Recent results from NA48/2 and NA62 experiments at CERN Nicolas Lurkin∗†on behalf of the NA48/2 and NA62 collaboration ‡ SchoolofPhysicsandAstronomy,UniversityofBirmingham E-mail: nicolas.lurkin@cern.ch The NA48/2 and NA62-R experiments at the CERN SPS collected a large sample of charged K kaon decays in flight. NA62-R was running in 2007–08 with a highly efficient minimum bias K 7 trigger for decays into electrons. A preliminary measurement of the electromagnetic transition 1 form factor slope of the π0 from 1.05×106 fully reconstructed π0 Dalitz decays is presented. 0 2 Theobtainedvaluea=(3.70±0.53stat±0.36syst)×10−2representsa5.8σ observationofanon- n zeroslopeinthetime-likeregionofmomentumtransfer. Anupperlimitontherateofalepton a numberviolatingdecayK±→π∓µ±µ± isreportedfrom∼1.6×1011K± decaysatNA48/2in J 2003–04: B <8.6×10−11 at 90% CL. Searches for heavy sterile neutrino N and inflaton χ 4 4 2 resonances in K± →πµµ decays are reported. No signal is observed and upper limits on the products B(K± → µ±N )B(N →π∓µ±) and B(K± →π±χ)B(χ → µ+µ−) are set in the ] 4 4 x range 10−10–10−9for resonance lifetimes up to 100ps. The result of a search for dark photon e - withthesamesampleofdecaysisalsoreported. Intheabsenceofobservedsignal,thelimitson p e themixingparameterε2intherange9–70MeV/c2areimproved. h [ XIIIInternationalConferenceonHeavyQuarksandLeptons 1 22-27May,2016 v Blacksburg,Virginia,USA 9 7 9 ∗Speaker. 6 †SupportedbyERCStartingGrant336581 0 . ‡F.Ambrosino,A.Antonelli,G.Anzivino,R.Arcidiacono,W.Baldini,S.Balev,J.R.Batley,M.Behler,S.Bifani, 1 C.Biino,A.Bizzeti,B.Bloch-Devaux,G.Bocquet,V.Bolotov,F.Bucci,N.Cabibbo,M.Calvetti,N.Cartiglia,A.Cec- 0 7 cucci, P. Cenci, C. Cerri, C. Cheshkov, J.B. Chèze, M. Clemencic, G. Collazuol, F. Costantini, A. Cotta Ramusino, 1 D.Coward,D.Cundy,A.Dabrowski,G.D’Agostini,P.Dalpiaz,C.Damiani,H.Danielsson,M.DeBeer,G.Dellacasa, : J. Derré, H. Dibon, D. Di Filippo, L. DiLella, N. Doble, V. Duk, J. Engelfried, K. Eppard, V. Falaleev, R. Fantechi, v i M.Fidecaro,L.Fiorini,M.Fiorini,T.FonsecaMartin,P.L.Frabetti,A.Fucci,S.Gallorini,L.Gatignon,E.Gersabeck, X A. Gianoli, S. Giudici, A. Gonidec, E. Goudzovski, S. Goy Lopez, E. Gushchin, B. Hallgren, M. Hita-Hochgesand, r M.Holder,P.Hristov,E.Iacopini,E.Imbergamo,M.Jeitler,G.Kalmus,V.Kekelidze,K.Kleinknecht,V.Kozhuharov, a W.Kubischta, V.Kurshetsov, G.Lamanna, C.Lazzeroni, M.Lenti, E.Leonardi, L.Litov, D.Madigozhin, A.Maier, I.Mannelli,F.Marchetto,G.Marel,M.Markytan,P.Marouelli,M.Martini,L.Masetti,P.Massarotti,E.Mazzucato, A.Michetti,I.Mikulec,M.Misheva,N.Molokanova,E.Monnier,U.Moosbrugger,C.MoralesMorales,M.Moulson, S.Movchan,D.J.Munday,M.Napolitano,A.Nappi,G.Neuhofer,A.Norton,T.Numao,V.Obraztsov,V.Palladino, M.Patel,M.Pepe,A.Peters,F.Petrucci,M.C.Petrucci,B.Peyaud,R.Piandani,M.Piccini,G.Pierazzini,I.Polenke- vich,I.Popov,Yu.Potrebenikov,M.Raggi,B.Renk,F.Retière,P.Riedler,A.Romano,P.Rubin,G.Ruggiero,A.Sala- mon, G. Saracino, M. Savrié, M. Scarpa, V. Semenov, A. Sergi, M. Serra, M. Shieh, S. Shkarovskiy, M.W. Slater, M. Sozzi, T. Spadaro, S. Stoynev, E. Swallow, M. Szleper, M. Valdata-Nappi, P. Valente, B. Vallage, M. Velasco, M.Veltri,S.Venditti,M.Wache,H.Wahl,A.Walker,R.Wanke,L.Widhalm,A.Winhart,R.Winston,M.D.Wood, S.A.Wotton,O.Yushchenko,A.Zinchenko,M.Ziolkowski. (cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommons Attribution-NonCommercial-NoDerivatives4.0InternationalLicense(CCBY-NC-ND4.0). http://pos.sissa.it/ RecentresultsfromNA48/2andNA62experimentsatCERN NicolasLurkin 1. TheNA48/2andNA62-R setup K TheNA48/2experimentoperatingin2003–04,anditssuccessorNA62-R phasein2007–08, K collectedchargedkaondecaysinflightattheCERNSPS.TheaimofNA48/2wasthemeasurement ofdirectCPviolation[1]withtheworld’slargestsampleofchargedkaonsdecays,whileNA62-R K aimedatmeasuringtheleptonuniversalityinthekaondecay[2]withaminimumbiasdatasample. Inbothcasestheamountofstatisticsrecordedallowedustostudyotherprocesses. Leptonnumber violatingdecaysandresonancesearchesinK±→πµµ anddarkphotonsearcheswithNA48/2are reported,aswellastheπ0 electromagnetictransitionformfactorwithNA62-R . K The beam line described in detail in [1] was designed to provide simultaneous K+ and K− beams. They were extracted from the 400GeV/c SPS proton beam impinging on a 40cm long berylliumtarget. Thefinalbeammomentumof(60±3)GeV/c(NA48/2)and(74.0±1.4)GeV/c (NA62-R ) was selected using a system of dipole magnets and a momentum-defining slit incor- K porated into a beam dump. This 3.2m thick copper/iron block provided the possibility to block eitheroftheK+ orK− beams. Thebeamswerefocusedandcollimatedbeforeenteringthe114m long cylindrical vacuum tank containing the fiducial decay volume. The beam contained mainly pions but included approximately 6% of kaons. The simultaneous K+/K− beams provided about 6.2×107particlesperspillof4.8sforNA48/2,whileNA62-R wasalsousingasinglemodebeam K withanintensity∼10timeslower,alternatingperiodswithK+,periodswithK−,andperiodswith both. The principal subdetectors in use were the same in both cases and a detailed description is given in [3]. The momenta of the charge particles were measured by a spectrometer housed in a tank separated from the decay volume and filled with helium. It was composed of four drift chambers(DCH)andadipolemagnetbetweenthesecondandthirdones. Themagnetprovideda horizontal transverse momentum kick of 120MeV/c in NA48/2 and 265MeV/c in NA62-R . A K hodoscope(HOD)composedoftwoplanesofplasticscintillatorwasplacedafterthespectrometer toprovideprecisetimingofthechargedparticlesandgeneratefasttriggersignalsforthelow-level trigger. A127cmthickquasi-homogeneouselectromagneticcalorimeterfilledwithliquidkrypton (LKr) was located downstream and used both as a photon detector and for particle identification. Thevolumeisdividedinto13248cellsof∼2×2cm2 crosssectionwithoutlongitudinalsegmen- √ (cid:0) (cid:1) tation. The energy resolution is σ /E = 3.2/ E⊕9/E⊕0.42 % and the position resolution is √ E (cid:0) (cid:1) σ = σ = 4.2/ E⊕0.6 mmwheretheparticleenergyE isgiveninGeV. Amuonvetosys- x y tem (MUV) was installed behind the LKr and consisted of three planes of scintillator orthogonal to the beam axis, each one preceded by a 80cm thick iron wall. They were made of 2.7m long and2cmthickstripsalternativelyarrangedhorizontallyandvertically. Thewidthofthestripswas 25cminthefirsttwoplanesand45cminthelastone. Thecentralstripsweredividedintwohalves to accommodate the beam pipe in a 22×22cm2 central hole. The detection efficiency was above 99%forsinglemuonevents,withatimeresolutionof350ps. The NA48/2 main trigger, used in the presented results, was optimised for three-track vertex topologies. Coincidence of hits in the two HOD planes was required in at least two of the 16 segments, or at least one segment accompanied by at least two clusters of energy deposition in the LKr. Fast algorithms were run to reconstruct tracks from the DCH hits and accept events with a three-track closest distance of approach below 5cm. Alternatively, the track assumed to 1 RecentresultsfromNA48/2andNA62experimentsatCERN NicolasLurkin be a π±, should have an energy E∗ <230MeV in the 60GeV/c K± rest frame. This condition suppressed the K± →π±π0 (E∗ =248MeV) while keeping the K± →π±π0π0. For NA62-R K the trigger operating in minimum-bias mode was optimised for decays into electrons: at least one coincidence of hits in the HOD, bounds on the hits multiplicity in the DCH, at least 10GeV of energy deposit in the LKr and at least one track with E/p>0.6 and p∈(5–90)GeV/c (with the energyE reconstructedintheLKrandthemomentum preconstructedintheDCH). 2. π0 electromagnetictransitionformfactorslopeattheNA62-R experiment K As the π0 is produced in four of the main K± decays, with about 2×1010collected kaon decays NA62-R is an ideal experiment to study the neutral pion. The Dalitz decay π0 →γe+e− K D [4] has a branching fraction of B =(1.174±0.035)% [5] and proceeds through the π0γγ vertex withoneoff-shellphoton. Thecommonlyusedkinematicvariablesdefinedintermsoftheparticle four-momentaare: (cid:18)M (cid:19)2 (p +p )2 2p ·(p −p ) x= ee = e+ e− , y= π0 e+ e− , m m2 m2 (1−x) π0 π0 π0 where p ,p ,p are respectively the π0 and e± four-momenta, m is the mass of the π0, and π0 e+ e− π0 M isthee+e− invariantmass. Thephysicalregionisgivenby ee (cid:18)2m (cid:19)2 (cid:114) r2 r2= e ≤x≤1, |y|≤ 1− . m x π0 Theπ0 differentialdecaywidthnormalisedtotheπ0 →γγ decaywidthreads: D 2γ 1 d2Γ(π0) α (1−x)3(cid:18) r2(cid:19) D = 1+y2+ (1+δ(x,y)) |F(x)|2 , Γ(π0 ) dxdy 4π x x 2γ whereF(x)isthesemi-off-shellelectromagnetictransitionformfactor(TFF)oftheπ0andδ(x,y) encodestheradiativecorrections. The TFF is usually expanded as F(x)=1+ax where a is the form factor slope parameter. This approximation is justified by the smallness of this parameter. It has been studied first in the vectormesondominance(VMD)model[6]whereitisdominatedbytheρ andω mesons,resulting (cid:16) (cid:17) inavaluea≈m2 m−2+m−2 /2≈0.03. FurtherstudiesextendingtheVMDmodel[7],orusing π0 ρ ω differentframeworks[8,9,10,11]areinagreementwiththeoriginalvalue. Another crucial aspect is the inclusion of the radiative corrections in the differential rate as they are of the same size as the TFF. The total radiative corrections have been first studied in [12], butthefirststudyofthecorrectionsonthedifferentialrateisdonein[13]inthesoft-photon approximation. Ithasbeenlaterextendedin[14]. Themostrecentcontribution[15],triggeredby thiswork,includesnewcontributionsandacodeimplementedattheMCeventgeneratorlevelfor thegenerationofradiativephotonfromtheinternalbremsstrahlungcontribution. 