Measurement of DE and INT in K±→±0g with NA48/2 Mauro Raggi FlaviaNet Workshop on K decays Frascati 18/05/2007

NA48/2 beam and detector LKr EM calorimeter Spectrometer - 4 Drift Chambers (DCH) - Magnet Mauro Raggi FlaviaNet Workshop

Gamma production mechanism DE IB IB DE INT Inner Bremsstrahlung(IB) : (2.75±0.15)·10-4 PDG (55<T*p<90 MeV) Direct Emission (DE) : (4.4±0.8)·10-6 PDG (55<T*p<90 MeV) Interference (INT) : not yet measured Mauro Raggi FlaviaNet Workshop

K±→p±p0g amplitudes Two type of contributions: Electric (J=l±1) dipole (E1) Magnetic (J=l) dipole (M1) Electric contributions are dominated by Inner Bremsstrahlung (EIB) Thanks to Low’s theorem IB contribution can be related to the non radiative decay p±p0 using QED corrections. DE shows up only at order O(p4) in CHPT Is generated by both E and M contributions: Magnetic contributions are dominated by chiral anomaly Electric contributions come from L4 CHPT lagrangian and loops L2 Present experimental results suggest a M dominated DE Mauro Raggi FlaviaNet Workshop

General expression for decay rate IB DE INT P*K=4-momentum of the K± P*p=4-momentum of the p± P*g=4-momentum of the radiated g Mauro Raggi FlaviaNet Workshop

Experimental status for DE and INT DE measurements All the measurements have been performed in the T*p region 55-90 MeV to avoid p±p0p0 BG All of them are assuming the Interference term to be = 0 INT estimates [20][21]: BNL E787 KEK E470 Mauro Raggi FlaviaNet Workshop

E787 2000 measurement 20K K+ events Fit in 8 bins 0.1-0.9 Fit function: with b set to 0 (INT=0). Mauro Raggi FlaviaNet Workshop

What’s new in NA48/2 measurement In flight Kaon decays Both K+ and K- in the beam (possibility to check CP violation) Very high statistics (220K p±p0g candidates of which 124K used in the fit ) Enlarged T*p region in the low energy part (0<T*p<80 MeV) Negligible background contribution <1% of the DE component Good W resolution mainly in the high statistic region 14 bins in the fit to enhance sensitivity to INT Order ‰ g mistagging probability for IB, DE and INT Mauro Raggi FlaviaNet Workshop

Enlarging T*p region T*p(IB) T*p(DE) T*p(INT) Standard region Standard region Standard region The standard region 55<T*p<90 MeV is safe WRT K3pn BG rejection Unfortunately the region below 55 MeV is the most interesting for DE and INT measurement This measurement has been performed in the region 0<T*p<80 MeV Mauro Raggi FlaviaNet Workshop

Reconstruction strategy We can get two independent determination of the K decay vertex: - The charged ZV(CHA) using the K and p flight directions (spectrom.) - The neutral ZV(NEU) choosing g pair with the best p0 mass (LKr) ZV(CHA) Once the neutral vertex has been chosen we also know which is the radiated g. g1 g3 g2 ZV(2-3) ZV(1-3) ZV(NEU) LKr Mauro Raggi FlaviaNet Workshop

Main BG sources Decay BR Background mechanism K±→p±p0 (21.13±0.14)% +1 accidental or hadronic extra cluster K±→p±p0p0 (1.76±0.04)% -1 missing or 2 overlapped gammas K±→p0 e±n (4.87±0.06)% +1 accidental g and e misidentified as a p K±→p0 m±n (3.27±0.06)% +1 accidental g and m misidentified as a p K±→p0 e±n(g) (2.66±0.2)·10−4 e misidentified as a p K±→p0 m±n(g) (2.4±0.85)·10−5 m misidentified as a p Physical BG rejection: For p±p0 we can relay on the cut in T*p< 80 MeV, MK and COG, cuts For p±p0p0 we have released the T*p cut, but we can anyway reach the rejection needed (MK COG (missing g) and overlapping g cut (fused g)) Accidental BG rejection (p±p0, Ke3(p0 e±n), Km3(p0 m±n)) Clean beam, very good time, space, and mass resolutions. Mauro Raggi FlaviaNet Workshop

BG rejection performance Thanks to the overlapping gamma cut, good MK and COG resolutions, the BG coming from p±p0p0 is under control. K±→p±p0p0 K±→p±p0g Selected region 220K events Source %IB %DE p±p0p0 ~1·10-4 ~0.61·10-2 Accidental <0.5·10-4 ~0.3·10-2 All physical BG can be explained in terms of the p±p0p0 events only. Very small contribution from accidentals neglected Total BG is less than 1% with respect to the expected DE contribution even in the range 0<T*p<80 MeV Mauro Raggi FlaviaNet Workshop

The miss tagged g events: a self BG ■ DE ▼IB The miss tagged gamma events behave like BG because they can induce fake shapes in the W distribution. In fact due to the slope of IB W distribution they tend to populate the region of high W simulating DE events. The identification of radiative gamma has 2 steps: 1. Compatibility of charged and neutral vertices (2.5% mistagging) 2. Distance between best and second best neutral vertices>xx cm The mistagging probability has been evaluated in MC as a function of the mistagging cut to be 1.2‰ at 400 cm Mauro Raggi FlaviaNet Workshop

Data MC comparison The IB dominated part of W spectrum is well reproduced by MC The radiated g energy (IB part of the spectrum) is well reproduced W(Data)/W (IBMC) Fitting region IB dominated region Data MC for Eg (W<0.5) Egmin cut Mauro Raggi FlaviaNet Workshop

Fitting algorithm To get the fractions of IB(a), DE(b), INT(g), from data we use an extended maximum likelihood approach: The fit has been performed in 14 bins, between 0.2-0.9, with a minimum g energy of 5 GeV, using a data sample of 124K events. To get the fractions of DE and INT the raw parameter are corrected for different acceptances Mauro Raggi FlaviaNet Workshop

Preliminary results Preliminary First measurement: in the region 0<T*p<80 MeV and of the DE term with free INT term. First evidence of INT term ≠0 The error is dominated by statistics and could be reduced using SS0 and 2004 data sets Unfortunately parameters are highly correlated r=0.93% Mauro Raggi FlaviaNet Workshop

INT=0 fit: just for comparison For comparison with previous experiments the fraction of DE, with INT=0 has been also measured. A likelihood fit using IB+DE MC only has been performed in the region 0<T*p<80 MeV and the result extrapolated to 55-90 MeV using MC The analysis of fit’s residuals shows a bad c2 Description in term of IB+DE only is unable to reproduce W data spectrum. Mauro Raggi FlaviaNet Workshop

Effect of the DE form factor Could the negative INT be faked by DE ff? No DE(no ff)-DE(ff) Only INT>0 could be faked DE could be underestimated Could IB+DE(ff) be enough to improve MC agreement with data spectrum? No Could the DE ff change the results for DE and INT? Yes (results moving on the correlation line) Mauro Raggi FlaviaNet Workshop

Conclusions A new measurement of the of fractions of DE and INT in K±→±0g has been performed with 124K events New kinematic region (T*p<55 MeV) explored First evidence of a non zero INT term found Preliminary Mauro Raggi FlaviaNet Workshop

Future prospects The total number of events in the final result (2003+2004) will be >500K A fit with form factor in DE is foreseen The CP violation will be investigated A statistical error on CP violation ≈ 10-3 could be reached using the full data sample The systematic error on CP violation will be probably <10-3 Mauro Raggi FlaviaNet Workshop