First observation and Study of the K to π e decay with ...

First observation and Study of the K± to π± π0 e+ e- decay with NA48/2 @ CERN

Brigitte Bloch-Devaux Università degli Studi di Torino

on behalf of the NA48/2 Collaboration

KAON201911thInterna1onalConferenceonKaonPhysicsUniversityofPerugia,Italy,September10-13

September10,2019 KAON2019/B.Bloch-Devaux 1

Outline

v NA48/2 description of experimental setup & detector performances…

any need to repeat it ? v  ChPT and the K± à π± π0 e+ e- decay mode v  Selection and backgrounds

v  Branching Ratio

v  ChPT contribution

v  Asymmetries v  Summary/Prospects


SPS

N

Geneva airport Switzerland

Jura mountains France

LHC

NA48/NA62

The NA48/NA62 experiments at CERN-SPS

The NA48/2 collaboration: still analyzing > 15 years old valuable data !!

~100 physicists from 15 Institutes in 8 countries Cambridge, CERN, Chicago, Dubna, Edinburgh, Ferrara, Firenze Mainz, Northwestern, Perugia, Pisa, Saclay, Siegen, Torino, Wien

NA48 direct CPV ε’/ε

NA48/1

NA48/2

NA62 (RK) NA62

1997: KL +Ks

1998: KL +Ks

1999: KL +Ks

2000: KL only

2001: KL +Ks

2002: Ks/hyperons

2003 K++ K –

2004 Ag(CPV)

2007 K++K –

2008 RK + tests

2007 design &

2013 construction

2012technical run

2013 long shutdown 2014 commissioning | & 2018 data taking


The NA48/2 experimental setup; Kaon beam

PK spectra, 60±3 GeV/c

Simultaneous K+ and K- beams: large charge symmetrization of

experimental conditions

Beams coincide within ~1mm all along the 114m decay volume

flux ratio K+/K- ~1.8

2003 + 2004 run: ~ 6 months, ~ 2 1011 K± decays in flight


NA48/2 detector and performances

Magnetic spectrometer : •  He tank to minimize multiple scattering •  4 high-resolution DCH’s + dipole magnet •  Δp/p = (1.02 ⊕ 0.044 p)% (p in GeV/c) •  Very good resolution for charged

invariant masses:σ(M3π±)= 1.7 MeV/c2

LKr electromagnetic calorimeter : • quasi-homogenous and high granularity • Δ E/E = (3.2/√E ⊕ 9.0/E ⊕ 0.42)% (E in GeV) σx=σy~1.5mmforE=10GeV• Very good resolution for neutrals(π0àγγ) σ(Mππ0π0)= 1.4 MeV/c2

• E/p ratio can be used for e/π separation

Hodoscope for fast charged trigger

precise time resolution σt= 150 ps

Decay region expands over ~114m Detector expands over ~50 m



Chiral Perturbation Theory and Kaon decays •  KaondecaysareaperfectlaboratorytostudyChPT(QCDatlowenergy)

-seetheorytalksthisweek-K± à π± l+ l- , K± à π± γ γ , K± à π± π0 γ , K± à π± π0 γ* à π± π0 e+ e- and many more..πeePLB677(2009),πµµPLB697(2011),πγγPLB730(2014),ππ0γ EPJC68(2010)•  Whatissospecialabout π± π0 e+ e- decay?H.Pichl,EPJC20(2001)371 L.Cappiello,O.Catà,G.D’Ambrosio,D.GaoEPJC72(2012)1872L.Cappiello,O.Catà,G.D’Ambrosio,EPJC78(2018)265

•  Neverobservedsofar:canBRmagnitudeconfirmChPTpredicònsofDEcontribuòn?

•  Ifafineanalysisispossible:

>signofinterferenceterm(IB,E)>Magne`ctermthrough(IB,M)interference>ChargeasymmetryasdirectCPviolaòn

DE(M,E)

IB

Event selection : signal and normalization

Signal:π± π0 e+ e- = π± γ γ e+ e- - Final state: 3 charged track and 2 photons forming a π0 pointing to the same decay vertex. One more γ wrt norm mode - Closed kinematics: constraints on Mπ0, MK - Subject to BR (π0γγ)=98.823%


Normaliza`on:π± π0D=π± e+ e- γ

- Final state: 3 charged tracks and 1 photon forming, with 2 opposite sign tracks, a π0 pointing to the same decay vertex - Closed kinematics: constraints on Mπ0D, MK - Very abundant: BR (ππ0) x BR (π0D)

20.67% x 1.174% = 2.425 10-3

SameTriggerchain:L1:>1HODquadrant(outof16)L2:DCHtrackbasedkinema`cs

HODLKr

Photons:ELKrwithin[3,60GeV]Chargedpar`cles:noPID,butusekinema`cconstrainttomaximizegeometricalacceptance

