Summary on CP Violation with Kaons

1
Summary on CP Violation with Kaons
Patrizia Cenci INFN Sezione di Perugia XX
Workshop on Weak Interaction and
Neutrinos Delphi, June 7th 2005 On
behalf of the NA48 Collaboration Cambridge,
CERN, Chicago, Dubna, Edinburgh, Ferrara,
Firenze, Mainz, Northwestern,
Perugia, Pisa, Saclay, Siegen,
Torino, Vienna
2
Overview

• Introduction
• CP Violation with Kaons
• Experiments KLOE, KTeV, NA48
• Results
• Direct CP Violation with neutral Kaons
• Charge Asymmetry in K0e3
• KS ? 3p0
• KS,L ? p0 ll-
• Direct CP Violation in K3p decays
• Prospects and conclusions

3
Introduction

• Why Kaons
• crucial for the present definition of Standard
  Model
• search for explicit violation of SM key element
  to understand flavour structure of physics
  beyond SM
• Motivation for Kaons experiments
• Test of fundamental symmetries
• CP Violation charge asymmetry, T violating
  observables
• CPT test tigher contraints from Bell-Steinberger
  rule, KS/KL semileptonic decays
• Sharpen theoretical tools
• Study low energy hadron dynamics ?PT tests and
  parameter determination, form factors
• Probe flavour structure of Standard Model and
  search for explicit violation
• Rare decays suppressed (FCNC 2nd order weak
  interactions) or not allowed by SM (e.g. Lepton
  Flavour Violation)
• Sensitivity to physics BSM

4
CP Violation with Kaons
CP Violation a window to physics beyond SM

• 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 decay with oscillation
  (Babar, Belle)
• 2004 Direct CP violation in B0 (Belle, Babar)

• CP Violation in Kaon decays can occur either in
  ?K0-K0 mixing or in the decay amplitudes
• Only Direct CP Violation occurs in K decays (no
  mixing)
• Complementary observables to measure Direct CP
  Violation in Kaons e/e, Ag , rare decays

5
Experiments with Kaons
CERN NA48 1997-2001 KL,KS NA48/1 2000,2002
KS NA48/2 2003-2004 K?
Active Experiments
Fermilab KTeV 1997,1999 KL,KS
Frascati - Da?ne KLOE gt 2000 KS ,KL ,K?
6
FNAL - KTeV Experiment

• Parallel K beams
• 2 proton lines ( 1012 ppp)
• KS from KL on Regenerator (scintillator plates),
  
• KS identification via x-y position
• switches beam line once per cycle
• ??- Magnetic Spectrometer
• ?(p)/p ? 0.17 ? 0.007 pGeV/c
• ?0?0 CsI calorimeter
• ?(E)/E ? 2.0/vE ? 0.45
• ?M(?0?0) ?M(??-) 1.5 MeV
• Photon veto and muon veto

?M(?0?0) ?M(??-) 1.5 MeV

Experimental Program KTeV 1997,1999 KL,KS
7
CERN - NA48 Experiment

• Simultaneous K beams (e/e)
• split same proton beam (1012 ppp)
• convergent KL-KS beams
• KS from protons on near target
• KS identification via proton tagging
• ??- Magnetic Spectrometer
  
• ?p/p 1.0 ? 0.044 x p GeV/c
• ?0?0 LKr Calorimeter
• ?E/E 3.2/vE ? 9/E ? 0.42 GeV
• Photon and muon veto

Beam pipe
Experimental Program NA48 1997-2001
KL,KS NA48/1 2000,2002 KS NA48/2 2003-2004 K?
?M(?0?0) ?M(??-) 2.5 MeV

8
LNF Da?ne the ? Factory

• ? Factory ee- collider_at_?s 1019.4 MeV M?
• ? Decays BR(??KLKS)34.3 BR(??KK-)49.31
• tagged K decays from ? ??KK ? pure K beams
• clean investigation of K decays and precision
  measurements
• KLOE data taking 2000-01-02-04-05

