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Recent Results From NA48/2 Experiment @ CERN-SPS
Recent Results From NA48/2 Experiment @ CERN-SPS
Simone BifaniUniversity of Turin – Experimental Physics Department
INFN – Turin
on Behalf of the NA48/2 CollaborationCambridge, CERN, Chicago, Dubna, Edinburgh, Ferrara, Firenze, Mainz,
Northwestern, Perugia, Pisa, Saclay, Siegen, Torino, Vienna
VI Latin American Symposium on High Energy PhysicsVI Latin American Symposium on High Energy Physics
Puerto Vallarta (Mexico), November 1Puerto Vallarta (Mexico), November 1stst-8-8thth 2006 2006
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 22
OutlineOutline
› NA48/2 Experimental Setup
› CP Violating Charge Asymmetry:
» “Charged” Mode: K± -> π±π+π-
» “Neutral” Mode: K± -> π±π0π0
› “Cusp” Effect in K± -> π±π0π0 Decay
› Rare Decay: K± -> π±π0γ
NA48/2 Experimental SetupNA48/2 Experimental Setup
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 44
Some HistorySome History
NA48 (1997-2000): Direct CP-Violation in neutral K
>Re(ε’/ε) = (14.7 ± 2.2)·10-4
NA48/1 (2002): Rare KS decays
>BR(KS -> π0e+e-) = (5.8+2.8-2.3 ±
0.8)·10-9
>BR(KS -> π0μ+μ-) = (2.8+1.5-1.2 ±
0.2)·10-9
NA48/2 (2003-2004): Direct CP-Violation in charged K
…and many other results on kaon and hyperon decays
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 55
Simultaneous BeamSimultaneous BeamBeams coincide within ~1mm all along 114m
decay volume
Simultaneous K+ and K- beams:
large charge symmetrization of
experimental conditions
2-3M K/spill (/K ~ 10) decay products stay in
pipeFlux ratio: K+/K– ~ 1.8
PK = 60±3 GeV/c
54 60 66
Width ~ 5 mm
K+/K- ~ 1 mm
Second achromat:Cleaning
Beam spectrometer
δPK/PK = 0.7%δx,y ~ 100 μm
~71011
p/spil400
GeV/cFront-end
achromat:
Momentum
selection
Quadrupole,
Quadruplet:
Focusing
sweeping
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 66
DetectorDetector
Beam pipe
Magnetic spectrometer (4 DCHs):>4 view / DCH -> high efficiency
>σP/P = 1.0% + 0.044%·P [GeV/c]
Hodoscope:>Fast trigger
>σt = 150ps
Electromagnetic calorimeter (LKr):>High granularity, quasi-homogeneous
>σE/E = 3.2%/√E + 9%/E + 0.42% [GeV]
Hadron calorimeter, muon veto and photon vetoes
Trigger:>Fast hardware trigger (L1): hodoscope & DCHs multiplicity>Level 2 trigger (L2): on-line processing of DCHs & LKr
information
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 77
Data TakingData Taking
A view of the NA48/2 beam line
Run periods:
>2003: ~ 50 days
>2004: ~ 60 days
Total statistics in 2 years:
>K± -> π±π+π-: ~ 4·109
>K± -> π±π0π0: ~ 1·108
-> >200 TB of data recorded
Rare K± decays can be measured down to BR ~ 10–9
Rare K± decays can be measured down to BR ~ 10–9
CP ViolatingCharge Asymmetry
CP ViolatingCharge Asymmetry
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 99
CP-Violation HistoryCP-Violation History
Major milestones in CP-Violation history:
>1964: Indirect CP-Violation in K0 (J.H. Christenson, J.W. Cronin, V.L. Fitch and R. Turlay)
>1988, 1999: Direct CP-Violation in K0 (NA31, E731, NA48, KTeV)
>2001: Indirect CP-Violation in B0 (BaBar, Belle)
>2004: Direct CP-Violation in B0 (Belle, BaBar)
Look for CP-Violation in K±
(no mixing -> only Direct CPV is possible)
Look for CP-Violation in K±
(no mixing -> only Direct CPV is possible)
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 1010
Introduction (I)Introduction (I)
Kinematics:
si = (PK - Pπi)2, i = 1,2,3 (3 = πodd)
s0 = (s1 + s2 + s3) / 3u = (s3 - s0) / mπ
2
v = (s2 - s1) / mπ2
Kaon rest frame:
u = 2mK ∙ (mK/3 - Eodd) / mπ
2
v = 2mK ∙ (E1 - E2) / mπ2
Matrix element:
|M(u,v)|2 ~ 1 + gu + hu2 + kv2
Direct CP violating quantity:
slope asymmetry
Ag = (g+ - g-) / (g+ + g-) ≠ 0
The best two K± decay modes:› BR(K± -> π±π+π-) = 5.57% “Charged”
› BR(K± -> π±π0π0) = 1.73% “Neutral”
π1even
π3oddπ2even
K±
“Charged” mode g = -0.2154 ±
0.0035|h|, |k| ~ 10-2
uv
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 1111
Introduction (II)Introduction (II)
Standard Model theoretical prediction in the range 10-
6÷5·10-5
Models Beyond the SM predict enhancement of the Ag value
Experimental results before NA48/2:
“Charged” mode:Ag =
(22±15stat±37syst)·10-4 (HyperCP - 54·106 evt.)
