New Results on Rare Kaon Decays in NA48

1K. Holtz • NA48 Collaboration Moriond 17. - 24.03.2001

New Results on Rare Kaon Decays in NA48

Kirsten HoltzUniversität Mainz

on behalf of the

NA48 Collaboration(Cagliari,Cambridge, CERN, Dubna, Edinburgh, Ferrara,Firenze,

Mainz, Orsay, Perugia, Pisa, Saclay, Siegen, Torino, Vienna, Warsaw)

K. Holtz • NA48 Collaboration

2Moriond 17. - 24.03.2001

Overview

New Results on rare KS and KL decays:

•KS

•KS 0 e+e-

•KS,L +- e+e-

•KL 30 Dalitz plot slope


3Moriond 17. - 24.03.2001

The NA48 Kaon Beams

1999: 2 days HIKs run: 2.3108 KS decays; no KL beam

1997-1999: 3.21010 KLdecays/year; 6.5107 KS

decays/year


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Magnetic Spectrometer:

Liquid Krypton Calorimeter:

The NA48 Detector

% 99 efficiency plane

50cm 2mm

[GeV/c] p % 009 0 %50 p/p

y x,

z

. .

σ σ

%50 EMeV100

]GeV[E

%23E/E ..

Kevlar window

Drift chamber 1

Anti counter 6

Drift chamber 2

Magnet

Drift chamber 3

Helium tank

Anti counter 7Drift chamber 4

Hodoscope

Liquid krypton calorimeter

Hadron calorimeter

Muon veto sytem


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• KS,L are sensitive to higher order loop effects of

Chiral Perturbation Theory, no short distance contributions

unambiguous prediction of branching ratio

• Br (KL already measured precisely

• Theoretical prediction (by PT):

Br (KS = (2.3 ± 0.2)×10-6

• NA31 measurement (1995):

Br (KS = (2.4 ± 0.9)×10-6

Ks


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• Two clusters in LKR calorimeter required

• Kaon energy between 60 GeV and 170 GeV

• LKR center-of-gravity < 7 cm

• no hadronic energy, no DCH hits, no AKS/AKL hits

• decay region: -2 < zvertex< 5 m

KS 0 0 with two missing

’s shifts vertex down-stream by > 7m

K

ijji

m

dEE

LKrvertex -zz

KS Event Selection

0

100

200

300

400

500

-1500 -1000 -500 0 500 1000 1500Zvertex [cm]

2

signa

l

from AKS

from


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KS Result

• 450 K candidates in signal region after all cuts• background subtraction:

11 ± 8 n(center-of-gravity side-band subtraction)

2 ± 2 KS

• maximum likelihood fit tosignal region + subtracting

KL contribution:

149 ± 21 K149 ± 21 KS S

• Normalization: 7.5 106 KS

0

100

200

300

400

500

-400 -200 0 200 400 600 800 1000 1200

Fitted KS KL

Data

background from 2(MC)

signal

Zvertex [cm]

Br (KS

statsyst10-6


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• KL 0 e+e- : CP violating and CP conserving contributions

• KS 0 e+e- directly connected to the amplitude of the indirect

CP violations in KL 0 e+e-

• Theoretical expectation:

Br (KS 0 e+e-) = 5.2 × 10-9 as ~ O(1)

aS: strength of the indirect CP violating component in KL 0 e+e-

important for future measurements of KL 0 e+e-

• NA31 measurement (1993):

Br (KS 0 e+e- < 1.1 × 10-6 (90% CL)

KS 0 e+e-

2Sa


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KS 0 e+e- Event Selection• two oppositely charged tracks • four clusters in LKR calorimeter:

two of them associated with tracks E/p > 0.9• center-of-gravity of the clusters in LKR < 10 cm

• |m - m| < 2.5 MeV/c2 (2.5 )

• |mee - mK0| < 2.5 MeV/c2 (3 )

• no hadronic energy, no AKS hits

background rejection:

• KS 0D 0

D (with 0D e+e-):

|me - m| > 30MeV/c2 (for all e combinations)

• KS 0 0

D (with 0D e+e-):

me+e- > 165 MeV/c2 (to take into account resolution

effects and multiple scattering)

forbidden region

forbidden region

Me[

GeV

/c

2]

Me[

GeV

/c

2]

Me[GeV/c2]

Me[GeV/c2]

KS 0 e+e- MC

KS 0D0

D MC


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• 9 candidates remain after all cuts but me+e-

• all candidates are compatible with Ks 0 0D background

expectation of 12 events

no signal event found

• Normalization: 8 104 Ks 0 0

D

(integrated flux of 3 108 Ks )

• indirect CP violating amplitudeof KL 0 e+e-

KS 0 e+e- Result

0

1

2

3

4

5

6

7

0 0.025 0.05 0.075 0.1 0.125 0.15 0.175 0.2

DataEn

trie

s

CutKs 00D MC

mee[GeV/c2]

Br (KS 0 e+e- 10-7

at 90% CL

Br (KL 0 e+e- indirect CPV 10-10


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• Main contributions to KL +- e+e- process:

• Interference of CP-even (IB) and CP-odd (M1) amplitudes causes large asymmetry in distribution of angle between +- and e+e- planes

• Ks +- e+e-: no significant asymmetry expected

KS,L +- e+e-

++ +

- -

-

KL KL

KL KSe+

e+

e+

e-

e-

e-

Direct Emission (E1,M1)

