Search for heavy neutrinos in K+ > mu+ nu(mu) decays

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Search

for

heavy

neutrinos

in

K

+

→

μ

+

ν

μ

decays

.

The NA62

Collaboration

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory: Received24May2017

Receivedinrevisedform25July2017 Accepted25July2017

Availableonline27July2017 Editor:W.-D.Schlatter Keywords:

Kaondecays Heavyneutrinos

TheNA62experimentrecordedalargesampleofK+→

μ

+

ν

μdecaysin2007.Apeaksearchhasbeen

performedinthereconstructedmissing massspectrum.Intheabsenceofasignal,limits inthe range 2×10−6_to10−5_{havebeensetonthesquaredmixingmatrixelement|U}

μ4|2betweenmuonandheavy

neutrinostates,forheavyneutrinomassesintherange300–375 MeV/c2_{.Theresultextendstherange}

ofmassesforwhichupperlimits havebeenset onthevalueof|Uμ4|2 inpreviousproductionsearch

experiments.

©2017TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

1. Introduction

Thefactthatneutrinososcillateimpliesthattheyhavenon-zero

masses. While in the Standard Model (SM) neutrinos are

mass-less by construction, the SM can be extended in various ways

to accommodate neutrino masses [1]. In a large class of mod-els, the see-saw mechanism is used to explain the lightness of the SM neutrinos by introducing additional heavy neutrino mass stateswhichmix withthe SM flavour states[2]. Oneexample of models including heavy neutrinos is the neutrino minimal Stan-dard Model (

ν

MSM), in which three right-handed neutrinos are addedto the SM withone ofthem beingat theGeVscale [3,4]. Forheavy neutrinoswithmassesbelowthekaonmass,limitson theirmixingmatrixelementscanbeplacedbysearchingforpeaks inthemissingmassspectrumof K± decays[5].In thefollowing,

two-bodykaondecaystoa muonanda SMneutrinoaredenoted

K+

→

μ

+

ν

μ,whilethosewithamuon anda heavyneutrinoare denoted K+

→

μ

+

ν

h; the notation K+

→

μ

+N indicates either

case.Limitson

|

Uμ4

|

2intheextendedneutrinomixingmatrix

us-ingtheprocessK+

→

μ

+

ν

hcomefromexperimentswithstopped

kaons,andareoftheorderof10−8upto300 MeV/c2[6]and10−6 upto330 MeV/c2 [7].

TheratiooftheK+decaywidthtoheavyneutrinotothedecay widthtoSMmuonneutrinosisrelatedto

|

Uμ4

|

2[5]:

B

(

K+

→

μ

+

ν

h

)

B

(

K+

→

μ

+

ν

μ

)

= |

U_μ4

|

2f

(

mh

) ,

(1)

wheremh isthemassoftheheavyneutrino,and f

(m

h

)

accounts

forthephasespacefactorandthehelicitysuppression,andvaries intherange1.5–4.0formh intheregion300–375 MeV/c2

consid-eredinthepresentanalysis.

Under theassumption that heavy neutrinos decayonly toSM particles, the lifetime of a heavy neutrino is determined by the mixing matrix elements and by its mass [8].For heavy neutrino massesintherange300–375 MeV/c2,thedominantdecaymodes are

ν

h

→

π

0

ν

e,μ,τ and

ν

h

→

π

+



−, where



=

e,

μ

. Assuming

|

U4

|

2

<

10−4 with



=

e,

μ

,

τ

,themeanfree pathofheavy

neu-trinosatNA62foranymassintherangeconsideredisgreaterthan 10km,andthereforetheirdecayscanbeneglected,sincethe prob-abilityofdecayinginthedetectorordecayvolumeisbelow1%.

2. Beam,detectoranddatasamples

The beam line and detector of the earlier NA48/2

experi-ment were reused by the NA62 experiment during 2007 data

taking; they are described in detail in [9,10].Primary protons of

400 GeV/c, extracted from the CERN SPS, impinged on a 40 cm

long,0.2 cmdiameterberylliumtarget.Secondarybeamsof posi-tivelyandnegativelychargedhadronswereproduced, momentum-selected, similarlyfocused andtransportedto the detector.These beamscouldberunsimultaneouslyorseparately.Thecentralbeam

momentumof74 GeV/c was selectedbythefirsttwomagnetsin

a four-dipole achromat andby momentum-defining slits incorpo-ratedintoa3.2mthickcopper/ironprotonbeamdump,whichalso provided thepossibilityofblockingeitherof thetwobeams.The beams hada momentum spread of

±

1

.

