Search for direct CP-violationin K± ±+– decays by
NA48/2Ivan Mikulec
HEPHY Vienna, AustriaOn behalf of the NA48/2 collaboration:
Cambridge, CERN, Chicago, Dubna, Edinburgh, Ferrara, Firenze, Mainz, Northwestern, Perugia, Pisa, Saclay, Siegen,
Torino, Vienna
XXXXth Rencontres de Moriond EWI&UTLa Thuile March 5-12 2005
I. Mikulec: Search for direct CP-violation in K± by NA48 2
Overview
Direct CP violation in K±→3 decaysNA48/2 experimental setupMeasurement principleSystematic effectsPreliminary result in K± ±+– decay mode
Outlook for K±±00 analysisConclusions
I. Mikulec: Search for direct CP-violation in K± by NA48 3
Brief history of CP violation
1964 - CP violation in K0 (Cronin, Christenson, Fitch, Turlay)1993-99 - Direct CP violation in K0 (NA31, NA48, KTeV)
2001 - CP violation in B0 (Babar, Belle)2004 - Direct CP violation in B0 (Belle, Babar)
CP violation and especially direct: A window to physics beyond SM
Complementary observables in Kaons: ’/↔Ag↔rare decays
Look for direct CP violation in K± !(only direct CPV in K± possible – no mixing)
I. Mikulec: Search for direct CP-violation in K± by NA48 4
2 21 gu v uM( , ) h ku v K±→3matrix element:
Direct CP-violation observable Ag
2 1
03 s
s
s
m
sm
u
v
2
10 3
i K i
i
s (P p )
s s
0gA
K+-K- asymmetry in g:
gg gg g
A
Direct CP violation
Dalitz variables
i=3 odd pion
Kπ+π-π±: g = -0.2154Kπ0π0π±: g = 0.652
|h|, |k| << |g|2even
1even
3oddK±
Kπ+π-π±
if
I. Mikulec: Search for direct CP-violation in K± by NA48 5
Experimental and theoretical status
10-6
|Ag|
Experimental results
10-5
10-4
10-3
10-2
Ford et al. (1970)
HyperCP prelim. (2000)
TNF prelim. (2002)“neutral” mode
NA48/2proposal
SM estimates of Ag vary within an order of
magnitude (few 10-6…8x10-5).
“charged”“neutral”
Theory
Asymmetry in decay widths A
expected to be smaller than in Dalitz-plot slopes Ag
(SM: ~10-7…10-6).
SMSM SUSYSUSY NewNewphysicsphysics
Models beyond SM predict substantial enhancements partially
within the reach of NA48/2.(theoretical analyses are by far not
exhaustive by now)
I. Mikulec: Search for direct CP-violation in K± by NA48 6
Goals and methodPrimary NA48/2 goals:
– Measure slope asymmetries in “charged” and “neutral” modes with precisions δAg<2.2x10-4, and δAg
0<3.5x10-4, respectively.
– Statistics required for this measurement: >2x109 in “charged” mode and >108 in “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;– Equalise K+ and K– acceptances by frequently alternating
polarities of relevant magnets.
I. Mikulec: Search for direct CP-violation in K± by NA48 7
Experimental setupPK spectra, 603 GeV/c
54 60 66
1cm
50 100
10 cm
200 250 m
He tank+ spectrometer
Front-end achromat
• Momentum selection
Quadrupole quadruplet• Focusing• sweeping
Second achromat
• Cleaning• Beam spectrometer
~71011
ppp
K+
K
Beams coincide within ~1mm
all along 114m decay volume
focusing beamsfocusing beams
Analysingmagnet
vacuum tank
not to scale
K+
K
beam pipebeam pipe
I. Mikulec: Search for direct CP-violation in K± by NA48 8
Data taking
2003 run: ~ 50 days2004 run: ~ 60 days
Total statistics in 2 years: K +: ~4x109
K 00: ~2x108
~ 200 TB of data recorded
First result based on 2003 K± ±+– sample will be presented here
I. Mikulec: Search for direct CP-violation in K± by NA48 9
Accepted statistics
U
|V|
even pionin beam pipe
Data-taking 2003: 1.61x101.61x1099 KK±± ±±++–– events
KK+ + : 1.03 x10: 1.03 x109 9 eventseventsKK : 0.58 x10: 0.58 x109 9 eventsevents
KK++/K/K–– ≈ 1.8≈ 1.8
odd pionin beam pipe
M=1.7 MeV/c2
Even
ts
Invariant mass
I. Mikulec: Search for direct CP-violation in K± by NA48 10
11
1N ( )N ( )
g uuu ug
R( ) R R( )u gu
Principle of the experiment
2g
g gA
• Build u-distributions of K+ and K-: N+(u),N-(u)• Make a ratio of these distributions: R(u)• Fit a linear function to this ratio: normalised slope ≈ g
e.g. uncertaintyδAg<2.2∙10-4
corresponds to δg<0.9∙10-4
But, achromat and spectrometer magnet unavoidable sources of apparatus asymmetry!
