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Evgueni Goudzovski (JINR, Dubna) CERN particle physics seminar 1 st March 2005

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The first results on direct CP-violation asymmetry measurement in K± π±π+π–decaysfrom the NA48/2 experiment

Evgueni Goudzovski

(JINR, Dubna)

CERN particle physics seminar

1st March 2005

on behalf of the NA48 Collaboration:

Cambridge, CERN, Chicago, Dubna, Edinburgh, Ferrara, Firenze, Mainz, Northwestern, Perugia, Pisa, Saclay, Siegen, Torino, Vienna

- Direct CP-violation in charged K3π decays;
- NA48/2 beam, detector and data-taking;
- NA48/2 method of CP-violation measurement;
- Preliminary result on 2003 data.

BR(K±±+)=5.57%; BR(K±±00)=1.73%.

- Matrix element:

- Kinematic variables

|M(u,v)|2 ~ 1 + gu + hu2+ kv2

Lorentz-invariants

u = (s3-s0)/m2;

v = (s2-s1)/m2;

si = (PK-Pi)2, i=1,2,3 (3=odd );

s0 = (s1+s2+s3)/3.

- Measured quantity sensitive to direct CP violation:

Centre of mass frame

u = 2mK∙(mK/3-Eodd)/m2;

v = 2mK∙(E1-E2)/m2.

Ag = (g+-g-)/(g++g-)≠0

- ”Charged” mode K±3π±:
- Ford et al. (1970) at BNL:Ag=(-7.0±5.3)∙10-3;
Statistics: 3.2M K±;

- HyperCP prelim. (2000) at FNAL:Ag=(2.2±1.5±3.7)∙10-3;
Statistics: 390M K+, 1.6M K-;

Systematics due to knowledge of magnetic fields;

Published as PhD thesis W.-S.Choong LBNL-47014 Berkeley 2000;

- Ford et al. (1970) at BNL:Ag=(-7.0±5.3)∙10-3;
- ”Neutral” mode K±π±π0π0:
- Smith et al. (1975) at CERN-PS:Ag=(1.9±12.3)∙10-3;
Statistics: 28000 K±;

- ISTRA+ (2004) at IHEP Protvino:Ag=(0.2±1.9)∙10-3;
Statistics: 0.52M K±;

- Smith et al. (1975) at CERN-PS:Ag=(1.9±12.3)∙10-3;

Theoretical calculations:

L.Maiani et al.

G.D’Ambrosio

G.Isidori

G.Martinelli

E.Shabalin

A.Bel’kov

I.Scimemi

Conclusions:

- Ag ~ 10-5:perfectly compatible with SM;
- 3∙10-5<Ag<5∙10-5:compatible with SM, but inbad agreement with ε’/ε;
- Ag>5∙10-5:SUSY /New Physics
- Asymmetry in decay widthsexpected to be smaller thanin Dalitz-plot slopes(SM: ~10-6…10-7).

Experimental limits by 2005:

10-2

Ford et al. (1970)

|Ag|

HyperCP prelim. (2000)

ISTRA+ (2004) “neutral” mode

10-3

NA48/2 proposal

10-4

New

physics

10-5

SUSY

SM

10-6

- NA48/2 goals:
- Measure slope asymmetries in both “charged” and “neutral” modes with an accuracy δAg<2∙10-4.
- Statistics required for this measurement: >2∙109 in “charged” mode and >108in “neutral” mode.

- NA48/2 method:
- Two simultaneous K+ and K- beams, superimposed in space, with narrow momentum spectra;
- Detect asymmetry exclusively considering slopes of ratios of normalized U distributions;
- Equalize averaged K+ and K–acceptances by frequently alternating polarities of relevant magnets.

54 60 66

PK spectra,

603GeV/c

BM

z

10 cm

200

250 m

1cm

50

100

magnet

K+

K+

focusing beams

K

~71011

ppp

K

Second

achromat

- Cleaning
- Beam spectrometer

Front-end

achromat

Quadrupole

quadruplet

Beams coincide within <1mm

all along 114m decay volume

- Momentum
- selection

- Focusing
- sweeping

vacuum

tank

He tank

+ spectrometer

not to scale

PK spectra,

603GeV/c

54 60 66

Pion decay products stay in beam pipe...

- 3 MICROMEGA gas chamber stations
- measure charged beam displacement
- within the magnetic field of the
- second achromat
- Measurement of kaon momentum:
- Reconstruct K3π with a lost pion;
- Redundancy in K3πanalysis;
- Resolve Ke4reconstruction ambiguity.

Not used yet for

K±3± analysis

- Main detector components:
- Magnetic spectrometer (4 DCHs):
- redundancy high efficiency;
- Δp/p = 1.0% + 0.044%*p [GeV/c]
- Hodoscope
- fast trigger;
- precise time measurement (150ps)
- Liquid Krypton EM calorimeter (LKr)
- High granularity, quasi-homogenious;
- ΔE/E = 3.2%/√E + 9%/E + 0.42% [GeV];
- electron/pion discrimination.
- Hadron calorimeter, photon vetos,
- muon veto counters

Beam pipe

- 3-track trigger
- K±3π± [“charged” mode]
- Ke4, events with Dalitz decay π0e+e–
- 4-leptonic: K±e+e–eν, K±e+e–μν, K±μ+μ–eν, K±μ+μ–μν

- 1-track missing mass trigger: (PK-Pπ)2>mπ02
- K±π0π0π± [“neutral” mode]
- K±π±π0, K±π±
- Suppress K±π±π0

- Control triggers (downscaled)
- Trigger efficiency measurements;
- Many 1-track channels: K±e±ν, e±ν, π0e±ν, etc…

- 2003run:~ 50 days
- (only 2003 data are presented)
- 2004run: ~ 60 days
- Total statistics in 2 years:
- K - + : ~4·109
- K 0 0 : ~2·108
- ~ 200 TB of data recorded

- Simple selection:
- Ghost track rejection;
- Select 3-track vertex with smallest 2;
- Track time consistency cuts;
- Decay vertex within the decay volume;
- Transverse momentum cut;
- -1<U<1;

- Only magnetic spectrometer involved
- No muon veto, EM calo, hadron calo;

- No significant background for this channel.

M=1.7 MeV/c2

Data-taking 2003:

1.61x109 events selected

|V|

even pion

in beam pipe

K+ : 1.03x109 events

odd pion

in beam pipe

K: 0.58x109 events

U

|M±(u,v)|2 ~ 1 + g±u + hu2+ kv2

- PDG:
- Kπ+π-π±: g = -0.2154±0.0035
- Kπ0π0π±: g = 0.652±0.031
- |h|, |k| << |g|

- Project onto U axis
- Neglect asymmetry in quadratic slopes h and k

If acceptance is equal for K+ and K–,

R(u) = N+(u)/N–(u) ≈

≈n∙(1+g+u)/(1+g–u) ≈

≈n∙(1+2gAgu)

Ag can be extracted from a

linear fit to the ratio R(u)

Ag=g/2g

- Supersample data taking strategy:
- achromat polarity (A) was reversed on weekly basis;
- analyzing magnet polarity (B) was reversed on daily basis.

Example: Data taking from August 6 to September 7, 2003

Achromat –

B+

B-

B+

B-

B+

B-

Week 1

Supersample 1

12 subsamples

B+

Achromat +

B+

B-

B+

B-

B-

Week 2

B-

B-

Achromat –

B+

B-

B+

B+

Week 3

Supersample 2

12 subsamples

Achromat+

B+

B+

B-

B+

B-

B-

Week 4

B-

Achromat –

B+

Week 5

Supersample 3

4 subsamples

Achromat+

B+

B-

Statistics selected for Ag measurement, million events

N(A+B+K+)

RUS=

N(A+B-K-)

N(A+B-K+)

RUJ=

N(A+B+K-)

N(A-B+K+)

RDS=

N(A-B-K-)

N(A-B-K+)

RDJ=

N(A-B+K-)

within supersample

Detector left-right asymmetry cancels

in 4 ratios of K+ over K U-spectra:

Y

(same kaon deviation direction

in numerator and denominator)

X

Achromats: K+ Up

B+

Jura

Z

B

Saleve

Achromats: K+Down

- Indeces of R’s correspond to
- beamline polarity (U/D);
- direction of kaon deviation in spectrometer (S/J).

1.Charge asymmetrydue to permanent magnetic fields

(Earth, tank magnetization) cancels integrating over azimuthal angle;

2. Cancellation ofeffects due to coupling of

charge asymmetrywith left-right asymmetry,

and effects of global time instabilities:

RU = RUS*RUJ

fit with f(u)=n∙(1+2AUu)

fit with f(u)=n∙(1+2ADu)

RD = RDS*RDJ

3. Cancellation ofbeam geometry differenceeffects:

fit with f(u)=n∙(1+2ASu)

RS = RUS*RDS

RJ = RUJ*RDJ

fit with f(u)=n∙(1+2AJu)

4.Maximum cancellation:

fit with f(u)=n∙(1+4∙u)

R = RUS*RUJ*RDS*RDJ

Normalization

Slope difference

The result is sensitive only to

time variation of left-right asymmetries of experimental conditions

with a time-scale of ~1 subsample

- Relation between and asymmetry Ag:
Δ = 2g∙Ag ≈ -0.43∙Ag

- Goal of the experiment:δAg<2∙10-4corresponds to δ<0.9∙10-4

(an example of cancellation)

Achromat reversal

reverses K+ and K–

beam spectra

K+

Systematic differences

of K+ and K–acceptance

due to beam spectra

mostly cancel in AU*AD

Systematic check:

Reweighting K+events

so as to equalise

momentum spectra

leads to negligible effect

Δ=0.03x10-4

K–

SS 3

Supersample 2

Supersample 1

Due to cancellations, theanalysis does not rely on Monte-Carlo. MC is used for systematics study.

- Based on GEANT;
- Full detector geometry and material description;
- Local DCH inefficiencies simulated;
- Beam geometry and DCH alignment variations are followed;
- Simulated statistics similar to experimental one.

Example of data/MC agreement:

Mean beams’ positions @DCH1

(1)Time variations of beam geometry

- Beams’ non-perfect superposition, and width differences coupled to acceptance selection;
(2)Time variations of spectrometer geometry

- Mainly variations of chambers’ transverse alignment;
(3)Variation of spectrometer magnetic field

- Magnet current can not be inverted with accuracy better than 10-3;

(4)Time variations of trigger efficiency

- Sensitivity to time-dependent local inefficiencies of DCHs and hodoscope;
(5)Stray magnetic fields in the decay region

- Earth field can not be inverted;
(6)Coupling ofπ±±decay to another systematic effects;

(7)Time variations of accidental rate.

- Geometrical acceptance is largely defined by pions undetected in the beam pipe;
- Beams’ geometry is variable, beams’ superposition is not perfect … asymmetric acceptance;
- Time-dependent cuts following beams’ movement are introduced.

pion in

beam pipe

“Virtual pipe” cut:

Cylinder following mean beam position measured by the spectrometer

|Rπi-<RK>|>R0 at DCH1, DCH4

<RK> = f (kaon sign, time, momentum)

Beam profile @DCH1

DCH1

Y, cm

Y, cm

0.8

0.4

K

K+

0

-0.4

X, cm

X, cm

-0.8

DCH4

Y, cm

-0.8

-0.4

0.8

0.4

0

4cm relative shift

due to analyzing magnet field

X, cm

- Typical scales:
- Beam width: ~5mm;
- Beam movement in time:~2mm.

X, cm

DCH4

- Upper and lower beam lines have slightly different geometric properties;
- Effect partially cancels by achromat inversion;
- Effect is corrected by “virtual pipe”.

A+KJ, A-KS

Y, cm

55 GeV/c

65 GeV/c

65 GeV/c

55 GeV/c

X, cm

A+KJ, A-KS

Split by analyzing

magnet field

Y, cm

Beam pipe

65 GeV/c

55 GeV/c

65 GeV/c

55 GeV/c

Beam positions

for various momenta

X, cm

Use kaon momentum measured by the beam spectrometer (KABES):

- Need only two detected pions to reconstruct event kinematics;
- Low sensitivity to beam pipe acceptance;
- Possible add events with only 2 tracks in acceptance (+60% events);
- Gain events in high-u region: higher sensitivity to Ag

The following sources were studied:

- Time binning used to follow mean beam position;
- Systematic shifts in measurement of mean beam position (pre-selection cuts, fitting limits);
- Variation of beam widths in time;
- Effect of Earth magnetic field on beam position.
Conclusion:

- Residual effect is <0.5·10-4.

- The observables AS and AJ are sensitive to time variations in spectrometer alignment; the effect does not cancel in ASJ.
- Spectrometer transverse alignment is fine-tuned for each subsample by imposing the K+ and K- to have the same reconstructed mean invariant mass.
- Sensitivity to DCH4 horizontal shift: M/x 1.5 keV/m.

Maximum

equivalent

horizontal shift:

~200m @DCH1

or

~120m @DCH2

or

~280m @DCH4.

- The observables AS and AJ are sensitive to non-perfect inversion of spectrometer magnetic field; however, the effect mostly cancels in ASJ.
- Effective momentum scale is adjusted for each subsample by imposing reconstructed mean invariant kaon mass to be equal to PDG mass.
- Sensitivity: M 100 keV for typical inversion precision on magnetic field integral of I/I 10-3.

The following sources were studied:

- Size of time binning used to follow calibration time-dependence;
- Systematic shift in kaon mass measurement (pre-selection cuts, fitting limits);
- Inhomogeneity of alignment correction;
- Misalignment hypothesis: effect of correction to drift chambers’ positions before/after magnet.
Conclusion:

- residual effect is <0.1·10-4.

- Trigger chain for K±3π±:
- L1: at least 2 hodoscope hits;
- L2: 3-track trigger (online vertex reconstruction);

- Only the geometry-related parts of inefficiency considered (rate-dependent parts maximally separated).

10-3

cut

cut

L1 inefficiency

stable: ~ 0.7x10-3

L2 inefficiency

varies:

0.2% … 1.8%

3x10-3

cut

cut

- L1 inefficiency: small and stable in time
- No correction is applied to the result;
- Uncertainty on Δ due to statistical error of inefficiency Δ = 0.4x10-4;

- L2 inefficiency: fluctuates in time,depends on local DCH inefficiency
- Correction applied to U-spectrumin each subsample.
- Resulting uncertainty due to statistical error: Δ = 0.8x10-4.

10-4

P kick(stray field)

P kick(spectrometer)

- Field map was directly measured and used invertex reconstruction: only residual systematics remain (Δ<10-5);
- Coupling of stray field with the “virtual pipe” acceptance cut: drift of pions and kaon trajectories in stray field. Relatively large effects (Δ~10-4) in AS, AJcancel in ASJ.

No magnetic field

correction

Field map in decay volume: Y projection

100T

Magnetic field

corrected for

x10-4

x10-4

Data

Monte-Carlo

20

20

0: Nominal data;

1: No decay before DCH1;

2: No decay before DCH2;

3: No decay before DCH4.

15

15

10

10

5

5

0

0

systematic error due to±±decay:

=0.4x10-4

(conservative estimation,

limited by MC statistics)

-5

-5

systematic error due toaccidental activity:

=0.3x10-4

(conservative estimation)

-10

-10

-15

-15

-20

-20

- Interaction of pions with the detector material
- <0.5x10-5

- Pion charge mis-identification
- <<10-5

Other possible sources found to have

negligible systematic effects on asymmetry:

2=37.5/38

AUS=(2.4±4.6)x10-4

AUJ=(-0.2±4.6)x10-4

2=50.5/38

U

U

2=35.6/38

ADS=(8.2±4.4)x10-4

2=34.8/38

ADJ=(-1.0±4.4)x10-4

U

U

2=38.1/38

=(2.3±2.2)x10-4

U

SS2: =(-3.1±2.5)x10-4

2=29.5/38

SS0: =(0.6±2.4)x10-4

2=39.7/38

U

U

SS3: =(-2.9±3.9)x10-4

SS1: =(2.3±2.2)x10-4

2=38.1/38

2=32.9/38

U

U

g x10-4

g x10-4

80

40

60

30

40

20

20

10

0

0

-20

-10

-40

-20

-60

-30

-80

-40

x10-4

20

Monte-Carlo describes

effects of

left-right asymmetries

and differences of

beamline properties

15

10

5

0

-5

-10

-15

-20

Result: Δx104

L2 trigger systematics included

Slope difference:

Δg = (-0.2±1.0stat.±0.9stat.(trig.)±0.9syst.)x10-4 =

(-0.2±1.7)x10-4

Charge asymmetry:

Ag = (0.5±2.4stat.±2.1stat.(trig.)±2.1syst.)x10-4 =

(0.5±3.8)x10-4

- Systematic errors are conservative for preliminary result;
- 2004 data:
- Statistics greater than in 2003 (~2x109 events);
- Expect smaller systematic effects (more frequent polarity alternation, better beam steering).

Ford et al. (1970)

10-2

HyperCP prelim. (2000)

|Ag|

10-3

NA48/2 prelim.: run 2003

NA48/2 goal: runs 2003-04

10-4

New

physics

10-5

SUSY

SM

10-6

K±±00

- U reconstructed with only LKr calorimeter;
- Statistics analyzed: 28.0x106events in 1 month of 2003 (SS1-3);
- Statistical error with analyzed data: Ag(stat)=2.2x10-4;
- Extrapolation to 2003+2004 data: Ag(stat)=1.3x10-4.

|v|

M=1.1 MeV/c2

g fit for SS1

2=45.3/50

=4.8x10-4

u

u

m(3), GeV/c2

- Preliminary NA48/2 2003 result on CP-violating charge asymmetry in K± π±π+π–:Ag = (0.5±2.4stat.±2.1stat.(trig.)±2.1syst.)x10-4
- There is a room to decrease trigger efficiency error and systematic error;
- 2004 statistics are another 2x109 events, and expected to have smaller systematic effects;
- Ultimate goal: reach the precision of Ag=2x10-4.Experimental precision is better than the theoretical one.

Spare slides

- Rough alignment made by interpolation between muon alignment runs [3 muon runs in 2003];
- Fine-tuning of momenta: P’ = P∙(1+β)∙(1+qbP)
- P0 – measured momentum;
- P – corrected momentum;
- q – track charge;
- b – magnetic field sign.

- Time-dependent corrections:
- β– for magnetic field integral;
- – for spectrometer misalignment.

The following sources were studied:

- Variations of fitting limits cut;
- Use of various U definitions with different resolution properties.
Conclusion:

- Effect is <0.5·10-4.

Cinematic fitting routine

is being implemented

K±±00

The charge exchange process +00is not negligible under threshold,

and interferes (destructively) with direct emission

30M events

MC: no rescattering

Data

4m+2

4m+2

1 bin = 0.00015 GeV2

M(00), GeV/c2

K±±00

One-loop exchange: 2 = 463/149

One and two loops: 2 = 159/147

Standard Dalitz-plot

parametrization

2 = 133/139

for M(00)>80 MeV/c2

Incl. + atoms: 2 = 144/146

M(00), GeV/c2