruz Torres1, R. Currie52, C. D’Ambrosio40, F. Da Cunha Marinho2, C.L. Da Silva72, E. Dall’Occo43, J. Dalseno48, A. Davis3, O. De Aguiar Francisco40, K. De Bruyn40,
S. De Capua56, M. De Cian12, J.M. De Miranda1, L. De Paula2, M. De Serio14,d,
P. De Simone19, C.T. Dean53, D. Decamp4, L. Del Buono8, H.-P. Dembinski11, M. Demmer10, A. Dendek28, D. Derkach35, O. Deschamps5, F. Dettori54, B. Dey65, A. Di Canto40,
P. Di Nezza19, H. Dijkstra40, F. Dordei40, M. Dorigo40, A. Dosil Su´arez39, L. Douglas53, A. Dovbnya45, K. Dreimanis54, L. Dufour43, G. Dujany8, P. Durante40, J.M. Durham72, D. Dutta56, R. Dzhelyadin37, M. Dziewiecki12, A. Dziurda40, A. Dzyuba31, S. Easo51,
M. Ebert52, U. Egede55, V. Egorychev32, S. Eidelman36,w, S. Eisenhardt52, U. Eitschberger10, R. Ekelhof10, L. Eklund53, S. Ely61, S. Esen12, H.M. Evans49, T. Evans57, A. Falabella15, N. Farley47, S. Farry54, D. Fazzini21,i, L. Federici25, D. Ferguson52, G. Fernandez38, P. Fernandez Declara40, A. Fernandez Prieto39, F. Ferrari15, L. Ferreira Lopes41,
F. Ferreira Rodrigues2, M. Ferro-Luzzi40, S. Filippov34, R.A. Fini14, M. Fiorini17,g, M. Firlej28, C. Fitzpatrick41, T. Fiutowski28, F. Fleuret7,b, M. Fontana16,40, F. Fontanelli20,h, R. Forty40, V. Franco Lima54, M. Frank40, C. Frei40, J. Fu22,q, W. Funk40, E. Furfaro25,j, C. F¨arber40, E. Gabriel52, A. Gallas Torreira39, D. Galli15,e, S. Gallorini23, S. Gambetta52, M. Gandelman2, P. Gandini22, Y. Gao3, L.M. Garcia Martin70, J. Garc´ıa Pardi˜nas39, J. Garra Tico49,
L. Garrido38, D. Gascon38, C. Gaspar40, L. Gavardi10, G. Gazzoni5, D. Gerick12, E. Gersabeck56, M. Gersabeck56, T. Gershon50, Ph. Ghez4, S. Gian`ı41, V. Gibson49, O.G. Girard41, L. Giubega30, K. Gizdov52, V.V. Gligorov8, D. Golubkov32, A. Golutvin55, A. Gomes1,a, I.V. Gorelov33, C. Gotti21,i, E. Govorkova43, J.P. Grabowski12, R. Graciani Diaz38,
L.A. Granado Cardoso40, E. Graug´es38, E. Graverini42, G. Graziani18, A. Grecu30, R. Greim9, P. Griffith16, L. Grillo56, L. Gruber40, B.R. Gruberg Cazon57, O. Gr¨unberg67, E. Gushchin34, Yu. Guz37, T. Gys40, C. G¨obel62, T. Hadavizadeh57, C. Hadjivasiliou5, G. Haefeli41, C. Haen40, S.C. Haines49, B. Hamilton60, X. Han12, T.H. Hancock57, S. Hansmann-Menzemer12,
N. Harnew57, S.T. Harnew48, C. Hasse40, M. Hatch40, J. He63, M. Hecker55, K. Heinicke10, A. Heister9, K. Hennessy54, P. Henrard5, L. Henry70, E. van Herwijnen40, M. Heß67,
A. Hicheur2, D. Hill57, P.H. Hopchev41, W. Hu65, W. Huang63, Z.C. Huard59, W. Hulsbergen43, T. Humair55, M. Hushchyn35, D. Hutchcroft54, P. Ibis10, M. Idzik28, P. Ilten47, R. Jacobsson40, J. Jalocha57, E. Jans43, A. Jawahery60, F. Jiang3, M. John57, D. Johnson40, C.R. Jones49, C. Joram40, B. Jost40, N. Jurik57, S. Kandybei45, M. Karacson40, J.M. Kariuki48, S. Karodia53,
N. Kazeev35, M. Kecke12, F. Keizer49, M. Kelsey61, M. Kenzie49, T. Ketel44, E. Khairullin35, B. Khanji12, C. Khurewathanakul41, T. Kirn9, S. Klaver19, K. Klimaszewski29, T. Klimkovich11, S. Koliiev46, M. Kolpin12, I. Komarov41, R. Kopecna12, P. Koppenburg43, A. Kosmyntseva32,
