Search for K + π + ν ν at NA62 Jacopo Pinzino CERN HQL2018 30/05/2018
The NA62 Experiment NA62: High precision fixed-target Kaon experiment at CERN SPS Main goal: measurement of BR(K + π + υ υ) Broader physics program: LFV / LNV in K + decays, hidden sector particles searches. NA62 NA62 Timeline 2008: NA62 Approval 2014: NA62 Pilot Run (partial layout) 2015: Commissioning run Full detector installation completed in September 2016 2016 : First πνν dataset in 2016 (This talk) Continuous data-taking until the end of 2018 ~ 200 participants from: Birmingham, Bratislava, Bristol, Bucharest, CERN, Dubna, GMU-Fairfax, Ferrara, Firenze, Frascati, Glasgow, Lancaster, Liverpool, Louvain, Mainz, Merced, Moscow, Napoli, Perugia, Pisa, Prague, Protvino, Roma I, Roma II, San Luis Potosi, Sofia, Torino, TRIUMF, Vancouver UBC 30/05/2018 JACOPO PINZINO 2
The K πυ υ decay High sensitivity to New Physics FCNC process forbidden at tree level Highly CKM suppressed (BR ~ V ts *V td 2 ) Very clean theoretically: Short distance contribution hadronic matrix element extracted from precisely measured BR(K + π 0 e + υ) SM predictions: BR(K + π + υ υ) = (8.4 ± 1.0) 10-11 BR(K L π 0 υ υ) = (3.4 ± 0.6) 10-11 [Buras et al. JHEP 1511 (2015) 33] Experimental Result: BR(K + π + υ υ) = (17.3 +11.5 10.5 ) 10-11 BR(K L π 0 υ υ) < 2.6 10-8 (90% C.L) [Phys. Rev. D 81, 072004 (2010)] [Phys. Rev. D 77, 052003 (2008), Phys. Rev. D 79, 092004 (2009)] 30/05/2018 JACOPO PINZINO 3
K πυ υ and New Physics Measurement of charged (K + π + υ υ) and neutral (K L π 0 υ υ) modes can discriminate among different NP scenarios Models with CKM-like flavor structure: Models with MFV Models with new flavour-violating interactions in which either LH or RH currents dominate: Z/Z models with pure LH/RH couplings Little Higgs with T parity Models without above constraints: Randall-Sundrum 30/05/2018 JACOPO PINZINO 4
K πυ υ Experimental State of the Art <<Kaon decay at rest>> technique E787+E949: K + decays: ~3.5 10 12, Single Event Sensitivity: ~0.8 10 10 BR(K + π + υ υ) = (17.3 +11.5 10.5 ) 10-11 [Phys. Rev. D 77, 052003 (2008), Phys. Rev. D 79, 092004 (2009)] 30/05/2018 JACOPO PINZINO 5
Analysis Strategy New Decay in flight technique Signal: 1 beam track, 1 charged track, notting else Background: K + decay modes; beam activity Kinematics: Key analysis requirements: 2 signal regions in 15 < <35 GeV/c 60 m long decay region Region I Region II Experimental principles: 1. Precise kinematic reconstruction 2. PID: K upstream, e / μ / π downstream 3. Hermetic γ detection 4. Sub-ns timing Keystone: O (100 ps) Timing between sub-detectors O (10 4 ) background suppression from kinematics > 10 7 Muon suppresion > 10 7 π 0 (from K + π + π 0 ) suppresion Signal and background control regions are kept blind throughout the analysis 30/05/2018 JACOPO PINZINO 6
NA62 Layout SPS Beam: 400 GeV/c protons 2 10 12 protons/spill 3.5s spill Secondary positive Beam: 75 GeV/c momentum 1 % bite 100 mrad divergence (RMS) 60x30 mm2 transverse size K + (6%)/π + (70%)/p(24%) 33x10 11 ppp on T10 (750 MHz at GTK3) Dipole spectrometer 4 straw-tracker stations μ Veto Fe/scint Beam tracking Si pixels, 3 stations γ veto Differential Cerenkov for K + ID in the beam γ veto Decay Region: 60 m long fiducial region ~ 5 MHz K + decay rate Vacuum ~ O(10-6 ) mbar Charged veto μ/π ID 1 atm Ne γ veto 30/05/2018 JACOPO PINZINO 7
Signal Selection πνν selection: K + Decay Event Fiducial Decay Region Particle ID: π + Photon rejection Multiple charged particle rejection Kinematic Selection of the Signal Regions Performance: ε μ + = 1 10-8 (64% π + efficiency) ε π 0 = 3 10-8 2 σ(m miss ) = 1 10-3 GeV 2 / c 4 σ T ~ O(100 ps) 30/05/2018 JACOPO PINZINO 8
Kinematic suppression 2 Three ways to compute the m miss 2 m miss (STRAW, GTK) 2 m miss (RICH, GTK) 2 m miss (STRAW, Beam) : Protects against mis-reconstruction Kinematic suppression: Measured using data Samples of K ππ and K μν Selected using calorimeters Fraction of Events in signal regions: K + π + π 0 ~ 1 10 3 K + μ + υ ~ 3 10 4 Radiative: π 0 + γ x 30 π 0 rejection in R2 23/05/2018 JACOPO PINZINO 9
Single Event Sensitivity (SES) 3% 1% Acceptance K + π + υ υ 0.04 ± 0.001 Normalization acceptance (K + π + π 0 ) 0.1 R2 R1 Source δses(10-10 ) Random Veto ±0.17 N K ±0.05 Trigger efficiency ±0.04 Definition of π + π 0 region ±0.10 Momentum spectrum ±0.01 Extra activity ±0.09 Simulation of π + interactions ±0.02 GTK Pileup simulation ±0.02 Total ±0.24 PNN trigger efficiency 0.87± 0.02 SES = (3.15 ± 0.01 stat ± 0.24 syst ) 10-10 Number of K + in the fiducial volume 1.21(2) x 10 11 23/05/2018 JACOPO PINZINO 10
