π 0 Dalitz Transition Form Factor and other news from NA62 Nicolas Lurkin University of Birmingham, 26/04/2017

Transcription

1 π 0 Dalitz Transition Form Factor and other news from NA62 Nicolas Lurkin University of Birmingham, 26/04/2017

2 Outline π 0 transition form factor slope from the π 0 Dalitz decay at NA The K + π + ν ν decay Status of the NA62 detector Status of the K + π + ν ν analysis Prospects for exotic searches at NA62 Nicolas Lurkin, 26/04/2017 2

3 CERN NA48/NA62 experiments Experiments history Switzerland Jura mountains SPS NA48/NA62: Centre of the LHC France LHC Earlier NA31 NA48 (K S /K L ) 2002 NA48/1 (K S /hyperons) Re ε /ε Discovery of direct CPV Rare K S and hyperon decays NA48/2 (K + /K ) Direct CPV, Rare K + /K decays Geneva airport NA62 RK (K + /K ) R K = K ± ± e2 /K μ2 1 st part 2014 NA62 (K + ) K + π + ν ν, Rare K + and π 0 decays 2 nd part Kaon decay in flight experiment NA62: currently ~200 participants, 29 institutions from 13 countries Nicolas Lurkin, 26/04/2017 3

4 A step back in the past Nicolas Lurkin, 26/04/2017 4

5 Experimental Setup (NA data) Principal subdetectors Scintillator hodoscope (HOD) Low-level trigger, time measurement (150 ps) Magnetic spectrometer (4DCHs) 4 views/dch high efficiency σ p p = 0.48% 0.009% p [GeV/c] Liquid Krypton EM calorimeter (LKr) High granularity, quasi-homogeneous σ E E = 3.2 E 9 E 0.42 % σ x = σ y = 4.2 E 0.6 mm ( GeV) [E in GeV] Data taking conditions P K = 74 ± 2 GeV/c Triggers: 1-track e ±, 1-track μ ± Alternate K + K beam, possibility to block both beams Nicolas Lurkin, 26/04/2017 5

6 π 0 TFF: Dalitz Decay Kinematic variables x = p e + +p e 2 m π 0 Differential decay width 1 Γ π0 2γ d 2 Γ π D 0 π 0 e + e γ 2, y = 2p π 0 p e + p e 2 dxdy = α 4π Form factor varies slowly: 1 x 3 x m π 0 Approximation F x 1 + ax 1 x 1 + y 2 + r2 x Radiative corrections 1 + δ x, y F x 2 Electromagnetic Transition Form factor Nicolas Lurkin, 26/04/2017 6

7 π 0 TFF: Motivations Computed in multiple theoretical framework: Chiral perturbation theory [K. Kampf et al., EPJ C46 (2006), 191]: a = 2.90 ± Dispersion theory [M. Hoferichter et al., EPJ C74 (2014), 3180]: a = 3.07 ± Two-hadron saturation (THS) model [T. Husek et al., EPJ C75 (2015) 12, 586]: a = 2.92 ± All roughly agree on a value close to 3% Measured in experiment: Space-like momentum transfer (CELLO) [H. J. Behrend et al., Z. Phys. C49 (1991), 401]: a = 3.26 ± 0.26 stat 10 2 Most precise, but model dependent extrapolation Time-like momentum transfer, many old results (latest result from 1992, but MAMI [P. Adlarson et al., Phys. Rev. C 95, ]) Limited by statistics and theoretical uncertainties on radiative corrections No single clear evidence of non-zero value Nicolas Lurkin, 26/04/2017 7

8 Better model independent measurement is an important test of the theory models Important input for: π 0 TFF: Motivations Hadronic light-by-light scattering contribution to g 2 μ [A. Nyffeler, Phys. Rev. D 94, (2016)] 3σ discrepancy between theory and experiment for g 2 μ The HLBL contribution is the second largest source of uncertainty The π 0 contribution is about 25% of the HLBL uncertainty π 0 e + e decay rate [A. E. Dorokhov and M. A. Ivanov, Phys. Rev. D 75, ] 3.3σ discrepancy between theory and experiment in the Br Hypothesis: transition form factor extrapolated from CELLO data is not valid (there are others) Nicolas Lurkin, 26/04/2017 8

