First search for K + →π + νν¯ using the decay-in-flight technique

(1)

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

First

search

for

K

+

→

π

+

ν

¯

using

the

decay-in-ﬂight

technique

.The

NA62

Collaboration

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received24November2018

Receivedinrevisedform21December2018 Accepted4January2019

Availableonline23February2019 Editor:W.-D.Schlatter

Thispaperisdedicatedtothememoryof ourcolleaguesS.BalevandF.Hahn.

TheNA62experimentattheCERNSPSreportstheﬁrstsearchforK+→

π

+

ν

¯ usingthedecay-in-ﬂight technique,basedonasampleof1.21_×1011 _K+_decays_collected_in_2016. _The_single_event_sensitivity is3.15×10−10_,_{corresponding}_to_0.267_Standard_Model_events._One_signal_candidate_is_observed_while

theexpectedbackgroundis0.152events.Thisleadstoanupperlimitof14×10−10_on_the_K+_→

_π

+

_ν

_¯ branchingratioat95%CL.

(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.

1. Introduction

The ﬂavour-changing neutralcurrent decay K+

→

π

+

ν

¯

pro-ceeds at the lowest order in the Standard Model (SM) through electroweak box and penguin diagrams largely dominated by t-quarkexchange.ThequadraticGIMmechanismandthesmallvalue ofthe CKM element

|

Vtd

|

describing the transitionofa top into

a down quark make thisprocess extremelyrare. Using tree-level elements of the CKM matrix as external inputs, the SM predicts [1] the branchingratiotobeBR

= (

8

.

4

±

1

.

0

)

×

10−11,wherethe uncertainty is dominated by the current precision on the CKM parameters. The intrinsic theoretical accuracy is at the 2% level, asthe computation includesNLO (NNLO)QCD corrections tothe top (charm) quark contribution [2,3] and NLO electroweak cor-rections [4]. Moreover, the hadronic matrix element largely can-celswhennormalisedtothepreciselymeasured BRofthe K+

→

π

0_e+

_ν

_decay, _with_isospin _breaking _and_{non-perturbative}_effects

calculatedindetail [4,5].

The K+

→

π

+

ν

¯

decayissensitivetophysicsbeyondtheSM. The largestdeviations fromthe SM are expectedin models with newsourcesofﬂavourviolation,whereconstraintsfromB physics

are weaker [6,7]. The experimental value of the CP-violation pa-rameter

ε

K limits the expected BR

(

K+

→

π

+

ν

¯

)

range within

models withcurrents ofdeﬁnedchirality, producing speciﬁc cor-relationpatterns betweenthe K+

→

π

+

ν

¯

and KL

→

π

0

ν

¯

de-cay rates [8]. Present experimental constraints limit the range of variation within supersymmetric models [9–11]. The K+

→

π

+

ν

¯

decaycanalsobesensitivetoeffectsofleptonﬂavour non-universality [12] and can constrain leptoquark models [13] aim-ing to explain the measured value ofthe CP-violation parameter

ε

/

ε

[14].

The E787 and E949 experiments at BNL [15,16] studied the

K+

→

π

+

ν

¯

decay usinga decay-at-resttechnique andobtained BR

= (

17

.

3+₋11₁₀._.5₅

)

×

10−11.The NA62experiment attheCERN SPS

aims to measure BR

(

K+

→

π

+

ν

¯

)

more precisely with a novel decay-in-ﬂighttechnique.Thisletterreportsaresultfromthe anal-ysisofdatacollectedbyNA62in2016,correspondingto2%ofthe full statistics accumulated during the 2016–2018 data-taking pe-riod.

2. Beamlineanddetector

Thechoiceofthedecay-in-ﬂight techniqueismotivatedbythe possibility of obtaining an integrated ﬂux of

O(

1013

)

kaon de-cays overa few yearsof data-taking witha signal acceptance of a few percent, leading to the collection of

O

(100) SM events in the K+

→

π

+

ν

¯

channel. This technique utilises a high energy beamtoboostthekaondecayproductstohighenergywheretheir detection and identification can be efficient. The boost folds the decayproductsintoasmallangularregionclosetothebeamaxis allowing fora classic fixed target geometry experiment.The de-tector should measure the incomingkaon andthe outgoingpion whilereducingbackgroundtothelevelof10−11_of_accepted_kaon

decaysin thehighrateenvironmentnecessaryto achieve the re-quiredsensitivity.Tothisend,thedetectorconsistsofacollection of sub-detectors, each designed to perform one main task, and

withcomplementaryperformance.

The NA62beamlineanddetectorlayout isdescribed indetail in [17] andshowninFig.1togetherwiththescaleandreference system:thebeamlinedeﬁnestheZaxiswithitsoriginatthekaon productiontargetandbeamparticlestravellinginthepositive di-rection,theYaxispointsverticallyup,andtheXaxisishorizontal anddirectedtoformaright-handedcoordinatesystem.

A secondary hadron beam of positive charge containing 70%

π

+, 23% protons and 6% K+, with a nominal momentum of

75 GeV/c and1% rmsmomentumbite,isderivedfrom400 GeV/c protons extractedfrom theSPS in spillsof 3 seffective duration andinteractingwitha40 cmlongberylliumtarget.Typical

inten-https://doi.org/10.1016/j.physletb.2019.01.067

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Fig. 1. SchematictopviewoftheNA62beamlineanddetector.Dipolemagnetsaredisplayedasboxeswithsuperimposedcrosses.Thetrajectoryisshownofanun-decayed beamparticleinvacuum,crossingthedetectorapertureswhichavoidinteractionswithmaterial.AdipolemagnetbetweenMUV3andSACdeﬂectsthechargedparticlesof thebeamoutoftheSACacceptance.

sitiesforthe presentmeasurement rangefrom 1

.

0 to 1

.

3

×

1012

protonsperpulse(ppp).

Thetime,momentumanddirectionofthechargedcomponents ofthe K+

→

π

+

ν

¯

decay are measured by the following detec-tors. A differential Cherenkov counter (KTAG, ﬁlled with N2 at

1.75 barpressure andread out by PMs groupedin eightsectors) tags incoming kaons with a 70 ps time resolution. Three silicon pixel stations (GTK) located before, between and after two pairs ofdipole magnets(beamachromat), formaspectrometer to mea-suremomentum, directionand time ofbeam particles with0.15 GeV/c,16

μ

rad and100 ps resolutions. Amagnetic spectrometer (STRAW,comprisingtwopairsofstrawchambersoneithersideof adipole magnet)measuresthemomentum-vectoroftheoutgoing particlewithamomentumresolution

σ

p

/

p inthe0.3–0.4%range.

Aring-imaging Cherenkov counter (RICH, ﬁlled withneon at at-mosphericpressure)tagsthedecayparticlewithaprecisionbetter than100 psandprovidesparticleidentiﬁcation.

Twoscintillator hodoscopes (CHOD,a matrixof tiles read out bySiPMsandNA48-CHOD,composedoftwo orthogonalplanesof slabs, reused fromthe NA48 experiment) are used for triggering andtiming purposes,providinga 99%eﬃcienttrigger anda time measurementwith200psresolutionforchargedparticles.

ThethirdGTKstation(GTK3)isimmediatelyprecededbyafinal collimator topartly block particlesproduced in upstreamdecays, andmarks thebeginningofa 117 m-longvacuumtank. Thefirst 80 mofthetankdefineafiducialvolume(FV)inwhich13%ofthe kaonsdecay.The beamhasa rectangulartransverse profileof52

×

24mm2 andadivergenceof0.11 mrad(rms)ineach planeat theFVentrance.Thetypicalbeamparticlerateis300 MHz.

