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Physics
Letters
B
www.elsevier.com/locate/physletb
First
measurement
of
jet
mass
in
Pb–Pb
and
p–Pb
collisions
at
the
LHC
ALICE
Collaboration
a
r
t
i
c
l
e
i
n
f
o
a
b
s
t
r
a
c
t
Articlehistory: Received26April2017
Receivedinrevisedform20October2017 Accepted21November2017
Availableonline23November2017 Editor: L.Rolandi
ThisletterpresentsthefirstmeasurementofjetmassinPb–Pb andp–Pb collisionsat√sNN=2.76 TeV and√sNN=5.02 TeV,respectively.Boththejetenergyandthejetmassareexpectedtobesensitiveto jetquenchinginthehotQuantumChromodynamics(QCD)mattercreatedinnuclearcollisionsatcollider energies. Jets are reconstructedfromcharged particles usingthe anti-kT jet algorithmand resolution parameter R=0.4.Thejetsaremeasuredinthepseudorapidityrange|
η
jet|<0.5 andinthreeintervals oftransversemomentumbetween60GeV/c and120GeV/c.Themeasurementofthejetmassincentral Pb–Pb collisions is compared to the jet mass as measured in p–Pb reference collisions, to vacuum event generators,and to models includingjet quenching. It is observed that the jet mass incentral Pb–Pb collisionsisconsistentwithinuncertaintieswithp–Pb referencemeasurements.Furthermore,the measuredjet massinPb–Pb collisionsisnot reproducedbythe quenchingmodelsconsidered inthis letterandisfoundtobeconsistentwithPYTHIAexpectationswithinsystematicuncertainties.©2017TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
ThisletterpresentsthefirstmeasurementofjetmassinPb–Pb and p–Pb collisions at
√
sNN=
2.
76 TeV and√
sNN=
5.
02 TeV, respectively. Both the jet energy and the jet mass are expected to be sensitive to jet quenching in the hot Quantum Chromody-namics(QCD) matter, theQuark–Gluon–Plasma (QGP), createdin ultra-relativisticnuclear collisions.Scattering processeswithlarge momentumtransfer, Q2,betweenthequarksandthegluons(par-tons)constituentsofcollidingnucleonsoccurearlyinthecollision (at a time
<
1fm/
c). Outgoing partons carry a net color charge and evolve from high to low virtuality producing parton show-ers,whicheventuallyhadronizeintocollimatedspraysofparticles, calledjets.Interactionsofthe outgoingpartonswiththe hot and denseQGPcreatedinheavy-ioncollisionsmaymodifytheangular andmomentumdistributionsofhadronicjetfragmentsrelativeto jetsfragmentinginvacuum.Thisprocess,knownasjetquenching, canbeusedtoprobethepropertiesofthehotQCDmedium[1–4]. Jet quenching has been investigated at the Relativistic Heavy IonCollider (RHIC)[5–9] and atthe LargeHadronCollider (LHC) [10–20] via measurements of high-pT hadrons and fullyrecon-structed jetsinnucleus–nucleus (AA)collisions and pp (vacuum) collisions. These measurements have shown a suppression of hadron and jet yields in AAcollisions and modest modifications ofthelongitudinalfragment distribution andtheradial profileof jets relative to jets produced in pp collisions within the typical
E-mailaddress:alice-publications@cern.ch.
jet cone of 0.3–0.4 at the LHC. The jet mass is sensitive to the initial virtuality of the parton at the origin of the shower [21]. Energy-momentumexchangewiththehotQCDmediummay tem-porarilyincreasethevirtualityofthepropagating partons,leading toalargergluonradiationprobability[22–25].Thiswouldresultin abroadeningofthejetprofileandanincreaseofthejetmass,ifa significantamountoftheradiatedgluonsarecapturedwithinthe jetconeusedforreconstruction.However,thevirtualityincreaseis temporaryanditisexpectedthattheleadingpartontraversinghot QCD matterexperiencessubstantial virtuality(or mass)depletion alongwithenergyloss[21].
Thejetmassofinclusivejetsandofjetsindijeteventshasbeen previouslymeasuredinhigh-energyppcollisionsat
√
s=
7 TeV at the LHC [26,27].Perturbative QCD predictions using higher-order matrix-elements for parton production combined with a Monte Carlo (MC) parton shower were found to be in good agreement withthedata.Thecommonlyusedleading-ordereventgenerators withfullshower evolution,PYTHIA[28,29] andHERWIG[30], re-producethejet massdistributioninpp collisionsreasonablywell inthe pT region200–600 GeV/c previouslystudied,howevertheyconsistently under- and over-predict the data, respectively, by a slightamount.
In this letter, measurements of the charged-jet mass are re-ported.Chargedjetsarejetsclusteredusingonlychargedparticles, reconstructed in the ALICE tracking system, opposed to full jets, reconstructed with both charged and neutral particles. The four momentum of the jet is defined as the sum of constituent four momenta.Thejetmassiscalculatedfromthejetfour-momentum,
https://doi.org/10.1016/j.physletb.2017.11.044
0370-2693/©2017TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3.
M
=
E2
−
p2T
−
p2z,
(1)whereE isthejetenergy,pT thetransverseand pzthe longitudi-nalmomentumofthejet.
The measurement is performed in Pb–Pb and p–Pb collisions and in three intervals of jet transverse momentum. Data-driven jet-by-jetbackgroundsubtractionschemesareusedtocorrectthe jet mass for the contribution of the Pb–Pb underlying event. In contrast, the p–Pb background is included in the response ma-trix andcorrected for in the unfolding, asdiscussed indetail in Sec.3.1and3.2.Thedataarecomparedatdetectorleveltoa sim-ulated reference without jet quenching effects. Furthermore, the measurementis correctedto particlelevel viaa two-dimensional unfoldingtechnique, accountingfor theremaining effectof back-ground fluctuations and detector effects. The fully corrected jet massdistributionincentralPb–Pb collisionsiscomparedto mod-elsandtothejetmassdistributionmeasuredinp–Pb collisions.
2. Datasample
The Pb–Pb collision data were recorded during the 2011LHC Pb–Pb runat
√
sNN=
2.
76 TeV. Thisanalysisused minimum-bias(MB) events,selectedonline by requiringa signal inthe forward V0 detectors,two arrays of scintillator tiles covering the full az-imuthwithin 2
.
8<
η
<
5.
1 (V0A)and−
3.
7<
η
<
−
1.