9 π 0 TFF: Radiative Corrections Corrections from NLO differential width encoded in d 2 Γ NLO d 2 Γ LO δ x, y = dxdy dxdy Mikaelian and Smith [Phys.Rev. D5 (1972) 1763] Husek, Kampf and Novotny [Phys.Rev. D92 (2015) 5, ] New δ 1γIR contribution Divergences cancel between δ virt and δ brem δ virt δ 1γIR δ brem Nicolas Lurkin, 26/04/2017 9

10 π 0 TFF: Radiative Corrections Corrections are of the same magnitude as TFF δ x, y x spectrum Nicolas Lurkin, 26/04/

11 π 0 TFF: Radiative Corrections δ brem is a 4-body decay π 0 e + e γγ discrepancies at low (Dalitz) photon energy New hybrid generator 3-body generator with δ 1γIR, δ virt radiative corrections and part of δ brem necessary to cancel divergences 4-body generator for remaining bremsstrahlung contribution At the generator level, selection based on a cut-off parameter 3 MC samples produced Main sample: blue Convergence test: red (agreement with main sample) green (not enough bremsstrahlung photons) γ energy spectrum Nicolas Lurkin, 26/04/

12 Measure the slope from a pure π D 0 sample from ~ K + decays Use the K ± π ± π 0 (K 2πD ) decay chain e + e γ 3-track vertex topology π 0 TFF: Selection Maximum three well reconstructed tracks Photon: single isolated cluster in LKr (away from (un)deflected track impact points) Remove track bremsstrahlung photons Nicolas Lurkin, 26/04/

13 π 0 TFF: Selection Particle identification from reconstructed kinematics 115 MeV/c 2 < M eeγ < 145 MeV/c MeV/c 2 < M π + π0 < 510 MeV/c2 Correct mass assignment Incorrect mass assignment Nicolas Lurkin, 26/04/

14 π 0 TFF: Selection Reconstructed Kaon compatible with beam properties and offline L2 and L3 trigger conditions Selected sample: fully reconstructed π D 0 events in the signal region (x > 0.01) x spectrum of selected sample Selection acceptances Nicolas Lurkin, 26/04/

15 π 0 TFF: Selection Reconstructed Kaon compatible with beam properties and offline L2 and L3 trigger conditions Selected sample: fully reconstructed π D 0 events in the signal region (x > 0.01) Reconstructed K + mass Reconstructed π 0 mass Nicolas Lurkin, 26/04/

16 π 0 TFF: Result Build x Dalitz distribution for data and MC (equal population bins) For each TFF slope value hypothesis, reweight simulated events (a sim = 0.032) Fit result illustration w a = 1+ax true 2 Data/MC(a=0) ratio 1+a sim x 2 25 equal population bins true Minimise χ 2 Points in bin barycentre a of Data/Simulation wrt. a a = 3.68 ± 0.51 stat ± 0.25 syst 10 2 = 3.68 ± (χ 2 /n.d.f: 54.8/49, p-value: 26.4%) Nicolas Lurkin, 26/04/

17 π 0 TFF: Result Systematic effects Trigger efficiency π 0 Dalitz decay generator Detector calibration and resolution Accidentals Particle identification Neglected π D 0 sources Beam momentum simulation Source Uncertainties Statistical Data 0.48 Statistical MC 0.18 Spectrometer momentum scale 0.16 Accidental background 0.15 Particle mis-id 0.06 Calorimeter trigger efficiency 0.06 Spectrometer resolution 0.05 LKr non-linearity and energy scale 0.04 Beam momentum spectrum 0.03 simulation Neglected π 0 D sources in MC 0.01 δa 10 2 Nicolas Lurkin, 26/04/