Othersub-detectorsareusedasvetoestosuppressdecaysinto photonsormultiplechargedparticles(electrons,pionsormuons) or as complementary particle-identiﬁers. Six stations of plastic scintillatorbars(CHANTI)detectwith99%eﬃciencyand1 nstime resolution extra activity including inelastic interactions in GTK3. Twelvestationsofring-shapedelectromagneticcalorimeters(LAV1 to LAV12, madeof lead-glass blocks) surround the vacuumtank andthedetectortoachievehermeticacceptanceforphotons

emit-ted in K+ decays in the FV at polar angles between 10 and

50 mrad. A 27 radiation length thick quasi-homogeneous liquid krypton electromagnetic calorimeter (LKr) detects photons from

K+ decaysemittedatanglesbetween1and10 mradand comple-mentsthe RICHforparticleidentiﬁcation. The LKrenergy, spatial andtime resolutions in NA62 conditions are

σ

E

/

E

=

1

.

4% at an

energydepositof25 GeV,1 mmandbetween0.5and1 ns,

respec-tively,depending ontheamountandtypeofenergyrelease.Two hadroniciron/scintillator-stripsamplingcalorimeters(MUV1,2)and an arrayofscintillator tileslocatedbehind 80cmofiron (MUV3, with400 pstimeresolution) supplementthepion/muon identiﬁ-cationsystem.Alead/scintillatorshashlikcalorimeter(IRC)located in front of the LKr, covering an annular region between 65 and 135 mmfromthe Zaxis,and asimilar detector(SAC) placed on theZaxisatthedownstreamendoftheapparatusensurethe de-tectionofphotonsdowntozerodegreesintheforwarddirection. Additionalcounters(MUV0,HASC)installedatoptimizedlocations providehermeticcoverageforchargedparticlesproducedin multi-trackkaondecays.

MostdetectorsarereadoutwithTDCs,exceptLKrandMUV1,2 readoutwith14-bitFADCs,andIRC,SACreadoutwithboth.With

the exception of GTK and STRAW, read out by speciﬁc boards,

the TDC modules are mounted on custom-made boards (TEL62),

whichbothproducetriggerinformationandperformdatareadout. Calorimetersuseadedicatedprocessorfortriggering [18].

Alow-leveltrigger(L0)exploitsthelogicalsignals(primitives)

produced by the RICH, CHOD, NA48-CHOD, LKr, LAV and MUV3.

Adedicated boardcombinesthe primitivesto build anddispatch triggerdecisionstothedetectorsfordatareadout [19].Asoftware trigger (L1) processes data from KTAG, LAV and STRAW to pro-ducehigherlevelinformationexploitingreconstructionalgorithms similartothoseusedoﬄine(Sec.3),butadaptedtotheonline en-vironment.

The data sample is obtained from about 5

×

104 SPS spills recordedduringonemonthofdata-takingin2016.Themain trig-gerchain(calledPNN)isdeﬁnedasfollows.TheL0triggerrequires a signal in RICH to tag a charged particle in coincidence within 10 nswith:asignalinatleastoneCHODtile;nosignalsin oppo-siteCHODquadrantstoreduceK+

→

π

+

π

+

π

−decays;nosignals inMUV3andLAV12toreduce K+

→

μ

+

ν

andK+

→

π

+

π

0

de-cays; and LKr energy deposit below 20 GeV to suppress K+

→

π

+

π

0 _decays._The _L1_trigger _requires:_a _kaon_identiﬁed _in_KTAG

andsignalsinatmosttwoblocksofeachLAVstation,within10 ns oftheL0triggerRICHtime;atleastoneSTRAWtrack correspond-ing toaparticlewithmomentumbelow50GeV/c whichformsa vertexwith thenominal beamaxis upstream ofthe ﬁrst STRAW chamber. Theanalysisalso usesdatatakenwitha minimum-bias L0triggerbasedonNA48-CHODinformationdownscaledbya fac-torof400(“controltriggers”)tomeasureeﬃcienciesandestimate backgrounds. Events collected withthe PNN (control) triggerare

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referredtoas“PNN-triggeredevents”(“controlevents”)inthe fol-lowing.

3. Reconstructionandcalibration

The KTAG channels are time-aligned,and signalsare grouped

within 2 ns wide windows to deﬁne candidates. The KTAG

can-didate time is used as a reference to adjust the time response oftheothersub-detectors.SignalsfromtheGTKstationsgrouped within 2 nsofaKTAG candidateformabeamtrack.Fully recon-structed K+

→

π

+

π

+

π

− decaysinthe STRAWspectrometer are used toalign the GTK stations transversally to a precision better than100

μ

mandtunetheGTKmomentumscale.

TheSTRAWreconstructionreliesontheNA48-CHODtime asa referenceto determine thedrift time. Space-points in the cham-bersdescribing apathcompatiblewiththemagneticbending de-fineatrack, anditsparametersareobtainedusingaKalman-filter fit.The

χ

2 _value _and_the_number_of_space-points _charaterize_the

trackquality.Straighttrackscollectedwiththemagnetoffserveto alignthestrawtubesto30

μ

maccuracy.Theaveragevalueofthe

K+ massreconstructedfor K+

→

π

+

π

+

π

− decaysprovidesﬁne tuningofthemomentumscaletoapermilleprecision.

TwoalgorithmsreconstructRICHcandidates,bothgrouping sig-nalsfromPMsintime aroundtheL0trigger.Theﬁrstone makes useofaSTRAWtrackasaseedtobuildaRICHringandcomputea likelihoodforseveralmasshypotheses(e+,

μ

+,

π

+andK+).The secondone (“single-ring”)ﬁtsthesignalsto aringassuming that they areproduced bya single particle,withtheﬁt

χ

2

character-izingthequalityofthishypothesis.Positronsareusedtocalibrate theRICH responseandalign thetwentyRICH mirrors toa preci-sionof30

μ

rad [20].

The CHOD candidatesare deﬁnedby the response ofthe two

SiPMsreadingoutthesametile.Signalsincrossinghorizontaland vertical slabs compatible with the passage of a charged particle formNA48-CHODcandidates;timeoffsetsdependingonthe inter-sectionpositionaccountfortheeffectoflightpropagationalonga slab.

GroupsofLKrcellswithdepositedenergywithin100 mmofa seedformLKrcandidates(clusters).Aseedisdeﬁnedbyacell in whichanenergyofatleast250 MeVisreleased.Clusterenergies, positions andtimesare reconstructedtakinginto account energy calibration, non-linearity, energy sharing for nearby clusters and noisycells.Theﬁnalcalibrationisperformedusingpositronsfrom

K+

→

π

0_e+

_ν

_decays._An_additional_{reconstruction}_algorithm_is

ap-pliedto maximisethephoton vetoeﬃciency.This isachievedby deﬁning candidates assets ofcells withatleast 40 MeV energy, closerthan100 mmandintimewithin40 nsofeachother.

The reconstruction of MUV1(2) candidates relies on the track impactpoint.Signalsinfewer than8(6)nearbyscintillator strips are groupedto forma candidate,the energyof whichis deﬁned asthesumoftheenergiesinthestrips,calibratedusingweighting factors extractedfrom dedicated simulations andtested on sam-plesof

π

+ and

μ

+.