7 (V0C).An onlinecentralitytriggerselectedthe10%most-centralPb–Pb colli-sionsusingthecentralitydeterminationasdescribedin[31],with 100%efficiencyforthe0–8%centralityinterval,and60%efficiency forthe8–10% interval. Thenumber ofPb–Pb eventsusedin this analysis,aftertheeventselectiondescribedbelow,is17 millionin the0–10%centralityinterval.CollisionsofprotonandleadbeamswereprovidedbytheLHC inthefirstmonthsof2013.Thebeamenergieswere4TeVforthe protonbeamand1.58TeVper nucleonfortheleadbeam, result-ing in collisions ata center-of-massenergy of
√
sNN=
5.
02 TeV. Thenucleon–nucleoncenter-of-masssystemmovesrelativetothe laboratoryframe withrapidity 0.
465 in the directionofthe pro-ton beam [32]. In the following,η
refers to the pseudorapidity in the laboratory frame. The V0 detectors were used for online minimum bias event triggering and offline event selection. The minimumbiastriggerrequiredasignalfromachargedparticlein boththeV0AandtheV0C.The totalintegratedluminosityofthe minimumbias eventsample is 37μb−1. Inaddition, eventstrig-geredbyanonlinejet triggerusingtheelectromagnetic calorime-ter (EMCal) [33,34] were used. The online jet patch covered an areaofapproximately0
.
2 sr andrequiredanintegratedpatch en-ergyofatleast20 GeV.Thetransversemomentumdistributionsof charged jetsin the triggered sample was compared to the min-imum bias one, showing that the trigger was fully efficient forpT,ch jet
60 GeV/c.Theminimumbiasandtriggeredsamplewereusedforunfolded pT,ch jet
<
80 GeV/c and pT,ch jet≥
80 GeV/c,re-spectively. The triggered sample correspond to a total integrated luminosityof1.6 nb−1.
Inadditionto theonlinetriggers inbothcollision systems,an offlineselectionwasappliedinwhichtheonlinetriggerwas vali-datedandremainingbackgroundeventsfrombeam–gasand elec-tromagneticinteractions were rejected. Toensure ahightracking efficiency for all considered events, the primary vertex was re-quiredto bewithin 10cm fromthecenterof thedetectoralong thebeamaxisandwithin1 cminthetransverseplane[35].
3. Jetreconstructionandbackgroundsubtraction
Jet reconstruction for both the p–Pb and Pb–Pb analysis was performedwiththekT[36]andanti-kT[37]sequential
recombina-tion jet algorithms as implemented in the FastJet package [38].
The anti-kT algorithm was used for the signal jets while
clus-ters reconstructed with the kT algorithm were used to estimate thebackgrounddensityoftheevents.Jetswerereconstructed us-ingchargedtracksdetectedintheTimeProjectionChamber(TPC) [39] and the Inner Tracking System (ITS) [40] which cover the full azimuthal angle and pseudorapidity
|
η
|
<
0.
9. Jets were re-constructed usingthe E-scheme to recombinethefour-vectorsof the constituents,assigning the charged-pion massfor each parti-cle. A resolution parameter, R, of 0.
4 was used, andthejet area was calculatedbytheFastJetalgorithmusingessentiallyzero mo-mentum particles, called ghosts, witharea 0.005 [41]. Jets were accepted ifthey were fullycontainedin thetracking acceptance: full azimuth and|
η
jet|
<
0.
5, to guaranteethat the reconstructedjetaxeswereatleastR awayfromtheedgeofthedetector accep-tance.
Reconstructedtrackswereacceptediftheirreconstructed trans-versemomentaexceeded0.15GeV/c,withatleast70spacepoints found in the TPC and at least 80% of the geometrically accessi-blespace-pointsintheTPC. Trackswererequiredtohaveatleast three hits in the ITS used in the fit to ensure good track mo-mentum resolution. To account for the azimuthally non-uniform response ofthetwo innermostlayers oftheITS, theSiliconPixel Detector(SPD),theprimary-vertexpositionwasaddedtothetrack fit,fortrackswithoutSPDpoints,inordertofurtherimprovethe momentumdeterminationofthetrack.Thetrackmomentum reso-lutionwasabout1%at1GeV/c andabout3%at50GeV/c[35].Jets whichcontainedatrackwithpTlargerthan100GeV/c,forwhich thetrackmomentumresolutionexceeded6.5%,wererejected.The tracking efficiency incentral Pb–Pb collisions was 80% for tracks with pT largerthan1GeV/c anddecreased to56%at0.15GeV/c. In p–Pb collisionsthetrackingefficiencywas 70% fortrackswith
pT
=
0.
15 GeV/c andincreasedto85%forpT≥
1 GeV/c.Tosuppressthecontributionofjetsconsistingmainlyof back-groundparticles (combinatorial jets),onlyjetscontaining a“hard core” were accepted.Ajet was selected onlyif itoverlapped ge-ometrically with a jet reconstructed with only constituents with
pT
>
4 GeV/c.In the kinematic region considered, the hard core selection had similar performance as the selection used in pre-vious works, namely demanding the jet leading track to have a transverse momentum of at least 5 GeV/c [17,18]. PYTHIA pp simulations showedthat applyingsuch a selection was 100% ef-ficientonthejetpopulationforcharged-jettransversemomentumpT,ch jet
≥
25 GeV/c.ThefluctuatingbackgroundinPb–Pb collisionsaffectedthejetenergyscaleincreasingthefull-efficiencythreshold topsubT,ch jet
=
60 GeV/c (wherepsubT,ch jetisthebackground-subtractedpT,ch jet,definedbelowinEq.(6)).Inminimumbiasp–Pb collisions
thefragmentationbiasvanishedforpT,ch jet
≥
30 GeV/c. 3.1. Jet-by-jetbackgroundsubtractioninPb–Pb collisionsJet measurements inPb–Pb collisions are severely affected by the underlyingevent. Areconstructedjet contains particles unre-lated tothe hard parton shower.In thisanalysisthe background wassubtractedjet-by-jet.Forthispurpose,meanbackground den-sitieswere determined by characterizingevent-by-event the con-taminationfromsoftparticlesunrelatedtothehardjetsignal.The backgroundtransversemomentumdensity,
ρ
,wasdefinedasρ
=
median pT,i Ai,
(2)wherei indicatestheith kT clusterintheevent, pT,i isthe
trans-versemomentumoftheclusterandAiisitsarea.ThetwokT
calculationofthemedian.The average
ρ
inthe10%mostcentral Pb–Pb collisionswas116 GeV/c.Furtherdetailsaregivenin[17].Totake into account theinfluence of backgroundparticles on thereconstructedjet mass,aquantity mδ,kcluster
T was evaluatedfor eachkTclusterfollowingtheprocedureoutlinedin[42]
mδ,kcluster T
=
j(
m2j+
p2T,jc2−
p T,jc),
(3)wherethesumrunsover all particlesinside thekT cluster, mj is themassand pT,j thetransversemomentum ofeach constituent.
