Pair-Symmetric Background
Analysis
Hall C Users’ Group Meeting,
Jefferson Lab, January, 2010
Luwani Z. Ndukum
Mississippi State University
Motivation
Q2
World data and
SANE kinematics

Recover data at low x
SANE
Outline
Introduction
 Effects of Background on DIS Asymmetry
 Asymmetry Analysis
 Pair Systematic Errors
 Trajectory of an electron-positron pair in SANE
 Summary

Introduction

Background mostly e+ e- pair
◦ Contamination in cerenkov
◦ Bethe-Heitler
(bremsstrahlung)
◦ π0 decay: π0 → γ e+e- (1.2%)
π0 → γγ (99%)

Correction will contribute to
SSF studies
Effect of Background on DIS Asymmetry

BETA is charge insensitive

The measured electron asymmetry must
be corrected for the background counts
and asymmetry

Need to estimate both background rate
and asymmetry
Asymmetry Analysis

The measured electron asymmetry
Nb
N
Am 
A
Ab where
Nm
Nm
fb 
Nb
Nm
 N  N m (1  f b )

A
Am  f b Ab
(1  f b )
N m  N  Nb
Am = measured asymmetry
A = Physics asymmetry,
Ab = Background asymmetry
N m= measured counts,
f b = fraction of background
Pair Systematic Error
If we assume Ab  0 , but has an uncertainty given by dAb
A
with f b = e+/e- ratio
 Systematic error is given by

A
Am
(1  f b )
 Ab
fb
A


A (1  f b )  A
2
  f b 
 

  f 
  b 
2
 Systematic uncertainty
> 20% for E' < 1.1 GeV
Data from table of Vipuli D., SANE report 4/21/05
Pair Systematic Error

Hall C analysis from
Jason Seely

Clear theta
dependence

Contamination is
small at large angles
From:
Vipuli Dharmawardane
SANE report 4/21/05
Trajectory of an e+e- pair for SANE
Example:
P  700 MeV

(each particle)

B
80 
2.78
Pair starting direction: 40°
(along BETA's axis)
40

Vertical deflection = 11.93°
(translates to 71cm in
BigCal)
Top
11.93

700 MeV/c corresponds to
x=0.2 for DIS electrons
142cm
224cm
11.93

side
Summary
Background for SANE is mostly e+e- pairs
 Clear theta dependence
 Small contamination at large angles
 Large f b magnifies error
 Need to estimate both the background rate
and the asymmetry
 Systematic uncertainty >20% for E' < 1.1 GeV