4 GeV

/c (rms).

Forabout 1

.

8

×

1012_{primary protonsincidentonthetargetper SPSspillof}

4.8 sduration, the secondarybeam fluxesattheentrance to the decayvolumewere,respectively,1

.

7

×

107and0

.

8

×

107 positively andnegativelychargedparticlesperspill.

The fraction of kaons ineach beamwas about6%. The beam

kaons decayed in a fiducial volume contained in a 114 m long

cylindricalevacuatedtank.The K+and K−beamsweredeflected

http://dx.doi.org/10.1016/j.physletb.2017.07.055

0370-2693/©2017TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

kickof265 MeV/c tochargedparticles,andthespectrometerhad amomentum resolutionof

σ

p

/p

=

0

.

48%

⊕

0

.

009%

·

p,wherethe

momentum p isexpressedinGeV/c.A hodoscope (HOD)

consist-ingoftwoplanesofplasticscintillatorstripsproducingfasttrigger signalswas placed afterthe spectrometer. A liquidkrypton (LKr) electromagnetic calorimeterof thickness 127 cm (27X0) was

lo-catedfurtherdownstream.Its13248readoutcellshadatransverse sizeof2

×

2 cm2_{eachwithnolongitudinalsegmentation.The}

en-ergyresolutionwas

σ

E

/

E

=

3

.

2%

/

√

E

⊕

9%

/E

⊕

0

.

42%,andthe spa-tialresolutionforthetransversecoordinatesx andy ofanisolated electromagnetic shower was

σ

x

=

σ

y

=

0

.

42 cm

/

√

E

⊕

0

.

06 cm, whereE isexpressedinGeV.Amuondetector(MUV)waslocated

further downstream.The MUVwas composed of three planesof

plasticscintillatorstrips(aligned horizontallyinthefirst andlast planes,andverticallyinthemiddleplane)readoutby photomulti-pliersatbothends.Eachstripwas2.7 mlongand1 cmthick.The widthsofthestripswere25 cminthefirsttwoplanes,and45 cm inthethirdplane.TheMUVwasprecededbyahadronic calorime-ter(6.7nuclearinteractionlengths)notusedforthepresent

mea-surement.Each MUVplane was precededby an additional0.8 m

thickironabsorber.

Generaldatatakingconditionsaredescribed in[11].Themain trigger condition for selecting the sample of K+

→

μ

+N decays requiredthecoincidence intime andspaceof signalsinthe two

HOD planes (HOD signal), and loose lower and upper limits on

theDCHhitmultiplicity(1-tracksignal),downscaledbyafactorof 150.Datatakingperiodswithsimultaneousbeamswere collected withalead barinstalled betweenthetwo HODplanes formuon identificationstudies.Fordatacollectedwiththeleadbarinplace, the vetoing power for backgrounds with photons is reduced, so thesedataareexcludedfromthepresentanalysis.Sincethemuon halobackgroundissmallerintheK+sample,thisanalysisisbased ondatawiththe K+ beamonly(43%oftheintegratedkaonflux, asusedin[12]),whiledatatakenwithonlytheK−beamareused tostudythebackgroundfromhalomuons.

3. Analysisstrategy

In the decay K+

→

μ

+N the neutrino mass can be

recon-structed asm2

h

=

m2miss

= (

pK

−

pμ)2,where pK and pμ arethe

four-momenta of the kaon andthe muon respectively. The kaon

momentumis not measured on an event-by-event basis,and pK

is obtained, assuming the kaon mass, from the average

three-momentummeasuredwithK+

→

π

+

π

+

π

−decaysapproximately

every 500 SPS spills. The muon four-momentum pμ is

deter-mined asthatof areconstructed chargedtrack, assumedto be a muon.

istsbelow300 MeV/c2 [7].Thereconstructedmissingmassrange 245–298 MeV/c2_,wherethe

_ν

h presenceisexcludedbythislimit,

isusedasacontrolregiontomeasurethetriggerefficiencyforthe backgroundevents.

