Background rejection in P326 (NA48/3) Giuseppe Ruggiero CERN K-Rare 2005 Workshop Frascati 26 / 05 / 2005 13/04/2019 Giuseppe Ruggiero - CERN

Giuseppe Ruggiero - CERN Overview Characterization of the background Kinematics and background rejection capability Muon rejection and Muon ID Requirements and results from simulations Photon rejection Electron ID Results from NA48 studies on data Results about background rejection Some thoughts about Charged Veto Conclusions 13/04/2019 Giuseppe Ruggiero - CERN

Background rejection Main task of a pnn experiment All the K+ decay modes are potentially dangerous Goal of P326: S/B = 10 ~10-12 rejection 2-Steps: Kinematic rejection Veto and Particle ID g, m, charged particles m – p - e separation As better as possible resolution in charged particle reconstruction High hermeticity 13/04/2019 Giuseppe Ruggiero - CERN

Background kinematically constrained Pion track hyp. Decay BR Km2 0.634 p+p0 0.211 p+p+p- (p0p0) 0.070 92% of total background Allows us to define the signal region 13/04/2019 Giuseppe Ruggiero - CERN

Background not kinematically constrained Decay BR Ke3 0.049 Km3 0.033 Km2g 0.006 p+p0g 0.001 Ke4 4 x 10-5 Km4 1 x 10-5 Pion track hyp. 8% of total background Spoils the signal region 13/04/2019 Giuseppe Ruggiero - CERN

Kinematics: Gigatracker + Double Spectrometer Tracking systems operating in vacuum Gigatracker: pixels Spectrometer: Straw tubes Gigatracker: 4x10-3 X0 per station PK measurement qK measurement Spectrometer: 5x10-3 X0 per chamber 2 Ptrack measurements qtrack measurement Resolution limited by MS 13/04/2019 Giuseppe Ruggiero - CERN

Kinematic reconstruction Total qpK qp PK Ptrack qK Two independent measurements of the downstream track momentum m2miss resolution ~1.1×10-3 GeV2/c4 Main contribution from QpK measurement 13/04/2019 Giuseppe Ruggiero - CERN

Giuseppe Ruggiero - CERN Kinematic rejection CUTS: Against Km2, p+p0 and p+p+p- Gaussian background < 10-6 Against Km2 RICH operational reasons Simulation and results p+p0 Simulation of the tracking systems GEANT - based Accidental PileUp in Gigatracker (150 ps resolution per station) Kinematic rejection inefficiency: (Limited by non gaussian tails from MS) Km2 ~5 x 10-6 (Region I mainly) p+p0 ~2 x 10-4 Reconstruction: Room for ×3 gain in rejection power (loss in signal acceptance) Cuts on Ptrack 13/04/2019 Giuseppe Ruggiero - CERN

Muon rejection: Physics Sources of m rejection inefficiency: “Catastrophic” m energy losses m bremsstrahlung e+e- pair production high Q2 m+ e- scattering m decay in flight Deep inelastic m – nucleon scattering m+ + N  m+ + hadrons (<10-6) electromagnetic shower (10-5) EM showers (from ICARUS) Hadronic shower (from ICARUS) 13/04/2019 Giuseppe Ruggiero - CERN

Giuseppe Ruggiero - CERN Muon rejection: MAMUD Detector: Sampling Calorimeter (m rejection) + Magnet (beam deflection) Goal: m rejection inefficiency < 10-5 Sensitivity to minimum ionizing particles (MIP) Distinguish hadronic and electromagnetic showers (longitudinal segmentation) Bending power: 5 Tm  75 GeV/c beam deflected by ~18 mrad 13/04/2019 Giuseppe Ruggiero - CERN

Simulation of muon rejection: results Complete GEANT simulation of MAMUD Rejection: MAMUD + LKr calorimeter Rejected events: MIP deposition in last section only EM cluster shape Inefficiency ~10-5 (>90% signal acceptance) (Inefficiency ~10-6 with 50% signal acceptance) 13/04/2019 Giuseppe Ruggiero - CERN

