Measurement%20of%20DE%20and%20INT%20in%20K

1
Measurement of DE and INTin K???0g with NA48/2

• Mauro Raggi
• FlaviaNet Workshop on K decays
• Frascati 18/05/2007

2
NA48/2 beam and detector
3
Gamma production mechanism
Inner Bremsstrahlung(IB) (2.750.15)10-4 PDG
(55ltTplt90 MeV) Direct Emission (DE)
(4.40.8)10-6 PDG (55ltTplt90 MeV)
Interference (INT) not yet measured
4
K?pp0g amplitudes

• Two type of contributions
• Electric (Jl1) dipole (E1)
• Magnetic (Jl) 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 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

5
General expression for decay rate
PK4-momentum of the K Pp4-momentum of the
p Pg4-momentum of the radiated g
6
Experimental status for DE and INT
DE measurements
All the measurements have been performed in the
Tp region 55-90 MeV to avoid pp0p0 BG All of
them are assuming the Interference term to be 0
INT estimates 2021
7
E787 2000 measurement

• 20K K events
• Fit in 8 bins 0.1-0.9
• Fit function
• with b set to 0 (INT0).

8
Whats 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 pp0g candidates of
  which 124K used in the fit )
• Enlarged Tp region in the low energy part
  (0ltTplt80 MeV)
• Negligible background contribution lt1 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

9
Enlarging Tp region
Tp(IB)
Tp(INT)
Tp(DE)
Standard region
Standard region
Standard region
The standard region 55ltTplt90 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 0ltTplt80 MeV
10
K?pp0g selection cuts

• Track Selection
• tracks 1.
• Pp gt 10 GeV
• E over P lt 0.85
• No muon veto hits
• 0 MeV lt Tp lt 80 MeV
• Gamma selection
• Ng 3. (well separated in time LKr clusters)
• Minimum g energy gt 3 GeV (gt5 for the fit)
• Gamma tagging optimization
• CHA and NEU vertex compatibility
• Only one compatible NEU vertex

• BG rejection cuts
• COG lt 2 cm
• Overlapping g cuts
• MK-MKPDG lt 10 MeV

11
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.
12
Main BG sources
Decay BR Background mechanism
K?pp0 (21.130.14) 1 accidental or hadronic extra cluster
K?pp0p0 (1.760.04) -1 missing or 2 overlapped gammas
K?p0 en (4.870.06) 1 accidental g and e misidentified as a p
K?p0 mn (3.270.06) 1 accidental g and m misidentified as a p
K?p0 en(g) (2.660.2)10-4 e misidentified as a p
K?p0 mn(g) (2.40.85)10-5 m misidentified as a p

• Physical BG rejection
• For pp0 we can relay on the cut in Tplt 80 MeV,
  MK and COG, cuts
• For pp0p0 we have released the Tp 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.

13
Fused g rejectionoverlapping g cut
Fused gamma events are very dangerous BG source
- MK, and COG cut automatically satisfied! -
Releasing the Tp cut they can give sizable
contribution
NA48 calorimeter is very good in rejecting them
- very high granularity (2x2 cm) cells - very
good resolution in vertex Z coordinate
Zv(p01)
Multi step algorithm looped over clusters
Zv(p02)
Split 1 out of the 3 clusters in 2 g of
energiesEg1xECL Eg2(1-x)ECLnow 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 vertex
Put x back in the Zv(p02) to get the real
ZV(neu)If the Zv(CHA)-ZV(neu)lt400 cm 2 g
are fused and so we discard the event
14
BG rejection performance
Thanks to the overlapping gamma cut, good MK and
COG resolutions, the BG coming from pp0p0 is
under control.
Source IB DE
pp0p0 110-4 0.6110-2
Accidental lt0.510-4 0.310-2
All physical BG can be explained in terms of the
pp0p0 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
0ltTplt80 MeV
15
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
steps1. Compatibility of charged and neutral
vertices (2.5 mistagging) 2. Distance between
best and second best neutral verticesgtxx cm
The mistagging probability has been evaluated in
MC as a function of the mistagging cut to be 1.2
at 400 cm
16
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
17
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
18
Preliminary results
Preliminary
First measurement in the region 0ltTplt80 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
r0.93
19
Systematic uncertainties
Effect Syst. DE Syst. INT
Energy scale 0.09 -0.21
Fitting procedure 0.02 0.19
LVL1 trigger 0.17 0.43
g Mistagging _ 0.2
LVL2 Trigger 0.17 0.52
Resolutions difference lt0.05 lt0.1
LKr non linearity lt0.05 lt0.05
BG contributions lt0.05 lt0.05
TOTAL 0.25 0.73
Systematic 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.
20
INT0 fit just for comparison
For comparison with previous experiments the
fraction of DE, with INT0 has been also
measured.A likelihood fit using IBDE MC only
has been performed in the region 0ltTplt80 MeV and
the result extrapolated to 55-90 MeV using MC
The analysis of fits residuals shows a bad
c2 Description in term of IBDE only is unable
to reproduce W data spectrum.
21
Effect of the DE form factor

• Could the negative INT be faked by DE ff? No
• DE(no ff)-DE(ff)
• Only INTgt0 could be faked
• DE could be underestimated
• Could IBDE(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)

22
Conclusions

• A new measurement of the of fractions of DE and
  INT in K???0g has been performed with 124K
  events
• New kinematic region (Tplt55 MeV) explored
• First evidence of a non zero INT term found

Preliminary
23
Future prospects

• The total number of events in the final result
  (20032004) will be gt500K
• 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 lt10-3

24
Backup slides
25
L1 trigger NT-PEAK
X OR Y gt 2 view
Y view
26
MBX1TR-P LVL2 trigger
Aim to reject K?pp0 and get pp0p0. Its based
on the online computation of Mfake