Table Of Content

PEN: a low energy test of lepton universality Dinko Pocˇanic´∗,a L.P. Alonzi,a V.A. Baranov,b W. Bertl,c M. Bychkov,a Yu.M. Bystritsky,b E. Frlež,a C.J. Glaser,a V.A. Kalinnikov,b N.V. Khomutov,b A.S.Korenchenko,b S.M.Korenchenko,b M.Korolija,d T.Kozlowski,e N.P.Kravchuk,b 7 N.A. Kuchinsky,b M.C. Lehman,a D. Mzhavia,bf A. Palladino,ac P. Robmann,g 1 A.M. Rozhdestvensky,b I. Supek,d P. Truöl,g A. van der Schaaf,g E.P. Velicheva,b 0 2 M.G. Vitz,a V.P. Volnykhb (The PEN Collaboration) n aUniversityofVirginia,Charlottesville,VA22904-4714,USA a bJointInstituteforNuclearResearch,Dubna,MoscowRegion,Russia J cPaulScherrerInstitute,Villigen-WürenlingenAG,Switzerland 8 1 d RudjerBoškovic´ Institute,Zagreb,Croatia eInstytutProblemówJa˛drowychim.AndrzejaSołtana,S´wierk,Poland ] x f InstituteforHighEnergyPhysics,TbilisiStateUniversity,Tbilisi,Georgia e gPhysik-Institut,UniversitätZürich,Zürich,Switzerland - p E-mail: [email protected] e h [ Allowed charged π meson decays are characterized by simple dynamics, few available decay channels, mainly into leptons, and extremely well controlled radiative and loop corrections. In 1 v thatsense, piondecaysrepresentaveritabletriumphofthestandardmodel(SM)ofelementary 4 particlesandinteractions. Thisrelativetheoreticalsimplicitymakeschargedpiondecaysasensi- 5 2 tivemeansfortestingtheunderlyingsymmetriesandtheuniversalityofweakfermioncouplings, 5 as well as for studying pion structure and chiral dynamics. Even after considerable recent im- 0 . provements, experimental precision is lagging far behind that of the theoretical description for 1 0 pion decays. We review the current state of experimental study of the pion electronic decay 7 π+ →e+ν (γ), or π , where the (γ) indicates inclusion and explicit treatment of radiative 1 e e2(γ) : decay events. We briefly review the limits on non-SM processes arising from the present level v i ofexperimentalprecisioninπ decays. FocusingonthePENexperimentatthePaulScherrer X e2(γ) Institute(PSI),Switzerland,weexaminetheprospectsforfurtherimprovementinthenearterm. r a XIIIInternationalConferenceonHeavyQuarksandLeptons 22-27May2016 Blacksburg,Virginia,USA ∗Speaker. (cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/ PEN:testofleptonuniversality DinkoPocˇanic´ 1. Motivation π mesons (pions), the lightest hadrons, occupy a special place for both the weak and the stronginteractions,andremainsubjectsofstudyalmost70yearsaftertheirdiscovery[1]. Charged piondecayshaveprovidedanimportantearlytestinggroundfortheweakinteractionandradiative correctionsduringthedevelopmentofmodernparticletheory. Decaysofthechargedpionproceed viatheweakinteraction,stronglyreflectingthepropertiesanddynamicsofthelatter. Inparticular, the failure of early searches to observe the direct electronic decay of the pion (π →eν, or π ) e2 led to an intense examination of the nature of the weak interaction. A low branching fraction of ∼1.3×10−4 was predicted [2] even before the decay’s discovery [3]. It is a direct consequence of theV−A nature of the weak interaction, through helicity suppression of the right-handed state of the electron. Furthermore, the predicted radiative corrections for the π decay [4,5] received e2 