Thebackgroundmassdensityisdefinedby
ρ
m=
median mδ,i Ai,
(4)wherethesubscripti indicatesagaintheith kTclusterintheevent
and Ai is the area of the kT cluster. As in the calculation of
ρ
,thetwo leading kT clusterswere excluded fromthe median cal-culation.ForcentralPb–Pb collisions,
ρ
mwas foundtobe about3.6 GeV/c2.
Thebackgrounddensities,
ρ
andρ
m,wereusedincombinationwithtwo backgroundsubtractiontechniquesforjetshapeswhich willbedescribedinthefollowing:
(i) Thearea-basedsubtractionmethodcorrectsjet-shape observ-ables forbackground orpileup effectson an event-by-event andjet-by-jetbasis[42].Themethodisvalidforanyjet algo-rithm andinfrared- and collinear- safe jet shapes. The back-groundischaracterizedby
ρ
andρ
m.Ghostsareaddedintheη
–ϕ
planetotheevent,eachofthemmimickingabackground component in a region of area Ag. The shape sensitivity to pileup is determined by considering its derivatives with re-specttothetransversemomentumandmassoftheghostsand extrapolatedby a Taylorseriesto zeropileuporbackground. Acompletedescriptionofthemethodcanbefoundin[42]. (ii) The constituent subtraction method is a particle-levelap-proachwhichremovesorcorrectsjetconstituents.The particle-by-particlesubtractionallowstocorrectboththe4-momentum of the jet andits substructure. Massless ghosts are added to the event such that they cover the
η
–ϕ
plane. Each jet will therefore contain the real particles and ghosts. A distance measure is defined for each pair of a real particle i and a ghostk:Ri,k
=
pT,i·
(
yi−
ykg)
2+ (
ϕ
i−
ϕ
kg)
2,
(5)where y istherapidityand
ϕ
theazimuthalangle.Aniterative background removal procedure starts fromthe particle-ghost pair with smallestdistance. Ateach step thetransverse mo-mentum and mass of each particle and ghost are modified. The backgrounddensitiesρ
andρ
m are used to assignmo-mentum andmass toeach ghost: pTg
=
Agρ
andmgδ=
Agρ
mwhere Ag istheareaofeach ghost.Ifthetransverse momen-tum ofparticle i islarger thanthe transversemomentum of the ghost,the ghostisdiscarded andthetransverse momen-tum of the ghost is subtracted from the real particle. If the transverse momentum of the ghost is larger than particle i,
therealparticleisdiscardedandthetransversemomentumof the ghost iscorrected. The sameprocedure is appliedto the massoftheparticlesandghosts.Allpairsareconsideredand theiterativeprocedureisterminatedwhentheendofthelist ofpairs isreached.Thefour-momentumofthejet is recalcu-latedwiththesamerecombinationschemeasusedforthejet finding procedure.A completedescriptionofthe methodcan befoundin[43].
The area-based subtraction method was used asthe nominal method for the Pb–Pb analysis to correct the reconstructed jet mass for the influence of backgroundsince it is expected to in-duce zero bias. On the other hand,since track-by-track it is not possibletodeterminewhethera softparticleisbackgroundoran effectoftheinteractionwiththemedium,theconstituentmethod couldpotentiallyremovenon-backgroundparticles.
Thereconstructedtransversemomentumofanti-kTjets,prawT,ch jet,
iscorrectedaccordingto[44],
psubT,ch jet
=
pTraw,ch jet−
ρ
·
A,
(6)where A istheareaofthejetand
ρ
isthe pT-densityofthecon-sideredevent,asdefinedinEq.(2).
3.2. Backgroundinp–Pb collisions
Inp–Pb collisionstheaverage
ρ
andρ
mwereabout1.
26 GeV/cand0
.
08 GeV/c2,respectively.Toaccountfortheregionsofthe de-tectorwithouteventactivity,anadditionalcorrection[45]was ap-pliedandthehardsignaljetswereexcludedfromthebackground estimate by excludingoverlap ofthekT clusterswithanti-kT jetswithpT,ch jet
>
5 GeV/c.Whiletheoverallbackgroundcontributionis significantly smaller than in Pb–Pb collisions, it was observed thatthewidthofthemassfluctuationscausedbythep–Pb back-groundwas increasedwhen subtractingthe backgroundona jet-by-jetbasiswithrespecttoincludingitintheresponse.Therefore, to minimize thiseffect present in sparse events and to mitigate thedifferentsensitivitiesoftheconsideredsubtractionmethodsto fluctuations,inp–Pb collisionsthebackgroundwasnotsubtracted jet-by-jet(onan event-by-eventbasis),butcorrectedforon aver-ageinthe unfolding,asexplainedin greaterdetail inSec.4.The systematicuncertaintyonthischoicewas assessedbysubtracting thebackgroundindatawiththeconstituentmethodand correct-ingonlyforthedetectoreffectsintheresponse(seeSec.6).
4. Jetscaleandresolution
For the Pb–Pb analysis, the jet energy and mass response werestudiedbyembeddingsimulatedppeventsatdetectorlevel, namely including the effects of the detector response, into real Pb–Pb events. The detector response was determined from a PYTHIA6simulation(tuneAwithinitialstateradiationparameter PARP(67)
=
2.5tofittheD0di-jetdata[46])followedbyadetailed particle transportusing GEANT 3[47] in a detectorconfiguration corresponding to the conditions during Pb–Pb data taking. Prior to embedding the reconstructed tracks from the simulation into Pb–Pb events,an additional pT-dependenttrackinginefficiency of2–4%wasappliedinordertoaccountforthelargertracking ineffi-ciencyduetothehighoccupancyforlargeparticledensities[18]. The combinationofPb–Pb andPYTHIAevents willbe referred to as‘hybridevents’.
The same jet reconstruction procedure as in data, see Sec. 3, was appliedto thehybridevents,resultingina sample ofhybrid jets. Thehybridjetswere matchedto theprobejets, whichwere obtained by reconstructing jets from only the PYTHIA events at thedetector level.Notall constituentsofan embedded probejet will necessarily be found in a hybrid jet. In order to relate the hybrid to theprobe jet, a matchingcondition was used.This re-quiredthattheconstituentsofthehybridjetthatcomesfromthe PYTHIAevent mustcarry at least50% of the transverse momen-tumofthePYTHIAjet.Inthecasethatahybridjetwaspairedto twoormoreprobejets,itwasmatchedtotheprobejetwith high-est pTandtheotherprobejetswereconsideredlost,reducingthe jet-findingefficiency.Theefficiencyinthe10%mostcentralevents
forchargedjetsincreasedfrom40%atpsubT,ch jet
=
10 GeV/c to100% forpsubT,ch jet
>
40 GeV/c.Region-to-regionfluctuationsofthejetmassandpT-scalewere characterizedby usingthe hybrideventsandcalculating
δ
pT andδ
M,definedasthedifferencebetweenthetransversemomentum ormassofthebackground-subtractedhybridjetandtheprobejet [48].Onajet-by-jetbasisalinearcorrelationbetweenδ
pTandδ
Mwasobserved.