18 π 0 TFF: World Data Theory expectations (reminder) Chiral perturbation theory: a = 2.90 ± Dispersion theory: a = 3.07 ± Two-hadron saturation model: a = 2.92 ± CELLO measurement: Extrapolation using VMD model: a = 3.26 ± 0.26 stat 10 2 NA62 measurement Compatible with previous measurements Compatible with theory (~1σ above VMD expectations) 15% relative uncertainty (2x better than previous measurement) Nicolas Lurkin, 26/04/

19 Back to present days! Nicolas Lurkin, 26/04/

20 The K + π + ν ν decay Ultra rare FCNC decay Highly CKM suppressed: Br ~ V ts V td 2 Theoretically very clean Largely dominated by top quark contribution Small contribution from the charm quark Small long-distance corrections Hadronic matric element from Br K + π 0 e + ν Largest uncertainties from CKM parameters Theoretically very clean Small SM contribution New physics contribution can be of the same order Nicolas Lurkin, 26/04/

21 Status of K + π + ν ν Theoretical status: Br K + π + ν ν SM = 9.11 ± Experimental status: Seven signal candidates from E787/E949 experiment at BNL Stopped kaon technique Br K + π + ν ν exp = [A.J. Buras et al., JHEP 1511 (2015) 033] [E787/E949, Phys.Rev.Lett.101, , 2008] Nicolas Lurkin, 26/04/

22 Impact of K + π + ν ν Impose bounds on new physics models Littlest Higgs with T-parity (LHT) Best probe of MSSM non-mfv Custodial Randall-Sundrum Z bosons [Acta Phys. Polonica B41(2010)657] [JHEP 0608 (2006) 064] [JHEP 0903 (2009) 108] [JHEP 1302 (2013) 116] Current exp. precision Measurement of V td complementary to LHCb 10% precision, same central value Searches complementary/alternative to LHC Accessible mass scales beyond those of LHC [arxiv: [hep-ph]] Nicolas Lurkin, 26/04/

23 NA62: The current experiment Operation started in Pilot run 2015 Commissioning run Including a minimum bias run at ~1% intensity 2016 Commissioning + Physics run Data taken at ~40% of the nominal intensity Nicolas Lurkin, 26/04/

24 Low level triggers acquired in /05 22/07: commissioning 3/07 13/09: Final commissioning (L1 Trigger, GTK) + Physics (exotic, rare/forbidden decays) 16/09 03/11: Physics (πνν, exotic, rare/forbidden decays) 2016 run statistics Data analysed and presented On disk [πνν period, 60m fiducial volume] ppp on T10 [average, 40% nominal] 10 3 Tbyte Data ~5% of 2016 data ( K + decays) All GTK stations operational Nicolas Lurkin, 26/04/

25 Main physics goal Measurement of Br K + π + ν ν O 10% precision O 10% signal acceptance Need K + decays in the fiducial volume Need intense beam Momentum 75 GeV/c 2, 1% bite Divergence (RMS) 100 μrad Transverse size mm 2 Composition K + 6%, π + 70%, p 24% Nominal rate ppp on T10 (750 MHz at GTK3). 10% decay in fiducial volume O 5/1 Signal/Background Need O background rejection factor Need powerful discriminant and vetoes Nicolas Lurkin, 26/04/

26 Beam Maximum achievable intensity and DAQ capability depends on the spill quality Spill frequency spectrum is computed online Bad spill Strong 50 Hz component (and harmonics) NA62 data Nicolas Lurkin, 26/04/

27 Region I Region II The NA62 Challenge Decay backgrounds Decay mode BR μ + ν γ 63.5% π + π 0 γ 20.7% π + π + π 5.6% π 0 e + ν 5.1% π 0 μ + ν 3.3% π + π 0 π 0 1.8% π + π e + ν Other backgrounds Beam-gas interactions Upstream interactions Time resolution Matching of upstream-downstream activity (< 100 ps time resolution) Kinematic rejection Kaon momentum (GigaTracker) π momentum (Straw) 2 m miss m K 2 PID and high efficiency Veto systems (< 10 5 inefficiency) Photons vetoes (LAV, LKr, IRC, SAC) Leptons vetoes (LKr, MUV) Accidentals vetoes (CHANTI) 1 P π + m 2 P π 1 P K 2 P K P K P π θ πk π PID (KTAG, RICH) Nicolas Lurkin, 26/04/

