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

2. 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

3. Background rejection Main task of a pnn experiment
All the K+ decay modes are potentially dangerous
Goal of P326: S/B = ~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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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 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)

13. 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 ( 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

14. 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
> 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)

15. 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

16. 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

17. 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

18. 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

19. 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