I. Mikulec: Search for direct CP-violation in K± by NA48 11
Four ratios
ZZ
XXYY
JuraJura(left)(left)
SaleveSaleve(right)(right)
Achromats: KAchromats: K+ + UpUp
Achromats: KAchromats: K++ DownDown
B+
B
Indeces of R’s’s correspond to• beamline polarity (UU/DD)• kaon deviation in spectrometer mag. field (SS/JJ).
B K
BJ K
AD A B K
A B
N( )N( )
N( )N
DJ
A B KUS A
A KU A B
B KS
KB
( )
N( )N( )
N( )N( )A K
R
R
R
R
Supersample data taking strategy:– achromat polarity (A) was reversed on weekly basis;– spectrometer magnet polarity (B) was reversed on daily basis.
1 Supersample ~ 2 weeks 2003 data 4 supersamples
I. Mikulec: Search for direct CP-violation in K± by NA48 12
Quadruple ratio
R = RUSRUJRDSRDJ ~ 1+4g·u3-fold cancellation of systematic biases:1) Global time-variable biases (K+,K- simultaneously recorded)2) Beam line biases (K+ beam up / K- beam up etc.)3) Detector asymmetries (K+ toward Saleve / K- toward Saleve etc.)
In addition, acceptance is defined respecting azimuthal symmetry4) Effects of permanent stray fields (earth, vacuum tank magnetisation) cancel
The result is sensitive onlyonly totime variationtime variation of asymmetriesasymmetries
in experimental conditionswith a characteristic time smaller than
corresponding field-alternation period (beam-week, detector-day)
I. Mikulec: Search for direct CP-violation in K± by NA48 13
Monte Carlo simulation
Still MC is used to study systematics. MC features:– Based on GEANT;– Full detector geometry and
material description;– Local DCH inefficiencies
simulated;– Variations of beam geometry
and DCH alignment are followed;– Simulated statistics similar to
experimental one.
Example of data/MC agreement:mean beam positions @DCH1
K+ dataK dataK+ MC K MC
K+
K
Due to acceptance cancellations, the analysis does not rely on Monte-
Carloto calculate acceptance
I. Mikulec: Search for direct CP-violation in K± by NA48 14
Beam systematicsTime variations of beam geometry
Acceptance largely defined by central beam hole edge (~10 cm radius)
Acceptance cut defined by (larger) “virtual pipe” centered on averaged beam positions as a function of charge, time and K momentum
Y, c
mSample beam profile at DCH1
0 0.4 0.8-0.4-0.8
0
0.4
0.8
-0.4
-0.8
X, cm X, cm
Beam movements: ~ 2 mm
Y, c
m
A-K+A+K-
A-K-A+K+
5565
55
65
x, cm
y, cm DCH1
A-K+A+K-
A-K-A+K+
5565
55
65
x, cm
y, cm DCH1
x,y-beam vs. K momentum
Beam widths: ~ 5 mm
5mm
2mm
I. Mikulec: Search for direct CP-violation in K± by NA48 15
Spectrometer systematicsTime variations of spectrometer geometry - Alignment is fine tuned by forcing mean reconstructed invariant masses to be equal for K+ and K-
Maximumequivalent
horizontal shift:~200m @DCH1
or~120m @DCH2
or~280m @DCH4
E.g. sensitivity to DCH4 horizontal shift: M/x 1.5 keV/m
Momentum scale variation due to limited control of spectrometer magnet current (10-3) cancels due to simultaneous beams
In addition, it is adjusted by forcing mean reconstructed invariant masses to PDG value of MK+
M
I. Mikulec: Search for direct CP-violation in K± by NA48 16
Trigger systematicsInefficiencies measured using control data from low bias triggers.Rate-dependent parts of trigger inefficiencies assumed to be symmetric.
