Limit on the production of a new vector boson in e+e-→Uγ, U→π+π- with the KLOE experiment

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Limit

on

the

production

of

a

new

vector

boson

in

e

+

e

−

→

U

γ

,

U

→ π

+

π

−

with

the

KLOE

experiment

The

KLOE-2

Collaboration

A. Anastasi

e

,

d

,

D. Babusci

d

,

G. Bencivenni

d

,

M. Berlowski

v

,

C. Bloise

d

,

F. Bossi

d

,

P. Branchini

r

,

A. Budano

q

,

r

,

L. Caldeira Balkeståhl

u

,

B. Cao

u

,

F. Ceradini

q

,

r

,

P. Ciambrone

d

,

F. Curciarello

e

,

b

,

l

,

∗

,

E. Czerwi ´nski

c

,

G. D’Agostini

m

,

n

,

E. Danè

d

,

V. De Leo

r

,

E. De Lucia

d

,

A. De Santis

d

,

P. De Simone

d

,

A. Di Cicco

q

,

r

,

A. Di Domenico

m

,

n

,

R. Di Salvo

p

,

D. Domenici

d

,

A. D’Uffizi

d

,

A. Fantini

o

,

p

,

G. Felici

d

,

S. Fiore

s

,

n

,

A. Gajos

c

,

P. Gauzzi

m

,

n

,

G. Giardina

e

,

b

,

S. Giovannella

d

,

E. Graziani

r

,

F. Happacher

d

,

L. Heijkenskjöld

u

,

W. Ikegami Andersson

u

,

T. Johansson

u

,

D. Kami ´nska

c

,

W. Krzemien

v

,

A. Kupsc

u

,

S. Loffredo

q

,

r

,

G. Mandaglio

f

,

g

,

∗

,

M. Martini

d

,

k

,

M. Mascolo

d

,

R. Messi

o

,

p

,

S. Miscetti

d

,

G. Morello

d

,

D. Moricciani

p

,

P. Moskal

c

,

A. Palladino

t

,

M. Papenbrock

u

,

A. Passeri

r

,

V. Patera

j

,

n

,

E. Perez del Rio

d

,

A. Ranieri

a

,

P. Santangelo

d

,

I. Sarra

d

,

M. Schioppa

h

,

i

,

M. Silarski

d

,

F. Sirghi

d

,

L. Tortora

r

,

G. Venanzoni

d

,

W. Wi´slicki

v

,

M. Wolke

u

a_{INFNSezionediBari,Bari,Italy} b_{INFNSezionediCatania,Catania,Italy}

c_{InstituteofPhysics,JagiellonianUniversity,Cracow,Poland} d_{LaboratoriNazionalidiFrascatidell’INFN,Frascati,Italy}

e_{DipartimentodiScienzeMatematicheeInformatiche,ScienzeFisicheeScienzedellaTerradell’UniversitàdiMessina,Messina,Italy} f_{DipartimentodiScienzeChimiche,Biologiche,FarmaceuticheedAmbientalidell’UniversitàdiMessina,Messina,Italy}

g_{INFNGruppoCollegato diMessina,Messina,Italy}

h_{DipartimentodiFisicadell’UniversitàdellaCalabria,Rende,Italy} i_{INFNGruppoCollegato diCosenza,Rende,Italy}

j_{DipartimentodiScienzediBaseedApplicateperl’Ingegneriadell’Università“Sapienza”,Roma,Italy} k_{DipartimentodiScienzeeTecnologieApplicate,Università“GuglielmoMarconi”,Roma,Italy} l_{NovosibirskStateUniversity,630090Novosibirsk,Russia}

m_{DipartimentodiFisicadell’Università“Sapienza”,Roma,Italy} n_{INFNSezionediRoma,Roma,Italy}

o_{DipartimentodiFisicadell’Università“TorVergata”,Roma,Italy} p_{INFNSezionediRomaTorVergata,Roma,Italy}

q_{DipartimentodiMatematicaeFisicadell’Università“RomaTre”,Roma,Italy} r_{INFNSezionediRomaTre,Roma,Italy}

s_{ENEAUTTMAT-IRR,CasacciaR.C.,Roma,Italy} t_{DepartmentofPhysics,BostonUniversity,Boston,USA}

u_{DepartmentofPhysicsandAstronomy,UppsalaUniversity,Uppsala,Sweden} v_{NationalCentreforNuclearResearch,Warsaw,Poland}

