recent results from na48/2 experiment @ cern-sps simone bifani university of turin – experimental...

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


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


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


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


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-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)


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


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|


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


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


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–


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+


› 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-


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


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


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


SS0

SS1

SS2

SS3

SS4

SS5

SS6

SS7

SS8

Δg Fit In SuperSamplesΔg Fit In SuperSamples


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


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


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


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


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


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



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”)


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

]


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]


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]


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


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


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


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


a2 = -0.041 ± 0.022stat ± 0.014syst

a0 = 0.220 ± 0.006stat ± 0.004syst ± 0.011ext


a0 = 0.220 ± 0.006stat ± 0.004syst ± 0.011ext


Based on partial

sample of 2003 data

2003 results

published: PLB 633 (2006)

173-182


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)


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


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


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


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


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


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


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)


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)


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


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]


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


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


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


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


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


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


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


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]


› 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


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


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


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)


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


a2 = -0.041 ± 0.022stat ± 0.014syst

g0 = 0.645 ± 0.004stat ± 0.009syst

h’ = -0.047 ± 0.012stat ± 0.011syst


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


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


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

recent results from na48/2 experiment @ cern-sps simone bifani university of turin – experimental...

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