quickexperimentalconfirmation[6,7]. Piondecayshavemorerecentlybeendescribedwithextraordinarytheoreticalprecision. Thanks totheunderlyingsymmetriesandtheassociatedconservationlaws,themorecomplicated,andthus more uncertain, hadronic processes are suppressed. If the experimental results reach a precision comparable to that of their theoretical description, pion decays offer an outstanding, clean testing groundofuniversalityofleptonandquarkcouplings. Astatisticallysignificantdeviationfromthe standardmodelexpectationswouldindicatethepresenceofprocessesorinteractionsnotincluded inthestandardmodel,affectingpiondecaysthroughloopdiagrams. 1.1 Theelectronicdecay,π+→e+ν (orπ ) e e2 The π− →(cid:96)ν¯ (or, π+ →(cid:96)¯ν ) decay connects a pseudoscalar 0− state (the pion) to the 0+ (cid:96) (cid:96) vacuum. Atthetreelevel,theratiooftheπ →eν¯ toπ →µν¯ decaywidthsisgivenby[2,8] Γ(π →eν¯) m2 (m2 −m2)2 Rπ ≡ = e · π e (cid:39)1.283×10−4. (1.1) e/µ,0 Γ(π →µν¯) m2 (m2 −m2)2 µ π µ The first factor in the above expression, the ratio of squared lepton masses for the two decays, comesfromthehelicitysuppressionbytheV−Alepton-W bosonweakcouplings. If, instead, the decay could proceed directly through the pseudoscalar current, the ratio Rπ would reduce to the e/µ second, phase-space factor, or approximately 5.5. A more complete treatment of the process in- cludesδRπ ,theradiativeandloopcorrections,andthepossibilityofleptonuniversalityviolation, e/µ i.e.,thatg andg ,theelectronandmuoncouplingstotheW,respectively,maynotbeequal: e µ Γ(π →eν¯(γ)) g2 m2 (m2 −m2)2 (cid:16) (cid:17) Rπ ≡ = e e π e 1+δRπ , (1.2) e/µ Γ(π →µν¯(γ)) g2 m2 (m2 −m2)2 e/µ µ µ π µ wherethe“(γ)”indicatesthatradiativedecaysarefullyincludedinthebranchingfractions. Steady improvements over time of the theoretical description of the π decay have produced greatly re- e2 finedcalculationsoftheSMprediction,culminatingattheprecisionlevelof8partsin105:  1.2352(5)×10−4 [9], (cid:12)  (cid:16) (cid:17)SM Γ(π →eν¯(γ))(cid:12)  Rπ = (cid:12) = 1.2354(2)×10−4 [10], (1.3) e/µ Γ(π →µν¯(γ))(cid:12) calc 1.2352(1)×10−4 [11]. 2 PEN:testofleptonuniversality DinkoPocˇanic´ Comparisonwithequation(1.1)indicatesthattheradiativeandloopcorrectionsamounttoalmost 4%ofRπ . Thecurrentexperimentalprecisionlagsbehindtheabovetheoreticaluncertaintiesby e/µ afactorof∼23: (cid:12) (cid:16)Rπ (cid:17)exp= Γ(π →eν¯(γ))(cid:12)(cid:12) =(1.2327±0.0023)×10−4, (1.4) e/µ Γ(π →µν¯(γ))(cid:12) exp dominatedbymeasurementsfromTRIUMFandPSI[12–15]. Becauseofthelargehelicitysuppressionoftheπ decay,itsbranchingratioishighlysuscep- e2 tible to small non-V−A contributions from new physics, making this decay a particularly suitable subject of study, as discussed in, e.g., Refs. [16–21]. This prospect provides the primary motivation for the ongoing PEN [22] and PiENu [23] experiments. Of all the possible “new physics” contributionsintheLagrangian,π isdirectlysensitivetothepseudoscalarone,whileothertypes e2 enter through loop diagrams. At the precision of 10−3, Rπ probes the pseudoscalar and axial e/µ vector mass scales up to 1,000TeV and 20TeV, respectively [20,21]. For comparison, unitarity testsoftheCabibbo-Kobayashi-Maskawa(CKM)matrixandprecisemeasurementsofseveralsu- perallowednuclearbetadecaysconstrainthenon-SMvectorcontributionsto>20TeV,andscalar onesto>10TeV[24]. AlthoughscalarinteractionsdonotdirectlycontributetoRπ ,theycando e/µ sothroughloopdiagrams,resultinginasensitivitytonewscalarinteractionsupto60TeV[20,21]. ThesubjectwasrecentlyreviewedatlengthinRef.