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Search for η → e+e− decay with the WASA experiment M.BerłowskiforWASA-at-COSYCollaboration1,a [1]NationalCentreforNuclearResearch,Warsaw,Poland Abstract. Nowadaysthefieldofsearchingforanewphysicsbecameaveryinteresting 3 subject inalightmeson decays duetoarecent resultsfromKTeVcollaboration which 1 foundthe3.3σdisagreement betweenStandardModeltheoryandtheirresultsofπ◦ → 0 e+e− branching ratiomeasurement [1].Theyproposetoexplainthisdiscrepancywitha 2 newU bosonparticlethatinteractsbothwithmesonandvirtualphotonproducing e+e− n pair [2]. The same effect could be observed in eta meson decay into electron-positron. a Thecurrentbranchingratiolimit[3,4]isfarawayfromthepredictednon-StandardModel J theoryandduetothatfactitcannotdistinguishbetweenStandardModelandmoreexotic 5 explanation.Thefollowingreportshowstheanalysishighlightsforsearchingforasuch 2 effectinpp→ pp(η→e+e−)at1.4GeVproducedinWASA@COSYexperiment. ] x 1 Introduction e - cl Due to the fact that very low branching ratio (BR) is expected in the Standard Model, η → e+e− u decay is very hard to observe. In the Standard Model, the decay proceeds dominantly through the n electromagneticinteraction(seefig.1left) [ 1 v 0 4 0 6 . 1 0 3 1 : v Fig.1. Thedominating conventional mechanism for η → e+e− decay (leftpanel), anexample of process with i intermediateUbosoninteraction(rightpanel). X r a anditissuppressedrelativetoη→γγbyα2andby(m /m )2fromhelicityconservation: e η BR[η→e+e−]∼ BR[η→γγ]·α2·(m /m )2 e η The upper limit for this decay BR < 2.7 × 10−5 at CL = 90% comes from CELSIUS/WASA exp experiment [3] and the most recent value comes from HADES collaboration and sets the limit to < 5.6×10−6 atCL = 90%[4].Thisnumbersareatleastfourordersofmagnitudelargerthenvalue predictedfromtheStandardModelcalculations(BR ∼10−9).Thesmallprobabilityofthisfourth- theo orderelectromagnetictransitionmakesthedecaysensitivetohypotheticalinteractionsthatarisefrom a e-mail:[email protected] EPJWebofConferences physicsbeyondStandard Model[2,5,6,7] (see fig.1 right). An observationof a signalabove ∼ 10−9 levelcouldbeevidenceforanunconventionalprocesswhichenhancesthisdecayrate. 2 Experiment The2weeksexperimenttookplaceatInstituteforNuclearPhysicsoftheForschungszentrumJuelich in Germany.ForthispurposeWASA detectionsysteminstalled atthe COSY storageringwas used. The protonbeam of energy1.4 GeV (2.14 GeV/c in momentum)was scattered on frozen hydrogen pelletscrossing the beam line.Thereactionproductsalong with scattered protonswere detected and measured in WASA detector(protonsin the Forward part and meson decay productsin the Central part).ThedetaileddescriptionoftheWASAdetectorcanbefoundin[8].Runswerededicatedforthe etamesondecayscomingfrompp→ ppηreaction.Forthisanalysisaspecialtriggersystemwasused demandingahighenergydepositineachofCentralDetectorshalves.Thistypeoftriggershouldtreat equallythefollowingdecaysoftheetameson:η→γγ,η→e+e−γ,η→e+e−e+e−,η→e+e−. 3 Analysis Inordertodeterminethenumberofetamesonsproduced,thedatasampleofη→γγdecayswereused. Fromthenumberofη→γγeventsreconstructed(seefig.2left),knowingtheBRforthedecayonewas abletoestimatethisnumbertoN =∼4.4×107.Asacrosschecktothisnormalizationprocedureη→ eta e+e−γdecaychannelwasused(seefig.2right).Thesecondchannelalsoprovidedessentialinformation aboutdetectorresponsetoelectron-positronpairsofvariousenergies.Bothchannelsservedalsoasan experimentalfieldforstudyingtheperformanceofthetriggersystem. Fig. 2. Missing mass of two protons versus invariant mass of two photons (left panel), Missing mass of two protonsversusinvariantmassofe+e−γ(rightpanel). Particleselectionfortheη→e+e−includeddemandingatleasttwoprotonsintheForwardDetec- tor(ifthereweremore,twoclosestintimewereused),vetoonneutralparticlesintheCentralDetector