1. The NA62 measurement of RK = GKe2/GKm2 R
The NA62 measurement of RK = GKe2/GKm2 R. Fantechi CERN and INFN – Sezione di Pisa on behalf of the NA62 Collaboration Bern ITP, Birmingham, CERN, Dubna, Fairfax, Ferrara, Florence, Frascati, IHEP, INR, Louvain, Mainz, Merced, Naples, Perugia, Pisa, Rome I, Rome II, San Luis Potosi, SLAC, Sofia, Triumf, Turin
SUSY10 – Bonn, August 24th, 2010

2. Physics motivations
Test of lepton universality
Excellent accuracy due to cancellation of hadronic uncertainties in the ratio
Helicity suppression of electronic mode, enhancement of sensitivity to non-SM effects
RK(SM) = (2.477 ± 0.001) · % precision!!
(V. Cirigliano, I. Rosell JHEP 0710:005 (2007))
dRK due to IB part of the radiative K ® eng process
K ® eng(IB) included by default in RK
R. Fantechi - SUSY /8/2010

3. RK beyond the Standard Model
The value of RK could be different in case of SUSY LFV
Masiero, Paradisi, Petronzio, Phys. Rev. D74 (2006)
“Charged Higgs mediated SUSY LFV contributions can be strongly enhanced in kaon decays into an electron or a muon and a tau neutrino”
D13= 6·10-3 , MH=500 GeV, tanb=40 ® RKLFV ~ RKSM · ( )
Analogous SUSY effects in pion decays are suppressed by a factor (mp/MK)4 ~ 6·10-3
B decays: large effects (MB/MK)4~104
Ben/Bet 10 times larger, but (Ben)SM~10-11
R. Fantechi - SUSY /8/2010

4. The experimental situation
Three esperiments from the 70’s
RK = (2.45 ± 0.11) ·10-5
dRK / RK = 4.5%
2009: KLOE (LNF) – data
~13800 Ke2 candidates, 16% bckg
RK = (2.493 ± ± 0.019) ·10-5
dRK / RK = 1.3%
2009: NA62 (CERN), part of 2007 data, preliminary result, candidates
RK = (2.500±0.016)×10−5
dRK / RK = 0.7%
NA62 final result, same sample, candidates, this talk
dRK / RK = 0.5%
NA62 goals from the proposal
~ Ke2 events
<0.4% accuracy
R. Fantechi - SUSY /8/2010

5. The NA62 Beam and Detector
The NA48/2 beam has been optimized for RK measurement
K+ and K- beams with PK = (74.0±1.6) GeV/c: simultaneous or alternate
Maximum spectrometer PT kick to improve missing mass resolution
Primary SPS protons (400 GeV/c): 1.8x1012/spill
At the entrance of the decay volume (114 m): 2.5x107/spill
Composition: K+(p+) = 5%(63%)
K+ decaying in the vacuum tank: 18%
Use the NA48 detector
LKr Calorimeter:
(E)/E  3.2%/√E  9%/E  0.42%
Energy resolution ~1%, spatial resolution 1 mm
Spectrometer:
(P)/P  0.48%  P[GeV/c]%
Scintillator hodoscope: fast trigger and good time resolution (150 ps)
R. Fantechi - SUSY /8/2010

6. 2007 and 2008 runs 2007 sample, 4 months 2008 sample, 2 weeks
Minimum bias trigger
~400 K SPS spills, 300 TB of raw data (90 TB recorded)
All data reprocessed and ready for the complete analysis
Beam halo background much higher for K- (~20%) than for K+ (~1%)
Due to the configuration of muon sweepers
90% of the data sample is K+ only
K+ only and K- only data allow precise cross-measurement of beam halo background
2008 sample, 2 weeks
Special data set for systematics uncertinties study and reduction
R. Fantechi - SUSY /8/2010

7. NA62 RK measurement strategy
Ke2 and Km2 decays collected concurrently
Do not rely on the kaon flux measurement
Many systematic effects cancel in the ratio
E.g. reconstruction/trigger efficiencies, time dependent effects
Limited use of MC simulations
Geometric acceptance correction
PID, trigger, readout efficiencies measured directly
Perform analysis in 10 lepton momentum bins
Background composition and event topology vary strongly with momentum
Selected signal events
PID efficiencies
Km2 trigger downscaling factor
Bckg events
Main syst source
Trigger efficiencies
LKR readout efficiency
Acceptances
R. Fantechi - SUSY /8/2010

8. Ke2 and Km2 selection Common selection Kinematical identification
One reconstructed track, 13 < P < 65 GeV/c
Common geometrical cuts
Decay vertex defined as closest distance of approach track-nominal K axis
Veto extra LKr energy deposition clusters
Kinematical identification
Use missing mass squared M2miss=(PK-Pl)2
PK (average) is measured from the data with K ® 3p
Enough Ke2/Km2 separation for Ptrack< 25 GeV/c
Lepton identification
ELKr/Ptrack < 0.85 for muons, between 0.95 and 1.10 for electrons
R. Fantechi - SUSY /8/2010

