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Measurement of DE and INT in K ± →  ±  0 g with NA48/2

Measurement of DE and INT in K ± →  ±  0 g with NA48/2. Mauro Raggi FlaviaNet Workshop on K decays Frascati 18/05/2007. LKr EM calorimeter. Spectrometer - 4 Drift Chambers (DCH) - Magnet. NA48/2 beam and detector. IB. DE. DE. IB. IB. INT. DE. Gamma production mechanism.

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Measurement of DE and INT in K ± →  ±  0 g with NA48/2

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

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

  3. IB DE DE IB IB INT DE Gamma production mechanism 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 FlaviaNet Workshop

  4. 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 FlaviaNet Workshop

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

  6. BNL E787 KEK E470 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±p0p0BG All of them are assuming the Interference term to be = 0 INT estimates [20][21]: FlaviaNet Workshop

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

  8. 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 FlaviaNet Workshop

  9. Enlarging T*p region T*p(IB) T*p(INT) T*p(DE) Standard region Standard region Standard region The standard region 55<T*p<90 MeV is safe WRT K3pnBG 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 FlaviaNet Workshop

  10. LKr g2 g3 ZV(NEU) g1 ZV(2-3) ZV(1-3) ZV(CHA) 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. FlaviaNet Workshop

  11. Main BG sources • 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. FlaviaNet Workshop

  12. K±→p±p0p0 • K±→p±p0g Selected region 220Kevents BG rejection performance Thanks to the overlapping gamma cut, good MK and COG resolutions, the BG coming from p±p0p0 is under control. All physical BG can be explained in terms of the p±p0p0events 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 FlaviaNet Workshop

  13. ■DE▼IB The miss tagged g events: a self BG 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 FlaviaNet Workshop

  14. Data MC for Eg (W<0.5) W(Data)/W(IBMC) IB dominatedregion Fitting region Egmincut Data MC comparison The radiated g energy (IB part of the spectrum) is well reproduced The IB dominated part of W spectrum is well reproduced by MC FlaviaNet Workshop

  15. 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, between0.2-0.9, 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 FlaviaNet Workshop

  16. 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% FlaviaNet Workshop

  17. 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 badc2 Description in term of IB+DE only is unable to reproduce W data spectrum. FlaviaNet Workshop

  18. 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) FlaviaNet Workshop

  19. 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 FlaviaNet Workshop

  20. 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 ≈ 10-3could be reached using the full data sample • The systematic error on CP violation will be probably <10-3 FlaviaNet Workshop

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