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory: Received18March2016

Receivedinrevisedform6April2016 Accepted7April2016

Availableonline11April2016 Editor:L.Rolandi

TherecentinterestinalightgaugebosonintheframeworkofanextraU(1)symmetrymotivatessearches inthemassrangebelow1GeV.Wepresentasearchforsuchaparticle,thedarkphoton,ine+e−→Uγ, U→ π+π− basedon28million e+e−→ π+π−γ events collectedatDANEbythe KLOEexperiment. The π+ π− productionbyinitial-state radiationcompensates foralossofsensitivity ofpreviousKLOE U→e+e−,μ+μ− searchesduetothesmallbranchingratiosintheρ–ωresonanceregion.Wefound noevidenceforasignalandsetalimitat90%CLonthemixingstrengthbetweenthephotonandthe

*

Correspondingauthors.

E-mailaddresses:fcurciarello@unime.it(F. Curciarello),gmandaglio@unime.it(G. Mandaglio).

http://dx.doi.org/10.1016/j.physletb.2016.04.019

0370-2693/©2016TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

isathermalrelicfromtheBigBang, accountingforabout24%of thetotalenergydensityoftheUniverse[2]andproducingeffects onlythroughits gravitationalinteractionswithlarge-scale cosmic structures.ToincludetheDMina particletheoreticalframework, theStandardModel(SM)isusuallycomplementedwithmany ex-tensions [3–7] that attribute to the DM candidates also strong self interactions and weak-scale interactions with SM particles. Amongthepossiblecandidates,a WeaklyInteractingMassive Par-ticle (WIMP) aroused much interest since a particle with weak-scaleannihilationcrosssectioncanaccountfortheDMrelic abun-danceestimatedthroughthestudyofthecosmicmicrowave back-ground[2].TheforcecarrierinWIMPannihilationscouldbeanew gaugevectorboson,knownasU boson,darkphoton,

γ

or A,with alloweddecaysintoleptons andhadrons.Its assiduousworldwide searchhasbeen stronglymotivatedbythe astrophysicalevidence recentlyobservedinmanyexperiments[8–14]andbyitspossible positiveone-loopcontributiontothetheoreticalvalueofthemuon magneticmomentanomaly [15],which couldsolve, partlyor en-tirely, the well known 3.6

σ

discrepancy withthe experimental measurement[16].

Inthispaperweassumethesimplesttheoreticalhypothesis ac-cordingtowhichthedarksectorconsistsofjustoneextraabelian gaugesymmetry,U

(

1

)

,withonegaugeboson,theUboson,whose decaysinto invisible light darkmatter are kinematically inacces-sible. Inthisframework thedark photonwould actlike a virtual photon, with virtuality q2

₌

_m2

U. It would couple to leptons and quarkswiththesamestrengthandwouldappear(itswidthbeing much smaller than the experimental mass resolution) as a nar-rowresonanceinanyprocessinvolvingrealorvirtualphotons.The couplingtotheSMphotonwouldoccurbymeansofavector por-talmechanism[3],i.e.loopsofheavydarkparticleschargedunder boththeSM andthedarkforce. Thestrengthofthe mixingwith thephotonisparametrized byasingle factor

ε

2

₌

_α



_/

_α

_whichis

theratio ofthe effective darkand SM photon couplings [3]. The sizeofthe

ε

2_{parameterisexpectedtobeverysmall(10}−2_–10−8₎ causingasuppressionoftheUbosonproductionrate.TheUboson decaysintoSM particleswouldhappen throughthesame mixing operator, withthe corresponding decay amplitude suppressed by an

ε

2 _{factor,butarestill expectedtobe detectableathigh} lumi-nositye+e− colliders[17–19].

