Transcript Recent results on kaon rare decays from the NA48/2

Recent results on kaon
rare decays from the
NA48/2 experiment
Massimiliano Fiorini
(Università degli Studi di Ferrara – INFN Ferrara)
on behalf of the NA48/2 Collaboration:
Cambridge, CERN, Chicago, Dubna, Edinburgh, Ferrara, Firenze,
Mainz, Northwestern, Perugia, Pisa, Saclay, Siegen, Torino, Vienna
High Energy Physics in the LHC Era
December 11-15 2006, Valparaiso – Chile
Outline
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The NA48/2 experiment at CERN SPS
The K±  π± π0 γ decay (Kπ2γ)
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The K±  π0 π0 e± ν decay (Ke400)
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Matrix element contributions
Experimental status
NA48/2 preliminary measurement of Direct Emission and
Interference Terms
Introduction and events selection
NA48/2 preliminary measurement of Branching Ratio and
Form Factors
Conclusions
Width ~ 5 mm
The NA48/2 simultaneous beams
PK spectra,
603 GeV/c
54
60
66
K+/K- ~ 1.8
K+
SPS
protons @
400 GeV/c
BM
K−
Simultaneus, unseparated, focused
beams
p/p ~ 1 %
x,y ~ 100 m
NA48/2 detector
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Spectrometer:
  p
p


0.044%
 1.02%
p
LKR calorimeter
 E

 3.2  0.2  %   0.09  0.01
E
E
  0.42  0.05  %
  x   y
0.4cm
 0.05cm
E
 t  300 ps
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E
Hodo, AKL
MUV, HAC
Kabes
NA48/2 data taking periods
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A view of the NA48/2 beam line
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2003 run: ~ 50 days
2004 run: ~ 60 days
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Total statistics in 2 years:

+ - ±
9
 K  π π π : > 3·10

0 0 ±
8
 K  π π π : > 1·10

Rare K± decays:
BR’s down to 10–9
can be measured
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>200 TB of data recorded
Primary goal:
Search for CP-violating charge asymmetries in K±  3 decays (see Dmitri
Madigozhin talk on Friday Plenary Session)
K±→π±π0γ decay
Gamma production mechanism
IB
DE
DE
IB
IB
INT
DE
Γ± depends on 2 variables (W e T*π), reduced to only W by integration
With this parametrization the ratio data/MC(IB) has the form 1+αW2+βW4
Lorentz invariant
variable:
*
*
*
*
(
P