Thejetmassdistributionsofhybridjetsmatchedtoprobejets within a certain jet pT-interval, showed on average a larger jet mass withrespect to the corresponding spectrum ofthe probes. Thisoffsetwas duetobackgroundfluctuationsandlimitedpurity andefficiency within a reconstructed jet pT-interval, resulting in
jetmigrationbetweenpT-intervals.
Detectoreffects onthe jetenergyandmass wereinvestigated bymatchingdetectorleveljetsandparticleleveljetsfromaPythia simulationandcomparingtheir properties(including Pb–Pb back-ground).Thejetswere matchedbasedon distance,inawaythat guarantees a one-to-one match. The constituents of the detector
Fig. 1. (Coloronline)Massresponseusingthearea-basedbackgroundsubtraction methodinthe10%mostcentralPb–Pb collisionsforbackgroundfluctuationsonly (black,shadedhistogram),comparedtothefullresponseincludingdetectoreffects (red,hashedhistogram),foranti-kTjetswithresolutionparameterR=0.4.Msub referstothebackground-subtractedreconstructedjetmasswhileMprobe isthejet massoftheembeddedprobe.Fromtoplefttobottomright,eachpanelrepresenta pT,ch jetregion,40–60,60–80,80–100,100–120 GeVc.
level jetsare all assignedthe pion mass,asis done forthe data analysis, while the particle mass is used for the particle level jet reconstruction. A comparison between the jet mass response dueto backgroundfluctuationsand thefull response,which also contains detector effects, is shown in Fig. 1. While background fluctuationsinduce a positive shiftofthe reconstructedjet mass, detector effects, which are dominated by the finite tracking effi-ciencyandthemassassumptionofthejetconstituents,reducethe reconstructed jetmass.Thiswas furthercharacterizedby extract-ing the meanand themostprobable value fromthe distribution in Fig. 1,givinga measureof therelative jetmass shift.The rel-ativemass shiftisshowninFig. 2forthe area-basedsubtraction method (left) and the constituent subtraction method (right). In the kinematic range of interest, the mass shift doesnot exhibit a strong dependence on jet momentum. The performance of the constituentsubtractionmethodisslightlybetterthanforthe area-based method sincethe constituent subtractioncorrects partially forthelocalbackgroundfluctuationswhilethearea-basedmethod onlycorrectsfortheaveragebackground.
Since embedding a full PYTHIA event, including the underly-ing event, into the sparse p–Pb event would significantly distort thep–Pb backgroundestimate,theaboveprocedure,devisedwith Pb–Pb collisionsinmind,wasmodifiedforp–Pb collisions.To min-imize the distortion, we instead embedded single tracks, whose 4-vectors correspond to jetsreconstructed froma PYTHIA simu-lation(tunePerugia2011[49]) atdetectorlevel,intop–Pb events. AfterrunningFastJetonthemeasuredeventsincludingembedded PYTHIA tracks, each resulting jet was matched with the particle levelPYTHIAjetassociatedtotheembeddedtrack.
Fig. 3showsthejetmassresolutionforPb–Pb andp–Pb colli-sions asafunctionofthejet massatparticlelevelforprobejets with60
<
pT,ch jet<
80 GeV/c.Astrong dependenceon Mprobe isobserved.The resolutionforjetswitha smallmassispoorwhile forlargerjet massesitimprovesto25%. Jetswithsmallmassare verycollimatedandtypicallyhaveasmallnumberofconstituents. The influence of the tracking inefficiency and the contamination of tracks fromthe background on thesejets are large. For large enough pT,ch jet (
>
40 GeV/c),jetswitha smalljet massare rareand therefore the poor resolution for very collimated jets with smallnumberofconstituentsisnotalimitingfactorinthis analy-sis,whichwasrestrictedtojetswithpT,ch jet
>
60 GeV/c forPb–Pbandp–Pb collisions.Forexample,onlyabout16% ofthejetshave amasssmallerthan6GeV/c2 withinthe60–80 GeV/c p
T,ch jet
in-tervalinPYTHIA.
The jet massscale andresolutionin p–Pb collisionsare dom-inated bytracking inefficiency,the massassumptionfor the con-stituents,and,lessstrongly,bytrackmomentumresolution.Thejet
Fig. 2. Jetmassscalecharacterizedbytherelativemeanandmostprobablevalueoftheresponse.Jetmassscaleisshownasafunctionofprobejet pTforbackground fluctuationsandthefullresponseincludingdetectoreffects,usinganti-kTPYTHIAjetswith R=0.4 embeddedintocentralPb–Pb collisions.Left:area-basedsubtraction method.Right:constituentsubtractionmethod.
massresolutioninp–Pb collisionsatsmalljetmassisbyafactor 2betterthaninPb–Pb collisionsduetothemuch smaller contri-butionoftheunderlyingevent.Atlargejet masstheresolutionis similarfor thetwo collision systems,25% for Pb–Pb and 20% for p–Pb,andmainlydrivenbydetectoreffects.
5. Uncorrectedjetmassdistributionsandcorrections
5.1.ComparisonofjetmassinPb–Pb toPYTHIAatdetectorlevel
Itis common useto compare uncorrected Pb–Pb results with embedded pp or PYTHIA events,including in the latter detector and background effects. We perform this comparison and then proceed with the full correction inorder to compare withp–Pb correctedresultsandparticle-leveleventgenerators.