L2 inefficiency
L2 trigger (online vertex reconstruction): time-varying inefficiency (local DCH inefficiencies) 1-≈ 0.2% to 1.8% flat in u within measurement precision
u-dependent correction applied
3x10-3
cut cut
statistical uncertainty from control sample
L1 trigger (2 hodoscope hits): stable and small inefficiency 1-≈ 0.7·10-3
(no correction)
L2correction Δgx104
SS0 0.5±1.8SS1 1.4±1.0SS2 -0.2±1.2SS3 -4.5±1.9
Possibility to use MC studied
I. Mikulec: Search for direct CP-violation in K± by NA48 17
Other systematics
No magnetic field correction
Magnetic fieldcorrected for
• Bias due to resolution in u calculation• Sensitivity to fitting interval and method• Effects connected to → decay • Effects due to event pile-up• +/- interactions in material• Track charge misidentification
Further systematic effects studied:
Residual effects of stray magnetic fields (magnetised vacuum tank, earth field)
minimised by explicit field map correction
Field map in decay volume: Y projection
Decay volume: Z coordinate
I. Mikulec: Search for direct CP-violation in K± by NA48 18
Fit linearity – four supersamples22=39.7/38=39.7/38SS0: SS0:
g=(0.6±2.4)x10g=(0.6±2.4)x10-4-4
22=38.1/38=38.1/38SS1: SS1: g=(2.3±2.2)x10g=(2.3±2.2)x10-4-4
22=29.5/38=29.5/38SS2: SS2: g=(-g=(-3.1±2.5)x103.1±2.5)x10-4-4
22=32.9/38=32.9/38SS3: SS3: g=(-g=(-2.9±3.9)x102.9±3.9)x10-4-4
U
U
U
U
I. Mikulec: Search for direct CP-violation in K± by NA48 19
Time stabilityQuadruple ratio with
K(right)/K(left) K(up)/K(down)K(+)/K(-)
4 supersamplesgive consistent
results
controlof detectorasymmetry
controlof beam lineasymmetry
By regrouping the components in quadruple ratio – check residual detector and beam line asymmetries (~few 10-4) – they cancel safely in g fits
g
MC can reproduce these apparatus
asymmetries
I. Mikulec: Search for direct CP-violation in K± by NA48 20
Systematics summary and resultPreliminary estimates of systematic uncertainties
Effect on
Δgx104
Acceptance and beam geometry
0.5
Spectrometer alignment 0.1Analyzing magnet field 0.1± decay 0.4U calculation and fitting 0.5Pile-up 0.3Syst. errors of statistical natureTrigger efficiency: L2 0.8Trigger efficiency: L1 0.4Total systematic error 1.3
Raw Corrected for L2 eff
SS0 0.0±1.5 0.5±2.4
SS1 0.9±2.0 2.2±2.2
SS2 -2.8±2.2 -3.0±2.5
SS3 2.0±3.4 -2.6±3.9
Total
-0.2±1.0
-0.2±1.3
2 2.2/3 3.2/3
Combined preliminary result: in Δgx104 units
(3 independent analyses) Including L2 trigger
correction
I. Mikulec: Search for direct CP-violation in K± by NA48 21
Preliminary result K±±+- (2003 data)
Δg =(-0.2±1.0stat.±0.9stat.(trig.)±0.9syst.)x10-4
Δg =(-0.2±1.7)x10-4
Ag = (0.5±2.4stat.±2.1stat.(trig.)±2.1syst.)x10-4
Ag = (0.5±3.8)x10-4• This is a preliminary result with conservative estimate of systematic uncertainties• Extrapolated statistical uncertainty 2003+2004: Ag=1.6x10-4
• Expect smaller systematic effects in 2004 data (due to more frequent polarity alternation, better L2 performance).