• New KLOE run in progress
• Lpeak 1.4 1032 cm-2s-1
• Goal collect 2 fb-1 by end 2005

May 2005
Integrated Luminosity
920106 f decays
550106 f decays
(Recent results from KLOE S. DellAgnello LNF
SC Open Meeting, may 05 and M. Martini Krare
Workshop_at_LNF, may 05)
9
LNF the KLOE detector
EM Calorimeter Lead and scintillating fibres
Drift Chamber Stereo geometry
10
Direct CP Violation experimental results on e/e

• Direct CPV established in K0?pp by NA48 and KTeV
  
• more results expected (KTeV, KLOE)
• no third generation experiments
• Result (roughly) compatible with SM
• Exclude alternative to CKM mechanism (superweak
  models and approximate-CP)
• Despite huge efforts, e/e not yet computed
  reliably due to large hadronic uncertainties
• Improvement of the calculation expected with
  lattice
• New physics may contribute as a correction to SM
  predictions

(16.7 ? 2.6) ? 10-4

• Final Result (1997-2001)

• Half Statistics (
  (1996-1997)

Re(??/?)
11
K0e3 Charge Asymmetry

• Charge Asymmetry in K0e3 is due to?K0-K0 mixing
  (Indirect CPV)
• Limits on CPT and ?S?Q
• Il CPT is conserved and ?S?Q
• Results in NA48 (2x108 Ke3) and KTeV (3x108
  Ke3)

? 2 ? Re(e)
KTeV dL(e) (3.322 ? 0.058stat ? 0.047syst) x
10-3
NA48 dL(e)(3.317 ? 0.070stat ? 0.072syst) x 10-3
PDG2004 dL(e) (3.27 ? 0.12) x 10-3
12
Semileptonic KS decays

• KLOE first measurement (2002), update in
  progress
• New preliminary result
• CPT Test new measurement of the charge
  asymmetry in KS
  
  (dL(e) 3.32 ? 0.07) ? 10-3 )

• Method
• KS tagged by opposite KL (? ??KK)
• Identify ?e pairs using TOF
• Event counting by fitting the E(?e)-P
  distribution (test for ?)
• Independent measurement of the two charge modes
• Selected 104 signal events per charge in the
  2001-02 data (0.5 fb-1)

Emiss Pmiss(MeV)
BR(KS ? pen) (7.09 ? 0.07stat ? 0.08syst) 10-4
dS(e) (-2 ? 9 ? 6) ? 10-3
13
CP Violation in KS? p0p0p0

• KS? 3p0 is CP violating CP(KS) ? 1, CP(3p0)
  - 1
• Allowed by SM, but never observed
• According to SM
• Last limit from direct search BR(KS?3p0) lt
  1.4x10-5 (SND, 1999)
• Can be parametrized with the amplitude ratio ?000
  
• The uncertainty on KS ? 3p 0 amplitude limits the
  precision on CPT test (Bell-Steinberger
  relation)

If CPT is conserved Re(?000) CPV in
mixing Im(?000) direct CPV
?000
?
14
KLOE search for KS?p0p0p0

• Direct search, new result
• Rarest decay studied by KLOE so far
• Data sample 0.5 fb-1 (2001-2002 run)
• 37.8 x 106 (KL-crash tag KS????)
• Require 6 prompt photons
• large background 40K events
• Kinematic fit, 2?0,3?0 estimators (z2, z3)
• After all analysis cuts (?3? 24.4)
• 2 candidate events found
• expected background 3.13 0.82 0.37

Submitted to Phys. Lett. hep-ex/0505012
Prospects with 2 fb-1 if background
negligible level UL will improve by factor 10
(down to few 10-8)
15
NA48/1 KS?p0p0p0 and ?000

• Measurement in NA48
• Sensitivity to ?000 from KS-KL interference
  superimposed on a huge flat KL?? ?0?0?0 component
• Aim O(1) error on Re(?000) and Im(?000)
• Method measure KS-KL interference near the
  production target
• use 3p0 events from near-target run for ?000
• normalize to KL? 3p0 from far-target run
• use MC to correct for residuals acceptance
  difference and Dalitz decays
• Time evolution of KL,S? 3p 0