“Neutral” mode:Ag = (2±19)·10-4
(TNF - 620·103 evt.)
10-5
10-4
10-3
10-2
SM
SUSY
New
Physics
Ford et al. (1970) “Charged”
HyperCP prelim. (2000) “Charged”
TNF (2005) “Neutral”
NA48/2proposal
“Charged”“Neutral”
Smith et al. (1975) “Neutral”
10-6
|Ag|
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 1212
Introduction (III)Introduction (III)
What’s new in NA48/2 measurement?
› Simultaneous K+ and K– beams, superimposed in space, with momentum spectra (60±3) GeV/c
› Equalize K+ and K– acceptances by frequently alternating polarities of relevant magnets
› Detect asymmetry exclusively considering slopes of ratios of normalized u distributions
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 1313
If K+ and K– acceptances are equal for the same u and v value, any difference between the experimental distributions would be a sign of Direct CP-Violation. Integrated over v, Ag can be extracted from a fit to the ratio R(u) using the PDG value for g:
› The normalization is a free parameter in the fit and Δg does not depend on it
› For the “charged” mode a fit with linear function is suitable due to smallness of the slope g
› u calculation:»“Charged” mode: only the magnetic spectrometer is used»“Neutral” mode: only the calorimeter is used
Measurement StrategyMeasurement Strategy
Δg = g+ - g- << 1
R(u) = = n ~ n 1 +N+(u)N-(u)
1 + g+·u + h·u2 +…1 + g-·u + h·u2 +…
Δg·u1 + g∙u + h∙u2 -> Ag = Δg/2g
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 1414
Acceptance (I)Acceptance (I)Magnetic fields present in both beam line and spectrometer: this leads to residual charge asymmetry of the setup
SuperSample (SS) data taking strategy:>Beam line polarity (A) reversed on weekly basis>Spectrometer magnet polarity (B) reversed on a more
frequent basis (~daily in 2003, ~3 hours in 2004)
The whole 2003+2004 data taking is subdivided in 9 SS in which all the field configurations are present
Example:data taking from August
6th to September 7th,
2003
SuperSample 1
SuperSample 2
SuperSample 3
12 subsamples
12 subsamples
4 subsamples
Week 1
Week 2
Week 3
Week 4
Week 5
B+ B–
B+
B+
B+
B+
B+
B+
B+
B+
B+
B–
B–
B–
B–
B–
B–
B–
B–
B–
B+
B+
B–
B–
B+
B+ B–
B–
A+
A–
A+
A–
A+
A–
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Acceptance (II)Acceptance (II)
>In each ratio the odd pions are deflected towards the same side of the detector (left-right asymmetry)
>In each ratio the event at the numerator and denominator are collected in subsequent period of data taking (global time variations)
N+(A-
B+)RDS =N-(A-B-)
N+
(A+B-)N-
(A+B+)
RUJ =
N+
(A+B+)N-
(A+B-)
RUS =
N+(A-
B-)N-(A-
B+)
RDJ =R indices:
› U/D: beam line polarity› S/J: πodd direction after the spectrometer magnet
field
z
xy
Up
Down
B+
B-
K+
K- A-
A+
SaleveSaleve
JuraJura
π-
π+
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 1616
› Double ratio: cancellation of global time instabilities (rate effects, analyzing magnet polarity inversion)
› Double ratio: cancellation of local beam line biases effects (slight differences in beam shapes and momentum spectra)
› Quadruple ratio: both previous cancellations + left-right detector asymmetry cancellation
RS = RUS × RDS
RJ = RUJ × RDJ
f2(u) = n2∙(1+ΔgSu)2f2(u) = n2∙(1+ΔgJu)2
R = RUS × RUJ × RDS × RDJ
f4(u) = n4∙(1+ Δgu)4
The method is independent of K+/K– flux ratio and relative sizes of the samples (important:
simultaneous beams)
The method is independent of K+/K– flux ratio and relative sizes of the samples (important:
simultaneous beams)
RU = RUS × RUJ
RD = RDS × RDJ
f2(u) = n2∙(1+ΔgUu)2f2(u) = n2∙(1+ΔgDu)2
Acceptance (III)Acceptance (III)
“Charged” Mode:K± -> π±π+π-
“Charged” Mode:K± -> π±π+π-
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Event Selection (I)Event Selection (I)
Main requirements (simplicity, charge symmetry):
>Identification of the best 3-track vertex
>zvertex > -18 m (downstream the last collimator)
>Track times: |ti – tj| < 10 ns -> probability of event pile-up ~ 10–4
>Pt < 0.3 GeV/c (suppression of background decays)
>|m3π - mK| < 9 MeV/c2 (5 times the resolution)
Ke4 background
MC K3π
MC K3π + π -> μν decay
MC K -> 3πγ
Data
m3π [GeV/c2]
Time difference for tracks pairs
Even
ts
ti-tj [ns]
zvertex [m]
z-coordinate of the decay vertex
Even
ts
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In the 2003+2004 data sample 3.11·109 K± have been selected:
Event Selection (II)Event Selection (II)
π -> μν
σm = 1.7 MeV/c2
Even
ts
K+: 2.00·109 events
m3π [GeV/c2]
K-: 1.11·109 events
π -> μν
Even
ts
m3π [GeV/c2]
odd pionin beam pipe
even pionin beam pipe
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 2020
Monte-Carlo SimulationMonte-Carlo Simulation
Example of Data/MC agreement:mean beam positions @ DCH1
K+
K
<x>
Run Number
Due to acceptance cancellations, the analysis does not rely on Monte-Carlo to calculate acceptance. Still Monte-Carlo is used to study systematic effects.