Candidates in MUV3are deﬁnedby time coincidencesof the

responseofthetwoPMsreadingthesametile.Thetimeofa can-didateisdeﬁnedbythelater ofthetwoPM signals,to avoidthe effectofthetimespreadinducedbytheCherenkovlightproduced byparticlestraversingthePMwindow.

TheCHANTIcandidatesaredeﬁnedbysignalsclusteredintime and belonging either to adjacent parallel bars or to intersecting orthogonalbars. Twothresholdsettingsdiscriminate theCHANTI, LAV,IRCandSACTDCsignals.Thusuptofourtimemeasurements areassociatedwitheach signal,corresponding totheleading and trailing edge times of the high andlow thresholds. The relation betweentheamplitudeoftheIRCandSACpulsesprovidedbythe

FADCreadoutandtheenergyreleaseisdeterminedforeach chan-nel afterbaselinesubtractionusingasample of K+

→

π

+

π

0

de-cays.

Signal timesmeasured byGTK,KTAG,CHOD andRICHare fur-theralignedonaspillbasistotheL0RICHtime,resultingin20 ps stability through the whole data sample. Early checks on recon-structeddatahelpidentifyingspillswithhardwaremalfunctioning, whichareexcludedfromtheanalysis.

SamplesofK+ decaysareproducedusingaGeant4-based [21] MonteCarlo(MC)simulationofthesetupandusedintheanalysis to validate thedetector response, compute acceptances and esti-mate backgrounds. The simulation includes the modeling of the time development of the signals, the response of the front-end electronic and theeffects of miscalibrationasderived fromdata. Accidental activityisaddedinGTK andKTAGassuming 300 MHz beamintensity,andusinga pileupbeamparticlelibrary.No acci-dentalactivityissimulatedinthedetectorsdownstreamofthelast GTK station.Simulateddataaresubjectedtothesame reconstruc-tionandcalibrationstepsasdescribedabove.

4. Eventselection

TheK+

→

π

+

ν

¯

signatureconsistsofaK+with4-momentum

pK intheinitialstate,anda

π

+with4-momentumpπ with

miss-ing energyin the ﬁnal state. The squared missing massm2 miss

≡

(

pK

−

pπ

)

2isusedtodiscriminatekinematicallythemainK+

de-caymodesfromthesignal.Thesignalissearchedforintwom2 miss

regions on each side of the K+

→

π

+

π

0 _peak _(Fig. ₂_, _left).

Se-lection criteriabased onm2_miss alone are not suﬃcient toreduce the backgrounds to the desired level,and additional suppression by

π

+ identiﬁcationandphoton rejectionisrequired.The princi-palselectioncriteriaarelistedbelow.

Up to two positively-charged STRAWtracks are allowed inan event, provided there are no negatively-chargedtracks andthese tracks do not form a vertex in the FV. To be accepted, a track must be within theRICH, CHOD, LKr,and MUV1,2,3sensitive re-gions, andmust be associatedin positionwith candidatesin the CHOD, NA48-CHOD, LKrandRICH. Association inthe RICH relies ontheconsistencybetweenthetrackdirectiondownstreamofthe spectrometer magnet andtheposition of thering centre.Timing constraints are applied to the LKr and RICH candidates with re-specttotheNA48-CHODcandidatetime.Tracktimesaremeasured with100 psresolutionby combiningthesub-detectorsignals as-sociatedwitheachtrack.

AKTAGcandidatewithCherenkovphotonsdetectedinatleast ﬁve out ofeight sectorstags a K+. Theparent K+ isdeﬁnedas that closest intime to the

π

+ candidatewithin 2 ns. The iden-tiﬁcation of the GTK track of the parent K+ relies on its time coincidence with theKTAG andRICH, andits geometric compat-ibility withtheSTRAWtrackquantiﬁed bythe closestdistanceof approach(CDA).ProbabilitydensityfunctionsofthetimeandCDA distributions forboth K+ andaccidentalbeam particlesobtained from reconstructed K+

→

π

+

π

+

π

− decaysare used to deﬁne a discriminant.The GTK trackwithin 0.6 nsoftheKTAG timewith the highestdiscriminant value is associatedwith theparent K+. An additionalrequirementis applied onthe discriminantleading to a K

/

π

association eﬃciency value of 75%, as measured with a K+

→

π

+

π

+

π

−datasample.Thecorresponding fractionof in-correct K

/

π

associations is below 1% when the K+ is correctly reconstructed.IftheK+isnotreconstructed,theprobabilityof as-sociatingapile-upbeamtracktothe

π

+is3.5%.

The K+ and

π

+ tracks deﬁne the decay vertex. Charged pi-ons originating from K+ decays upstream ofthe ﬁnal collimator or from interactions of beam particles in the GTK stations can mimic a K+

→

π

+

ν

¯

decay if an accidental beam particle in

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Fig. 2. Left:truem2

miss distributionofthe K+→π+νν¯ decayandthemainK+decays,computedunderπ+masshypothesisforthechargedparticleintheﬁnalstate.

Signal(red)ismultipliedby1010_for_visibility_and_the_dashed_areas_show_the_signal_search_{regions. Right:}_{reconstructed}_m2

missasafunctionofπ+momentumforcontrol

events selectedwithoutapplyingπ+identiﬁcationandphotonrejection.Signalregions1and2,aswellasthe3π,π+π0_and_μ+_ν_background_regions_are_also_shown._The controlregionslocatedbetweenthesignalandbackgroundregionsareindicatedbydashedlines.

GTK matches the

π

+, leading to incorrectvertex reconstruction. Inadditionto K

/

π

association,furtherconditions suppressthese events: Zvertex

>

Z0,where Zvertex isthevertexlongitudinal

coor-dinateand Z0 liesintherange110–115 m dependingonthe

π

+

direction;the

π

+extrapolatedtotheﬁnalcollimatorZplanemust lieoutsidea200

×

1000 mm2_area_centred_around_the_Z_axis_(“box

cut”);noactivityinCHANTIisallowedwithin3 nsofthe

π

+;and fewerthanﬁveGTKtracksmaybereconstructedwithin2 nsofthe KTAG time. To reduce backgroundfrom K+

→

π

+

π

+

π

− decays andoptimizethe

π

0 _rejection, _Z

vertex is requiredtobe upstream

of160–165 m,dependingonthetrackslope.

Iftwo STRAWtracksin thesameeventsatisfy theabove con-ditions, the one closer in time to the trigger and the associated

K+ isconsidered.The CHOD,KTAG,RICHandLKrcandidatesand theGTKtrackmatchedtothe

π

+arerequiredtobetheclosestto thetriggertime.Theanalysisisrestrictedtothepionmomentum ( Pπ+)rangeof

(

15

,

35

)

GeV/c.Thisconditionensuresthepresence ofatleast40GeVenergyinadditionto the

π

+,whichimproves the rejection of backgrounds such as K+

→

π

+

π

0_. _It _also

en-hancesthe kinematic separation betweensignal and K+

→

μ

+

ν

decay,andmakestheoptimaluseoftheRICHfor

π

+

/

μ

+ separa-tion.Them2_miss isreconstructedfromtheK+ and

π

+4-momenta measuredbytheGTKandtheSTRAW.Them2_miss resolutionatthe

K+

→

π

+

π

0 _peak _is_about ₁₀−3 _GeV2_/c4_, _which_determines _the

choice of two m2_miss signal regions deﬁned as

(

0

,

0

.

