1. Recent results of the NA48/2 experiment
Mauro Raggi LNF
On behalf of the NA48/2 Collaboration PHI PSI 08 International Workshop on e+e- collisions Laboratori Nazionali di Frascati, Italy 7-10 April 2008

2. Outline NA48 history and detector
NA48/2 recent results in charged Kaon decays
BR and FF in K±→p±e+e- (New preliminary)
BR and kinematics in K±→p±gg (New preliminary)
Measurement of K±→p±e+e-g (New Final)
Lepton universality in RK=Ke2/Km2
NA62 prospects for RK
Mauro Raggi
PHI PSI 08, LNF 7-10 April 2008

3. 10 years of History NA48(1997-2001): Direct CP-Violation in neutral K
Ke2/K2 run
NA62
NA48( ): 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( ): Direct CP-Violation in charged K
Ag(K± → π±π+π-) = (-1.5±2.2)·10-4
Ag(K± → π±π0π0) = (1.8 ± 1.8)·10-4
NA62(2007-?):
RK(Ke2/Ke2)
Mauro Raggi
PHI PSI 08, LNF 7-10 April 2008

4. Simultaneous K± Beam Simultaneous K+ and K- beams:
~71011
p/spill
400 GeV/c
PK = 60±3 GeV/c
Width ~ 5 mm
K+/K- ~ 1 mm
Simultaneous K+ and K- beams:
Flux ratio: K+/K– ~ 1.8
large charge symmetrization of experimental conditions
Mauro Raggi
PHI PSI 08, LNF 7-10 April 2008

5. NA48 detector layout LKr EM calorimeter Spectrometer
- 4 Drift Chambers - Magnet
Trigger Hodoscope
Mauro Raggi
PHI PSI 08, LNF 7-10 April 2008

6. K±→l+l– Theoretical framework
K  *  l+l–: suppressed FCNC process Usefull test for cPT
dee/dz ~ P(z)·|W(z)|2
z=(Mee/MK)2, P(z) is a phase space factor
Form-factor models:
(2) D’Ambrosio et al. JHEP 8 (1998) 4, (3) Dubnickova et al. hep-ph/
(1) polynomial: W(z) = GFMK2∙f0∙(1+z)
(2) ChPT O(p6): W(z) = GFMK2∙(a++b+z) + W(z)
(3) Dubna ChPT: W(z) = W(Ma, Mρ, z)
(f0,) or (a+,b+) or (Ma,Mρ) determine a model-dependent BR
Mauro Raggi
PHI PSI 08, LNF 7-10 April 2008

7. Measurement technique and goals
Previous Measurements
Sample BR
Bloch et al., PL 56 (1975) B201
41 (K+)
(2.700.50)·10-7
Alliegro et al., PRL 68 (1992) 278
500 (K+)
(2.750.26)·10-7
Appel et al. [E865], PRL 83 (1999) 4482
10,300 (K+)
(2.940.15 )·10-7
NA48/2 analysis goals
Model-independent BR(z>0.08) in visible kinematic range
Model’s parameters and BRs in full kinematic range
The BR is measured normalizing to K0D
To reject K0D background a Kinematic cut has been applied: Mee>140 MeV
This cut suppress the low Mee region (z<0.08), most interesting for model dependent BR determination
Mauro Raggi
PHI PSI 08, LNF 7-10 April 2008

8. NA48/2 K±→p±e+e- data sample
Kaon decay flux ( ): K=1.701011
FlaviaNet ‘08 BR(K2) used
K±→p±e+e-
K±→p±p0D
7146 events (Mee>140 MeV) Background = 0.6% (K0D K0De + particle misID)
12.23 mln events
Background = 0.15% (K0D)
Mauro Raggi
PHI PSI 08, LNF 7-10 April 2008