NA48/2 MC properties:› Based on GEANT
› Full detector geometry and material description
› Local DCH inefficiencies
› Variations of beam geometry and DCH alignment
Simulated statistics similar to experimental
one
Simulated statistics similar to experimental
one
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 2121
SS0
SS1
SS2
SS3
SS4
SS5
SS6
SS7
SS8
Δg Fit In SuperSamplesΔg Fit In SuperSamples
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 2222
RunSuperSam
pleΔg·104 Χ2 of the R4(u)
fit
2003
0 -0.8±1.
8
30/26
1 -0.5±1.
8
24/26
2 -1.4±2.
0
28/26
3 1.0±3.3
19/26
2004
4 -2.0±2.
2
18/26
5 4.4±2.6
20/26
6 5.0±2.2
26/26
7 1.5±2.1
10/26
8 0.4±2.3
23/26
Combined
0.6±0.7
Results In SuperSamplesResults In SuperSamples
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 2323
SystematicsSystematicsSystematic effect Effect on
Δg·104
Spectrometer alignment ±0.1
Momentum scale ±0.1
Acceptance and beam geometry ±0.2
Pion decay ±0.4
Accidental activity (pile-up) ±0.2
Resolution effects ±0.3
Total systematic uncertainty ±0.6
L1 trigger: uncertainty only ±0.3
L2 trigger: correction -0.1±0.3
Total trigger correction -0.1±0.4
Systematic & trigger uncertainty
±0.7
Raw Δg 0.7±0.7
Δg corrected for L2 inefficiency
0.6±0.7
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 2424
ResultsResults
› A factor ~20 better precision than the previous measurements
› Uncertainties dominated by those of statistical nature
› Design goal reached. There is still some room to improve the systematic uncertainty
› Result compatible with the Standard Model predictionsBased on the full
2003+2004 data sample
Ford
et
al.
(1970)
Hyp
erC
P (
2000)
Pre
lim
inary
2003
Fin
al 2003
Cu
rren
t 2003+
2004
NA48/2(results superseding
each other)
Ag
10
4
Measurements of Ag
Final 2003 result
published: PLB634 (2006)
474-482
Δg = (0.6 ± 0.7stat ± 0.4trig ± 0.6syst)·10-4
Δg = (0.6 ± 1.0)·10-4
Ag = (-1.3 ± 1.5stat ± 0.9trig ± 1.4syst)·10-4
Ag = (-1.3 ± 2.3)·10-4
Δg = (0.6 ± 0.7stat ± 0.4trig ± 0.6syst)·10-4
Δg = (0.6 ± 1.0)·10-4
Ag = (-1.3 ± 1.5stat ± 0.9trig ± 1.4syst)·10-4
Ag = (-1.3 ± 2.3)·10-4
Preliminary
“Neutral” Mode:K± -> π±π0π0
“Neutral” Mode:K± -> π±π0π0
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 2626
IntroductionIntroduction
M002 - s0
mπ2
u =
s0 = (s1 + s2 + s3) / 3
u
|v|
odd pionin beam pipe
Statistical precision in Ag similar to “charged” mode:
› Ratio of “neutral” to “charged” statistics: N0/N± ~ 1/30 (91·106 K± have been selected in the 2003+2004 data sample)
› Ratio of slopes: |g0/g±| ~ 1/3
› More favourable Dalitz-plot distribution (gain factor ~1.5)
For u calculation only the energy of the two neutral pions in laboratory frame is used (only calorimeter information)
σm=0.9 MeV/c2
Even
ts
m3π [GeV/c2]
π -> μν
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 2727
Results In SuperSamplesResults In SuperSamples
RunSuperSam
pleΔg·104
2003
0 4.3±3.8
1+2 0.5±5.0
3 -2.0±8.