[25]. Inaddition,(Rπ )exp provideslimitson e/µ themassesofcertainSUSYpartners[19],andonanomaliesintheneutrinosector[18]. 1.2 Radiativeelectronicdecayπ →eνγ (orπ ) e2γ It is impossible to discuss the π electronic decay without taking into account its radiative e2 variant that becomes indistinguishable in the infrared limit E → 0. The decay π+ → e+ν γ γ e proceeds via a combination of QED (inner bremsstrahlung, IB) and direct, structure-dependent (SD) amplitudes [8,26]. Under normal circumstances, as in the π → µνγ decay, the direct amplitudes are hopelessly buried under an overwhelming inner bremsstrahlung background. How- ever,thestronghelicitysuppressionoftheprimarynon-radiativeprocess,π →eν,alsosuppresses the bremsstrahlung terms, making the direct structure dependent amplitudes measurable in cer- tain regions of phase space [26,27]. (The same helicity suppression makes sensitive searches for non-V−A interaction terms possible in precision measurements of the primary π →eν decay, as discussed above.) The relative accessibility of hadronic structure amplitudes is of keen interest to effectivelow-energytheoriesofthestronginteraction,primarilychiralperturbationtheory(ChPT), which rely on the SD amplitudes to provide important input parameters. Whereas the IB amplitude is completely described by QED, the structure-dependent amplitude can be parametrized in terms of the pion form factors. As seen in the tree-level Feynman diagrams in Figure 1, standard V−A electroweak theory requires only two pion form factors, F , axial vector, and F , vector (or A V polar-vector), to describe the SD amplitude. Thus, a proper experimental description of the π e2γ decayservesseveralimportantgoals: (a)improvingtheaccuracyoflowenergyeffectivehadronic theories,suchasChPT,(b)enablingaprecisedeterminationoftheprimaryπ decayratebycon- e2 trolling the systematics of the radiative decay event subset, and (c) providing the opportunity to searchforevidenceofnewparticleswithofnon-SMcoupling,suchasaputativetensor-interacting 3 PEN:testofleptonuniversality DinkoPocˇanic´ SM Figure 1: Left: tree-level Feynman diagrams of the inner bremsstrahlung (IB) and structure-dependent (SD) amplitudes determined by the vector and axial-vector form factors, F and F , respectively. A new V A interactiontype,e.g.,sonemediatedbyahypotheticaltensorparticlediscussedintheliteraturethroughout the1990sand2000s[28], wouldaddanSDamplitudedefinedbyacorrespondingformfactor, F . Right: T relativestrengthsofthephysicsamplitudesinπ→eνγdecay:structure-dependentamplitudesSD+∝(F + V F )2andSD−∝(F −F )2,innerbremsstrahlungIB,andinterferenceINT ∝(F +F )S+ +(F −F )S−, A V A V A int V A int plottedasfunctionsofE andE . γ e boson[28]towhichtheprimary,π decayprocessisnotsensitive. Arecentreviewofthesubject e2 canbefoundin[29]. The most comprehensive study to date of the π decay has been performed by the PIBETA e2γ collaboration [30]. This work has provided a narrow constraint on the sum F +F of the vector V A and axial vector form factors of the pion, a sub-percent precision measurement of the branching