4. Eventselection

Charged particle trajectories and momenta are reconstructed fromhitsanddrifttimesinthespectrometerusingadetailed mag-neticfieldmap.Thereconstructed K+

→

π

+

π

+

π

−invariantmass is usedfor fine calibrationof thespectrometer momentum scale andDCHalignmentthroughoutthedatataking.Clustersofenergy depositionintheLKrcalorimeterarefoundbylocatingmaximain space andtime in the digitized pulses fromindividual cells. Re-constructed energies are corrected forenergy outside thecluster boundaries,energylostinisolatedinactive cells(0.8% ofthetotal number),sharingofenergybetweenclusters,andnon-linearityfor clusterswithenergybelow11 GeV.

Theselectionrequiresexactlyonepositivelychargedtrackwith the following characteristics: within the DCH, LKr calorimeter

andMUVgeometricalacceptance; momentum p between 10and

65 GeV/c;within 20 nsofthetriggertime recordedbytheHOD;

distance of closest approach (CDA) between the track and the

beam axis, as monitored with K+

→

π

+

π

+

π

− decays, smaller than3 cm; trackextrapolationassociatedintimeandspacewith MUVsignalsfromthefirsttwoplanes.

Selected events are required to be free of clusters of energy deposition inthe LKrcalorimeterexcept foranyofthe following configurations:theclusterenergyislowerthan2 GeV;thecluster time ismorethan12 nsaway fromthe tracktime;thecluster is consistentwithbremsstrahlungfromthetrackbeforedeflectionby thespectrometermagnet(within6 cm ofthestraight-line extrap-olatedupstreamtrack);theclusterpositioniswithin40 cmofthe extrapolateddownstreamtrack.

5. Backgroundcontributions

Thebackgroundreceivescontributions frommuon halo, evalu-ated withthecontrol data sample,and fromkaondecays, evalu-atedwithsimulation.

5.1. Muonhalobackground

A data driven approach is used in modelling the muon halo

contribution,andindesigningaselectionthatminimizesthis back-groundwhilepreservingsignalacceptance.Thedistributionofhalo backgroundeventsisestimatedusingthecontrolsample(see Sec-tion 3). The majority of reconstructed

μ

+ in the control sample

Fig. 1. Distributionofhaloeventsinthe(zvertex,θ)plane(left)and(zvertex,p)plane(right).Thecontoursshowtheprojectionsofthefive-dimensionalselectioncriteria.The

eventsoutsidethecontoursarerejected.Thearrowindicatesthestartofthefiducialvolume.SeeSection5fordetails.

comesfrommuonhalowithtwosourcesofcontamination:1) K+

in specific momentum bands pass through the beam absorbers

(witha probability ofup to 5

×

10−4 _{depending on momentum)}

anddecayinto K+

→

μ

+

ν

μ; asimulation showsthatthe recon-structedmmiss calculatedassuming thenominalkaonmomentum

islowerthan280 MeV/c2_{,andthereforethiscomponentdoesnot}

enter the signal region; 2) the contribution from mis-identified positivelychargedpions from K−

→

π

−

π

−

π

+ decaysentersthe signalregion,andissimulatedandsubtracted.

To studythe halo, the event selection described in Section 4

is used with a relaxed CDA condition (CDA

<

8 cm). The distri-bution ofthe eventsin thecontrol sample passing thisselection is shown in Fig. 1 in the variables zvertex, track momentum p

and

θ

, where

θ

isthe anglebetweenthe K+ beamaxisandthe measuredmuondirection.Tomimimizethehalocontribution, ad-ditionalselection criteria are applied ina five-dimensional space (zvertex

,

θ,

p,CDA

,

φ

), where

φ

isthetrackazimuthal angleinthe

transverseplane.ThecontoursinFig. 1showexampleprojections ofthesefive-dimensionalcriteria;theeventsoutsidethecontours are rejected. The signal acceptance reduction due to the multi-dimensionalcriteriawithrespecttotheselectiondescribedin Sec-tion4isintherange40–45%dependingonmh.

The estimated number of halo background events in the

fi-nal sample is obtained from the number of events observed in

the control sample, normalized to the K+ data in the range

m2

miss

>

0

.