2
2004
5 5.6±6.8
6 4.7±5.1
7 3.5±5.6
8 -1.4±5.
8
Combined
2.7±2.0
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 2828
ResultsResults
Based on the full 2003+2004 data
sample
Final 2003 result
published: PLB638 (2006)
22-29
Δg = (2.7 ± 2.0stat ± 1.2syst ± 0.3ext)·10-4
Δg = (2.7 ± 2.4)·10-4
Ag = (2.1 ± 1.6stat ± 1.0syst ± 0.2ext)·10-4
Ag = (2.1 ± 1.9)·10-4
Δg = (2.7 ± 2.0stat ± 1.2syst ± 0.3ext)·10-4
Δg = (2.7 ± 2.4)·10-4
Ag = (2.1 ± 1.6stat ± 1.0syst ± 0.2ext)·10-4
Ag = (2.1 ± 1.9)·10-4
› A factor ~10 better precision than the previous measurements
› The errors are dominated by statistics
› Design goal reached. Further improvements of the analysis are possible
› Result compatible with the Standard Model predictions
Ag
10
4
Measurements of Ag
Preliminary
Fin
al 2003
NA48/2(results superseding
each other)
Cu
rren
t 2003+
2004
Sm
ith
et
al.
(1975)
TN
F (
2005)
“Cusp” Effect inK± -> π±π0π0 Decay
“Cusp” Effect inK± -> π±π0π0 Decay
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 3030
A “Cusp”A “Cusp”
› From K± -> π±π0π0 decay we observed an anomaly in the M00
2 invariant mass distribution in the region around M002 =
(2mπ+)2 = 0.07792 GeV2
› This anomaly has been interpreted as a final state charge exchange scattering process of K± -> π±π+π- (π+π- -> π0π0)
› The parameter a0-a2 (difference between the S-wave ππ scattering lengths in the isospin I=0 and I=2 states) can be precisely measured using this sudden anomaly (“cusp”)
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 3131
Reconstruction:› At least 4 clusters 15 cm away from any track and 10 cm away
from other clusters› Select γ pairs with smallest distance between vertices
› M002 computed using average vertex of two π0
Standard Dalitz plot parameterization shows deficit in data before “cusp”:
Event SelectionEvent Selection
Data
FitData
cusp cusp
Whole region:c2/ndf=9225/149!Above cusp:c2/ndf=133/110
DataFitData
Δ
Standard parametrization
M002 [(GeV/c2)2] M00
2 [(GeV/c2)2]
Even
ts /
0.0
00
15
[(G
eV
/c2)2
]
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 3232
Instrumental Effects (I)Instrumental Effects (I)
Good resolution and linear acceptance near the “cusp” region:
σ ~ 0.5 MeV/c2 @ M00 = 2mπ+
cusp
Resolu
tion
(M
C)
cusp
Accep
tan
ce
(MC
)
M002 [(GeV/c2)2]M00
2 [(GeV/c2)2]
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 3333
Instrumental Effects (II)Instrumental Effects (II)
Data-MC comparisons above and below “cusp”:
Event deficit is a real effect
Event deficit is a real effect
a/b ratios:Data (dot)vs.
MC (full)
Data distributions across the “cusp” agree with MC predictions without “cusp”
I+/I-
Eγ [GeV]
min rγ [cm] max rγ [cm]
min dγγ [cm] min dγ-track [cm]
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 3434
Re-scattering model: two amplitudes contribute to K± -> π±π0π0
› M0: Direct emission
› M1: Charge exchange in final state of K± -> π±π+π- (π+π- -> π0π0)
The singularity in the invariant mass spectrum at π+π- threshold is mainly caused by the destructive interference of M0 and M1
The effect is present below the threshold and not above it (re-scattering model at one-loop (N. Cabibbo: PRL 93 (2004) 121801))
Interpretation (I)Interpretation (I)
M(K± -> π±π0π0) = M0 + M1
CEDE
π201 maaMug
21
1M 00
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 3535
Interpretation (II)Interpretation (II)
› More complete formulation of the model including all re-scattering processes at one-loop and two-loop level (N.