fractionforE >10MeVandθ >40◦,aswellasastringentupperboundonF ,thetensorform γ eγ T factor, previously the subject of considerable controversy [29]. In addition to determining F and V F individuallywithgreatlyincreasedprecisioncomparedtopreviousmeasurements,thePIBETA A collaborationalsoevaluated,forthefirsttime,theF dependenceonq ,thee+-ν invariantmass. V eν e Infact,thePIBETAlimitonF providesthemoststringentconstraintonthestrengthofthetensor T weakinteraction[31],assumedtobezerointheSM. ThePENexperiment[22],discussedinmoredetailbelow,buildsonthemethods,resultsand accomplishmentsofthePIBETAcollaboration. 2. ThePENexperimentatPSI In2006anewmeasurementofRπ wasproposedatthePaulScherrerInstitutebyacollabo- e/µ rationofseveninstitutionsfromtheUSandEurope[22],withtheaimtoreach ∆Rπ /Rπ (cid:39)5×10−4. (2.1) e/µ e/µ 4 PEN:testofleptonuniversality DinkoPocˇanic´ PEN detector 2009-10 pure CsI PH MWPC1 + π mTPC VACUUM PMT beam AD AT BC MWPC2 ~3 m flightpath (cid:24)(cid:24)(cid:24)(cid:24)(cid:24)(cid:24)(cid:24)(cid:24)(cid:24)(cid:58) 10 cm Figure2: SchematiccrosssectionofthePENapparatus,showninthe2009-10runningconfiguration,with itsmaincomponents: beamentrywiththeupstreambeamcounter(BC),5mmthickactivedegrader(AD), minitimeprojectionchamber(mTPC)followedbyapassiveAlcollimator,andactivetarget(AT),cylindrical multiwire proportional chambers (MWPCs), plastic hodoscope (PH) detectors and photomultiplier tubes (PMTs), 240-element pure CsI electromagnetic shower calorimeter and its PMTs. BC, AD, AT and PH detectorsaremadeofplasticscintillator. Fordetailsconcerningthedetectorperformancesee[32]. The target precision of PEN falls short of matching the theoretical uncertainties given in equa- tion1.3,byafactorofabout6. Nevertheless,PEN’stargetuncertaintieswillconsiderablyexpand the mapped area of non-SM parameter space, as discussed in Section 1.1. PEN has acquired data inthreeruns,in2008,2009and2010. 2.1 PENapparatusandmeasurementmethod The PEN experiment uses the key components of the PIBETA apparatus with additions and modifications suitable for a dedicated study of the π and π decay processes. The PIBETA e2 e2γ detectorhasbeendescribedindetailin[32],andusedinaseriesofmeasurementsofrareallowed pionandmuondecaychannels[29,30,33,34]. ThemajorcomponentofthePENapparatus,shown in Figure 2, is a spherical large-acceptance (∼ 3πsr) electromagnetic shower calorimeter. The calorimeter consists of 240 truncated hexagonal and pentagonal pyramids of pure CsI, 22cm or 12 radiation lengths deep. The inner and outer diameters of the sphere are 52cm and 96cm, respectively. Beam particles entering the apparatus with p(cid:39)75MeV/c are first tagged in a thin upstream beam counter (BC) and refocused by a triplet of quadrupole magnets. After a ∼ 3m long flight path they pass through a 5mm thick active degrader (AD) and a low-mass mini time projectionchamber(mTPC),finallytoreacha15mmthickactivetarget(AT)wherethebeampions stop. Decay particles are tracked non-magnetically in a pair of concentric cylindrical multiwire 5 PEN:testofleptonuniversality DinkoPocˇanic´ proportionalchambers(MWPC1,2)andanarrayoftwenty4mmthickplastichodoscopedetectors (PH), all surrounding the active target. The BC, AD, AT and PH detectors are all made of fast plastic scintillator material and read out by fast photomultiplier tubes (PMTs). Signals from the beam detectors are sent to waveform digitizers, running at 2GS/s for BC, AD, and AT, and at 250MS/sforthemTPC. 