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

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) ·10-5 0.04% 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 - SUSY10 - 24/8/2010

RK beyond the Standard Model The value of RK could be different in case of SUSY LFV Masiero, Paradisi, Petronzio, Phys. Rev. D74 (2006) 011701 “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 · (1+0.013) 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 - SUSY10 - 24/8/2010

The experimental situation Three esperiments from the 70’s RK = (2.45 ± 0.11) ·10-5 dRK / RK = 4.5% 2009: KLOE (LNF) – 2001-2005 data ~13800 Ke2 candidates, 16% bckg RK = (2.493 ± 0.025 ± 0.019) ·10-5 dRK / RK = 1.3% 2009: NA62 (CERN), part of 2007 data, preliminary result, 51100 candidates RK = (2.500±0.016)×10−5 dRK / RK = 0.7% NA62 final result, same sample, 60000 candidates, this talk dRK / RK = 0.5% NA62 goals from the proposal ~150000 Ke2 events <0.4% accuracy R. Fantechi - SUSY10 - 24/8/2010

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%  0.009 P[GeV/c]% Scintillator hodoscope: fast trigger and good time resolution (150 ps) R. Fantechi - SUSY10 - 24/8/2010

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 - SUSY10 - 24/8/2010

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 - SUSY10 - 24/8/2010

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 - SUSY10 - 24/8/2010

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 ~ 40000 ® 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 - SUSY10 - 24/8/2010

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 - SUSY10 - 24/8/2010

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 - SUSY10 - 24/8/2010

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 - SUSY10 - 24/8/2010

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 - SUSY10 - 24/8/2010

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 - SUSY10 - 24/8/2010

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 - SUSY10 - 24/8/2010

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 - SUSY10 - 24/8/2010

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 - SUSY10 - 24/8/2010

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 - SUSY10 - 24/8/2010