Cabibbo and G. Isidori: JHEP 0503 (2005) 21) has been used to extract NA48/2 results
› Experimental work in progress on an effective field theory model (CGKR: hep-ph/0604084) valid in whole decay region
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 3636
Try fitting different theoretical models to M00
2 distribution and evaluate:
› Fitting up to 0.097 (GeV/c2)
› 5 fitting parameters: norm, g0, h’, a0-a2 and a2
› For final results pionium set to theoretical expectation and 7 bins around the “cusp” excluded from the fit in order to reduce sensitivity to Coulomb corrections
DataFitData
Δ
Results (I)Results (I)
One loop Х2/ndf=420/148
One+two loop Х2/ndf=155/146
Pionium Х2/ndf=149/145
Exclude 7 bins around cusp Х2/ndf=145/139
M002 [(GeV/c2)2]
Δ
Δ
Δ
Δ
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 3737
Systematic effects: acceptance determination, trigger efficiency and fitting interval
Predictions in ChPT (PLB 488 (2000) 261):› (a0-a2)·mπ+ = 0.265 ± 0.004› a2 = -0.0444 ± 0.0010
Fit imposing ChPT constraint between a0 and a2 (PRL 86 (2001)
5008)
Results (II)Results (II)
g0 = 0.645 ± 0.004stat ± 0.009syst
h’ = -0.047 ± 0.012stat ± 0.011syst
(a0-a2)·mπ+ = 0.268 ± 0.010stat ± 0.004syst ± 0.013ext
a2 = -0.041 ± 0.022stat ± 0.014syst
g0 = 0.645 ± 0.004stat ± 0.009syst
h’ = -0.047 ± 0.012stat ± 0.011syst
(a0-a2)·mπ+ = 0.268 ± 0.010stat ± 0.004syst ± 0.013ext
a2 = -0.041 ± 0.022stat ± 0.014syst
a0 = 0.220 ± 0.006stat ± 0.004syst ± 0.011ext
(a0-a2)·mπ+ = 0.264 ± 0.006stat ± 0.004syst ± 0.013ext
a0 = 0.220 ± 0.006stat ± 0.004syst ± 0.011ext
(a0-a2)·mπ+ = 0.264 ± 0.006stat ± 0.004syst ± 0.013ext
Based on partial
sample of 2003 data
2003 results
published: PLB 633 (2006)
173-182
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 3838
Results (III)Results (III)
A factor ~2 better precision than the previous measurement
Measurements of (a0-a2)·mπ+
(a0-a
2)·
mπ
+
a) b) c) d)
a) NA48 result PLB 633 (2006)b) DIRAC result PRL 619 (2005)c) G.Colangelo et al. NPB 603 (2001)d) J.R.Pelaez et al. PRD 71 (2005)
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 3939
Change of Dalitz variables, from (s3, s2-s1) to (s3, cosθ). Define θ as angle between π± and π0 in π0π0 COM:
g0 and h’ changeNo change in (a0-a2)·mπ+ and a2
Results (IV)Results (IV)
k’ = 0.0097 ± 0.0003stat ± 0.0008syst
k’ = 0.0097 ± 0.0003stat ± 0.0008syst
Fitting new Dalitz plot above “cusp” finds evidence for k’ > 0 term
Based on a partial sample of
2003 data
...k'v21
uh'21
ug21
1M 220
200
Data/MC comparison for different k’ values
cosθEven
ts
Preliminary
Rare Decay:K± -> π±π0γRare Decay:K± -> π±π0γ
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 4141
Rare DecaysRare Decays
Statistics usually at least one order of magnitude above previous experiments. Several channels not yet observed
› K± -> π+π-e±ν
› K± -> π0π0e±ν
› K± -> π+π-μ±ν
› K± -> π±π0γ
› K± -> π±γγ
› K± -> π±e+e-γ
› K± -> π±π0γγ
› K± -> π±e+e-
Ke4
Kμ4
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 4242
Two amplitudes:› Inner Bremsstrahlung (IB)› Direct Emission (DE)
Two type of contributions:>Electric (j=l±1) dipole (E1)
>Magnetic (j=l) dipole (M1)
Electric contributions come from L4 CHPT Lagrangian, loops L2 and are dominated by the IB termMagnetic contributions are dominated by Chiral AnomalyDE shows up only at order O(p4) in ChPT: is generated both by E and M contributions. Present experimental results seem to suggest a M dominated DE
Interference (INT) is possible between IB and electric part of DE:
› Measuring at the same time DE and INT gives measurement of M and E
› CP-Violation could appear in INT
Introduction (I)Introduction (I)
DE
IB
PDG (55 MeV < T*π < 90 MeV)
IB:DE:INT:
(2.75 ± 0.15)·10-4
(4.4 ± 0.8)·10-6
not yet measured
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 4343
Introduction (II)Introduction (II)
IBfrom K± ->
π±π0
INTsensitive to electric dipole
DEsensitive to electric &
magnetic dipole
W21 W4
P*K = 4 momentum of the K±
P*π = 4 momentum of the π±
P*γ = 4 momentum of the
radiative γ
W W W
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 4444
Introduction (III)Introduction (III)
Interference found to be compatible with 0:
-> Set INT = 0 and fit only DE (all measurements have been performed in the T*
π region 55÷90 MeV to avoid K± -> π±π0π0 background)
BNL E787KEK E470
Recent history of BR(DE)
4.7
3.53.2
3.8
0.0
1.0
2.0
3.0
4.0
5.0
6.0
1999 2000 2001 2002 2003 2004 2005 2006 2007
year
BR
(DE)
10
-6BNL E787KEK E470
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 4545
Introduction (IV)Introduction (IV)
What’s new in NA48/2 measurement?
