Limit on the production of a new vector boson in $e^{+}e^{-} \rightarrow U\gamma$, $U\rightarrow \pi^{+}\pi^{-}$ 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

aINFNSezionediBari,Bari,Italy bINFNSezionediCatania,Catania,Italy

cInstituteofPhysics,JagiellonianUniversity,Cracow,Poland dLaboratoriNazionalidiFrascatidell’INFN,Frascati,Italy

eDipartimentodiScienzeMatematicheeInformatiche,ScienzeFisicheeScienzedellaTerradell’UniversitàdiMessina,Messina,Italy fDipartimentodiScienzeChimiche,Biologiche,FarmaceuticheedAmbientalidell’UniversitàdiMessina,Messina,Italy

gINFNGruppoCollegato diMessina,Messina,Italy

hDipartimentodiFisicadell’UniversitàdellaCalabria,Rende,Italy iINFNGruppoCollegato diCosenza,Rende,Italy

jDipartimentodiScienzediBaseedApplicateperl’Ingegneriadell’Università“Sapienza”,Roma,Italy kDipartimentodiScienzeeTecnologieApplicate,Università“GuglielmoMarconi”,Roma,Italy lNovosibirskStateUniversity,630090Novosibirsk,Russia

mDipartimentodiFisicadell’Università“Sapienza”,Roma,Italy nINFNSezionediRoma,Roma,Italy

oDipartimentodiFisicadell’Università“TorVergata”,Roma,Italy pINFNSezionediRomaTorVergata,Roma,Italy

qDipartimentodiMatematicaeFisicadell’Università“RomaTre”,Roma,Italy rINFNSezionediRomaTre,Roma,Italy

sENEAUTTMAT-IRR,CasacciaR.C.,Roma,Italy tDepartmentofPhysics,BostonUniversity,Boston,USA

uDepartmentofPhysicsandAstronomy,UppsalaUniversity,Uppsala,Sweden vNationalCentreforNuclearResearch,Warsaw,Poland

a r t i c l e i n f o a b s t ra 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 collectedatDA}NEbythe KLOEexperiment.

The π⁺ π^{− productionby}initial-state radiationcompensates foralossofsensitivity ofpreviousKLOE U→^e+e−,μ⁺μ^{− searchesduetothesmallbranchingratiosinthe}ρ^–ω^{resonanceregion.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)isusuallycomplementedwithmanyex- tensions [3–7] that attribute to the DM candidates also strong self interactions and weak-scale interactions with SM particles.

Amongthepossiblecandidates,a WeaklyInteractingMassivePar- ticle (WIMP) aroused much interest since a particle with weak- scaleannihilationcrosssectioncanaccountfortheDMrelicabun- danceestimatedthroughthestudyofthecosmicmicrowaveback- 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, partlyoren- tirely, the well known 3.6

σ

discrepancy withthe experimental measurement[16].

Inthispaperweassumethesimplesttheoreticalhypothesisac- 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 couplingtotheSMphotonwouldoccurbymeansofavectorpor- 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

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

ε

² factor,butarestill expectedtobe detectableathighlumi- nositye⁺e⁻ colliders[17–19].

KLOEhasalreadysearchedforradiativeUbosonproductionin thee⁺e⁻→^Uγ^,U→^e+e−,μ⁺μ^{−processes[20,21].The leptonic} channelsareaffectedbyadecreaseinsensitivityintheρ^–ω^region dueto the dominant branching fractioninto hadrons. For a vir- tualphotonwithq²<1GeV²,thecouplingtothechargedpionis givenbytheproductoftheelectricchargeandthepionformfac- tor,eFπ(q²).TheeffectivecouplingoftheUbosontopionsisthus predictedtobegivenbytheproductofthevirtualphotoncoupling andthekinetic mixingparameter

ε

²eFπ(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 DANE, the Frascati φ^-factory.

DANEis an e⁺e⁻ colliderusually operated ata centerof mass energym_φ¹.019GeV.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.

Calorimetermodulesarereadoutatbothendsby4880photomul- 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 MeVintwosectorsofthebarrelcalorime- ter[24].

3. Eventselection

We selectedπ⁺π⁻γ ^{candidates byrequesting eventswithtwo} oppositely-chargedtracksemitted atlargepolarangles,50^◦< θ <

130^◦,withtheundetectedISR photon missingmomentumpoint- ing –accordingto theπ⁺π⁻γ ^{kinematics –at}small polar angles (θ <15^◦,θ >165^◦).Thetrackswererequiredtohavethepointof closestapproachtothez axiswithinacylinderofradius8cmand length15cmcentered attheinteractionpoint.Inordertoensure goodreconstruction andefficiency,we selectedtrackswithtrans- verseandlongitudinalmomentumintherangep_⊥>160MeV or p>90MeV,respectively.

Sincetheπ⁺π⁻γ ^crosssectionbehavior asafunctionoftheISR photonpolarangleisdivergent(∝¹/θ_γ⁴), thetrackandthephoton acceptanceselectionsmake thefinal-stateradiation(FSR)andthe φ^{resonantprocessesrelatively}unimportant,leavinguswithahigh purityISRsampleandincreasingoursensitivitytotheU→ π⁺π⁻ decay[25].

