Measurement of Measurement of → e → µν BR(K → )/BR(K → ν e µν µ e ν ) BR(K e )/BR(K µ ) Roberto Piandani INFN Perugia & CERN Work supported by for the NA62 collaboration (Bern ITP, Birmingham, CERN, Dubna, Fairfax, Ferrara, Florence, Frascati, IHEP Protvino, INR Moscow, Louvain, Mainz, Merced, Naples, Perugia, Pisa, Rome I, Rome II, Saclay, San Luis Potosí, SLAC, Sofia, TRIUMF, Turin) Outline: 1) Motivation & experimental status; 2) Beam, detector and data taking; 3) Backgrounds & systematic effects; 4) Preliminary results and prospects. Pheno 2010 Madison Wisconsin • 10 -12 May 2010
R K =K e2 /K µ in the SM R K =K e2 /K 2 in the SM µ 2 Observable sensitive to lepton flavour violation and its SM expectation: (similarly, R π in the pion sector) Radiative correction (few %) due to K + → e + νγ (IB) process, Helicity suppression: f~10 –5 by definition included into R K ✗ SM prediction: excellent sub-permille accuracy due to cancellation of hadronic uncertainties. ✗ Measurements of R K and R π have long been R K SM = (2.477 ± 0.001) × 10 –5 considered as tests of lepton universality. R π SM = (12.352 ± 0.001) × 10 –5 ✗ Recently understood: helicity suppression of Phys. Lett. 99 (2007) 231801 R K might enhance sensitivity to non-SM effects to an experimentally accessible level.
R K =K e2 /K µ beyond the SM R K =K e2 /K 2 beyond the SM µ 2 PRD 74 (2006) 011701, 2 Higgs Double Models – tree level (including SUSY) JHEP 0811 (2008) 042 K l2 can proceed via exchange of charged Higgs H ± instead of W ± ➢ Does not affect the ratio R K 2 Higgs Double Models – one-loop level Dominant contribution to ∆ R Κ : H ± mediated Analogous SUSY effect LFV (rather than LFC) with emission of ν τ in pion decay is suppressed ➢ R K enhancement can be experimentally accessible by a factor (M π /M K ) 4 ≈ 6 × 10 –3 (see also PRD76 (007) 095017) Large effects in B decays Up to ~1% effect in large (but not extreme) due to (M B /M K ) 4 ~10 4 : tan β regime with a massive H ± B µν /B τν ~50% enhancement; Example: B e ν /B τν enhanced by ( ∆ 13 =5 × 10 –4 , tan β =40, M H =500 GeV/c 2 ) ~one order of magnitude. lead to R K MSSM = R K SM (1+0.013). Out of reach: Br SM (B e ν ) ≈ 10 –11
Experimental status Experimental status R K world average (March 2009) PDG’08 average (1970s measurements): R K =(2.45±0.11)·10 –5 ( ∆ R K /R K =4.5%) Recent improvement: KLOE (Frascati). Data collected in 2001–2005, 13.8K K e2 candidates, 16% background. R K =(2.493±0.031)·10 –5 ( ∆ R K /R K =1.3%) (EPJ C64 (2009) 627) NA62 (phase I) goal: dedicated data taking strategy, ~150K K e2 candidates, <10% background, δ R K /R K <0.5% : a stringent SM test.
NA62 data taking 2007/08 NA62 data taking 2007/08 Data taking: • Four months in 2007 (23/06–22/10): ~400K SPS spills, 300TB of raw data ~400K SPS spills, 300TB of raw data (90TB recorded) (90TB recorded) ; reprocessing & ; reprocessing & data preparation finished. data preparation finished. • Two weeks in 2008 (11/09–24/09): special data sets allowing reduction of special data sets allowing reduction of the systematic uncertainties. the systematic uncertainties. Principal subdetectors for R K : • Magnetic spectrometer (4 DCHs): 4 views/DCH: redundancy 4 views/DCH: redundancy ⇒ ⇒ efficiency; efficiency; Δ Δp/p = 0.47% + 0.020%*p [GeV/c] p/p = 0.47% + 0.020%*p [GeV/c] Vacuum beam pipe: • Hodoscope non-decayed kaons fast trigger, precise t measurement (150ps). fast trigger, precise t measurement (150ps). • Liquid Krypton EM calorimeter (LKr) High granularity, quasi-homogeneous; High granularity, quasi-homogeneous; Decay volume He filled tank, σ σ E /E = 3.2%/E 1/2 + 9%/E + 0.42% [GeV]; E /E = 3.2%/E 1/2 + 9%/E + 0.42% [GeV]; is upstream atmospheric pressure σ σ x = σ σ y =0.42/E 1/2 + 0.6mm (1.5mm@10GeV). x = y =0.42/E 1/2 + 0.6mm (1.5mm@10GeV).
Measurement strategy Measurement strategy (1) K e2 /K µ 2 candidates are collected simultaneously: ➔ the result does not rely on kaon flux measurement; ➔ several systematic effects cancel at first order (e.g. reconstruction/trigger efficiencies, time-dependent effects). (2) counting experiment, independently in 10 lepton momentum bins (owing to strong momentum dependence of backgrounds and event topology) N(K e2 ) – N B (K e2 ) A(K µ 2 ) × f µ × ε (K µ 2 ) 1 R K = N(K µ 2 ) – N B (K µ 2 ) A(K e2 ) × f e × ε (K e2 ) f LKr N(K e2 ), N(K σ 2 ): numbers of selected K l2 candidates; N B (K e2 ): main source N B (K e2 ), N B (K µ 2 ): numbers of background events; of systematic errors A(K e2 ), A(K µ 2 ): MC geometric acceptances (no ID); f e , f µ : directly measured particle ID efficiencies; ε (K e2 )/ ε (K µ 2 ) >99.9%: E LKr trigger condition efficiency; f LKr =0.9980(3): global LKr readout efficiency. (3) MC simulations used to a limited extent only: ➔ Geometrical part of the acceptance correction (not for particle ID); ➔ simulation of “catastrophic” bremsstrahlung by muons.