2.2 PENdataandtheiranalysis Measurements of pion decay at rest, such as the one made with the PEN detector, must deal withthechallengeofseparatingtheπ→eν andπ→µ→eeventswithgreatconfidence. Hence,as in earlier π studies at rest, a key source of systematic uncertainty in PEN is the hard to measure e2 low energy tail of the detector response function. The tail is caused by electromagnetic shower leakage from the calorimeter, mostly in the form of photons. If not properly identified and suppressed, other physical processes contribute events to the low energy part of the spectrum; unlike showerleakagetheycanalsoproducehighenergyevents. Oneprocessistheordinarypiondecay intoamuoninflight,beforethepionisstopped,withtheresultingmuondecayingwithinthetime gateacceptedinthemeasurement. Anotheristheunavoidablephysicalprocessofradiativedecay. The latter is well measured and properly accounted for in the PEN apparatus, as in the PIBETA studies of [30,34]. Shower leakage and pion decays in flight can only be well characterized if the π →µ →e chain can be well separated from the direct π →e decay in the target. Therefore much effort has been devoted to digitization, filtering and analysis of the target waveforms [35], as illustrated in Figure 3. The method used to separate the 2-peak (π ) and 3-peak (π →µ →e) e2 events is illustrated and explained in Figure 4. The input is provided by the beam and MWPC detectors, used to predict the pion and positron energy depositions in the target, and the times of 30000 TGT waveform TGT waveform 25000 TGT waveEEfonnhhrtt__mrriittee ggfssilttt wweraa e11vvd88ee77119911 25000 TGT whhav__ettfoggrttmww faailtevvreeed11 TGT waveMMfoeeraamnn, p ion 55a88n88d.. e77+ prediction TGT EEwnnavttrreiieefossrm , pi o55n99 a33n44d e+ prediction MMeeaann 660044..55 RRMMSS 2288..11 20000 TGT waveform, found muon TGT RRwMMavSSef o r m, fo 22u66nd..55 m88uon 20000 15000 15000 10000 10000 5000 5000 0 0 550 600 650 700 550 600 650 700 target waveform bin target waveform bin Figure3: Fullandfilteredactivetarget(TGT)waveforminthePENexperimentfortwochallengingπ → µ→esequentialdecayeventswithanearlyπ→µ decay(left)andearlyµ→edecay(right). Thefiltering procedureconsistsofasimplealgebraicmanipulationofthesignal. Tothenakedeyebothrawwaveforms appeartohavetwopeaksonly. Theseparationofeventswith/withoutamuonsignaldependscriticallyon theaccuracyofthepredictionsforthepionandpositronsignals. Forthepionthepredictionisbasedonthe timesandenergiesobservedinBCandAD.ForthepositronthepredictiondependsonthePHtimingand onthepathlengthreconstructedwiththepionandpositrontrackingdetectors(seeFigure4). 