KLOEhasalreadysearchedforradiativeUbosonproductionin thee+e−

→

U

γ

,U

→

e+e−,

μ

+

μ

−processes[20,21].The leptonic channelsareaffectedbyadecreaseinsensitivityinthe

ρ

–

ω

region dueto the dominant branching fractioninto hadrons. For a vir-tualphotonwithq2

_<

_1GeV2_{,thecouplingtothechargedpionis} givenbytheproductoftheelectricchargeandthepionform fac-tor,eFπ

(

q2

)

.TheeffectivecouplingoftheUbosontopionsisthus predictedtobegivenbytheproductofthevirtualphotoncoupling andthekinetic mixingparameter

ε

2_e

Fπ

(

q2

)

[19].Beingfarfrom the

π

+

π

− massthreshold (see Section 3) finite mass effects can be safelyneglected. We thus searched fora short livedU boson

The KLOE detector operates at DA



NE, the Frascati

φ

-factory. DA



NEis an e+e− colliderusually operated ata centerof mass energym_φ



1

.

019GeV.Positronandelectronbeamscollideatan angleof

π

−

25mrad,producing

φ

mesonsnearlyatrest.TheKLOE detectorconsistsofalargecylindricaldriftchamber(DC)[22], sur-rounded by a lead scintillating-fiber electromagnetic calorimeter (EMC) [23]. A superconducting coil around the EMC provides a 0.52 T magnetic field along the bisector of the colliding beams. Thebisectoristakenasthez axisofourcoordinatesystem.Thex

axisishorizontal, pointingtothecenterofthecolliderringsand the y axisisvertical,directedupwards.

The EMC barrel and end-caps cover 98% of the solid angle.

Calorimetermodulesarereadoutatbothendsby4880 photomul-tipliers. Energy andtime resolutions are

σ

E

/

E

=

0

.

057

/

√

E

(

GeV

)

and

σ

t

=

57 ps

/

√

E

(

GeV

)

⊕

100ps, respectively. The drift

cham-berhasonlystereowiresandis4 mindiameter,3.3 mlong.Itis builtout ofcarbon-fibersandoperates witha low- Z gasmixture (heliumwith10%isobutane).Spatialresolutionsare

σ

xy

∼

150μm

and

σ

z

∼

2mm.Themomentumresolutionforlargeangletracksis

σ

(

p_⊥

)/

p_⊥

∼

0

.

4%.ThetriggerusesbothEMCandDCinformation. Events used inthis analysisare triggered by atleast two energy depositslargerthan50 MeVintwosectorsofthebarrel calorime-ter[24].

3. Eventselection

We selected

π

+

π

−

γ

candidates byrequesting eventswithtwo oppositely-chargedtracksemitted atlargepolarangles,50◦

< θ <

130◦,withtheundetectedISR photon missingmomentum point-ing –accordingto the

π

+

π

−

γ

kinematics –atsmall polar angles (

θ <

15◦,

θ >

165◦).Thetrackswererequiredtohavethepointof closestapproachtothez axiswithinacylinderofradius8cmand length15cmcentered attheinteractionpoint.Inordertoensure goodreconstruction andefficiency,we selectedtrackswith trans-verseandlongitudinalmomentumintherangep_⊥

>

160MeV or p

>

90MeV,respectively.

Sincethe

π

+

π

−

γ

crosssectionbehavior asafunctionoftheISR photonpolarangleisdivergent(

∝

1

/θ

4

γ), thetrackandthephoton

acceptanceselectionsmake thefinal-stateradiation(FSR)andthe

φ

resonantprocessesrelativelyunimportant,leavinguswithahigh purityISRsampleandincreasingoursensitivitytotheU

→ π

+

π

− decay[25].

