Soft-Dielectron Excess in Proton-Proton Collisions at √s = 13 TeV

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ Soft-Dielectron Excess in Proton-Proton Collisions at √s = 13 TeV © 2021 CERN Published version ALICE Collaboration ALICE Collaboration. (2021). Soft-Dielectron Excess in Proton-Proton Collisions at √s = 13 TeV. Physical Review Letters, 127(4), Article 042302. https://doi.org/10.1103/PhysRevLett.127.042302 2021 Soft-Dielectron Excess in Proton-Proton Collisions at ffiffi s p=13 TeV S. Acharya et al.* (A Large Ion Collider Experiment Collaboration) (Received 15 June 2020; revised 12 April 2021; accepted 8 June 2021; published 23 July 2021) A measurement of dielectron production in proton-proton (pp) collisions at ffiffiffi s p¼13 TeV, recorded with the ALICE detector at the CERN LHC, is presented in this Letter. The data set was recorded with a reduced magnetic solenoid field. This enables the investigation of a kinematic domain at low dielectron (ee) invariant mass mee and pair transverse momentum pT;ee that was previously inaccessible at the LHC. The cross section for dielectron production is studied as a function of mee,pT;ee, and event multiplicity dNch=dη. The expected dielectron rate from hadron decays, called hadronic cocktail, utilizes a parametrization of the measured η=π0ratio in pp and proton-nucleus collisions, assuming that this ratio shows no strong dependence on collision energy at low transverse momentum. Comparison of the measured dielectron yield to the hadronic cocktail at 0.15 <m ee <0.6GeV=c2and for pT;ee <0.4GeV=c indicates an enhancement of soft dielectrons, reminiscent of the “anomalous”soft-photon and soft-dilepton excess in hadron-hadron collisions reported by several experiments under different experimental conditions. The enhancement factor over the hadronic cocktail amounts to 1.61 0.13ðstatÞ0.17ðsyst;dataÞ 0.34ðsyst;cocktailÞin the ALICE acceptance. Acceptance-corrected excess spectra in mee and pT;ee are extracted and compared with calculations of dielectron production from hadronic bremsstrahlung and thermal radiation within a hadronic many-body approach. DOI: 10.1103/PhysRevLett.127.042302 The study of lepton pair production is an important tool for investigating the properties of hadronic and nuclear collisions as they can leave the strongly interacting system at any stage of its evolution. In order to single out possible medium contributions to the dilepton yield in nucleusnucleus collisions on top of those from hadron decays, studies in hadronic collision systems are instrumental in obtaining a medium-free reference. Recent measurements of dielectron (eþe−) production at midrapidity in protonproton (pp) collisions at the Large Hadron Collider (LHC) at CERN [1–3] and at the Relativistic Heavy-Ion Collider (RHIC) at BNL [4–6] are compatible with the expectations from hadron decays, i.e., with the hadronic cocktail, and show no indication of medium effects within the experimental uncertainties. In contrast to this, recent measurements of hadronic observables in small collision systems at the LHC [7–10] and at RHIC [11–13] reveal signs of collectivity and equilibration of the final-state particles at high multiplicities. This suggests that considerable interaction in an intermediate state may, indeed, be at work even in pp collisions, which should also give rise to the emission of electromagnetic radiation. The production of soft photons in hadronic collision systems was extensively studied in fixed-target experiments at beam momenta ranging from 10.5 to 450 GeV=c. Except for the lowest collision energies [14], most experiments reported an excess of soft photons compared with the expectation from hadron decays that could not be explained by initialand final-state bremsstrahlung [15–17]. The emergence of a photon excess in a transverse momentum (pT) range far below 0.2GeV=c was dubbed the softphoton puzzle because bremsstrahlung from initialand final-state particles should dominate over the radiation from any intermediate state in the soft limit, as stated by the Low theorem [18]. This raised speculations about the existence of a radiating intermediate state with characteristic time and length scales well above 1 fm [19]; a scenario that can be largely ruled out by more recent measurements of the source size in pp collisions from particle interferometry [20–22]. Several possible mechanisms were proposed to explain the observations, including the annihilation of soft partons [23–28], the production of a cold nonequilibrium state of quarks and gluons [29,30], and the emission of synchrotron radiation off quarks that are accelerated in the chromomagnetic fields of the colliding hadrons [31,32].A final conclusion on the interpretation of the soft-photon excess has not been reached though [33,34]. In the dilepton sector, an enhancement over the hadronic cocktail was observed for both electron and muon pairs at *Full author list given at the end of the article. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. PHYSICAL REVIEW LETTERS 127, 042302 (2021) 0031-9007=21=127(4)=042302(13) 042302-1 © 2021 CERN, for the ALICE Collaboration small invariant masses in pp collisions at the intersecting storage rings (ISRs) [35], and in fixed-target experiments with πand p beams from 10 to 400 GeV=c [36–46]. Similar to the case of real photons, the excess yield could not be reconciled with the expectation from hadronic bremsstrahlung. These observations are supported by findings of an enhanced eþ=πratio at the ISR [47]. However, the observations in the dilepton sector remained controversial because other experiments reported results that were compatible with bremsstrahlung and hadron decays only [48–50]. The question of anomalous softdilepton production in hadronic collisions awaits further experimental input since three decades. In a dedicated campaign during pp operation at ffiffiffi s p¼13 TeV, the ALICE central-barrel detectors [51] were operated inside a lower magnetic solenoid field, which increased the sensitivity for electrons at low pT (the term “electron”is used here for electrons and positrons). This makes a reassessment of soft dielectron production possible that could not be performed in a previous analysis at nominal field [2]. A detailed description of the ALICE apparatus and its performance can be found in [52]. The tracking of charged particles is performed by the inner tracking system (ITS) [53] and by the time projection chamber (TPC) [54], which are located in the central barrel and are surrounded by a solenoid, providing a homogeneous magnetic field along the beam direction. The TPC is used for particle identification (PID) via the measurement of the specific ionization energy loss (dE=dx). Additional PID information is provided by the time of flight (TOF) [55] system. Collision events are selected using the V0 detectors located on either side of the interaction point. Furthermore, the events are classified on the basis of the V0 signal amplitude. The event classes are reported in terms of dNch=dηat midrapidity [56]. The data samples analyzed for this Letter were recorded in 2016–2018 in pp collisions at ffiffiffi s p¼13 TeV with ALICE, employing a setup where the magnetic solenoid field was reduced from 0.5 T to 0.2 T. This increases the acceptance and efficiency of the tracking and TOF detectors, extending the single electron selection from pT;e ≥0.2GeV=c down to pT;e ≥0.075 GeV=c and providing access down to pair transverse momenta pT;ee ≥0 for invariant masses mee >0.15 GeV=c2. The minimum bias (MB) event trigger is constructed using a coincident signal in both V0 scintillators. Interaction vertices are reconstructed by extrapolation of ITS track segments toward the nominal interaction point. Events with multiple reconstructed vertices are tagged as pileup and rejected. The requirement on the vertex position to be within 10 cm of the nominal interaction point in beam direction is employed to ensure a uniform detector performance. After event selection, a total of 5.42 ×108MB pp events remain for further analysis, corresponding to an integrated luminosity of Lint ¼9.38 0.47 nb−1based on the visible cross section observed by the V0 trigger extracted from a van der Meer scan [57]. The electron candidates used in this analysis are selected in the transverse momentum range pT;e >0.075 GeV=c and pseudorapidity jηej<0.8. Further track and PID selection criteria are identical to those described in [2] with the exception of a stronger requirement on the maximum distance of closest approach (DCA) to the primary vertex in the longitudinal direction (DCAz<0.3cm) to remove a contribution of looping tracks in the TPC. Since pairs of electrons originating from the same source cannot be identified unambiguously, a statistical approach is applied to extract the yield of correlated pairs. To this end, a combinatorial pairing of all electron candidates in an event is