05 GeV

2

_/c

4 _and3

_<

_CDA

_<

_{8 cm.}

5.2. Kaondecaybackground

Thetotal numberofkaondecaysinthefiducialregion, NK, is

used to scale the simulated distributions of the expected back-grounds. It is measured with a sample of K+

→

μ

+

ν

μ decays using the selection described in [11] after adding the kinematic criteria; the numberof events in the missing mass squared dis-tribution within

|

m2_miss

|

<

0

.

015 GeV2

/c

4 _{is evaluated after}

sub-tractinga sub-percent contributionfrombeam halo.The squared

missing mass distribution is shown in Fig. 2. The number of

K+

→

μ

+

ν

μ decays after background subtraction is 9

.

45

×

106 andthe corresponding acceptance is 24.88%. The resulting num-berofkaondecaysinthefiducialvolumeintheanalyseddataset isNK

= (

5

.

977

±

0

.

015

)

×

107.

The decay K+

→

μ

+

ν

μ forms a peak at zero m2_miss with a

widthdeterminedbythewidthofthekaonmomentumspectrum,

Fig. 2. Reconstructedsquaredmissingmassdistributionfordatapassingthefinal eventselection.

thebeamdivergenceandthespectrometerresolution;thepeakis well outside the300–375 MeV/c2 signal region.The contribution fromK+

→

μ

+

ν

μ

γ

decayappearsasahigh-masstailinthem2_miss distributionandistakenintoaccountbythesimulation.The domi-nantbackgroundfromkaondecaysinthesignalregioncomesfrom K+

→

π

0

_μ

+

_ν

μ decays with an undetected

π

0 due to the non-hermeticgeometricalacceptance.ThehadronicdecayK+

→

π

+

π

0

isonlyreconstructedassignalifthe

π

0 _{isundetectedandthe}

_π

+

ismis-identifiedasamuonordecaysintoamuon.

The backgrounds due to kaon decays to three pions are

naturally suppressed because they involve either three tracks (K+

→

π

+

π

+

π

−)orphotons(K+

→

π

+

π

0

_π

0_{).Theeventswhich}

pass theselection typicallyappear at theupperendofthem2 miss

spectrum. Decays with positrons in the final state (K+

→

e+

ν

e,

K+

→

π

0_e+

_ν

e)arerejectedwithparticleidentification. 6. Systematicuncertaintiesonthebackgroundestimate

The uncertainty on kaon decaybackground receives contribu-tions from theuncertainty onthe numberofkaon decaysin the fiducialvolume, NK,andtheindividual kaondecaybranching

missing mass,

π

+ momentum and direction spectra. From this comparisonitis inferred thatthe uncertaintyon thebackground estimatein the K+

→

μ

+

ν

h signal regiondoesnot exceed6% of

thetotalexpectedbackground;thisuncertaintyaffectsmostlythe low

ν

h massregion.

The systematic uncertaintyattributed to the halo background arisesfromthelimitedsizeofthecontrolsample (halostatistical contribution),andfromtheassumptionthat thehalodistribution inthe control sample accurately reproduces that of the K+ data (halomodelcontribution).Thehalostatisticalcontributionis2–4% of the total expected background in the range 300–360 MeV/c2

andrises to 16%inthe range360–375 MeV/c2.The control sam-ple is divided into sub-samples according to selection variables

and each sub-sample is normalised to the K+ data. The halo

modelcontributionisevaluated bycomparing thenormalizations obtained with the different sub-samples with that obtained for theentiresample. Thiscontributionis1–3%ofthetotalexpected backgroundinthe range300–360 MeV/c2 andrises to 8% inthe range360–375 MeV/c2.Theuncertaintydueto thesubtractionof K−

→

π

−

π

−

π

+eventsisnegligible.

A K+

→

μ

+

ν

μ sample is used to measure the MUV muon identificationefficiencyasafunctionoftrackmomentum.This

ef-ficiencyvariesbetween 96% and98% over the momentum range

between10and 65 GeV/c. The simulationis tuned to reproduce thisefficiency to 1% precision, andtherefore a systematic uncer-taintyof1%isassignedtothetotalexpectedbackground.

TheHODtriggerinefficiencyis

(

1

.