>Simultaneous K+ and K- beams -> check for CP-Violation
>Enlarged T*π region in the low energy part (0÷80 MeV)
>Negligible background contribution (<1% of the DE component)
>γ miss-tagging probability ~ ‰ for IB, DE and INT
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 4646
Event Selection (I)Event Selection (I)
Event selection:
>Select 1 track and any number of clusters
>Require 3 γs with Eγ > 3 GeV outside 35 cm radius from π @ LKr γs and 10 cm away from other clusters
>Charged vertex (zc): calculate the K decay point as the position where the π± track intersects the beam line
>Selecting the γ pairing for the π0:
»Three combinations are possible (choosing the wrong combination for the π0 -> choosing the wrong odd γ (miss-tagging) -> distorts W)
»Two possible methods used: select the combination giving the best π0 or K± invariant mass
>Neutral vertex (zn): from imposing π0 mass to γ pairs; must be in agreement with charged vertex (within 400 cm)
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 4747
Event Selection (II)Event Selection (II)
Pmiss-tagging < 1.2 ‰
@ Δzncut = 400 cm
Pmiss-tagging < 1.2 ‰
@ Δzncut = 400 cm
Miss-tagged events move to large W: this could induce a DE component if difference between Data-MC
› Demanding the charged vertex compatible with the best neutral vertex gives Pmiss-tagging ~ 2.5%
› Rejecting events with a second solution for neutral vertex close to the best one, |zn
second - znbest| < Δzn
cut, reduces the Pmiss-
tagging
■ DE
▼ IB
0.00
0.50
1.00
1.50
2.00
2.50
3.00
0 50 100 150 200 250 300 350 400 450 500 550
Δzncut [cm]
Pro
bab
ilit
y %
Miss-tagging probability (MC)
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 4848
Background (I)Background (I)
Decay BR Background mechanismK± -> π±π0 (21.13 ± 0.14)
%1 accidental γ or hadronic extra
cluster
K± -> π±π0π0
(1.76 ± 0.04) %
1 missing or 2 overlapped γs
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
e misidentified as a π
K± -> π0μ±ν(γ)
(2.4 ± 0.85)·10−5
μ misidentified as a π
Backgrounds can be rejected using particle ID, COG, mass and time cuts
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 4949
Background (II)Background (II)
After all cuts the background estimation is <1% of DE and can be explained in terms of
K± -> π±π0π0
After all cuts the background estimation is <1% of DE and can be explained in terms of
K± -> π±π0π0
› For every of the three γs in the event assume that its energy Ei is really the overlap of 2 γs of energies E’ = x·Ei and E’’ = (1-x)·Ei
› Solve for sharing fraction (x) imposing that the two π0 must come from the same vertex
› Reject event if any of the reconstructed π0 vertex is compatible with charged vertex (within 400 cm)
In addition need to use MUV detector to avoid miss-reconstruction of track momentum due to π -> μν decay in flight
Cut on overlapping γs (allows avoiding T*π > 55 MeV):
K± -> π±π0π0
K± -> π±π0γ
mK [GeV/c2]
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 5050
Data/MC ComparisonData/MC ComparisonIn the 2003 data sample (~30% of the whole statistics) 220·103 K± have been selected:
› After trigger efficiency correction good agreement between Data and MC for Eγ, in particular for Eγ > 5 GeV (used for final result)
› The ratio W(Data)/W(MCIB) is in good agreement for IB dominated region and clearly shows DE
Eγ [GeV]
Eγ [GeV]
W(Data)/W(MCIB)
Fitting region
IB dominatedregion
W
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 5151
Systematic effect
Effect on DE
Effect on INT
Miss-tagging - ±0.2
Energy scale +0.09 -0.21
Resolutions difference
< 0.05 < 0.1
LKr non linearity < 0.05 < 0.05
BG contributions < 0.05 < 0.05
Fitting procedure 0.02 0.19
L1 trigger ±0.17 ±0.43
L2 trigger ±0.17 ±0.52
Total ±0.25 ±0.73Systematic effects dominated by the trigger
(both L1 and L2 have been modified in 2004)
Systematic effects dominated by the trigger
(both L1 and L2 have been modified in 2004)
Systematic checks have been performed using both Data and MC
SystematicsSystematics
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 5252
Results (I)Results (I)
Use extended Maximum Likelihood for 0.2 < W < 0.9 to fit in the region 0 MeV < T*π < 80 MeV (based on 124·103 events)