9. Raw fit results (preliminary)
polynomial: W(z) = GFMK2∙f0∙(1+z)
ChPT O(p6): W(z) = GFMK2∙(a++b+z) + W(z)
Dubna ChPT:W(z) = W(Ma, Mρ, z)
 =
2.420.15
f0 =
0.5290.012
ρ(, f0) =
–0.963
a+ =
–0.5760.012
b+ =
–0.8300.053
ρ(a+, b+) =
–0.913
Ma =
(0.9510.028) GeV
Mρ =
(0.7050.010) GeV
ρ(Ma, Mρ) =
0.998
(1)
(2)
Analysis cut: z>0.08, or Mee>140MeV/c2
(3)
Available data set unable to distinguish among models
Model-Independent BRMI(z>0.08) can be computed by integrating dG/dz
Mauro Raggi
PHI PSI 08, LNF 7-10 April 2008

10. Preliminary BR results
BRMI=
2.26 
0.03stat
0.03syst
0.06ext =
(2.26
0.08) 10-7 z>0.08
 =
2.35
0.15stat
0.09syst
0.18
f0 =
0.532
0.012stat
0.008syst
0.007ext
0.016
BR1=
3.02
0.04stat
0.04syst
0.08ext
(3.02
0.10) 10-7
a+ =
–0.579
b+ =
–0.798
0.053stat
0.037syst
0.017ext
0.067
BR2=
3.11
(3.11
Ma =
0.965
0.028stat
0.018syst
0.002ext
[GeV/c]
Mρ =
0.711
0.010stat
0.007syst
[GeV/c]
BR3=
3.15
(3.15
(1)
(2)
(3)
Including uncertainty due to the model dependence, (full z range)
BR=(3.080.04stat0.04syst0.08ext 0.07model)10–7=(3.080.12)10–7
CPV parameter (first measurement! correlated K+/K– uncertainties excluded):
(K±ee) = (BR+–BR–) / (BR++BR–) = (–2.1±1.5stat±0.3syst)%
Mauro Raggi
PHI PSI 08, LNF 7-10 April 2008

11. Results comparison  Measurement BR(10-7) BR(10-7) Bloch et al.,
New “naïve” WA
NA48/2
‘75
‘92
‘99
‘08
BR(10-7)
NA48/2
VMD models [PRD60 (1999) ]
‘92
‘99
‘00
‘08 
Measurement
BR(10-7)
Bloch et al.,
2.700.50
Alliegro et al.
2.750.26
Appel et al.
2.940.15
NA48/2 prel. (08)
3.080.12
Measurement
Process
d value
Alliegro et al
K++e+e–
1.310.48
Appel et al.[E865]
2.140.20
Ma et al.[E865]
K+++–
–0.95
NA48/2 prel. (08)
Ke+e–
2.350.18
Mauro Raggi
PHI PSI 08, LNF 7-10 April 2008

12. K±→p±gg physics
Relevant low mgg
O(p4) in cPT: B=D=0; A loop diagrams contribution, C WZW anomaly (10%)
O(p6) in cPT: unitarity corrections can increase the BR by % [G. D’Ambrosio and J. Portoles, Nucl., Phys. B386 (1996), 403]
Mauro Raggi
PHI PSI 08, LNF 7-10 April 2008

13. The spectrum dependence will be used to extract the ĉ value
BR dependence on ĉ
Both the BR and the mgg spectrum shape depend on ĉ:
The spectrum dependence will be used to extract the ĉ value
Mauro Raggi
PHI PSI 08, LNF 7-10 April 2008

14. Preliminary BR result:
K±→p±gg data sample
Previous measurement by E787: BR=(1.10.32)·10– ĉ=1.8±0.6 Based on 31 events 5 BG events
1164 candidates 3.3% BG
1164 events in 40% of the full data ~40 times larger wrt to world sample
BG mainly from ppg(IB)
good agreement with MC O(p6), ĉ=2 measurement of ĉ is ongoing
Preliminary BR result:
MC O(p6), ĉ=2
BR(O(p6),ĉ=2)=(1.070.04sta0.08sys)·10–6
Mauro Raggi
PHI PSI 08, LNF 7-10 April 2008