16
NA48/1 results on KS?p0p0p0

• Data samples (run 2000)
• Near-target run 4.9?106 KL,S ?
  3p0 data 90?106 KL? 3p0 MC
• Far-target (KL) run 109?106 KL? 3p0
  data 90?106 KL? 3p0 MC
• Fit method fit double ratio
• Final Results (2004)
• Re(?000) -0.002 0.011stat. 0.015syst
• Im(?000) -0.003 0.013stat. 0.017syst
• ? lt 0.045 90 CL
• Br(KS?3p0) lt 7.4 ? 10-7 90 CL
• If Re(?000) Re(e) 1.66 ? 10-3 (CPT)
• Im(?000)CPT -0.000 0.009stat. 0.017syst
• ?CPT lt 0.045 90 CL
• Br(KS?3p0)CPT lt 2.3 ? 10-7 90 CL

Ratio of near-target and far-target data
corrected for acceptance (3 Energy intervals)
17
KLOE search for KS?pp-p0

• Motivation
• Present status
• BR(CPC) 3x10-7, BR(CPV) 1.2x10-9
• Direct measurement of CPC part possible with
  ultimate 2fb-1
• Measurement tests of prediction (untested) of ?PT
• Data sample 740 pb-1
• 373 pb-1 (2001/2 data) 367 (2004 data)
• Assuming BR3x10-7 230 signal events produced
• Prospect with 2 fb-1
• 16 events, of which 9 background
• 60 statistical accuracy on BR(KS ?pp-p0 )
• BR with accuracy below 50 competitive with
  other measurements, and the only direct search

E621 (1996) 4.82.2-1.6(stat)
1.1(syst) ? 10-7 CPLEAR (1997)
2.51.2-1.0(stat)0.5-0.6(syst) ? 10-7
PDG2004 (average) 3.2 1.2-1.0
? 10-7 ?PT 2.4
0.7 ? 10-7
18
Direct CP Violation in K3?

• K ? 3? matrix element
• M(u,v)2 1 gu hu2 kv2

p1even
BR(K?????)5.57 BR(K???0?0)1.73
p3odd
K
p2even
K? pp-p g -0.2154 K? p0p0p g
0.652 h, k ltlt g
Dalitz variables
K-K- asymmetry in g

• Direct CP violation
• if

i1,2,3
K? pp-p
i3 is the odd pion
NA48/2 search for Direct CPV by comparing the
linear slopes g for K
19
Experimental and theoretical status
Experimental results
Ford et al. (1970)
SM estimates of Ag vary within an order of
magnitude (few 10-6 to 8x10-5).
10-2
HyperCP prelim. (2000)
Ag
TNF prelim. (2002) - neutral mode
10-3
Models beyond SM predict substantial enhancements
partially within the reach of NA48/2. (theoretical
analyses are by far not
exhaustive by now)
NA48/2 proposal
neutral
10-4
charged
CPV asymmetry in decay width is much smaller
than in Dalitz-plot slopes Ag (SM 10-710-6)
10-5
Theory
10-6
SM
New physics
SUSY
20
NA48/2 goal and method

• Primary NA48/2 goal
• Measure slope asymmetries in charged and
  neutral modes with precisions dAg lt 2.2x10-4,
  and dAg0 lt 3.5x10-4, respectively
• Statistics required for this measurement gt 2x109
  in charged mode and gt 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

21
NA48/2 experimental set-up
magnet
K
K
beam pipe
focusing beams
K?
7?1011 ppp
K?

• Second Achromat
• Cleaning
• Beam spectrometer

• Front-end Achromat
• Split /-
• Select momentum
• Recombine /-

/- beams overlap within 1mm all along 114m
decay volume

• Quadrupole quadruplet
• Focusing
• µ swapping

He tank spectrometer
vacuum tank
not to scale
22
NA48/2 Data Taking
Data taking finished 2003 run 50 days 2004
run 60 days Total statistics in 2 years
K?? ????? 4x109 K?? ?0?0?? 2x108 200
TB of data recorded
This presentation first result based on 2003
K? ????? sample
23
K3? statistics
Data taking 2003 1.61x109 K ? ????? events
?M1.7 MeV/c2
Events
V
even pion in beam pipe
????
Invariant ??? mass
odd pion in beam pipe
Accepted statistics K 1.03 x109 events K?
0.58 x109 events K/K 1.8
U
24
Ag measurement strategy - 1