The Monte Carlo simulation of the Mππ spectrum was

pro-ducedwiththe

PHOKHARA

eventgenerator [26] withtheKühn–

Santamaria (KS) [27] pion form factor parametrization and

in-cluded a full description of the KLOE detector (GEANFI

pack-age [28]). The collected data were simulated including

φ

decays andleptonicprocessese+e−

→ 

+



−

γ(γ)

,



=

e

,

μ

.Themain back-groundcontributionsaffectingtheISR

π

+

π

−

γ

samplearethe res-onante+e−

→ φ

→ π

+

π

−

π

0 processandtheISRandFSRe+e−

→

Fig. 1. ExampleofMtrk distributionsforthe Mππ=820–840MeV bin.Measured data arerepresented inblack, simulatedπ+π−γ and μ+μ−γ inred. Simulated

μ+_μ−_{γ + π}+_π−_{γ inblue.Eventsat theleftoftheverticallinearerejected.(For} interpretationofthereferencestocolorinthisfigurelegend,thereaderisreferred tothewebversionofthisarticle.)

background” inthe following). We reduced their contribution by applying kinematic cuts in the Mtrk–M_ππ2 plane, as explained in Refs. [29,30]. M2_ππ is the squared invariant mass of the two se-lected tracksinthe pionmasshypothesis whileMtrk isthemass ofthechargedparticles associatedtothetracks,computedinthe

equalmasshypothesisandassumingthatthemissingmomentum

oftheeventpertainstoasinglephoton.1 DistributionsoftheMtrk variable for data andsimulation are shown in Fig. 1, wherethe

Mtrk

>

130MeV cuttodiscriminatemuonsfrompions isalso in-dicated.

AparticleIDestimator(PID),L_±,definedforeachtrackwith as-sociatedenergyreleasedinEMCandbasedonapseudo-likelihood function,uses calorimeterinformation(sizeandshapeofthe en-ergy depositionsandtime offlight)to suppressradiativeBhabha scatteringevents[29–31].

Electrons deposit their energy mainly at the entrance of the

calorimeter while muons andpions tend to have a deeper

pen-etration in the EMC. Events with both tracks having L_±

<

0 are identifiedase+e−

γ

eventsandrejected.The efficiencyofthis se-lectionislargerthan99.95%asevaluatedusingmeasureddataand simulated

π

+

π

−

γ

samples.

After these selections, about 2

.

8

×

107 events are left in the measured datasample. We then applied the sameanalysischain

to the Monte Carlo simulated data: most of the selected

sam-ple consists of

π

+

π

−

γ

events, with residual ISR



+



−

γ

,



=

e

,

μ

and

φ

→ π

+

π

−

π

0. Fig. 2showsthe fractional components ofthe residualbackground, FBG,individually foreach contributing chan-nelandtheir sum. The residualbackground rises upto about6% at low invariant masses andto 5% above 0.9 GeV, decreasing to lessthan1%intheresonanceregion,anditisdominatedby

μμγ

eventsinthewholeinvariantmassrange.

A very good description of the

ρ

–ω interference region (see theinsertof

Fig. 3

)wasachievedbyproducingadedicatedsample

using

PHOKHARA

aseventgenerator withthe Gounaris–Sakhurai

(GS) pion form factor parametrization [32]. The generation pro-cessused properly smeared distributions in order to account for the dipion invariant mass resolution(1.4–1.8 MeV). In Fig. 3the measureddataspectrumiscomparedwiththeresultsofthis sim-ulationprocess,whichincludestheresidualbackground.

1 _M

trkis computedfrom themeasured momentaofthetwo particles p± as-suming they have the same mass:

_√ s−  | p+|2+M2 trk−  | p−|2+M2 trk 2 −  p++ p−2=M2 γ=0.