In this section, the jet mass distributions measured in cen-tralPb–Pb collisionsarecomparedtohybriddetector-levelPYTHIA jets. Thebackground was subtracted fromthe jettransverse mo-mentumandmass usingthe area-basedandconstituent
subtrac-Fig. 3. JetmassresolutionasafunctionoftruejetmassMprobech jetforprobeswith60<
pprobeT,ch jet(GeV/c)<80GeV/c.Pb–Pb:Anti-kTPYTHIAjetswithR=0.4 embeddedinto centralPb–Pb collisionstakinginto accountbackgroundfluctuationsanddetector effects.p–Pb:4-vectorscorrespondingtodetector-levelPYTHIAjetsembeddedinto p–Pb events.
tion methods. A comparison of the jet distributions (normalized per jet) is shown in Fig. 4. It can be observed that the Pb–Pb and PYTHIA distributions are similar, which supports the valid-ity of using embedded PYTHIA for the corrections as discussed inSec. 5.2.The constituentmethodgivessystematicallylower jet mass than the area-based method,due to the different effect of background fluctuations for the two subtraction algorithms, see Sec. 4.The lower panelsofFig. 4 show the ratiobetweenPb–Pb and PYTHIA embedded jets. The ratio asa function of jet mass showsthatthemeasureddistributionsareverysimilartothe em-beddedPYTHIAjets,orpossiblyasmallshifttolowermass,which ishowevermorepronounced fortheconstituentbackground sub-traction method. The hint of a shift is more pronounced in the meanjet mass,which isslightlysmallerin Pb–Pb collisionsthan embedded PYTHIAevents,asshowninFig. 5.Alsowhen compar-ingthecorrectedresultswithPYTHIAatparticlelevellaterinthis letter,thedatashowahintofashifttowardssmallermasseswith respecttoPYTHIAwhenconsideringonlystatisticaluncertainties.
Fig. 5. ComparisonofthemeanjetmassinPb–Pb collisionstodetector-level embed-dedPYTHIAjets,foranti-kTchargedjetswithR=0.4 in10%mostcentralPb–Pb collisions.Backgroundsubtractionwiththearea-basedmethod.
Fig. 4. Detector-leveljetmassdistributionsinPb–Pb dataandPYTHIA(tuneA)embeddedintoPb–Pb collisions.Centrality:0–10%.Anti-kT withR=0.4.Left:area-based backgroundsubtraction.Right:constituentbackgroundsubtraction.
5.2. Correctionofjetmasstoparticlelevel
Forthecorrectionofthejetmassmeasurementtoparticlelevel, atwo-dimensionalBayesianunfoldingtechnique[50]from RooUn-fold[51] was used.Afour-dimensionalresponse matrixwas con-structed withthe following axes: particle-level pT,ch jet,
detector-level pT,ch jet, particle-level Mch jet and detector-level Mch jet. For
thePb–Pb analysis,detector-leveljetswereobtainedbyembedding detector-levelPYTHIAjetsintoPb–Pb events,runningthejetfinder and applyingthe background subtractionas explained in Sec. 3. A projectionoftheresponseonthedetectorlevelmass isshown inFig. 4.Asdiscussed inSec.4,theembedded detector-leveljets werematchedtothedetector-leveljetswithoutPb–Pb background. Thelatterwerematchedtoparticle-leveljetsinsuchawayto ob-tainauniquematchingbetweeneachdetector-levelembeddedjet andthecorrespondingparticle-leveljet.
For the p–Pb analysis, detector-level jets were obtained from embedding detector-level jet four-momentum vectors into p–Pb events(seeSec.4).Thereconstructedembeddedjetswerematched with the particle-level four-momentum vectorscorresponding to thedetectorlevelembedded fourmomenta.Thefour-dimensional matrix contains the smearing in jet pT and mass due to back-groundanddetectoreffects.
The four-dimensionalresponse matrixwas usedto unfoldthe jet pT andmasssimultaneously,takingadvantageoftheobserved strongcorrelationbetweenthejettransversemomentumandmass fluctuations caused by the residual region-to-region background fluctuations,whichreducesoff-diagonalelements intheresponse matrix. The relationship betweenthe transverse momentum and massofthejetatparticlelevelintheresponse,calledtheprior,is obtainedfromPYTHIAsimulations(tuneAforPb–Pb andPerugia 2011forp–Pb).Avariation ofthisassumptionwas consideredin thesystematicuncertainties(Sec.6).
Theunfoldingprocedurewas validatedusingaMCclosuretest by applying the correction procedure to PYTHIA embedded jets. For the signal and the response matrix, statisticallyindependent datasetswereused.Thebackgroundsubtractedandunfoldedand true distributions agree witheach other to a precision of 5% for
pT,ch jet
>
40 GeV/c. The refolded distribution, obtained bycon-voluting the unfolded solution with the response matrix, is in agreement with the measured distribution within the statistical uncertainty.
6. Systematicuncertainties
The systematic uncertainties for the jet mass measurement were determined by varying parameters and algorithmic choices ofthe measurement, corrections fordetectorresponse and back-ground fluctuations. The main systematic uncertainties originate fromtheregularizationoftheunfoldingalgorithm,thebackground subtractionmethodandtheuncertaintyonthedetectorresponse. ForthePb–Pb analysis,alsothechoiceoftheprior,therelation be-tweenmassandpT attheparticlelevel,usedintheunfoldinghas animportanteffect.Inthissectionthemethodtoestimatethe sys-tematicuncertaintyforeachsourceandtheirmagnitudeincentral Pb–Pb collisionsandp–Pb collisionswillbediscussed.
The unfolding procedure converges after a certain number of iterations. Only relatively small variations in the results are ex-pected when the convergence is reached. The sensitivity to the numberofiterations chosen asdefaultwas estimatedby varying their number over a wide range, where the convergence of the resultis verified. The nominalnumber of iterations used forthe Pb–Pb measurementis6andthenumberofiterationswas varied from3to10.Forp–Pb collisionsthedefaultis3andthenumber was varied between1 and5. Changing the numberof iterations
shifts the full jet mass distribution to higher or lower jet mass, resultingin ananti-correlated shapeuncertainty. Therelative un-certaintyislargestinthetailsofthejetmassdistributionwhereit amounts to20% inPb–Pb collisions and5–20%inp–Pb collisions fordifferent pT ranges.In thepeak regionof thejet mass distri-butions theuncertaintydoesnot exceed5%(2%) inPb–Pb (p–Pb) collisions.Thesizeoftheuncertaintyinthenumberofiterationsis correlated withthestatistical uncertaintyandtheuncertainty on thedatapointsiscorrelatedpoint-to-point.
The prior used for the Bayesian unfolding was taken from PYTHIA simulations. The mean jet mass as a function of uncor-rectedbutbackground-subtractedjet pT is1–4%smallerinPb–Pb collisions thaninPYTHIAsimulationsasshowninFig. 5.The sec-ondcentral momentsofthedistributions arestatistically compat-ible indicating that the shape of the distribution is unchanged. Therefore it isreasonable to apply a shift ofatmaximum 4% on the jet massin the prior toestimate a systematicuncertainty to themeasurementduetothepriorchoice.Thisresultsina system-aticuncertaintyof10%aroundthejetmasspeak,whichincreases gradually to50%inthetails.Forthep–Pb analysis, asmearingof the massatparticle levelinthe responsematrix wasperformed. ThenewparticlelevelmassisextractedrandomlyfromaGaussian centeredattheoriginalmasswitha
σ
of2%,roughly correspond-ing tothemaximumspreadobservedintheratioofthejet mass distributionintheresponseatdetectorlevelandinthedata.The resultinguncertaintyrangesfrom4%to6%,withthelargestvalue reachedinthefirst pTrange.For the jet-by-jet background subtraction in Pb–Pb collisions, theresultfromthearea-basedmethodwascomparedtothe con-stituent subtraction. The response matrix forthe methodsis dif-ferentsincethejetmassscalediffersaswas showninFig. 2.The response matrixinboth caseswas obtainedusingtheembedding technique presentedin Sec.4. The systematicuncertainty dueto thebackgroundsubtractionmethodvariesbetween5%atthe cen-terofthedistributionand30%inthetails.