4

±

0

.

1

)

% asdiscussedin[11];

since the inefficiency depends mainly on the number of tracks

whichisthesameforsignal, K+

→

μ

+

ν

μ decaysandmain back-grounds,itcancelsout toagoodapproximation.The1-track trig-ger inefficiency for K+

→

μ

+

ν

μ decays was measured with

re-spect to the HOD trigger to be much smaller that the HOD

in-efficiency [11] and can be neglected. Conversions of undetected photons from K+

→

π

0

_μ

+

_ν

_{decays cause a 1-track inefficiency}

due to events with high multiplicity of hits in the DCH cham-bers. The 1-track trigger efficiency for the background could be evaluated directly in the signal region, or by extrapolating the measurement performed in the control region to the signal re-gion. However, the possible presence of K+

→

μ

+

ν

h decays in

thesignal regionwould increase the apparent efficiency, thereby affecting the signal sensitivity. Therefore the 1-track trigger effi-ciencyforthebackgroundisevaluatedinthecontrolmmiss region

245–298 MeV/c2_{,since strong limits on the heavy neutrino}

pro-duction in this region already exist. In this control region the 1-track efficiencyis

(

89

.

8

±

0

.

6

)

% andwas shownnot to depend onthemissingmass.Theuncertaintyonthetriggerefficiencyfor thebackgroundtranslatesintoa contributionof0.7%onthetotal expectedbackground.

Fig. 3. Missingmass distributionsfor data,showing statisticaluncertainties,and fortheestimatedbackgroundcontributions,inbothsignalandcontrolregions.The lowerplotshowsthetotaluncertaintyonthebackgroundestimate.

7. Upperlimitsonheavyneutrinoproduction

The eventselection described in Section 4 with the addition ofthefive-dimensionalcriteriadescribedinSection5.1constitutes the final selection. Fig. 3 shows the mmiss distribution of events

passingthefinalselectionandtheestimatedbackgroundspectrum. Thehalocontributionvariesasafunctionofmmissbetween5%and

20% ofthe backgroundandcarriesthe largestrelative systematic uncertainty.

For each neutrino mass mh under consideration in the signal

region 300–375 MeV/c2, a window of

±

σ

h in the missing mass

spectrum is defined centred on mh, where

σ

h is the resolution

parametrized as

σ

h

=

12 MeV

/c

2

−

0

.

03

·

mh. For each window,

thewidthis roundedto thenearest multipleof10−4 _GeV2

_/c

4 _in

m2

miss.The signal acceptance,evaluated forarangeofheavy

neu-trino masses with simulation, is about 0.20 up to 360 MeV/c2

anddropstozeroforlargermasses.Thestatisticalanalysisis per-formedby applyingtheRolke–Lopez method[13]tofindthe90% confidenceintervalsonthenumberofreconstructed K+

→

μ

+

ν

h

eventsforthecaseofaPoissonprocessinthepresenceofGaussian backgrounds.Inputstothecomputationineachmasswindoware the numberofdata eventsobserved,andthe estimate ofthe to-talnumberofbackgroundeventswithitsuncertainty.Thesquared uncertaintiesonthenumbersofexpectedeventsineachmass hy-pothesisareshowninFig. 4,wherethevariouscontributionscan beseen.

No signal is observed, the maximum local significance being 2.67 standard deviations at 357 MeV/c2. The upper limits (UL) at 90% CL on the numbers of reconstructed K+

→

μ

+

ν

h events

is indicated asnU L.The expected upperlimitsare calculated

as-sumingthat thenumberofeventsobserved isequaltothe num-ber of events expected, i.e. the number of background events. These upperlimits are convertedto upper limitson the branch-ing ratio

B(

K+

→

μ

+

ν

h

)

as shown in Fig. 5, using the relation

nU L

=

B

U L

(K

+

→

μ

+

ν

h

)A(m

h

)N

K,where A(mh

)

isthe signal

ac-ceptance,

B

U L isthe upperlimit on thebranching ratio,and NK

is given in Section 5. The branching ratio is related to the neu-trinomixing-matrix element squared

|

Uμ4

|

2 by equation (1).The

obtainedupperlimitson

|

Uμ4

|

2 areshowninFig. 6,togetherwith

Fig. 4. Upperplot:squareduncertaintiesonthenumbersofexpectedbackground eventsateachheavyneutrinomass.Lowerplot:squaredstatisticaluncertaintyfor data.