Fig. 2. Fractional backgrounds, normalizedto the π+π−γ contribution, from the

π+π−π0_,e+_e−_γ,andμ+_μ−_{γchannelsafterallselectioncriteria.}

Fig. 3. Comparisonofmeasureddata(bluesquares)andsimulationperformedwith theGounaris–Sakurai|Fπ|2parametrization(redopensquares)fortheMπ π

invari-antmassspectrum.Thefigureinsertshowsindetailtheagreementachievedinthe

ρ–ωmixingregion(779–791 MeV).

4. Irreduciblebackgroundparametrizationandestimate

Exceptforthe

ρ

–ωregion,weestimatedtheirreducible back-grounddirectlyfromdata.ForeachUmasshypothesisthedataare fitted ina Mππ intervalcentered at MU and18–20times wider

than the Mππ resolution

σ

Mπ π.The backgroundismodeled by a

monotonic function using Chebyshev polinomials up to the sixth orderandisestimatedusingthesidebandtechnique,byexcluding fromthefitthedataintheregion

±

3

σ

Mπ π around MU[20].The procedureisrepeatedinstepsof2MeVinMU.

Fits withthebest reduced

χ

2 _{areselected ashistograms} rep-resenting the background.Forall usedmass intervals, the distri-butions werefound tobe smooth,withno“wiggles”inanymass sub-range. An example ofthe fit procedure isreported in

Fig. 4

.

Fig. 5 shows the distribution of the differences (pulls) between data andthe fittedbackground normalizedto the datastatistical error. Alsoshownisa Gaussianfit ofthisdistribution.The mean andwidthparametersoftheGaussianfitarearoundzeroandone, respectively.

The regionof

ρ

–ωinterferenceis notsmooth (see Fig. 3)and then not easy to be fittedwiththe sidebandtechnique. Wethus estimatedthe backgroundinthisregionby usingthe

PHOKHARA

generator with smeared distributions, as explained in Section 3

Fig. 4. ExampleofaChebyshevpolinomialsidebandfitfortheMU=936MeV hy-pothesis.

Fig. 5. Distributionofthedifferences(pulls)data-backgroundnormalizedtothedata statisticalerror(bluepoints)andrelativeGaussianfit(redcurve).

5. Systematicerrorsandefficiencies

The main systematic uncertainties affecting this analysis are relatedtotheevaluationoftheirreduciblebackground.Astwo dif-ferentprocedureswereusedindifferentmassranges,theestimate ofthesystematicerrorisaccountedfortwoindependentsources:

•

systematicuncertaintiesduetothesidebandfittingprocedure;

•

systematic uncertainties due to the evaluation of the

back-ground with the

PHOKHARA

generator and to the smearing

procedureinthe779–791MeVmassrange.

The evaluation of the systematic uncertainties on the fitted backgroundwas performed by adding in quadrature,bin by bin, thecontributionsduetotheerrors ofthefitandasystematic er-rorduetothefit procedure.Thefirstis obtainedby propagating, foreachfitinterval,thecorrespondingerrorsofthefitparameters. Thesecondisevaluatedbyvaryingthefitparametersby

±

1σ and

computingthemaximumdifference betweenthestandardfitand

thefit derived by usingthe modified parameters. Thesystematic errorislessthan1%inmostofthemassrange.

Inthe

ρ

–ωregionthesystematicerroriscomputedbyadding inquadraturethecontributionsduetothetheoreticaluncertainty oftheMonteCarlogenerator(0

.

5%[26]),thesystematicerrordue totheresidualbackgroundevaluation(0.3%,computedbychanging the analysis cuts within the corresponding experimental resolu-tions),thecontributionofthesmearingprocedure(0.8%, obtained

Fig. 6. Fractionalsystematicerrorontheestimatedbackground.Bluepoints:errors fromthesidebandfitprocedure;blackpoints:errorsestimatedfromthePHOKHARA

MonteCarlosimulationfortheρ–ωinterferenceregion.(Forinterpretationofthe referencestocolorinthisfigure,thereaderisreferredtothewebversionofthis article.)

Table 1

Summaryof the systematic uncertaintiesaffecting the π+_π−_γ analysis.

