Search for η-->e+e- decay with the WASA experiment

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by  Marcin Berlowski

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.
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