As mentionedinSec. 3.2,inp–Pb eventsthe background sub-traction introduces additional fluctuations due to the region-to-regionfluctuationsofthebackground,whichleadstoabroadening ofthejet massdistributionaftersubtraction.Itwas therefore de-cidednottoperformthesubtractionevent-by-eventandjet-by-jet, and instead include the background in the response matrix and correct for in it the unfolding. As an extreme variation for the systematic uncertainty, the background was subtracted event-by-eventinthedatawiththeconstituentmethod,whichisless sen-sitivetofluctuationsthantheareamethod,andcorrectedonlyfor detectoreffectsusingthePYTHIAresponse.Thejetmass distribu-tionscorrectedwiththetwoassumptionsdifferby5%inthepeak regionandthedifferenceincreasesgraduallyupto40%inthe low-masstail.Thesevariationsweretakenassystematicuncertainties.
The uncertainty in the detector response was dominated by theuncertaintyinthetrackingefficiency,whichwasestimatedby varyingtrackqualitycutsandfoundtobe3–4%.Thetracking effi-ciencyinthedetectorsimulationwasvariedaccordingly,providing analternativeresponsematrixwithwhichtorepeattheunfolding. Observeddifferenceswithrespecttothenominalresultvaryfrom 10% to 40%and5% to 30%in Pb–Pb and p–Pb,respectively,with thelargestuncertaintyinthetailsofthedistributions.
Allsystematicuncertaintieswereaddedinquadratureforeach
Mch jet bin.Theuncertaintiesaffecttheshapeofthejetmass
dis-tribution and the normalization applied causes long-range anti-correlations. The uncertainty on the mean jet mass as a func-tion of pT,ch jet was evaluated onthe unfolded distribution using
the variations mentioned above and shown in Table 1. The to-tal systematic uncertainty in the mean jet mass increases from 6% for jets with 60
<
pT,ch jet<
80 GeV/c to 9.0% for jets withTable 1
Systematicuncertaintyinmeanjetmassfromdifferentsourcesinthe10%mostcentralPb–Pb collisions(left)andminimum-biasp–Pb collisions(right). SourcepT,ch jet (GeV/c) Pb–Pb p–Pb 60–80 80–100 100–120 60–80 80–100 100–120 Prior 1.0% 3.0% 5.0% 0 0 0 Background 3.0% 3.0% 5.0% 1.0% 0.5% 1.0% Tracking efficiency 5.0% 5.0% 5.0% 3.0% 3.0% 3.0%
Unfolding (iterations, range) 1.0% 3.0% 4.0% 0.5% 1.0% 4.0%
Total 6.0% 8.0% 9.0% 3.5% 3.5% 4.5%
Fig. 6. Fully-correctedjetmassdistributionforanti-kT jetswith R=0.4 inp–Pb collisions,comparedtoPYTHIAandHERWIGsimulationsfor threerangesof pT,ch jet. Statisticaluncertaintiesindataaresmallerthanthemarkersandinthemodelsaresmallerthanthelinewidth.
Fig. 7. Fully-corrected jet mass distribution for anti-kTjets with R=0.4 in minimum bias p–Pb collisions compared to central Pb–Pb collisions for three ranges of pT,ch jet. 100
<
pT,ch jet<
120 GeV/c inPb–Pb centralcollisions.Thesystem-aticuncertaintyinp–Pb collisionsisabouttwotimessmallerthan in central Pb–Pb collisions due to the much smaller underlying eventcontribution.
7.Resultsanddiscussion
7.1.JetmassmeasurementsinPb–Pb andp–Pb collisions
Thefullyunfolded jet massdistributions includingall system-aticuncertainties,measuredinp–Pb collisionsat
√
sNN=
5.
02 TeV inthreeranges of pT,ch jet between60and120 GeV/c are shownin Fig. 6 and compared with PYTHIA Perugia 2011 and
HER-WIGEE5C[30,52]. Minimum-biastriggered events were used for
pT,ch jet
<
80 GeV/c, while the online jet triggered eventsam-plewas usedfor pT,ch jet
≥
80 GeV/c. The agreementof data andPYTHIAis within 10–20% for mostof the Mch jet range. The
de-viationsincrease for the low and highmass tail andcan exceed 30–50% for the intermediate pT,ch jet range. The agreement with
HERWIGisslightlyworse,mostly inthelowmasstailofthe dis-tributionandinthehighestpT,ch jet interval.Consideringthegood
agreementwithsimulations andthatthejet nuclearmodification factors RpPb and QpPb measurements show nocold nuclear
mat-tereffects[45,53–55],thep–Pb measurement(andPYTHIA)canbe
usedasa referencefortheassessmentof thehot nuclearmatter effectsinPb–Pb collisions.
Fig. 7 showsthe comparisonofthejet massdistribution, nor-malizedperjet,incentralPb–Pb collisionsat
√
sNN=
2.
76 TeV and the p–Pb collision measurement. It can be observed that the jet mass distribution in Pb–Pb collisionsis shifted to smaller values withrespecttothemeasurementinp–Pb collisions forpT,ch jet<
100GeV/c.
Fig. 8 shows the ratio between the jet mass distribution in the 10% most central Pb–Pb collisions and p–Pb collisions. The systematic uncertainties are propagated into the ratio as uncor-related. The center-of-massenergyatwhich thePb–Pb and p–Pb collisions weretakenisdifferent,
√
sNN=
2.
76 TeV for Pb–Pb and√
sNN
=
5.
02 TeV forp–Pb collisions.Thisisexpectedtointroduce a smalldifferenceinthe jetmassdistributions duetoa different shapeintheunderlyingjet pT-spectrumandadifferent quark-to-gluon ratio.Therefore,the figureshows alsothesame ratiofrom particlelevel simulatedPYTHIApp collisions (tunePerugia 2011) at the two energies. Considering statistical uncertainties only in the ratio, a shift to lower jet masses in Pb–Pb is observed forpT,ch jet
<
100 GeV/c,consistentwiththePYTHIAembeddedresultsinSec.5.1.Includingthesystematicuncertaintiesinour measure-ments,thedecreasingtrendoftheratioasafunctionofMch jet is
Fig. 8. Ratiobetweenfully-correctedjetmassdistributionforanti-kTjetswithR=0.4 incentralPb–Pb collisionsandminimumbiasp–Pb collisions.Theratioiscompared totheratioofmassdistributionsofPYTHIA(tunePerugia2011)at√s=2.76 TeV and√s=5.02 TeV (widthofthebandrepresentsthestatisticaluncertainties).