• Use only the slopes of ratios of normalized
  u-distribution
• Build u-distributions of K and K- events
  N(u), N-(u)
• Make a ratio of these distributions R(u)
• Fit a linear function to this ratio normalised
  slope ?g

e.g. uncertainty dAg lt 2.210-4corresponds to
d?g lt 0.910-4

• Compensate unavoidable detector asymmetry
  inverting periodically the polarity of the
  relevant magnets
• Every day magnetic field B in the spectrometer
  (up/down B/B-)
• Every week magnetic field A of the achromat
  (up/down A/A-)

25
Ag measurement strategy - 2
Four ratios are used to cancel acceptances
Spectrometer field
Y
X
beam line K Up
B
Jura (left)
Achromat
Z
B?
Saleve (right)
beam line K Down

• beam line polarity (U/D)
• direction of kaon deviation in the spectrometer
  (S/J)

• 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
26
Ag measurement strategy - 3
Quadruple ratio is used for further cancellation
R RUS ? RUJ ? RDS ? RDJ 1 4 ? Dg ? u

• Cancellation of systematic biases
• Beam rate effects global time-variable biases
  (K and K- simultaneously recorded)
• Beam geometry difference effects beam line
  biases (K beam up / K- beam up etc)
• Detector asymmetries effects (K and K-
  illuminating the same detector region)
• Acceptance is defined respecting azimuthal
  symmetry
• Effects of permanent stray fields (earth, vacuum
  tank magnetisation) cancels

The result is sensitive only to time variation
of asymmetries in experimental conditions
(beamdetector) with a characteristic time
smaller than the corresponding field-alternation
period (e.g. the
supersample time scale beam-week, detector-day)
27
Beam systematics

• Time variations of beam geometry
• Acceptance largely defined by central beam hole
  edge (R10 cm)
• Acceptance cut defined by a (larger) virtual
  pipe - centered on averaged beam positions - as
  a function of charge, time and K momentum

Y, cm
Sample beam profile at DCH1
Beam movements 2 mm
5mm
2 mm
Beam widths 5 mm
X, cm
28
Spectrometer systematics

• Time variations of spectrometer geometry
• DCH drifts by O(100µm) in a 3 month run
  asymmetry in p measurement
• alignment is fine tuned by forcing the average
  value of the reconstructed invariant 3? masses
  to be equal for K and K-

• Momentum scale
• due to variations of the magnet current (10-3)
• sensitivity to a 10-3 error on field integral
  ?M 100 keV
• mostly cancels due to simultaneous beams
• in addition, it is adjusted by forcing the
  average value of reconstructed invariant 3?
  masses to the PDG value of MK

DMppp
Maximum equivalent horizontal shift 200?m
_at_DCH1 or 120?m _at_DCH2 or 280?m _at_DCH4
29
Trigger systematics

• Measure inefficiencies using control data from
  low bias triggers
• Assume rate-dependent trigger inefficiencies
  symmetric