backgrounds : K + π + π 0 (γ) Data driven background estimation Control region validation: 1 event observed (1.5 expected) bg N ππ(γ) = 0.064 ± 0.007 stat ± 0.006 syst 23/05/2018 JACOPO PINZINO 11
backgrounds : K + μ + ν μ (γ) Data driven background estimation Control region validation: 2 event observed (1.1 expected) bg N μ+ ν μ (γ) = 0.020 ± 0.003 stat ± 0.003 syst 23/05/2018 JACOPO PINZINO 12
Upstream background Accidental particles from the beam line Pions from interactions with beam spectrometer material Kaon-pion matching and geometrical cuts effective Data driven estimation bg N upstream +0.090 = 0.050 0.030 23/05/2018 JACOPO PINZINO 13
Background summary Process K + π + υ υ (SM) Total Background K + π + π 0 (γ) IB K + μ + ν μ (γ) IB Expected events in R1+R2 0.267 ± 0.001 stat ± 0.020 syst ± 0.032 ext 0.15 ± 0.09 stat ± 0.01 syst 0.064 ± 0.007 stat ± 0.006 syst 0.020 ± 0.003 stat ± 0.003 syst K + π + π - e + ν e 0.018 0.017 stat ± 0.009 syst K + π + π + π - 0.002 ± 0.001 stat ± 0.002 syst Upstream Background 0.050 +0.090 0.030 stat 23/05/2018 JACOPO PINZINO 14
Result 23/05/2018 JACOPO PINZINO 15
Result R2 R1 One event observed in R2 23/05/2018 JACOPO PINZINO 16
Result: RICH ring for the observed event 23/05/2018 JACOPO PINZINO 17
Result 1 observed Events SES = (3.15 ± 0.01 stat ± 0.24 syst ) 10-10 Expected Background = 0.15 ± 0.09 stat ± 0.01 syst BR(K + π + υ υ) < 14 10-10 @ 95% CL BR(K + π + υ υ) < 11 10-10 @ 90% CL BR(K + π + υ υ) = 2.8 +4.4 2.3 10-10 @ 68% CL For comparison: SM prediction: BR(K + π + υ υ) SM = (0.84 ± 0.10) 10-10 BNL E949/E787 (Kaon decays at rest): BR(K + π + υ υ) exp = 1.73 +1.15 1.05 10-10 23/05/2018 JACOPO PINZINO 18
Prospects Processing of 2017 data is on going: 20 more than the presented statistic Upstream background reduction expected Improvements on reconstruction efficiency 2018 data taking is started on 09/04/2018 218 days including stops On going studies to improve signal acceptance ~20 SM events expected on tape before LS2 Running after 2018 to be approved Conditions for ultimate sensitivity under evaluation 23/05/2018 JACOPO PINZINO 19
Summary The new NA62 decay in flight technique works One events observed in the 2016 data BR(K + π + υ υ) < 14 10-10 @ 95% CL 23/05/2018 JACOPO PINZINO 20
Spares 30/05/2018 JACOPO PINZINO 21
backgrounds : K + π + π - e + ν e (K e4 ) K physics alone can fully constrain the CKM unitarity triangle Comparison with B physics can provide description of NP flavour dynamics 23/05/2018 JACOPO PINZINO 22
NA62 Luminosity 2016 Run 13 10 11 ppp on target (40% normal) ~1 10 11 K + decays useful for π + υ υ 2017 Run 20 10 11 ppp on target (60% normal) >3 10 12 K + decays collected 30/05/2018 JACOPO PINZINO 23
Kinematic Resolution 23/05/2018 JACOPO PINZINO 24
backgrounds : K + π + π + π - N(π + π + π - ): Event in π + π + π region after π + νν selection f kin measured on a π + π + π control sample selected tagging the π + π pair Kinematic rejection factor corrected for biases induced by the control sample selection using MC f kin (R2) 10 4 bg N π+ π + π = 0.002 ± 0.001 stat ± 0.002 syst 23/05/2018 JACOPO PINZINO 25
NA62 Physics beside K + π + ν ν Standard kaon physics: ChPT studies: K + π + γγ, K + π + π 0 e + e, K + π + l + l Searches for lepton-flavor or -number violating decays K + π + μ ± e, K + π μ + e +, K + π l + l + Heavy neutral lepton production searches K + l + ν h (analysis with 2015 data published in Phys.Lett. B778 (2018) 137-145) ν h from upstream K, D decays with ν h πl Searches for long-lived dark sector particles Dark photon γ produced in π/ρ decays in target, with γ l + l Axion-like particle A 0 produced in target/beam dump, with A 0 γγ π 0 decays rare and forbidden/lfv, dark photon production: π 0 invisible, π 0 3/4γ, π 0 ee,eee, π 0 μe, π 0 γ γ 30/05/2018 JACOPO PINZINO 26
An Example:K + π + μ + μ - 50% 2016 + 25% 2017 Data: N k = 6.3 10 11 World-largest K + π + μ + μ - sample: ~4.6 10 3 events (BR ~10-7 ) Improvements on NA48/2: mass resolution better by ~ a factor of 2 Search for LNV decay K + π - μ + μ +, not limited by background: SES = 2 10-11 Sensitivity on hidden sector search, K + π + χ, χ μ + μ - for lifetimes up to O(1ns) 30/05/2018 JACOPO PINZINO 27