01

)

GeV2/c4 (“Region 1”)and

(

0

.

026

,

0

.

068

)

GeV2/c4 (“Region 2”).Three back-ground regions (

π

+

π

0_,

_μ

+

_ν

_and ₃

_π

₎ _and _suitable _control

re-gionsarealsochosen(Fig.2,right).Signal andcontrolregionsare kept maskedforPNN-triggeredevents until thecompletion ofthe analysis.The m2

miss isalsocomputedeitherassuming theaverage

K+ beammomentumanddirection,orusingpπ measuredbythe RICHsingle-ringalgorithmassumingthe

π

+mass.Conditions im-posedonthem2_miss computedinthesealternativewaysreﬁnethe deﬁnitionofsignal regions,reducingthe K+

→

π

+

ν

¯

acceptance byafurtherrelative7%andprovidingadditionalbackground sup-pressionincaseofmis-reconstructioninSTRAWorGTK.

AmultivariateclassiﬁerbasedonaBoostedDecisionTree algo-rithm [22] combines13variablesdescribingtheenergyassociated with the

π

+, the shape of the clusters and energy sharing be-tweenthecalorimeters.PionidentiﬁcationwiththeRICH exploits theratio oflikelihoods under the

π

+ and

μ

+ hypotheses. Addi-tionalconstraintsareappliedontheparticlemasscalculatedfrom the ring radius computedby the single-ringRICH algorithm and

themomentummeasuredbytheSTRAW.The

π

+identiﬁcation ef-ﬁciencymeasured inthe

(

15

,

35

)

GeV/c momentum rangeis78% (82%) withcalorimeters (RICH), and the corresponding probabil-ityof

μ

+mis-identiﬁcationas

π

+is0

.

6

×

10−5 ₍₂

_.

₁

_×

₁₀−3_)._MC

simulations reproduce these results with10–20% accuracy. A re-quirement of no particle detected in MUV3 within 7 ns of the

π

+ reinforcestheMUV3triggercondition.

Photon rejection suppresses K+

→

π

+

π

0 _decays _which _can

mimic the signal if the two-body kinematics is not well recon-structed. Themain requirementsare: noenergydeposited inany LAVstation(IRCandSAC)within3(7) nsofthe

π

+time;no clus-tersintheLKrbeyond100 mmfromthe

π

+impactpointlocation withintime windowsrangingfrom

±

5 nsforclusterenergies be-low5 GeV to

±

50 ns above15GeV.Multiplicityrejectioncriteria against photons interacting in the detector material upstream of theLKrinclude:noin-timeactivityintheCHODandNA48-CHOD unrelated to the

π

+ but in spatial coincidence with an energy depositofatleast40MeVintheLKr;no additionalsegments re-constructed in the STRAWcompatible withthe decay vertex; no in-timesignalsinHASC andMUV0;fewerthan fourextrasignals intheNA48-CHOD intimewiththe

π

+.Theresulting

π

0

_→

_{γ γ}

rejection ineﬃciency is measured to be 2

.

5

×

10−8 by counting selected control(PNN-triggered)events inthe

π

+

π

0 _region _before

(after) photon and multiplicity rejection.Alternatively the single photondetectioneﬃcienciesofLAV,LKr,IRCandSACaremeasured using K+

→

π

+

π

0_decays_with_one_{reconstructed}_photon_used_to

tagtheother photon,andconvolvedwitha K+

→

π

+

π

0

simula-tion;thisleadstoasimilar

π

0 _rejection_estimate._The

correspond-ingsignallossis34%,with10%(24%)dueto

π

+interactions (acci-dentals).Photonandmultiplicityrejectionisalsoeffectiveagainst

K+

→

π

+

π

+

π

−andK+

→

π

+

π

−e+

ν

backgrounds.

5. Singleeventsensitivity

The single eventsensitivityis deﬁnedasSES

=

1

/(

NK

· ε

π νν

)

,

where NK isthenumberof K+ decaysintheFVand

ε

π νν is the

signaleﬃciency.TheformerquantityiscomputedasNK

= (

Nππ

·

D

)/(

Aππ

·

BRππ

)

,where Nππ is thenumberofK+

→

π

+

π

0

de-cays selected from controlevents using the K+

→

π

+

ν

¯

criteria except photon and multiplicity rejection, and requiring m2_miss to be in the

π

+

π

0 _region; _Aππ

_{= (}

₉

_.

₉

_±

₀

_.

₃

₎

_% _is _the _acceptance

of this selection estimated from MC simulation, where the er-ror is due to theaccuracy in the simulation ofm2_miss resolution; BRππ is the K+

→

π

+

π

0 _branching _ratio _[₁₄_{] and} _D

₌

_{400 is}

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Fig. 3. Left:Totalacceptance Aπ νν inbinsofπ+ momentum,andinRegions1,2separatelywiththeirrespectiveuncertainties. Right:signaleﬃciencyεR V inbinsof

instantaneousbeamintensityafterphotonandmultiplicityrejectionwithtotaluncertainty,afterphotonrejection,afterIRCandSACvetoonly,afterLAVvetoonly,afterLKr vetoonly.Linesareforeyeguidanceonly.Theinstantaneousintensity(estimatedfromout-of-timeactivityinGTK)canvaryuptoafactoroftwowithinaspillwithrespect totheaverageintensity.

thedownscaling factor ofthe control trigger.This leads to NK

=

(

1

.

21

±

0

.

04syst

)

×

1011, where the uncertainty is dominated by

theerroron Aππ .

Thesignaleﬃciencyisevaluatedinfour5 GeV/c wide_Pπ+bins as

ε

π νν

=

Aπνν

· ε

trig

· ε

R V.Here Aπνν istheselectionacceptance

forthe signal;

ε

trig is the PNN trigger eﬃciency; 1

−

ε

R V isthe

fractionofsignal eventsdiscardedby thephotonandmultiplicity rejectionas a consequenceof accidentalactivity in thedetectors (Random Veto). Other effects inducing signal loss are either in-cludedin Aπνν orcancelintheratioto Aππ whencomputingthe SES.Anexampleisthesignallossduetorandomvetoinducedby accidentalcountsintheMUV3detector.

TheK+

→

π

+

ν

¯

decayissimulatedusingformfactorsderived from the K+

→

π

0_e+

_ν

_decay. _The _selection _acceptance _Aπνν

₌

(

4

.

0

±

0

.

1

)

% is evaluated using MC simulation and includes the particleidentiﬁcationeﬃciency (Fig.3, left).Themain sources of acceptance losses are: detector geometry,

π

+ momentum range,

m2_missregions,particleidentiﬁcationandK+

/

π

+associationinthe FV.The uncertaintyon Aπνν isduetotheaccuracy ofthe simu-lationof thesignal losses resultingfromphoton and multiplicity rejectioninducedby

π

+interactionswiththedetectormaterial.

Thequantity

ε

trig is theproductofL0 andL1trigger

efficien-cies.TheefficiencyofeachL0componentismeasuredwithcontrol events. These efficiencies, with the exception of the LKr condi-tion, are evaluated with the K+

→

π

+

π

0 _sample _used _for _the

NK computation, while the LKr eﬃciency is measured using a

K+

→

π

+

π

0 _sample _selected _with_both _photons_in _{LAV1–LAV11.}

The resulting L0 eﬃciency ranges from 0.93 to 0.85 depending

on _Pπ+, where the main losses come from the LKr and MUV3

conditions, witha systematic uncertainty of 0.02. The L1 trigger eﬃciencyismeasuredtobe0

.