performed. Additional photon conversion rejection is achieved by removing pairs based on their characteristic orientation relative to the magnetic field [1]. The combinatorial background estimate is constructed from same-event pairs with the same charge sign, corrected for charge-dependent acceptance effects, and subtracted from the opposite-sign pair distribution, following the approach described in [2]. To correct the signal for the finite reconstruction efficiency, a Monte Carlo (MC) simulation is used as described in [2]. Proton-proton events are generated using the Monash 2013 tune of PYTHIA 8.1 [58] to simulate light-hadron decays, while the Perugia 2011 tune of PYTHIA 6.4 [59] is utilized to embed heavyflavor hadrons that decay to electrons. The generated particles are propagated through the detector using GEANT 3[60]. The final efficiency as a function of mee and pT;ee is the average of the efficiencies of the different dielectron sources, weighted by their expected contribution to the hadronic cocktail (see below). The systematic uncertainties of the data are evaluated as described in [2] by simultaneous variation of the singleelectron tracking and PID selection criteria. The track sample is varied by changing the criteria on the number of space points in TPC and ITS, the χ2of the track fits, and the criteria used for electron selection and hadron rejection. These variations imply changes of the pair efficiency by up to about 30%. The systematic uncertainty is calculated as the root-mean-square of the resulting data points. Similar to [2], additional uncertainties related to the conversion rejection criteria, the isolation criterion in the ITS and the requirement of a hit in the first ITS layer, as well as on the TPC-ITS matching efficiency, the V0 trigger efficiency and the vertex reconstruction efficiency are added in quadrature. The resulting total systematic uncertainties are 12% for mee <0.04 GeV=c2and 11% for larger invariant masses, independent of pT;ee. The global 5% uncertainty resulting from the luminosity measurement is not included in the systematic uncertainties of the data points. PHYSICAL REVIEW LETTERS 127, 042302 (2021) 042302-2 The dielectron measurement is compared with the sum of expected contributions from light (π0,η,η’,ω,ρ,ϕ) and heavy-flavor hadron decays within the kinematic range under study. The hadronic cocktail is constructed as described in [2], with the following exceptions. The pT spectrum of πin pp collisions at ffiffiffi s p¼13 TeV [61] is parametrized using a modified Hagedorn function [62]. The difference between π0and πdue to isospin-violating decays, mainly of the ηmeson, is estimated using an effective model that describes measured hadron spectra at low pTand includes strong and electromagnetic decays [63]. This leads to a pT-dependent scaling factor applied to the πparametrization, which implies an upward shift by 18% 6% for pT→0that drops monotonically to below 1% at pT>1GeV=c. The uncertainty of this correction is estimated from variations of the model parameters and propagated into the final cocktail uncertainty. The dominant contribution to the hadronic cocktail in the kinematic region of interest is given by the ηmeson. Therefore, a parametrization of the ALICE measurement of η=π0ratio as a function of pTin pp collisions at ffiffiffi s p¼7TeV [64],8TeV[65], and in p-Pb collisions at a center-of-mass energy per nucleon-nucleon collision ffiffiffiffiffiffiffiffi sNN p¼5.02 TeV [66] is performed and extended to low pT, using data from CERES/TAPS [67] below pT¼0.4GeV=c and assuming energy independence of the ratio. The estimated uncertainty is about 15% at pT>0.5GeV=c, where data from LHC exist. At smaller pT, a conservative pT-dependent uncertainty of up to 40% is assigned, covering the full spread of the data points and a possible weak energy dependence of the η=π0ratio. The resulting η=π0parametrization including the estimated uncertainties is shown in Fig. 1. It also illustrates that mTscaling [68] fails to describe the measured η=π0ratio at low pT, as reported earlier [65,69]. The contribution from correlated semileptonic decays of open charm and beauty hadrons is estimated based on the decay distributions from the Perugia 2011 tune of PYTHIA 6.4, normalized to the measured cross sections at midrapidity, dσc¯ c=dyjy¼0¼974 138ðstatÞ140ðsystÞμb and dσb¯ b=dyjy¼0¼79 14ðstatÞ11ðsystÞμb, from the dielectron analysis in pp collisions at ffiffiffi s p¼13 TeV at