2 RecentresultsfromNA48/2andNA62experimentsatCERN NicolasLurkin 2.1 Eventselection The selection is tuned to select a pure Dalitz decay sample from the dominant K± →π+π0 D decaychain(K ),featuringtwosame-signtracks,oneopposite-signtrackandaphoton. Exactly 2πD one three-track vertex must be reconstructed within the fiducial decay region and no additional track is allowed. The impact point of the three tracks should be in the acceptance of the DCH chambers and separated by at least 2cm in the first one. The momenta are required to be in the range (2–74)GeV/c. A single cluster of energy deposition with more than 2GeV of energy, and separatedbyatleast20cmfromthesame-signtracksand10cmfromtheremainingtrackisallowed intheevent. Itisusedtoreconstructaphotoncandidateassumingitoriginatesfromthevertex. The totalreconstructedmomentumofthethreetracksandthephotoncandidateshouldbeintherange (70–78)GeV/candthesquaredtotaltransversemomentumwithrespecttothenominalbeamaxis p2<5×10−4(GeV/c)2. Themassassignmenttothetracksisdoneusingthekinematicproperties t of this decay chain. The opposite-sign track is assigned an electron mass and two hypotheses are built for the possible mass assignments of the same-sign tracks. Only events with at most one of the hypotheses satisfying the following conditions on the reconstructed π0 and K± masses andx,ykinematicvariablesareselected: M ∈(115–145)MeV/c2, M ∈(465–510)MeV/c2, eeγ π±π0 and x,|y|<1. The trigger conditions described in section 1 must be reproduced, but with tighter criteria to eliminate edge effects due to different resolution between online and offline analysis. ThetotalLKrelectromagneticenergyshouldbehigherthan14GeVandatleastoneofthetracksis requiredtohave p>5.5GeV/candE/p>0.6. Duetotheacceptancenotbeingwellreproduced in the simulation for events with low x, the signal region is defined as x > 0.01, equivalent to M >13.5MeV/c2. ee The final selected sample amounts to 1.1×106 events. The acceptances, evaluated from Monte-Carlosimulations,are1.81%forK and0.02%forK±→π0µ±ν (K ). 2πD D µ3D 2.2 Preliminaryresult A χ2 fitofthexdistributionofdataandsimulationisusedtoextracttheTFFslopevalue. An equipopulous binning is used. The different hypotheses are tested by reweighing the MC events simulated with a known slope a = 0.032. The main systematic uncertainties arise from the sim simulation of the beam spectrum and from the calibration of the spectrometer global momentum scale. Thepreliminaryresultobtainedis: a=(3.70±0.53 ±0.36 )×10−2, stat syst with χ2/ndf=52.5/49, which has a p-value of 0.34. This measurement represents a significant measurement of a positive π0 electromagnetic TFF slope of more than 5σ in the time-like region of momentum transfer. An illustration of the best fit result and the comparison with previous measurementsfromπ0 DalitzareshowninFig. 1. 3. SearchesforleptonnumberviolationandresonancesinK± →πµµ atthe NA48/2experiment Lepton number violation has never been observed. However heavy Majorana neutrinos are 3 RecentresultsfromNA48/2andNA62experimentsatCERN NicolasLurkin Ratio1.05 Data / MC(a=0) Geneva-Saclay (1978) C Form factor: best fit Fischer et al. 30k events M1.04 Form factor: – 1s band a / Saclay (1989) at1.03 Fonvieille et al. D 32k events 1.02 SINDRUM I @ PSI (1992) Meijer Drees et al. 54k events 1.01 TRIUMF (1992) Farzanpay et al. 8k events 1 0.99 NA62 (2016) 1.1M events 10-2 10-1 1 - 0.1 - 0.05 0 0.05 0.1 x p 0 TFF slope Figure1: (a)Illustrationofthefitresultshowingtheratiodata/MC,withtheMCsampleweighted