Converted to asymmetry:
I. Mikulec: Search for direct CP-violation in K± by NA48 22
Comparison K±±+-
NA48/2 prelim.: 2003 data
10-6
|Ag|
10-5
10-4
10-3
10-2
SMSM SUSYSUSY
Ford et al. (1970)
HyperCP prelim. (2000)
NA48/2 goal:2003-04 data
NewNewphysicsphysics
This preliminary
result is already an
order of magnitude better than previous
experiments
I. Mikulec: Search for direct CP-violation in K± by NA48 23
K±±00 analysis∙ u can be reconstructed with LKr calorimeter only∙ Statistics analyzed: 28 × 106 events (1 month of 2003)∙ Statistical error with analyzed data: Ag(stat)= 2.2 × 10−4
∙ Extrapolation to 2003+2004 data: Ag(stat)= 1.3 × 10−4
∙ Similar statistical precision as in “charged” mode∙ Possibly larger systematic errors
v
-1.5 -1.0 -0.5 0 0.5 1.0 u
0
-1
-2
1
2
3Dalitz-plot
Statistical precision in Ag0
similar to “charged” mode: - Ratio of “neutral” to “charged” statistics: N0/N±~1/20 (√=1/4.5) - Ratio of slopes: |g0/g±|3 - More favourable Dalitz-plot distribution (gain factor f~1.5)
I. Mikulec: Search for direct CP-violation in K± by NA48 24
Observation of scattering effect in K→3 decays
1 bin = 0.00015 GeV2
MC (no rescattering)
Data
K±±00
M(00) GeV/c 2
4mπ+2
Charge exchange process +00 not negligible under 2m threshold,destructive interference generates a cusp in the Dalitz plot,
not seen earlier by lower precision experiments
30M events
4mπ+2
Great potential for a new accurate measurement of ππ scattering lengths
from this data
New preliminary result at Moriond QCD!
I. Mikulec: Search for direct CP-violation in K± by NA48 25
Conclusions Preliminary NA48/2 result (2003 data) on direct
CP−violating charge asymmetry in K± ±+– decays is Ag = (0.5 ± 2.4stat. ± 2.1stat.(trig.) ± 2.1syst.)×10-4
>10 times better precision than previous measurements Further room to decrease systematic uncertainties 2004 data contain another 2×109 K± ±+– events, with
higher quality
Neutral mode asymmetry: complementary, comparable sensitivity
Design goal is within reach in both decay modes
Other interesting results will follow ( scattering lengths, other CP asymmetries, rare decays)
I. Mikulec: Search for direct CP-violation in K± by NA48 26
Spare slides
I. Mikulec: Search for direct CP-violation in K± by NA48 27
Theoretical predictions of Ag
StandardModel
L.Maiani, N.Paver ’95 (2.3±0.6)x10-6
A. Bel’kov ’95 <4x10-4
G.D’Ambrosio, G.Isidori ’98 <10-5
E.Shabalin ’01 <3x10-5
E.Gamiz, J.Prades, I.Scimemi ’03
(-2.4±1.2)x10-
5
E.Shabalin ’05 (La Thuile’05) <8x10-5
SUSY G.D’Ambrosio, G.Isidori, G.Martinelli
~10-4
Newphysics
E.Shabalin ’98 [Weinberg model of extended Higgs doublet]
~4x10-4
I.Scimemi ’04 >3x10-5
I. Mikulec: Search for direct CP-violation in K± by NA48 28
Experimental results so far””Charged” mode KCharged” mode K±±33ππ±± : :
Ford et al. (1970) at BNL: Ag=(-7.0±5.3)∙10-3; Statistics: 3.2M KStatistics: 3.2M K±±;;
HyperCP prelim. (2000) at FNAL: Ag=(2.2±1.5±3.7)∙10-3; Statistics: 41.8M KStatistics: 41.8M K++, 12.4M K, 12.4M K;; Systematics due to knowledge of magnetic fields;Systematics due to knowledge of magnetic fields; Published as PhD thesis W.-S.Choong LBNL-47014 Berkeley Published as PhD thesis W.-S.Choong LBNL-47014 Berkeley 2000;2000;
””Neutral” mode KNeutral” mode K±±ππ±±ππ00ππ00 : :Smith et al. (1975) at CERN-PS: Ag
0=(1.9±12.3)∙10-3; Statistics: 28000 KStatistics: 28000 K±±;;
TNF (2004) at IHEP Protvino: Ag0=(0.2±1.9)∙10-3;
Statistics: 0.52M KStatistics: 0.52M K±±..