nominal field [2]. Finally, the detector resolution in pT;e,ηe and azimuthal angle φeis extracted as a function of pT;e from the same MC simulation and applied to all decay electrons [70]. To construct the cocktail in intervals of dNch=dη, the light-flavor pTspectra of the MB cocktail are scaled by the ratio of the charged-particle pTspectra measured in multiplicity intervals to all events having at least one charged particle produced in the pseudorapidity interval jηj<1(INEL >0events) [61]. The open-charm contribution is weighted according to the measured enhancement of Dmesons at pT>1GeV=c in pp collisions at ffiffiffi s p¼7TeV [71]. The overall systematic uncertainties of the hadronic cocktail are estimated by adding in quadrature the uncertainties of the following contributions: the input data parametrizations as a function of pT, the π0=πcorrection factor, the uncertainty of the η=π0,ω=π0 [58], and ρ=π0[58] ratios, the scaling parameters used for η0 [59] and ϕ[72], the branching fractions of the different light-flavor decay channels, the measured cross sections, as well as the estimation of dNch=dη. This results in a systematic uncertainty of the hadronic cocktail between 13% in the π0-Dalitz region and up to 24% in the mass region dominated by the ηmeson. The dielectron cross section as a function of mee in the range pT;ee <0.4GeV=c and within the ALICE single0.5 1 1.5 2 2.5 3 3.5 4 0 π/η 2− 10 1− 10 ALICE = 7 TeVs pp = 8 TeVspp = 5.02 TeV NN sp-Pb 0 π scaled T m from η = 13 TeVsat CERES/TAPS = 29.1 GeV NN sp-Au = 29.1 GeV NN sp-Be HELIOS = 29.1 GeV NN sp-Be Parametrization ALICE & CERES/TAPS sys. uncertainty )c (GeV/ T p FIG. 1. The ratio η=π0as a function of pT, measured in pp and proton-nucleus collisions at different center-of-mass energies [64–67]. Also shown is the parametrization used for the construction of the hadronic cocktail (solid line), its uncertainty (dotted line), and the expectation from mTscaling (dashed line). 0 0.1 0.2 0.3 0.4 0.5 0.6 ) 2 c (GeV/ ee m 3− 10 2− 10 1− 10 1 10 2 10 ) 2 c (mb / GeV/ ee m/d σ d ALICE = 0.2 TB = 13 TeV, s pp c < 10 GeV/ T,e p0.075 < c < 0.4 GeV/ T,ee p e η | < 0.8, | 0 0.1 0.2 0.3 0.4 0.5 0. 6 ) 2 c (GeV/ ee m 1 1.5 2 Data/Cockt. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 )c (GeV/ T,ee p 5 10 15 20 25 30 35 40 45 3− 10× )c (mb / GeV/ T,ee p/d σ d Data Cocktail sum - e + eγ→ 0 πe + eγ→η - e + e→cc ')η+φ+ω+ρLight flavor ( 2 c 0.6 GeV/≤ ee m≤ 0.15 not shown 5% Global unc.± 0 0.2 0.4 0.6 0.8 1 1.2 1.4 )c (GeV/ T,ee p 1 2 3 Data/Cockt. FIG. 2. Differential dielectron cross sections as a function of mee (left) and pT;ee (right). The different components of the hadronic cocktail are shown as solid lines. The error bars and boxes indicate the statistical and systematic uncertainties of the data points. The cocktail uncertainties are shown as gray bands. In the bottom panels, the ratios of data and cocktail are shown. PHYSICAL REVIEW LETTERS 127, 042302 (2021) 042302-3 electron acceptance is shown in the left panel of Fig. 2. The data points are compared to the hadronic cocktail. Within the uncertainties, data and cocktail are in good agreement at mee <m πwhile an excess over the hadronic cocktail is observed at larger masses. The representation of the data as a function of pT;ee in the invariant mass region 0.15 <m ee <0.6GeV=c2(right panel of Fig. 2) illustrates that the excess is most pronounced at pT;ee <0.4GeV=c, while the hadronic cocktail agrees well with the data at higher pT;ee. In the mass region 0.15 <m ee <0.6GeV=c2and for pT;ee <0.4GeV=c, the enhancement factor amounts to 1.61 0.13ðstatÞ0.17ðsyst;dataÞ0.34ðsyst;cocktailÞ. The systematic uncertainty is dominated by the uncertainty of the ηcontribution to the hadronic cocktail. The study of the multiplicity dependence of the observed excess may help to unravel the nature of the underlying dielectron production mechanisms [26]. To this end, four intervals of the event multiplicity are selected, based on the V0 signal, and the dielectron data are integrated over different regions of mee and pT;ee. The upper part of Fig. 3shows the dielectron yield per event in the interval 0.15 <m ee <0.6GeV=c2 and pT;ee <0.4GeV=c compared with the hadronic cocktail, integrated over the same mee and pT;ee interval, as a function of the relative charged-particle multiplicity at midrapidity, ðdNch=dηÞ=hdNch=dηiINEL>0, where hdNch=dηiINEL>0¼7.60.5is the mean multiplicity in INEL >0pp collisions at ffiffiffi s p¼13 