ofenergymorethan20MeV,atleasttwooppositelycharged(ifmore,twogivingthegreatestopening angle were used) and time correlation between both chargedin Central Detector and protonsin the ForwardDetector.Asfor η → e+e− thefirststepwasdeterminationofthemainbackgroundsources whichappearedtobe: pp → ppπ+π−, pp → pp(η → e+e−γ)and pp → p(∆ → p[γ∗ → e+e−]).The directtwochargedpionproductionhas100timelargercross-sectionthenfortheetamesonproduction andthesamenumberofchargedparticlesinthefinalstate.ThesingleDalitzdecayoftheetameson hasthesamefinalstateparticles(comingfromthesameasin η → e+e− initialstage),ifthemassof virtual photon is large enough and real photon is not observed. The radiative decay of Delta(1232) resonancehas the same finalstate as η → e+e−. Accordingto the Monte Carlo distributionsforthe channelsaboveand signalsimulation optimizationprocedureforcuts were evoked.The particlesin the Central Detector were identified in two different methods: energy deposited in electromagnetic calorimeter(seefig.3andalsobyusingratioofdepositedenergytoparticlemomentum. MESON2012-12thInternationalWorkshoponMesonProduction,PropertiesandInteraction GeV]0.8 GeV]0.8 C [0.7 C [0.7 Energy deposit in the right side of SE000000......02345610 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Energy deposit in the right side of SE000000......02345610 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Energy deposit in the left side of SEC [GeV] Energy deposit in the left side of SEC [GeV] Fig.3.EnergydepositsinbothsidesoftheenergycalorimeterforMonteCarlosimulatedη →e+e− signal(left panel)andMonteCarlodirectproductionoftwochargedpions(rightpanel). Aftertheanalysisweareleftwith148eventcandidatesinwholedatasampleontheplotofmissing mass of two protonsusing cut optimization,obtainingMonte Carlo signal acceptance = 5.5%. If in addition we use reduction of backgroundcoming from pp → ppπ+π− and pp → p(∆ → p[γ∗ → e+e−])withthepolynomialfitsubtractionandtheremainingexpectednumberofeventscomingfrom pp → pp(η → e+e−γ)background= 4.6±1.5events(seefig.4left).Afteradditionalcuts(ontotal missing mass, momentumand energyand other) the possible signalis consistent within errorswith theexpectednumberofeventscomingfromη→e+e−γ(seefig.4right).Thelackofthesignalevents leadsustothepreliminaryBRlimit: BR =4.6×10−6atCL90%(preliminary), limit which is an orderof magnitudebelow present limit [3] for branchingratio of η → e+e− decay and atthesamelevelasthemostrecentmeasurementfromHADESCollaboration[4].Eighttimeslarger statisticofthesamereactionwithsimilartriggerwasrecentlycollectedandanalysishasstarted. N entries111246 preliminary EMREMRnnMMeettaaSSrrnnii ee s s 00 ..0000..2255 110044445555884477 N Entries1.82 preliminary EMREMRnnMMeettaarrSSiinnee ss 00 ..0000..11559944 000011993300 1.6 10 1.4 8 1.2 1 6 0.8 4 0.6 2 0.4 0.2 00.5 0.51 0.52 0.53 0.54 0.55 0.56 0.57 0.5M8Mpp0 [.G59eV/c02].6 00.5 0.51 0.52 0.53 0.54 0.55 0.56 0.57 0.58MM0p.p5 [9GeV/c02].6 Fig.4.Plotofthemissingmassoftwoprotons.Blackcurve-polynomialfittothebackground,redcurve-the shapeofexpectedMonteCarloη → e+e− signalpluspolynomialbackground(leftpanel).Thefinaldistribution ofthemissingmassoftwoprotons(rightpanel). 4 Acknowledgements We acknowledge support by the Polish Ministry of Science and Higher Education under grant No. 86/2/N-DFG/07/2011/0. References 1. A.E.DorokhovandM.A.Ivanov,Phys.Rev.D75:114007,(2007)arXiv:0704.3498[hep-ph] 2. Y.Kahn,M.Schmitt,andT.M.P.Tait,Phys.Rev.D78:115002,(2008)arXiv:0712.0007v1[hep-ph] EPJWebofConferences 3. M.Berłowskietal.,Phys.Rev.D77:032004,(2008)arXiv:0711.3531v3[hep-ex] 4. G.Agakishievetal.,Eur.J.Phys.A48:64,(2012)arXiv:1203.1333[nucl-ex] 5. P.Fayet,Phys.Rev.D74:054034,(2006)arXiv:0607318v1[hep-ph] 6. Q.Chang,Y.-D.Yang,arXiv:0808.2933v1[hep-ph] 7. L.Bergstroem,Zeit.Phys.C14,129,(1982),CERN-TH-3439,Oct1982 8. H.H.Adamsetal.,arXiv:0808.2933v1[hep-ph]