toobtaina=0. Theeventsaredividedinto20equipopulousbinswiththepointsatthebarycentre of each bin. The solid line represents the TFF function with a slope value equal to the fit central value. Dashedlinesindicatethe1σ band. (b)Comparisonofthisresultwithpreviousexperiments measuringtheTFFslopefromπ0 decays. D possiblemediatorsforsuchprocesses. TheνMSMmodel[16]introducesthreeheavysterileneu- trinosN. ThelightestoneN hasamassofO(keV/c2)andisalsoacandidatefordarkmatter. The i 1 twoothershavemassesrangingfrom100MeV/c2 tofewGeV/c2. Themodelcanexplainbaryon asymmetrythroughCP-violatingsterileneutrinooscillationsandmixingwiththeactiveneutrinos, and the low neutrino masses through see-saw mechanism. Effective vertices with theW±,Z and the SM leptons can be built, with a mixing matrix U describing the mixing between sterile and SM neutrinos. A simple extension of this model with a real scalar field (inflaton χ) [17] can also explainthehomogeneityandisotropyoftheuniverseonlargescalesandtheexistenceofstructures onsmallerscales. The particular case of the lepton number violating (LNV) decay K± →π∓µ±µ± is investi- gated, with the data sample of NA48/2 containing ∼1.6×1011 K± decays in vacuum. It can be mediatedbythesterileneutrinosiftheyareproducedinthekaondecayasK± →µ±N, thensub- sequently decay as N →π∓µ±. For a neutrino produced as a resonance (indicated as N ) with a 4 mass m , the possible mass range is m +m 10−7 and m > 10MeV/c2 the mean free path does not exceed 10cm and can be ne- A(cid:48) glected, and the dark photon is assumed to decay promptly. The searched decay chain is then K± →π±π0 ,π0 →γA(cid:48) ,A(cid:48) →e+e− and the final state is therefore similar to the Dalitz decay of theπ0,whichrepresentsanirreduciblebackground. Asecondselectionfocusedonthedecaychain startingwiththeK±→π0µ±ν (K )decayisalsobuilt. µ3 7 RecentresultsfromNA48/2andNA62experimentsatCERN NicolasLurkin 4.1 Eventselection Thelogicoftheeventsselectionissimilartotheπ0 onepresentedinsection2withonenotable D difference: the E/p based particle separation method is used and therefore all tracks should be in theLKracceptanceandwellseparatedontheLKrplane. ThedistanceD betweenthetrackimpact ij points on the LKr are requested to be D >10cm for electron candidates and D >25cm for ee eπ/µ (cid:12) (cid:12) electron to pion/muon candidates. The reconstructed masses should also satisfy (cid:12)Meeγ−mπ0(cid:12)< 8MeV/c2and|M −m |<20MeV/c2. FortheK therequirementonthetotalinvariantmass π±π0 K µ3 is replaced by |M2 |<0.01(GeV/c2)2, where M2 =(P −P −P )2 is the squared missing miss miss K µ π0 mass, P and P are the reconstructed momenta of the µ± and π0, and P is the nominal beam µ π0 K momentum. Thetotalselectedsampleamountsto1.67×107fullyreconstructedπ0 candidates. Theoverall D acceptancesare3.82%forK and4.20%forK . 