Event selection: signal and normalization


Signal: π± γ γ e+ e- Normaliza`on=π± e+ e- γRequire3goodqualitytracksformingavertexinthefiducialdecayregion

+twogoodqualityphotonclusters+onegoodqualityphotoncluster•  AssignelectronmasstothetrackwithQoppositetovertexcharge(K+orK-)•  Forboth(me,mπ) assignmentstosame-chargetracks,computereconstructedMπ0and

MKtobeinawiderangeandcheckkinema`ccorrela`on

|Mπ0–MPDG|<15MeV/c2|MK–MPDG|<45MeV/c2

|Mπ0–0.42MK+73.2MeV/c2|<6MeV/c2(massesinMeV/c2)

>99%intheband

96.5%intheband


9

Backgrounds

Normalization: •  Semileptonic Kl3D = Ke3D, Kmu3D πo

D is correctly reconstructed (e+e- γ)π  mass assigned to the e or mu track

•  Resulting fractions: Nbn / Nn = (0.106 ± 0.001)% Kmu3D : 0.064% Ke3D : 0.042%

Signal: •  KππoπoD with one photon lost/

merged: suppressed by (Mππo)2 > 0.12 GeV2/c4

•  KππoD with an extra photon: suppressed by |Meeγ – Mπo| > 7 MeV/c2

•  Resulting fractions: Nbs / Ns = (4.9 ± 0.4)% K3piD : 2.7% K2piD : 2.1 % Ke3D : 0.1%

•  Large simulated samples used to compute acceptances •  Contribution estimated from acceptances and BRs

Normalization : large very pure sample

Normalisa`onKππ0D:•  Kππ0generatorcodeincluding1realphotonemission-GapEPJC45(2006)•  π0Ddecaysincluding1extraphotonemission-Husek,Kampf,NovotnyPRD92(2015)•  alsoπ0Ddecaysincludingextraphoton(s)emission-PhotosWasetalCPC79(1994)


B/(S+B)=0.106%IncludeHKN(“Prague”)radcor.

rms=4.5MeV/c2

Normaliza`oncandidates

16316690(4039)

Background 17292(159)

A(radcorHKN) 3.981(2)%

Mee(GeV/c2)

rms=1.8MeV/c2

Signal : small clean sample

SignalKππ0ee:dominatedby•  IB(99%),thenDE(M)(1.39%)andINT(IB,E)(0.39%nega`ve):independent

genera`ons(IB,M,INT(BE))withdifferentacceptancescombinedasAeffaccordingtorela`vecontribu`ons

•  Rad.cor.addingextraphoton(s)emissionusingPhotos


rms=3.0MeV/c2 rms=7.8MeV/c2

Signalcandidates 4919(70)

Background 241(20)

A(radcor)IB 0.645(1)%

A(radcor)eff 0.662(1)%

B/(S+B)=4.9%IncludePhotosradcor

Mee(GeV/c2)


Branching ratio measurement ingredients

Normaliza`oncandidates

16316690(4039)

Background 17292(159)

A(radcorHKN) 3.981(2)%

L1efficiency 99.767(3)%


Signalcandidates 4919(70)

Background 241(20)

A(radcor)eff 0.662(1)%



Source δBR/BRx102

Ns 1.426

Nbs 0.416

Nn 0.025

Nbn Negl.

As(MCstat) 0.171

An(MCstat) 0.051

L1nxL2n(MCstat) 0.007

L1sxL2s(MCstat) 0.023

A(geometrycontrol) 0.083

A(`mevaria`on) 0.064

Triggerefficiency 0.400

Modeldependence 0.285

Radia`veeffects 0.490

BRππ0 0.87

BRπ0D/BRπ0γγ 2.946

stat1.486

syst0.777

external2.971


Branching ratio measurement @ NA48/2

BR=(4.237±0.063stat± 0.033syst±0.126ext)10-6=(4.237±0.145)10-6

CurrentlydominatedbyexternalerroronBR(π0D)InperfectagreementwithChPTcalcula`onsfromEPJC78(2018),noRadCor,noIsospinbreakingCor

IBonly IB+DE+INT4.18310-6 Total4.22910-6

Gooddata/MCagreementofthemeespectraoverseveralordersofmagnitude

Further studies


•  CurrentBRmeasurement:~3%precision•  Expectedrela`vecontribuònofDE(M)term:~1%•  Expectedrela`vecontribuònofINT(BE)term:evensmallerNowaytoextractDEorINTcontribuònsfromtheBRmeasurementBUT•  Cappielloetalsuggestedtolookatmee~50MeV/c2whereIB,DE,INTcontribuòns

populatedifferentlythe(Eγ*,T*π)plane.•  Toofewsta`s`cs,insteadstudythewhole3D-space(q2,Eγ*,T*π)inagridof3D-

boxesofequaldatapopula1ons(N1bandsinq2,thenN2bandsinT*π,thenN3bandsinEγ*)

•  Distributesimulatedeventsandbackgroundoverthesamegridasthedata. •  Minimizea χ2 es`matortofinda=DE/IB,b=INT/IBandN(overallnorm)

In3D-boxi:Ni(δNi)=dataevents(error)Mi(δMi)=MCevents(error)Ni

Bkg=Bkgevents


3D-kinematic space study (N1=3, N2=5, N3=6)

Fitresult:χ2=98.2/87,p-value=19% ChPTpredic`onsM/IB=0.0114±0.0043stat M/IB=0.0141±0.0014extBE/IB=-0.0014± 0.0036stat BE/IB=-0.0039± 0.0028ext Goodagreementbutlimitedprecision!

q=37.7521.11MeV/c2


What else ? Asymmetries !

