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### Tracking(1)Kp2 decay-in-flight BG simulation

JPARC TREK Collaboration meeting at Saskatchewan

K. Horie

Osaka University

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

Target and tracking

This talk

MC simulation (GEANT3) for Kp2 BG rejection (1) fiber size

(2) target thickness

(3) Kp2 BG fraction

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.

Analysis of target fiber

Maximum energy deposit in 1 fiber (MeV)

Energy deposit in each fiber (MeV)

- fiber geometry square type □2mm20 cm lengthtarget radius : 3cm
- K+ beam800 MeV/c, pencil beam

2 different type of ADC or amplifier are needed.

total energy deposit (MeV)

number of hit fiber

H

L

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.

x

x

x

Method 1

Method 2

Spatial resolution of K+ stopping position as a function of fiber size

rdiff =true – measured

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 1L=2mm

method 1L=1.5mm

L

method 1L=2.5mm

L (cm)

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

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=3cm

rtarg=3cm

rtarg=3cm

rtarg=5cm

rtarg=5cm

rtarg=5cm

rtarg=5cm

Km3

0.12 cm in RMS

0.07 cm in RMS

Kp2

Kp2 BG could be rejected by target diff. cut.

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)

Results of Kp2 BG fractionevent survive factor for target diff. cut

red: 5cm radius

blue: 3 cm radius

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. 0.07cm in RMS
- Fraction of Kp2 BG is controlled to be 0.1-0.5% level.

Consistency of Kpi2 BG fraction between last reported value and this talk

- previous reported at KEKBG fraction = 0.3 %
- Difference Feb. Aug.GEM s=200micron 100micron r= 5cm 8cmtarget r=3cm 3cm, 5cmtracking c2 cut adequate adequatetarget diff. cut loose adequate

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

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