Tracking(1)K2 decay-in-flight BG simulation

K. Horie

Osaka University

Sep. 1, 2007JPARC TREK Collaboration meeting at Saskatchewan

Simple estimation of K2 BG fraction

Fraction of K2 decay-in-flight in the target is BR(K2)/BR(K3) * decay probability ~ * (1. exp (target radius / c

momentum of K2 + 205MeV/c; = 0.84c ~ 100cmtarget radius ~ 10cm1- exp(-10/100) ~ 0.1

The number of K2 decay-in-flight in the target is comparable to the number of K3 events

Target and tracking

Last meeting at KEK

Source of K2 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 K2 BG rejection (1) fiber size

(2) target thickness

(3) K2 BG fraction

Analysis of target fiber

• fiber geometry square type □2mm20 cm lengthtarget radius : 3cm

• K+ beam800 MeV/c, pencil beam

2 different type of ADC or amplifier are needed.

Energy deposit in each fiber (MeV)

Maximum energy deposit in 1 fiber (MeV)

total energy deposit (MeV)

number of hit fiber

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 Method 1 Method 2

x xx

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

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.

L

method 1L=1.5mm

method 1L=2.5mm

method 1L=2mm

rdiff =true – measured

L (cm)

K2-dif fraction in K3 sample

• K2 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, =0.01cm

4 point tracking (C0, C2, C3, C4) with a 2 cut

requirement by TOF P>185 MeV/c or back-to-b

ack events are rejected.

K3K2

tracking2

Target diff. spectra

rtarg=5cm rtarg=3cm• Most of survived K2 events

after K3 selection is + decay-in-flight before C0 chamber.

• Resolution of K+ stopping position (=0.07cm) is not considered.

rtarg=5cm rtarg=3cmrtarg=5cm

K+ stop. pos. (hit-fit) (cm)

rtarg=3cmrtarg=5cm

K3

K2

0.12 cm in RMS

0.07 cm in RMS

K2 BG could be rejected by target diff. cut.

Results of K2 BG fraction

red: 5cm radiusblue: 3 cm radius

• K2 background fraction can be obtained as

BG fraction=

• Ω is the detector acceptance with K3 selection conditions.

Br(K2)* Ω(K)

Br(K3)* Ω(K3)

event survive factor for target diff. cut

Summary

• Fraction of K2 dif BG is estimated by using a GEANT3 for target radius of 3cm and 5cm .

• Most of survived K2 events after K3 selection is + 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 K2 BG is controlled to be 0.1-0.5% level.

backup

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

• previous reported at KEKBG fraction = 0.3 %

• Difference Feb. Aug.

GEM =200micron 100micron r= 5cm 8cmtarget r=3cm 3cm, 5cmtracking 2 cut adequate adequatetarget diff. cut loose adequate

Simple estimation of K2 BG events

• momentum of K2 + 205MeV/c; = 0.84 c is nearly 100cm• target radius is neary 10cm• 1- exp(-10/100) is nearly prob= 0.1• fraction of K2 decay in flight in the target is

BR(K2)/BR(K3) * prob is neary 1.

The number of K2 decay-in-flight in the target is comparable to the number of Kmu3 events