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Ke2/K m 2

Ke2/K m 2. Riunione di Gruppo I Napoli 16/12/08 Paolo Massarotti On behalf of NA62 Collaboration: Bern, Birmingham, CERN, Dubna, Fairfax, Ferrara, Florence, Frascati, Mainz, Menlo Park, Merced, Moscow, Naples, Perugia, Protvino, Pisa, Rome, Saclay, San Luis Potosi, Sofia, Triumf, Turin.

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Ke2/K m 2

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  1. Ke2/Km2 Riunione di Gruppo I Napoli 16/12/08 Paolo Massarotti On behalf of NA62 Collaboration: Bern, Birmingham, CERN, Dubna, Fairfax, Ferrara, Florence, Frascati, Mainz, Menlo Park, Merced, Moscow, Naples, Perugia, Protvino, Pisa, Rome, Saclay, San Luis Potosi, Sofia, Triumf, Turin NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  2. K+→e+νe in the Standard Model In the Standard Model(SM), the decay K+→e+νe (Ke2) is strongly helicity-suppressed compared to the muonic channel K+→μ+νμ(Kμ2). The ratio of the decay rates can be predicted very precisely and is given as where δRQED= -3.6% is a correction due to Kl2γ(IB) and virtual photon processes (Cirigliano, Rosell, JHEP 0710:005, 2007). NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  3. K+→e+νe in Supersymmetry In Supersymmetry (SUSY), K+→e+νe can proceed via exchange of a charged Higgs H+ instead of W+, but in this case, at tree level, the ratio Γ(K+→e+νe)/Γ(K+→μ+νμ) remains unchanged. However, loop effects are predicted to lead to lepton flavour violating (LFV) couplings lH±ντ, which noticeably change only the rate of K+→e+νe (Masiero, Paradisi, Petronzio, PRD 74, 2006): The LFV parameter Δ13 should be of o(10-4 – 10-3) (similar to LFV in the neutrino sector). For given values of MH± and tan β, deviations up to a few percent on RK are possible. As a result of the helicity suppression of the SM contribution, the Ke2 decay rate is sensitive to New Physics. Tree-level diagram forK+→l+ν Loop diagram for K+→e+ντ NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  4. Main actors (experiments) in the challenge to push down precision on RK: KLOE Preliminary result with 2001 – 2005 data: RK = (2.55 ± 5stat ± 5syst)×10-5 from 8000 Ke2 candidates (3% accuracy), perspectives to reach 1% error after analysis completion. NA48/2 Preliminary result with 2003 data: RK = (2.416 ± 43stat ± 24syst)×10-5, from ~ 4000 Ke2 candidates, statistical error dominating (2% accuracy). Preliminary result with 2004 data: RK = (2.455 ± 45stat ± 41syst)×10-5, from ~ 4000 Ke2 candidates from special minimum bias run (3% accuracy). NA62 Collected ~ 100,000 Ke2 events in dedicated 2007 run, aims at breaking the 1% precision wall, possibly reaching < ~ 0.5 %. Entering the precision realm for RK NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  5. Scintillator hodoscope: establish event time (σ~150 ps), initiate trigger LKr calorimeter: efficient vetoing, e.m. energy resolution σE/E = 3.2%/ √E[GeV] 9%/E[GeV] 0.42% σx,y = 4.2mm/√E[GeV] 0.6 mm, granularity of ~13,000 2×2 cm2 cells Hadron calorimeter, Muon veto system NA48/2 Experiment • Unseparated, simultaneous K± highly collimated beams , designed • to precisely measure K±→π+π-π± (π0π0π±) dalitz-plot density: • pK ~ 60 GeV/c, σp ~ 3 Gev/c (3.8% p-bite) • spot of ~ 5 mm width DCH1 entrance Track decay products with 4 DCH’s: • PT kick of 121 MeV/c after DCH2 • σp/p ~ 1.02% 0.044% p [GeV/c] NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  6. Beam + Detector: Charged kaons are produced by 400 GeV/c protons impinging on a Be target. An achromatic system of dipole magnets selects charged particles ( ~ 90 % π±, 10 % K±) with momentum of (75±2) GeV/c. The decay volume is housed in a ~ 110 m long vacuum tank. The decay products are measured in the detector of the former NA48/2 experiment. Data taking: 120 days run period, June – October 2007. Minimum Bias-Trigger: Require minimum number of hits in drift chambers and hodoscope + energy deposition in the LKr calorimeter > 10 GeV. > 120000 K± → e±ν decays recorded Goal of NA62: Measurement of RK with accuracy < 0.5 % NA62 Physics Run 2007 NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  7. Data 2003 Data 2004 NA62 2007 Data Ptrk(GeV) NA62 Physics Run 2007 Design of NA62 run optimized for Ke2/K2, major parameters tuned: PK: ~60 GeV  ~75 GeV Momentum bite: 3.8%  2.5% P kick: 120 MeV  263 MeV MM2 resolution improved Better separation for Ke2 and K2 (MM2) (GeV2) Data 2007: 2003/4 par’s 2007 par’s Expected: MM2(e) (GeV2) P (GeV) Resolution contributions: PK, P, K NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  