97

±

0

.

01 usingaK+

→

π

+

π

0

sam-plepassingthePNN L0condition. Thequoteduncertaintyreﬂects thestabilityduringthedatataking.

Thefactor

ε

R V ismeasured asthefractionof K+

→

μ

+

ν

de-cays in the data surviving the photon and multiplicity rejec-tion. The selection used for this measurement is similar to the

K+

→

π

+

ν

¯

selection,apartfromparticleidentiﬁcation(replaced by positive

μ

+ identiﬁcationinMUV3and calorimeters)andthe

m2

miss range requirement. The result is

ε

R V

=

0

.

76

±

0

.

04; this

quantity is independent of _Pπ+ and depends on beam intensity asshownin Fig. 3,right. The quoted value includes a correction of

+

0

.

02 basedonsimulationtoaccountforactivityinCHODand LAV induced by

δ

-rays produced by muons in the RICH mirrors. Theuncertaintyon

ε

R V isevaluatedasthedifferencebetweenthe

loss ofacceptance ascalculated by simulationand themeasured

ε

R V extrapolatedtozerointensity.

TheSESandthecorrespondingnumberofSM K+

→

π

+

ν

¯

de-caysexpectedinthesignalregionsare:

SES

= (

3

.

15

±

0

.

01stat

±

0

.

24syst

)

×

10−10

,

(1)

N_{π νν}exp

(

SM

)

=

0

.

267

±

0

.

001stat

±

0

.

020syst

±

0

.

032ext

.

(2)

SystematicuncertaintiesincludethoseonNK,Aπνν ,

ε

trigand

ε

R V.

An additional uncertainty is assigned to beam pileup effects; it isevaluatedbycomparingtheacceptancescomputedincludingor not thepileupsimulation.Theexternalerroron Nπνν_exp (SM)comes fromtheuncertaintyoftheSMprediction.

6. Expectedbackground

Backgroundfrom K+decaysintheFVismainlydueto K+

→

π

+

π

0

₍

_γ

₎

_,_K+

_→

_μ

+

_ν

₍

_γ

₎

_{, K}+

_→

_π

+

_π

+

_π

−_and_K+

_→

_π

+

_π

−_e+

_ν

decays.The ﬁrstthreeprocesses mayenterthesignal regions via

m2_miss mis-reconstruction.Theestimateofthecorresponding back-grounds relies on the assumption that

π

0 _rejection _for _K+

_→

π

+

π

0_,_particle _{identiﬁcation}_for _K+

_→

_μ

+

_ν

_and_multiplicity

re-jectionfor K+

→

π

+

π

+

π

− are independentofthem2_miss criteria deﬁning thesignalregions. Possibleviolationsofthisassumption, suchastheimpactoftheradiativecomponentofK+

→

π

+

π

0

₍

_γ

₎

decay, are investigated separately in a dedicated study. In this

framework, the number of expected background events in each

signal region (1 and 2) from these processes is computed as

Nbkg

·

fkin. Here Nbkg is the number of PNN-triggeredevents

re-maining in the corresponding background m2_miss region after the

K+

→

π

+

ν

¯

selection; fkin _{(“tails”)} _is _the _proportion _of

back-ground events entering the signal region through tails of m2_miss

whichare modelledseparately.Theabove procedureisappliedin four binsof _Pπ+ for K+

→

π

+

π

0 _and _K+

_→

_μ

+

_ν

_backgrounds.

Backgroundinthecontrolregionsisevaluatedsimilarly.

K+

→

π

+

π

0

₍

_γ

₎

_{background: forty-two}_events _remain _in _the

π

+

π

0 _region _after _the _K+

_→

_π

+

_ν

_¯

_selection. _The _m2 miss tails

in Regions 1 and 2 are at the 10−3 level and do not depend on _Pπ+.Theyare evaluated from K+

→

π

+

π

0 _control_events

se-lected with the same criteriaas for NK computation, except for

the m2_miss condition. The two photonsfrom the

π

0

_→

_{γ γ}

_decay

are required to be in the LKr acceptance, and K+

→

π

+

π

0

(6)

Fig. 4. Left:reconstructedm2

missdistributionofthe K+→π+π

0_control_{events selected}_from_data_by_tagging_the_π0_(dots,_see_text_for_details)._{Two K}+_→_π+_π0_MC_samples

aresuperimposed:oneselectedasindata(redline),theotherselectedas K+→π+νν¯(blueline,referredtoasMCK+→π+π0₍_γ₎_in_the_legend)._Signal_regions₁_and₂

arealsoshown.TheMCdistributionsarenormalisedtothedataintheπ+π0_{region. Right:}_{expected K}+_→_π+_π0₍_γ₎_background_in_bins_of_π+_momentum_compared_to theexpectednumberofSM K+→π+νν¯events.

miss distributionofthe K+→μ+ν(γ)controleventsselectedbyassigningtheπ+ masstotheμ+fordata(dots),andfortwoMC K+→

μ+ν(γ)samplessuperimposed:oneselectedas indata(redline),theotherselectedas K+→π+νν¯ withoutparticle identiﬁcation(blueline,referredasMC K+→

μ+ν(γ)inthelegend).Signalregions1and2arealsoshown. Right:expected K+→μ+ν(γ)backgroundinbinsofπ+momentumcomparedtotheexpectednumberof SM K+→π+νν¯events.

K+ tracks by imposing

π

0 _mass,_nominal_beam _momentum_and

missingmassconstraints.Simulationreproducesthetailstoan ac-curacy better than 20%. The kinematic constraints do not affect thereconstruction tailsinRegion 1, butsuppressthecontribution fromK+

→

π

+

π

0

_γ

_decays_in_{Region 2}_(Fig.₄_,_left)._The_estimate

of K+

→

π

+

π

0

_γ

_(inner_{bremsstrahlung)} _background_is_based_on

simulation[23] andsinglephoton detectioneﬃciencies measured fromdata.Asystematicuncertaintyisassignedtoaccountforthe precisionoftheabove measurement.Thebackgrounddependence on _Pπ+ is shownin Fig. 4, rightand isdue tophoton detection ineﬃciency at small angles. After unblinding the control regions betweenthe

π

+

π

0 _and_the_two _signal_regions _(Fig.₂_{, right),}_one

eventis observed,while 1

.

46

±

0

.

16stat

±

0

.

06syst events are

ex-pected.

K+

→

μ

+

ν

(

γ

)

background: forty-ﬁve events remain in the

μ

+

ν

regionafterthe K+

→

π

+

ν

¯

selection.AK+

→

μ

+

ν

sample selectedasforthe

ε

R V measurement butwithout them2_miss

con-ditionisusedtoevaluatethereconstructiontails(Fig.5,left).Tails inRegion 1rangefrom5

×

10−5 intheﬁrst _Pπ+ binto5

×

10−4 in the last bin; tails in Region 2 are 2

×

10−5 in all _Pπ+ bins. Simulations reproduce theseresults to an accuracy of30–40% in Region 1andthecontrolregion,andbetterthan10%inRegion 2. The radiative contribution isincluded in the measured tails. The calorimetric conditions used to identify

μ

+ in the control

sam-pleleadtounderestimationofthetailsinRegion 2byup to50%, limitedbythecurrentdatastatistics,andacorresponding system-aticuncertaintyisassignedtothetailmeasurementinthisregion.

