First observation of K+ → p+p0e+e- Mauro Raggi Laboratori Nazionali di Frascati NA48/2 rare decay session CERN 14/07/2011

Outline Motivations Data and MC samples Selection cuts BG estimates and BG cuts Data MC comparison BR error estimate

K± → p±p0e+e- theory The interference thanks to the possibility of measuring the plane of polarization of the e+e- pair is splitted into 3 terms: IB/E, IB/M and E/B. Where IB/M cancels when integrated over f as in ppg, while E/B is only non-zero if CP violation is allowed. The only theoretical paper on ppee is currently: H. Pichl, ``K --> pi pi0 e+ e- decays and chiral low-energy constants,'’ Eur. Phys. J. C 20, 371 (2001) [arXiv:hep-ph/0010284]. L. Cappiello, O. Cata, G. D'Ambrosio and D. Gao are currently working on a revised theoretical paper on ppee. There is no observation of the decay at the moment

Theory different contributions From private communication with D’Ambrosio

Analysis targets Implement and test the IB generator to be used in the NA62FW Measure the IB BR to better than 5% precision Measure the plane asymmetry f to better than 2% Try to spot for deviations from the IB spectrum Explore the sensitivity of NA62 setup to other components Establish a trigger recipe to collect ppee in the NA62 experiment (S. Giudici is investigating)

Data and MC samples used Data sample: whole 2003+2004 data Total Kaon flux ~1x1011 MC background generated run by run in SS0 43.2M p±p0g corresponding to 6x1012 K ??M p-p0D corresponding to ??x1011 K 94.5M p±p0p0D corresponding to 2.3x1010 K MC p+p0e+e- signal Only 100K events generated with pure phase space

Pre-selection cuts This set of cuts is common to ppee and ppd: SEVT_NCLUSTER <8 SEVT_NTRACK <8 SEVT_NVTX <8 SEVT_NTRACK >3 SEVT_NCLUSTER >3 SEVT_NVTX >0 Loose tracks and clusters quality cuts NgoodTr ≥3 ngoodcl ≥3 simele ≥2 The same set of cuts has been used to run a data filter (ss0 only) which reduces the data size by factor 100.

Basic signal selection cuts Signal signature cuts: N good Tracks = 3 1p + 2e N good Clusters = 4 2g + 2e N associated tracks ≥ 2 at least 2e N non associated clusters = 2 2g Only 1 vertex with -1000 cm < ZV < 8000 cm PID definitions and cuts: Electron = track with 0.9< E/p < 1.1 Pion = track with E/p < 0.85 Photon = cluster without an associated track N of electrons = 2 with total electron charge = 0 N of pions = 1 N photons = 2

BG rejection cuts General cuts abs(Mp0 - Mp0PDG) < 10 MeV COG < 2.5 cm abs(MK - MKPDG) < 10 MeV abs(ETOT - PK) < 6 GeV Mee > 1MeV (physical lower bound) Cut against gamma conversion (rejects p+p0g) sqrt(DXe(DCH1)^2-DYe(DCH1)^2) >?? cm Radiative clusters cut (p+p0D): Distance extr. e- and gamma cluster >2 cm

First signal evidence ~5000 candidates p+p0e+e- in the peak Are those geniune p±p0e+e-?

Background estimates

K→pp0p0D background K+→p+p0p0D = p+p0e+e-g need a lost g MK, COG and PK cuts are effective BR(p+p0p0D) = 2x(2.07±0.06)x10-4

K+→p+p0D background K →p+p0D= p+e+e-g need an additional g Can profit by accidentals or radiated g from p or e MK, COG and PK cuts are effective

K+→p+p0g background Very dangerous BG K→pp0gIB with g→e+e- Same final state of the signal Pass all the mass cuts (mp0, MK) PK and COG cuts Can get rid of it using dedicated cut on distance of the tracks at DCH1

Summary of BG contribution Decay %BG p+p0D 2.1±0.03 p+p0p0D 4.7±0.3 p+p0gIB < 0.1 Accidental Total BG 6.8 This is calculated on the whole 2003-2004 data sample

Wrong sign events data MC p±p0p0D MC Wrong signs events have total electron charge of +2 or -2 The mass distribution of WS events is well reproduced by MC of p±p0p0D only

BG estimate data comparison p+p0D p+p0p0D Data points

How to reduce p±p0p0D (Mee) p±p0p0D BG shows a clear peak for Mee above 100MeV The ppee IB BR becomes so small at so high Mee mass to allow us to have statistics above 100 MeV Can apply a cut at Mee>100 to reject ~90% of the residual p±p0p0D Mee distribution Data p±p0p0D