S. Kotriakhova31, M. Kozeiha5, L. Kravchuk34, M. Kreps50, F. Kress55, P. Krokovny36,w, W. Krzemien29, W. Kucewicz27,l, M. Kucharczyk27, V. Kudryavtsev36,w, A.K. Kuonen41, T. Kvaratskheliya32,40, D. Lacarrere40, G. Lafferty56, A. Lai16, G. Lanfranchi19,
C. Langenbruch9, T. Latham50, C. Lazzeroni47, R. Le Gac6, A. Leflat33,40, J. Lefran¸cois7, R. Lef`evre5, F. Lemaitre40, E. Lemos Cid39, O. Leroy6, T. Lesiak27, B. Leverington12,
P.-R. Li63, T. Li3, Y. Li7, Z. Li61, X. Liang61, T. Likhomanenko68, R. Lindner40, F. Lionetto42,
V. Lisovskyi7, X. Liu3, D. Loh50, A. Loi16, I. Longstaff53, J.H. Lopes2, D. Lucchesi23,o,
M. Lucio Martinez39, H. Luo52, A. Lupato23, E. Luppi17,g, O. Lupton40, A. Lusiani24, X. Lyu63, F. Machefert7, F. Maciuc30, V. Macko41, P. Mackowiak10, S. Maddrell-Mander48, O. Maev31,40, K. Maguire56, D. Maisuzenko31, M.W. Majewski28, S. Malde57, B. Malecki27, A. Malinin68, T. Maltsev36,w, G. Manca16,f, G. Mancinelli6, D. Marangotto22,q, J. Maratas5,v,
J.F. Marchand4, U. Marconi15, C. Marin Benito38, M. Marinangeli41, P. Marino41, J. Marks12, G. Martellotti26, M. Martin6, M. Martinelli41, D. Martinez Santos39, F. Martinez Vidal70, A. Massafferri1, R. Matev40, A. Mathad50, Z. Mathe40, C. Matteuzzi21, A. Mauri42, E. Maurice7,b, B. Maurin41, A. Mazurov47, M. McCann55,40, A. McNab56, R. McNulty13, J.V. Mead54, B. Meadows59, C. Meaux6, F. Meier10, N. Meinert67, D. Melnychuk29, M. Merk43, A. Merli22,40,q, E. Michielin23, D.A. Milanes66, E. Millard50, M.-N. Minard4, L. Minzoni17, D.S. Mitzel12, A. Mogini8, J. Molina Rodriguez1, T. Momb¨acher10, I.A. Monroy66, S. Monteil5, M. Morandin23, M.J. Morello24,t, O. Morgunova68, J. Moron28, A.B. Morris52, R. Mountain61, F. Muheim52, M. Mulder43, D. M¨uller56, J. M¨uller10, K. M¨uller42, V. M¨uller10, P. Naik48, T. Nakada41, R. Nandakumar51, A. Nandi57, I. Nasteva2, M. Needham52, N. Neri22,40, S. Neubert12, N. Neufeld40, M. Neuner12, T.D. Nguyen41, C. Nguyen-Mau41,n, S. Nieswand9, R. Niet10, N. Nikitin33, T. Nikodem12, A. Nogay68, D.P. O’Hanlon50, A. Oblakowska-Mucha28, V. Obraztsov37, S. Ogilvy19, R. Oldeman16,f, C.J.G. Onderwater71, A. Ossowska27,
J.M. Otalora Goicochea2, P. Owen42, A. Oyanguren70, P.R. Pais41, A. Palano14, M. Palutan19,40, A. Papanestis51, M. Pappagallo52, L.L. Pappalardo17,g, W. Parker60, C. Parkes56,
G. Passaleva18,40, A. Pastore14,d, M. Patel55, C. Patrignani15,e, A. Pearce40, A. Pellegrino43, G. Penso26, M. Pepe Altarelli40, S. Perazzini40, D. Pereima32, P. Perret5, L. Pescatore41, K. Petridis48, A. Petrolini20,h, A. Petrov68, M. Petruzzo22,q, E. Picatoste Olloqui38,
B. Pietrzyk4, G. Pietrzyk41, M. Pikies27, D. Pinci26, F. Pisani40, A. Pistone20,h, A. Piucci12, V. Placinta30, S. Playfer52, M. Plo Casasus39, F. Polci8, M. Poli Lener19, A. Poluektov50, I. Polyakov61, E. Polycarpo2, G.J. Pomery48, S. Ponce40, A. Popov37, D. Popov11,40, S. Poslavskii37, C. Potterat2, E. Price48, J. Prisciandaro39, C. Prouve48, V. Pugatch46,
A. Puig Navarro42, H. Pullen57, G. Punzi24,p, W. Qian50, J. Qin63, R. Quagliani8, B. Quintana5,