I. Mikulec: Search for direct CP-violation in K± by NA48 29
NA48 detector
Main detector components:
• Magnetic spectrometer (4 DCHs): redundancy redundancy high efficiency; high efficiency; ΔΔp/p = 1.0% + 0.044%*p [GeV/c]p/p = 1.0% + 0.044%*p [GeV/c]
• Hodoscope fast trigger;fast trigger; precise time measurement (150ps).precise time measurement (150ps). • Liquid Krypton EM calorimeter (LKr) High granularity, quasi-High granularity, quasi-homogenious;homogenious; ΔΔE/E = 3.2%/√E + 9%/E + 0.42% E/E = 3.2%/√E + 9%/E + 0.42% [GeV].[GeV].
• Hadron calorimeter, muon veto counters, photon vetoes.
Beam pipe
I. Mikulec: Search for direct CP-violation in K± by NA48 30
KAon Beam Spectrometer (KABES)
3 MICROMEGA gas chamber stations
measure beam particle• charge (prob. mis-ID~10-2);• momentum (p/p=0.7%);• position in the 2nd achromat (x,y100m).
Not used yet forK±3± analysis
Measurement of kaon momentum:
• Reconstruct K3π with a lost pion;• Redundancy in K3π analysis;• Resolve Ke4 reconstruction ambiguity.
I. Mikulec: Search for direct CP-violation in K± by NA48 31
Break down of K±±+- statistics
Dates Sub-sample
s
Achromat A+ Achromat A–KK++ KK– KK++ KK–
0 22.06-25.07
26 229.6229.6 125.9125.9 201.0201.0 114.0114.0
1 6.08-20.08 12 122.5122.5 68.168.1 135.1135.1 75.475.42 20.08-3.09 12 147.2147.2 81.881.8 105.5105.5 58.958.93 3.09-7.09 4 40.640.6 22.622.6 54.554.5 30.430.4
Total 54 Total events selected 1613.2
Statistics selected for Ag measurement, events x106
I. Mikulec: Search for direct CP-violation in K± by NA48 32
Cancellation of beam spectraAchromat reversalAchromat reversalreverses reverses KK++ and and KK––
beam spectrabeam spectraSystematic differencesSystematic differencesof of KK++ and and KK–– acceptanceacceptance
due to beam spectradue to beam spectramostly cancelmostly cancel in in RRUU**RRDD
Supersample 1Supersample 1 Supersample 2Supersample 2 SS 3SS 3
Systematic check:Systematic check:Reweighting KReweighting K++ eventseventsso as to equalise so as to equalise momentum spectramomentum spectraleads to negligible effectleads to negligible effectΔΔg=0.03x10g=0.03x10-4-4
KK++
KK––
I. Mikulec: Search for direct CP-violation in K± by NA48 33
Fits to the “cusp” effect in K±±00
datafit - data
M(00), GeV/c2
Standard Dalitz-plot parametrization
2 = 133/139for M(00)>80
MeV/c2
One-loop exchange: 2 = 463/149
One and two loops: 2 = 159/147
Incl. + atoms: 2 = 144/146
M(00), GeV/c2
Cabibbo: PRL 93 (2004) 121801
Cabibbo, Isidori: hep-ph/0502130