-> First evidence of Interference between Inner Bremsstrahlung and Direct Emission amplitudes
Frac(DE) = (3.35 ± 0.35stat ± 0.25syst) %Frac(INT) = (-2.67 ± 0.81stat ± 0.73syst) %
Frac(DE) = (3.35 ± 0.35stat ± 0.25syst) %Frac(INT) = (-2.67 ± 0.81stat ± 0.73syst) %
Based on a partial sample of 2003
data
ρ = -0.92 Frac(DE)
Fra
c(I
NT)
Preliminary
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 5353
Results (II)Results (II)
Frac(DE) = (0.85 ± 0.05stat ± 0.02syst) %
Setting INT = 0 for comparison, fitting between 0 MeV < T*π < 80 MeV and extrapolating to 55÷90 MeV
Fraction of DE(INT=0)
0.00
0.50
1.00
1.50
2.00
2.50
1999 2000 2001 2002 2003 2004 2005 2006 2007
year
% D
E
BNL E787KEK E470NA48/2
A description in term of IB and DE onlyis unable to reproduce the W data
spectrum
A description in term of IB and DE onlyis unable to reproduce the W data
spectrum
The analysis of fit residuals
shows a bad Χ2
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 5454
Summary (I)Summary (I)
› The preliminary result on the Direct CP violating charge asymmetry in K± -> π±π+π- based on the 2003+2004 data sample (whole statistics) is:
Ag = (-1.3 ± 1.5stat ± 0.9trig ± 1.4syst)·10-4
= (-1.3 ± 2.3)·10-4
› The preliminary result for Ag in K± -> π±π0π0 based on the 2003+2004 data sample (whole statistics) is:
Ag = (2.1 ± 1.6stat ± 1.0syst ± 0.2ext)·10-4
= (2.1 ± 1.9)·10-4
› Both results have ~10 times better precision than the previous measurements
› The errors are dominated by statistics
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 5555
Summary (II)Summary (II)
› A new “cusp” structure in K± -> π±π0π0 was observed (ππ final state charge exchange process of K± -> π±π+π-) which provides a new method for the extraction of the ππ scattering lengths:(a0-a2)·mπ+ = 0.268 ± 0.010stat ± 0.004syst ± 0.013theor
› The measurement is based on a 2003 data sample and agrees both with another independent measurement and with the theoretical predictions
› Parameter a2 directly measured for the first time even though with low accuracy:
a2 = -0.041 ± 0.022stat ± 0.014syst
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 5656
Summary (III)Summary (III)
› The first measurement of Direct Emission and Interference terms in K± -> π±π0γ based on a 2003 data sample (~30% of the whole statistics) has been performed in the region 0 MeV < T*π < 80 MeV:
Frac(DE) = (3.35 ± 0.35stat ± 0.25syst) %
Frac(INT) = (-2.67 ± 0.81stat ± 0.73syst) %
› A first evidence of a negative Interference has been found and therefore a non negligible contribution of electric term to Direct Emission amplitude
SparesSpares
CP ViolatingCharge Asymmetry
CP ViolatingCharge Asymmetry
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 5959
Time variations of spectrometer geometry do not cancel in the result.Alignment is fine-tuned by scaling π± momenta (charge-asymmetrically)to equalize the reconstructed average K+, K- masses
Sensitivity to DCH4 horizontal shift: |ΔM/Δx| ~ 1.5 KeV/μm
The imperfect inversion of spectrometer field cancels in double ratio. Momentum scale adjusted anyway by constraining average reconstructed 3π masses to the PDG value
Sensitivity to 10-3 error on field integral:
ΔM ~ 100 KeV/c2
Transverse alignment
Magnetic field
Systematics - SpectrometerSystematics - Spectrometer
Maximumequivalent
transverse shift:
~200m @DCH1or
~120m @DCH2or
~280m @DCH4
Much more stable alignment in 2004
Max. effectin 2004
Subsample
ΔM
3π [
KeV
/c2]
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 6060
› Acceptance largely defined by central hole edge (R ~ 10cm)
› Geometry variations, non-perfect superposition: asymmetric acceptance
› Additional acceptance cut defined by a “virtual pipe” (R = 11.5cm) centered on averaged reconstructed beam position as a function of charge, time and K momentum -> statistics loss: 12%
Beam widths:~ 1 cmBeam
movements:~ 2 mm
Sample beam profile at DCH1
0 0.4 0.8-0.4-0.8
0
0.4
0.8
-0.4
-0.8
x [cm]
Y [
cm
]
“Virtual pipe” also corrects for the
differences between the upper
and lower beam paths
Y [
cm
]
x [cm]
2mm
[Special treatment of permanent magnetic fields effect on measured beam positions]
Systematics - Beam GeometrySystematics - Beam Geometry
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 6161