L2 trigger (online vertex reconstruction)
time-varying inefficienciy ( 0.2 to 1.8) flat
in u within measurement precision
u-dependent correction applied
L1 trigger (2 hodoscope hits) stable and small
inefficiency ( 0.710-3) no correction
L2 inefficiency
3x10-3
cut
cut
30
Fit linearity 4 Supersamples
SS2 ?g(-3.12.5)x10-4
?229.5/38
?239.7/38
SS0 ?g(0.62.4)x10-4
U
U
SS1 ?g(2.32.2)x10-4
SS3 ?g(-2.93.9)x10-4
?238.1/38
?232.9/38
U
U
31
Systematics summary and results
Conservative estimation of systematic uncertainties Effect on ?gx104
Acceptance and beam geometry 0.5
Spectrometer alignment 0.1
Analyzing magnet field 0.1
p??? decay 0.4
U calculation and fitting 0.5
Pile-up 0.3
Systematic errors of statistical nature
Trigger efficiency L2 0.8
Trigger efficiency L1 0.4
Total systematic error 1.3
Combined result in ?g x
104 units (3 independent analyses)
Raw Corrected for L2 eff
SS0 0.01.5 0.52.4
SS1 0.92.0 2.22.2
SS2 -2.82.2 -3.02.5
SS3 2.03.4 -2.63.9
Total -0.21.0 -0.21.3
?2 2.2/3 3.2/3
L2 trigger correction included
32
Stability of the result
?g (-0.21.0stat.0.9stat.(trig.)0.9syst.)x10-4
?g (-0.2 1.7) x 10-4
?g
Quadruple ratio with
K(right)/K(left)
K(up)/K(down)
K()/K(-)
?g
EK (GeV)
?g
4 supersamples give consistent results
control of detector asymmetry
control of beam line asymmetry
MC reproduces these apparatus asymmetries
zvertex (cm)
33
Preliminary result on Ag
Ag (0.52.4stat.2.1stat.(trig.)2.1syst.)x10-
4 Ag (0.53.8) x 10-4 NA48/2 - 2003 data
Smith et al. (1975) (N)
10-2
Ford et al. (1970) (C)

• This is a preliminary result, with conservative
  estimate of the systematic errors
• The extrapolated statistical error 200304 is
  ?Ag1.610-4
• 2004 data expected smaller systematic effects
  (more frequent polarity inversion, better beam
  steering)

Ag
HyperCP prelim. (2000) (C)
TNF (2004) (N)
10-3
NA48/2
N
C
10-4
10-5
New physics
SM
SUSY
10-6
34
Neutral mode asymmetry K???0?0

• Statistics analyzed 28 x 106 events (1 month of
  2003)
• Statistical error with analyzed data ?Ag 2.2
  10-4
• Extrapolation to 2003 2004 data ?Ag 1.3
  10-4
• Similar statistical precision as in charged
  mode
• Possibly larger systematics errors

35
Search for KS???0ee?
Motivation determination of the indirect CP
violating amplitude of the decay KL???0ee?
KL???0ee?

• BR(SM)3-10x10-12, BR(??ee?)6?10-7
• 3 contribution to this decay (?PT)
• A1(CPC) not predicted, derived from KL???0 ??
  (K2??0????0ee?, NA48, KTeV)
• A2(CPVInd) not predicted, measured by
  KS ??0ee- (NA48/1)
• A3(CPVDir) predicted in terms of CKM phase
  (electroweak penguins and W boxes with top)

KTeV limits (90CL) BR(p0ee) lt
2.8x10-10 BR(p0mm) lt 3.8x10-10
36
NA48/1 K0S ? p0ll-
Main motivation for the NA48/1 proposal
KS ?p0 ee
KS ?p0 mm
6 events, bkg. 0.220.19-0.12
7 events, bkg. 0.150.10-0.04
First measurement BR(p0ee) 5.8 2.8-2.3(stat)
0.8(syst) ? 10-9 as1.060.26-0.21 (stat)
0.07 (syst) PLB 576 (2003)
First measurement BR(p0mm) 2.9 1.4-1.2(stat)
0.2(syst) ? 10-9 as1.550.38-0.32 (stat)
0.05 (syst) PLB 599 (2004)
37
SM prediction for K0L ? p0ll-

• From KL measurement small CPC contribution
• From KS measurement indirect CPV contribution
  dominates
• Sensitivity of BR to CKM phase depends on the
  (unmeasurable) relative sign of the two CPV terms
  
• Theory constructive interference favored
• Sensitivity to new physics enhanced electroweak
  penguins would enhance the BR