8. Basic principles for data reduction: Filter Ke2 (Km2) from events triggered by 1TRKLM & ELKr > 10GeV (1TRKLM, downscaled by 150) Common to Ke2 and Km2 selections: 1 track reconstructed by DCH, 15 < p < 65 GeV No cluster of significant energy unconnected to track For Ke2, one connected cluster with 0.95 < E/P < 1.1 For Km2, require E/P < 0.2 or no cluster connected Analysis method: selection algorithm NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  9. Ke2Identification and Background Rejection Most relevant background: Decays of K± → μ±ν (Br = 63 %). All other decays and processes are excluded kinematically or measured in data (e.g. beam halo background). P Rejection of K±→μ±ν: Events with ptrack < 35 Gev/c (40 %): Kinematic separation by “Missing mass” M2 =(PK – Ptrack)2 Events with ptrack > 35 GeV/c (60 %): Separation by ratio E/p: Electrons have E/p ~ 1, (NAPLES) Muons E/p << 1. NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  10. M2Miss(l) Analysis Data taking lasted 4 months: NA62 has the world largest data set, exceeding 100000 Ke2 events Largest homogeneus period (K+ data), lasted ~1 month: > 40000 Ke2events NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  11. E/P: “Katastrophic Bremsstrahlung” Problem: “Katastrophic Bremsstrahlung” Avery small fraction of muons deposit all energy in the calorimeter by emission of a high energy bremsstrahlung photon. • A few 10-6 of muons with E/p > 0.95 being mis-identified as electrons. Solution: Cover part of the calorimeter with lead plate (see sketch), allowing only muons to pass. • Direct measurement of muon background by explicit E/p measurement for muons. NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  12. E/P : muons contamination Check probability for  to fake e, directly measuring it: Subsample of data taken with Pb wall between HOD’s Use HOD pulse heights to select ’s with MIP energy loss in Pb Number of events Ptrack (GeV) NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  13. Muons contamination: Data-MC comparison Use MC for checks of any bias due to the presence of Pb wall Can use Pb wall data to validate results of G4 simulation Data-MC agreement looks good , 106 units, Pb wall present Data MC Ptrack (GeV) NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  14. Backgrounds subtracion Will subtract background from beam halo directly measuring it: Use K-less data normalized in side-band of negative Mmiss2 Expect ~ 7% total bkgto Ke2, dominated by K2 NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  15. Perspective: same RK, RK=0.3% RK – Present and future sensitivity to NP Sensitivity shown as 95%-CL excluded regions in the tan- MH plane, for fixed values of the 1-3 slepton-mass matrix element,13 = 10-3,0.510-3,10-4 Present world avg: RK= 2.457(32)10-5 tan tan MH (GeV) MH (GeV) NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  16. Golden LFV observable is RK: Preliminary mmt 2% from NA48/2, 3% from KLOE Results from preliminary measurement in agreement with SM Huge effort to enter the precision realm for RK: KLOE expects to push error to ~1.3%, after analysis completion NA62 dedicated 2007 run aims at reaching <0.5% error Conclusions NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  17. F. Ambrosino, P. Massarotti, F. Perfetto. E/P evaluation for high momentum electron using theKL→p ±eneruns 20433 - 20435 ± NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  18. Event section We use the KL runs because the KL momentum is between 30 until 100 GeV. The electon momentum (KLe3) is comparable with the K±e2 one Our goal is to select the KLe3 decay. The main backgrounds are due to KLm3 and KL3p decays g . KL Lkr e+ p- NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  19. Signal selection KLe3 The signal is obtained asking • one particle with 0.2 ≤E/p ≤ 0.7, the other is the electron/positron candidate track-cluster. • Track Muon status = -1 • No clusters in the LKr or only one in the direction of the not track (big box) • Associated cluster quality status <= 4 NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  20. Searching for a photon from external bremsstrahlung KLe3 Only one cluster in the LKr in the direction of the not deflected elctron candidate track We define two boxes: The BIG one The small one DX < 2 cm DX < 0.75 DY < 2 cm DY < 0.8 The first is used for the selection of the global signal, the second one is used the selection of a pure KLe3g distribution (external bremsstrahlung) NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  21. Pure electrons