Fig. 5. Expectedandobservedupperlimits(at90%CL)onthebranchingratioin 10−5_unitsoftheK+_→μ+_{νhdecayateachassumedνhmass.}

8. Conclusions

Apeak search has beenperformedin themissingmass spec-trumobservedin K+

→

μ

+N decaysusingpartoftheNA622007 dataset.Limitsintherange2

×

10−6to10−5 havebeensetonthe mixingmatrixelementsquaredbetweenmuonandheavyneutrino

Fig. 6. Expected and observedupper limits (at90% CL)on the matrix element squared|Uμ4|2 ateach assumedνh mass.TheexistinglimitfromKEKE089[6]

isalsoshown(dottedline).Below300 MeV/c2 _{thereisalimitofO(10}−8_)from

BNLE949[7],notshown.

statesforassumedneutrinomassesintherange300–375 MeV/c2_.

Theresultextendstherangeofmassesforwhichupperlimitshave been set on the value of

|

Uμ4

|

2 by previous

ν

h production

ex-periments.Thanks tothedesignandexcellentperformanceofthe currentNA62setup [14],a substantialimprovementinsensitivity isexpected.

Acknowledgements

WegratefullyacknowledgetheCERNSPSacceleratorand beam-linestafffortheexcellent performanceofthebeamandthe tech-nical staff of the participating institutes for their efforts in the maintenanceandoperationofthedetector,anddataprocessing.

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NA62Collaboration

C. Lazzeroni

∗

,

1

,

N. Lurkin

∗

,

2

,

F. Newson,

A. Romano

UniversityofBirmingham,Edgbaston,Birmingham,B152TT,UnitedKingdom

A. Ceccucci,

H. Danielsson,

V. Falaleev,

L. Gatignon,

S. Goy Lopez

3

,

B. Hallgren

4

,

A. Maier,

A. Peters,

M. Piccini

5

,

P. Riedler

DipartimentodiFisicadell’UniversitàeSezionedell’INFNdiFirenze,I-50019SestoFiorentino,Italy

A. Antonelli,

M. Moulson,

M. Raggi

11

,

T. Spadaro

LaboratoriNazionalidiFrascati,I-00044Frascati,Italy

K. Eppard,

M. Hita-Hochgesand,

K. Kleinknecht,

B. Renk,

R. Wanke,

A. Winhart

4

InstitutfürPhysik,UniversitätMainz,D-55099Mainz,Germany12

R. Winston

UniversityofCalifornia,Merced,CA95344,USA

V. Bolotov

†

,

V. Duk

5

,

E. Gushchin

InstituteforNuclearResearch,117312Moscow,Russia

F. Ambrosino,

D. Di Filippo,

P. Massarotti,

M. Napolitano,

V. Palladino

13

,

G. Saracino

DipartimentodiFisicadell’UniversitàeSezionedell’INFNdiNapoli,I-80126Napoli,Italy

G. Anzivino,

E. Imbergamo,

R. Piandani

14

,

A. Sergi

4

DipartimentodiFisicadell’UniversitàeSezionedell’INFNdiPerugia,I-06100Perugia,Italy

P. Cenci,

M. Pepe

Sezionedell’INFNdiPerugia,I-06100Perugia,Italy

F. Costantini,

N. Doble,

S. Giudici,

G. Pierazzini

†

,

M. Sozzi,

S. Venditti

DipartimentodiFisicadell’UniversitàeSezionedell’INFNdiPisa,I-56100Pisa,Italy

S. Balev

†

,

G. Collazuol

15

,

L. Di Lella,

S. Gallorini

15

,

E. Goudzovski

1

,

2

,

4

,

G. Lamanna

16

,

I. Mannelli,

G. Ruggiero

17

ScuolaNormaleSuperioreeSezionedell’INFNdiPisa,I-56100Pisa,Italy

C. Cerri,

R. Fantechi

Sezionedell’INFNdiPisa,I-56100Pisa,Italy

S. Kholodenko,

V. Kurshetsov,

V. Obraztsov,

V. Semenov,

O. Yushchenko

G. D’Agostini

DipartimentodiFisica,UniversitàdiRomaLaSapienzaeSezionedell’INFNdiRomaI,I-00185Roma,Italy