6 PEN:testofleptonuniversality DinkoPocˇanic´ EEnnttrriieess 22..118888007722ee++0077 105 eV] 4 104 M y [ 104 erg 3 103 n e et 103 g 2 102 ar n t sitro 1 10 102 po Ee+ EE£nn 5ttrr0iiee Mss eV, t :00 50 - 200 ns 0 1 10 Ee+ ‡ 60 MeV, t £ 20 ns 5 10 15 20 25 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 positron target pathlength [mm] D (c 2) Figure4: Left: correlationbetweenobservedpositronenergyinthetargetwaveformandthee+pathlength in the target, reconstructed from the observed π+ and e+ trajectories. Shown are events with proper π → µ→esequencesforwhichthee+signaliswellseparatedfromothersignals. Right:differenceinχ2forthe assumptions of a target waveform with/without a muon pulse present. The observable is normalized such thatπ →eν eventspeakat+1,andπ →µ →eat−1. Shownareeventsfortwodifferentcombinationsof e+ energyanddecaytimeresultinginalmostpuresamplesofπ →eν andπ →µ →e,respectively. Tiny admixtures of the other, suppressed process are readily identified and are of considerable help in reducing thesystematicuncertainties. theirrespectivesignals. Oncethepredictedwaveformissubtracted,theremainingnetwaveformis scannedforthepresenceofa4.1MeVmuonpeak. Thedifferencebetweentheminimumχ2values withandwithoutthemuonpeakisreportedas∆(χ2),constructedsothatclean2-peakand3-peak fitsreturnvaluesof+1and−1, respectively. Thescanisfast andreturnsa∆(χ2)valueforevery event,asillustratedinthefigure. AparticularlytellingfigureregardingthePENdataqualityisthedecaytimecomparisonofthe π →eν decayandπ →µ →esequence,showninFigure5forasubsetofdatarecordedin2010. Theπ →eν datafollowtheexponentialdecaylawovermorethanthreeordersofmagnitude,and perfectly predict the measured π →µ →e sequential decay data once the latter are corrected for random (pile-up) events. Both event ensembles were obtained with minimal requirements (cuts) ondetectorobservables,noneofwhichbiasestheselectioninwaysthatwouldaffectthebranching ratio. The probability of random µ →e events originating in the target can be controlled in the data sample by making use of multihit time to digital converter (TDC) data that record past pion stop signals. With this information one can strongly suppress events in which an “old” muon was present(“piledup”)inthetargetbythetimeofthepionstopthattriggeredthereadout. The“intrinsic”lowenergytailofthePENresponsefunctionbelow∼50MeV,duetoshower lossesforπ decayeventsforpionsatrest,amountstoapproximately2%ofthefullyield. Events e2 with π → µ decays in flight, with subsequent ordinary Michel decay of the stopped muon in the target,addacomparablecontributiontothetail. Thetwocontributionscanbesimulatedaccurately, withtherespectivedetectorresponsesindependentlyverifiedthroughcomparisonswithmeasured data in appropriately selected processes and regions of phase space. Although verified through comparisonswithMonteCarlosimulations,theintrinsictailitselfisnotdirectlymeasurableatthe 7 PEN:testofleptonuniversality DinkoPocˇanic´ 105 104 s n 5 0. 103 r e p nts 102 EtEEn =tnnr Ett2iEerr6niiseen.t0 ssrtri3 eie sns s , 0.5 ns sm 0 e00ar 0e 0d e p fi en v e p fi en distribution folded with m decaytime 10 p fi m fi e, probability of extra m below 2.5% 0 50 100 150 200 1.1 observed/predicted p fi en events 1.05 1 0.95 EEnnttrriieess 777766 1.04 observed/predicted p fi m fi e events 1.02 1 0.98 0 50 100 150 200 EEnnttrriieess 660000 decay time [ns] Figure5: Decaytimehistogramsforasubsetof2010PENdata: π →eν eventsandπ →µ →esequential decay events. The two processes are distinguished primarily by the total e+ energy and by the absence or presence, respectively, of an extra 4.1MeV (muon) in the target due to a π →µ decay. The π data are e2 shownwithapionlifetimeτ =26.03nsexponentialdecayfunctionsuperimposed. Theπ →µ →edata π wereprescaledbyafactorof∼1/64;theyareshownwiththecutontheprobabilityof<2.5%forasecond, pile-upmuontobepresentinthetargetatt=0,thetimeofthenominalpionstop. Theturquoisehistogram givestheπ→µ→eyieldconstructedentirelyfromthemeasuredπ→eν datafoldedwiththeµdecayrate, and corrected for random muons; it perfectly matches the bold dark blue histogram. The two lower plots showtheobservedtopredictedratiosforπ andπ →µ →eevents, respectively. Thescatterintheratio e2 plotsisstatisticalinnature. requiredprecisionbecauseofthestatisticaluncertaintiesarisinginthetaildataselectionprocedure. Radiativedecayprocessesaredirectlymeasurableandaccountedforinthebranchingfractiondata analysis. MoreinformationaboutthePEN/PIBETAdetectorresponsefunctionsisgivenin[32]. Wenextturnourattentiontothestronglyradiativedecayevents,i.e.,thosewithenergeticpho- tonsthatleadtoclearlyseparatede+ andγ initiatedshowersinthePENCsIcalorimeter. ThePEN collaborationhasrecordedasubstantialnewdatasetofsuchevents,tobeaddedtothepreviously recorded PIBETA π data set. Since the PEN beam stopping rate is lower than that used in the e2γ cleanest,2004PIBETArun,theimpactonthestatisticaluncertainties,whilesignificant,willnotbe profound. However,thecleanerrunningconditionsofPENallowforeasieraccesstothekinematic regionE ,E <50MeV,previouslystronglycontaminatedbythemuondecaybackground. These γ e 8 PEN:testofleptonuniversality DinkoPocˇanic´ are the regions needed to probe the SD− amplitude critically, as seen in Figure1. Accessing SD− openstheprospectsforanewdeterminationofthequantityF −F ,poorlyconstrainedinthemain V A PIBETA result [30], and, hence, for an improved model-independent experimental determination ofF . Theseresultswillbeforthcominginthenearfuture. V 3. Conclusions Duringthethreeproductionruns,from2008to2010,thePENexperimentaccumulatedsome 2.3×107 π → eν, and more than 1.5×108 π → µ → e events, as well as significant numbers of pion and muon radiative decays. A comprehensive blinded analysis is under way to extract a new experimental value of Rπ . As of this writing, there appear to be no obstacles that would e/µ preventthePENcollaborationfromreachingaprecisionof∆R/R<10−3. ThecompetingPiENu experiment at TRIUMF [23] has a similar precision goal. The near to medium future will thus bring about a substantial improvement in the limits on e-µ lepton universality, and in the related limitsonnon-SM,non-V−Aprocessesandcouplings. ItisimportanttonotethatevensubsequenttothecompletionofthePENandPiENudataanal- yses, there will remain considerable room for improvement of experimental precision with high payoffintermsoflimitsonphysicsnotincludedinthepresentStandardModel. Iffullysuccessful, the current experiments will bridge the current gap between the experimental and theoretical un- certaintylevelsonlyhalf-way,leavingroomfornewexperiments. Furthermore,thisworkremains relevant and complementary to the direct searches on the energy frontier currently underway at particlecolliders,providingvaluabletheoreticalcrosschecks. Acknowledgments This work has been supported by grants PHY-0970013 and PHY-1307328 from the United StatesNationalScienceFoundation. References [1] C.M.G.Lattes,H.Muirhead,G.P.S.OcchialiniandC.F.Powell,Processesinvolvingchargedmesons, Nature159(1947)694. [2] R.P.FeynmanandM.Gell-Mann,TheoryofFermiinteraction,Phys.Rev.109(1958)193. [3] T.Fazzini,G.Fidecaro,A.W.Merrison,H.PaulandA.V.Tollestrup,Electrondecayofthepion,Phys. Rev.Lett.1(1959)247. [4] S.M.Berman,Radiativecorrectionstopionbetadecay,Phys.Rev.Lett.1(1958)468. [5] T.Kinoshita,Radiativecorrectionsto pi-edecay,Phys.Rev.Lett.2(1959)477. [6] H.L.Anderson,T.Fujii,R.H.MillerandL.Tau,Branchingratiooftheelectronicmodeofpositive piondecay,Phys.Rev.119(1960)2050. [7] E.DiCapua,R.Garland,L.PondromandA.Strelzoff,Studyofthedecayπ →e+ν,Phys.Rev.133 (1964)B1333. 9 PEN:testofleptonuniversality DinkoPocˇanic´ [8] Foradetaileddiscussionoftheπ decayssee,e.g.,D.A.Bryman,P.DepommierandC.Leroy, e2(γ) π →eν,π →eνγ decaysandrelatedprocesses,Phys.Rep.88(1982)151. [9] W.J.MarcianoandA.Sirlin,Radiativecorrectionstoπ decays,Phys.Rev.Lett.71(1993)3629. l2 [10] M.Finkemeier,Radiativecorrectionstoπ andK decays,Phys.Lett.B387(1996)391 l2 l2 [hep-ph/9505434]. [11] V.CiriglianoandI.Rosell, Two-loopeffectivetheoryanalysisofπ(K)→eν¯ [γ]branchingratios, e Phys.Rev.Lett.99(2007)231801[hep-ph/0707.3439]. [12] D.I.Brittonetal.,Measurementoftheπ+→e+ν branchingratio,Phys.Rev.Lett.68(1992)3000. [13] D.I.Brittonetal.,Improvedsearchformassiveneutrinosinπ+→e+ν decay,Phys.Rev.D46(1992) R885. [14] G.Czapeketal.,Branchingratiofortherarepiondecayintoe+andν ,Phys.Rev.Lett.70(a993)17. e [15] A.Aguilar-Arevaloetal.,Improvedmeasurementoftheπ →eν branchingratio,Phys.Rev.Lett.115 (2015)071801[hep-ex/1506.05845]. [16] R.E.Shrock,Generaltheoryofweakprocessesinvolvingneutrinos,Phys.Rev.D24(1981)1232. [17] O.U.Shanker,π ,K andK0–K¯0constraintsonleptoquarksandsupersymmetricparticles,Nucl. (cid:96)2 (cid:96)3 Phys.B204(1982)375. [18] W.Loinazetal.,NuTeVanomaly,leptonuniversality,andnonuniversalneutrino-gaugecouplings, Phys.Rev.D70(2004)113004[hep-ph/0403306]. [19] M.J.Ramsey-Musolf,S.SuandS.Tulin,Pionleptonicdecaysandsupersymmetry,Phys.Rev.D76 (2007)095017[hep-ph/0705.0028]. [20] B.A.CampbellandD.W.Maybury,Constraintsonscalarcouplingsfromπ±→(cid:96)±ν ,Nucl.Phys. (cid:96) B709(2005)419[hep-ph/0303046]. [21] B.A.CampbellandA.Ismail,Leptonicpiondecayandphysicsbeyondtheelectroweakstandard model,hep-ph/0810.4918. [22] http://pibeta.phys.virginia.edu,andlinkstherein. [23] http://pienu.triumf.ca,andlinkstherein. [24] K.A.Oliveetal.(ParticleDataGroup),ReviewofParticlePhysics,Chin.Phys.C40(2016)100001. [25] D.A.Bryman,W.J.Marciano,R.TschirhartandT.Yamanaka,Rarekaonandpiondecays: Incisive probesfornewphysicsbeyondthestandardmodel,Annu.Rev.Nucl.Part.Sci.61(2011)331. [26] Foradetaileddiscussionsee,e.g.,J.F.Donoghue,E.GolowichandB.R.Holstein,Dynamicsofthe StandardModel,CambridgeUniversityPress,Cambridge1992. [27] A.M.BernsteinandB.R.Holstein,NeutralpionlifetimemeasurementsandtheQCDchiralanomaly, Rev.Mod.Phys.85(2013)49[hep-ph/1112.4809]. [28] M.V.Chizhov,Discoveryofnewphysicsinradiativepiondecays?,PismaFiz.Elem.Chast.Atom. Yadra2(2005)no.4,7-16,[hep-ph/0402105],andreferencestherein. [29] D.Pocˇanic´,E.FrležandA.vanderSchaaf,Experimentalstudyofrarechargedpiondecays,J.Phys. G:Nucl.Part.Phys.41(2014)114002[hep-ex/1407.2865]. [30] M.Bychkov,D.Pocˇanic´,B.A.VanDevender,etal.,Newprecisemeasurementofthepionweakform factorsinπ+→e+νγ decay,Phys.Rev.Lett.103(2009)051802[hep-ex/0804.1815]. 10