Isidori et al. EPJC36 (2004)
Destructive
Constructive
J. Buras et al. hep-ph/0402112 NP B697 (2004)
Two indeopendent analysis G. Buchalla, G.
DAmbrosio, G. Isidori, Nucl.Phys.B672,387 (2003)
- S. Friot, D. Greynat, E. de Rafael,
hep-ph/0404136, PL B 595
38
Prospects and conclusions

• Kaon was central in the definition of SM
• Quantitative tests of CKM mechanism and search
  for new physics beyond SM are possible with rare
  Kaon decay mesurements
• High level of precision is attainable
• Constraints to CKM variables and further test of
  CPV from FCNC processes (golden decays)
• KL???0ee? decays
• K ? p?nn decays

39

• SPARE

40
The golden K ? p?l l decays

• Motivation
• FCNC processes, no tree level, proceed via loop
  diagrams
• access to quark level physics with small
  theoretical uncertanties
• dominant short distance contributions
• long distance only for charged lepton modes
• matrix elements of quark operators related to
  Ke3 decays
• CPV KL decays
• Charged leptons final states easier lepton
  identification but high levels of radiative
  background
• Best change K ? pnn decays
• no long distance contributions
• clean theoretical predictions
• no radiative background
• KL decay dominated by direct CPV

41
Why Kaon again?
Unitarity Triangle Vud Vub Vcd Vcb Vtd
Vtb 0
? and ? precise measurements important as
precision test of the CMK formalism - used to
describe CP and quark mixing -and search for new
physics
KL? ?0??? KOPIO_at_BNL KS ? ?0ee- KL?
?0ee- KL ? ?0?? KL ? ee- ??
h
(?,?)
K ? ???? P326_at_CERN
The study of K mesons allows quantitative tests
of the SM independent and complementary to B
physics
r
(1,0)
(0,0)
(1.4,0)
KL ? ??- KL ? ?? KL ? ee- ? KL ? ee-
ee- KL ? ee- ??-
42
Experimental prospects

• K0L ? p0nn
• Large window of opportunity exists.
• Upper limit is 4 order of magnitude from the SM
  prediction
• Expect results from data collected by E391a
  (proposed SES3 10-10)
• Next experiment KOPIO_at_BNL
• Future JPARC and KLOD_at_IHEP
• K0L ? p0ee(mm)
• Long distance contributions under better control
• Measurement of KS modes has allowed SM prediction
• KS rates to be better measured (KLOE?)
• Background limited (study time dep.
  Interference?)
• 100-fold increase in kaon flux to be envisaged
• K? pnn
• The situation is different 3 clean events are
  published
• Experiment in agreement with SM
• Next round of exp. need to collect O(100) events
  to be useful P326 at CERN
• Move from stopped to in flight experiments

43
KAon BEam Spectrometer (KABES)

• 3 MICROMEGA gas chamber stations
• measure beam particle
• charge (prob. mis-ID10-2)
• momentum (?p/p0.7)
• position in the 2nd achromat (?x,y?100?m).

• Measurement of kaon momentum
• Reconstruct K3p with a lost pion
• Redundancy in K3p analysis
• Resolve Ke4 reconstruction ambiguity.

Not used yet for K?3? analysis
E. Goudzovski / CERN, 1 March 2005
44
KL,S ? ????e?e? why?
For KL interference gives indirect CP-violating
asymmetry in the orientation of ???? and e?e?
decay planes Easier access to polarization
asymmetry in K ? ??? Large (? 14) asymmetries
predicted
45
KL,S ? ????e?e?
A? (13.3 ? 1.7)
KTeV KL
NA48A? (14.2 ? 3.6)
1.5K events
BR (3.63 ? 0.18) ? 10-7
NA48 BR (3.08 ? 0.2) ? 10-7
BR (4.69 ? 0.30) ? 10-5
NA48 KS,KL
No asymmetryA? (?1.1 ? 4.1)
46
KTeV ?- measurement

• Direct CPV
• Assuming G(KS?pe?)G(KL?pe?), the result is

(hep-ex/0406002)
? 2.7s discrepancy with PDG average