Only charge-asymmetric trigger inefficiency dependent
on u can bias the result
Trigger efficiencies measured using control data samples triggered by downscaled low bias
triggers
Statistical errors due to limited sizes of the control samples are propagated into the
result
0.4
0.2
0.6
0.8
1.0
1.2
1.4
1.6
Ineffi
cie
ncy x
10
3 L2 inefficiency vs u (normal conditions)
1.0 1.50.50.0-0.5-1.0-1.5u
0.4
0.2
0.6
0.8
1.0
1.2
1.4
1.6
Ineffi
cie
ncy %
L2 inefficiency vs time (2003)
Beginning of 2003 run:L2 algorithm tuning
Max. inefficiency in 2004
Systematics - TriggerSystematics - Trigger
L2 triggertime-varying inefficiency
(local DCH inefficiencies, tuning)1–ε = 0.06% ÷ 1.5%
u-dependent correction applied
L1 triggersmall and stable
inefficiency1–ε ~ 0.9·10-3
no correction
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 6262
Field map in decay volume (y projection)
Decay volume: z coordinate
0.4
0
-0.4
-0.8
-1.2
[Gau
ss]
Residual effects of stray magnetic fields (magnetized
vacuum tank, earth field) minimized by explicit field map
correction
Further systematic effects studied:
› Accuracy of beam tracking, variations of beam widths
› Bias due to resolution in u
› Sensitivity to fitting interval and method
› Coupling of π -> μν decays to other effects
› Effects due to event pile-up
› π+/π- interactions with the material
Other SystematicsOther Systematics
0.5MeV/c2
No magneticfield correction
Magnetic fieldcorrected
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 6363
Time-Stability & Control Quantities
Time-Stability & Control Quantities
2003
(results consistent)
2004 RLR(u)=RS/RJ RUD(u)=RU/RDControl of setup time-variable
biasesControl of differences of the
two beam paths
Monte-Carlo
(reproduces apparatus asymmetries)
Physics asymmetry
Control quantities canceling in the result
quadruple ratio components rearranged(smallness demonstrates 2nd order effects
negligible)
“Cusp” Effect inK± -> π±π0π0 Decay
“Cusp” Effect inK± -> π±π0π0 Decay
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 6565
a0 (UB) = 0.256 ± 0.008stat ± 0.007syst ± 0.018th
-> a2 = -0.031 ± 0.015stat ± 0.015syst ± 0.019th
a0 (UB) = 0.256 ± 0.008stat ± 0.007syst ± 0.018th
-> a2 = -0.031 ± 0.015stat ± 0.015syst ± 0.019th
Preliminary
Results From K±e4 Results From K±e4
g0 = 0.645 ± 0.004stat ± 0.009syst
h’ = -0.047 ± 0.012stat ± 0.011syst
(a0-a2)·mπ+ = 0.268 ± 0.010stat ± 0.004syst ± 0.013ext
a2 = -0.041 ± 0.022stat ± 0.014syst
g0 = 0.645 ± 0.004stat ± 0.009syst
h’ = -0.047 ± 0.012stat ± 0.011syst
(a0-a2)·mπ+ = 0.268 ± 0.010stat ± 0.004syst ± 0.013ext
a2 = -0.041 ± 0.022stat ± 0.014systPredictions in ChPT (PLB 488 (2000) 261):› a0= 0.220 ± 0.005› a2 = -0.0444 ± 0.0010› (a0-a2)·mπ+ = 0.265 ± 0.004
“Cusp”
Rare Decay:K± -> π±π0γRare Decay:K± -> π±π0γ
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 6767
L2 trigger:› Using DCH information and assuming 60 GeV K along z axis
on-line processors compute a sort of missing mass of the K-π system
› Cut events with T*π > 90 MeV (to keep away of edge resolution effects require T*π < 80 MeV in analysis)
TriggerTrigger
L1 trigger:› Require 1 track and LKr information
(peaks) compatible with at least 3 clusters
› This introduces an energy dependence -> distortion of W distribution
› Correction found using all 3γs events (K± -> π±π0π0 with γ lost) and applied to MC
L1 requires nx>2 or ny>2
VI SILAFAEVI SILAFAESimone BifaniSimone Bifani 6868
Overlapping γsOverlapping γs
Overlapping γs would give right K mass
For any of the 3 γs with energies E1, E2, E3 do the following:
› Assume that the γ with energy E1 is really the overlap of 2 γ s of energies E’=x·E1 and E’’=(1-x)·E1
› Suppose E’ comes from a π01 together with cluster 2 and
E’’ was coming from π02 with cluster 3. Then the vertices
would be:zπ01 = √(Dist12·E’·E2)/mπ0 = √(Dist12·x·E1·E2)/mπ0
zπ02 = √(Dist13·E’’·E3)/mπ0 = √(Dist13·(1-x)·E1·E3)/mπ0
› As zπ01 = zπ02 we can solve for x
› We reject the event if |zπ01 - zc| < 500 cmThanks to this cut we can extend the fit region to
0 MeV < T*π < 80 MeV keeping the needed rejection
Thanks to this cut we can extend the fit region to0 MeV < T*π < 80 MeV keeping the needed
rejection
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