P
)(
P

P
K


 )
2
W 
2
(mk m )
P*K = 4-momentum of the K±
P*π = 4-momentum of the π±
P*γ = 4-momentum of the radiative γ
W distributions for IB, DE, INT
IB and DE are well separated in W
Inner Bremsstrahlung(IB) : (2.75±0.15)×10-4 PDG 2006 (55<T*π<90
MeV)
Direct Emission (DE)
: (4.4±0.8)×10-6
PDG 2006 (55<T*π<90
MeV)
Interference (INT)
: not yet measured
Kπ2γ amplitudes
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Two amplitudes:
 Inner Bremsstrahlung (IB)
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Direct Emission (DE)
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Calculable: QED corrections to K± π±π0
Suppressed by ΔI=1/2
Insight on weak vertex structure
Electric contribution (E) comes from L4 ChPT lagrangian and loops L2
(non predictable)
Magnetic contribution (M) has contributions from reducible chiral
anomaly (WZW calculable) and also direct contributions (non
predictable)
Interference (INT) possible between IB and electric part of DE
 Measuring at the same time DE and INT gives measurement of both
M and E
 In addition CPV could appear in INT
Present experimental results seem to suggest a M dominated DE
Experimental measurements
Experiment
Year
# Events
E787
E470
E787
E470
ISTRA
2000
2003
2005
2006
2006
19836
4434
20571
10154
930
BR(DE) × 106
4.7 ± 0.8 ± 0.3
3.2 ± 1.3 ± 1.0
3.5 ± 0.6 + 0.3-0.4
3.8 ± 0.8 ± 0.7
3.7 ± 3.9 ± 1.0
All the measurements have been
performed:
 in the T*π region 55-90 MeV to
avoid π± π0 π0 and π± π0 BG
 assuming INT term = 0
of DE BR ratio
History ofHistory
DE Branching
Interference measurements:
6.0
5.0
BNL E787
BR(DE) 10-6
4.7
4.0
3.8
KEK E470
3.5
3.0
3.2
BNL E787
KEK E470
2.0
1.0
0.0
year
1999 2000 2001 2002 2003 2004 2005 2006 2007
NA48/2 has analyzed
> 5 times more events
What’s new in NA48/2 measurement
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In-flight Kaon decays
Both K+ and K- in the beam (possibility to check CP violation)
Very high statistics (220K π±π0γ candidates of which 124K
used in the fit )
Enlarged T*π region in the low energy part (0 < T*π < 80 MeV)
Negligible background contribution < 1% of the DE
component
Good W resolution mainly in the high statistic region
More bins in the fit to enhance sensitivity to INT
Order ‰ γ mistagging probability for IB, DE and INT
Fit with free interference term
Enlarged
T*π(IB)
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T*π(INT)
T*π(DE)
Standard
region
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*
T  region
Standard
region
Standard
region
Using standard region 55 < T*π < 90 MeV is a safe choice for
background rejection (π±π0π0 and π±π0)
But the region below 55 MeV is the most interesting for DE and
INT measurements
The presented measurement is performed in the region
0 < T*π< 80 MeV to improve statistics and sensitivity to DE
±
±
0
K →π π γ
Track Selection
• # tracks = 1.
• Pπ+ > 10 GeV
• E/p < 0.85
selection
BG rejection cuts
• COG < 2 cm
• Overlapping γ cuts
• |MK-MKPDG| < 10 MeV
• No muon hits
• 0 MeV < T*π+ < 80 MeV
Gamma selection
• Nγ = 3. (well separated in time LKr
clusters)
• Minimum γ energy > 3 GeV (>5 for the
fit)
Gamma tagging optimization
• CHA and NEU vertex compatibility
• Only one compatible NEU vertex
200K
events
Reconstruction strategy
We can get two independent determination of the K decay vertex:
- The charged vertex ZV(CHA) using the K and π flight directions (DCH)
- The neutral vertex ZV(NEU) imposing π0 mass to gamma couples (LKr)
For the neutral vertex we have 3
values: ZV(12), ZV(23), ZV(13)
ZV(CHA)
LKr
ZV(NEU)
γ2
γ3
γ1
ZV(2-3)
ZV(1-3)
We evaluate the kaon mass for all of
them and then choose the vertex
giving the best kaon mass.
Once the neutral vertex has been
chosen we also know what the
radiated γ is.
Main BG sources
Decay
BR
Background mechanism
K±→π±π0
(21.13±0.14)% +1 accidental or hadronic extra cluster
K±→ π±π0π0
(1.76±0.04)%
-1 missing or 2 overlapped gammas
K±→π0e±ν
(4.87±0.06)%
+1 accidental γ and e misidentified as a π
K±→ π0μ±ν
(3.27±0.06)%
+1 accidental γ and μ misidentified as a π
K±→ π0e± ν(γ)
(2.66±0.2)·10−4
K±→ π0μ±ν(γ)
(2.4±0.85)·10−5 μ misidentified as a π
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e misidentified as a π
Physical BG rejection:
 For π±π0 we can rely on the cut in T*π < 80 MeV, MK and COG cuts
 For π±π0π0 we have released the T*π cut, but we can anyway reach
required rejection with kaon mass cut (missing γ) and overlapping γ
cut (fused γ)
Accidental BG rejection: mainly π±π0, Ke3(π0e±ν) and Kμ3(π0μ±ν)
 Clean beam, very good time, space, and mass resolutions.
Fused  rejection:overlapping  cut
Fused gamma events are very dangerous BG source:
- MK and COG cuts automatically satisfied!
- Releasing the T*π cut they can give sizable contribution
NA48 LKr is very good to reject them:
- very high granularity (2×2 cm) cells
- very good resolution in Z vertex
Zv(π01)
Multi step algorithm looped over clusters:
Zv(π02)
Split 1 out of the 3 clusters in two γ of energies:
εγ1=xECL εγ2=(1-x)ECL
now we got 4 γ’s and we start reconstructing the event as a π± π0 π0.
Evaluate the ZV(x) pairing the gammas and extract x imposing that:
Zv(π01)= Zv(π02)
same K decay vertex: the 2π0 came from the same K!
Put x back in the Zv(π02) to get the real ZV(neu)
If the |Zv(CHA)-ZV(neu)| < 400 cm
discard the event
the γ are really fused and so we
BG rejection performance
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All physical background can be
explained in terms of the π±π0π0
events only.
Very small contribution from
accidentals
Source
%IB
%DE
π± π0 π0
~1·10-4
~0.61·10-2
Accidentals <0.5·10-4
~0.3·10-2
total background < 1% DE component
K±  π±π0π0
K±  π±π0γ
Selected
region
220K
events
The mistagged  events: a self BG
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Mistagging problem:
 mistagged gamma events
behave like BG because they
can induce fake shapes in the W
distribution
 they tend to populate the region
of high W simulating DE events
 must keep mistagging
probability as small as possible
Simply demanding compatibility
between zvc and best zvn gives
2.5% mistagging probability
Solution: Reject events with a
second solution for neutral
vertex close to best one
Require |zvn (second)-zvn
(best)| Dzvn> xx cm
3,000
■ DE
▼IB
2,500
mistagging prob %
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2,000
1,500
1,000
0,500
0,000
0
50
100
150
200
250
300
350
400
450
500
550
Mistagging cut (cm)
The mistagging probability has been
evaluated in MC as a function of the
mistagging cut to be 1.2‰ at 400 cm
The K± π±π0  decay. Trigger
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L1 trigger
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L1 requires nx>2 or ny>2
Require one track and LKr information
(peaks) compatible with at least 3 clusters
This introduces an energy dependence
 distortion of W distribution
Correction found using all 3  events
(K± π± π0π0 with  lost) and applied to
Monte Carlo
L2 trigger (rejects K± π±π0)
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Using DCH information and assuming 60 GeV kaon along z axis on-line
processors compute a sort of missing mass of the K-π system
Cut events whose missing mass is compatible with π0. Equivalent to
T*π < 90 MeV cut
To avoid edge resolution effects require T*π < 80 MeV in analysis
Data MC comparison
The IB dominated part of the data W
spectrum is very well reproduced by
MC
The radiated γ energy (for the IB part
of
the spectrum) is very well
reproduced
W(Data)/W (IBMC)
Data MC for Eγ (W<0.5)
IB dominated
region
Fitting region
Eγmin
cut
Fitting algorithm
To get the fractions of IB(α), DE(β), INT(γ), from data we use an
extended maximum likelihood approach:
 k  k  k   jk
    1
nbin
ln L   nk ln  k   k  bk ln  k   k  d k ln  k   k  I k ln k  k 
k 1
The fit has been performed in 14 bins in W, between 0.2-0.9, with a
minimum γ 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
Systematic uncertainties
Many systematic checks have been performed using both data and MC
Effect
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Syst. DE
Syst. INT
Energy scale
+0.09
-0.21
Fitting procedure
0.02
0.19
L1 trigger
±0.17
±0.43
Mistagging
_
±0.2
L2 Trigger
±0.17
±0.52
Resolutions difference
<0.05
<0.1
LKr non linearity
<0.05
<0.05
BG contributions
<0.05
<0.05
TOTAL
±0.25
±0.73
Trigger efficiency dominates
In 2004 both L1 and L2 triggers have been modified in
order to reduce systematics
Fit results
Frac( DE )0T * 80 MeV  (3.35  0.35stat  0.25syst )%