28 The NA62 detector Momentum measurement (GTK, STRAW) Particle ID (KTAG, RICH) Photon vetoes (LAV, IRC, SAC, LKr) Muon vetoes (MUV1,2, MUV3) Accidentals vetoes (CHANTI) Trigger (CHOD) Decay region Fiducial region 60 m [NA62 Detector Paper, arxiv: ] Nicolas Lurkin, 26/04/

29 Gigatracker All stations fully operational since September 2 Final m miss resolution using GTK 2 σ m miss vs. P π + σ p π + p π + = 0.3% 0.005% p π + σ p K p K = 0.2% σ θ π + = μrad σ θ K = 15 μrad Design resolution GeV 4 /c 2 Nicolas Lurkin, 26/04/

30 Downstream Particle Identification Particle identification using the RICH detector Best separation for 15 GeV/c 2 < p π + < 35 GeV/c 2 Ring radius vs. P π + RICH ring-finding algorithm Ring efficiency: ε ring ~90% Maximum likelihood with π + hypothesis, using ring radius, p π from Straw ε μ ~ 10 2 ε π ~ 80% Pion ID efficiency Muon ID efficiency Nicolas Lurkin, 26/04/

31 Downstream Particle Identification Particle identification using calorimeters (LKr, MUV1,2, MUV3) BDT technique, using E, E sharing, cluster shape, track-cluster distance ε μ ~ 10 5 ε π ~ 80% Pion ID efficiency Combined efficiency Pion efficiency: 60% Muon rejection: 10 7 Muon ID efficiency Nicolas Lurkin, 26/04/

32 K + π + ν ν analysis Expectations In 2016, study single event sensitivity Signal & background (events/year) Signal 45 K + π + π 0 5 K + μ + ν 1 K + π + π + π <1 Other 3-track decays <1 K + π + π 0 γ (IB) 1.5 K + μ = νγ (IB) 0.5 Total background <10 Start selection with single track kaon decay Single track topology Apply timing cuts to reject accidentals KTAG signal GTK track Straw track matching Trigger PNN: Kaon signal, Single track, no muon signal, no electromagnetic energy Control: CHOD (at least one track, D=400) Nicolas Lurkin, 26/04/

33 Straw Fiducial decay region single track kaon decay p π + vs. z vtx for single track kaon decays Decay origin K + decay downstream of GTK3 K + interaction in GTK3 K + decay upstream of GTK3: early decays Fiducial region GTK3 Design Nicolas Lurkin, 26/04/

34 Fiducial decay region Early decays are source of π + tracks in GTK Possible mismatching of the straw track (from genuine K + decay) with the π GTK track Fake vertex: distributions are different for background and signal 110/115 < z vtx < 165 m R straw1 vs. z vtx π GTK track R straw1 vs. z vtx (signal) Nicolas Lurkin, 26/04/

35 Signal region Region I: Region II: 15 < p π + < 35 GeV/c 2 0 < m miss < 0.01 GeV 4 /c < m miss < GeV 4 /c 2 single track kaon decay Fiducial region 2 m miss vs. p π + (fiducial region) K + π + π + π K + π + π 0 K + μ + ν Nicolas Lurkin, 26/04/

36 Signal region 2 m miss RICH : pion track 3-momentum measured by RICH 2 m miss No GTK : Assume nominal kaon track 3-momentum 2 2 m Apply particle ID miss RICH vs. m miss RICH Calorimeters K + π + π + π region K + π + π 0 region single track kaon decay Fiducial region Particle ID K + μ + ν region Nicolas Lurkin, 26/04/