TeV [56]. The dielectron yield is systematically above the cocktail in all multiplicity intervals. The enhancement of the data over the cocktail is shown in the lower part of Fig. 3. Within the experimental accuracy, no clear trend for the multiplicity dependence is found. Figure 3also shows the multiplicity dependence in control regions at smaller mee or larger pT;ee, where no excess is observed. To further characterize the observed dielectron enhancement, the hadronic cocktail is subtracted from the measured mee and pT;ee spectra. The extracted excess spectra are corrected for the single-electron acceptance in pT;e and ηe, assuming isotropic decay in the pair center-ofmass frame, which enables the measurement of the excess cross section in mee >0.15 GeV=c2and pT;ee >0at midrapidity. The corresponding excess spectra as a function of mee and pT;ee are shown in Fig. 4. The data points are compared with a calculation of bremsstrahlung from initialand final-state hadrons following the approach in [73] using a mean charge transfer hΔQ2i¼1.32 in units oftheelectricchargeesquared and the inelastic hadronic cross section [57]. Also shown is a calculation of the thermal dielectron yield from a hadronic many-body model [74–76], assuming a fireball lifetime of 2fm=c, an initial temperature of 216 MeV=c and a freeze-out temperature of 170 MeV=c. While the hadronic manybody approach is successful in describing the dilepton production in heavy-ion collisions at the SPS [77,78],at RHIC [79–81],andattheLHC[82], it fails to describe the present dielectron results in pp collisions. An enhancement of dielectrons at very low pT;ee in peripheral Au-Au collisions at ffiffiffiffiffiffiffiffi sNN p¼200 GeV, reported by the STAR collaboration [83], could be explained by coherent twophoton production of lepton pairs in the strong electric fields of the colliding nuclei [84–86]. Owing to the strong Zdependence, this mechanism is not sufficient to describe the present enhancement in pp collisions. 0 0.5 1 1.5 2 2.5 3 3.5 4 ) 2 c (GeV/ ee m 600− 500− 400− 300− 200− 100− 0 3− 10× ) 2 c (mb / GeV/yd ee m/d σ d 0.2 0.4 0.6 3− 10× ALICE = 0.2 TB = 13 TeV, s pp c < 10 GeV/ T,e p0.075 < | < 0.8 e η | Cocktail sum 0 0.5 1 1.5 2 2.5 INEL>0 〉 η /d ch Nd〈)/ η /d ch N(d 1 1.5 2 2.5 Data/Cocktail Data MB Data mult. dependent 2 c < 0.6 GeV/ ee m0.15 < c < 0.4 GeV/ T,ee p 2 c < 0.6 GeV/ ee m0.15 < c < 6 GeV/ T,ee p1 < 2 c < 0.15 GeV/ ee m c < 0.4 GeV/ T,ee p ev / ee NN FIG. 3. Upper panel: Dielectron yield per event in the excess region as a function of the event multiplicity compared with the hadronic cocktail. Lower panel: Enhancement factor data or cocktail in three different kinematic regions. Error bars and boxes show the statistical and systematic uncertainties of the data points. The cocktail uncertainties are indicated as vertical bars around one in the lower panel. 0.2 0.3 0.4 0.5 2 c(GeV/ ee m 4 10 3 10 2 10 1 10 ALICE = 0.2 TB= 13 TeV, s pp c< 0.4 GeV/ T,ee p| < 0.8, e | 0.2 0.4 0.6 0.8 1 4 10 3 10 2 10 1 10 0.2 0.4 0.6 0.8 1 )c(GeV/ T,ee p Data in-med. hadr. [Rapp] hadr. bremsstrahlung 2 c0.6 GeV/ ee m0.15 0 . ) 1 10 1 10 ) 2 c(mb / GeV/yd ee m/dd )c(mb / GeV/yd T,ee p/d 2 d FIG. 4. Dielectron excess spectra as a function of mee (left) and pT;ee (right) after subtraction of the hadronic decay cocktail. The error bars and boxes represent statistical and combined systematic uncertainties from data and cocktail. Arrows indicate upper limits at 90% confidence level. Also shown as lines are calculations of bremsstrahlung from initialand final-state hadrons [73], and thermal dielectron production [74–76]. PHYSICAL REVIEW LETTERS 127, 042302 (2021) 042302-4 The results reported here are expected to encourage further theoretical work. In conclusion, an excess of soft dielectrons over the expectation from hadron decays is observed in pp collisions at ffiffiffi s p¼13 TeV. The enhancement factor shows no dependence on the event multiplicity, and the acceptance-corrected excess yield cannot be explained by bremsstrahlung from initialand final-state hadrons or by thermal dielectron production. The excess of soft dielectrons in pp is an intriguing observation, although its significance is presently limited to 1.6σ, mostly by the uncertainty of the hadronic cocktail. Forthcoming precision measurements with the upgraded ALICE detector will help to further elucidate this finding, including a possible connection to earlier observations of anomalous