Fig. 9. Fully-correctedmeanjetmassasafunctionofpT,ch jetforanti-kTjetswith
R=0.4 inminimumbiasp–Pb collisionsat√sNN=5.02 TeV comparedtocentral Pb–Pb collisionsat√sNN=2.76 TeV.
compatiblebetweendataandPYTHIAandnosignificantreduction injetmassinPb–Pb collisionsisobserved.
Thecomparisonof thejet massinPb–Pb collisions relativeto p–Pb collisions isfurther established by presenting themean jet massasafunctionofpT,ch jetinFig. 9.Thedifferenceinthemean
jetmass forthe twocollision energiesconsidered isbetween0
.
2 and 0.
5 GeV/c2 in the PYTHIA simulation.This difference in themean jet mass isindicated by a filled box attachedto the p–Pb datapoints inFig. 9.Forthe lowest pT,ch jet rangeinPb–Pb
colli-sionsthemeanjetmassexhibitsareductionwithrespecttop–Pb measurements,limitedtoaboutonestandarddeviation.Forhigher
pT,ch jet themeanjetmassinthetwosystemsiscompatiblewithin
systematicuncertainties.
7.2. Modelcomparisonanddiscussion
The jet mass measurements for central Pb–Pb collisions for three pT,ch jet intervalscompared to severalevent generators are
shownin Fig. 10. PYTHIArepresents the expectationwithout jet quenchingwhileJEWEL[56,57]andQ-PYTHIA[58](withPQM ge-ometry [59]) are two models withmedium-induced energyloss. InJEWEL each scatteringofthe leading partonwithconstituents from the medium is computed giving a microscopic description of the transport coefficient, q.
ˆ
By default, JEWEL does not keep track ofthe momenta of the recoiling scattering centers (“recoil off”). This leads to a net loss of energy and momentum out of thedi-jetsystem,andisexpectedtomostlyaffectlow-pT-particleproduction.Forthe jet massmeasurement, low-momentum frag-mentsareimportant,soJEWELwasalsoruninthemodeinwhich itkeepstrackofthescatteringcenters(“recoilon”).Inthatmode,
more softparticlesare generated,some ofwhichhavevery large angles with the jet and will contribute to the background esti-mateintheevent.TheJEWELauthorsimplementedabackground subtraction infull jetsby introducing “fake” neutral constituents used for the 4-momentum subtraction. Since the pp charged jet mass distribution isreproduced by shifting thefull jet mass dis-tributiontowardslowermasses,theJEWELbackground-subtracted chargedjetmassisobtainedbyshiftingthebackground-subtracted full jet mass. Q-PYTHIA modifies the splitting functions in the PYTHIAeventgenerator,resultinginmedium-inducedgluon radia-tionfollowingthemultiplesoftscatteringapproximation.Both jet quenchingmodelsreproducethesuppressionobservedininclusive high-pTparticleandjetproduction[57,58].
ThejetmassisstronglyoverestimatedbyQ-PYTHIAduetothe strongbroadeningofthejetprofileclosetothejetaxis.AlsoJEWEL with “recoil on” significantly overestimates the jet mass. JEWEL “recoil off”underestimates the jetmass duetothe large amount of out-of-cone radiation,which doesnot hadronize in thismode of the generator. The vacuum expectation from PYTHIA, while slightlyoverestimating the jet massforlower pT,ch jet when
con-sideringstatisticaluncertaintiesonly,iscompatiblewiththePb–Pb measurementwithinsystematicuncertainties.ThePb–Pb meanjet massasafunctionofpT,ch jetiscomparedtotheeventgenerators
inFig. 11.Thelinearincreaseofthemeanjetmasswithjet pT is expectedfromNLOpQCDcalculations[60].
Previous jet shape and jet fragmentation function measure-ments clearlyfavorJEWELwith“recoilon”overQ-PYTHIA[20,19, 61–64]. Despite the difference in the fragment distributions be-tween Q-PYTHIA and JEWELwith “recoilon”, Fig. 10shows that bothmodelspredictasimilarlargeincreaseofthejetmass,which is excluded by the measurement. JEWEL “recoil off”,which does not describetheprevious measurementswell becauseitdoesnot includeallsoftradiation,givesabetterdescriptionofthejetmass thanJEWEL“recoilon”.Thedifferencebetweenthejetmass distri-butionsinJEWELwith“recoilon”an“recoiloff”indicatesthatthe jetmassissensitivetothesoftfragmentsatlargeanglewhichare producedbyhadronisationofrecoilpartonsintheJEWELmodel.
7.3. Summary
The first jet mass measurement in heavy-ion collisions for charged jets (60
<
pT,ch jet<
120 GeV/c) was reported andcom-paredtop–Pb referencemeasurementsandmodelswithand with-outquenching.Thepresentedresultsarethefirstattempttoaccess thevirtualityevolutionofthehardpartonsinheavy-ioncollisions. By constraining both energyand virtualityexperimentally, differ-ential jet mass measurements could provide further non-trivial testsformodelsofin-mediumshowerevolution.
Fig. 10. Fully-correctedjetmassdistributionfor anti-kT jetswith R=0.4 inthe 10%mostcentralPb–Pb collisions comparedtoPYTHIA withtunePerugia2011and predictionsfromthejetquenchingeventgenerators(JEWELandQ-PYTHIA).Statisticaluncertaintiesarenotshownforthemodelcalculations.
Fig. 11. Fully-correctedmeanjetmasscomparedtoPYTHIAPerugia2011andthejet quenchingeventgenerators(JEWELandQ-PYTHIA)foranti-kTjetswithR=0.4 in the10%mostcentralPb–Pb collisions.
