Measurement of DE and INTin K0g with NA48/2Mauro RaggiFlaviaNet Workshop on K decaysFrascati 18/05/2007

NA48/2 beam and detector

Gamma production mechanismInner Bremsstrahlung(IB): (2.750.15)10-4 PDG (55

Kpp0g amplitudesTwo type of contributions:Electric (J=l1) dipole (E1)Magnetic (J=l) dipole (M1) Electric contributions are dominated by Inner Bremsstrahlung (EIB)Thanks to Lows theorem IB contribution can be related to the non radiative decay pp0 using QED corrections.DE shows up only at order O(p4) in CHPTIs generated by both E and M contributions:Magnetic contributions are dominated by chiral anomalyElectric contributions come from L4 CHPT lagrangian and loops L2Present experimental results suggest a M dominated DE

General expression for decay rateP*K=4-momentum of the K P*p=4-momentum of the p P*g=4-momentum of the radiated g

Experimental status for DE and INTAll the measurements have been performed in the T*p region 55-90 MeV to avoid pp0p0 BGAll of them are assuming the Interference term to be = 0INT estimates [20][21]:DE measurements

E787 2000 measurement20K K+ eventsFit in 8 bins 0.1-0.9Fit function:

with b set to 0 (INT=0).

Whats new in NA48/2 measurementIn flight Kaon decaysBoth K+ and K- in the beam (possibility to check CP violation)Very high statistics (220K pp0g candidates of which 124K used in the fit ) Enlarged T*p region in the low energy part (0

Enlarging T*p regionT*p(IB)T*p(DE)T*p(INT)Standard regionStandard regionStandard regionThe standard region 55

Kpp0g selection cutsTrack Selection# tracks = 1.Pp+ > 10 GeVE over P < 0.85No muon veto hits0 MeV < T*p+ < 80 MeVGamma selectionNg = 3. (well separated in time LKr clusters)Minimum g energy > 3 GeV (>5 for the fit)Gamma tagging optimizationCHA and NEU vertex compatibilityOnly one compatible NEU vertexBG rejection cutsCOG < 2 cmOverlapping g cuts|MK-MKPDG| < 10 MeV

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) Once the neutral vertex has been chosen we also know which is the radiated g.

Main BG sourcesPhysical BG rejection:For pp0 we can relay on the cut in T*p< 80 MeV, MK and COG, cutsFor pp0p0 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 (pp0, Ke3(p0 en), Km3(p0 mn)) Clean beam, very good time, space, and mass resolutions.

DecayBRBackground mechanismKpp0(21.130.14)%+1 accidental or hadronic extra clusterKpp0p0(1.760.04)%-1 missing or 2 overlapped gammasKp0 en(4.870.06)%+1 accidental g and e misidentified as a pKp0 mn(3.270.06)%+1 accidental g and m misidentified as a pKp0 en(g)(2.660.2)104e misidentified as a pKp0 mn(g)(2.40.85)105m misidentified as a p

Fused g rejection:overlapping g cutSplit 1 out of the 3 clusters in 2 g of energies: Eg1=xECL Eg2=(1-x)ECL now we got 4 gs and we reconstruct the event as a pp0p0.Evaluate the ZV(x) pairing the gammas and extract x imposing that: Zv(p01)= Zv(p02) same K decay vertexZv(p02)Zv(p01)Put x back in the Zv(p02) to get the real ZV(neu) If the |Zv(CHA)-ZV(neu)|

BG rejection performanceTotal BG is less than 1% with respect to the expected DE contribution even in the range 0

The miss tagged g events: a self BGThe 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 mistagging probability has been evaluated in MC as a function of the mistagging cut to be 1.2 at 400 cmThe 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

Data MC comparisonThe IB dominated part of W spectrum is well reproduced by MCThe radiated g energy (IB part of the spectrum) is well reproduced

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

Preliminary results First measurement:in the region 0

Systematic uncertaintiesSystematic effects dominated by LVL1 and LVL2 triggers!Learning from experience both LVL1 and LV2 triggers have been modified in 2004. We are confident that both systematics will be smaller in 2004 data set.

EffectSyst. DESyst. INTEnergy scale+0.09-0.21Fitting procedure0.020.19LVL1 trigger0.170.43g Mistagging_0.2LVL2 Trigger0.170.52Resolutions difference

INT=0 fit: just for comparisonFor 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

Effect of the DE form factorCould the negative INT be faked by DE ff? NoDE(no ff)-DE(ff)Only INT>0 could be fakedDE could be underestimated

Could IB+DE(ff) be enough to improve MC agreement with data spectrum? NoCould the DE ff change the results for DE and INT? Yes (results moving on the correlation line)

ConclusionsA new measurement of the of fractions of DE and INT in K0g has been performed with 124K eventsNew kinematic region (T*p

Future prospectsThe total number of events in the final result (2003+2004) will be >500KA fit with form factor in DE is foreseenThe CP violation will be investigatedA statistical error on CP violation 10-3 could be reached using the full data sampleThe systematic error on CP violation will be probably

Backup slides

L1 trigger NT-PEAKY viewX OR Y > 2 view

MBX1TR-P LVL2 triggerAim to reject Kpp0 and get pp0p0. Its based on the online computation of Mfake: