The first results on direct CP-violation asymmetry measurement in K
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The first results on direct CP-violation asymmetry measurement in K ±  π ± π + π – decays from the NA48/2 experiment. Evgueni Goudzovski (JINR, Dubna) CERN particle physics seminar 1 st March 2005 on behalf of the NA48 Collaboration:

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

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Evgueni goudzovski jinr dubna cern particle physics seminar 1 st march 2005

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


Overview

Overview

  • 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.


Direct cp violation in k 3

Direct CP-violation in K3

BR(K±±+)=5.57%; BR(K±±00)=1.73%.

  • Matrix element:

  • Kinematic variables

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

Lorentz-invariants

u = (s3-s0)/m2;

v = (s2-s1)/m2;

si = (PK-Pi)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)/m2;

v = 2mK∙(E1-E2)/m2.

Ag = (g+-g-)/(g++g-)≠0


Previous measurements of a g

Previous measurements of Ag

  • ”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;

  • ”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±;


Theoretical expectations

Theoretical expectations

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 and method

NA48/2 goals and method

  • 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.


Na48 2 experiment

54 60 66

PK spectra,

603GeV/c

BM

z

10 cm

200

250 m

1cm

50

100

NA48/2 experiment

magnet

K+

K+

focusing beams

K

~71011

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


Na48 2 narrow band beams simultaneous coaxial focused

PK spectra,

603GeV/c

54 60 66

NA48/2 narrow-band beams: simultaneous, coaxial, focused

 Pion decay products stay in beam pipe...


Kaon beam spectrometer kabes

KAon BEam Spectrometer (KABES)

  • 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


Na48 detector

NA48 detector

  • 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


L2 trigger for k data taking

L2 trigger for Kdata-taking

  • 3-track trigger

    • K±3π± [“charged” mode]

    • Ke4, events with Dalitz decay π0e+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…


Data taking finished

Data taking: finished

  • 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


K 3 selection

K3π selection

  • 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.


Selected statistics 2003

Selected statistics 2003

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


Measurement strategy

Measurement strategy

|M±(u,v)|2 ~ 1 + g±u + hu2+ kv2

  • PDG:

  • Kπ+π-π±: g = -0.2154±0.0035

  • Kπ0π0π±: 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


Acceptance cancellation

Acceptance cancellation

  • 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-


Supersamples 2003

Supersamples 2003

Statistics selected for Ag measurement, million events


Acceptance cancellation1

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-)

Acceptance cancellation

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).


More cancellations 1

More cancellations (1)

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)


More cancellations 2

More cancellations (2)

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


Beam spectra difference effect

Beam spectra difference effect

(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


Monte carlo simulation

Monte-Carlo simulation

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


Systematic effects 1

Systematic effects (1)

(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;


Systematic effects 2

Systematic effects (2)

(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.


1 beam geometry

(1): Beam geometry

  • 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)


Beams movement in time

Beams’ movement in time

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


Momentum dependence of beams

Momentum dependence of beams

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


Alternative to the virtual pipe not included into the analysis yet

Alternative to the “virtual pipe”[not included into the analysis yet]

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


Residual systematics

Residual systematics

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.


2 spectrometer alignment

(2): Spectrometer alignment

  • 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:

~200m @DCH1

or

~120m @DCH2

or

~280m @DCH4.


3 spectrometer magnetic field

(3): Spectrometer magnetic field

  • 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.


Spectrometer residual systematics

Spectrometer: residual systematics

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.


4 trigger efficiency

(4): Trigger efficiency

  • 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


Trigger efficiency systematics

Trigger efficiency systematics

  • 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.


5 stray magnetic fields

10-4

P kick(stray field)

P kick(spectrometer)

(5): Stray magnetic fields

  • 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

100T

Magnetic field

corrected for


6 7 decay accidentals

(6,7): π±±decay, accidentals

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


Other second order effects

Other second-order effects

  • 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:


Example of fits supersample 1

Example of fits: supersample 1

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


Asymmetry fits in supersamples

Asymmetry fits in supersamples

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


Stability of the result

Stability of the result

g x10-4

g x10-4

80

40

60

30

40

20

20

10

0

0

-20

-10

-40

-20

-60

-30

-80

-40


Time stability g vs supersample

Time-stability: g vs supersample

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 systematics

Result & systematics

Result: Δx104

L2 trigger systematics included


Preliminary result 2003 data

Preliminary result: 2003 data

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).


Comparison with other results

Comparison with other results

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


Neutral mode analysis

“Neutral” mode analysis

K±±00

  • 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


Conclusions

Conclusions

  • 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.


Evgueni goudzovski jinr dubna cern particle physics seminar 1 st march 2005

Spare slides


Theoretical predictions on a g

Theoretical predictions on Ag


Spectrometer calibration

Spectrometer calibration

  • Rough alignment made by interpolation between muon alignment runs [3 muon runs in 2003];

  • Fine-tuning of momenta: P’ = P∙(1+β)∙(1+qbP)

    • P0 – measured momentum;

    • P – corrected momentum;

    • q – track charge;

    • b – magnetic field sign.

  • Time-dependent corrections:

    • β– for magnetic field integral;

    • – for spectrometer misalignment.


Systematics u calculation fitting

Systematics: U calculation & fitting

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


Observation of cusp effect 1

Observation of “cusp-effect” (1)

K±±00

The charge exchange process +00is 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(00), GeV/c2


Observation of cusp effect 2

Observation of “cusp-effect” (2)

K±±00

One-loop exchange: 2 = 463/149

One and two loops: 2 = 159/147

Standard Dalitz-plot

parametrization

2 = 133/139

for M(00)>80 MeV/c2

Incl. + atoms: 2 = 144/146

M(00), GeV/c2


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