Giuseppe Ruggiero - CERN Muon – Pion ID Detector: RICH Goal: Muon – Pion separation with 10-2 ineff. over a wide momentum range As low X0 as possible (RICH before LKr) 1st option: P.S. Cooper – FERMILAB-CONF-05-015-CD Some Brain – Storming : O. Ullaland (CERN) 0.1 ~1 in 2 m Ar : ~22 pe and c=23.7 mrad Argon 0.01 Helium Dq m / p (rad) 0.001 ~1 in 15 m He : ~21 pe and c=8.2 mrad 0.0001 5 10 15 20 25 30 35 40 13/04/2019 Giuseppe Ruggiero - CERN Momentum (GeV/c)

Energy of photons from p0 in p+p0 events Giuseppe Ruggiero - CERN Photon Rejection Detectors: lead-scint sandwich (ANTI), LKr, lead-scint sandwich (IRC, SAC) Goal: 10-8 level of veto inefficiency on p0 (requirement from p+p0) Decays with p0: energy correlation between gs’ from p0 decay Decays with single photon (radiative): hermeticity (0 - 50 mr coverage) Energy of photons from p0 in p+p0 events ANTI LKr Eg > 1 GeV IRC / SAC Eg > 6 GeV 1 2 1 2 3 4 5 6 1 2 3 4 5 6 7 8 9 10 Eg (GeV) Eg (GeV) Eg (GeV) 13/04/2019 Giuseppe Ruggiero - CERN

Simulation of photon rejection: results Simulation of geometrical layout Parametrization of the g inefficiencies Inefficiency: 2 x 10-8 on p0 from p+p0 5 x 10-8 on p0 from Kl3 10-3 on g from radiative GEANT simulation of each device started Simulation results validating on existing detector configurations where data are available Detector E range Inefficiency ANTI < 50 MeV 1 (0.5, 1) GeV 10-4 > 1 GeV 10-5 LKR < 1 GeV (1,3) GeV (3,5) GeV 10-4 - 10-5 > 5 GeV IRCs, SAC All 10-6 2mm lead / 6mm Scintillator DATA: S. Ajimura et al., NIM A435 (1999) 408 MC: Our GEANT4 simulation Inefficiency 10-4 10-5 13/04/2019 Giuseppe Ruggiero - CERN 200 400 600 800 1000 Photon Energy (MeV)

Giuseppe Ruggiero - CERN Electron ID Detector: LKr Inefficiency of e ID studied in NA48 with e from p0 Dalitz decay. Background from hadronic showers hID = 1% for E/p < 0.9. Improvement of a factor 10 if E/p < 0.85 (about 2% signal lost) Room for improvements using E/p + other informations about clusterization (NN technique). We assume hID = 10-3 13/04/2019 Giuseppe Ruggiero - CERN

Signal acceptance and Kaon flux Fast simulation of the complete layout Signal acceptance (Geometry, kinematic cuts, FF): Region I: 4.5% Region II: 14.5% Assumed signal BR = 10-10 Detailed simulation of the beam line Expected kaon decays in fiducial region per year: 4.8 x 1012 13/04/2019 Giuseppe Ruggiero - CERN

RESULTS: Events collected per year Total Region I Region II Signal 91.2 21.6 69.6 Km2 1.6 1.4 0.1 p+p0 4.4 2.3 2.1 Ke3 0.2 Km3 <0.1 Km2g 0.4 0.3 p+p0g p+p+p- In progress Ke4 Km4 < 10-2 ×Ke4 Background 8 + charged 4 4 + charged Without Form Factor 13/04/2019 Giuseppe Ruggiero - CERN

Thoughts about Charged Vetoes Goal: at least 5 x 10-3 on a single track Reject Ke4, Km4, p+p+p- The most dangerous one: Ke4 Task at high angle 2 Gigatracker stations 13/04/2019 Giuseppe Ruggiero - CERN

Giuseppe Ruggiero - CERN Conclusions The experimental layout is being finalised Background estimation almost complete Region I: well understood, a RICH is needed for m background rejection. S/B = ~5, but room for improvements. Region II: S/B > 10, (but with Ke4 and Kp3 contributions missing) Charged vetoes to be optimised Once dead-time and selection cuts are taken into account, a 10% signal acceptance is plausible (i.e. 40 events/year for Br~10-10) 13/04/2019 Giuseppe Ruggiero - CERN