An easy handle…Meeg Meeg mass Can easily get rid of all the Daliz background just cutting on Meeg Data MC sum Introduce a bias in the Mee the variable you want to study I can’t still evaluate the bias due to absence of MC matrix element

Data MC comparison (mass, mee, Zvtx)

MK values data and MC MKdat=493.331 DPDG=-0.346 MeV Still not on the PDG mass. Need to insert the blue tube corrected directions for the track Thanks to Evgueni for the suggestion MK data MK MC MKdat=493.331 DPDG=-0.346 MeV s(MK) dati= 4.30852 MeV MKMC=493.381 DPDG= -0.296 MeV s(MK)MC= 3.86467 MeV

Mass of the p0 data and MC Mp dati= 135.331 MeV DMPDG= 0.364 MeV Mp0 data Mp0 MC Mp dati= 135.331 MeV DMPDG= 0.364 MeV sMp dati= 2.55 MeV Mp MC= 135.059 MeV DMPDG= 0.092 MeV sMp MC= 2.10 MeV

BR extraction roadmap

Normalization channel p+p0D Br(p+p0)=(20.66±0.08)·10-2; Br(p0)D=(1.174±0.035 ) ·10−2 Br(p+p0)D= (2.43±0.07)·10−3 Same trigger chain 3 track, same tracks p+e+e- p±p0D mass plot No significant BG found Precision to flux measurement limited to 3.% due to p0 Dalitz BR

Montecarlo simulation signal Montecarlo for p+p0e+e- does not exist in CMC I wrote a pure phase space generator Can provide a guess of the signal acceptance Ask Giancarlo for ppee IB matrix element Acceptance I’m calculating up to now is pure phase space therefore not correct!

L2 trigger efficiency measurement Trigger type used Bit 1 1VTX (SEVT_TRGW) Bit 2 2VTX (SEVT_TRGW) Bit 4 C-PRE (SEVT_TRGW) Bit 5 MB1TR-P (SEVT_TRGW) Trigger definition used for efficiency Control trigger C-PRE Signal trigger: 1Vtx or 2Vtx or MB1TR-P To be improved Got the recipe by Evgueni K3p measurement

Trigger efficiency estimates Used a p±p0p0D with lost gamma and p±p0D for the measurement Signal BG p+p0D 1VTX 88.42± 0.08 2VTX 6.29±0.06 1VTX or 2VTX or MB1TR-P 97.5±0.5 96.92±0.04 No evidence for differences in trigger efficiency Maximum trigger correction is order 1% Should be done using PU for the control trigger. Need to measure mee dependence of trigger efficiency

Naïve estimate of BR uncertainties Source Value Statistics 1.7% Background subtraction 0.4% Normalization (BR p0->Dalitz) 3.0% Total Uncertainty 3.5% With the present sample of ~5000 events the total error is already dominated by Dalitz decay BR. Changing the normalization will introduce difficult systematics due to trigger efficiency which will increase a lot the work to reach a result Neglecting the trigger efficiency will now change the error by only 0.1%!

Measuring the f angle f is the angle in between electron and pion pair Still trying to understand if this is a CP violating observable. Measurement uses both calorimeter and spectrometer due to p0 momentum p+ e+ e- p0

f Asymmetry sensitivity study Source Raw asym values N sigma Signal data - 0.037 ± 0.015 2.48 Signal phase space MC - 0.0004 ± 0.02 0.02 BG p±p0p0D - 0.045 ± 0.05 0.93 BG p±p0D 0.14 ± 0.22 0.65 Based on partial data set and MC (numbers are not final) No statistically relevant asymmetry is observed in data Phase space MC show no asymmetry in the detector and in the reconstruction Contribution of BG asymmetries to data asymmetry are: BG(p±p0D)xAsym(p±p0D)=0.14*0.02 = 0.0028 BG(p±p0p0D)xAsym(p±p0p0D)=0.045*0.047 = 0.0021

Conclusions First evidence of p+p0e+e- ~5000 events found in 2003-2004 BG is order 6% Easy handles to reduce BG have been identified ppee MC simulation is not available in CMC007 Pure phase space MC wrote (not enough) Got a recipe from Evgueni to implement the new generator First attempt to measure the f angle Sensitivity is order 2% and no detector bias found