37 Photon rejection Apply photon veto condition (LKr, LAV, IRC, SAC) π + π 0 suppression: ε π 0 = N after γ rej,pnn trigger D N before γ rej,min.bias = 1.2 ± π + π 0 region, before γ rejection (Control trigger) π + π 0 region, after γ rejection (PNN trigger) single track kaon decay Fiducial region Particle ID Signal region Nicolas Lurkin, 26/04/

38 K + π + ν ν result N exp πνν = D control N control ππ Br πνν A πνν ε trigg = Br ππ A ππ Normalisation: K + π + π 0 control trigger data passing signal selection but the photon rejection Event in the box has m miss outside the signal region 2 Background level ~ 0.6/0.86 from MC No GTK K + π + π K + μ + ν K + π + π + π Total background Estimated S/B 80% 2 m miss ~ 85% (preliminary) measured with data 2 RICH vs. m miss More improvements in signal efficiency and background rejection expected in the future Nicolas Lurkin, 26/04/

39 Multiple opportunities for exotic searches in the current conditions Search for dark photon Search the decay chain K + π + π 0, π 0 A γ, A invisible Br π 0 A γ = 2ε 2 1 m A 2 2 Peak search in M miss Analysis: PNN trigger Selection: Exotic searches: Dark photon Same π + as in π + ν ν 1γ in LKr Missing momentum in LKr Veto for extra γ 2 m π 0 3 = p K p π p γ 2 Br π 0 γγ Min. bias data 2 M miss spectrum m A = 40 MeV/c 2 m A = 60 MeV/c 2 m A = 80 MeV/c 2 m A = 100 MeV/c 2 m A = 120 MeV/c 2 A MC Nicolas Lurkin, 26/04/

40 Dark photon Black line: na62 search result (3% of 2016 data) Red line: assume equal counts for data and background K πνν: model dependent limit from E787/E949 Nicolas Lurkin, 26/04/

41 Heavy Neutral Lepton 2 Peak search in M miss spectrum of K + l + ν l l = μ, e decays Using 2015 minimum bias sample: 23M K + μ + ν μ ; 1500 K + e + ν e Background 100x lower than NA Can set worlds most stringent limits on heavy neutrino production 2015 preliminary 2015 preliminary Nicolas Lurkin, 26/04/

42 Measurement of rare decays K + π + l + l l = μ, e Search for LFV/LNV modes K + π l + l +, K + π + μ ± e Use Multi-track trigger Rare/Forbidden decays 2016A dataset event yield comparable to NA48/2, but much lower background level (~2k K + π + μ + μ, ~1k K + π + e + e ) Nicolas Lurkin, 26/04/

43 2 runs (17h total) dedicated to ALP searches Beam dump mode (remove target, close collimators) ALP couples to 2γ and produced in the upstream collimator Long-lived exotic particles Other exotic searches protons on target/year: π/η/η /Φ/ ρ/ω and charmed mesons produced Can decay into long-lived exotic particles, decaying inside the NA62 decay volume Nicolas Lurkin, 26/04/

44 Nicolas Lurkin, 26/04/

45 Stable data taking for K + π + ν ν Plan for 2017 More exotics, rare and forbidden decays Run at 40% to 60% of nominal intensity (depending on beam quality) Expect SM K + π + ν ν events in 2017 Nicolas Lurkin, 26/04/

46 Summary New result from NA : π 0 transition form factor slope from π 0 Dalitz decay: a = 3.68 ± 0.51 stat ± 0.25 syst 10 2 NA62 detector, beam line and trigger fully commissioned GTK is fully operational, performances are nominal Veto capabilities at the level of 10 7 K + π + ν ν analysis on-going 5% of 2016 data analysed Signal efficiency lower and background larger than expected Improvements are expected Exotic searches being performed with 2015 and 2016 data Already some worlds best limits in some channels First results expected this year, many more in the coming years Nicolas Lurkin, 26/04/