The K e2 and K µ selection The K e2 and K 2 selection µ 2 Large common part (topological similarity) Missing mass vs lepton momentum electron mass hypothesis ➢ one reconstructed track; ➢ geometrical acceptance cuts; K µ 2 (data) ➢ K decay vertex: closest approach of track & nominal kaon axis; ➢ veto extra LKr energy deposition clusters; K e2 ➢ track momentum: 15GeV/c<p<65GeV/c. (data) …poor separation at high p Kinematic separation missing mass Log scale : average measured with K 3 π decays Sufficient K e2 /K µ 2 separation at p track <25GeV/c Separation by particle ID E/p = (LKr energy deposit/track momentum). 0.95<E/p<1.10 for electrons, E/p<0.85 for muons. Powerful µ ± suppression in e ± sample: f~10 6
K e2 : partial (40%) data set K e2 : partial (40%) data set K e2 candidates 10 4 10 3 10 2 10 1 Log scale 51,089 K + → e + ν candidates, NA62 estimated total K e2 sample: 99.2% electron ID efficiency, ~120K K + & ~15K K – candidates. B/(S+B) = (8.0 ± 0.2)% Proposal (CERN-SPSC-2006-033): 150K candidates cf. KLOE: 13.8K candidates (K + and K – ), ~90% electron ID efficiency, 16% background
K µ background in K e2 sample K 2 background in K e2 sample µ 2 Main background source Muon “catastrophic” energy loss in LKr by Lead (Pb) wall emission of energetic bremsstrahlung photons. Theoretical bremsstrahlung cross-section [Phys. Atom. Nucl. 60 (1997) 576] must be validated in the region (E γ /E µ )>0.9 by a direct measurement of P( µ →e) to ~10 –2 relative precision. P( µ→ e): measurement vs Geant4-based simulation model validation ~10X 0 (Pb+Fe) Thickness: Width: 240cm (=HOD size) Height: 18cm (=3 counters) Area: ~20% of HOD area 2007 special muon run Duration: ~50% of R K runs + special muon runs P( µ →e) is modified by the Pb wall Used for via two competing mechanisms: background subtraction 1) ionization losses in Pb (low p); 2) bremsstrahlung in Pb (high p). analysis momentum range Result: B/(S+B) = (6.28 ± 0.17)%
ν e γ (SD) background + ν e γ K + →e e + (SD) background K + → ➔ Background by definition of R K , no helicity suppression. ➔ Rate similar to that of K e2 , limited precision: BR=(1.52±0.23)·10 –5 . K e2 γ (SD) Dalitz plot distribution Only energetic electrons (E e * >230MeV) E e, GeV are compatible to K e2 kinematic ID and contribute to the background K e3 endpoint This region of phase space is accessible for direct BR and form-factor measurement (being above the E e * =227 MeV SD – component endpoint of the K e3 spectrum). ChPT O(p 6 ), SD background contamination form factor with measured kinematic dependence (EPJC64 627) B/(S+B) = (1.02 ± 0.15)% E γ , GeV (uncertainty due to PDG BR, K e2 γ (SD – ) background is negligible, will be improved using a recent KLOE measurement, EPJC64 627) peaking at E e = E max /2 ≈ 123 MeV
Backgrounds: summary Backgrounds: summary Backgrounds Statistics in lepton momentum bins Source B/(S+B) K µ 2 (6.28 ± 0.17)% K µ 2 ( µ→ e) (0.23 ± 0.01)% K e2 γ (SD + ) (1.02 ± 0.15)% Beam halo (0.45 ± 0.04)% K e3 0.03% K 2 π 0.03% Total (8.03 ± 0.23)% Record K e2 sample: 51,089 candidates x5 with low background x5 x25 B/(S+B) = (8.0 ± 0.2)% Lepton momentum bins are differently affected by backgrounds (selection criteria, e.g. Z vertex and M miss 2 , and thus the systematic are optimised individually in each P track bin) uncertainties.
K µ : 40% of data set K 2 : 40% of data set µ 2 K µ 2 candidates 15.56M candidates with low background B/(S+B) = 0.25% (K µ 2 trigger was pre-scaled by D=150) The only significant background source is the beam halo. Log scale
Preliminary result (40% data set) Preliminary result (40% data set) R K = (2.500 ± 0.012 ± 0.012 stat ± 0.011 syst ) × × 10 10 –5 R K = (2.500 stat ± 0.011 syst ) –5 (arXiv:0908.3858) = (2.500 ± 0.016) = (2.500 ± 0.016) × × 10 10 –5 –5 Uncertainties Independent measurements Source δ R K × 10 5 in lepton momentum bins Statistical 0.012 K µ 2 0.004 Beam halo 0.001 K e2 γ (SD + ) 0.004 Electron ID 0.001 SM IB simulation 0.007 Acceptance 0.002 Trigger timing 0.007 Total 0.016 NA62 preliminary (0.64% precision) The whole 2007 sample will allow statistical uncertainty ~0.3%, total uncertainty of 0.4–0.5%.
Recommend
More recommend