B. Rachwal28, J.H. Rademacker48, M. Rama24, M. Ramos Pernas39, M.S. Rangel2, I. Raniuk45,†, F. Ratnikov35, G. Raven44, M. Ravonel Salzgeber40, M. Reboud4, F. Redi41, S. Reichert10, A.C. dos Reis1, C. Remon Alepuz70, V. Renaudin7, S. Ricciardi51, S. Richards48, M. Rihl40,
K. Rinnert54, P. Robbe7, A. Robert8, A.B. Rodrigues41, E. Rodrigues59,
J.A. Rodriguez Lopez66, A. Rogozhnikov35, S. Roiser40, A. Rollings57, V. Romanovskiy37, A. Romero Vidal39,40, M. Rotondo19, M.S. Rudolph61, T. Ruf40, P. Ruiz Valls70,
J. Ruiz Vidal70, J.J. Saborido Silva39, E. Sadykhov32, N. Sagidova31, B. Saitta16,f,
V. Salustino Guimaraes62, C. Sanchez Mayordomo70, B. Sanmartin Sedes39, R. Santacesaria26, C. Santamarina Rios39, M. Santimaria19, E. Santovetti25,j, G. Sarpis56, A. Sarti19,k,
C. Satriano26,s, A. Satta25, D.M. Saunders48, D. Savrina32,33, S. Schael9, M. Schellenberg10, M. Schiller53, H. Schindler40, M. Schmelling11, T. Schmelzer10, B. Schmidt40, O. Schneider41, A. Schopper40, H.F. Schreiner59, M. Schubiger41, M.H. Schune7, R. Schwemmer40, B. Sciascia19, A. Sciubba26,k, A. Semennikov32, E.S. Sepulveda8, A. Sergi47, N. Serra42, J. Serrano6,
L. Sestini23, P. Seyfert40, M. Shapkin37, I. Shapoval45, Y. Shcheglov31, T. Shears54,
L. Shekhtman36,w, V. Shevchenko68, B.G. Siddi17, R. Silva Coutinho42, L. Silva de Oliveira2, G. Simi23,o, S. Simone14,d, M. Sirendi49, N. Skidmore48, T. Skwarnicki61, I.T. Smith52,
J. Smith49, M. Smith55, l. Soares Lavra1, M.D. Sokoloff59, F.J.P. Soler53, B. Souza De Paula2, B. Spaan10, P. Spradlin53, S. Sridharan40, F. Stagni40, M. Stahl12, S. Stahl40, P. Stefko41, S. Stefkova55, O. Steinkamp42, S. Stemmle12, O. Stenyakin37, M. Stepanova31, H. Stevens10,
S. Stone61, B. Storaci42, S. Stracka24,p, M.E. Stramaglia41, M. Straticiuc30, U. Straumann42, J. Sun3, L. Sun64, K. Swientek28, V. Syropoulos44, T. Szumlak28, M. Szymanski63,
S. T’Jampens4, A. Tayduganov6, T. Tekampe10, G. Tellarini17,g, F. Teubert40, E. Thomas40,
J. van Tilburg43, M.J. Tilley55, V. Tisserand5, M. Tobin41, S. Tolk49, L. Tomassetti17,g, D. Tonelli24, R. Tourinho Jadallah Aoude1, E. Tournefier4, M. Traill53, M.T. Tran41, M. Tresch42, A. Trisovic49, A. Tsaregorodtsev6, P. Tsopelas43, A. Tully49, N. Tuning43,40, A. Ukleja29, A. Usachov7, A. Ustyuzhanin35, U. Uwer12, C. Vacca16,f, A. Vagner69, V. Vagnoni15,40, A. Valassi40, S. Valat40, G. Valenti15, R. Vazquez Gomez40,
P. Vazquez Regueiro39, S. Vecchi17, M. van Veghel43, J.J. Velthuis48, M. Veltri18,r, G. Veneziano57, A. Venkateswaran61, T.A. Verlage9, M. Vernet5, M. Veronesi43,
M. Vesterinen57, J.V. Viana Barbosa40, D. Vieira63, M. Vieites Diaz39, H. Viemann67, X. Vilasis-Cardona38,m, M. Vitti49, V. Volkov33, A. Vollhardt42, B. Voneki40, A. Vorobyev31, V. Vorobyev36,w, C. Voß9, J.A. de Vries43, C. V´azquez Sierra43, R. Waldi67, J. Walsh24, J. Wang61, Y. Wang65, D.R. Ward49, H.M. Wark54, N.K. Watson47, D. Websdale55, A. Weiden42, C. Weisser58, M. Whitehead40, J. Wicht50, G. Wilkinson57, M. Wilkinson61, M. Williams56, M. Williams58, T. Williams47, F.F. Wilson51,40, J. Wimberley60, M. Winn7, J. Wishahi10, W. Wislicki29, M. Witek27, G. Wormser7, S.A. Wotton49, K. Wyllie40, Y. Xie65, M. Xu65, Q. Xu63, Z. Xu3, Z. Xu4, Z. Yang3, Z. Yang60, Y. Yao61, H. Yin65, J. Yu65, X. Yuan61, O. Yushchenko37, K.A. Zarebski47, M. Zavertyaev11,c, L. Zhang3, Y. Zhang7, A. Zhelezov12, Y. Zheng63, X. Zhu3, V. Zhukov9,33, J.B. Zonneveld52, S. Zucchelli15.
