1. New Results from NA48/1 KS  p0e+e- KS  p0m+m- Richard Batley
(University of Cambridge)
University of Sussex,
10th March 2004
KS  p0e+e-
Rare KS Decays :
KS  p0m+m-
… and their relevance to CP violation

2. Rare K Decays : CP Violation
(r,h) a
K+  p+ n n
KL  p0 n n h
KL  p0 e+e- g b
KS  p0 e+e-
(0,0)
(1,0)
(1.4,0) r
KL  p0 g g
KL  g g e+e-
KL  g g, g e+e-
KL  m+ m-
KL  e+e-e+e-, e+e-m+m-
(or m+m-)

3. Rare K Decays : Present Constraints (!)
G. Isidori, hep-ph/

4. Rare K Decays : The Future ?
Expect :
BR(K+  p+ n n ) ~ 0.8 x 10-10
± 7 %
intrinsic theoretical uncertainty
BR(KL  p0 n n ) ~ 2.8 x 10-11
± 2 %
BR(KL  p0 e+ e- )SD ~ 4.5 x 10-12
± 3 %
Theoretically very clean
B(KLp0 e+e-)
precise measurement of area of unitarity triangle :
A.Buras, hep-ph/

5. KL  p0e+e- Contains three components .... 1) Direct CP violating d dW s d
Dominated by top quark contribution : t t g e+
determine h e-

6. KL  p0e+e- 2) Indirect CP violating 3) CP conserving g e+ KL g p0 e-
must measure KS  p0 e+ e-
must measure KL  p0 g g
Expect :

7. The NA48 Experiment

8. Neutral Kaon Beams KS KL Also hyperons: L0, X0 p+p- p0p0 pen pmn
Sources of background for rare K decays
pen
pmn
p0p+p-
p0p0p0
KS  p0e+e-
KS  p0m+m-
~10-9

9. NA48 1997 - 2001: Direct CP violation (e'/e) and rare KL decays KS KL
DETECTOR
7.2 cm KL
NA48
120 m
120 m

10. NA48/1 2002 : Rare KS decays Fluxes : KS ~ 3 x 1010 L0 ~ 4 x 109
X0 ~ 2 x 109 KS
DETECTOR
7.2 cm
NA48
120 m
120 m

11. absorber plug for KL beam
NA48/1 Modifications
new sweeping magnet
dismount
AKS counters
Plus: DAQ x 2
New DCH r/o
absorber plug for KL beam
add photon converter
6 m
(1.5 m long, bronze)

12. The NA48 Detector LKr EM calo. sp /p ~ 1.0% sE /E ~ 0.8% sx ~ 1 mm
st ~ 230 ps

13. NA48 Events
K0  p+p-
K0  p0p0

14. Detector Performance
p0  gg
K0  p+p-

15. (Also used as normalisation channel)
Search for KS  p0e+e-
Main cuts :
0 < t/tS < 2.5 ,
40 < EK < 240 GeV
e± ID :
0.95 < E/p < 1.05
Blind analysis :
Signal region :
2.5s box in (Mp,MK)
kept hidden while cuts developed
Control region :
6.0s box in (Mp,MK)
Main background from Dalitz decays :
KS  p0 p0
(Also used as normalisation channel)
e+ e- g
lost g
require
m(e+e-) > 165 MeV

16. KS  p0e+e- : KS p0p0D Background
SIGNAL
BGD
cut

17. KS  p0e+e- : KS p0p0D Background
OPPOSITE-SIGN
SAME-SIGN
Negligible background for mee > GeV

18. KS  p0e+e- : KS  p0Dp0D Background
SIGNAL
BGD
Residual background = events

19. KS  p0e+e- : KL e+e-gg Background
Use 2001 data (KL beam, x KL flux) to estimate bgd :
Residual background = events

20. KS  p0e+e- : Accidental Background=
accidental superposition of two separate decay fragments which happen to be close together in time
Signal Region :
Control Region :
Extrapolate from CR into SR assuming flat Dt
Background = events