NA48/2hasaccumulatedK+andK-decaysconcurrentlyallowingasymmetrystudieswithreducedsystema`cs.MeasureBR(K+)andBR(K-)asindependentquan``esandevaluate:Sta`s`csisevenmorelimitedasK+/K-~1.8atproduc`ontargetBR(K+)=(4.151±0.078stat)x10-6 BR(K-)=(4.394± 0.108stat)x10-6

ACP=-0.0284±0.0155statorsingle-sidedUL


Other CP Asymmetries

Par`alintegra`onintheangularspaceallowstobuildotherasymmetries:


Asymmetries

Compu`ngBRforK+andK-ineachΦ“sector”oftheangularspace:ACP(Φ*)=-0.0119±0.0150statACP(Φ~)=0.0058±0.0150statBothareconsistentwith0,givingsinglesidedULAnotherP-viola`ngasymmetrycanbedefinedforK+,K-andcombinedifconsistentK+:AP

(L)=0.0059±0.0180,K-:AP(L)=-0.0166± 0.0237,K±:AP

(L)=-0.0023± 0.0144

Summary and prospects •  NA48/2hascollectedacleansampleof~5000π π0e+e-decay candidateswithless

than5%background:firstobservaònleadingtoa3%BRmeasurementinperfectagreementwithChPTpredicòns.

smallsamplebutrichsourceofinformaòn• BR=(4.24± 0.07exp±0.13ext)x10-6totaluncertaintydominatedbyexternalerrorexperimentalerrordominatedbysignalsta`s`cssystema`cscouldbenefitfrombeyerconsideraònofradia`veeffects•  FirstevaluaònoftheMcontribuòn(1.1±0.4)%,inagreementwithChPTpredicòn•  Severalasymmetriesinves`gatedwithsingle–sidedULof~2%@90%CLdiscussionwiththeoristsismostimportantforacorrectandpreciseformulaònofradia`veandisospinbreakingcorrecòns

PLB 788 (2019) 552


ProspectstocollectmoredecaysinthecurrentNA62run:•  requiring3tracks+largeelectromagne`cenergyincompa`blewithπ νν trigger•  parasi`c3-tracktriggerdownscaledbyalargefactor:nowaytocollectmore

datathanin2003-2004withthistrigger(1.741011chargedkaondecaysanalyzed)•  couldbestudiedinRun3a{erLS2(2021-2023)withadedicatedtriggerandthe

caveatthatgeometricalacceptanceislowerthan@NA48/2


Spares


Background estimate

AKl3~10-4,An~410-2

AK3πD~210-6,AK2πD~0.210-6


Acceptance

Aeff = [A(IB) + 1/71 A(DE) – 1/253 A(INT)]/[1 + 1/71 – 1/253] INT(B-M) and INT(E-M) cancel in the angular integration E term is order of magnitude smaller than M 1/71 = (M/IB) predicted using NM

(0) = +0.0285(14) -1/253 = (B-E/IB) predicted using NE

(0) = +0.0022(7) and “large” % NE(1,2) values

NE(1) = −0.0167(13),NE

(2) =+0.089(11), L9 = 0.0069

NM

(0) comes from XM = (254 ± 6 ± 6) GeV-4

NE(0) comes from XE = ( -24 ± 4 ± 4) GeV-4 , both from EPJ C68

A(IB) = (0.645 ± 0.001)% A(DE) = (1.723 ± 0.003)% A(INT>0) = (1.933 ±0.004)% A(INT<0) = (1.805 ± 0.005)% A(INT) = (0.288 ± 0.001)% (56% INT>0, 44% INT<0) Aeff = (0.662 ± 0.001)%


3D-space: IB,DE,INTpopulatedifferentlythe(Eγ*,T*π)plane

BEcontribuònis<0atlowq2values

BEcontribuònchangessignwithincreasingq2valuesBEcontribuònIs>0forlargeq2values


Asymmetries

ACP≈sinδ sinΦΕ

ACPΦ*≈cosδ sinΦΜ

ACPΦ~≈cosδ sinΦAP

(L)≈NM(0) sinδ

Strongphaseoftheππ0system,Φ CPviola`ngphase

ΦE,ΦM

First observation and Study of the K to π e decay with ...

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