Systematic source Relative uncertainty (%)

Mtrkcut 0.2 Acceptance 0.6–0.1 as Mπ πincreases Trigger 0.1 Tracking 0.3 Generator 0.5 Luminosity 0.3 PID negligible Total 0.9–0.7 as Mπ πincreases

Fig. 7. Global analysis efficiency as a function of Mπ π.

byvaryingtheappliedsmearingof

±

1

σ

),andthesystematic un-certaintyontheluminosity(0.3%[26]).Theresultingtotal system-aticerrorisabout1%.

The total systematicuncertainty due to the background eval-uation is shown in Fig. 6. The full list of the systematic effects takenintoaccountissummarizedin

Table 1

.Theydonotaffectthe irreduciblebackgroundestimate performedwiththesideband fit-tingtechnique,butpartiallycontributetothebackgroundestimate inthe

ρ

–ωregion (seeabove)andenter inthe determinationof the selection efficiency andthe luminosity measurement. Finally, in

Fig. 7

weshowtheglobalanalysisefficiencyasestimatedwith thefull

π

+

π

−

γ

simulation(

PHOKHARA

generator

+

GEANFI[28]).

Fig. 8. Maximum number of U boson events excluded at 90% CL.

Thisincludes contributions fromkinematic cuts, trigger, tracking, acceptance and PID-likelihood effects. The total systematic error ontheglobalanalysisefficiencyrangesbetween0.7%and0.4% as

Mππ increases. 6. Upperlimits

We did not observe anyexcess of events withrespect to the estimatedbackgroundwithsignificancelargerthanthreestandard

deviations over the whole MU explored spectrum. We thus

ex-tractedthe massdependent limitson

ε

2 _{at90% confidencelevel} (CL)bymeansoftheCLStechnique[33].Theprocedurerequiresas inputsthemeasured invariantmassspectrum,theestimated irre-ducibletotalbackgroundandtheUbosonsignalforeachMππ bin.

Themeasuredspectrumisusedasinputwithoutanyefficiencyor backgroundcorrection.ThesignalisgeneratedvaryingtheUboson masshypothesisinstepsof2 MeV.Ateachstep,aGaussianpeak isbuiltwithawidthcorresponding tothe invariantmass resolu-tionofthedipionsystem.Thesystematicuncertaintiesweretaken intoaccountby performing aGaussiansmearing oftheevaluated backgroundaccordingtotheestimatesinSection5and

Fig. 6

.The resultsofthestatisticalprocedureareshownin

Fig. 8

intermsof thenumberNCLS ofUbosonsignaleventsexcludedat90%CL.

Wecomputedthelimitonthemixingstrength

ε

2 _bymeansof thefollowingformula[20,21]:

ε

2

=

α



α

=

NCLS

eff

·

L

·

H

·

I

,

(1)

where

effistheglobalanalysisefficiency(see

Fig. 7

), L isthe in-tegratedluminosity, H is theradiator function computedatQED NLO corrections with a 0.5% uncertainty [34–37], I is the effec-tive e+e−

→

U

→

π

+

π

− cross sectionintegrated overthe single mass bin centered at Mππ

=

MU with

ε

=

1. The uncertainties on H ,

eff, L, and I, propagate to the systematicerror on

ε

2 via eq.(1).Theresultinguncertaintyon

ε

2 _{islower than1% andhas} beentaken intoaccount in the estimatedlimit. Fig. 9showsthe results fromeq. (1) after a smoothing procedure (tomake them morereadable), comparedwithlimits fromother experiments in themassrange0–1 GeV. Our90%CL upperlimiton

ε

2 _reachesa maximumvalueof1

.

82

×

10−5at529 MeVandaminimumvalue of1

.

93

×

10−7 _{at 985 MeV.The sensitivityreduction due tothe}

ω

→ π

+

π

−

π

0 _{decayisofthesameorderofthestatistical} fluctua-tionsandthusnotvisibleafterthesmoothingprocedure.