Mis-measurement of _Pπ+ in the STRAW may introduce a

corre-lation betweenthe reconstructedm2_miss and

π

+ identiﬁcation in theRICH.Theeffectofthiscorrelation isestimatedby comparing the RICHperformance measured withdata K+

→

μ

+

ν

eventsin

μ

+

ν

and signal regions. This leads to a 30% uncertainty on the backgroundestimate.Thebackgrounddependenceon _Pπ+ (Fig.5, right)isdrivenbytheincreaseofboththemuonmis-identiﬁcation probability and the tails with _Pπ+. After unblinding the control region,betweenRegion 1andthe

μ

+

ν

region (Fig.2,right),two eventsareobservedwhile1

.

02

±

0

.

16stat

±

0

.

31syst eventsare

ex-pected.

K+

→

π

+

π

+

π

− background: it populates mainly Region 2

due to the large m2_miss. Twenty events remain in the 3

π

region afterthe K+

→

π

+

ν

¯

selection.A K+

→

π

+

π

+

π

− sample used to evaluate them2_miss tails is selected fromcontrolevents usinga

π

+

π

−pairtotagthedecay.Simulationsreproducethem2_miss dis-tribution of thissample over four orders ofmagnitude. The tails areconservativelyestimatedtobe10−4_,_with_a _systematic

uncer-tainty of 100% assigned to account for the bias induced by the aboveselection,asdemonstratedbysimulations.Multiplicity

(7)

rejec-Fig. 6. Left:transversepositionattheFVentranceofpionsfromadatasampleenrichedwithupstreamevents.Bluelinescorrespondtothecontourofthelastdipoleofthe secondachromat;redlinesshowthecontouroftheﬁnalcollimator;theblacklineindicatestheacceptanceregioncoveredbyCHANTI. Right:timedifferencebetweenRICH andKTAGversusthatofGTKandKTAGforthesamesampleofpions.

tionandkinematiccutsare effectiveagainst K+

→

π

+

π

+

π

− de-cays,andtheexpectedbackgroundisalmostnegligible.

K+

→

π

+

π

−e+

ν

background: it is characterised by large

m2_miss and therefore enters Region 2. It is suppressed by its

O(

10−5

)

branchingratio,multiplicity rejection,particle identiﬁca-tionandkinematics. Theapproach adoptedto estimatethe other majorbackgroundscannot beusedinthiscasebecausethe num-berofparticlesinthedetectoracceptanceandthusthemultiplicity rejection are correlated with the kinematics. The background is thereforeestimatedfromsimulation.Out of6

×

108 simulated de-cays,twoeventspasstheK+

→

π

+

ν

¯

selection.Modiﬁcationsto the K+

→

π

+

ν

¯

selection,suchasrequiringa

π

−insteadof

π

+, orinvertingspeciﬁcmultiplicityrejectionconditions,deﬁne exclu-sivesampleswith

O

(10)dataeventspassingthesenewselections.

The agreement between the observed and expected numbers of

eventsinthesesamplesvalidatesthesimulation.

Otherbackgroundsfrom K+ decays: the contributions from

K+

→

μ

+

π

0

_ν

_,_K+

_→

_π

0_e+

_ν

_and _K+

_→

_π

+

_{γ γ}

_decays_are_found

tobe negligibleconsidering particleidentiﬁcationandphoton re-jectionperformanceappliedtosimulatedsamples.Upperlimitsof

O(

10−3

₎

_events_are_obtained_for_these_{contributions.}

Upstreambackgrounds: they are dueto pionsoriginating

up-streamoftheFVandareclassiﬁedasfollows.

• π

+ from K+ decaysbetweenGTK stations2and3,matched toanaccidentalbeamparticle;

• π

+ frominteractionsofabeam

π

+ atGTK stations2and3, matchedtoanaccidentalK+;

• π

+ frominteractionsofa K+ withmaterialinthebeamline, produced eitherpromptly orasa decayproduct ofa neutral kaon.

The interpretation ofthe upstream events in terms ofthe above mechanisms is supported by a detailed analysis of a sample se-lected as K+

→

π

+

ν

¯

apart fromtheconditionsdeﬁning theFV andthe K

/

π

association, whichare inverted to enrich the sam-ple with upstream events. The

π

+ position distribution at the GTK3 Zplane (Fig. 6,left) indicates that they originate from up-streamdecaysorinteractionsintheGTKstationsandjustiﬁesthe 200

×

1000 mm2_box_cut_(Sec.₄_)._The_distribution_of_RICH–KTAG_vs

GTK–KTAGtimedifference suggeststheaccidental originofthese events(Fig.6,right).Installationofanadditionalshieldinginsertin 2017anda newcollimator oflargertransversesize in2018have

Table 1

Summaryofthebackgroundestimatessummedoverthetwosignalregions.

Process Expected events

K+→π+π0₍_γ₎ ₀_.₀₆₄_±₀_.₀₀₇ stat±0.006syst K+→μ+ν(γ) 0.020±0.003stat±0.006syst K+→π+π+π− 0.002±0.001stat±0.002syst K+→π+π−e+ν 0.013₋+00..017012|stat±0.009syst K+→π0_μ+_ν_{, K}+_→_π0_e+_ν _<₀_.₀₀₁ K+→π+γ γ <0.002

Upstream background 0.050+₋00..090030|stat

Total background 0.152₋+00..092033|stat±0.013syst

substantially reduced the backgrounds at

|

Y

|

>

100 mm (Fig. 6, left).

Upstream backgroundestimationisperformedwithdatausing a bifurcation technique [16]. The K

/

π

matching (cut1) and the box cut (cut2) are the two selection criteriato be inverted. The combination of cut1 and cut2 deﬁnes four samples, denoted as

A

(

cut1

·

cut2

)

correspondingtosignal, B

(

cut1

·

cut2

)

,C

(

cut1

·

cut2

)

andD

(

cut1

·

cut2

)

.Thenumberofexpectedeventsinsample A is NA

=

NB

·

NC

/

ND assumingcut1 and cut2 areuncorrelated. This

assumptionisvalidatedwithdatabyapplyingthesameprocedure to differentsamples obtainedmodifying the selection conditions. Theaccuracyoftheresultsislimitedbythesizeofthebifurcation samples.

Thebackgroundestimatesinthesignalregionsaresummarized inTable1.Errorsareaddedinquadraturetoobtaintheuncertainty onthetotalexpectedbackground.

7. Resultandconclusion

Afterunblinding the signal regions, one eventisfound in Re-gion 2, as shownin Fig. 7, left. The STRAW track momentum is 15.3 GeV/c.TheRICHresponseisconsistentwitha

π

+ hypothesis (Fig.7, right).Thetrackdeposits50%(20%)ofitsenergyintheLKr (MUV1),andthe p-valueofthemuonhypothesisbasedon calori-metricidentiﬁcationis0.2%.ThecandidatesintheRICH,CHODand LKr associated to the

π

+, as well as the KTAG and GTK candi-datesassociatedtotheK+,aremutuallyconsistentintimewithin onestandarddeviationofthecorrespondingresolutions.TheKTAG candidateisidentiﬁedbysignalsinsevenoftheeightsectors,and the kinematics of the K+ measured with the GTK is consistent withthenominalbeamproperties.Thedecayvertexpropertiesare

(8)

missasafunctionofπ+momentumforPNN-triggeredevents (markers)satisfyingthe K+→π+νν¯selection,exceptthem2missandπ+momentum

criteria.Thegreyareacorrespondstotheexpecteddistributionof K+→π+νν¯MCevents.Redcontoursdeﬁnethesignalregions.TheeventobservedinRegion 2isshown togetherwiththeeventsfoundinthecontrolregions. Right:signalinRICH(opencircles)detectedintheK+→π+νν¯candidateeventwithringssuperimposedasbuilt underdifferentparticlemasshypotheses.

