Recent CP violation measurements at CERN-NA48 experiment For the NA48 and NA48/2 collaborations: Cagliari, Cambridge, CERN, Chicago, Dubna, Edinburgh,

Recent CP violation measurements at

CERN-NA48 experiment

For the NA48 and NA48/2 collaborations: Cagliari, Cambridge, CERN, Chicago, Dubna, Edinburgh, Ferrara, Firenze, Mainz, Northwestern, Orsay,

Perugia, Pisa, Saclay, Siegen, Torino, Vienna, Warsaw

Cristina BiinoINFN, Torino

ICHEP 2008Philadelphia, Jul.29-Aug.5, 2008

ICHEP08 Cristina Biino 2

A brief history of NA48

2007 Ke2/K

2 runNA62

• NA48 (1997-2001):Direct CP-Violation in neutral K

Re(ε’/ε) = (14.7 ± 2.2)·10-4

• NA48/1 (2002):Rare KS decays and hyperons

• NA48/2 (2003-2004):Direct CP-Violation in charged K

• NA62 (2007-2008) and P326 (2008…): R(Ke2/K2), and proposal K+ +

KTeV (2008) 19.2 ± 2.1 10-4


NA48 detector

1 m

Decay productsFrom K decays

• Magnetic spectrometer

4 high-resolution DCHs

• Scintillator hodoscope

150 ps time resolution - fast trigger

• LKr electromagnetic calorimeter Quasi-homogeneous, high granularity,

excellent e/ discrimination

• Hadron calorimeter, veto counters, vetos

p/p = (1.0 0.044 p)% (p in GeV/c)

E/E = (3.2/√E 9.0/E 0.42)% (E in GeV/)


CP violation in K0 decays:

Measurement of | | with KL → decays


The CP violation parameter |

|

= + ’ = A(KL +–)

A(KS +–)

is a fundamental observable of CP violation

Indirect CPV Direct CPV

Experimental method:

Measure precisely the ratio R = BR(KL +–) / BR(KL e). Extract | | computed as: =

using the best single KS and KL lifetimes measurements and normalisation BR(KL e) and BR(KS +–) from KLOE, KTeV and NA48.

BR(KL +–)BR(KS +–)

KS

KL

KL ~ K2 + K1

CP = +1CP = 1

CP(+–) = + 1

KS ~ K1 + K2


| |: event selection

KL →

CP-violating process need to suppress dominant background channels (Ke3, K3, K3 ) by 4-5 orders of magnitude small background achieved: 0.5% data well described by MC about 47k events selected

selection based on electron identification in the LKr calorimeter ( E/p ≈ 1) about 5 x 106 Ke3 events selected with 0.5% background

KL e

Dedicated 2-day run with a pure, low intensity KL beam: ~80 x 106 2-tracks events recorded.


Results on | |

Uncertainty source Correction Uncertainty

Particle ID +1.34% 0.05%

K2 background –0.49% 0.03%

Muon ID +0.48% 0.18%

Trigger –1.29% 0.11%

Energy spectrum 0.20%

Radiative corrections

0.10%

MC statistics 0.10%

Total +0.04% 0.33%

Experimental value:

R = BR(KL +–) / BR(KL e ) = (4.835 0.022stat. 0.016syst.) 10–3

BR(KL+–)=(1.941 0.019) 10–3

Published in Phys. Lett. B645 (2007) 26

Radiative corrections: - +– Inner Bremsstrahlung (IB) component included

- direct emission (DE) component subtracted (CP conserving)

Corrections and systematics on R

CP violation parameter:

= (2.223 0.012) 10–3 BR(KL+–)

BR(KS+–)|+-| = KL

KS

ICHEP08 Cristina Biino

| |: comparison of results

The NA48 result is in very good agreement with KTeV and KLOE experiments

These new measurements contradict the PDG’ 04 value !

BR(KL+–) / BR(KLe) [10–3] |+–| [10–3]


Search for direct CP violation:

Measurement of the Ag and Ag

0 asymmetries in K and K 0 0 decays


CP Violation in K± 3 decays

-

-g

g g ΔgA

g g 2g

+

+

-= =

+

|M(u,v)|2 1 + gu + hu2 + kv2 + …

Dalitz plot forK± 3

Only direct CP violation is involved in charged kaon decays!