selection KLe3 with e→eg The signal is obtained asking • one particle with 0.2 ≤E/p ≤ 0.7, the other is the electron/positron candidate track-cluster. • Track Muon status = -1 • One “neutral” cluster in the direction of the not deflected track in the small box • Associated cluster quality status <= 4 NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  22. Pions selection KLp3 The pions distribution signal is obtained asking • Track Muon status = -1 • Two “neutral”clusters in the LKr • p0 invariant mass between 125 and 145 MeV • KL invariant mass between 480 and 510 MeV NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  23. Mouns Selection KLm3 The muons distribution is obtained asking • one particle with 0.2 ≤E/p ≤ 0.7, the other is the muon candidate track-cluster. • Track Muon status = 1 NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  24. Signal selection KLe3 The signal is obtained asking • one particle with 0.2 ≤E/p ≤ 0.7, the other is the electron/positron candidate track-cluster. • Track Muon status = -1 • No clusters in the LKr or only one in the direction of the not track (big box) • Associated cluster quality status <= 4 m e p NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  25. Strategies: background evaluations • Using the shape of the different decays (KLe3g, KLp3, KLm3) we can: • fit whole the region before the peak and the peak in E/P using the 3 contributions • fit the region before the peak, between 0.0 and 0.5 in E/P, using only the background contributions (KLp3, KLm3) • fit whole the region before the peak and the peak, between 0.6 and 1.0 in E/P using the KLe3g and KLp3 contributions • subtract formally the two backgrounds (KLp3, KLm3) NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  26. Fit results: very good agreement! • fit whole the region before the peak and the peak in E/P using the 3 contributions (KLe3g, KLp3, KLm3) • fit the region before the peak, between 0.0 and 0.5 in E/P, using only the background contributions (KLp3, KLm3) • fit whole the region before the peak and the peak, between 0.6 and 1.0 in E/P using the KLe3g and KLp3 contributions • subtract formally the backgrounds (KLp3, KLm3) !!!!Agreement for the four methods in 0.1-0.2 % !!!! NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  27. Result: good agreement between the different procedures Our goals: extend these fits to the very high momentum region (55 to 65 GeV) evaluate the E/P inefficiency in the semileptonic charged channel (K+e3) Conclusions NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  28. SPARES NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  29. Background from bremsstrahlung photons Number of events Ptrack (GeV/c) NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  30. Total Background Estimation Will subtract background from beam halo directly measuring it: Use K-less data normalized in side-band of negative Mmiss2 Expect ~ 7% total backgroundto Ke2, dominated by Kμ2 NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  31. Analysis of Ke2/K2 at NA62 Data taking lasted 4 months: NA62 has the world largest data set, exceeding 100 K Ke2 evts Largest homogeneus period (K+ data), lasted ~1 month: > 40 K Ke2 evts N(Ke2) (Kevts) Data taking shifts (8 hours) Ke2 K2 MM2() (GeV2) MM2(e) (GeV2) NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  32. Ke2 Identification and Background Rejection Use MC for checks of any bias due to the presence of Pb wall Can use Pb wall data to validate results of G4 simulation Data-MC agreement looks good NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  33. Searching for a photon from external bremsstrahlung Only one cluster in the LKr in the direction of the not deflected elctron candidate track NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  34. Searching for a photon from external bremsstrahlung NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  35. Experimental Status PDG2006 + three new preliminary measurements by NA48/2 and KLOE give 1.3% total accuracy and are in agreement with SM prediction: RK=(2.475±0.032)×10-5 (MH,GeV) SUSY limits by RK measurements NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

  36. Cluster efficiency for electrons: Control check: independent efficiency evaluation from data KLe3 (Mainz. Independent analysis from Neaples is in progress) Difference between Ke3c and Ke3n correction factors is 0.025% Sources of systematic error:eLKr(Ke2) NA 62 Analysis Paolo Massarotti Riunione GRI Napoli 16/12/2008

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