2π µ3 4.2 Searchfordarkphotonresonance ThesamemassscanframeworkusingtheRolke-Lopezmethodasinsection3isusedtosearch for narrow peaks in the reconstructed e+e− invariant mass spectrum M . The steps and width of ee the search windows are determined by the resolution σ(M ), which has been estimated from a ee K MCsampleandisapproximatelygivenbyσ(M )≈0.011×M . Themassstepforthescan 2πD ee ee is set to 0.5σ(M ) rounded to the nearest multiple of 0.02MeV/c2. The window is centered on ee the searched dark photon mass with a width equal to 3σ(M ). A total of 404 m hypotheses are ee A(cid:48) testedintherange9MeV/c2≤m ≤120MeV/c2. A(cid:48) The90%CLlimitsareobtainedfromtherelation B(π0→γA(cid:48))= NA(cid:48) (cid:2)B(K )A(K )+B(K )A(K )(cid:3)−1 , 2π 2π µ3 µ3 N K where N is the number of dark photon candidates, N is the total number of kaon decays in the A(cid:48) K fiducial region, and B(K ),A(K ),B(K ) and A(K ) are the branching fraction and accep- 2π 2π µ3 µ3 tances of the K and K decays respectively. The largest uncertainty (3%) comes from the π0 2π µ3 D branching fraction entering in the computation of N and is neglected. The 90% CL upper limits K ontheε2 mixingparameterarecalculatedfromeq. 4.1. Nodarkphotonsignalisobservedinthescannedrange. Itconstitutesanimprovementonthe existing limits in the range m ∈(9–70)MeV/c2. The sensitivity is limited by the irreducible π0 A(cid:48) D background and the upper limits are 2–3 orders of magnitude above the single event sensitivity. Theresultingexclusionlimitsassociatedtoconstraintsfromotherexperiments,asshowninFig.3, completely rules out the dark photon as an explanation for the muon (g−2) problem within the theoreticalassumptionsmadeintheanalysis. Conclusions The preliminary measurement of the electromagnetic transition form factor slope of the π0 using1.05×106π0→e+e−γ decaycollectedattheNA62experimentin2007–08isreported. The obtainedresulta=(3.70±0.53 ±0.36 )×10−2isasignificantmeasurementofmorethan5σ stat syst ofapositiveslopeinthetime-liketransfermomentumregion. 8 RecentresultsfromNA48/2andNA62experimentsatCERN NicolasLurkin ε2 KLOE WASA σ) (3 −2)e g 10-5 ( HADES X E P (g−2) A µ A1 10-6 BaBar E774 NA48/2 10-7 E141 10 102 m (MeV/c2) A’ Figure3: Upperlimitsat90%CLonthemixingparameterε2foreachdarkphotonmasshypothesis comparedwithexclusionlimitssetbypreviousexperiments. SearchesfortheLNVK±→π∓µ±µ± decayandresonancesinK±→πµµ decaysusingthe 2003–04 data sample collected at NA48/2 are also presented. No signals are observed. An upper limitonthebranchingratiooftheLNVchannelissetat8.6×10−11,improvingthepreviousbest limitbyanorderofmagnitude. UpperlimitsaresetontheproductsofbranchingratiosB(K±→ µ±N )B(N → π∓µ±), B(K± → µ±N )B(N → π±µ∓) and B(K± → π±χ)B(χ → µ+µ−) 4 4 4 4 as functions of the resonance mass and lifetime. These limits are in the range 10−9–10−10 for resonancelifetimesbelow100ps. Usingthesamedatasample,upperlimitsaresetonthemixingparameterε2 ofadarkphoton A(cid:48). The dark photon is assumed to be produced in the decay π0 →γA(cid:48) and decays only into an e+e− pair. Thelimitsareoftheorderof10−6 inthemassrangem ∈(9–70)MeV/c2, excluding A(cid:48) thisdarkphotonasanexplanationforthemuon(g−2)anomaly. References [1] R.Batleyetal.,Eur.Phys.J.C52(2007)875. [2] C.Lazzeronietal.,Phys.Lett.B719(2013)326. [3] V.Fantietal.,Nucl.Instrum.Meth.A574(2007)433. [4] R.H.Dalitz,Proc.Phys.Soc.A64(1951)667. [5] ParticleDataGroup,K.A.Oliveetal.,ChinesePhysicsC38(2014)090001. [6] M.Gell-MannandF.Zachariasen,Phys.Rev.124(1961)953. [7] P.Lichard,Phys.Rev.D83(2011)037503. [8] K.Kampfetal.,Eur.Phys.J.C46(2006)191. [9] T.HusekandS.Leupold,Eur.Phys.J.C75(2015)586. 9

Recent results from NA48/2 and NA62 experiments at CERN

Similar Recent results from NA48/2 and NA62 experiments at CERN