1Centro Brasileiro de Pesquisas F´ısicas (CBPF), Rio de Janeiro, Brazil 2Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil 3Center for High Energy Physics, Tsinghua University, Beijing, China
4Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IN2P3-LAPP, Annecy, France 5Clermont Universit´e, Universit´e Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France 6Aix Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France
7LAL, Univ. Paris-Sud, CNRS/IN2P3, Universit´e Paris-Saclay, Orsay, France
8LPNHE, Universit´e Pierre et Marie Curie, Universit´e Paris Diderot, CNRS/IN2P3, Paris, France 9I. Physikalisches Institut, RWTH Aachen University, Aachen, Germany
10Fakult¨at Physik, Technische Universit¨at Dortmund, Dortmund, Germany 11Max-Planck-Institut f¨ur Kernphysik (MPIK), Heidelberg, Germany
12Physikalisches Institut, Ruprecht-Karls-Universit¨at Heidelberg, Heidelberg, Germany 13School of Physics, University College Dublin, Dublin, Ireland
14Sezione INFN di Bari, Bari, Italy 15Sezione INFN di Bologna, Bologna, Italy
16Sezione INFN di Cagliari, Cagliari, Italy 17Universita e INFN, Ferrara, Ferrara, Italy 18Sezione INFN di Firenze, Firenze, Italy
19Laboratori Nazionali dell’INFN di Frascati, Frascati, Italy 20Sezione INFN di Genova, Genova, Italy
21Sezione INFN di Milano Bicocca, Milano, Italy 22Sezione di Milano, Milano, Italy
23Sezione INFN di Padova, Padova, Italy 24Sezione INFN di Pisa, Pisa, Italy
25Sezione INFN di Roma Tor Vergata, Roma, Italy 26Sezione INFN di Roma La Sapienza, Roma, Italy
27Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Krak´ow, Poland 28AGH - University of Science and Technology, Faculty of Physics and Applied Computer Science,
Krak´ow, Poland
29National Center for Nuclear Research (NCBJ), Warsaw, Poland
30Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania 31Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia
32Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia
33Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow, Russia
34Institute for Nuclear Research of the Russian Academy of Sciences (INR RAN), Moscow, Russia 35Yandex School of Data Analysis, Moscow, Russia
36Budker Institute of Nuclear Physics (SB RAS), Novosibirsk, Russia 37Institute for High Energy Physics (IHEP), Protvino, Russia
38ICCUB, Universitat de Barcelona, Barcelona, Spain
39Instituto Galego de F´ısica de Altas Enerx´ıas (IGFAE), Universidade de Santiago de Compostela,
Santiago de Compostela, Spain
40European Organization for Nuclear Research (CERN), Geneva, Switzerland
41Institute of Physics, Ecole Polytechnique F´ed´erale de Lausanne (EPFL), Lausanne, Switzerland 42Physik-Institut, Universit¨at Z¨urich, Z¨urich, Switzerland
43Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands
44Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, The
Netherlands
45NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine
46Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine 47University of Birmingham, Birmingham, United Kingdom
48H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom 49Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom 50Department of Physics, University of Warwick, Coventry, United Kingdom 51STFC Rutherford Appleton Laboratory, Didcot, United Kingdom
52School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom 53School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom 54Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom 55Imperial College London, London, United Kingdom
56School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom 57Department of Physics, University of Oxford, Oxford, United Kingdom
58Massachusetts Institute of Technology, Cambridge, MA, United States 59University of Cincinnati, Cincinnati, OH, United States
60University of Maryland, College Park, MD, United States 61Syracuse University, Syracuse, NY, United States
62Pontif´ıcia Universidade Cat´olica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil, associated to 2 63University of Chinese Academy of Sciences, Beijing, China, associated to3
64School of Physics and Technology, Wuhan University, Wuhan, China, associated to3
65Institute of Particle Physics, Central China Normal University, Wuhan, Hubei, China, associated to3 66Departamento de Fisica , Universidad Nacional de Colombia, Bogota, Colombia, associated to8 67Institut f¨ur Physik, Universit¨at Rostock, Rostock, Germany, associated to 12
69National Research Tomsk Polytechnic University, Tomsk, Russia, associated to32
70Instituto de Fisica Corpuscular, Centro Mixto Universidad de Valencia - CSIC, Valencia, Spain,
associated to 38
71Van Swinderen Institute, University of Groningen, Groningen, The Netherlands, associated to43 72Los Alamos National Laboratory (LANL), Los Alamos, United States, associated to 61
aUniversidade Federal do Triˆangulo Mineiro (UFTM), Uberaba-MG, Brazil bLaboratoire Leprince-Ringuet, Palaiseau, France
cP.N. Lebedev Physical Institute, Russian Academy of Science (LPI RAS), Moscow, Russia dUniversit`a di Bari, Bari, Italy
eUniversit`a di Bologna, Bologna, Italy fUniversit`a di Cagliari, Cagliari, Italy gUniversit`a di Ferrara, Ferrara, Italy hUniversit`a di Genova, Genova, Italy iUniversit`a di Milano Bicocca, Milano, Italy jUniversit`a di Roma Tor Vergata, Roma, Italy kUniversit`a di Roma La Sapienza, Roma, Italy
lAGH - University of Science and Technology, Faculty of Computer Science, Electronics and
Telecommunications, Krak´ow, Poland
mLIFAELS, La Salle, Universitat Ramon Llull, Barcelona, Spain nHanoi University of Science, Hanoi, Vietnam
oUniversit`a di Padova, Padova, Italy pUniversit`a di Pisa, Pisa, Italy
qUniversit`a degli Studi di Milano, Milano, Italy rUniversit`a di Urbino, Urbino, Italy
sUniversit`a della Basilicata, Potenza, Italy tScuola Normale Superiore, Pisa, Italy
uUniversit`a di Modena e Reggio Emilia, Modena, Italy vIligan Institute of Technology (IIT), Iligan, Philippines wNovosibirsk State University, Novosibirsk, Russia †Deceased