TheratioofthejetmassdistributionincentralPb–Pb collisions andminimum-biasp–Pb collisionsiscomparedtothatinPYTHIA Perugia2011simulationsatthetwo center-of-massenergies. The dataratio iscompatiblewith thePYTHIA expectationatthe two center-of-massenergies withinsystematicuncertainties. Ahintof a difference within statisticaluncertainties only in the ratioand inthemeanjet massinthelowest pT,ch jet rangeisofinterestto
motivatefurtherworkonreducing thesystematicuncertaintiesin ordertoincrease theprecision injetmassmeasurements aswell as pursuing more differential studies, for example with respect to hard fragmenting jets. The fully-corrected results are consis-tentwiththeobservationbasedondetectorlevelcomparisonwith PYTHIAembeddedjets.ThemeasuredjetmassinPb–Pb collisions isnotreproducedbythequenchingmodelsconsideredinthis let-terandisfoundtobeconsistentwithPYTHIAvacuumexpectations withinsystematicuncertainties.Theseresultsarequalitatively con-sistentwithpreviousmeasurements ofjetshapesattheLHC[20, 62],whichshowonlyrelatively smallchangesoftheparticle dis-tributionsinjetsinPb–Pb collisionscomparedtoppcollisions.The JEWELmodelwith“recoil on”,whichdescribestheexisting mea-surementsoffragmentdistributionsinjets[19,20]reasonablywell [61,63],predicts asignificantincrease ofthejet mass,contraryto whatisobservedinthemeasurement.
The observed suppression of jet yields in the presence of a dense medium, RAA
<
1 [65], is interpreted as due to radiated partons lost or scattered out of the jet cone. Therefore, one re-constructsasubset oftheentirepartonshower within ajet with resolutionparameter0.4.Intheextremecasethatonlytheleading partonwereto escapethe medium,andthen showerin vacuum, onewouldreconstructthemassoftheleadingpartonatthepointofexit.Since alsothevirtualityevolutionofthepartonshower is modified in the presence of jet quenching, one would expect in suchascenariothattheescaping(reconstructed)jetsexhibita re-ducedjet mass withrespectto thepp andp–Pb references [21]. The data show that the jet mass is consistent within uncertain-tiesin Pb–Pb and p–Pb collisions within a fixed pT,ch jet-interval,
implyingthatthesoftradiationoutsidethejetconedoesnot sig-nificantlyaltertherelationbetweenpTandthemassoftheparton.
Acknowledgements
The ALICE Collaboration would like to thank all its engineers andtechniciansfortheir invaluablecontributions tothe construc-tion of the experiment and the CERN accelerator teams for the outstanding performance ofthe LHC complex.The ALICE Collab-oration gratefully acknowledges the resources and support pro-videdbyallGridcentresandtheWorldwideLHCComputingGrid (WLCG) collaboration. The ALICE Collaboration acknowledges the followingfunding agenciesfortheir supportinbuildingand run-ningtheALICEdetector:A.I.AlikhanyanNationalScience Labora-tory(YerevanPhysicsInstitute)Foundation(ANSL),State Commit-teeofScienceandWorldFederationofScientists(WFS),Armenia; Austrian AcademyofSciences andNationalstiftungfür Forschung, Technologie und Entwicklung, Austria; Ministry of Communica-tions and High Technologies, National Nuclear Research Center, Azerbaijan; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Universidade Federal do Rio Grande do Sul (UFRGS), Financiadora de Estudos e Projetos (Finep) and Fun-dação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Brazil; Ministry of Science & Technology of China (MSTC), Na-tional Natural Science Foundation of China (NSFC) and Ministry of Education of China (MOEC), China; Ministry of Science, Edu-cation and Sportand Croatian Science Foundation, Croatia; Min-istryofEducation,YouthandSportsoftheCzech Republic,Czech Republic; The Danish Council for Independent Research | Natu-ral Sciences, the Carlsberg Foundation and Danish National Re-search Foundation (DNRF), Denmark; HelsinkiInstitute of Physics (HIP),Finland;Commissariatàl’EnergieAtomique(CEA)and Insti-tut National de Physique Nucléaire et de Physique des Particules (IN2P3) andCentre Nationalde la Recherche Scientifique(CNRS), France; Bundesministerium für Bildung, Wissenschaft, Forschung undTechnologie (BMBF)andGSI Helmholtzzentrum für Schweri-onenforschung GmbH, Germany; Ministry of Education, Research andReligiousAffairs,Greece;NationalResearch,Developmentand Innovation Office, Hungary; Department of Atomic Energy Gov-ernment of India (DAE) and Council of Scientific and Industrial Research (CSIR), New Delhi, India; Indonesian Institute of Sci-ence, Indonesia;CentroFermi–MuseoStoricodellaFisica e Cen-tro Studi e RicercheEnrico FermiandIstituto Nazionale diFisica
Nucleare (INFN), Italy; Institute for InnovativeScience and Tech-nology, Nagasaki Institute of Applied Science (IIST), Japan Soci-ety for the Promotion of Science (JSPS) KAKENHI and Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan; Consejo Nacional de Ciencia (CONACYT) y Tec-nología,throughFondode CooperaciónInternacionalenCienciay Tecnología(FONCICYT)andDirecciónGeneralde Asuntosdel Per-sonal Academico (DGAPA), Mexico; Nationaal instituut voor sub-atomaire fysica (Nikhef), Netherlands; The Research Council of Norway,Norway;CommissiononScienceandTechnologyfor Sus-tainableDevelopmentintheSouth(COMSATS),Pakistan;Pontificia UniversidadCatólicadelPerú,Peru;MinistryofScienceandHigher EducationandNationalScience Centre, Poland;Korea Institute of Science andTechnology InformationandNationalResearch Foun-dation of Korea (NRF), Republic of Korea; Ministry of Education andScientificResearch,Institute ofAtomicPhysicsandRomanian National Agency for Science, Technology and Innovation, Roma-nia;JointInstituteforNuclear Research(JINR), Ministryof Educa-tionandScienceofthe RussianFederationandNationalResearch CentreKurchatovInstitute, Russia;MinistryofEducation,Science, ResearchandSportofthe SlovakRepublic, Slovakia; National Re-search Foundation of South Africa, South Africa; Centro de Apli-cacionesTecnológicasyDesarrolloNuclear(CEADEN),Cubaenergía, Cuba, Ministerio de Ciencia e Innovacion and Centro de Investi-gacionesEnergéticas, Medioambientales yTecnológicas (CIEMAT), Spain;SwedishResearchCouncil(VR)andKnut&AliceWallenberg Foundation(KAW),Sweden;EuropeanOrganizationforNuclear Re-search,Switzerland;NationalScienceandTechnologyDevelopment Agency(NSDTA),SuranareeUniversityofTechnology(SUT)and Of-fice of the Higher Education Commission under NRU project of Thailand,Thailand;TurkishAtomicEnergyAgency(TAEK),Turkey; National Academy of Sciences of Ukraine, Ukraine; Science and TechnologyFacilitiesCouncil(STFC),UnitedKingdom;National Sci-enceFoundationoftheUnitedStatesofAmerica(NSF)andUnited StatesDepartmentof Energy,Office ofNuclear Physics (DOE NP), UnitedStatesofAmerica.
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