Zvertex

=

146 m andCDA

=

1

.

7 mm.The

π

+ transverse position

extrapolatedattheFVentranceis

(

x

,

y

)

= (−

373

,

30

)

mm. The hybrid Bayesian-frequentist approach [24] and the CLs method [25] areused for the statisticalinterpretation of the re-sult.A countingexperimentanalysisis performedconsidering an expected signal of 0.267 events and an expected background of 0

.

152

±

0

.

090 events,whereasymmetricuncertaintyisconsidered. Thecorresponding expectedupperlimit isBRexp

(

K+

→

π

+

ν

¯

)

<

10

×

10−10 at 95% CL.Considering the observationof one event, the p-valueof the signal andbackground hypothesis is 15% and thecorrespondingobservedupperlimitis

BR

(

K+

→

π

+

ν

¯

) <

14

×

10−10at 95% CL

.

(3)

The result, basedon 2% ofthe total NA62 exposure in2016– 2018,demonstratesthevalidityofthedecay-in-ﬂighttechniquein termsofbackgroundrejectionandinviewofthemeasurementin progressusingthefulldatasample.

Acknowledgements

Itis apleasure toexpressour appreciationto thestaff ofthe CERNlaboratory andthetechnicalstaff oftheparticipating labo-ratories and universities fortheir efforts in the operation of the experimentanddataprocessing.

The cost ofthe experiment andof its auxiliary systems were supported by the funding agencies of the Collaboration Insti-tutes. We are particularly indebted to: F.R.S.-FNRS (Fonds de la Recherche Scientiﬁque - FNRS), Belgium; BMES (Ministry of Ed-ucation, Youth and Science), Bulgaria; NSERC (Natural Sciences and Engineering Research Council), Canada; NRC (National Re-search Council) contribution to TRIUMF, Canada; MEYS (Ministry

of Education, Youth and Sports), Czech Republic; BMBF

(Bun-desministeriumfür Bildung und Forschung)contracts 05H12UM5

and 05H15UMCNA, Germany; INFN (Istituto Nazionale di Fisica

Nucleare), Italy; MIUR (Ministero dell’Istruzione, dell’Università e dellaRicerca),Italy;CONACyT(ConsejoNacionaldeCienciay Tec-nología),Mexico;IFA (InstituteofAtomicPhysics), Romania; INR-RAS(InstituteforNuclearResearchoftheRussianAcademyof Sci-ences),Moscow,Russia;JINR(JointInstituteforNuclearResearch), Dubna, Russia; NRC (National Research Center) “Kurchatov Insti-tute”andMESRF(MinistryofEducationandScienceoftheRussian

Federation), Russia; MESRS (Ministry of Education, Science, Re-searchandSport),Slovakia;CERN (EuropeanOrganizationfor Nu-clearResearch),Switzerland;STFC(ScienceandTechnology Facili-tiesCouncil), UnitedKingdom;NSF (NationalScience Foundation)

Award Number 1506088, U.S.A.; ERC (European Research

Coun-cil)“UniversaLepto”advancedgrant268062,“KaonLepton”starting grant336581,Europe.

Individuals have received support from: Charles University (project GAUK number404716),CzechRepublic;Ministry of Ed-ucation,UniversitiesandResearch(MIUR“Futuro inricerca2012” grant RBFR12JF2Z,Project GAP), Italy; RussianFoundation for Ba-sicResearch(RFBRgrants18-32-00072,18-32-00245),Russia;the

Royal Society (grants UF100308, UF0758946), United Kingdom;

STFC(RutherfordfellowshipsST/J00412X/1,ST/M005798/1),United Kingdom;ERC(grants268062,336581).

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(2006)064.

[10]T.Blazek,P.Matak,Nucl.Phys.B,Proc.Suppl.198(2010)216. [11]M.Tanimoto,K.Yamamoto,Prog.Theor.Exp.Phys.2016(2016)123B02. [12]M.Bordone,D.Buttazzo,G.Isidori,J.Monnard,Eur.Phys.J.C77(2017)618. [13]C.Bobeth,A.J.Buras,J.HighEnergyPhys.1802(2018)101.

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(9)

NA62Collaboration

E. Cortina Gil,

E. Minucci,

S. Padolski

1

,

P. Petrov,

B. Velghe

2

UniversitéCatholiquedeLouvain,Louvain-La-Neuve,Belgium

G. Georgiev

3

,

V. Kozhuharov

3

,

L. Litov

FacultyofPhysics,UniversityofSoﬁa,Soﬁa,Bulgaria

T. Numao

TRIUMF,Vancouver,BritishColumbia,Canada

D. Bryman,

J. Fu

4

UniversityofBritishColumbia,Vancouver,BritishColumbia,Canada

T. Husek

5

,

K. Kampf,

M. Zamkovsky

CharlesUniversity,Prague,CzechRepublic

R. Aliberti,

G. Khoriauli

6

,

J. Kunze,

D. Lomidze

7

,

R. Marchevski

∗

,

8

,

L. Peruzzo,

M. Vormstein,

R. Wanke

InstitutfürPhysikandPRISMAClusterofExcellence,UniversitätMainz,Mainz,Germany

P. Dalpiaz,

M. Fiorini,

E. Gamberini

8

,

I. Neri,

A. Norton,

F. Petrucci,

H. Wahl

DipartimentodiFisicaeScienzedellaTerradell’UniversitàeINFN,SezionediFerrara,Ferrara,Italy

A. Cotta Ramusino,

A. Gianoli

INFN,SezionediFerrara,Ferrara,Italy

E. Iacopini,

G. Latino,

M. Lenti

DipartimentodiFisicaeAstronomiadell’UniversitàeINFN,SezionediFirenze,SestoFiorentino,Italy

A. Bizzeti

9

,

F. Bucci,

R. Volpe

10

INFN,SezionediFirenze,SestoFiorentino,Italy

A. Antonelli,

G. Lamanna

11

,

G. Lanfranchi,

G. Mannocchi,

S. Martellotti,

M. Moulson,

M. Raggi

12

,

T. Spadaro

LaboratoriNazionalidiFrascati,Frascati,Italy

F. Ambrosino,

T. Capussela,

M. Corvino,

D. Di Filippo,

P. Massarotti,

M. Mirra,

M. Napolitano,

G. Saracino

DipartimentodiFisica“EttorePancini”eINFN,SezionediNapoli,Napoli,Italy

G. Anzivino,

F. Brizioli,

E. Imbergamo,

R. Lollini,

C. Santoni

DipartimentodiFisicaeGeologiadell’UniversitàeINFN,SezionediPerugia,Perugia,Italy

M. Barbanera

13

,

P. Cenci,

B. Checcucci,

V. Duk

14

,

P. Lubrano,

M. Lupi

8

,

M. Pepe,

M. Piccini

INFN,SezionediPerugia,Perugia,Italy

F. Costantini,

L. Di Lella,

N. Doble,

M. Giorgi,

S. Giudici,

E. Pedreschi,

M. Sozzi

DipartimentodiFisicadell’UniversitàeINFN,SezionediPisa,Pisa,Italy

C. Cerri,

R. Fantechi,

R. Piandani

15

,

J. Pinzino

8

,

L. Pontisso,

F. Spinella

(10)