softphoton and soft-dielectron production at lower collision energies. The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources and support provided by all Grid centres and the Worldwide LHC Computing Grid (WLCG) collaboration. The ALICE Collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: A. I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation (ANSL), State Committee of Science and World Federation of Scientists (WFS), Armenia; Austrian Academy of Sciences, Austrian Science Fund (FWF): [Grant No. M 2467-N36] and Nationalstiftung für Forschung, Technologie und Entwicklung, Austria; Ministry of Communications and High Technologies, National Nuclear Research Center, Azerbaijan; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Financiadora de Estudos e Projetos (Finep), Fundação de Amparo `a Pesquisa do Estado de São Paulo (FAPESP) and Universidade Federal do Rio Grande do Sul (UFRGS), Brazil; Ministry of Education of China (MOEC), Ministry of Science and Technology of China (MSTC) and National Natural Science Foundation of China (NSFC), China; Ministry of Science and Education and Croatian Science Foundation, Croatia; Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaenergía, Cuba; Ministry of Education, Youth and Sports of the Czech Republic, Czech Republic; The Danish Council for Independent Research | Natural Sciences, the VILLUM FONDEN and Danish National Research Foundation (DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland; Commissariat `al’Energie Atomique (CEA) and Institut National de Physique Nucl´eaire et de Physique des Particules (IN2P3) and Centre National de la Recherche Scientifique (CNRS), France; Bundesministerium für Bildung und Forschung (BMBF) and GSI Helmholtzzentrum für Schwerionenforschung GmbH, Germany; General Secretariat for Research and Technology, Ministry of Education, Research and Religions, Greece; National Research, Development and Innovation Office, Hungary; Department of Atomic Energy Government of India (DAE), Department of Science and Technology, Government of India (DST), University Grants Commission, Government of India (UGC) and Council of Scientific and Industrial Research (CSIR), India; Indonesian Institute of Science, Indonesia; Centro Fermi —Museo Storico della Fisica e Centro Studi e Ricerche Enrico Fermi and Istituto Nazionale di Fisica Nucleare (INFN), Italy; Institute for Innovative Science and Technology, Nagasaki Institute of Applied Science (IIST), Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Japan Society for the Promotion of Science (JSPS) KAKENHI, Japan; Consejo Nacional de Ciencia (CONACYT) y Tecnología, through Fondo de Cooperación Internacional en Ciencia y Tecnología (FONCICYT) and Dirección General de Asuntos del Personal Academico (DGAPA), Mexico; Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; The Research Council of Norway, Norway; Commission on Science and Technology for Sustainable Development in the South (COMSATS), Pakistan; Pontificia Universidad Católica del Perú, Peru; Ministry of Science and Higher Education, National Science Centre and WUT ID-UB, Poland; Korea Institute of Science and Technology Information and National Research Foundation of Korea (NRF), Republic of Korea; Ministry of Education and Scientific Research, Institute of Atomic Physics and Ministry of Research and Innovation and Institute of Atomic Physics, Romania; Joint Institute for Nuclear Research (JINR), Ministry of Education and Science of the Russian Federation, National Research Centre Kurchatov Institute, Russian Science Foundation and Russian Foundation for Basic Research, Russia; Ministry of Education, Science, Research and Sport of the Slovak Republic, Slovakia; National Research Foundation of South Africa, South Africa; Swedish Research Council (VR) and Knut and Alice Wallenberg Foundation (KAW), Sweden; European Organization for Nuclear Research, Switzerland; Suranaree University of Technology (SUT), National Science and Technology Development Agency (NSDTA) and Office of the Higher Education Commission under NRU project of Thailand, Thailand; Turkish Atomic Energy Agency (TAEK), Turkey; National Academy of Sciences of Ukraine, Ukraine; Science and Technology Facilities Council (STFC), United Kingdom; National Science Foundation of the United States of America (NSF) and United States Department of Energy, Office of Nuclear Physics (DOE NP), United States of America. 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