1. Tracking(1) Kp2 decay-in-flight BG simulation
Sep. 1, 2007
JPARC TREK Collaboration meeting at Saskatchewan
Tracking(1) Kp2 decay-in-flight BG simulation
K. Horie
Osaka University

2. Simple estimation of Kp2 BG fraction
Fraction of Kp2 decay-in-flight in the target is BR(Kp2)/BR(Km3) * decay probability ~ * (1.- exp (target radius / bgct) ) ~1.
momentum of Kp2 p+ 205MeV/c; b= 0.84
bgct ~ 100cm
target radius ~ 10cm
1- exp(-10/100) ~ 0.1
The number of Kp2 decay-in-flight in the target is comparable to the number of Km3 events

3. Target and tracking Last meeting at KEK
Source of Kp2 BG (1)scatter in the spectrometer (2)dif in the spectrometer can be rejected by installing C0.
(3)dif before C0
… to be rejected by target fiber.
This talk
MC simulation (GEANT3) for Kp2 BG rejection (1) fiber size
(2) target thickness
(3) Kp2 BG fraction

4. Analysis of target fiber
Energy deposit in each fiber (MeV)
Maximum energy deposit in 1 fiber (MeV)
fiber geometry square type □2mm 20 cm length target radius : 3cm
K+ beam 800 MeV/c, pencil beam
2 different type of ADC or amplifier are needed.
total energy deposit (MeV)
number of hit fiber

5. Fiber size and spatial resolution of K+ stopping position
K+ stopping position was determined as a function of fiber size by GEANT3.
Analysis method Method 1 : K+ stopping position was determined by 1 fiber position which has the maximum energy deposit. Method 2: K+ stopping position was determined by energy weighted average of 2 fiber positions which has the maximum and 2nd maximum energy deposit.
Energy deposit H L x x x
Method 1
Method 2

6. Spatial resolution of K+ stopping position as a function of fiber size
rdiff =true – measured
method 1 L=1.5mm
method 1 L=2mm L
open : method 1 (using 1 fiber info.)
closed: method 2 (using 2 fibers info.) K+ stopping position was obtained to be 0.07cm in RMS.
method 1 L=2.5mm
L (cm)

7. Kp2-dif fraction in Km3 sample
Kp2 decay-in-flight fraction was obtained as a function of target radius by a GEANT3 code.
C0(GEM) position was fixed and more realistic than the previous study. rGEM= 8cm, s=0.01cm
tracking c2
4 point tracking (C0, C2, C3, C4) with a c2 cut
m requirement by TOF
P>185 MeV/c or back-to-back events are rejected.
Km3
Kp2

8. Target diff. spectra
K+ stop. pos. (hit-fit) (cm)
Most of survived Kp2 events after Km3 selection is p+ decay-in-flight before C0 chamber.
Resolution of K+ stopping position (s=0.07cm) is not considered.
rtarg=5cm
rtarg=5cm
rtarg=5cm
rtarg=5cm
rtarg=3cm
rtarg=3cm
rtarg=3cm
Km3
0.12 cm in RMS
0.07 cm in RMS
Kp2
Kp2 BG could be rejected by target diff. cut.

9. Results of Kp2 BG fraction
Kp2 background fraction can be obtained as
BG fraction=
Ω is the detector acceptance with Km3 selection conditions.
Br(Kp2)* Ω(Kp2)
Br(Km3)* Ω(Km3)
event survive factor for target diff. cut
red: 5cm radius
blue: 3 cm radius

10. Summary
Fraction of Kp2 dif BG is estimated by using a GEANT3 for target radius of 3cm and 5cm .
Most of survived Kp2 events after Km3 selection is p+ decay-in-flight before C0 chamber. Arrangement of target fiber with C0 is key point to reject.
Resolution of K+ stopping position is evaluated cm in RMS
Fraction of Kp2 BG is controlled to be % level.

11. backup

12. Consistency of Kpi2 BG fraction between last reported value and this talk
previous reported at KEK BG fraction = 0.3 %
Difference Feb Aug. GEM s=200micron micron r= 5cm cm target r=3cm 3cm, 5cm tracking c2 cut adequate adequate target diff. cut loose adequate

13. Simple estimation of Kp2 BG events
momentum of Kp2 p+ 205MeV/c; b= 0.84
bgct is nearly 100cm
target radius is neary 10cm
1- exp(-10/100) is nearly prob= 0.1
fraction of Kp2 decay in flight in the target is BR(Kp2)/BR(Km3) * prob is neary 1.
The number of Kp2 decay-in-flight in the target is comparable to the number of Kmu3 events