21. KS  p0e+e- : Accidental Background
Main component of bgd :
(relax E/p and t/tS cuts)

22. KS  p0e+e- : X0 Background

23. KS  p0e+e- : X0 Background
SIGNAL
BGD
Residual background negligible

24. KS  p0e+e- Signal
Finally allowed to open the box :
7 events

25. KS  p0e+e- Signal

26. KS  p0e+e- : Result KS flux : (from p0p0D) Acceptance : p0p0Dp0D
0.007
Background :
eegg
0.075
accidental
0.069
Events :
(mee > GeV)
(all mee )

27. Search for KS  p0m+m- Main cuts : 0 < t/tS < 3.0 ,
60 < EK < 200 GeV ,
Muon ID
Blind analysis :
Signal region :
2.5s box in (Mp,MK)
kept hidden while cuts developed
Control region :
6.0s box in (Mp,MK)
Main background from pion decay in flight :
KL  p0 p+ p-
m-n
m+n
Suppress using cuts on Mp, MK, t/tS , CDA, pT , …

28. KS  p0m+m- : KL  p0p+p- Background
With t/tS cuts removed : MK Mp
Residual background < events

29. KS  p0m+m- : Accidental Background
SR :
CR :
Main components :

30. KS  p0m+m- : Accidental Background
Extrapolate from Control Region into Signal Region
bgd = events

31. KS  p0m+m- : X0 Background
Residual background negligible

32. KS  p0m+m- : Trigger Efficiency
First 40% of data :
Final 60% of data :
Overall: e = 0.78 ± 0.02

33. KS  p0m+m- : Signal
Finally allowed to open the box :
6 events

34. KS  p0m+m- Signal

35. KS  p0m+m- Result KS flux : (from p+p-) Acceptance : p0m+m- < 0.02
Background :
mmgg
0.04
accidental
0.18
Events :

36. Interpretation of Results
Form factor : p0 KS e- g e+
Chiral Perturbation Theory prediction :

37. Mass Distributions KS  p0e+e- KS  p0m+m- +3 bS/aS = 0 +3 -6 -3 -6 -3+3 +3 -6 -3 -6 -3
Shape determined by ratio bS/aS :

38. Signal Acceptance
VMD
KS  p0e+e- :
KS  p0m+m- :

39. Determination of aS
Assume VMD :
KS  p0e+e- :
KS  p0m+m- :

40. Determination of aS and bS
Statistics too low to determine bS accurately
Consistent with VMD

41. Implications for KL  p0e+e-
direct
interference
indirect
For central aS values :
What about the CP conserving components ?? ….

42. Implications for KL  p0e+e-

43. KL  p0gg p0
g g
remove KL p0 p0
KTeV :
(1.68  0.07  0.08) x 10-6

44. CPC Component of KL  p0e+e-
KTeV
NA48
Direct CPV
~ 5 x 10-12 aV
Donoghue + Gabbiani,
hep-ph/

45. CKM Unitarity d d Vus s u W e+ n 0+ b-decay : K → pln : (2.0 s) E865 :
Hyperons :
N.Cabibbo, E.Swallow, R.Winston, hep-ph/ ,

46. Vus from K-decay
hep-ex/

47. Hyperon Signals in NA48/1
L0 → pp-

48. X0 Signals in NA48/1 X0 → L0 g X0 → L0 p0 ~ 80,000 events
(PDG 2002: 147 events)
X0 → L0 p0
~ 800,000 events

49. X0 Signals in NA48/1 pp0 pp0 X0 → S+ e- n X0 → S+ m- n
(7 x world statistics)
(first observation)

50. Summary (Vus ?) First observation of KS  p0e+e-, KS  p0m+m-
(7 events)
(6 events)
Implications for CP violation in KL  p0e+e-
Also in NA48/1 :
KS  p0p0p0, gg, …
(Vus ?)
Hyperons
: simultaneous K+, K- beams (NA48/2)