R. Arcidiacono

18

,

B. Bloch-Devaux,

M. Boretto

8

,

E. Menichetti,

E. Migliore,

D. Soldi

DipartimentodiFisicadell’UniversitàeINFN,SezionediTorino,Torino,Italy

C. Biino,

A. Filippi,

F. Marchetto

INFN,SezionediTorino,Torino,Italy

J. Engelfried,

N. Estrada-Tristan

19

InstitutodeFísica,UniversidadAutónomadeSanLuisPotosí,SanLuisPotosí,Mexico

A.M. Bragadireanu,

S.A. Ghinescu,

O.E. Hutanu

HoriaHulubeiNationalInstituteofPhysicsandNuclearEngineering,Bucharest-Magurele,Romania

T. Enik,

V. Falaleev,

V. Kekelidze,

A. Korotkova,

D. Madigozhin,

M. Misheva

20

,

N. Molokanova,

S. Movchan,

I. Polenkevich,

Yu. Potrebenikov,

S. Shkarovskiy,

A. Zinchenko

†

JointInstituteforNuclearResearch,Dubna,Russia

S. Fedotov,

E. Gushchin,

A. Khotyantsev,

A. Kleimenova

10

,

Y. Kudenko

21

,

V. Kurochka,

M. Medvedeva,

A. Mefodev,

A. Shaikhiev

10

InstituteforNuclearResearchoftheRussianAcademyofSciences,Moscow,Russia

S. Kholodenko,

V. Kurshetsov,

V. Obraztsov,

A. Ostankov,

V. Semenov,

V. Sugonyaev,

O. Yushchenko

InstituteforHighEnergyPhysics- StateResearchCenterofRussianFederation,Protvino,Russia

L. Bician,

T. Blazek,

V. Cerny,

M. Koval

8

,

Z. Kucerova

FacultyofMathematics,PhysicsandInformatics,ComeniusUniversity,Bratislava,Slovakia

A. Ceccucci,

H. Danielsson,

N. De Simone

22

,

F. Duval,

B. Döbrich,

L. Gatignon,

R. Guida,

F. Hahn

†

,

B. Jenninger,

P. Laycock

1

,

G. Lehmann Miotto,

P. Lichard,

A. Mapelli,

K. Massri,

M. Noy,

V. Palladino

23

,

M. Perrin-Terrin

24

,

25

,

V. Ryjov,

S. Venditti

CERN,EuropeanOrganizationforNuclearResearch,Geneva,Switzerland

M.B. Brunetti,

V. Fascianelli

26

,

F. Gonnella,

E. Goudzovski,

L. Iacobuzio,

C. Lazzeroni,

N. Lurkin,

F. Newson,

C. Parkinson,

A. Romano,

A. Sergi,

A. Sturgess,

J. Swallow

UniversityofBirmingham,Birmingham,UnitedKingdom

H. Heath,

R. Page,

S. Trilov

UniversityofBristol,Bristol,UnitedKingdom

(11)

UniversityofGlasgow,Glasgow,UnitedKingdom

J.B. Dainton

27

,

J.R. Fry

14

,

L. Fulton,

D. Hutchcroft,

E. Maurice

28

,

G. Ruggiero

∗

,

27

,

B. Wrona

UniversityofLiverpool,Liverpool,UnitedKingdom

A. Conovaloff,

P. Cooper,

D. Coward

29

,

P. Rubin

GeorgeMasonUniversity,Fairfax,VA,USA

*

Correspondingauthor.

E-mailaddresses:radoslav.marchevski@cern.ch(R. Marchevski),giuseppe.ruggiero@cern.ch(G. Ruggiero).

† _Deceased.

1 _Present_address:_Brookhaven_National_Laboratory,_Upton,_NY_11973,_USA 2 _Present_address:_TRIUMF,_Vancouver,_British_Columbia,_V6T_2A3,_Canada 3 _Also_at_Laboratori_Nazionali_di_Frascati,_I-00044_Frascati,_Italy

4 _Present_address:_UCLA_Physics_and_Biology_in_Medicine,_Los_Angeles,_CA_90095,_USA 5 _Present_address:_IFIC,_Universitat_de_València_{- CSIC,}_E-46071_València,_Spain 6 _Present_address:_Universität_Würzburg,_D-97070_Würzburg,_Germany 7 _Present_address:_Universität_Hamburg,_D-20146_Hamburg,_Germany

8 _Present_address:_CERN,_European_Organization_for_Nuclear_Research,_CH-1211_Geneva_23,_Switzerland 9 _Also_at_Dipartimento_di_Fisica,_Università_di_Modena_e_Reggio_Emilia,_I-41125_Modena,_Italy 10 _Present_address:_Université_Catholique_de_Louvain,_B-1348_{Louvain-La-Neuve,}_Belgium 11 _Present_address:_Dipartimento_di_Fisica_{dell’Università}_e_INFN,_Sezione_di_Pisa,_I-56100_Pisa,_Italy 12 _Present_address:_Dipartimento_di_Fisica,_Università_di_Roma_La_Sapienza,_I-00185_Roma,_Italy 13 _Present_address:_INFN,_Sezione_di_Pisa,_I-56100_Pisa,_Italy

14 _Present_address:_School_of_Physics_and_Astronomy,_University_of_Birmingham,_Birmingham,_B15_2TT,_UK 15 _Present_address:_Dipartimento_di_Fisica_e_Geologia_{dell’Università}_e_INFN,_Sezione_di_Perugia,_I-06100_Perugia,_Italy 16 _Also_at_Department_of_Industrial_Engineering,_University_of_Roma_Tor_Vergata,_I-00173_Roma,_Italy

17 _Also_at_Department_of_Electronic_Engineering,_University_of_Roma_Tor_Vergata,_I-00173_Roma,_Italy 18 _Also_at_Università_degli_Studi_del_Piemonte_Orientale,_I-13100_Vercelli,_Italy

19 _Also_at_Universidad_de_Guanajuato,_Guanajuato,_Mexico

20 _Present_address:_Institute_of_Nuclear_Research_and_Nuclear_Energy_of_Bulgarian_Academy_of_Science_(INRNE-BAS),_BG-1784_Soﬁa,_Bulgaria

21 _Also_at_National_Research_Nuclear_University_(MEPhI),₁₁₅₄₀₉_Moscow_and_Moscow_Institute_of_Physics_and_Technology,₁₄₁₇₀₁_Moscow_region,_Moscow,_Russia 22 _Present_address:_DESY,_D-15738_Zeuthen,_Germany

23 _Present_address:_Physics_Department,_Imperial_College_London,_London,_SW7_2BW,_UK

24 _Present_address:_Centre_de_Physique_des_Particules_de_Marseille,_Université_Aix_Marseille,_CNRS/IN2P3,_F-13288,_Marseille,_France 25 _Also_at_Université_Catholique_de_Louvain,_B-1348_{Louvain-La-Neuve,}_Belgium

26 _Present_address:_Dipartimento_di_Psicologia,_Università_di_Roma_La_Sapienza,_I-00185_Roma,_Italy 27 _Present_address:_Physics_Department,_University_of_Lancaster,_Lancaster,_LA1_4YW,_UK

28 _Present_address:_Laboratoire_Leprince_Ringuet,_F-91120_Palaiseau,_France