The Monte Carlo simulation of the Mππ spectrum was pro- ducedwiththe PHOKHARAeventgenerator [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^{.Themainback-} groundcontributionsaffectingtheISRπ⁺π⁻γ ^{samplearetheres-} onante⁺e⁻→ φ→ π⁺π⁻π^{0 processandtheISRandFSRe+e−}→

⁺⁻γ(γ)^, =^e,μ ^{processes (they will be defined as “residual}

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 M_trk–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 DistributionsoftheM_trk variable for data andsimulation are shown in Fig. 1, wherethe M_trk>130MeV cuttodiscriminatemuonsfrompions isalsoin- dicated.

AparticleIDestimator(PID),L_±,definedforeachtrackwithas- 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 efficiencyofthisse-} lectionislargerthan99.95%asevaluatedusingmeasureddataand simulatedπ⁺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 andφ→ π⁺π⁻π^{0. Fig. 2showsthe fractional components ofthe} residualbackground, FBG,individually foreach contributingchan- 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 theinsertofFig. 3)wasachievedbyproducingadedicatedsample usingPHOKHARA 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 measureddataspectrumiscomparedwiththeresultsofthissim- ulationprocess,whichincludestheresidualbackground.

1 Mtrkis 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

=^M2γ=^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 πinvari- antmassspectrum.Thefigureinsertshowsindetailtheagreementachievedinthe ρ–ωmixingregion(779–791 MeV).

4. Irreduciblebackgroundparametrizationandestimate

Exceptforthe

ρ

–

ω

region,weestimatedtheirreducibleback- grounddirectlyfromdata.ForeachUmasshypothesisthedataare fitted ina Mππ intervalcentered at MU and18–20times wider than the Mππ resolution

σ

M_{π π}.The backgroundismodeled by a monotonic function using Chebyshev polinomials up to the sixth orderandisestimatedusingthesidebandtechnique,byexcluding fromthefitthedataintheregion±³

σ

Mπ π around M_U[20].The procedureisrepeatedinstepsof2MeVinM_U.

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 inFig. 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

–

ω

interferenceis notsmooth (see Fig. 3)and then not easy to be fittedwiththe sidebandtechnique. Wethus estimatedthe backgroundinthisregionby usingthePHOKHARA generator with smeared distributions, as explained in Section 3 andshowninFig. 3forthe779–791MeVmassrange.

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.Astwodif- 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 ofthefitandasystematicer- rorduetothefit procedure.Thefirstis obtainedby propagating, foreachfitinterval,thecorrespondingerrorsofthefitparameters.

Thesecondisevaluatedbyvaryingthefitparametersby±¹

σ

and computingthemaximumdifference betweenthestandardfitand thefit derived by usingthe modified parameters. Thesystematic errorislessthan1%inmostofthemassrange.

Inthe

ρ

–

ω

regionthesystematicerroriscomputedbyadding 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

),andthesystematicun- certaintyontheluminosity(0.3%[26]).Theresultingtotalsystem- aticerrorisabout1%.

The total systematicuncertainty due to the background eval- uation is shown in Fig. 6. The full list of the systematic effects takenintoaccountissummarizedinTable 1.Theydonotaffectthe irreduciblebackgroundestimate performedwiththesidebandfit- tingtechnique,butpartiallycontributetothebackgroundestimate inthe

ρ

–

ω

region (seeabove)andenter inthe determinationof the selection efficiency andthe luminosity measurement. Finally, inFig. 7weshowtheglobalanalysisefficiencyasestimatedwith thefull

π

⁺

π

⁻

γ

simulation(PHOKHARAgenerator+^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,theestimatedirre- ducibletotalbackgroundandtheUbosonsignalforeachMππ bin.

Themeasuredspectrumisusedasinputwithoutanyefficiencyor backgroundcorrection.ThesignalisgeneratedvaryingtheUboson masshypothesisinstepsof2 MeV.Ateachstep,aGaussianpeak isbuiltwithawidthcorresponding tothe invariantmassresolu- tionofthedipionsystem.Thesystematicuncertaintiesweretaken intoaccountby performing aGaussiansmearing oftheevaluated backgroundaccordingtotheestimatesinSection5andFig. 6.The resultsofthestatisticalprocedureareshowninFig. 8intermsof thenumberNCLS ofUbosonsignaleventsexcludedat90%CL.

Wecomputedthelimitonthemixingstrength

ε

² bymeansof thefollowingformula[20,21]:

ε

²

= α

^

α ⁼

N_CLS

eff

·

^L

·

^H

·

^I

,

(1)

where

effistheglobalanalysisefficiency(seeFig. 7), L isthein- 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 decayisofthesameorderofthe}statisticalfluctua- tionsandthusnotvisibleafterthesmoothingprocedure.

Fig. 9. 90%CLexclusionplotforε²asafunctionoftheU-bosonmass(KLOE(4)).The limitsfromtheA1[38]andAPEX[39]fixed-targetexperiments;thelimitsfromthe φDalitzdecay (KLOE(1))[40,41]ande⁺e⁻→^Uγ processwheretheUbosonde- 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⁻→^Uγ^,U→ π⁺π^{−continuumprocess.Nosignalhasbeenobservedandalimit} at 90% CL hasbeen set on the coupling factor

ε

² in the energy rangebetween527and987 MeV.Thelimitismorestringentthan otherlimitsintheρ^–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 DANE 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.