15. First observation of K±→p±e+e−g
Never observed before
Like in gg ĉ determines BR and meeg shape
Theoretical expectation (Gabbiani 99): BR(K±→p±e+e−g)=( )·10-8
Normalization channel: K±→ p±p0D Kaon decay flux: K=1.48·1011 (’03-’04)
Event sample:
120 candidates events with
6.1% Background
Model-independent BR(Mgee>260MeV/c2):
BR(±e+e−g)=(1.19±0.12stat±0.04sys)·10−8
Using Gabbiani ChPT O(p4) model:
ĉ=0.90±0.45
[final result published, PLB659 (2008) 493]
Mauro Raggi
PHI PSI 08, LNF 7-10 April 2008

16. Ke2/Km2 measurement physics
Test of lepton universality
V-A couplings
Within SM
Theoretical prediction for
RK=RK(0)(1+dRK)=(2.477±0.001)·10-5
PDG average: RK=(2.45±0.11)·10-5
Experimental error (4.5%) 100 times bigger wrt theoretical
112 events
534 events
404 events
[V. Cirigliano and I Rosell, Phys.Rev.Lett.99:231801,2007]
Mauro Raggi
PHI PSI 08, LNF 7-10 April 2008

17. SUSY LFV mechanism Ke2/Km2
If tanb=40 and MH+=500 GeV with |DR31|2= 5·10-4
The effect can be as high as 2% and therefore measurable PR D74 (2006) (A. Masiero, P. Paradisi, R. Petronzio)
Mauro Raggi
PHI PSI 08, LNF 7-10 April 2008

18. Present status of NA48/2 Ke2/Km2
2003 data taking: - 1 month of data taking with downscaled trigger - trigger efficiency
2004 data taking: - 56 hours of dedicated run with minimum bias trigger - K2 background subtraction in Ke2 sample
2 new preliminary result by Na48/2: NA48/2 (‘03 data 4670 ev): RK=(2.416±0.043sta±0.024sys)·10−5
NA48/2 (‘04 data 3407 ev): RK=(2.455±0.045sta±0.041sys)·10−5
Mauro Raggi
PHI PSI 08, LNF 7-10 April 2008

19. NA62 2007 data taking Ke2/Km2 Dedicated 2007 data taking:
4 months of data taking (excluding detectors prototype tests) - More than ~1105 Ke2 events collected - Aims to a precision better than 0.5 % on RK
Reduction of systematics
New beam and spectrometer configuration:
Kaon momentum: (60±3) GeV/c → (75.0 ± 2.5) GeV/c
PT kick from spectrometer magnet: 120 MeV/c → 263 MeV/c
→ improved missing mass resolution
→ improved Ke2 / K2 kinematic separation
Direct measurement of K2 background and electron mis-identification probability
Minimum bias 1 track trigger (as for 2004 run)
Mauro Raggi
PHI PSI 08, LNF 7-10 April 2008

20. Summary Preliminary BRs in K→pee Preliminary BR of K→pgg
BR =(3.080.12)10–7 (including model dependence errors)
BRmi(z>0.08)=(2.26±0.08)·10-7
Preliminary BR of K→pgg
BR(O(p6),ĉ=2)=(1.070.04sta0.08sys)·10–6
Final BR and FF of K→peeg
BR(±e+e−g)=(1.19±0.12sta±0.04sys)·10−8
ĉ=(0.90±0.45)
Preliminary results and prospect in Ke2/Km2
2003 preliminary: RK=(2.416±0.043sta±0.024sys)·10−5
2004 preliminary: RK=(2.455±0.045sta±0.041sys)·10−5
More than 110K Ke2 events collected in 2007 dRK/RK~0.5%
Mauro Raggi
PHI PSI 08, LNF 7-10 April 2008