Inner Bremsstrahlung (IB) K Charge Radius


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KS +- e+e- Event Selection

• two positive and two negative tracks• electron (E/p > 0.85) and pion (E/p < 0.85) identification

• Ks identified by using the tagging detector

• center-of-gravity of the clusters in LKR < 8 cm• no AKS hits, no hit in MUV detector

• 477.7 MeV/c2 < mee < 512.7 MeV/c2

background rejection:

• KS +- with in time conversion: 490.7 < m < 504.7 MeV

• KS + - 0

D (0D e+e-) with missing : p

2 < 0.02 GeV2/c2

0 0D (0

D e+e-) : |mp- m4 MeV/c2

p


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PRELIMINARY

RESULTS

KS +- e+e- Result

• 1998 data:

56 KS +- e+e- events

first observation!

• Combined 1998+1999 (incl. Ks high intensity run 99)

MC Ks +- e+e-

MC Ks +- 0D

Data

Br (KS +-e+e- 4.50.7stat 0.4syst) 10-5

Br (KL +-e+e- ) IB =1.3 0.1) 10-7

Br (KS +-e+e- = 4.3 0.2stat 0.3syst) 10-5

No asymmetry is observed: Asee= (-0.2 3.4stat

1.4syst


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• 1998 and 1999 data

• similar cuts applied like in KS +- e+e-

• additionally: 482.7 MeV/c2 < mee < 507.7 MeV/c2

• and: p2 < 510-4 GeV2/c2

KL +- e+e- Result

PRELIMINARY

RESULTS

ALee= (13.9 2.7stat

2.0syst

Br (KL +-e+e- ) = (3.1 0.1stat 0.2syst) 10-7

221

0.17

0.20M1

GeV 03.072.0/aa and

1.35g~ using

As measured by KTeV (Phys. Rev. Lett. 84(2000) 408) Data

MC

KL +-e+e-

=cos()sin()

1/

d/d


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KL 30 Dalitz Plot Slope

• K 3Dalitz plot distributions:|M(u,v)|2 1 + gu + hv + ju2 + kv2 +…

• KL 30: simplification to

|M(R2,)|2 1 + h R2

with R2 = (u2+v2/3) and = arctan(u/ v)

only quadratic slope h left!

• Combining slope h with slope parameters in K± decays

allows to probe the validity of the I = 1/2 rule

)p(ps ),ss(ss and

m/)s(sv , )/ms(su with

1,2,3K1,2,332131

0

221

203

π

ππ

3


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theoretical predictions unclear:

• J. Kambor et al. (PRL 68,1818 (1992))

h = (-12 ± 4) 10-3

• G. D’Ambrosio et al. (Phys. Rev. D50, 5767 (1994))

h +1.4 10-3

• L. Maiani, N. Paver (2. DANE Phys. Handbook (1995))

h = (-1.2 ± 3.6) 10-3

only one previous measurement:

• E731, 1992: h = (-3.3 ± 1.1stat ± 0.7syst) 10-3

based on 5 106 KL 30 decays


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• six clusters in LKR calorimeter• center-of-gravity of clusters in LKR < 10 cm

very clean events, no background left• ~ 14.5 106 30 events

11.8 106 MC events

fit of slope h:• divide data by normalized MC

(h=0) and fit slope in region 0 < R2 < 1.9 (to avoid resolution

effects at the edge)N

orm

aliz

ed

Even

tsD

ata

/ n

orm

aliz

ed

MC

R2

R2

Fit region

DataMonte Carlo

PRELIMINARY

RESULTh = (-6.1 0.9stat 0.5syst) 10-3


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Summary

KS Br (KS ) = (2.6 0.4stat 0.2syst ) 10-6

(published in Phys. Lett. B493 (2000) 29-35)

KS 0 e+e-: Br (KS 0 e+e-1.4 10-7 at 90% CL

(final result to be published soon)

KS +- e+e-: Br (KS +-e+e- = 4.5 0.7stat 0.4syst)10-5

based on 1998 data (published in Phys. Lett.496 (2000) 137-144)

Br (KS +-e+e- = 4.3 0.2stat 0.3syst)10-5 preliminary result

Asee= (-0.2 3.4stat 1.4systpreliminary result

based on combined 1998 and 1999 data

KL +- e+e-: ALee= (13.9 2.7stat 2.0syst preliminary result

Br (KL +-e+e- ) =( 3.1 0.1stat 0.2syst ) 10-7 preliminary result

based on combined 1998 and 1999 data KL 30 Dalitz plot slope:

h = (-6.1 0.9stat 0.5syst) 10-3 preliminary result


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Outlook

2000: No drift chambers running for neutral decays only Ks : ~ 30 1999 data

Ks 30 : expect statistical error on 000of ± 0.02

KL 30 Dalitz plot slope: more statistics, less systematics

2001: new drift chambers NA48 finishes Re(’/) measurement hoping for 50% - 100% more data for rare KL decays

2002: full year of high intensity Ks and hyperon data taking

the statistic of 1999 data

observation of Br (Ks 0e+e-)

neutral hyperon decays: expecting ~ 30000 0 beta decays

2003/2004: K± program

measurement of CP violation in K± ±±

form factor measurement of K± ±e±

New Results on Rare Kaon Decays in NA48

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