9. The major background to Ke2 - 1
Catastrophic bremsstrahlung of muons in the LKr
Gives E/p > 0.95 ® tag the event as Ke2
Probability Pme ~3·10-6 (momentum dependent)
Km2 /Ke2 ~ ® background O(10%)
Need a direct measurement to validate the muon bremsstrahlung cross section in this region
Measure it putting 9.2 X0 of lead in front of the calorimeter
For about 50% of the run time, 18% acceptance reduction
Suppress 10-4 contamination of m ® e decays
Tracks traversing the lead are pure muons
Pme is modified by the Pb wall
Ionization losses in the Pb (at low p)
Bremsstrahlung in Pb (at high p)
The correction is evaluated with a dedicated Geant4 simulation
R. Fantechi - SUSY /8/2010

10. The major background to Ke2 - 2
Pme vs momentum (Pb wall installed)
Correction for Pb wall: fPb = Pme/PmePb
MC precision
dfPb/fPb = 2%
MC precision
dPme/Pme = 10%
Contribution to background: B(S+B) = (6.10±0.22)%
Uncertainty ~3 times smaller than simulation alone
Contributions to uncertainty:
Limited data sample 0.16%
MC correction 0.12%
Missing mass vs P correlation 0.08%
R. Fantechi - SUSY /8/2010

11. Km2 with m ® e decay in flight
With NA62 conditions (Pbeam = 74 GeV, decay length = 100m), it seems at first sight a huge background
N(Km2 ,m ® e)/N(Ke2) ~ 10
Muons from Km2 decay are fully polarized, following Michel distribution:
d2G/dxd(cosQ) ~x2[(3-2x)-cosQ(1-2x)]
x= Ee/Emax ~ 2Ee/Mm
Only energetic forward positrons are selected as Ke2 candidates
Natural suppression by the muon polarisation
Additional 10% suppression by radiative corrections
Result: B/(S+B) = (0.27±0.04)%
Important, but not dominant background
R. Fantechi - SUSY /8/2010

12. Other backgrounds to Ke2 & systematics
Beam halo
Beam halo muons decaying to electrons
Halo higher for Ke2- (~20%) than Ke2+ for (~1%), lower for Km2
~90% of the data sample is K+ only, ~10% is K- only
K- halo component measured using K- sample and viceversa
Ke2g (SD+)
Default background, RK includes only IB
Rate similar to Ke2, the theory is form factor model-dependent
Use the KLOE FF measurement to get BR=(1.37±0.06)·10-5
Improvement with a direct measurement from an ongoing analysis of 2007 NA62 data
Positron ID efficiency fe
Measured with pure electron samples from K± ® p0e±n collected during the main data taking (P<50 GeV/c) and with KL ® p±e±n, collected with a special KL run
Good agreement between the two, fe= (99.27±0.05)%
1.14 ± 0.06 %
1.15 ± 0.17 %
R. Fantechi - SUSY /8/2010

13. Summary of the backgrounds
Source
B/(S+B)
Km2
(6.10±0.22)%
Km2 (m ® e)
(0.27±0.04)%
Ke2g (SD+)
(1.15±0.17)%
Beam halo
(1.14±0.06)%
Ke3(D)
(0.06±0.01)%
K2p (D)
Total
(8.78±0.29)% x5
x50
Record Ke2 sample:
59963 candidates
B/(S+B) = (8.8 ±0.3)% x5 x5
Different effects in lepton momentum bins
R. Fantechi - SUSY /8/2010

14. Ke2 candidates Only 40% of data used
B/(S+B) = (8.8 ±0.3)%
Positron ID efficiency (99.27 ±0.05)%
Cfr. KLOE: 13.8Kevents, ~90% electron id efficiency, 16% background
Only 40% of data used
Total estimated sample: ~130k K+ and ~20k K- candidates
Proposal: 150k candidates
R. Fantechi - SUSY /8/2010

15. Km2 candidates Source B/(S+B) Beam halo (0.38±0.01)% Total
18 M candidates with low background
B/(S+B) = 0.38%
Km2 trigger prescaled by a factor 150
R. Fantechi - SUSY /8/2010

16. NA62 final result (40% data set)
RK = (2.486 ±0.011stat ± 0.007syst)x10-5 = (2.486 ±0.013)x10-5 (June 2010)
Uncertainties
Source
dRK*105
Statistical
0.011
Km2
0.005
BR(Ke2g SD+)
0.004
Beam halo
0.001
Acceptance
0.002
DCH alignment
Positron ID
1-track trigger
Total
0.013
R. Fantechi - SUSY /8/2010

17. Result summary Still in agreement within 1s with the SM
Exclusion limits at 95% confidence level from the new RK average
World average
RK*105
Precision
March 2009
2.467±0.024
0.97%
Today
2.487±0.012
0.48%
Still in agreement within 1s with the SM
Any significant enhancement wrt the SM value would be an evidence of New Physics
R. Fantechi - SUSY /8/2010

18. Conclusions
NA62 has taken data in 2007 and 2008 to reach better than 0.4% precision on RK
The analysis of a partial data set (40% of the data) has been completed
The measured value is RK = (2.486±0.013)*10-5
The precision achieved is 0.52%
The analysis of the complete data set (~150K Ke2 events) will bring the statistical error below 0.3% and the total one below 0.4%
R. Fantechi - SUSY /8/2010