E. Leonardi,

M. Serra,

P. Valente

Sezionedell’INFNdiRomaI,I-00185Roma,Italy

A. Fucci,

A. Salamon

Sezionedell’INFNdiRomaTorVergata,I-00133Roma,Italy

B. Bloch-Devaux

19

,

B. Peyaud

DSM/IRFU–CEASaclay,F-91191Gif-sur-Yvette,France

J. Engelfried

InstitutodeFísica,UniversidadAutónomadeSanLuisPotosí,78240SanLuisPotosí,Mexico20

D. Coward

SLACNationalAcceleratorLaboratory,StanfordUniversity,MenloPark,CA94025,USA

V. Kozhuharov

21

,

L. Litov

FacultyofPhysics,UniversityofSofia,BG-1164Sofia,Bulgaria22

R. Arcidiacono

23

,

S. Bifani

4

DipartimentodiFisicadell’UniversitàeSezionedell’INFNdiTorino,I-10125Torino,Italy

C. Biino,

G. Dellacasa,

F. Marchetto

Sezionedell’INFNdiTorino,I-10125Torino,Italy

T. Numao,

F. Retière

TRIUMF,Vancouver,BritishColumbia,V6T2A3,Canada

*

Correspondingauthors.

E-mailaddresses:cristina.lazzeroni@cern.ch(C. Lazzeroni),nicolas.lurkin@cern.ch(N. Lurkin).

† _Deceased.

1 _{SupportedbyaRoyalSocietyUniversityResearchFellowship.} 2 _{SupportedbyERCStartingGrant336581.}

3 _{Presentaddress:CIEMAT,E-28040Madrid,Spain.}

4 _{Presentaddress:SchoolofPhysicsandAstronomy,UniversityofBirmingham,Birmingham,B152TT,UK.} 5 _{Presentaddress:Sezionedell’INFNdiPerugia,I-06100Perugia,Italy.}

6 _{Presentaddress:Ruprecht-Karls-UniversitätHeidelberg,D-69120Heidelberg,Germany.}

7 _{Presentaddress:InstituteofNuclearResearchandNuclearEnergyofBulgarianAcademyofScience(INRNE-BAS),BG-1784Sofia,Bulgaria.} 8 _{FundedbytheNationalScienceFoundationunderawardNo.0338597.}

9 _{AlsoatDipartimentodiFisica,UniversitàdiModenaeReggioEmilia,I-41125Modena,Italy.} 10 _{AlsoatIstitutodiFisica,UniversitàdiUrbino,I-61029Urbino,Italy.}

11 _{Presentaddress:UniversitàdiRomaLaSapienza,I-00185Roma,Italy.}

12 _{FundedbytheGermanFederalMinisterforEducationandResearch(BMBF)undercontract05HA6UMA.} 13 _{Presentaddress:PhysicsDepartment,ImperialCollegeLondon,London,SW72BW,UK.}

14 _{Presentaddress:Sezionedell’INFNdiPisa,I-56100Pisa,Italy.}

15 _{Presentaddress:DipartimentodiFisicadell’UniversitàeSezionedell’INFNdiPadova,I-35131Padova,Italy.} 16 _{Presentaddress:DipartimentodiFisicadell’UniversitàeSezionedell’INFNdiPisa,I-56100Pisa,Italy.} 17 _{Presentaddress:DepartmentofPhysics,UniversityofLiverpool,Liverpool,L697ZE,UK.}

18 _{PartlyfundedbytheRussianFoundationforBasicResearchgrant12-02-91513.} 19 _{Presentaddress:DipartimentodiFisicadell’Università,I-10125Torino,Italy.}

20 _{FundedbyConsejoNacionaldeCienciayTecnología(CONACyT)andFondodeApoyoalaInvestigación(UASLP).} 21 _{AlsoatLaboratoriNazionalidiFrascati,I-00044Frascati,Italy.}

22 _{FundedbytheBulgarianNationalScienceFundundercontractDID02-22.} 23 _{AlsoatUniversitàdegliStudidelPiemonteOrientale,I-13100Vercelli,Italy.}