Matrix element:

Asymmetry parameter:

K± : g = - 0.21134

0.00017

K± : g = 0.626 0.007

If Ag 0 Direct CP Violation

2even

1even

3oddK±

SM predictions: |Ag| : 10- 5 - 10- 6

If Ag ~ 10-4 New Physics !

u = 2mK(mK/3-E3*)/m

2

v = 2mK(E1*-E2

*)/m2|h|, |k| << |g|

If CP invariance holds: gK+ = gK-


The NA48/2 approach

• NA48/2 method:- Use of intense simultaneous K+ and K- beams, superimposed in space, with narrow PK spectra

- Equalization of averaged K+ and K- acceptances by frequent alternations of magnet polarities (magnetic spectrometer, K beam lines)

- Measurement of asymmetry from slopes of normalized u-distribution ratios (no MC acceptance correction required! ) :

R(u) = N+(u) / N-(u) n ( 1+g u)

Ag = g / 2g

Induced instrumental asymmetry can only be due to charge-asymmetric and u-dependent effects that vary with time

(-70 53) 10-4 BNL AGS (1970)

Ag=

( 22 40) 10-4 HyperCP (2000)prelim.

{ (19 125) 10-4 CERN PS (1975)

Ag0 =

(2 19) 10-4 Protvino IHEP(2005)

{

• NA48/2 main goal: Measure linear slope (g) asymmetries for K

and K decays with accuracies Ag 2.2 10-4 and Ag0 3.5

10-4 respectively. significant improvement w.r.t. present measurements

Statistics required >2109 events in “charged” mode and >108 in “neutral” mode

Selection of K events

U

|V| even pion

in beam pipe

2.00x109 K+

events1.11x109 K– events

K+/K– ≈ 1.8

odd pionin beam pipe

Invariant mass

Based only on Hodoscope and Magnetic Spectrometer

Negligible

background

m = 1.7 MeV/c2

Events 3-tracks vertex in decay volume

Acceptance limited mostly by beam pipe through DCHs K±


Acceptance equalization for K+ and K-

Fit of R(u) is sensitive to the time variation of asymmetries in experimental conditions with characteristic time smaller than corresponding field alternation periods (beam week,detector day (2003), 3 hours(2004 ) data taken in 9 SUPER-SAMPLES of ~2 weeks each

Cancellation of systematic biases: - Detector L-R asymmetries ( K+ Salève / K- Salève and K+ Jura / K- Jura ) - Beam line biases ( K+ beam Up / K- beam Up and K+ beam Down / K- beam Down) - Global time-variable biases (K+ and K- simultaneously recorded)

ZZ

XXYY

Achromats: KAchromats: K+ + UpUp

Achromats: KAchromats: K++ DownDown

B+B+

B-B-

Detector Left-Right asymmetry cancellation in the four K+/K ratios:

US

N ( , ,u)R (u)

N ( , ,

A

u)

B

BA

+

-

+ +-+

=

SD

N ( , ,u)R (u)

N ( , ,u)

A

B

B

A

+

-

+-- -

=

JU

N ( , ,u)R (u)

N ( , ,u)

A

B

B

A

+

-

-++ +

=

Jura Jura (Left)(Left)

SalèveSalève(Right)(Right)

Quadruple ratio:

R(u) = RUS(u)·RUJ(u)·RDS(u)·RDJ(u) ~ n· (1 + 4 Δg u) DJ

N ( , ,u)R (u)

N ( , ,

A

u)

B

BA

+

-

- -+-

=

♦

♦

♦

♦


K± → ± : Ag final result

Δg = (0.6 ± 0.7stat. ± 0.4stat. (trig.) ± 0.5syst.) 10-4

K+/K-

Result compatible with SM predictions No evidence for New Physics

Published in Eur. Phys. J. C 52, 875 (2007)

Δg

9 super-samples give consistent results

Detector and beam line asymmetry effects at 10-4 level, reproduced by MC

Ag = (-1.5 ± 2.1) 10-

4A factor ~ 20 better precisionthan previous experiments

Systematic effects on Δg

Δg

(10-4)

Spectrometer alignment ± 0.1

Spectrometer magnetic field

± 0.3

Beam geometry and stray magnetic fields

± 0.2

Resolution and fitting ± 0.2

Accidental activity

(pile-up)± 0.2

Total systematics ± 0.5

L1 trigger efficiency ± 0.3

L2 trigger efficiency-0.1 ±

0.3

Time stability of result:


Selection of K 0 0 events

Consider two 0 → decays which define

K± decay vertex position

- no geometrical information from the ± track to avoid charge-asymmetric biases - kaon momentum and invariant 3 cuts

Computation of u variable from the

0 0 invariant mass only:

charge-symmetric procedure

Based on Charged Spectrometer and LKr Electromagnetic Calorimeter

m = 0.9 MeV/c2

|v| Odd pion

in beam pipe

and wrong photon

pairings

M(0)2 EiEk (Dik)2/(Zik)2

9.13 107 events

K±

uEven

ts

M(3) [GeV/c2]


K± → ± 0 0 : Ag0 final result

Statistical precision in Ag0 similar to K± → ±

mode despite 34 times less events: |g0 / g±| ~ 3.

Ag0 = (1.8 ± 1.8) x 10-4

Systematic effects on Δg

Δg

(10-4)

Overlap of LKr showers ± 0.5

L1 HOD trigger efficiency ± 0.1

L1 LKr trigger efficiency ± 0.1

L2 trigger efficiency ± 0.3

Stray magnetic fields ± 0.1

Accidental activity(pile-up) ± 0.2

Total ± 0.6

Final result consistent with SM predictions:

Published in Eur. Phys. J. C 52, 875 (2007)

Δg = (2.2 ± 2.1stat. ± 0.6syst.) 10-4

Time stability of result:

K+/K-

Δg

7 super-samples give consistent results

Detector and beam line asymmetry effects at 10-4 level,reproduced by MC


|Vus | Measurement from K±l3


|Vus | Measurement from K ± l3

K± collected during a special low intensity minimum bias run

Measurement of leptonic and semileptonic decays.Method of Measurement:

• Normalize Ke3 and K3 to K2 very similar topologies and selection criteria• Select one track and two photons, consistent with a 0, from a common decay vertex• Distinction of Kl3 and K2 mainly through kinematics• Particle identification for electrons ( ~ 98.6%), pions ( ~ 99.5%) and muons ( ~ 99.8%)



Channel Acc x P-Id K+ (K-)

Ke3: 56k (31k)

Kμ3: 49k (28k)

π± π0 462k (257k)

Acceptance x Particle ID K+

(K-)

6.98 ± 0.01 % (6.94 ± 0.01)%

9.27 ± 0.01 % (9.25 ± 0.01)%

14.18 ± 0.01 % (14.12 ± 0.01)%

Background

< 0.1%

~ 0.2%

~ 0.3%



RKe3/K2 = 0.2470 0.0009stat 0.0004sys

RK3/K2 = 0.1636 0.0006stat 0.0003sys

• Quadratic expansion for f+(t) (’+ = 0.02485(163), ”+ = 0.00192(62)) and linear expansion for f0(t) (0 = 0.00196(34) from PDG2006 • Form factor variations within their errors and differences to pole model parametrization taken as systematic uncertainties

Results:

Sistematics:

Ke3/K2: Mainly Ke3, K2 acceptance, trigger efficiency

K3/K2: Mainly K3 form factors and Ke3, K2 acceptance

Accuracy 0.4%

Published in Eur. Phys. J. C 50, 329 (2007) & erratum

Form factors:

Using new KLOE measurement of

BR(K2()) = 0.2065(5)(8)

BR(K± e3) = 0.05104 0.00019stat 0.00008sys

BR(K±3) = 0.03380 0.00013stat

0.00006sys



Ke3 : |Vus | f+(0) = 0.21794(43)exp (52)norm, (61)ext = 0.2179(9)

K3 : |Vus | f+(0) = 0.21818(46)exp (52)norm, (66)ext = 0.2182(10)

Use K = 12.370(19) ns (average PDG2006 & Kloe

2008), ......

Very good agreement between Ke3 and K3

Test of lepton universality:

Ratio(K3/Ke3) = 0.663 0.003stat 0.001sys

Most precise measurement so far and consistent with lepton

universality (SM prediction = 0.661(3))

Combination of Ke3 and K3 (taking into account correlation) :

| Vus | f+(0) = 0.2180 0.0008and finally using f+(0) = 0.964 0.005

(RBC-UKQCD’07) :

| Vus | = 0.2261 0.0014

Determination of |Vus |:


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Recent CP violation measurements at CERN-NA48 experiment For the NA48 and NA48/2 collaborations: Cagliari, Cambridge, CERN, Chicago, Dubna, Edinburgh,