Frac( INT )0T * 80 MeV  (2.67  0.81stat  0.73syst )%

First measurement in the region
0 < T*π< 80 MeV of the DE term
with free INT term.
First evidence of a non zero INT term
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The error on the results is still
dominated by statistics and we could
profit of 2004 data (and the remaining
fraction of 2003 data) to reduce
statistical uncertainties.
High correlation (ρ=-0.92)
INT=0 fit: just for comparison
For comparison with other experiments, we have also extracted the fraction of
DE, with the INT term fixed to 0 in the region 55-90 MeV. A likelihood fit using
IB and DE MC only has been performed in the region 0 < T*π < 80 MeV.
Extrapolating to the 55-90 MeV region using MC we get:
Frac( DE ) INT *0
55T 90 MeV
 (0.85  0.05stat  0.02 syst )%
Fraction of DE in % of IB (INT=0)
The analysis of fit
residuals shows a bad χ2
2.50
% DE
2.00
1.50
BNL E787
KEK E470
NA48/2
1.00
0.50
0.00
1999
2000
2001
2002
2003
2004
2005
2006
year
2007
Description in term of
IB+DE only is unable to
reproduce W data
spectrum.
±
K→
0
0
±
ππeν
decay
Introduction to
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Ke4 decays
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00
Ke4
form factors are good constraints on ChPT Lagrangian
parameters
clean sources of π-π pairs at low energy (extraction of
scattering lengths)
theoretically related to each other by isospin arguments
Ke400 is the simplest because decay kinematics is
described by only one form factor (2 identical π0’s in
final state  no P wave)
Best measurement so far by KEK-E470 (stopped kaon
decays) based on 216 events
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Measured branching ratio (2.29  0.33) × 10-5
Signal selection
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Ke400 Signal Topology:
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1 charged track
2 π0’s (reconstructed from 4γ’s in LKr)
1 electron
some missing energy and pT (neutrino)
Analysis cuts:
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≥ 4 good γ clusters in LKr
2 good π0’s with similar vertex positions
Cut on E/p > 0.95 and on shower width for e/π separation
Assume 60 GeV/c Kaon momentum traveling on z axis for
neutrino reconstruction
Elliptical cut in plane mK (hypothetical π±π0π0 mass) and pT in
order to eliminate K±  π±π0π0 decays
Background estimate
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Background: main sources
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ππ0π0 decay + π misidentified as e (dominant)
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π0eνγ (Ke3γ) decay + accidental γ
In total 9642 selected events (2003 data only) with the
following BG:
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260 ± 94 from K±→π±π0π0
16 ± 2 accidental BG form Ke3(γ)
Total contamination: 3%
Branching Ratio Measurement
Using only a fraction of 2003 data (9642 signal events with 276  94
background events) and ππ0π0 as normalization channel, the Branching Ratio
of the Ke400 decay has been measured:
 
BR Ke004  2.587  0.026stat  0.019syst  0.029ext 105
BR Ke400
2,9
BR*10-5
2,7
2,5
2,3
PDG 2005
KEK E470
2,1
NA48/2
1,9
1,7
1,5
2003
Year
2004
2005
2006
2007
The result has been crosschecked
using also Ke3 as normalization
channel leading to consistent
result.
The statistical error can be further
reduced using the 2004 data set
(28K events)
The
00
Ke4
form factors
In this case we have only 1 form factor F (2 identical 0):

Fs  f s  f s' q 2  f s'' q 4  f e Se / 4m
2
 ..
The fit has been performed using both 2003 and 2004 data (~38K events)
No sensitivity to fe reached → fe set to 0.
Under this assumption we get:
f s' f s  0.129  0.036 stat  0.020 syst
f s" f s  0.040  0.034 stat  0.020 syst
Those values are consistent with the ones measured by NA48/2 in
“charged” Ke4  see detailed presentation by Dmitri Madigozhin
Conclusions

NA48/2 has performed the first measurement of the DE and INT
terms in the region 0 < T*π< 80 MeV for the decay K±→π±π0γ:
Frac( DE )0T * 80 MeV  (3.35  0.35stat  0.25syst )%

Frac( INT )0T * 80 MeV  (2.67  0.81stat  0.73syst )%

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The results seem to indicate the presence of a negative, non
vanishing, interference and therefore a non negligible
contribution of E terms to the DE
An improved measurement of the Ke400 BR has been achieved, to
be compared with recent published value:


BR K e004   2.587  0.026stat  0.019syst  0.029ext  105

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In addition Form Factors are measured, consistent with the
charged Ke4 measurement
The results can be further improved using the remaining fraction
of 2003 data and the full 2004 statistics
SPARE SLIDES
MBX1TR-P LVL2 trigger
Aim to reject K±→π±π0 events and get
K±→π±π0π0.
It’s based on the online computation of Mfake:
Only events with MFAKE < 475 MeV, are
collected by the trigger
Ke4 neutral decays : formalism
Branching fraction and Form factors measurements:
Two identical π0 (no P wave)  only ONE form factor Fs

Fs  f s  f s' q 2  f s'' q 4  f e Se / 4m
2
 ..
Sπ/4m2 π0
Se/4m2π0
background