Fig. 9. 90%CLexclusionplotforε2_{asafunctionoftheU-bosonmass(KLOE}

(4)).The

limitsfromtheA1[38]andAPEX[39]fixed-targetexperiments;thelimitsfromthe φDalitzdecay (KLOE(1))[40,41]ande+e−→Uγ processwheretheUboson

de-caysine+e−orμ+μ−(KLOE(3)andKLOE(2)respectively)[20,21];theWASA[42],

HADES[43],BaBar[44]andNA48/2[45]limitsarealsoshown.Thesolidlinesare the limitsfromthemuonandelectronanomaly[15],respectively.The grayline showstheUbosonparametersthatcouldexplaintheobservedaμdiscrepancywith

a2σ errorband(graydashedlines)[15].(Forinterpretationofthereferencesto colorinthisfigure,thereaderisreferredtothewebversionofthisarticle.)

7. Conclusions

Weusedanintegratedluminosityof1

.

93fb−1 ofKLOEdatato search for darkphoton hadronicdecays inthe e+e−

→

U

γ

,U

→

π

+

_π

−_{continuumprocess.Nosignalhasbeenobservedandalimit}

at 90% CL hasbeen set on the coupling factor

ε

2 _{in the energy} rangebetween527and987 MeV.Thelimitismorestringentthan otherlimitsinthe

ρ

–

ω

regionandabove.

Acknowledgements

We warmly thank our former KLOE colleagues for the

ac-cess to the data collected during the KLOE data taking

cam-paign. We thank the DA



NE team for their efforts in

main-taining low background running conditions and their

collabo-ration during all data taking. We want to thank our

techni-cal staff: G.F. Fortugno and F. Sborzacchi for their dedication

in ensuring efficient operation of the KLOE computing

facili-ties; M. Anelli for his continuous attention to the gas system and detector safety; A. Balla, M. Gatta, G. Corradi and G. Pa-palino for electronics maintenance; M. Santoni, G. Paoluzzi and R. Rosellini for generaldetectorsupport; C. Piscitelli for his help

during major maintenance periods. This work was supported in

part bythe EU IntegratedInfrastructure InitiativeHadron Physics Project under contract number RII3-CT-2004-506078; by the

Eu-ropean Commission under the Seventh Framework Programme

through the ‘Research Infrastructures’ action of the ‘Capacities’

Programme,Call:FP7-INFRASTRUCTURES-2008-1,GrantAgreement

No. 227431; by the Polish National Science Centre through the GrantsNos. 2011/03/N/ST2/02652,2013/08/M/ST2/00323,2013/11/ B/ST2/04245,2014/14/E/ST2/00262,2014/12/S/ST2/00459.

References

[1]F.Zwicky,Helv.Phys.Acta6(1933)110–127; F.Zwicky,Astrophys.J.86(1937)217.

[2]K.Olive,etal.,Reviewofparticlephysics,Chin.Phys.C38(2014)090001. [3]B.Holdom,Phys.Lett.B166(1985)196.

[4]C.Boehm,P.Fayet,Nucl.Phys.B683(2004)219. [5]P.Fayet,Phys.Rev.D75(2007)115017.

[6]Y.Mambrini,J.Cosmol.Astropart.Phys.1009(2010)022. [7]M.Pospelov,A.Ritz,M.B.Voloshin,Phys.Lett.B662(2008)53.

[25]L.Barzè,etal.,Eur.Phys.J.C71(2011)1680.

[26]H.Czy ˙z,A.Grzelinska,J.H.Kühn,G.Rodrigo,Eur.Phys.J.C39(2005)411. [27]J.H.Kühn,A.Santamaria,Z.Phys.C48(1990)445.

[43]G.Agakishiev,etal.,HADESCollaboration,Phys.Lett.B731(2014)265. [44]J.P.Lees,etal.,BaBarCollaboration,Phys.Rev.Lett.113(2014)201801. [45]J.R.Batley,etal.,NA48/2Collaboration,Phys.Lett.B746(2015)178.